Recent Advances in Biomimetic Related Lubrication

Abstract

Friction is ubiquitous in industry and daily life, which not only leads to the wear and tear of equipment and machinery, but also causes a lot of energy waste. Friction is one of the significant factors leading to energy loss in mechanical systems. Therefore, it is essential to minimize friction losses. Creatures in nature have evolved various surfaces with different tribological characteristics to adapt to the environment. By studying, understanding, and summarizing the friction and lubrication regulation phenomena of typical surfaces in nature, various bionic friction regulation theories and methods are obtained to guide the development of new lubrication materials and lubrication systems. This article primarily discusses the study of lubrication mechanisms through biomimetic design, which is mainly divided into chemical approaches, structural strategies, and chemical–structural coupling approaches. From the chemical point of view, this paper mainly summarizes joint lubrication and engineering lubrication in biomedicine, with inspiration from lotus leaves, fish skin, and snake skin, each with unique antifriction structures which are famous for their super hydrophobicity in nature. Finally, chemical–structural coupling simulates the lubrication mechanism of natural organisms from the joint action of biological structures and chemical substances, and is applied to coating design, so as to reduce the friction and wear on coating surfaces, improve the durability and anti-pollution ability of coatings, significantly improve the tribological performance of mechanical systems, promote scientific innovation, and promote energy conservation, emission reduction, and sustainable development.

1. Introduction

Lubrication technology, as a new energy-saving strategy, plays a significant role in engineering design, biomedical applications, and aerospace fields [1,2,3]. However, as quality of life improves and technological innovations advance rapidly, traditional lubrication methods sometimes struggle to meet the demands of specific friction systems [4,5,6]. Bionic lubrication technology emerges from the study of the friction and lubrication phenomena related to the microstructures and surface characteristics of biological entities in nature [7,8,9]. This research offers a variety of bionic friction regulation theories and methods, guiding the development of novel lubricating materials and systems [10,11,12]. It also provides potential solutions for effectively lubricating complex structures such as modern devices and human joints [13,14,15]. Peta et al. investigated lubrication and wetting phenomena in human joints and gave the optimal observation scales as 203 μm², 664 μm², and 429 μm² at 3%, 5%, and 7% CMC fluid, respectively [16].

Currently, many lubrication strategies based on tribological theory have emerged and been implemented at both macro- and microscales to reduce friction and subsequent wear [17,18,19]. Lubrication is important with relation to sliding, erosion, wear, and impact. It reduces friction, lowers wear, retards erosion, and cushions impact forces by forming a protective film between contact surfaces. Lubrication not only improves the operational efficiency and reliability of equipment, but also extends service life, reduces maintenance costs, and ensures smooth operation.

This is especially true since the Industrial Revolution, with technology encompassing oil lubrication, lubricating additives such as graphite, nanoparticle lubrication, and hydrated lubrication [20,21,22]. Moreover, metal surfaces can undergo plasma treatment or be coated with Teflon^® or other lubricating polymers to enhance lubrication performance [23,24,25]. We find the lubrication of biological surfaces particularly intriguing and challenging. Recent advancements in cartilage demonstrate bioinspired polymers for lubrication and wear resistance, as well as polymer-based lubricating materials for functional hydrated lubrication [26,27,28]. For example, polymer-based lubricants can be used as drug carriers to realize controlled release of drugs. Applications range from the lubrication of soft oral surfaces to the development of lubricants for osteoarthritis treatment, including those used in medical devices like artificial joints [29,30,31]. Nature supplies diverse forms of lubrication that inspire our work. A typical example includes the fabrication of biomimetic surfaces, such as lotus leaves, shark skin, and cicada wings [32,33,34]. However, constructing micro/nanostructures necessitates costly techniques like photolithography, multi-jet fusion, and laser powder bed fusion [35,36,37]. Also, the surface microtopography significantly impacts lubrication efficacy and may fail under high temperature or pressure conditions [38,39,40]. Notably, the uniqueness of biological systems determines the specificity of material molecules and lubrication structures in biomimetic systems, complicating the development of biomimetic lubrication technologies [41,42,43]. Thus, meticulous research into material selection and surface structure is crucial for advancing biomimetic lubrication techniques [44,45,46].

Research indicates that polymer lubricants serve as sustainable lubrication materials with low friction coefficients and high wear resistance in biological lubrication [47,48,49]. Polymer solutions exhibit excellent lubricating effects in natural environments, primarily due to nonspecific interactions [50,51,52]. For instance, natural polymer glycoproteins (mucin), proteoglycans, and lipids are essential components in joint lubrication [53,54,55]. In practical applications, polymers are favored due to their cost-effectiveness and ease of mass production. Additionally, hydrogels, as macromolecular adhesive materials, play a critical role in biological lubrication [56,57,58]. Hydrogels can accommodate significant volumes of liquid, allowing them to be finely tuned for controlled release mechanisms and jelly-like lubricants for joints, thereby minimizing friction and degradation between moving bone and cartilage surfaces, ultimately allowing prolonged use to protect joints [59,60,61]. Beyond lubrication, wear protection is equally vital for the effective and sustainable function of various systems [62,63,64]. In natural systems such as teeth, claws, shells, exoskeletons, bones, nacre, and enamel, wear-resistant structures typically consist of organic materials with specialized architectures that contribute to the study of biomaterials’ abrasion resistance [65,66]. Clearly, the unique properties of lubricating materials and their intricate wear-resistant structures enable biomimetic lubrication technologies to overcome challenges posed by the simplistic chemical nature of materials and suboptimal interfacial contact conditions [67,68]. Moving forward, integrating the advantages of various materials to develop high-performance or easily modified functional materials, achieving on-demand design, and manufacturing biomimetic lubricating materials and structures will remain a significant challenge [69,70,71].

In this review, we aim to investigate various biomimetic lubrication strategies (Figure 1). These include chemical, surface structural, and multifactorial coupling approaches [72,73,74]. We will explore the lubrication mechanisms from a chemical perspective, particularly in biomedical and engineering contexts. Furthermore, this article will delve into the impact of surface structures on lubrication, such as the superhydrophobic properties of lotus leaf structures and the streamlined resistance-reducing patterns found in snake and fish skins [75]. The research demonstrates that introducing biomimetic textures on material surfaces, combined with solid lubricants, effectively reduces the coefficient of friction and wear rate [76,77,78]. This chemical–structural coupling strategy not only enhances the self-lubricating properties of materials but also improves their stability and durability under extreme conditions [79,80,81]. In this paper, bionic lubrication inspired by nature is reviewed. Whether it is to reduce liquid resistance (superhydrophobicity) or reduce solid-to-solid friction (lubrication), its essence is to reduce friction and play a lubricating effect. We will summarize the progress made in the practical application of biomimetic lubrication technology and discuss current challenges along with future development directions [82].

2. Chemistry Aspect of Tribology Lubrication

The application of biomimetic design in lubrication has made significant advancements in the field of chemistry. This section highlights how scientists have developed various novel lubricating materials by mimicking the lubrication mechanisms found in nature, such as cartilage and the mucus secretion of mollusks. It summarizes several typical materials, including biomimetic joint nanomaterials and drilling fluids, which exhibit excellent lubrication performance and wear resistance. Furthermore, these materials demonstrate good environmental adaptability and biocompatibility, providing readers with fresh ideas for researching high-performance lubricants.

2.1. Biomedical Treatment

Biomedical engineering significantly influences contemporary healthcare. It emphasizes disease prevention, treatment efficacy, biomaterials, and tissue engineering [83,84,85]. By meticulously analyzing human biological traits, biomedical engineering facilitates the development of enhanced strategies for disease prevention and management. This encompasses a comprehensive understanding of disease pathology, innovative diagnostic methodologies, and tailored therapeutic approaches. This section will examine the investigations and applications of biomimetic lubricating materials, particularly in joint lubrication, optimizing artificial joint performance, minimizing friction and wear, and enhancing patient quality of life. Cartilage mainly exists in the joints of the human body, especially those joints that need to bear weight and provide a smooth moving surface, such as the knee joint: this is one of the largest and most complicated joints in the human body. Articular cartilage is an elastic load-bearing tissue, which covers the articular surface and can effectively reduce the friction of the articular surface during sliding. The friction coefficient of articular cartilage can be as low as 0.001 under physiological high pressure, and this extremely low friction coefficient contributes to the smoothness of joint movement. In addition, the surface of articular cartilage is very smooth, which is due to the single distribution of chondrocytes on the surface of articular cartilage, which is small in size and oval, and its long axis is parallel to the surface of cartilage. This arrangement makes the surface of cartilage smoother. Deep cartilage connects with osseous tissue, where the cartilage matrix undergoes calcification, thus providing additional support and strength. Simultaneously, the collagen fibrils within the matrix adopt an arched orientation, reinforcing the cartilage while also supplying significant stress support, offering both compressive resistance and a degree of elasticity. The low friction coefficient between articular cartilages is crucially supported by lubricating substances, such as synovial fluid and hyaluronic acid, which ensure smooth joint motion and minimize friction and wear. This section explores a range of biomimetic nano-materials for joint applications, including polymer brushes and nano-composite hydrogels.

Polymer brushes represent nanoscale materials characterized by elongated polymer molecular chains. One end of these chains’ anchors to a solid substrate, while the other end remains free to move. This configuration resembles a bristle brush and facilitates the formation of a low-friction interface between contacting surfaces. Polymer brushes create a stable hydrated layer at the bone–cartilage interface, accommodating the relative motion of adjacent surfaces and minimizing friction and wear. This property is critical for joint lubrication. To enhance lubrication between bone and cartilage, using anchoring groups proves beneficial. By reacting carboxyl, aldehyde, or epoxy groups with the amine groups on cartilage surfaces, these anchoring groups improve the polymer’s adhesion to cartilage. Notably, polymers containing epoxy groups can spontaneously react with amine groups at room temperature, resulting in a significant reduction in the coefficient of friction. Research by Karolina et al. [86] has explored the roles of carboxyl, aldehyde, or epoxy groups incorporated into polymer brush macromolecules as anchoring blocks or cartilage-binding moieties. The structure of the natural lubricant protein Lubricin consists of three brush-like structures. It features a mucin-like central domain and two non-glycosylated termini. In their study, researchers synthesized the structure of brush polymers. The anchoring groups, such as epoxy groups, can react with amino groups on the cartilage surface, forming a stable lubricating layer (Figure 2a). This is due to the ability of the anchored groups of the polymer brushes to form chemical bonds with the amino and hydroxyl groups on the cartilage surface, or to physically adsorb through hydrogen bonds and electrostatic interactions. Such interactions enhance the adhesion of the polymer brushes to the cartilage surface. Given the significant differences in friction properties between biological samples, researchers first conducted baseline lubrication tests on paired cartilage tissues in phosphate-buffered saline (PBS). They then introduced the brush polymer solution onto the same contact area to evaluate its lubrication effect. The enhancement of lubrication performance was measured by comparing the decrease in friction coefficients before and after using the polymer (Figure 2b). Results showed that polymers without anchoring groups reduced the friction coefficient by 22%, while polymers with anchoring groups exhibited even more pronounced lubricating effects. Specifically, polymers modified with carboxyl, aldehyde, and epoxy groups reduced the friction coefficient by 35% to 45%. Different structures exhibit varying lubrication effects. Compared to linear polymers, cyclic polymers, lacking free chain ends, create a denser and more lubricated brush layer on joint surfaces. Zhang et al. [87] designed a super-lubricating ring brush of zwitterionic polymer (CB) for the first time. They utilized poly(2-hydroxyethyl methacrylate) (c-p(HEMA)) as a core template. Through a “one-pot” method involving atom transfer radical polymerization (ATRP), they co-polymerized SBMA and DMAEMA to produce (cb-P(HEMA-g-P(DMAEMA-st-SBMA))). The zwitterionic groups on the CB, such as positively charged quaternary ammonium salts and -SO₃-, effectively attract large quantities of water molecules via ionic coupling, forming a stable hydrated lubrication layer. This hydrated layer reduces friction and protects contact surfaces during frictional processes. Additionally, the DMAEMA groups in the polymer brush possess pH responsiveness, facilitating drug release under acidic conditions, such as those found in the joint cavities of OA patients. This further enhances the lubrication performance of the polymer brush (Figure 2c). They conducted tribological experiments using UMT-3 in reciprocating mode (Figure 2d) to investigate the lubricating properties of CB and bottle brush (BB). And, through subsequent cell and animal experiments, the effects of different frequencies and loads (simulating the frequency and load at the joints of human body when walking and running) on the lubrication performance of the hydration layer were further evaluated. CB and BB both showed a lower coefficient of friction (COF) than water, but under the same conditions, the lubrication performance of CB was obviously better than BB, because the side chain of circulating brush polymer was short and had no end, so a denser and more lubricated brush layer could be formed on the cartilage surface. This structure can reduce the mutual interference between polymer chains and improve the spatial stability, thus providing better lubrication effect under pressure. To minimize direct contact between cartilage surfaces, certain nanofibers can selectively recognize and bind to fibronectin and type II collagen in cartilage. This interaction forms a stable hydrated layer, thereby reducing friction. Xie et al. [88] investigated a brush-like nanofiber lubricating composite, where HA serves as the backbone. These composite features two critical side chains, which include brush-like lubricants and lipids attached to the main chain. By covalently grafting highly hydrophilic lubricant analogs (PAMPS) and lipid-like polymers (PMPC) as side chains onto the HA backbone, two distinct types of brush-like nanofibers, named HA/PA and HA/PM, were synthesized. HA/PA and HA/PM engage in intermolecular interactions, binding to key cartilage proteins, such as fibronectin and type II collagen, through electrostatic forces and hydrophobic interactions. This binding creates a stable hydrated layer, functioning as a lubricating layer, which in turn minimizes direct contact and friction between cartilage surfaces. Furthermore, HA/PA and HA/PM aid in restoring joint lubrication during early osteoarthritis and effectively alleviate the condition (Figure 2e). To further enhance lubricating performance, the incorporation of PSPMK brushes with negative charges within nanoparticles can develop a hydrated layer in aqueous environments, thereby reducing friction. Liu et al. [89] developed functionalized poly(3-sulfonatopropyl methacrylate potassium salt) (PSPMK) brushes grafted onto temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm) microgels (Figure 2f). These nanoparticles mimic the lubrication mechanism of natural cartilage by facilitating lubrication at the bone–cartilage interface. The main steps are as follows: firstly, HEMA-Br monomer containing ATRP initiator is synthesized, then PNIPAAm-Br microgel is prepared by emulsifier-free emulsion polymerization, and finally, PSPMK polymer brush is grafted on the microgel by surface-initiated ATRP. These brush layers, rich in sulfonic acid groups, can create a hydrated layer in water, providing lubrication. Because PSPMK brush has excellent hydration ability, it can form a stable hydration lubrication layer in an aqueous environment. Because of the negative charge of PSPMK brush, water molecules are attracted and exist around polymer brushes, and a hydration layer can be formed around microgels, thus achieving effective hydration lubrication. The PNIPAAm microgels exhibit temperature sensitivity (Figure 2g), enabling a volume phase transition at specific temperatures, thus modulating lubrication properties with temperature changes. Under charge influence, the negative charge of PSPMK enhances repulsion among nanoparticles through electrostatic and steric effects, helping maintain the stability of the lubrication layer under pressure.

Hydrogels are hydrophilic polymers with a three-dimensional network structure. They can absorb and retain significant amounts of water without dissolving in aqueous environments. Nanocomposite hydrogels integrate nanomaterials into traditional hydrogels to enhance their properties. These nanocomposite hydrogels can develop a hydrated layer on surfaces based on hydrogen bonding and electrostatic interactions. This layer improves lubrication through the mechanism of hydrated lubrication, effectively reducing friction during everyday joint movement. Yang et al. [90] conducted a comprehensive review on joint lubrication from the perspective of hydrogels. They discussed chitosan-based “mussel-inspired” multifunctional hydrogels, which significantly improve adhesion to cartilage tissue through catechol modification. Additionally, they examined hydrogels with tannic acid and nucleotide crosslinking, noting their exceptional mechanical durability and stage-dependent drug release behaviors [91]. This section simulates normal cartilage tissue to construct a directed graded structure, resulting in multilayered hydrogels with low friction and high load-bearing capacity. The underlying anisotropic structure enhances mechanical strength, while the top layer provides lubrication. The synergistic effect of these layers enables the hydrogels to exhibit biotribological properties that closely resemble natural cartilage under hard/soft and soft/soft contact conditions. Chen et al. [92] developed a dual-layer hydrogel intended to replicate the directional stratified structure of cartilage. The hydrogel consists of two layers made from anisotropic hydrogels with different orientation structures. The bottom layer (A-CF hydrogel) features a vertical orientation (Figure 3a), akin to the deep layer of cartilage, providing efficient energy dissipation and high mechanical strength. The upper layer (A-MMT hydrogel) exhibits a horizontal orientation, resembling the superficial layer of cartilage, contributing a low friction coefficient. The vertical orientation of the PDA-Fe₃O₄-CF in the bottom layer enhances both the mechanical strength and energy dissipation capacity, while the horizontal orientation of PDA-Fe₃O₄-MMT in the upper layer effectively resists shear forces, resulting in excellent friction performance. This structural design allows the hydrogel to maintain lubrication at the surface while bearing loads. Additionally, incorporating two-dimensional materials into the hydrogel reduces the friction coefficient and wear rate through interlayer sliding. Fu et al. [93] developed a biomimetic-inspired hydroxyapatite (HA)-coated hydrogel. This hydrogel achieves antibacterial and self-lubricating properties by integrating graphene oxide (GO) with supermolecular hydrogels. Through rheological tests, they found that the mechanical strength and crosslinking density of graphene oxide (GO) were significantly improved after it was added to the hydrogel. This enhancement effect is attributed to the fact that GO acts as a physical crosslinking point in the hydrogel, which enables the hydrogel to maintain its structural integrity more effectively when subjected to shear stress. At the onset of the sliding test (Figure 3b), the hybrid hydrogel stored within the textured pores initially undergoes a gel–sol transition, triggered by friction or frictional heat. The flowing sol state of the hydrogel migrates to the surface under the influence of frictional forces. Hydrogel quickly forms a thin film on the friction surface. The cyclodextrin (CD)-based pseudo-polyrotaxane (PPR) superstructure in the film not only has good lubricity, but also can adsorb nearby water molecules through hydrogen bonds to form a hydration layer, which reduces friction. However, when the external pressure is too high, the hydrogen bond may be broken, which will weaken the effect. The presence of graphene oxide (GO) in the film enhances the layer sliding mechanism, further decreasing the coefficient of friction and wear rate. When the hydrogel is expelled from the pores, the vacated space captures wear debris, thus preventing three-body wear and increased friction. Additionally, substances such as HA and PPA can be incorporated into the designed multilayer hydrogel materials. This not only boosts the hydrogel’s bioactivity and cell adhesion capabilities but also strengthens its bonding strength, tensile strength, self-repair ability, and friction wear resistance. Zhang et al. [94] developed a hydrogel consisting of PVA-HA (polyvinyl alcohol-hydroxyapatite) and PVA-HA-PAA (polyvinyl alcohol-hydroxyapatite-polyacrylic acid) composite hydrogels. This design mimics the gradient changes found in natural articular cartilage, from superficial to deep layers (Figure 3c). The superficial layer exhibits lower friction properties, while the inner layer demonstrates substantial load-bearing capacity. This is attributed to the presence of PAA (polyacrylic acid) within the upper hydrogel, which contains numerous carboxylate hydrophilic groups that form crosslinked networks with other polymers, enhancing the lubricity of the hydrogel. Meanwhile, the lower layer of PVA-HA benefits from the inclusion of HA, which improves its mechanical strength and load-bearing ability. HA, as a primary component of natural bone, enhances the bioactivity and cell adhesion of the hydrogel, thereby bolstering the material’s load-bearing capacity. Ultimately, through physical crosslinking, the multilayer hydrogel is integrated with a substrate of ultra-high molecular weight polyethylene (UHMWPE), creating an interface similar to that of bone–cartilage structures. This architecture not only provides excellent lubricating performance but also boasts high mechanical strength. To further reduce the friction coefficient on the hydrogel surface, a hydrated shell layer can be formed. This hydrated layer can interact strongly with water molecules via charged groups present on the zwitterionic polymer chains. During shear, these hydrated layers contribute to an exceptionally low friction coefficient due to their favorable flow properties and lack of adhesion. Wang et al. [95] synthesized a hybrid hydrogel using glycerol ether (GE) and sulfobetaine methacrylate (PSBMA) through chemical crosslinking and transesterification methods. This hybrid hydrogel established a stable three-dimensional network structure via chemical crosslinking. Within this structure, charged groups on PSBMA polymer chains interact strongly with water molecules, forming a hydration shell on the hydrogel surface, which is essential for lubrication (Figure 3d). However, under constant positive and negative loads, internal water seepage can reduce the thickness of the hydration layer. Nonetheless, the hydrogel maintains its lubrication performance, as the hydration layer can rapidly reform to counteract continuous shear forces. The added glycerol ether (GE), due to its hydrophilicity, promotes a more stable hydration layer on the hydrogel surface. This implies that the hydration layer formed on the hydrogel surface can maintain greater thickness even under shear forces, offering improved lubrication and lower friction coefficients. Components like anionic hyaluronic acid, chondroitin sulfate, and lubricin in cartilage provide a moist environment, suggesting the design of a hydrogel rich in cartilage anionic lubricating components. Peng et al. [96] developed polyanionic hydrogels (“CS” hydrogels) by copolymerizing acrylate (AAc) and potassium 3-sulfopropyl methacrylate (SPMK). These “CS-Fe” hydrogels (Figure 3e) exhibited mechanical adaptability akin to natural cartilage and maintained stability under varying loads. They utilized ultraviolet (UV) irradiation on the “CS-Fe” hydrogels, triggering the photoreduction of Fe³⁺ to Fe²⁺, producing a less dense outer layer enriched with free carboxylate and sulfonate groups. This layer could bind more water molecules, creating a highly hydrated hydrogel network, which significantly reduced friction coefficients and offered controllable lubrication properties. Furthermore, the “CS-Fe” hydrogels inhibited the overexpression of hydroxyl radicals (·OH) and nitric oxide (NO) in macrophages, thus safeguarding chondrocytes and fibroblasts from inflammatory damage. This mechanism further enhanced lubrication between bone and cartilage. As temperature increased, the adsorption capacity of the microgels improved, aiding the formation of a stable lubricating film during friction. Additionally, surface chemical reactions could be employed to enhance the lubricating properties of the nano/microgel particles. Yang et al. [97] utilized a surface chemical reaction to graft negatively charged chondroitin sulfate (CHI) onto chitosan nanoparticles (CS NPs), resulting in the formation of CS-CHI NPs. These CS-CHI NPs can establish a stable hydration layer in aqueous media (Figure 3f), capable of carrying loads during shear, thereby reducing friction. The excellent hydrophilicity and lubricity of CHI enhance the lubricating properties of the nanoparticles due to its incorporation into the CS NPs. Furthermore, the negative charges (SOO−) on the CHI chains interact with water molecules, forming a hydration layer that facilitates effective hydrating lubrication. To ensure continuous generation of the hydration layer on the surface of the hydrogel microspheres, specific liposomes can be integrated into hyaluronic acid-based hydrogel microspheres (HMs). Under friction wear, these liposomes are perpetually exposed on the external surface, creating a self-renewing hydration layer. Lei et al. [98] developed a hyaluronic acid (HA)-based hydrogel microsphere (HMs) using microfluidic technology and photo-induced polymerization. These microspheres feature a coating of liposomes, where the phosphatidylcholine head groups interact strongly with water molecules, forming an effective hydration layer that reduces friction. The hydrogel microspheres can reduce sliding interface friction through a rolling mechanism, similar to ball bearings (Figure 3g). The frictional wear on the surface of the microspheres continuously exposes new liposomes, creating a self-renewing hydration layer that ensures stable lubrication. To characterize the performance of Lipo@HMs throughout the lubrication process, a 3600 s friction test was conducted using a Universal Material Tester (UMT-3) (Figure 3h). During the second phase (blue region), specifically between 300 to 1200 s of the test, the friction coefficient remained at approximately 0.03, indicating a stable curve. This observation suggests that the Lipo@HMs surface has sufficiently displayed numerous liposomes. The strong interactions between the phosphatidylcholine head groups of these liposomes and surrounding water molecules generate a self-renewing hydration layer on the surface of the microspheres. It is this hydration layer that provides the microspheres with durable and stable lubrication.

In summary, the application of polymer brushes and hydrogels in bone–cartilage lubrication represents a significant research frontier in materials science and biomedical engineering. The design inspiration of these materials usually comes from the natural lubrication mechanism of human articular cartilage, such as the natural lubrication of synovial joints in the classical lubrication model. The main synovial joints (such as hips and knees) are unique and effective tribological systems, and there is a very low friction coefficient between articular cartilage. Lubricating substances play a vital role, such as synovial fluid and hyaluronic acid, which ensure smooth movement of joints and reduce friction and wear, so as to imitate or enhance their low friction characteristics in joints. Polymer brushes, formed by grafting high-density polymer chains onto solid surfaces, provide an effective boundary lubrication mechanism. They reduce direct contact between surfaces, thereby lowering friction. In articular cartilage lubrication, polymer brushes increase the load support from interstitial fluid, significantly enhancing cartilage lubrication and reducing vertical deformation of the tissue by maintaining fluid pressure. Hydrogels, composed of high water content crosslinked polymer networks, exhibit soft and elastic characteristics, making them ideal for simulating articular cartilage. Their lubricating properties partly result from a non-fluid, exposed lipid boundary layer. Researchers have designed hydrogels with low lipid concentration that continuously exude lipids to form a self-lubricating layer, markedly reducing friction and wear. Animal models play an important role in the study of osteoarthritis (OA) because they can simulate some aspects of human diseases, thus helping scientists to understand the disease mechanism, test new treatment methods, and evaluate drug effects. Although animal models can provide valuable information, they cannot completely replicate all aspects of human diseases. There are differences in joint mechanics between animal models (such as rat models) and humans, so it is necessary to be cautious when applying the research results to human cartilage. And if possible, these findings should be verified by other research methods (such as clinical research). This research opens new avenues for developing biomaterials with excellent biocompatibility, efficient lubrication performance, and drug delivery capabilities. As research progresses and technology advances, we anticipate that these materials will yield outstanding applications across various fields.

2.2. Engineering

The application of bionic lubrication technology in engineering field is mainly reflected in the development of lubricating materials and coatings with ultra-low friction coefficient, excellent liquid repellency, self-repairing ability, and high pressure stability by imitating the lubrication mechanism of organisms. This section mainly summarizes some applications in drilling fluids and gears. Drilling fluids serve a crucial role during the drilling process. They aid in cooling and lubricating the drill bit, transporting cuttings to the surface, and maintaining the stability of the wellbore [99]. Typically, a drilling fluid consists of a mixture of water, clay, additives, and other components. Drilling fluids can be categorized into water-based and oil-based types. Water-based fluids utilize water as the primary carrier and are suited for most drilling environments. In contrast, oil-based fluids rely on oil as the carrier and excel in high-temperature, high-pressure conditions or environments requiring the mitigation of water-sensitive formation swelling [100]. Various types of drilling fluids may incorporate different additives to address specific drilling requirements. Bionic bio-lubricated drilling fluids enhance performance by emulating the unique characteristics of natural organisms, such as marine mussels, earthworms, and Nepenthes secretions. These secretions can create a lubricating layer on rock surfaces, reducing resistance to physical movement while increasing the cohesion of rock particles, thus improving rock strength and wellbore stability. This ultimately leads to enhanced drilling efficiency and increased hydrocarbon yield. Bioinspired lubricants find applications not only in drilling fluids but also across various engineering projects [101]. These bioinspired lubricants effectively reduce friction in mechanical systems, minimize wear, and ensure efficient and long-term operation of equipment. Bioinspired coatings can delay the transition of boundary layers from laminar to turbulent flow, thereby reducing underwater resistance and enhancing the operational efficiency and speed of underwater transport vehicles like ships. This technology mimics the lubricating properties of biological entities found in nature, resulting in materials that significantly decrease friction, lower energy consumption, improve mechanical efficiency, and enhance wear resistance and biofouling resistance [102]. Furthermore, these lubricants are environmentally friendly and possess self-repairing capabilities, leading to improved equipment performance and durability, while simultaneously reducing maintenance costs and environmental impact. This section delves into a range of bioinspired lubricant materials, including their roles in drilling fluids and various engineering applications.

Mussels represent a unique marine organism with the remarkable ability to securely attach to various surfaces in moist environments, such as seawater. This adhesive capability primarily derives from the proteins in their byssal threads, notably those containing 3,4-dihydroxyphenylalanine (DOPA). DOPA is a catechol compound that facilitates strong adhesion in underwater settings. To replicate the robust adhesion of mussel foot proteins to various surfaces and the formation of stable lubricating films on metal surfaces, Yang et al. [103] synthesized a synthesizes ortho-hydroxy lubricant (L_3,4) that emulates these properties. This lubricant effectively forms a lubricating film in water, significantly enhancing adhesion to metal surfaces. The catechol group interacts with metal ions, such as Fe³⁺, to establish bidentate metal–ligand bonds, which exhibit greater binding energy and stability than hydrogen bonds (Figure 4a). Consequently, this leads to the formation of a dense organic lubricating film on metal surfaces. Additionally, L_3,4 demonstrates excellent lubrication performance in bentonite, with a friction coefficient (COF) as low as 0.06 at a 1% concentration and a wear scar diameter (WSD) of 0.365 mm, while withstanding temperatures of up to 210 °C. This indicates that L_3,4 maintains effective lubrication under varying temperatures and pressures, making it suitable for water-based drilling fluids, particularly in high-temperature and high-pressure drilling environments. Inspired by the adhesive proteins secreted by mussels, some scientists are also examining the mucus secreted by earthworms, which allows for movement through soil without clinging to it. This is attributed to the mucus’s properties that reduce viscosity and minimize friction. To achieve this, Quan et al. [104] developed a novel bionic water-based drilling fluid technology. They grafted polyacrylic acid onto polyvinyl alcohol by initially attaching acrylic acid and using the functional elements -COOH and -NH₂ as reactive functional groups through acylation reactions. This process enabled the incorporation of catechol groups (Figure 4b). Consequently, monomers with catechol functional groups attached to partial polyacrylic acid on the polyvinyl alcohol side chains. This structure allows chelation and crosslinking with divalent metal ions, such as Fe³⁺, Ca²⁺, and Mg²⁺, found in the wellbore rock. It forms a microbial bionic network that enhances the adhesion and cohesion of the rock by solidifying the well walls. During drilling, the motion of the drill bit and tools in the formation closely resembles that of soil-dwelling animals like earthworms. Therefore, inspired by the principle of soil animals reducing viscosity and drag, a biolubricating adhesive is developed via esterification reactions. This lubricant can form metal chelate rings and multiple hydrogen bonds with the well walls and drill bit surfaces. Notably, this lubricant can transition from hydrophilic to hydrophobic and contains trace amounts of extreme pressure elements, such as sulfur and phosphorus. When amino and carboxyl groups within the lubricant interact with silanol (Si-OH) and aluminol (Al-OH) groups on the rock surface, numerous hydrogen bonds form (Figure 4c). Multiple hydrogen bonds can enhance the adsorption strength of the lubricating film and improve its shear resistance, which helps reduce friction between drilling tools and rock surfaces. Furthermore, bioinspired adhesive lubricants effectively control inherent vortices at flow interfaces, lower friction during drilling, and increase mechanical drilling rates, thereby improving overall drilling efficiency. Coconut shells, often discarded, contribute to pollution and resource waste. To address this issue, some scientists have started utilizing graphene nanoplatelets (GNPs) derived from coconut shells to enhance the lubrication and shale inhibition properties of water-based drilling fluids (WBM). Yahya et al. [105] extracted graphene nanosheets from discarded coconut shells. They modified the surface of coconut-based graphene (GN-CS) using Triton X-100 to create modified graphene (GN-TX). This modification enhanced its dispersibility in water-based drilling fluids, improving its lubrication properties as an additive. The lubrication mechanisms include rolling effects and protective film formation, which reduce friction between drill tools and formations. The nanosheets of GN-CS and GN-TX possess a thin and flexible flake structure, enabling them to roll on metal surfaces. This rolling effect transforms sliding friction into rolling friction, thereby decreasing friction and wear. The nanosheets of GN-CS and GN-TX can adhere to metal and cake surfaces, forming a protective film. This film effectively isolates liquids and gases, such as moisture and oxygen, slowing oxidation and corrosion rates (Figure 4d). Additionally, hydroxyl groups (-OH) in GN-TX may adsorb onto metal drill tools and cake surfaces through hydrogen bonding and electrostatic attraction, forming a thin lubricating layer that further reduces wear. This function helps extend the lifespan of drilling tools.

Friction not only leads to significant energy loss—accounting for approximately one-fifth of global annual energy consumption—but also causes material degradation and limits equipment durability. Thus, achieving effective and long-lasting lubrication to reduce friction has remained crucial for enhancing system reliability and minimizing maintenance efforts. This makes lubrication essential in mechanical systems. Dopamine and cholesterol, found in various organisms and foods, have been synthesized into an oil-soluble dopamine-cholesterol compound (DA) through molecular design by Zhang et al. [106] This compound capitalizes on the widespread presence and exceptional adhesive properties of dopamine and cholesterol in biological systems. The hydroxyl and amide groups in dopamine can adsorb onto metal surfaces through electrostatic and coordination interactions, forming a substantial physical adsorption film that effectively reduces the contact between lubricating oil and metal surfaces, thereby minimizing friction. They discussed the lubrication performance of DA composites and PAO 10. Figure 5a shows that PAO 10 exhibits weak adsorption on metal substrates, with minimal adsorption present. However, when DA is added as an additive to PAO 10, the phenolic hydroxyl groups in dopamine coordinate with the metal substrate to facilitate chemical adsorption. Through the optimized SRV-IV friction and wear tester, it is found that DA additive can significantly reduce the friction coefficient and wear amount. The optimal concentration of DA as additive is 3% of that of PAO 10. At this concentration, it shows the best friction reduction (RF) and wear resistance (AW) compared with PAO 10. Before the actual friction contact occurs, the hydroxyl groups in the DA molecules can pre-anchor to the metal substrate surface through electrostatic interactions and coordination effects (Figure 5b). This process facilitates the formation of a relatively thick physical adsorption layer on the metal substrate, laying the groundwork for the subsequent formation of a boundary lubrication film through tribochemical reactions. In the friction zone, the amide groups and hydroxyls in DA may engage in tribo-chemical reactions with the metal substrate, generating a complex tribo-chemical film. The formation of a boundary lubrication film effectively prevents direct contact between friction pairs, thus enhancing the wear resistance and friction-reducing properties of the lubricating oil. Utilizing biomimetic lubricating materials can enhance performance and longevity in various engineering internal mechanical systems, such as bearings, gears, and pistons. Wang et al. [107] drew inspiration from the natural regenerative mechanisms of cartilage to propose an innovative in situ gelation strategy based on instant crosslinking technology. This aims to establish a highly efficient aqueous biomimetic lubrication system. The core of this system lies in the generation of polyvinyl alcohol (PVA) hydrogel, which cleverly employs borax as a crosslinking catalyst to trigger the crosslinking reaction directly at the friction interface. By adding borax with different contents, the effects of borax on the mechanical properties of polyimide (PI) substrate and the lubricating properties related to the crosslinking degree of hydrogel were studied. It was found that adding borax can significantly reduce the friction coefficient. Compared with water lubrication, the friction coefficient of in situ hydrogel decreased by 58%. Upon the onset of friction, borax acts as a “smart” crosslinking agent and gradually releases into the PVA solution. In the dynamic environment of friction, the two rapidly combine to form a robust and elastic three-dimensional network hydrogel (Figure 5c). This instant transformation process not only provides immediate supply of lubricant but also ensures that the lubrication layer can adapt to the changing conditions of the friction interface, continuously offering protection. The specific binding of borax molecules with the cis-hydroxyl groups on the PVA molecular chains constructs stable crosslinked network nodes through reversible coordination bonds (Figure 5d). This dynamic equilibrium mechanism imparts unique adaptability to the hydrogel; as it bears load, shear force, and frictional heat, some crosslinking points may temporarily dissociate. However, the immediate replenishment of boronic acid ions quickly reconstructs the crosslinked network, maintaining the stability and continuity of the hydrogel structure. Importantly, this in situ gelation strategy enables the self-maintenance and renewal of the lubrication layer under extreme conditions. Even under severe conditions of high load and high-speed sliding, the hydrogel network can sustain its high viscosity and outstanding load-bearing capacity through its distinct dynamic equilibrium mechanism, providing long-lasting and stable lubrication effects at the friction interface. This innovation not only enhances lubrication efficiency but also extends the lifespan of the lubrication system, paving the way for a new generation of green and efficient lubrication solutions in engineering applications.

In summary, biomimetic lubrication plays a crucial role in drilling fluids and lubricants for engineering machinery. These lubricants, as remarkable products of modern technology, feature intricate designs that create a stable adsorption barrier on metal surfaces. This barrier effectively prevents direct contact between mechanical components, significantly reducing friction and wear. This innovation notably enhances energy efficiency and fuel economy in engineering machinery, while demonstrating exceptional energy-saving capabilities in heavy industrial fields such as drilling operations. It directly lowers operational costs and boosts the competitiveness of enterprises. Notably, biomimetic lubricants maintain their excellent performance stability under extreme conditions, whether in high-temperature environments or high-pressure challenges, ensuring the long-term smooth operation of mechanical components. This characteristic not only extends the service life of the lubricant but also minimizes inconvenience and costs associated with frequent maintenance or replacement, thus improving overall operational efficiency and reliability. Additionally, many developments in biomimetic lubricants are rooted in environmental principles, either drawing from the smart extraction of natural materials or employing biodegradable advanced formulations. This approach minimizes negative impacts on the natural environment. Such practices in green chemistry not only respond to the global demand for sustainable development but also set new environmental benchmarks for the lubricant industry, steering the future of industrial lubrication technology toward greener transformations [108,109,110,111].

3. Structure Aspect of Tribology Lubrication

This section primarily explores biologically inspired surface designs to achieve super hydrophobicity, self-healing, and drag reduction properties. Researchers have emulated the structures of natural organisms, such as lotus leaves, shark skin, and pufferfish skin, to develop surfaces with micro/nano composite structures. These advancements enhance lubrication performance in underwater environments, reduce hydrodynamic drag, and improve mechanical durability and self-healing capabilities. This research provides theoretical foundations and experimental data for readers aiming to design novel drag-reducing materials and surface treatment technologies.

3.1. Bionic Lotus Leaf Lubrication

Friction arises as a resistance and energy dissipation process during the relative motion or attempted motion of two contacting surfaces [112,113,114]. It has undeniable benefits and drawbacks. Friction plays a crucial role in ensuring the smooth operation of daily activities and production processes. However, it also significantly contributes to wear and substantial energy loss. When designing and utilizing mechanical devices, one must consider both the advantages and disadvantages of friction. Implementing effective lubrication and structural design can reduce unnecessary friction, enhancing the performance and reliability of equipment. Therefore, exploring and developing diverse lubricating materials to precisely control interface friction has become an urgent necessity. For a long time, humanity’s in-depth exploration of friction has propelled the continuous advancement of industrial production and fostered the flourishing of mechanical and materials science [115,116,117]. However, faced with increasingly complex friction environments and the urgent demand for high-performance friction systems, the limitations of traditional lubricating materials have become apparent. This situation calls for more innovative and precise friction control strategies. In this context, nature’s wisdom serves as a source of inspiration for human innovation. The diverse tribological characteristics exhibited on the surfaces of plants and animals are a remarkable interpretation of environmental adaptability, shaped through billions of years of evolution. Researchers, through meticulous observation and in-depth study of these natural creations, have extracted a series of unique friction and lubrication regulation mechanisms. This work has led to the development of biomimetic friction regulation theories and methods. These theories and methods not only unveil the mysteries of biological friction regulation but also provide guidance for the development of novel lubricating materials and systems. This section delves into the superhydrophobic and self-cleaning mechanisms of lotus leaves.

The surface of lotus leaves possesses a unique micro-level papilla structure. These papillae are covered with nanoscale waxy crystals, creating a “micro/nano” dual structure. This micro-structure effectively repels water droplets and dirt, allowing for self-cleaning capabilities. By emulating the microscopic structure of lotus leaves, scientists have produced similar superhydrophobic surfaces. When water contacts these surfaces, it forms nearly spherical droplets, which roll off and carry away dirt and debris, thus maintaining cleanliness. In the field of coatings, this self-cleaning function can be achieved through specific chemical formulations. For instance, modifying surfaces with fluorinated or silane compounds reduces surface energy. Alternatively, employing nanotechnology allows for the formation of micro/nano dual structures on material surfaces, imparting superhydrophobic and oleophobic properties. The first discovery of the superhydrophobic nature of lotus leaves emerged in 1977 from Barthlott and Neinhuis at the University of Bonn [118]. They revealed this through scanning electron microscopy, introducing the well-known “Lotus Effect”, which describes the super hydrophobicity and self-cleaning ability of lotus leaves. Building on the branched surface structures of lotus leaves, Peng et al. [119] employed a green, versatile manufacturing strategy successfully used in creating superhydrophobic surfaces with layered structures. They initially observed fresh lotus leaves, noticing the spherical formation of water droplets on their surfaces. They then used magnified scanning electron microscope (SEM) images to elucidate the microscopic structure of the leaf, which includes microscale pillars and nanoscale branched structures (Figure 6a). Using lotus leaves as a biological template, researchers created materials with hierarchical structures and superhydrophobic properties. They employed techniques such as replication, coating with candle soot, secondary replication, and plant wax modification (Figure 6b). To test the hydrophobicity of the fabricated materials, they fixed the superhydrophobic samples on glass slides and positioned them 10 cm below a water outlet at a 45-degree angle. Their results, depicted in Figure 6c, demonstrated that the artificial superhydrophobic surfaces, modeled after lotus leaves, maintained their superhydrophobic characteristics even after enduring continuous water flow. The synthesized superhydrophobic surfaces (SHS) exhibited excellent mechanical stability and durability. This durability arose from the surface composition of micron-scale pillars (mastoids) and nanoscale soot particles, which enhanced surface roughness and facilitated the formation of a stable air layer, thereby boosting super hydrophobicity (Figure 6d). Additionally, a layer of plant wax, characterized by low surface energy, coated the surface to further reduce the contact area between water and the surface, enabling water droplets to roll off more easily. The presence of micro/nano structures coupled with low surface energy plant wax created a discontinuous three-phase interface (air, water, and solid surface) between the water droplets and the surface. This air layer inhibited direct contact between the droplets and the surface, promoting easier movement of droplets across the surface. As droplets rolled on the inclined surface, they could effectively carry away impurities. The unique micro/nano structures of the lotus leaf not only increased surface roughness but also captured air, forming an air cushion that reduced the contact area with water droplets. Yan et al. [120] utilized an electrochemical deposition method to fabricate layered structures with micro- and nanoscales, mimicking the microscopic morphology of lotus leaves while employing air film lubricant to minimize resistance during fluid flow. They first electrodeposited copper micro-pillar arrays onto copper substrates. Then, they varied the concentrations of polyethylene glycol (PEG) and temperature to achieve different distributions of copper micro-pillars. Finally, they formed nickel nanobrush structures on the copper micro-architectures via electro-deposition (Figure 6e). This process also involved using stearic acid to reduce surface energy, enhancing surface superhydrophobicity. To assess the hydrophobic properties of the nickel nanobrush structures, they placed a droplet of water on the smooth copper surface, the micro-pillar structure (MHCP), and the surface with nickel nanobrushes deposited (MHCPNBN) (Figure 6f). The measured contact angles were 76°, 140°, and 160°, respectively, indicating hydrophobic characteristics. Notably, the deposited nickel nanobrush structures exhibited a contact angle of approximately 160°, indicating a stronger hydrophobicity. Furthermore, the density of the micro-pillar distribution significantly affected the friction coefficient and air fraction of the micro/nano hierarchical structures. As the micro-pillar density increased, the air fraction also increased, leading to the formation of an air cushion at the solid–liquid interface, which reduced friction.

The micro- and nanoscale protrusions on the surface of lotus leaves typically exhibit irregular arrangements, observable through electron microscopy. To investigate the lubricating effects of systematically arranged micro/nano protrusions on lotus leaf surfaces, Rong et al. [121] employed laser etching technology to create controllable bioinspired super hydrophobics, bionic superhydrophobic surface (BSSs), and bioinspired superhydrophobic/hydrophilic surfaces (BSHSs) on an aluminum–magnesium (Al-Mg) substrate. In their preparation process, they initially degreased the samples before washing and drying them in deionized water. Subsequently, they applied laser etching to specific areas of the samples to form micron-sized protrusions. Afterward, they immersed the samples in a fluorinated alkyl silane (FAS) solution to further reduce surface energy and enhance superhydrophobic characteristics (Figure 7a). By manipulating the patterns of laser etching and the distribution of hydrophilic stripes, they achieved precise control over the micro/nano structures on the surface. To test the lubrication level of the fabricated surfaces, they subjected the surfaces to a flowing impact, revealing that at the interface between the superhydrophobic surface and the hydrophilic stripes, bubbles became trapped and adhered to the hydrophilic regions (Figure 7b). This phenomenon arises from the difference in surface energy between the hydrophilic bands, forming an energy barrier that stabilizes the three-phase contact line (gas–liquid–solid) and maintains bubble stability at high flow rates. Furthermore, a stable bubble layer forms between the superhydrophobic surface and the hydrophilic bands, reducing direct contact between the liquid and solid, partially converting it to gas–liquid contact. This interfacial slip phenomenon diminishes fluid resistance in the boundary layer, thereby increasing fluid velocity and achieving drag reduction. Measurements indicated that untreated surfaces and laser-treated BSS samples demonstrated different frictional resistance at varying flow rates, with BSS samples exhibiting lower frictional resistance than untreated surfaces within a specific range of flow rates. This difference likely results from their unique superhydrophobic micro/nano structures that facilitate interfacial slip, thereby decreasing hydrodynamic drag. The lotus leaf’s surface features a waxy layer, granting it self-cleaning properties. Zhang et al. [122] conducted extensive research on this phenomenon. They placed water droplets on untreated lotus leaves (Figure 7c) and tilted them at a 90° angle. Despite this extreme inclination, the water droplets retained a spherical shape and rolled off the leaf surface. This shows that the surface of lotus leaf is super hydrophobic; that is, it has an extremely high static water contact angle (about 150°) and rolling angle (<5°), which makes lotus leaf have excellent self-cleaning ability. In contrast, the thermally treated lotus leaves had their nanoscale hairy structures removed, leaving only a microscale textured surface. This alteration significantly reduced the static water contact angle (approximately 126.3°) and increased the adhesion of water droplets. Even at a 90° tilt or when inverted, the droplets became sticky and adhered to the surface. This indicates that the nanoscale hairy structures of the lotus leaf are crucial for minimizing surface adhesion and maintaining self-cleaning characteristics. Through multiple experiments, they elucidated the mechanism of self-lubrication in lotus leaves. When water droplets possess a very high contact angle, they can roll off and carry away contaminant particles, achieving self-cleaning. However, with a lower contact angle, droplets cannot effectively clean the surface (Figure 7d). Inspired by the irregular microstructures of lotus leaves, they developed superhydrophobic PTFE (polytetrafluoroethylene) films (Figure 7e). They introduced ZnAc₂ and NaCl into a commercial PTFE emulsion, followed by drying, baking, and washing with acetic acid. This process yielded a surface with micro- and nanoscale porous structures, demonstrating superhydrophobic properties with a static contact angle greater than 150° and a sliding angle less than 10°. Even after oil contamination, water droplets formed ‘beads’ on the coated surface, indicating that the self-cleaning properties remained intact despite contamination (Figure 7f). Wetting is different from lubrication; its purpose is to change the lyophilicity of solid surface and make the liquid spread on the solid surface more easily. The excellent hydrophobicity and self-cleaning properties of lotus leaf surfaces arise from their irregular nanostructures. Inspired by these irregular nanostructures, Tong et al. [123]. designed and fabricated micrometer-scale square and pillar textures on silicon surfaces. They developed a N-[3-(Trimethoxysilyl) propyl] ethylenediamine-Lauroyl chloride (DA-LA) double self-assembled monolayer at the nanoscale, creating a micro/nano composite structure. In the Cassie state, droplets partially suspend on the rough structure, whereas in the Wenzel state, droplets completely wet the rough surface. Transitions between these two states are often influenced by surface energy, roughness, and droplet volume. In the Wenzel state, increased contact area reduces the contact angle (Figure 7g), while in the Cassie state, trapped air within the rough surface structure increases the contact angle. The pillar textures show a gradual increase in contact angle at varying spacings compared to irregular textures, indicating that increased spacing contributes to enhanced hydrophobicity. The super hydrophobicity of the pillar texture is attributed to its 54.7° hanging angle and the rough structure of the sidewalls, which help maintain the Cassie state even under external disturbances. Experimental results reveal the mechanism behind superhydrophobic stability; in the Cassie state, the water droplet remains stably supported due to the hanging angle and rough sidewalls of the pillar texture. However, under external disturbances such as vibration or increased pressure, water droplets may tend to shift towards the Wenzel state, potentially leading to a loss of superhydrophobic properties. Figure 7h is critical for understanding how surface design can enhance the stability of superhydrophobic surfaces. Adhesion tests on flat silicon surfaces, square textures, pillar textures, and those modified with DA-LA SAMs reveal that adhesion on pillar textured surfaces is lower than on flat silicon (Figure 7i). This reduction likely results from the structure decreasing the contact area between the droplet and the surface. The modified surfaces of DA-LA SAMs exhibit reduced adhesion, likely due to the low surface energy of the SAMs and their potential lubricating properties. This further minimizes the adhesion of water droplets to the surface. Ultimately, the combination of micro/nano composite structures with low surface energy chemical modifications enables the super hydrophobicity of silicon surfaces, enhancing their stability and friction performance under dynamic conditions.

Inspired by the unique properties of lotus leaf surfaces, biomimetic lotus leaf lubrication technology has emerged. Its essence lies in mimicking the remarkable super hydrophobicity and self-cleaning abilities of the lotus leaf. The exceptional features of the lotus leaf stem from its intricate dual structure at the micro- and nanoscale. This intricate design has spurred innovative exploration in the field of lubrication materials. At the forefront of lubrication technology, researchers have successfully developed a series of biomimetic lubrication materials that exhibit low friction coefficients and high load-bearing capabilities by carefully imitating the natural structure of the lotus leaf. These materials not only demonstrate significant potential in industrial applications but also profoundly influence our daily lives. In household and consumer electronics, the prospects for superhydrophobic surfaces are vast. For instance, anti-fog surfaces are widely applied on rearview mirrors and vehicle windows, significantly boosting visibility during driving by preventing moisture condensation, thereby enhancing driving safety. Furthermore, the adoption of such surfaces in kitchenware and restroom facilities drastically improves self-cleaning abilities, effectively reducing the frequency and burden of cleaning tasks for users. Turning to the industrial sector, large transportation vehicles like ships and aircraft could benefit from superhydrophobic technology, significantly lowering hydrodynamic drag and thus improving transport efficiency while reducing energy consumption. The implementation of this technology not only aids in lowering operational costs but also plays a proactive role in promoting green transportation and energy conservation. In conclusion, biomimetic lotus leaf lubrication technology, characterized by its unique innovative concept and outstanding performance, is gradually transforming our lifestyles and production methods, making significant contributions to the sustainable development of society [124,125,126,127].

3.2. Bionic Fish Skin Lubrication

In the long course of natural selection and evolution, biological species have developed specialized surface structures, material properties, and efficient drag reduction mechanisms. Recently, scientists have conducted in-depth studies on how aquatic organisms achieve efficient swimming in underwater environments, particularly focusing on the intricate relationship between their skin structure and drag performance [128,129,130]. Research reveals that fish skin possesses not only fine microscopic morphological features but also outstanding mechanical properties. The synergistic effects of these characteristics form an effective strategy for reducing surface friction resistance in nature. This section will explore the latest advancements in biomimetic fish skin and snake skin technologies, including their manufacturing processes, drag reduction mechanisms, design optimizations, and potential practical applications. It will unveil how humans draw inspiration from the marvels of nature to create innovative drag-reducing materials and surface treatment technologies.

Fish, having evolved over 3.7 billion years, exhibit streamlined bodies and unique skin structures that enhance swimming efficiency and reduce drag. These traits inspire modern biomimetics, particularly in studies of shark skin and pufferfish dermal denticles. Recent advancements stemmed from exploring additional fish skin features and fabrication techniques, which broaden the application of biomimetic solutions. Researchers noted that pufferfish skin possesses tiny, spine-like structures, prompting questions about their role in reducing drag during movement. Fan et al. [131] employed Particle Image Velocimetry (PIV) to investigate the turbulent boundary layer (TBL) structures on biomimetic spine-covered samples. They analyzed pufferfish dorsal skin samples (Figure 8a), discovering that the spines aligned in a staggered pattern along the flow direction, with smaller, denser spines at the front and larger, sparser ones at the rear. This arrangement likely positively impacts fluid dynamics, such as drag reduction. Subsequently, they 3D-printed samples mimicking pufferfish dermal projections for testing in turbulent boundary layers. Figure 8b illustrates the interaction between the skin’s projections and airflow, which creates small-scale vortices that stabilize the boundary layer and reduce drag via vortex separation. The study visualizes how the boundary layer develops and separates across the pufferfish shape, revealing the influence of rough projections on flow patterns and wake formation. The study reveals that the protrusions on the skin of the pufferfish, such as small spines, interact with turbulent flow at the front end. This interaction stabilizes the turbulent boundary layer (TBL) and generates numerous small-scale vortices. The denser, larger spines at the rear induce vortex separation, facilitating the evolution of the small-scale vortices created at the front. This process rapidly enhances the low pressure above the pufferfish, increasing buoyancy to counteract gravity. Thus, the micro-structures on the pufferfish skin can effectively reduce drag by influencing the turbulent boundary layer. Inspired by the pufferfish skin, Feng et al. [132] developed a novel biomimetic drag-reducing surface (CPES). This surface combines conical protrusions and an elastic layer to diminish fluid dynamic resistance (Figure 8c). They created graphite molds based on the spacing and size of the pufferfish’s conical protrusions, filled with copper powder, and subjected them to sintering at 850 °C for one hour to produce CPPCS. Subsequently, they immersed the samples in a mixture of PDMS, curing agents, and SiO₂, ensuring uniform application before vacuum processing for four hours and heating at 70 °C for an additional four hours, ultimately yielding CPES biomimetic samples (Figure 8d). Analyzing the designed models in turbulent boundary layers indicated a similar drag reduction mechanism; the conical protrusions act as independent vortex generators, reducing viscous drag through immersion in the boundary layer. Meanwhile, the low permeability and porous structure of the CPPCS samples exhibited significant drag reduction effects in turbulence, as the penetration phenomenon resulted in greater flow direction and lateral direction permeability compared to the vertical direction, thereby influencing the turbulent characteristics within the channel. It was also determined that at various Reynolds numbers, the turbulent kinetic energy of samples with conical protrusions (CPRS, CPPCS, and CPES) was lower than that of smooth samples (PCS and CS), indicating that conical structures effectively minimize turbulent energy exchange. The CPES samples exhibited the lowest turbulent kinetic energy, highlighting their superior performance in reducing turbulent energy. This design not only reduces frictional resistance but also enhances fluid dynamic performance by controlling vortex structures and improving boundary layer stability.

Snakes primarily achieve locomotion through the bending and stretching of their bodies, enabling them to move slowly and steadily across various terrains. The scales of snakes feature microscopic textural structures, such as tiny scale-like protrusions or grooves (Figure 8e). These structures minimize friction during movement by reducing actual contact area, decreasing adhesion, and optimizing the motion interface. Zhao et al. [133] designed a polymer surface with similar micro-grooves by mimicking the keratin structure of Achalinus spinalis. They extracted key characteristics from the snake’s keratin, such as width, length, depth, and wall thickness, applying these features in the fabrication of biomimetic surfaces. By employing a Nd:YVO₄ picosecond laser system to engrave periodic patterns on nickel sheets (Figure 8f), they adjusted laser parameters to control microstructural dimensions. Subsequently, the patterns on the nickel mold transferred to PMMA sheets via thermal embossing to create negative molds. They cast PUA prepolymer into the PMMA negative molds and cured them using ultraviolet light, resulting in PUA surfaces with the desired microstructure. Friction tests utilized a smooth steel ball to evaluate three different microstructural surfaces (Figure 8g). Structure-C displayed a significantly higher minimum friction coefficient compared to AITPS and Structure-B. The friction coefficients of AITPS and Structure-B were similar, suggesting that uninterrupted continuous micro-grooves, like those in Structure-B, can provide a drag reduction effect comparable to that of AITPS. By imitating the microscopic structure of snake keratin in nature, artificial surfaces with exceptional drag reduction performance can be designed. Additionally, this indicates that optimizing the friction performance of materials can occur by adjusting the geometric parameters of the micro-grooves, such as depth, width, and length.

As research on biomimetic fish skin surfaces progresses, some researchers are turning their focus to practical applications. Liu et al. [134] developed a biomimetic polydimethylsiloxane (PDMS) film by mimicking the structure and surface energy of shark skin, utilizing the surface-initiated atom transfer radical polymerization (SI-ATRP) technique. This film exhibits super hydrophobicity, self-healing capabilities, and drag reduction properties. The fabrication process begins with replicating shark skin to obtain microstructures (Figure 9a), followed by surface functionalization and initiator immobilization, culminating in the polymerization of PFMA brushes. Scanning electron microscopy (SEM) images reveal the features of the shark skin, the KH550-treated PDMS, the BIB-treated PDMS, and the PFMA-grafted PDMS. By analyzing these images (Figure 9b), one can observe changes in the surface structure of the PDMS film. Initially, the shark skin displays a distinct V-shaped ridge structure, but treatment with KH550 and BIB introduces reactive groups on the surface, preparing it for subsequent polymerization. After grafting PFMA, more protruding structures appear, confirming the successful incorporation of PFMA onto the PDMS film. PFMA’s low surface energy is crucial for achieving superhydrophobic properties. Reynolds number (Re) is a dimensionless quantity, which is usually used to predict the flow patterns under different flow conditions. The lower the Re, the more stable the water flow. To assess the drag reduction performance of the PDMS film (Figure 9c), they evaluated the drag reduction rates of the shark skin surface and the superhydrophobic surface at varying Reynolds numbers (Re), finding that drag reduction increases with Re, peaking at 21.7%. This indicates that drag reduction is more pronounced at higher flow rates. In the context of developing drag-reducing hydrogel coatings based on biomimetic fish skin, Zhang et al. [135] designed a Janus hydrogel coating (JHC) inspired by specific fish skin. JHC comprises two distinct components. The lower segment features a sticky hydrogel (STH) that exhibits strong adhesion to various surfaces, including metals, ceramics, and polymers. The upper section consists of a slippery hydrogel (SLH) that mimics the scale structure of fish, while the properties of the hydrogel replicate the slime on fish skin. This configuration aids in reducing water resistance and offers antifouling capabilities. The preparation process (Figure 9d) initiates with the application of a prepolymer solution of the sticky hydrogel (STH) onto a glass substrate, which undergoes curing through ultraviolet (UV) irradiation. Subsequently, the STH coats a soft template filled with the precursor of the slippery hydrogel (SLH), followed by another round of UV-induced polymerization. A critical aspect of this preparation employs citric acid (CA) and ammonium persulfate (APS) as initiators (Figure 9e). Under pH 4 conditions, UV light triggers a free radical polymerization reaction, while chitosan quaternary ammonium salt (QCS) acts as a physical crosslinker. This increases the mechanical stability of the STH layer through ionic bonding and hydrogen bonding, while varying the relative concentrations of acrylamide (AM) and acrylic acid (AA) adjusts the viscosity of the hydrogel. Researchers tested the capability of JHC in reducing fluid resistance using Particle Image Velocimetry (PIV). They compared the velocity distribution over the surface of JHC to that over a planar SLH surface. The results indicated that at every measured distance above JHC, the flow velocity surpassed that above the planar SLH surface, demonstrating that JHC effectively decreases fluid resistance.

In the rapidly evolving field of engineering technology, biomimetic drag-reducing surface technology is emerging as a pivotal force driving industry advancement. Moreover, compared with the surfaces obtained by traditional manufacturing techniques (such as EDM, milling and polishing), bionic surfaces have significant differences in friction and wear resistance. First of all, the design of bionic surface is inspired by creatures in nature, which have formed excellent antifriction and wear-resistance characteristics in the long-term evolution process. Although EDM has high precision and is suitable for machining hard metals, it is not as good as bionic surface technology in the creation of micro/nano structures on the surface. Recently, a particularly noteworthy trend has surfaced: scientists and engineers collaborating to delve into and replicate the intricate structures found in nature [136,137,138]. A prime focus has been on the unique ribbed texture of shark skin, leading to the development of a series of bioinspired raised surfaces with superior drag-reduction capabilities. This cutting-edge technology finds exemplary application in aerospace and other high-tech domains. By employing precision cutting and manufacturing techniques, engineers have skillfully duplicated the natural form of shark skin, producing drag-reducing surfaces that are both aesthetically pleasing and highly efficient. These innovative designs not only push the boundaries of traditional materials science but also instigate profound changes in the field of fluid dynamics. A case in point is the Airbus A320, which incorporates advanced film technology inspired by shark skin [139]. The integration of this biomimetic design represents a significant milestone in the aviation industry. After three years of rigorous flight testing, this technology has demonstrated exceptional drag-reduction effects, significantly enhancing fuel efficiency. It is estimated that each A320 aircraft utilizing this technology could save up to 350 tons of fuel annually. This achievement showcases the immense potential of biomimetic technology in enhancing energy utilization efficiency and environmental performance, while also illustrating the boundless possibilities arising from the harmonious integration of nature’s wisdom with modern engineering techniques.

4. Chemical–Structural Coupling Aspect of Tribology Lubrication

This section explores various biomimetic strategies to enhance the lubricating performance of materials. Researchers draw inspiration from the surface structures of organisms found in nature, such as the blue-ringed octopus, earthworms, and clams, to develop smart coatings with self-healing and self-lubricating properties. These coatings typically integrate micro/nano structural designs along with composite solid lubricants, enabling superior tribological performance and contamination resistance in diverse environments, including marine and mechanical engineering settings. By mimicking the lubrication mechanisms of natural organisms, this research provides novel ideas for developing eco-friendly and efficient lubricating materials [140,141,142,143]. Furthermore, environmentally friendly and efficient lubricants offer significant advantages over traditional lubricants in terms of environmental impact. The production of traditional lubricants involves extensive oil extraction, high energy consumption, and substantial waste emissions. They are prone to leakage, volatility, and are difficult to degrade, complicating waste disposal. In contrast, environmentally friendly and efficient lubricants utilize renewable resources, optimize production processes to reduce energy use and emissions, and exhibit low leakage rates and volatility during use. Some materials even possess degradability, making them easier to recycle and dispose of in a non-harmful manner. Therefore, selecting and using environmentally friendly and efficient lubricants contributes to minimizing negative environmental impacts and achieving sustainable development.

4.1. Application of Antifouling

Bionic biostructural–chemical coupling lubrication represents a scientific approach that designs and manufactures lubricants by mimicking the lubrication mechanisms found in nature. This method utilizes the inherent lubricating properties of organisms, such as the low friction characteristics of aquatic plant and animal surfaces. By replicating their structures and functions, researchers develop novel lubricant materials. This section mainly reviews the application of bionic biostructural–chemical coupling lubrication in coatings. Its effectiveness lies in significantly reducing the friction coefficient on coated surfaces, enhancing wear resistance and durability, while also exhibiting excellent self-healing capabilities and liquid-repellent qualities. Such coatings efficiently lubricate various substrate surfaces by mimicking lubrication strategies of natural organisms, like the micro/nano structures of earthworms, and find extensive applications in the fields of antifouling, anti-adhesion, anti-icing, and drag reduction [144].

Biomimetic lubricating coatings can form a stable lubricating film on surfaces, reducing wear and extending the lifespan of mechanical components. Wong et al. [145] proposed an innovative concept where they inject smooth liquids, such as cost-effective Teflon, into porous surfaces. They successfully developed surfaces showcasing exceptional stability, liquid-repellent properties, and anti-adhesion performance. Inspired by Wong’s findings, Lee et al. [146] created a biomimetic lubricating interface structure (LIS) with a unique surface topology. This topology features inverted three-dimensional cavities, inspired by the mucus secretion and storage systems of eels, sea cucumbers, and algae. Figure 10a displays the SEM image of the mucus secretion pores from eels. Additionally, detailed steps for fabricating biomimetic LIS with inverted cavity structures are outlined (Figure 10b). These steps consist of emulsion preparation, assembly, oil droplet removal, surface crosslinking, plasma etching, fluorination, and lubricant injection. The focus of this process lies in forming surfaces with specialized micro/nano structures, which are critical for maintaining the lubricating layer, thus achieving sustained drag reduction under external shear flow. The durability of R-LIS and S-LIS was compared under continuous exposure to high shear flow conditions (Figure 10c). R-LIS demonstrated a small reduction in drag (approximately 6.73%) even after multiple exposures, while S-LIS exhibited a significant drag reduction (approximately 46.81%) after the third exposure, indicating substantial lubricant layer loss. This outcome suggests that R-LIS possesses greater durability in resisting lubricant loss under high shear flow compared to S-LIS. By injecting lubricants into biomimetic porous surfaces, researchers reduced friction inspired by nature. Zhou et al. [147] applied this mechanism to antifouling coatings, drawing from the blue-ringed octopus, which secretes slippery skin mucus and toxins when threatened to aid in escape. They developed a bioinspired lubrication coating that exhibits synergistic antifouling characteristics. The preparation process involves blending polydimethylsiloxane (PDMS), hydroxyl-terminated silicone oil (HTSO), a coupling agent (APTES), and capsaicin in specific ratios (Figure 10d). This mixture dissolves in ethyl acetate and then coats the substrate using the dip-coating method, followed by curing at 80 °C for six hours or at room temperature for one day, resulting in a lubricant coating with synergistic antifouling properties. To evaluate the antifouling efficacy, they tested different coatings (including original aluminum, PDMS, and S2C1 coatings) against organic (such as algae, BSA, coffee, ink, and milk) and inorganic contaminants (like 3.5 wt% NaCl solution and CuCl₂·2H₂O particles). Experimental results (Figure 10e) showed that S2C1 coating exhibited excellent self-cleaning properties and strong anti-adhesion against common pollutants, leaving minimal residues. Measurements of the shear force required to remove simulated barnacles (Figure 10f) revealed that S2C1’s adhesion strength to barnacles significantly decreased by 99% compared to the original aluminum, indicating exceptional antifouling performance. This finding is crucial for marine applications like hull coatings, as it signifies that the coating surface can more easily shed attached biological fouling, thus reducing maintenance costs and labor. Tong et al. [148] drew inspiration from the defensive behavior of blind eels secreting mucus. They developed an intelligent SLIPS antifouling coating mimicking the blind eel. The core concept involves the transition between cis and trans azobenzene (Azo) and their supramolecular interactions with α-cyclodextrin (α-CD) to modulate the lubrication of SLIPS (Figure 10g). This coating can responsively adjust surface lubrication based on environmental conditions such as light and temperature to switch between different antifouling modes (Figure 10h). During the rapid growth of biological fouling in the daytime or warmer seasons, the coating releases lubricants and activates the “enhanced” mode. Conversely, at night or in cooler seasons, it halts the lubricant release and reverts to “normal” mode. Researchers demonstrated the coating’s inhibitory effects on algal attachment by testing attachment performance under varying conditions. Moreover, after exposure to visible light or heat treatment, the water contact angle on the coating surface decreased, and the sliding speed of water droplets increased significantly (Figure 10i). This indicates that the responsive release of lubricants enhanced surface lubrication and reduced friction with the coating substrate. The increased presence of lubricants improved the macroscopic lubrication performance of the coating, thereby enhancing its antifouling capabilities. Zhao et al. [149] addressed the challenge of replenishing lubricant depletion on coated surfaces. They observed that earthworms can navigate through clay without leaving stains, leading to the conclusion that their epidermis continuously secretes mucus, forming a thick slippery layer on their textured skin (Figure 10j). By mimicking the coarse skin of earthworms, they created a textured structure on the polymer-coated surface. The lubricants within the coating were stored as discrete droplets within a supramolecular polymer matrix made of urea and polydimethylsiloxane (uPDMS). The gel film on the coating surface was produced by casting a polymer solution under humid conditions (Figure 10k), where solvent evaporation induced water condensation on the forming polymer surface, directly creating a rough structure. The textured surface of the gel film developed within the first two hours, with the encapsulated liquid released to cover the surface. When localized pressure was applied to the rough surface, oil immediately released to cover the compressed area, resulting in the formation of a stable lubricating layer. The study determined the kinetic friction coefficients (KFC) of various films in relation to the applied load, as illustrated in Figure 10l. Notably, the EWI surface, where the rough structure was entirely filled with oil, exhibited the lowest KFC value. Moreover, as the applied load increased, the KFC remained constant, attributed to the locking mechanism of the oil layer provided by the surface’s microstructure.

In summary, this research focuses on developing innovative anti-pollution and drag-reducing materials. Researchers drew inspiration from the surface characteristics of marine organisms to design a surface exhibiting lasting lubrication effects. This design significantly reduces fluid friction while enhancing material durability. Simultaneously, they mimicked the defensive mechanisms of the blue-ringed octopus to create a hybrid physical-chemical antifouling smooth coating that demonstrates excellent stability and anti-pollution properties. Furthermore, they developed a smart SLIPS antifouling coating that adjusts lubrication based on changing environmental conditions and possesses self-repair functions. Finally, inspired by the traits of earthworms, they created a polymer coating that adapts to reduce friction and prevent pollution on solid surfaces. The significance of this research lies in providing environmentally friendly and sustainable solutions aimed at reducing energy consumption and environmental pollution [150] during marine transportation, while also extending the lifespan and improving the efficiency of materials.

4.2. Application of Drag Reduction

Energy loss due to friction represents a global issue. It is prevalent in industrial machinery, vehicles, and everyday goods. Friction between metal surfaces can lead to wear, energy loss, reduced equipment performance, increased maintenance costs, and potential equipment failure, alongside a shortened lifespan [151]. To enhance the tribological properties of metal surfaces, the integration of surface texturing and solid lubrication techniques has emerged as a crucial strategy [152]. This approach revolves around strategically incorporating carefully designed geometric patterns, which adhere to specific dimensions and layout principles, into the metallic surfaces to significantly optimize their wear resistance. Most of these strategies are inspired by the surface structures of some animals and plants in nature, such as lotus leaves, fish skins, and mussels. Through this innovative design, the actual contact area on wear surfaces minimizes effectively, resulting in reduced friction and stabilized friction coefficients. Ultimately, this enhances the overall performance of materials in friction environments. This section will focus on reviewing the applications of structured chemical co-coupling inspired by biological structures for friction reduction on metal surfaces.

Biological organisms have developed various complex biocoupling effects throughout their long evolutionary history. These adaptations allow organisms to display remarkable resilience in changing environments. In the field of biomimetic lubrication, researchers emulate these biocoupling effects to enhance human-engineered products. By mimicking the lubrication mechanisms found in nature, effective drag reduction methods for metal surfaces emerge. Zhang et al. [153] drew inspiration from the micro-texture properties of clam shells, using sinusoidal functions to fit discrete data on shell textures. The fitting outcomes revealed periodic variations in the shell’s texture characteristics (Figure 11a). They designed a bionic textured composite surface with a “brick-mud” staggered structure, employing laser engraving technology and ultra-high-speed laser melting deposition technology on 42CrMo substrates to create BTS and BTCS with varying densities (Figure 11b). During the lubrication process, three stages occur: hydrodynamic lubrication, starvation lubrication, and lubricant-solid cooperative lubrication. Initially, the base oil and MXenes within the MLCG lubricate the friction contact area. As the base oil is depleted, MXenes begin to form a boundary film in this region, providing lubrication. During this stage, the superabsorbent component stored within the bionic texture is extruded and diffuses into the wear area, creating a uniform lubrication film. In the final stage, SAC particles can fill defects in the wear zone, facilitating self-repair. Furthermore, these spherical aggregates act as sub-micrometer rolling bearings, transforming sliding friction into rolling friction (Figure 11c), thereby significantly enhancing the tribological performance of rotary support raceways. Qin et al. [154] drew inspiration from the surface architecture of shells. They employed curved fitting methods to extract surface textures. Subsequently, they fabricated corresponding biomimetic corrugated textures on the TC4 surface and infused them with a SnAgCu-WS2 composite solid lubricant. Compared to grooved textures, the biomimetic corrugated textures created greater equivalent pressure at the texture edges and within the grooves. This phenomenon enhanced the lubricant’s deposition from the texture to the wear track surface. The sliding effect of WS2 during friction alleviated the direct stress influence on the friction interface. It reduced shearing and loss of soft metal lubricants. This facilitated the spread and repair of solid lubricants on the wear track surface (Figure 11d), resulting in a more complete and dense lubricating film. Figure 11e illustrates the wear cross-sections of TWs-SAC and TWs-SACW02 samples. The wear cross-section of TWs-SAC shows an interrupted lubricating film. This discontinuity likely arises from the susceptibility of soft metal lubricants, like SnAgCu, to shearing and peeling. In contrast, the wear cross-section of TWs-SACW02 displays a clear friction layer with a continuous and intact lubricating film on top. This indicates that the addition of WS2 can enhance the film formation quality of composite solid lubricants. The synergistic interaction of biomimetic textures and composite solid lubricants can improve the tribological performance of TC4 alloys. Surfaces with different textures in metal friction will influence varying coefficients of friction. Thus, rational selection of surface textures under different conditions is crucial. Inspired by the Phrynosoma cornutum lizard and its abdominal scales (Figure 11f), Huang et al. [155] designed a biomimetic rhombic microstructure surface texture for M50 steel. The rhombic texture can distribute loads during friction, thus enhancing wear resistance. They studied the tribological behavior under dry sliding conditions, analyzing the drag reduction effects of 45° and 60° rhombic textures under different loads. Initially, the introduction of SnAgCu and MXene-Nb2C as solid lubricants on the surface of biomimetic rhombus microstructures made from M50 steel helps form a uniformly dense lubricating layer within the textured design. Subsequent investigations focus on the conditions applicable to 60° and 45° rhombus textures, as well as the role of lubricants under varying loads. Research findings (Figure 11g) indicate that under a 15 N load, the composite materials MSN-45 demonstrate impressive tribological performance, and at 25 N, the MSN-60 exhibits similar results. This implies that strategically designing textures and selecting appropriate solid lubricants can significantly enhance the wear resistance and frictional properties of M50 steel. The optimization of biomimetic textures remains equally crucial. Inspired by the microstructure of tree frog surfaces, Huang et al. [156] developed biomimetic microtextures integrated with the solid lubricant SnAgCu to improve the tribological performance of AISI 4140 steel. They employed the Response Surface Methodology (RSM) to identify optimized parameters for these biomimetic textures, followed by tribological testing. Tree frogs can maneuver freely across various surfaces due to their foot microstructures and secreted mucus (Figure 11h). The microstructure found on tree frog feet not only enhances the distribution of forces but also proficiently manages the transportation of bodily fluids. First, solid lubricant SnAgCu fills the microstructures. During friction, this lubricant migrates to the substrate surface. The biomimetic textures’ grooves capture wear debris and promote the spread of lubricant. By altering the stress distribution at the contact interface, it influences the formation of the lubricant layer. The control group (CG) without textures shows a worn surface with deep grooves and scattered debris; scratches appear on the substrate surface, indicating significant abrasive wear behavior (Figure 11i). In contrast, the biomimetically textured OT samples show that SnAgCu forms an uneven lubricant layer during friction. Due to this unevenness, it is difficult to prevent direct contact between friction pairs, resulting in the presence of more flaky debris on the worn surface. Additionally, deep pits appear at the spacing locations of the hexagonal texture, with some pits filled with SnAgCu. In comparison, the OP samples feature more lubricant at the hexagonal centers, resulting in a uniform, dense lubricant film approximately 2.4 microns thick, providing better isolation from direct contact of the friction pairs. Therefore, the worn surface of OP shows significantly reduced discontinuous layers, with a more even distribution of lubricant over the surface. The optimized biomimetic texture (OP) effectively maintains the uniformity and continuity of the lubricant film, thereby reducing the friction coefficient and enhancing wear resistance.

This study explores the effects of biomimetic structures and chemical coupling on enhancing the tribological performance of materials. By mimicking microstructures found in nature, such as the pads of tree frogs and the surfaces of shells, researchers designed surfaces with specific microtextures. They also incorporated solid lubricants like SnAgCu and MXene-Nb2C to improve the friction and wear properties of metal alloys, including AISI 4140 steel, TC4 alloy, and M50 steel [157]. The findings reveal that these biomimetic textures significantly reduce the coefficient of friction and wear rate, enhancing the wear resistance and antifriction properties of materials under both dry and lubricated conditions. These advancements hold substantial application value in fields such as aerospace, automotive manufacturing, and wind turbine technology [158], contributing to the reliability and longevity of mechanical systems. The results indicate that the strategy of combining biomimicry with solid lubricants has great potential for improving the tribological performance of materials [159].

In conclusion, these studies have made important progress in the field of biomimetic related tribology.

Table 1. Summary of typical researches of Bionic lubrication.

Category	Materials	Fabrication	Characteristic	Application	Ref.
Chemistry	CPBDT, AIBN, BIBB, CuCl, CuCl2, bpy.	Reversible inactivation radical polymerization method	Reduce the friction coefficient of joints and effectively simulate the lubrication performance of cartilage surface.	Biological lubricant, drug delivery	[85]
	Poly-2-2-hydroxyethyl methacrylate.	One-pot method	It has lubrication maintenance function and can effectively slow down the progress of osteoarthritis.	Treating osteoarthritis, achieve accurate drug delivery	[86]
	PAMPS, PMPC.	Covalent connection	It has the structural characteristics of natural joint lubrication compound nanofibers.	Treat early osteoarthritis, restore cartilage lubrication	[87]
	HEMA, 2-bromoisobutyryl bromide, NIPAAm, MBA, triethylamine, aspirin.	1H NMR	It shows good mechanical properties such as elastic recovery, creep resistance, fatigue resistance, and impact resistance.	Synthetic synovial fluid	[88]
	PVA, HNO3, PAA.	One-step method	With layered structure, high strength and toughness and low friction coefficient.	Artificial cartilage	[91]
	GO/PEG-NH2, α-CDs.	Vacuum infiltration method	It has good antibacterial and self-lubricating properties.	Artificial joint replacement surgery	[92]
	UHMWPE, PVA, PEG, PEG-400	Oxidative esterification	High tensile strength, excellent compressibility, thermal stability.	Artificial cartilage	[93]
	SBMA, EGDMA, APS, TMEDA.	One-step method	Excellent mechanical properties, low friction coefficient, and good biocompatibility.	Articular cartilage substitute	[94]
	AAc, potassium 3-sulfopropyl methacrylate, N, N′-methylenebis.	Ultraviolet irradiation, swelling method	Excellent mechanical adaptability, controllable lubrication performance, and anti-inflammatory adjustment ability.	Cartilage tissue engineering	[95]
	CS, CHI, GA, EDC, NHS.	Coupling reaction	Excellent water retention, improving the lubricity of damaged cartilage.	Treat rheumatoid arthritis	[96]
	RAPA@ Lipo@ HMs.	Microfluid technology and photopolymerization process	It can effectively reduce friction, delay the progress of osteoarthritis, and maintain cell homeostasis.	Relieve osteoarthritis and possibly treat friction-related diseases	[97]
	Na-BT, p-toluenesulfonic acid, sodium bicarbonate, Oleanol, 3,4-dihydroxy benzoic acid.	One-step method	Excellent lubrication performance, good mechanical resistance, low friction, high load, and excellent wear resistance.	Water-based drilling fluid	[102]
	Acrylic acid, polyvinyl alcohol, catechol.	Acylation reaction	Drilling fluid additive with super adhesion and excellent lubrication performance.	Water-based drilling fluid	[103]
	Barite, Na2CO3, KCl, bentonite, PAC-UL, octyl phenol ethylene oxide condensate.	High temperature preparation	Thermal stability, good lubricity, enhanced shale inhibition effect, and environmental friendliness.	Water-based drilling fluid	[104]
	PAO 10, dopamine hydrochloride.	One-step synthesis	It has good oil solubility, thermal stability, and tribological properties.	Lubricating oil of mechanical equipment	[105]
	DMAC, PMDA, Borax.	Mixed preparation	Fast response, strong adaptability, and good lubrication effect.	Industrial processes and biomedical fields	[106]
Structure	PDMS, T3, Foncepi, rice bran wax ethyl acetate.	Hydrophobic modification method of “impregnation-drying-impregnation”	Has mechanical durability and repairability.	Oil–water separation, fluid transportation, anti-corrosion, anti-icing, and microfluidic equipment	[118]
	Copper, PdCl2, PEG.	Electroplate	It has low friction, high bearing capacity, and excellent wear resistance.	Microfluidic equipment and controllable oil transportation systems	[119]
	Acetone, alcohol, oxalic acid solution.	Laser ablation	Friction resistance decreases steadily at high speed.	Marine ships and pipeline transportation	[120]
	Si, N-[3-(trimethoxysilyl) propyl], DA-LA.	Etching method	It shows low friction, high load and excellent wear resistance.	The superhydrophobic surface has moisture resistance, which can reduce the adhesion of tiny droplets. In MEMS devices, mechanical failure or performance degradation caused by liquid adhesion is reduced. Self-cleaning surface, antifouling coating	[122]
	White resin.	3D printing	It has low friction, high bearing capacity and excellent wear resistance.	Underwater vehicle	[130]
	SiO2, PDMS, CPPCS.	Mold finishing	Has excellent drag reduction performance and good mechanical stability.	Underwater vehicle or ship	[131]
	Polyurethane acrylate.	Picosecond pulse laser engraving technology	The surface has regularly arranged quasi-rectangular microchannels.	Drilling machines and robots	[132]
	PMDET, FMA, CuBr, BIB.	Imitation molding process	It has the characteristics of superhydrophobicity, self-repair and drag reduction.	Hull coatings, surface treatment of medical equipment, waterproof and antifouling coatings for textiles	[133]
	PDMS, AA, CQAS.	3D printing	It has many functions such as drag reduction, anti-pollution, anti-swelling.	Pipeline transportation, bioengineering and shipbuilding industry	[134]
Chemical–structural coupling	Polyvinyl alcohol, silicone oil, toluene, acetone.	Filling method	The surface is injected with lubricant with special surface morphology.	Marine coatings	[145]
	PDMS, Sylgard 184B, Al, hydroxyl silicone oil, APTES, capsaicin, ethyl acetate.	Impregnation preparation	Has physical and chemical synergistic antifouling performance.	Surface of ships and underwater facilities	[146]
	AzoPU, AcCD, Azo(OH)2.	Mixed preparation	Can intelligently adjust the surface lubricity in response to external stimuli (such as visible light or heating).	Marine antifouling coatings	[147]
	Urea, uPDMS, methyl-terminated uPDMS, silicone oil, THF.	Solution casting	Capable of responsively releasing lubricating oil through mechanical stimulation in a solid matrix environment.	Agricultural machinery, micro-robot equipment	[148]
	MLCG, MXenes.	Laser carving technique	It has excellent antifriction and wear resistance, and can realize self-repair.	Engineering equipment and mechanical systems	[152]
	TC4, SnAgCu-WS.	3D printing	Excellent self-adaptive wear-resisting and antifriction performance.	Aviation, biomedicine, automobile manufacturing	[153]
	M50, SnAgCu, MXene-Nb.	Laser marking technology	Reduce the friction coefficient and wear depth of M50 steel under dry sliding condition.	Aeroengine bearing	[154]
	AISI 4140, solid lubricant SnAgCu.	Optical fiber laser marking	Decrease average friction coefficient, friction coefficient fluctuation, and wear rate of AISI 4140 steel.	Wind turbine bearing	[155]

summarizes the categories, materials, fabrication, characteristics, and applications of biomimetic tribology. It is anticipated that this comprehensive framework will stimulate increased interest among researchers to delve into this field.

5. Conclusions and Prospect

This review comprehensively and deeply analyzes bionic biological lubrication and its broad prospects for cross-domain application, especially its innovation potential in two key fields: industrial manufacturing and biomedical research. In the biomedical field, especially for the chemical analysis of joint lubrication mechanism, this review profoundly reveals the subtle cooperation of unique chemical components in articular cartilage and synovial fluid, and how to jointly build an efficient and low-friction barrier to protect joints from wear and ensure smooth and lasting movement of living beings. This discovery not only provides a new perspective for understanding the pathogenesis of joint diseases, but also lays a solid theoretical foundation for developing new artificial joint materials and lubricants, which indicates a revolutionary breakthrough in joint replacement and repair technology in the future. At the same time, from the perspective of structural bionics, this review makes a detailed analysis of typical natural surfaces such as the super-hydrophobic self-cleaning of lotus leaves, the drag reduction and sliding of fish skin, and the flexibility and wear resistance of snake skin. Through hundreds of millions of years of evolution and optimization, these biological surfaces have formed unique microstructures, which not only realize the precise regulation of friction, but also give them multiple functions such as self-cleaning, antibacterial, and antifouling. Inspired by this, we explored the possibility of integrating these natural characteristics into the design of engineering materials, and created a series of bionic surfaces with excellent properties, which brought remarkable efficiency improvement and cost savings to high-tech fields such as mechanical transmission and the aerospace indsutry (Figure 12).

In order to further promote the development of bionic lubrication technology, this review also innovatively puts forward the concept of chemical–structural coupling, emphasizing that the synergistic effect of biological structure and chemical components should be fully considered in the design of lubrication system. By simulating the lubrication mechanism of natural organisms, we skillfully introduce this synergistic effect into the engineering system, which not only significantly improves the tribological properties of mechanical parts, but also promotes the cross-integration of materials science, chemical engineering, mechanical engineering, and other disciplines, opening up a new territory for scientific and technological innovation. However, the future development of bionic lubrication technology still faces many challenges. Aiming at the problems of its specificity limitation, high cost, and insufficient durability and stability, we put forward a multi-functional and intelligent solution. By developing intelligent materials with the characteristics of self-repair, self-lubrication, and corrosion resistance, bionic lubrication technology can adapt to complex and changeable working environment, prolong equipment service life, and reduce maintenance costs.

In addition, in terms of environmental adaptability and sustainability, we will continue to pay attention to the research and application of biodegradable materials, reduce the use of harmful chemicals, and promote the development of green lubrication technology. This not only helps to alleviate the problem of environmental pollution, but also meets the strategic needs of global sustainable development. Looking into the future, the in-depth application of nanotechnology and surface science will provide more powerful technical support for the design and preparation of bionic lubricating materials. By fine-tuning the nanostructure and surface properties of materials, we will be able to create new materials with better lubrication performance, lower cost, higher stability, and environmental adaptability. The continuous advancement of these frontier researches will further verify the effectiveness and practicability of bionic lubrication technology and lay a solid foundation for its wide application in the future.