PDMS Microstructures Research Articles

High-performance flexible self-powered triboelectric pressure sensor based on chemically modified micropatterned PDMS film

Flexible pressure sensors have played an increasingly important role in the Internet of Things (IoT) and human–machine interaction (HMI) systems. However, it is in a desire to fabricate the sensor with high sensitivity and low power consumption. In this paper, we have developed a self-powered triboelectric pressure sensor based on chemically modified micropatterned PDMS. Different wire meshes serve as template to fabricate microstructures on the triboelectric layer. To enhance the triboelectric output performance, introducing functional groups to the surface of the triboelectric layer to expand triboelectric polarity difference. By virtue of the cooperative effect of enhanced contact area variations from the PDMS microstructure’s gradual deformation, the reduced dielectric thickness as well as the increased charge density from the extended triboelectric polarity difference originated from chemical modification, the self-powered triboelectric sensor displays a sensitivity of 44.31 V kPa -1 in the range of 0.006-2 kPa and 7.16 V kPa -1 in the range of 2-8.5 kPa. Additionally, the self-powered triboelectric sensor is efficient in human motions measurement. This work provides a route to fabricate high-performance pressure sensor with morphological and chemical modification on the triboelectric polymers. • A self-powered triboelectric pressure sensor based on chemically modified micropatterned PDMS was developed. • Amino-containing and fluorine-containing functional groups were introduced to the mPDMS surface to expand triboelectric polarity difference to enhance the triboelectric performance. • The pressure sensor shows a high sensitivity, fast response time and high reliability.

<em>In Vitro</em> Reconstitution of Self-Organizing Protein Patterns on Supported Lipid Bilayers

Many aspects of the fundamental spatiotemporal organization of cells are governed by reaction-diffusion type systems. In vitro reconstitution of such systems allows for detailed studies of their underlying mechanisms which would not be feasible in vivo. Here, we provide a protocol for the in vitro reconstitution of the MinCDE system of Escherichia coli, which positions the cell division septum in the cell middle. The assay is designed to supply only the components necessary for self-organization, namely a membrane, the two proteins MinD and MinE and energy in the form of ATP. We therefore fabricate an open reaction chamber on a coverslip, on which a supported lipid bilayer is formed. The open design of the chamber allows for optimal preparation of the lipid bilayer and controlled manipulation of the bulk content. The two proteins, MinD and MinE, as well as ATP, are then added into the bulk volume above the membrane. Imaging is possible by many optical microscopies, as the design supports confocal, wide-field and TIRF microscopy alike. In a variation of the protocol, the lipid bilayer is formed on a patterned support, on cell-shaped PDMS microstructures, instead of glass. Lowering the bulk solution to the rim of these compartments encloses the reaction in a smaller compartment and provides boundaries that allow mimicking of in vivo oscillatory behavior. Taken together, we describe protocols to reconstitute the MinCDE system both with and without spatial confinement, allowing researchers to precisely control all aspects influencing pattern formation, such as concentration ranges and addition of other factors or proteins, and to systematically increase system complexity in a relatively simple experimental setup.

In Vitro Reconstitution of Self-Organizing Protein Patterns on Supported Lipid Bilayers.

Many aspects of the fundamental spatiotemporal organization of cells are governed by reaction-diffusion type systems. In vitro reconstitution of such systems allows for detailed studies of their underlying mechanisms which would not be feasible in vivo. Here, we provide a protocol for the in vitro reconstitution of the MinCDE system of Escherichia coli, which positions the cell division septum in the cell middle. The assay is designed to supply only the components necessary for self-organization, namely a membrane, the two proteins MinD and MinE and energy in the form of ATP. We therefore fabricate an open reaction chamber on a coverslip, on which a supported lipid bilayer is formed. The open design of the chamber allows for optimal preparation of the lipid bilayer and controlled manipulation of the bulk content. The two proteins, MinD and MinE, as well as ATP, are then added into the bulk volume above the membrane. Imaging is possible by many optical microscopies, as the design supports confocal, wide-field and TIRF microscopy alike. In a variation of the protocol, the lipid bilayer is formed on a patterned support, on cell-shaped PDMS microstructures, instead of glass. Lowering the bulk solution to the rim of these compartments encloses the reaction in a smaller compartment and provides boundaries that allow mimicking of in vivo oscillatory behavior. Taken together, we describe protocols to reconstitute the MinCDE system both with and without spatial confinement, allowing researchers to precisely control all aspects influencing pattern formation, such as concentration ranges and addition of other factors or proteins, and to systematically increase system complexity in a relatively simple experimental setup.

Selective etching of PDMS: Etching as a negative tone resist

In this work authors present for the first time how to apply the additive-free, cured PDMS as a negative tone resist material, demonstrate the creation of PDMS microstructures and test the solvent resistivity of the created microstructures.The PDMS layers were 45 μm and 100 μm thick, the irradiations were done with a focused proton microbeam with various fluences. After irradiation, the samples were etched with sulfuric acid that removed the unirradiated PDMS completely but left those structures intact that received high enough fluences. The etching rate of the unirradiated PDMS was also determined. Those structures that received at least 7.5 × 1015 ion × cm−2 fluence did not show any signs of degradation even after 19 h of etching.As a demonstration, 45 μm and 100 μm tall, high aspect ratio, good quality, undistorted microstructures were created with smooth and vertical sidewalls.The created microstructures were immersed into numerous solvents and some acids to test their compatibility. It was found that the unirradiated PDMS cannot, while the irradiated PDMS microstructures can resist to chloroform, n-hexane, toluene and sulfuric acid. Hydrogen fluoride etches both the unirradiated and the irradiated PDMS.

Axonal growth model in PDMS microstructures for in vitro neuro science applications.

Axonal growth model in PDMS microstructures for in vitro neuro science applications.

Microdevice for continuous flow magnetic separation for bioengineering applications

A novel continuous flow microfluidic device, integrated with soft-magnetic wire (permalloy), is fabricated and tested for magnetophoresis based separation. The flow-invasive permalloy wire, magnetized using an external bias field, is positioned perpendicular to the external magnetic field and with its length traversing the introduced sample flow. The microfluidic device is realized in PDMS; the mold for PDMS microstructures is cut out of Plexiglas® sheets with controllable dimensions. Microfluidic devices with microchannel height ranging between 0.5 mm and 2 mm are fabricated. Experiments are carried out with and without sheath flow; with sheath flow the microparticles are focused at the center of the microchannel.When focusing is not employed, the microdevice can exhibit a complete separation (or filtration) with the introduction of the sample at rates lower than a maximum threshold. However, this complete separation is attributed to the fact that part of the particles, once they approach the repulsive field of the wire, will find their way into the attractive region of the wire while the remaining will be indefinitely trapped at the channel walls. On the other hand, when the focused sample is flowing at the same rate but alongside an appropriate sheath flow, the complete separation can be achieved with all (initially repelled) particles being captured on the attractive region of the wire itself.This microdevice design is well suited for purification, enrichment, and detection of microparticles in lab-on-a-chip devices due to its ability to handle high throughput without compromising capture efficiency while exhibiting excellent reliability and flexibility.

Double casting prototyping with a thermal aging step for fabrication of 3D microstructures in poly(dimethylsiloxane)

The paper describes a cheap and accessible technique of a poly(dimethylsiloxane) (PDMS) master treatment by thermal aging as a step of double casting microfabrication process. Three-dimensional PDMS microstructures could have been achieved using this technique. It was proved, that thermal aging changes nanotopology of a PDMS surface and thus enhances efficiency of double casting prototyping. The thermally aged PDMS master could have been used for multiple and correct replication of over 98% of the fabricated microstructures. Moreover, lack of chemical modification preserved the biocompatibility of PDMS devices. The fabricated microstructures were successfully utilized for 3D cell culture.

Bio-inspired design of hierarchical PDMS microstructures with tunable adhesive superhydrophobicity.

In this paper, bio-inspired PDMS films with different hierarchical microstructures were designed and tunable adhesive super-hydrophobicity was achieved on these films. The adhesive forces between a water droplet and the PDMS film can be adjusted from extremely low (about 8.3 μN) to very high (about 57 μN), and the tunable effect can be ascribed to different wetting states for the water droplets that result from different microstructures on the films. Noticeably, the obtained super-hydrophobic surfaces are acid/alkali-resisting, and water droplets with different pH values have similar contact angles and adhesive forces on the same surface. Finally, the application of the obtained surfaces for microdroplet transportation and self-cleaning are also discussed. The results reported herein provide a new method to obtain super-hydrophobic surfaces with controlled adhesion, and significantly improve our understanding of the relationship between surface adhesion, surface microstructures and the fabrication principle of tunable adhesive super-hydrophobic surfaces.

A Soft PDMS/Metal-Film Photo-Mask for Large-Area Contact Photolithography at Sub-Micrometer Scale With Application on Patterned Sapphire Substrates

This paper reports a new type of soft PDMS/metal-film photo-mask that can be applied in contact photolithography with a resolution at sub-micrometer scale and a patterning area over a 4-in wafer. This new type of photo-mask is made from a soft PDMS mold that contains a patterned metal film on the concave surface of its microstructures. The metal film can selectively block incident UV light, while the convex PDMS microstructures can guide the incident UV light to expose a photo-resist (PR) layer. Due to its soft and compliant property, this new soft photo-mask can form intimate contact with a substrate and carry out UV exposure to form PR microstructures. It is particularly useful in patterning slightly curved substrates such as sapphire wafers, and therefore has a great potential on manufacturing patterned sapphire substrates (PSSs) in light-emitting diodes. In this paper, both 2 and 4 in PSSs with sub-micrometer feature sizes are successfully achieved. This new type of soft photo-mask and its contact photolithography can be easily implemented at a low cost for large-area, nonflat, and sub-micrometer scaled patterning, and therefore has great potential in many applications.

Bioinspired design of a photoresponsive superhydrophobic/oleophilic surface with underwater superoleophobic efficacy

Surfaces with underwater superoleophobic efficacy and superhydrophobic/oleophilic property in air were created with ZnO nanostructures on PDMS microstructures.

Silicon oxide deposition for enhanced optical switching in polydimethylsiloxane-liquid crystal hybrids

We report an optical switch based on a diffraction grating by combining PDMS microstructures with a photo-responsive Nematic Liquid Crystal (NLC). The grating was realized via replica molding and was subsequently coated with a thin SiO layer. SiO induced a full planar alignment of the liquid crystal. The induced parallel alignment of the LC reduces the response time of the structure by approximately an order of magnitude compared to the same structures without SiO. We explored the effect of the pump intensity on the transmission properties and time response of the switch and identified a strong dependence on the probe polarization, due to the full planar alignment in this structure. The aforementioned inclusion of the SiO layer enables enhanced performance of optical devices based on the fusion of nematogens with soft and flexible substrates.

Benzophenone Absorption and Diffusion in Poly(dimethylsiloxane) and Its Role in Graft Photo-polymerization for Surface Modification

Following the great success of traditional microfluidic devices across many disciplines, a new class of microfluidic systems emerged in recent years, which features finely tuned, localized surface modifications within the microstructures in order to keep up with the demand for devices of ever increasing complexity (lab on chip, assay on chip, etc.). Graft photopolymerization has become a powerful tool for such localized surface modifications particularly in combination with poly(dimethylsiloxane) (PDMS) devices, as it is compatible with many functional monomers and allows for high spatial resolution. However, application within enclosed PDMS microstructures and in particular well-controlled surface-directed polymerization remains challenging. Detailed understanding of the interaction between photoinitiator, benzophenone (BP), and polymer matrix is needed. We have developed a visualization technique, which allows for observation of reacted BP in situ within the PDMS matrix. We present a detailed study on solvent-driven BP diffusion providing results essential to successful surface treatment. We also identified and investigated photoinitiator inhibition by oxygen and provide appropriate mitigation strategies.

Facile creation of hierarchical PDMS microstructures with extreme underwater superoleophobicity for anti-oil application in microfluidic channels

Composition modification and surface microstructures have been widely utilized in interface science to improve the surface performance. In this paper, we observed a significant improvement of oil contact angle (CA) from 66 ± 2° to 120 ± 4° by introducing a radical silanol group on a flat PDMS surface through oxygen plasma pretreatment. By combining surface microstructures and plasma modification, we produced three kinds of superoleophobic surfaces: 20 μm pitch micropillar arrays, 2.5 μm pitch micropillar arrays and gecko foot-like hierarchical microstructures. Among them, the hierarchical surface with high surface roughness showed extreme underwater superoleophobicity, which featured ultrahigh CA (175 ± 3°) and ultrasmall sliding angle (<1°). Quantitative measurements demonstrated that these superoleophobic surfaces exhibited distinct adhesive behaviors, by which they were interpreted as Wenzel's, Cassie's and the Lotus state, respectively. A microfluidic channel with superoleophobic microstructures was further created by novel curve-assisted imprint lithography, and the characterization based on anti-oil contamination applications was carried out and discussed. We believe that the superoleophobic surfaces will power broad applications in oil microdroplet transportation, anti-oil channels and droplet microfluidic systems.

Capillary force assembly of giant vesicles on a microstructured substrate

Convective capillary force assembly has shown to efficiently assemble nm to μm solid colloids onto a surface. We show here that this technique can also efficiently trap liposomes with 10μm mean size or films of lipid into microstructures. We hypothesize that whether liposomes or lipids are trapped in the template depends on the interfacial interaction of the objects with the surface. After solvent evaporation, ruptured and deflated liposomes are observed onto oxidized (hydrophilic) PDMS while uniform films of lipids are found in untreated (hydrophobic) PDMS microstructures. This assumption is supported by the analysis of the evaporation of a droplet containing a dispersion of liposomes in free environment.

Benchtop fabrication of PDMS microstructures by an unconventional photolithographic method

Poly(dimethylsiloxane) (PDMS) microstructures have been widely used in bio-microelectromechanical systems (bio-MEMS) for various types of analytical, diagnostic and therapeutic applications. However, PDMS-based soft lithographic techniques still use conventional microfabrication processes to generate a master mold, which requires access to clean room facilities and costly equipment. With the increasing use of these systems in various fields, the development of benchtop systems for fabricating microdevices is emerging as an important challenge in their widespread use. Here we demonstrate a simple, low-cost and rapid method to fabricate PDMS microstructures by using micropatterned poly(ethylene glycol) diacrylate (PEGDA) master molds. In this method, PEGDA microstructures were patterned on a glass substrate by photolithography under ambient conditions and by using simple tools. The resulting PEGDA structures were subsequently used to generate PDMS microstructures by standard molding in a reproducible and repeatable manner. The thickness of the PEGDA microstructures was controllable from 15 to 300 µm by using commonly available spacer materials. We also demonstrate the use of this method to fabricate microfluidic channels capable of generating concentration gradients. In addition, we fabricated PEGDA microstructures by photolithography from the light generated from commonly available laminar cell culture hood. These data suggest that this approach could be beneficial for fabricating low-cost PDMS-based microdevices in resource limited settings.

Light induced patterning of poly(dimethylsiloxane) microstructures

A new method for direct patterning of Poly(dimethylsiloxane) (PDMS) microstructures is developed by taking advantage of photorefractive effect in a functionalized substrate. Here we show that when a x-cut Iron doped Lithium Niobate (LN) crystal is exposed to appropriate structured laser light, a charge density pattern builds-up in the crystal and a space charge field arise that is able to induce self-patterning of the PDMS liquid film deposited on its surface via the dielectrophoretic effects. Proper heating treatment allows to achieve polymeric linking process creating a solid and stable PDMS microstructures. The self-patterned structures replicate the illuminating light pattern. We show that 1D and 2D patterning of PDMS gratings can be achieved. This new soft-lithographic approach can pave the way for realizing PDMS micro-structures with high degree of flexibility that avoids the need of moulds fabrication.

Fabrication of three-dimensional microarray structures by controlling the thickness and elasticity of poly(dimethylsiloxane) membrane

In this paper, we propose a method to construct three-dimensional curved microstructures with easy control of the size, position and shape, by exploiting the elasticity of poly(dimethylsiloxane) (PDMS) membranes and basic physics. For this end, we developed the method to handle thin PDMS membrane safely, and to replicate PDMS microstructure from the PDMS mold. Using this method, we demonstrated two potential applications: (1) the use of concave well for the formation of embryoid body (EB) to differentiate into neuronal cells, and (2) the fabrication of SU-8 and hydrogel microparticles having diverse curved shapes. The curved structures were successfully fabricated with simple process, and EBs were formed in the concave well and differentiated into the neuronal cells. Microparticles with diverse shapes were fabricated from a range of materials for potential use as drug carrier and pH responsive micro-actuator elements.

Controlled double emulsification utilizing 3D PDMS microchannels

This paper presents a PDMS emulsification device that is capable of generating water-in-oil-in-water double emulsions in a controlled manner. Specially designed 3D microchannels are utilized to steer the independently driven water- and oil-phase flows (especially to restrict the attachment of the middle oil-phase flow on the channel surfaces), and to break the continuous flows into monodisperse double emulsions. In addition to channel geometries and fluid flow rates, surfactants and osmotic agents are employed to facilitate the breakup process and stabilize the resulting emulsion structures. In the prototype demonstration, two-level SU-8 molds were fabricated to duplicate PDMS microstructures, which were surface treated and bonded irreversibly to form 3D microchannels. Throughout the emulsification trials, dripping was intentionally induced to generate monodisperse double emulsions with single or multiple aqueous droplets inside each oil drop. It is found that the overall and core sizes of the resulting double emulsions could be adjusted independently, mainly by varying the outer and inner fluid flow rates, respectively. As such, the presented double emulsification device could potentially realize the controllability on emulsion structure and size distribution, which is desired for a variety of biological and pharmaceutical applications.

Influence of Chemistry and Topology Effects on Superhydrophobic CF4-Plasma-Treated Poly(dimethylsiloxane) (PDMS)

Superhydrophobic surfaces are gaining considerable interest in a lot of different applications, and nonetheless, precise control over the wettability properties of such surfaces is still a challenge due to difficulties when controlling the effects independently induced on superhydrophobicity by the chemical and topological surface characteristics. We have fabricated engineered superhydrophobic surfaces onto poly(dimethylsiloxane) (PDMS) substrates by means of suitable CF4-plasma treatments. These treatments allowed the modification of both the morphological properties of the PDMS surface, due to a preferential etching of certain components of its macromolecules, and the chemical ones, by the deposition of a fluorinated layer. Chemical effects were separated from topological ones by performing a double replica molding process of the CF4-plasma-treated surfaces. This allowed us to obtain positive copies of the structured surfaces without the overlaying fluorinated coating affecting the surface chemistry. Such replicated surfaces showed a decrease of the contact angle if compared to the treated ones and therefore evidenced chemistry's weight in superhydrophobicity effects. In particular, we found that, for highly dense columnar-like PDMS microstructures, the effect of the plasma-deposited fluorinated layer covering surfaces produces an enhancement of the contact angle of about 20 degrees .

A novel method of microfabrication and manipulation of bacterial teamsters in low Reynolds number fluidic environments

The flagellated bacteria Serratia marcescens have been employed as fluidic actuators to propel custom designed microstructures through the use of a swarm blotting technique. The novel methodology for microfabrication, manipulation, and experimentation is described in detail, and the advantages and drawbacks of alternative techniques are considered. Our results with PDMS and silicon microstructures led to the discovery of SU-8 as a suitable material. A microstructure-tracking algorithm was developed to quantify the motion. The methodology is applied in a study of effects of microstructure geometry on velocity and trajectory in an open fluidic channel. Additionally, relationships between structure dimension and velocity are discussed.