A Review of Biomechanical Studies of Heart Valve Flutter

Abstract

This paper reviews recent biomechanical studies on heart valve flutter. The function of the heart valves is essential for maintaining effective blood circulation. Heart valve flutter is a kind of small vibration phenomenon like a flag fluttering in the wind, which is related to many factors such as a thrombus, valve calcification, regurgitation, and hemolysis and material fatigue. This vibration phenomenon is particularly prevalent in valve replacement patients. The biomechanical implications of flutter are profound and can lead to micro-trauma of valve tissue, accelerating its degeneration process and increasing the risk of thrombosis. We conducted a systematic review along with a critical appraisal of published studies on heart valve flutter. In this review, we summarize and analyze the existing literature; discuss the detection methods of frequency and amplitude of heart valve flutter, and its potential effects on valve function, such as thrombosis and valve degeneration; and discuss some possible ways to avoid flutter. These findings are important for optimizing valve design, diagnosing diseases, and developing treatment strategies.

1. Introduction

As a key component of heart function, heart valves undertake the important task of controlling the direction of blood flow and maintaining effective blood circulation. Their health and function are crucial to this process. The human heart contains four main valves, the mitral, tricuspid, pulmonary, and aortic valves, which each function between different chambers of the heart. These valves prevent blood backflow by coordinating the opening and closing, ensuring the smooth flow of blood from the heart to the body.

In the 1960s, allograft heart transplants were used to treat valvular diseases, but due to difficulties in preservation and limited availability, researchers began exploring xenografts, which led to the development of bioprosthetic heart valves (BHVs), typically made from bovine or porcine pericardium [1]. BHVs exhibit high biocompatibility, do not require long-term medication after implantation, and have a structure very similar to natural valves. Compared to mechanical heart valves (MHVs), BHVs significantly reduce the risk of thrombosis, and clinical data show that they can last around 20 years in adult patients [2]. However, the functionality of BHVs is affected by structural valve degeneration (SVD), which manifests primarily as calcification, tearing, and other damage that leads to stenosis or regurgitation [3,4,5]. SVD is closely related to the processing and design of the valve. For example, biological tissue requires chemical treatment, such as with glutaraldehyde, for preservation, but studies have shown that this treatment can negatively impact the tissue’s shear and bending properties, thereby altering the stress distribution on the valve [6,7]. Furthermore, the method of suturing BHVs to the stent can cause stress concentration at the sutures, increasing the risk of tearing along the suture edges [8].

The development of synthetic polymer heart valves (PHVs) can be traced back to the 1950s when poly(dimethylsiloxane) (PDMS) was widely used in the medical field due to its excellent biocompatibility, flexibility, and fatigue resistance [9]. However, early clinical trials revealed that PHVs made from PDMS had high postoperative mortality rates, thrombosis formation, and structural valve degeneration [10], eventually leading to the abandonment of PDMS for PHV production. Another early material was polytetrafluoroethylene (PTFE) and expanded PTFE (ePTFE), which demonstrated good biocompatibility and stability but resulted in high mortality rates, severe regurgitation, and leaflet tearing during clinical applications [11,12]. The improved version, ePTFE, renewed researchers’ confidence due to its surface properties, which helped reduce thrombosis and improve long-term durability, though its applications remain primarily limited to pediatric patients, and its long-term impact on hemodynamics remains unclear [13,14,15,16].

Polyurethanes (PUs) were another early material used in PHVs, known for their excellent mechanical properties and biocompatibility, with widespread medical applications. PHVs made from PUs performed well in vitro, with models based on polycarbonate urethane (PCU) showing a lifespan of 400 to 650 million cycles after accelerated fatigue testing [17]. In vitro tests also indicated that PHVs made from PUs exhibited large opening areas and minimal regurgitation [18]. However, clinical and animal studies showed that PU valves had insufficient resistance to thrombosis and calcification, which led to their reduced clinical use [19].

To overcome the limitations of early PHVs, researchers began developing a new generation of composite materials. These composites combined biocompatible base materials with mechanically superior fillers, addressing the weaknesses of earlier materials. Macro-composites, for example, used large-scale polymer fibers to reinforce materials with good biocompatibility but poor mechanical properties, mimicking the anisotropy and fibrous structure of native aortic valves. This reinforcement helped redistribute the load on the leaflets and reduce maximum stress, thereby extending the valve’s lifespan and preventing crack propagation [20,21]. Meanwhile, nanocomposite materials, which not only improve the mechanical properties of polymers but also enhance their anti-calcification and blood compatibility characteristics, have become a research focus for the next generation of PHVs [22,23].

In recent years, the phenomenon of heart valve flutter has received extensive attention because of its significant role in valvular disease and dysfunction. A heart valve flutter, a tiny vibration similar to a flag fluttering in the wind, is commonly associated with calcification, regurgitation, hemolysis, and material fatigue [24]. This phenomenon is common in the healthy population; especially in certain groups, such as pregnant healthy young women, the occurrence rate is as high as 78.6% [25], and it is even more significant in patients after heart valve surgery. But native heart valve flutter does not usually lead to valve dysfunction. In transplanted biological valves, these vibrations are often closely related to valve material, thickness, and design [1] and are also influenced by the type of heart valve surgery. For example, after Straight Graft (SG), Valsalva Graft (VG), Anticommissural Plication (ACP), and Stanford Modification surgery (SMOD), the frequency and characteristics of valve tremors vary [26].

Contrary to common belief, although aortic valve stenosis significantly impacts hemodynamic parameters, leading to coronary artery disease, rheumatic aortic valve stenosis, or calcification, it typically does not exhibit fluttering phenomena [27]. This indicates that fluttering may be closely related to the health status and biomechanical properties of the valve, including the strength, toughness, and design of the materials.

For example, in in vitro experiments, the heart valves taken from fresh pig hearts had a flutter frequency of about 32 Hz, while bovine valves were relatively less prone to flutter, showing differences in valve performance between species [20]. In addition, the study also observed the changes in tremor frequency corresponding to different Reynolds values and the changes in tremor frequency of valves with different diameters under the same flow conditions, indicating that valves with smaller diameters showed higher blade flutter frequency [13]. In particular, the Perceval biological valve has greater strength and toughness, but its valve edge also has a large motion deformation during ventricular contraction, sometimes reaching a frequency of 15 Hz or about 1000 times/minute, reflecting that even if the strength of the biomaterial is sufficient, it may cause vibration due to design [28]. In in vitro tests, when normal saline was used instead of blood, the valve blades showed obvious flutter during systole, with a frequency of about 200 Hz and a maximum amplitude of 4 mm [29]. These findings show that the phenomenon of flutter is very common in artificial biological valves, and the frequency of flutter vibration is high, revealing the importance of flutter in heart valve health and disease.

High-frequency fluttering phenomena may cause microtrauma to valve tissues, accelerating their degeneration process and increasing the risk of thrombosis, thereby severely impacting the patient’s quality of life. Therefore, a deep understanding of the characteristics of heart valve flutter and its potential impact on valve function is of great significance for optimizing valve design, improving disease diagnoses, and developing effective treatment strategies.

This review summarizes the biomechanical properties of heart valve flutter in recent years, including its frequency and amplitude and how it is affected by the parameters of the valve itself and the hemodynamic state. In addition, we will explore the potential impact of flutter on valve function and how it can lead to thrombosis, valve degeneration, and other valve diseases. By summarizing and analyzing the existing literature, we hope to provide a scientific basis for the optimization of valve design, early warning of disease diagnoses, and formulation of treatment strategies.

2. Detection and Quantification of Heart Valve Flutter

The detection of heart valve flutter is a key component of the evaluation of heart valve function. Precise measurements of frequency and amplitude are critical to understanding the performance and durability of the heart valve.

First, applying a high-pass filter is critical because it removes the contribution of natural shape changes and valve movements, allowing researchers to more clearly identify and quantify non-physiological tremors that may negatively affect valve performance. During normal movement, the valve experiences certain shape changes and motions, which are typical physiological phenomena. These natural movements should not be considered when analyzing abnormal tremor energy, which is often caused by internal tissues or material defects. By applying a high-pass filter, frequencies associated with normal movement are removed, enabling researchers to focus on the higher-frequency abnormal vibrations that may lead to valve dysfunction or damage. This process allows for a more precise assessment of both the valve design’s performance and potential long-term durability issues.

The high-pass filter distinguishes between the desired low-frequency signals, corresponding to normal valve behavior, and higher-frequency signals that may represent noise or abnormal vibrations. Using techniques like a Fourier transform, researchers can isolate these frequency components and remove unwanted noise, resulting in cleaner data for a further analysis. This filtering not only helps in identifying harmful high-frequency vibrations but also improves the overall reliability of the data by minimizing interference from natural valve motion.

In addition to filtering, a flutter analysis is performed using a high-speed camera combined with a C++ algorithm that tracks the contours of the cusps in each video frame, enabling more accurate measurements of both frequency and amplitude. enabling more accurate measurements of both frequency and amplitude. This technique allows researchers to capture intricate details of valve movement and quantitatively assess the properties of flutter through a C++ algorithm analysis [30,31]. The combination of high-pass filtering and high-speed imaging provides a comprehensive toolset for evaluating valve function and detecting potential failure modes at an early stage.

At present, the main detection methods include high-speed imaging and a motion capture analysis, two-position particle image velocimetry (PIV), computational fluid dynamics and fluid–structure coupling simulation, bending deformation index calculation, and a cardiac cycle analysis. Each of these methods has its own characteristics and together constitute a comprehensive toolset for the assessment and understanding of cardiac valve flutter phenomena.

High-speed photography provides intuitive image data for the study and analysis of the flutter by capturing the high-speed motion of the heart valve. Combined with image processing technology, detailed parameters of valve motion, such as the vibration frequency, amplitude, and vibration mode, can be extracted from these high-speed videos, providing important visual information for an in-depth analysis of flutter.

Zhu et al. [32]. conducted motion tracking of the blade of the heart valve in a biomechanical study on the preservation of the aortic root. For this purpose, five points were selected along the leading edge of each valve blade, including the Arantius nodule. Next, the original spatio-temporal data were converted from pixel units to the international system of units in MATLAB. It provides an accurate basis for the subsequent analysis. A displacement map of each tracking point is then generated, with a particular focus on the critical stages of rapid blade opening and closing. In order to quantify these movements, a linear regression model was applied, through which the average instantaneous speed of each tracking point was calculated, and the average rapid opening and closing speed of the blade was obtained accordingly. In addition, in order to evaluate the relative forces of the blades during rapid opening and closing, the derivatives of the velocity plot for each tracking point were calculated and these derivatives were averaged. These acceleration data were then standardized with reference to aortic valves in control pigs to estimate relative forces. The analysis of the flutter is conducted by performing a Fourier transform on each displacement graph. This step allows us to identify the basic frequency and power as well as the specific frequency corresponding to each tracking point. This comprehensive methodology not only reveals the motion pattern of the blade, including the rapid opening and closing speed, but also provides an in-depth understanding of the flutter frequency and power, increasing our understanding of the phenomenon of heart valve flutter [20].

The application of a two-dimensional particle image velocimetry (PIV) experiment further improves the accuracy of flutter detection. PIV technology provides a dynamic and intuitive way to observe and measure valve flutter by tracking the movement of tiny particles in the fluid [2]. This technique is able to capture the flutter phenomenon more precisely, providing a detailed dynamic view for the analysis.

Moore et al. [18] used PIV technology to track the movement of the valve tip in their study of the spatio-temporal complexity of the aortic sinus vortices. Specifically, the researchers used a high-speed CMOS camera (Photronix Inc., San Diego, CA, USA) to record videos of valve motion. This camera is set at an angle of approximately 60 degrees to capture the flow within a single sinus in the best view. To analyze these videos, the researchers used commercial particle image velocimetry (PIV) software DaVis ((DaVis 7.2, LaVision, Göttingen, Germany) for data acquisition and processing. The tracking of the valve tip motion was conducted manually with an accuracy of 56 microns (equivalent to ±2 pixels). By recording four tests under four heart rate conditions and then calculating the average, the researchers were able to calculate the average frequency and amplitude of the valve vibrations.

The bending deformation index (BDI) provides another perspective for the evaluation of flutter. BDI quantifies the degree of flutter by measuring the ratio of the maximum bending depth of the blade edge to the overall length of the blade. Specifically, the maximum bending depth of the blade edge reflects the maximum bending degree of the blade along its length during flutter, while the overall blade length is the distance from the blade edge to the edge. The BDI value reflects the severity of flutter: a larger bending depth produces a higher BDI value, indicating more serious flutter; a smaller bending depth results in a lower BDI value, indicating less flutter. This measure is used not only to assess the health of the heart valve, but also to judge the quality of the implant, as excessive flutter can lead to the early damage or dysfunction of the valve [5].

The use of computational fluid dynamics (CFD) and fluid–structure coupling (FSI) simulations has enhanced our understanding of the flutter behavior of artificial valves. These simulations take into account the physical properties of the valve material, such as thickness and flexibility, thus allowing us to predict the valve’s flutter behavior under different conditions [33]. In his study of the valve, Nelson Michael Wiese utilized an immersive geometric fluid–structure interaction (FSI) analysis framework based on IGA (Isogeometric Analysis) to track the movement of the free edge of the blade during valve opening with high precision, as shown in Figure 1. This method is designed to capture the energy spectral density of blade flutter and associated bending motion. For this purpose, the free edge of each blade is monitored with 100 uniformly distributed tracking points, as shown in Figure 2, and the flutter of the blade is quantified by calculating the energy spectral density through the discrete Fourier transform of the free edge displacement data.

Finally, the frequency and amplitude of the flutter can also be calculated by analyzing the duration of the flutter after the valve is fully opened during the heart cycle. This method provides an alternative measure of flutter frequency and amplitude by calculating the number of peaks over duration and the average distance between adjacent peaks and valleys [34].

In most detection methods, the Fourier transform is used to process the data. A Fourier analysis plays an important role in the detection and quantification of bioprosthetic heart valve flutter. Specific applications include the use of a discrete Fourier transform to process displacement data of valve blades. This method allows us to calculate the energy spectral density from the displacement data and accurately quantify the frequency and power of the flutter. The importance of a Fourier analysis is that it provides an accurate quantitative means to understand the characteristics of flutter under different conditions, and is a key tool to analyze the complex dynamics of heart valve flutter.

In addition to the methods mentioned above, a theoretical analysis can also be employed to study the conditions under which flutter occurs and to quantify flutter. A theoretical analysis aims to simplify the heart valve flutter process into a mathematical model that incorporates valve material properties and flow field boundary conditions, thereby elucidating the flutter behavior of the valve. Currently, extensive research has been conducted both domestically and internationally on flexible cantilever plate models. Future improvements and the in-depth exploration of this model are expected to provide unique insights into the theoretical analysis of valve flutter, as shown in Figure 3 [25].

In general, the detection methods of heart valve flutter range from high-precision imaging techniques to complex computational simulations, each of which provides us with unique and important information about the characteristics of valve flutter. The combined application of these methods allows us to comprehensively evaluate the function of the heart valve, ensuring the optimization of cardiac surgery and valve design.

3. Problems with Heart Valve Flutter

Valve flutter not only directly affects the mechanical properties of the valve, such as its efficiency in opening and closing, but also affects the long-term stability and durability of the valve, and can increase the risk of thrombosis. Prolonged high-frequency vibration may lead to hemodynamic abnormalities, which increase the risk of thrombosis, and may also accelerate the wear and degradation of valve materials, thereby shortening the effective life of the valve.

3.1. Blood Flow Disturbance Caused by Heart Valve Flutter

Many studies have shown that blood flow disturbance caused by vibration may increase the risk of thrombosis, but no specific explanation has been provided [21,34,35,36,37].

Zhu et al. [32] described this mechanism in more detail in their study. Flutter can cause blood flow disturbances and other abnormal fluid phenomena, such as local turbulence and local high wall shear stress, which can damage red blood cells and platelets, which are more sensitive to shear. When platelets rupture under stress, enzymes in their cytoplasm promote blood clotting, increasing the risk of blood clots. In addition, turbulent blood flow creates high wall shear stresses, especially in the ascending aorta, which may lead to damage to vascular endothelial cells. Endothelial cells, the inner lining of blood vessels, are essential for maintaining blood flow and blood vessel health. Damage to these cells may lead to vascular dysfunction and increase the risk of cardiovascular disease [38].

Becsek et al. [34] found in their study of the relationship between local perturbations and thrombosis that the total turbulent dissipation accounted for 26% of the total valve pressure loss, indicating that turbulence is a significant and adverse factor in hemodynamic valve performance, and also found turbulent viscous shear stress of up to 14 Pa, which may be related to shear-induced platelet activation. This suggests that a bioprosthetic heart valve’s (BHV) propensity for thrombosis may be related to the turbulent flow of the valve. Using computational models, the researchers studied in detail for the first time the pattern of wall shear stress along the aortic wall. This analysis revealed a potential link between BHV turbulence and endothelial damage. The levels of turbulent wall shear stress found in these models suggest that turbulence may cause damage to endothelial cells, which in turn affects the long-term health of the aorta. In addition, this study provides theoretical support for understanding the association between local disturbances, such as valvular flutter and vorticity shedding, and thrombosis. These findings highlight the importance of hemodynamic parameters in heart valve research and cardiovascular health assessment, especially in exploring ways to reduce valvular-induced blood flow abnormalities and their potential health effects.

NYGAARD et al. [36] suggested that complications associated with artificial heart valves, such as thrombosis, hemolysis, and calcification, are thought to be related to blood flow disturbances caused by the valves. Two artificial heart valves, Hancock Porcine (HAPO) and Ionescu–Shiley Pericardial Standard (ISPS), were used to measure fluid velocity using a hot film anemometer. Flow rates were measured in a diameter range downstream of both valves. The experiment also involves a turbulence analysis, calculating axial turbulence energy as a function of time, and estimating turbulence shear stress. A relative blood damage index (RBDI) is proposed, which combines the estimated magnitude of shear stress and exposure time, covering the entire cross-sectional area. RBDI is calculated in a way that makes it easier to compare different heart valves.

3.2. Mechanical Problems and Valve Degeneration

Due to high-frequency cyclic loading, flutter may accelerate other mechanical problems with valve implants, such as degradation, reduced durability, and fatigue. Blade flutter of the bio-valve introduces additional mechanical load cycles during contraction. Repeated bending of the blade material and stress during opening and closing can lead to early blade wear or mechanical failure. Long-term exposure to abnormal mechanical stress may result in valve degeneration, increasing the risk of calcification and accelerating the deterioration of valve tissue [35,36].

In summary, the problems caused by valve flutter are multifaceted and serious. It not only causes hemodynamic problems, such as thrombosis, but may also cause mechanical problems with the valve, such as valve degradation, wear, and deterioration. Therefore, in the process of heart valve design, it is necessary to consider how to minimize the vibration phenomenon to improve the performance of the valve and extend its service life.

Table 1. Problems with heart valve flutter: thrombus and fatigue.

Description	Reference
The subtle flutter may represent a local turbulent effect on the flexible aortic valve tip in an altered hemodynamic state	[39]
The flutter can cause blood flow disturbances and other abnormal fluid phenomena that may lead to the formation of a thrombus or embolisms	[35]
The study explored the effects of altered hemodynamics on the aortic valve leaflets, including aspects such as wall shear stress (WSS), which can lead to fatigue and a thrombus	[40]
Other mechanical problems with the valve implant, such as degradation, reduced durability, and fatigue, may be accelerated due to high-frequency cyclic loading	[35]
Leaflet flutter of a biological valve can have an impact on the durability of the valve as it introduces additional cycles of mechanical load during contraction, which can lead to early blade wear or mechanical failure due to repeated bending of the blade material and the forces experienced during opening and closing	[37]
Heart valve flutter is thought to be associated with calcification, regurgitation, hemolysis, and material fatigue. A stronger tremor observed in the TF valve may lead to a reduced peak flow rate on the one hand and reduced long-term durability of the valve on the other	[41]

summarizes the problems caused by flutter.

4. Parameters Related to Heart Valve Flutter

4.1. Heart Valve Material Parameters

In heart valve design and biomechanical research, it is important to understand the phenomenon of valve flutter and its relationship with material parameters. Valve flutter is affected by many factors, especially material parameters.

First, the material strength and toughness of the valve have a direct effect on the vibration. Ael et al. [35]. simulated the effects of different mass, thickness, and stiffness on flutter, and concluded that valve mass and membrane stiffness had little influence on flutter, while flexural stiffness was the main factor leading to valve flutter. The study also found that plasma coatings formed during valve implantation or used in combination with fixatives may harden the valve blades enough to minimize vibration at the edges of the blades. This suggests that the plasma coating may reduce the valve’s flutter by increasing its rigidity.

In addition, the elasticity of the valve also has a significant effect on the flutter. Because of the lack of a clear resonance frequency, the less elastic valve may produce irregular and rapid vibration under the excitation of blood flow, which is called urgent flutter. Therefore, optimizing the elastic properties of the valve is an important consideration in reducing flutter [33].

In summary, the phenomenon of valve flutter is affected by several parameters of the valve material, including its strength, toughness, thickness, and elasticity. Reasonable design of these parameters not only plays an important role in reducing flutter, but also helps to improve the overall performance and durability of the valve. Therefore, the comprehensive consideration of these material parameters during valve design is essential to optimize valve function and extend its service life.

4.2. Heart Valve Design Parameters

While stronger and more ductile materials may help reduce flutter, this does not mean that flutter can be avoided entirely. Flutter is also influenced by many factors such as valve design and fluid dynamics [42]. For example, the unique design of the Perceval bioprosthetic valve, despite the material’s sufficient strength and toughness, may still cause a vibration phenomenon due to its design [29,43]. In the case of aortic valve stenosis, contractile vibration may not be seen due to the limitation of blade motion due to the thickening of the valve blade. This suggests that the thickness of the valve may be related to the tremor [39]. The results of most in vitro experiments and simulation showed that the thinner the valve, the higher the flutter frequency and the more obvious the flutter phenomenon.

Condurache et al. [44] used high-pass filters to remove the contribution of natural shape and valve motion to flutter and used a discrete Fourier transform to calculate the displacement amplitude data. They found that there were multiple significant contributions of higher frequencies in the case of thinner valves, indicating that thinner valves would have more significant flutter [18]. In addition, a considerable amount of literature has concluded that thinner valves are more likely to cause flutter [18,32,33,41], but there are also researchers who hold opposing opinions. For instance, Jea et al. [31] concluded in their research that the thicker the valve is, the more intense the flutter is Jea explained that the previous in vitro experiments only considered the stable flow without controlling the changes in control device diameter, lobular thickness, biomechanics, or operating conditions, resulting in inaccurate experimental results.

Table 2. Influence of heart valve thickness on flutter.

Description	Reference
In the case of aortic valve stenosis, the incidence of flutter decreased compared to normal due to the limitation of leaflet motion due to the thickening of the valve leaflet	[25]
The flutter frequency of valve tissue with different thickness is lower with greater thickness	[45]
The heart valve flutter is related to thickness, but the correlation is not specified	[46]
The study took into account different thicknesses of biomaterials and found that thinner valves were more likely to cause flutter	[33]
For fixed-diameter devices, a thinner leaflet will produce a lower tremor frequency than a thicker leaflet	[31]
Thinner biological tissue can induce valve flutter during aortic heart valve replacement	[47]

summarizes some descriptions of the effects of thickness on flutter in the literature.

Jea et al. [31] also analyzed the effect of valve diameter on flutter. Smaller-diameter valves were found to show a higher frequency of the leaflet tremor. This means that the diameter of the valve is also an important factor in its flutter behavior. Smaller-diameter valves may face greater mechanical stress and potential wear problems due to a higher frequency of the tremor.

In addition, the length of the free edge of the valve and the degree of curvature of the abdominal curve can affect the flutter. Some scholars have used the structural finite element method to optimize various geometric parameters, including the valve height, angle, connection distance, reflection angle with the valve shaft, and reflection angle with the valve radius, as shown in Figure 4 [2], and some scholars have optimized the thickness distribution of a valve through fluid–structure coupling calculation to further improve the durability and bending stiffness of the valve [47].

While the geometry of the native valve can be easily cast and replicated, this does not necessarily lead to the best performance of the polymer valve. It is worth noting that for a long time, the design optimization of a valve has been guided by intuition, as shown in the following figure; the cut-out part of a hemisphere intersecting with a cylinder is used as the valve, which obviously has a lot of room for improvement, as shown in Figure 5 [43].

4.3. Influence of Non-Valvular Parameters on Cardiac Valve Flutter

In biomechanical studies of heart valves, fluid viscosity is an important consideration because it directly affects the flutter behavior of the valve blades. According to the research, when the viscosity of the fluid is closer to that of blood, such as in a glycerin mixture, the flutter phenomenon is not obvious. This suggests that in flow conditions that mimic blood, higher fluid viscosity may help reduce flutter in the valve blades. This phenomenon may be due to the greater damping effect provided by the higher-viscosity fluid, which reduces the vibration of the blade [30].

The research also showed that fluid viscosity had no significant effect on the valve closing process. In experiments with Tyrode’s solution, a clear solution used to replace blood, high-frequency vibrations were observed in the valve, with some flapping at frequencies between 50 and 100 Hz. However, the total deflection observed was very small (about 0.6 mm) and did not differ significantly between high- and low-viscosity conditions. This indicates that although the fluid viscosity may affect the vibration frequency of the valve blade, it has little effect on the overall amplitude or deflection of its vibration [26].

In summary, fluid viscosity is an important factor affecting cardiac valve flutter. Although it has a limited effect on the valve closing process, it can reduce the flutter phenomenon of the valve blade under high-viscosity conditions that mimic blood. Therefore, it is critical to consider the effect of fluid viscosity on flutter behavior in the design and testing of heart valves, especially in environments that mimic blood flow in the human body. By optimizing valve design, flutter can be minimized, thereby improving valve performance and durability.

5. Discussions

5.1. Material and Thickness

The material properties and thickness of a valve have a direct effect on flutter. For example, bovine pericardium materials exhibit lower flutter frequency and better opening synchronization than porcine pericardium [30].

It is not advisable to simply increase the elastic modulus of the material. Bernacca et al. [48] concluded that the valve thickness was 0.1–0.3 mm (about 20% of normal thickness), and the durability of the valve was the best [48]. They also tested the effect of thickness and Young’s modulus on the valve, which is 5–63.5 MPa (an order of magnitude higher than normal), and found that thickness has a more significant effect on the fluid dynamic function of the valve, so it is necessary to make the artificial valve much thinner than the original film, and a thinner valve is more prone to flutter. Therefore, an attempt was made to simply increase the elastic modulus to observe the effect of the elastic model on flutter. We adopted the fluid–structure coupling method and set the thickness of the valve to 0.1 mm and the elastic modulus to 70 MPa in the LS-DYNA solver. Experimental results showed that although the valve exhibited a very low flutter frequency, its opening area was not up to standard. Therefore, it is not possible to simply strengthen the material without changing the valve design to avoid vibration.

In addition, although increasing the thickness of the valve can reduce the internal stress level, it may have adverse effects on the hemodynamic performance, such as increasing the differential pressure across the valve [41]. In addition, as previously mentioned, even if the use of materials with sufficient strength and toughness can reduce or control tremors to some extent, it cannot be completely avoided.

5.2. Design Optimization

Valve design, especially the optimization of blade design, is the key to reducing flutter. Different valve designs, such as TFUR and MADV, show significant kinematic differences in the closing stage, indicating that valve material, thickness, and design have a significant impact on their dynamic behavior [36].

5.3. Fiber Enhancement in the Heart Valve

Anisotropic valves (with fibers) vibrate less than isotropic valves because anisotropy results in a smaller pressure difference across valves and enhances the material properties of the valves. Reinforcing structures, such as by embedding special fibers or using high-strength materials, can significantly reduce flutter [49]. A mesh of fibers is embedded in a polymer valve. In the manufacturing process, the fiber-reinforced layer is placed between the top and bottom of the polymer matrix layer to form a composite structure. These fiber layers are arranged in specific directions and angles, such as transverse, longitudinal, and reticular arrangements. In order to mimic the arrangement of collagen fibers in natural heart valves, researchers developed a specific method. In the mesh fiber-reinforced model, both layers of fibers are given different orientations to form cross-angles, a design that significantly reduces stress in the polymer matrix.

To achieve this fiber reinforcement, special manufacturing techniques, such as lamination or braiding, are used to integrate the fibers directly into the valve material, as shown in Figure 6. In this way, when the valve is under pressure or dynamic load, the embedded fibers can share some of the stress, improving the overall strength and durability of the valve, while promoting a more uniform stress distribution, thereby improving the functional performance and longevity of the valve [50].

In other studies, Nitinol wire was used to make use of its superelastic properties to enable the valve to open rapidly during cardiac contraction and close effectively during diastole, thus improving blood flow efficiency. In addition, the wire provided additional strength and support, increased the flexural stiffness of the valve, and effectively reduced the occurrence of flutter [43].

6. Conclusions

In this paper, the biomechanical characteristics of heart valve flutter were comprehensively analyzed, and the phenomena, detection methods, and problems caused by and related to biomechanical parameters were discussed in order to provide a basis for clinical treatment and research.

First of all, this paper discusses the phenomenon of heart valve flutter and elucidates the negative effects of flutter on heart function, especially in inducing a thrombus and valve fatigue damage. In addition, this paper discusses multiple methods for detecting heart valve flutter, such as the high-pass filter and PIV technology, which are essential for early diagnoses and timely treatment. A variety of biomechanical parameters related to heart valve flutter, such as valve stiffness and tissue elasticity, were analyzed. These parameters are of great significance for understanding the mechanism and development of heart valve flutter. Finally, the methods to prevent and reduce heart valve flutter are discussed, including optimizing valve design, increasing valve material strength, and implanting fiber-reinforced structures.

In conclusion, heart valve flutter is a complex and multifaceted problem that needs comprehensive consideration from multiple perspectives such as biomechanics, clinical medicine, and treatment strategies. The studies in this paper not only improve the understanding of heart valve flutter, but also provide a new perspective and ideas for future research and treatment.To this end, a summary diagram has been created, which includes all the factors and variables discussed in this article, including those that affect vibration, as shown in Figure 7.