Micromachines

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22 pages, 5368 KiB

Open AccessArticle

Bistable Threshold Humidity Sensor Switch with Rectangular Bimorph Bending Plate

by Nikolai Gulnizkij and Gerald Gerlach

Micromachines 2020, 11(6), 569; https://doi.org/10.3390/mi11060569 - 3 Jun 2020

Cited by 7 | Viewed by 2678

Energy-autonomous bistable threshold sensor switches have the potential to reduce costs because they do not need any electrical energy supply for monitoring physical quantities, such as relative humidity. In previous work, a bistable beam-like sensor switch with switching hysteresis was manufactured from sheet [...] Read more.

Energy-autonomous bistable threshold sensor switches have the potential to reduce costs because they do not need any electrical energy supply for monitoring physical quantities, such as relative humidity. In previous work, a bistable beam-like sensor switch with switching hysteresis was manufactured from sheet metal and a partially coated water vapor-sensitive hydrogel (poly(vinyl alcohol)/poly(acryl acid)). Based on the beam theory, a corresponding mechanical model was developed. However, bending plates should be used instead of bending beams to separate the humidity to be measured from the electrical contacts. For this reason, this work deals with the development and realization of a mechanical model based on the plate theory to describe the deflection of a silicon bimorph bending plate partially coated with hydrogel that swells with increasing humidity. For implementing a switching hysteresis a plasma-enhanced chemical vapor deposition silicon dioxide (SiO₂) layer is used, which was deposited and structured on top of the silicon plate. The hydrogel layer itself is patterned on the surface of the bending plate using a stamp technique. To validate the mechanical model, the switching hysteresis of the miniaturized sensor switch was measured optically by a camera measurement device. Full article

(This article belongs to the Special Issue Selected papers from the APMM 2019–Active Polymeric Materials and Microsystems Conference)

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14 pages, 5985 KiB

Open AccessArticle

Shell-Forming Stimulus-Active Hydrogel Composite Membranes: Concept and Modeling

by Adrian Ehrenhofer and Thomas Wallmersperger

Micromachines 2020, 11(6), 541; https://doi.org/10.3390/mi11060541 - 26 May 2020

Cited by 16 | Viewed by 2512

Abstract

The swelling of active hydrogels combined with passive layers allows the design of shell-forming structures. A shell-like structure offers different properties than a flat structure, e.g., variations in bending stiffness across different directions. A drastic increase of the bending stiffness is favorable e.g., [...] Read more.

The swelling of active hydrogels combined with passive layers allows the design of shell-forming structures. A shell-like structure offers different properties than a flat structure, e.g., variations in bending stiffness across different directions. A drastic increase of the bending stiffness is favorable e.g., in rollable/flexible displays: in their unrolled form, they have to be stiff enough to resist bending due to dead weight. At the same time, they have to be flexible enough to be rolled-up. This can be achieved by shell-forming. In the current modeling and simulation work, we present a basic concept of combined active–passive composites and demonstrate how they form shells. As the example material class, we use hydrogels with isotropic swelling capabilities. We demonstrate how to model the combined mechanical behavior with the Temperature-Expansion-Model. Afterwards, we show numerical results obtained by Finite Element simulations. We conclude that the envisioned structure has a great potential for obtaining soft rollable sheets that can be stiffened by intrinsic activation. Full article

(This article belongs to the Special Issue Selected papers from the APMM 2019–Active Polymeric Materials and Microsystems Conference)

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20 pages, 7508 KiB

Open AccessFeature PaperArticle

Hydrogel Patterns in Microfluidic Devices by Do-It-Yourself UV-Photolithography Suitable for Very Large-Scale Integration

by Anthony Beck, Franziska Obst, Mathias Busek, Stefan Grünzner, Philipp J. Mehner, Georgi Paschew, Dietmar Appelhans, Brigitte Voit and Andreas Richter

Micromachines 2020, 11(5), 479; https://doi.org/10.3390/mi11050479 - 2 May 2020

Cited by 20 | Viewed by 6141

Abstract

The interest in large-scale integrated (LSI) microfluidic systems that perform high-throughput biological and chemical laboratory investigations on a single chip is steadily growing. Such highly integrated Labs-on-a-Chip (LoC) provide fast analysis, high functionality, outstanding reproducibility at low cost per sample, and small demand [...] Read more.

The interest in large-scale integrated (LSI) microfluidic systems that perform high-throughput biological and chemical laboratory investigations on a single chip is steadily growing. Such highly integrated Labs-on-a-Chip (LoC) provide fast analysis, high functionality, outstanding reproducibility at low cost per sample, and small demand of reagents. One LoC platform technology capable of LSI relies on specific intrinsically active polymers, the so-called stimuli-responsive hydrogels. Analogous to microelectronics, the active components of the chips can be realized by photolithographic micro-patterning of functional layers. The miniaturization potential and the integration degree of the microfluidic circuits depend on the capability of the photolithographic process to pattern hydrogel layers with high resolution, and they typically require expensive cleanroom equipment. Here, we propose, compare, and discuss a cost-efficient do-it-yourself (DIY) photolithographic set-up suitable to micro-pattern hydrogel-layers with a resolution as needed for very large-scale integrated (VLSI) microfluidics. The achievable structure dimensions are in the lower micrometer scale, down to a feature size of 20 µm with aspect ratios of 1:5 and maximum integration densities of 20,000 hydrogel patterns per cm². Furthermore, we demonstrate the effects of miniaturization on the efficiency of a hydrogel-based microreactor system by increasing the surface area to volume (SA:V) ratio of integrated bioactive hydrogels. We then determine and discuss a correlation between ultraviolet (UV) exposure time, cross-linking density of polymers, and the degree of immobilization of bioactive components. Full article

(This article belongs to the Special Issue Selected papers from the APMM 2019–Active Polymeric Materials and Microsystems Conference)

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13 pages, 3036 KiB

Open AccessArticle

Comparison of Dynamic Light Scattering and Rheometrical Methods to Determine the Gel Point of a Radically Polymerized Hydrogel under Mechanical Shear

by Katinka Kohl

Micromachines 2020, 11(5), 462; https://doi.org/10.3390/mi11050462 - 28 Apr 2020

Cited by 6 | Viewed by 2925

Abstract

The phase transition of nanocomposite hydrogels made of N-isopropylacrylamide (NIPAm) and clay (Laponite^® XLS) was investigated under mechanical shear influencing the gelation. The hydrogels were synthesized by free radical polymerization. For the processing of cross-linked gels, the phase transition (liquid–solid) and its [...] Read more.

The phase transition of nanocomposite hydrogels made of N-isopropylacrylamide (NIPAm) and clay (Laponite^® XLS) was investigated under mechanical shear influencing the gelation. The hydrogels were synthesized by free radical polymerization. For the processing of cross-linked gels, the phase transition (liquid–solid) and its dependence on mechanical stress are of paramount importance. On the one hand, the determination of the gel point (tg) is possible with rheometry and, on the other hand, with dynamic light scattering (DLS). With rotational rheometry, by identifying the abrupt increase of viscosity, the gel point is evaluated. The DSL is an alternative method to rheometry, to investigate hydrogels under the action of the shear flow, to make results comparable to the rheometric investigations, with and without shear. Experimental parameters were chosen based on preparatory work to obtain comparable results regarding the determination of the gel point of a radically polymerized NIPAm hydrogel. Full article

(This article belongs to the Special Issue Selected papers from the APMM 2019–Active Polymeric Materials and Microsystems Conference)

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14 pages, 5211 KiB

Open AccessFeature PaperArticle

Swelling Studies of Porous and Nonporous Semi-IPN Hydrogels for Sensor and Actuator Applications

by Daniela Franke and Gerald Gerlach

Micromachines 2020, 11(4), 425; https://doi.org/10.3390/mi11040425 - 18 Apr 2020

Cited by 11 | Viewed by 3389

Abstract

In this article, we present a semi-interpenetrating network (IPN) hydrogel of reasonable size with improved swelling behavior. The semi-IPN is composed of N-isopropylacrylamide and 2-acrylamido-2-methyl-1-propanesulfonic acid. Porosity was generated chemically by a surfactant-based template method. The swelling behavior was measured after an [...] Read more.

In this article, we present a semi-interpenetrating network (IPN) hydrogel of reasonable size with improved swelling behavior. The semi-IPN is composed of N-isopropylacrylamide and 2-acrylamido-2-methyl-1-propanesulfonic acid. Porosity was generated chemically by a surfactant-based template method. The swelling behavior was measured after an abrupt change of the temperature to 25 °C or 40 °C or after an abrupt change of the salt concentration of the aqueous medium surrounding the hydrogel samples. A set of static swelling degrees was determined from swelling measurements in salt solutions of varying concentrations and at different temperatures. Introducing porosity to the semi-IPN decreases the swelling times for most measurements while the sensor and actuator characteristics of the hydrogel found in previous studies are preserved. Additionally, we propose theoretical assumptions and explanations regarding the differences in the swelling kinetics of the porous and the nonporous semi-IPN and deduce implications for sensor and actuator applications. Full article

(This article belongs to the Special Issue Selected papers from the APMM 2019–Active Polymeric Materials and Microsystems Conference)

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16 pages, 3793 KiB

Open AccessArticle

A Biomimetic Fish Fin-Like Robot Based on Textile Reinforced Silicone

by Sascha Pfeil, Konrad Katzer, Anas Kanan, Johannes Mersch, Martina Zimmermann, Michael Kaliske and Gerald Gerlach

Micromachines 2020, 11(3), 298; https://doi.org/10.3390/mi11030298 - 12 Mar 2020

Cited by 32 | Viewed by 4409

Abstract

The concept of merging pre-processed textile materials with tailored mechanical properties into soft matrices is so far rarely used in the field of soft robotics. The herein presented work takes the advantages of textile materials in elastomer matrices to another level by integrating [...] Read more.

The concept of merging pre-processed textile materials with tailored mechanical properties into soft matrices is so far rarely used in the field of soft robotics. The herein presented work takes the advantages of textile materials in elastomer matrices to another level by integrating a material with highly anisotropic bending properties. A pre-fabricated textile material consisting of oriented carbon fibers is used as a stiff component to precisely control the mechanical behavior of the robotic setup. The presented robotic concept uses a multi-layer stack for the robot’s body and dielectric elastomer actuators (DEAs) on both outer sides of it. The bending motion of the whole structure results from the combination of its mechanically adjusted properties and the force generation of the DEAs. We present an antagonistic switching setup for the DEAs that leads to deflections to both sides of the robot, following a biomimetic principle. To investigate the bending behavior of the robot, we show a simulation model utilizing electromechanical coupling to estimate the quasi-static deflection of the structure. Based on this model, a statement about the bending behavior of the structure in general is made, leading to an expected maximum deflection of 10 mm at the end of the fin for a static activation. Furthermore, we present an electromechanical network model to evaluate the frequency dependent behavior of the robot’s movement, predicting a resonance frequency of 6.385 Hz for the dynamic switching case. Both models in combination lead to a prediction about the acting behavior of the robot. These theoretical predictions are underpinned by dynamic performance measurements in air for different switching frequencies of the DEAs, leading to a maximum deflection of 9.3 mm located at the end of the actuators. The herein presented work places special focus on the mechanical resonance frequency of the robotic setup with regard to maximum deflections. Full article

(This article belongs to the Special Issue Selected papers from the APMM 2019–Active Polymeric Materials and Microsystems Conference)

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14 pages, 4191 KiB

Open AccessFeature PaperArticle

A Non-Cytotoxic Resin for Micro-Stereolithography for Cell Cultures of HUVECs

by Max J. Männel, Carolin Fischer and Julian Thiele

Micromachines 2020, 11(3), 246; https://doi.org/10.3390/mi11030246 - 26 Feb 2020

Cited by 21 | Viewed by 5126

Abstract

Three-dimensional (3D) printing of microfluidic devices continuously replaces conventional fabrication methods. A versatile tool for achieving microscopic feature sizes and short process times is micro-stereolithography (µSL). However, common resins for µSL lack biocompatibility and are cytotoxic. This work focuses on developing new photo-curable [...] Read more.

Three-dimensional (3D) printing of microfluidic devices continuously replaces conventional fabrication methods. A versatile tool for achieving microscopic feature sizes and short process times is micro-stereolithography (µSL). However, common resins for µSL lack biocompatibility and are cytotoxic. This work focuses on developing new photo-curable resins as a basis for µSL fabrication of polymer materials and surfaces for cell culture. Different acrylate- and methacrylate-based compositions are screened for material characteristics including wettability, surface roughness, and swelling behavior. For further understanding, the impact of photo-absorber and photo-initiator on the cytotoxicity of 3D-printed substrates is studied. Cell culture experiments with human umbilical vein endothelial cells (HUVECs) in standard polystyrene vessels are compared to 3D-printed parts made from our library of homemade resins. Among these, after optimizing material composition and post-processing, we identify selected mixtures of poly(ethylene glycol) diacrylate (PEGDA) and poly(ethylene glycol) methyl ethyl methacrylate (PEGMEMA) as most suitable to allow for fabricating cell culture platforms that retain both the viability and proliferation of HUVECs. Next, our PEGDA/PEGMEMA resins will be further optimized regarding minimal feature size and cell adhesion to fabricate microscopic (microfluidic) cell culture platforms, e.g., for studying vascularization of HUVECs in vitro. Full article

(This article belongs to the Special Issue Selected papers from the APMM 2019–Active Polymeric Materials and Microsystems Conference)

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16 pages, 2024 KiB

Open AccessArticle

Hydrogel Microvalves as Control Elements for Parallelized Enzymatic Cascade Reactions in Microfluidics

by Franziska Obst, Anthony Beck, Chayan Bishayee, Philipp J. Mehner, Andreas Richter, Brigitte Voit and Dietmar Appelhans

Micromachines 2020, 11(2), 167; https://doi.org/10.3390/mi11020167 - 5 Feb 2020

Cited by 17 | Viewed by 3872

Abstract

Compartmentalized microfluidic devices with immobilized catalysts are a valuable tool for overcoming the incompatibility challenge in (bio) catalytic cascade reactions and high-throughput screening of multiple reaction parameters. To achieve flow control in microfluidics, stimuli-responsive hydrogel microvalves were previously introduced. However, an application of [...] Read more.

Compartmentalized microfluidic devices with immobilized catalysts are a valuable tool for overcoming the incompatibility challenge in (bio) catalytic cascade reactions and high-throughput screening of multiple reaction parameters. To achieve flow control in microfluidics, stimuli-responsive hydrogel microvalves were previously introduced. However, an application of this valve concept for the control of multistep reactions was not yet shown. To fill this gap, we show the integration of thermoresponsive poly(N-isopropylacrylamide) (PNiPAAm) microvalves (diameter: 500 and 600 µm) into PDMS-on-glass microfluidic devices for the control of parallelized enzyme-catalyzed cascade reactions. As a proof-of-principle, the biocatalysts glucose oxidase (GOx), horseradish peroxidase (HRP) and myoglobin (Myo) were immobilized in photopatterned hydrogel dot arrays (diameter of the dots: 350 µm, amount of enzymes: 0.13–2.3 µg) within three compartments of the device. Switching of the microvalves was achieved within 4 to 6 s and thereby the fluid pathway of the enzyme substrate solution (5 mmol/L) in the device was determined. Consequently, either the enzyme cascade reaction GOx-HRP or GOx-Myo was performed and continuously quantified by ultraviolet-visible (UV-Vis) spectroscopy. The functionality of the microvalves was shown in four hourly switching cycles and visualized by the path-dependent substrate conversion. Full article

(This article belongs to the Special Issue Selected papers from the APMM 2019–Active Polymeric Materials and Microsystems Conference)

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