Polydimethylsiloxane Microstructures Research Articles

Combining stereolithography and replica molding: On the way to superhydrophobic polymeric devices for photovoltaics

A strategy combining stereolithography (SL) and soft-lithography for the straightforward fabrication of superhydrophobic bulk devices is reported. Microtextured masters are rapidly prototyped by SL and passivated with a perfluorosilane. Such surface treatment enables the faultless fabrication of negative microstructured polydimethylsiloxane molds ultimately utilized to obtain bulk polymeric micropatterned structures by replica molding. As illustrative proof of concept, this approach is employed in the field of photovoltaics to realize the first example of superhydrophobic luminescent solar concentrators (LSCs) showing superior self-cleaning properties. Following our strategy, a new dye-doped acrylate mixture is developed and optimized to ensure complete wetting of the hollow microstructures present on the mold. By judiciously tailoring the photoinitiator concentration and by implementing a tailored double-step UV-irradiation process, complete UV-photopolymerization is achieved despite the significant thickness of the target samples. The high fidelity replication of the original SL-printed features on the daughter replicas as well as their super water-repellency are successfully demonstrated. The performance of the resulting superhydrophobic LSCs is investigated at varying device dimensions and found to be comparable with state-of-the-art systems. This study demonstrates the potential of high-resolution SL-printing in combination with replication techniques as a versatile tool to reproducibly fabricate microstructured superhydrophobic polymeric bulk devices in a straightforward fashion.

High Sensitivity Flexible Capacitive Pressure Sensor Using Polydimethylsiloxane Elastomer Dielectric Layer Micro-Structured by 3-D Printed Mold

3-D printing is used to fabricate mold for micro-structuring the polydimethylsiloxane (PDMS) elastomeric dielectric layer in capacitive flexible pressure sensors. It is shown that, despite of the limited resolution of the used commercial 3-D printer for producing the mold, the fabricated sensor with the micro-structured PDMS layers can achieve sensitivity higher than previous work using micro-fabricated silicon wafer molds. The devices also present very low detection limit, fast response/recovery speed, excellent durability and good tolerance to variations of ambient temperature and humidity, which enables to reliably monitor weak human physiological signals in real time. As an application example, the flexible pressure sensor is integrated in a wearable system to monitor wrist pulse.

A highly sensitive, flexible SERS sensor for malachite green detection based on Ag decorated microstructured PDMS substrate fabricated from Taro leaf as template

A bio-inspired flexible surface enhanced Raman spectroscopy (SERS) sensor for malachite green (MG) detection has been fabricated by coating Ag on a structured surface of polydimethylsiloxane (PDMS) substrate using textured nano/microstructured Taro leaf as template. The Ag-coated microcavity structured PDMS exhibits high adhesion, hydrophobic behavior (water contact angle ∼128±1°) and serves as a concentrator for the analyte/probe molecule responsible for enhanced SERS intensity. The SERS activity of the fabricated Ag/PDMS flexible sensor having MG as probe molecules shows an enhancement factor of ∼2.06×105 over bulk MG molecules. The fabricated SERS sensor is showed the high sensitivity and good signal reproducibility. Sensitivity for very low MG concentration (∼10−11M) is reported which is attributed to the presence of high local field produced by Ag granular film and nanovoids onto PDMS surface. AFM and Raman characterizations have revealed the formation of nanovoids on Ag/PDMS structures to create plasmonic hotspots that results in enhanced SERS activity and high sensitivity. Further, the SERS signal under bending strain (tested between 180° and 100° bending), is also detectable for very low concentration of MG (as low as 10−7M) even at substrate bending angles of 100° that establishes the proposed SERS sensor system as an exceptional candidate for flexible MG SERS sensor applications.

Gecko-inspired adhesion controlled by light

A team of German researchers have created an adhesive pad inspired by the sticky feet of geckos that unsticks from objects when triggered by ultraviolet light. The pad could lead to robotic fingers controlled by light that can pick up and release delicate objects. Geckos can scurry up and down walls thanks to about half a million fibers called seta on each of their feet. Each setae is divided into hundreds of spatula-like nanostructures that individually grip a surface through weak van der Waals interactions. Collectively, these millions of interactions lead to strong adhesion. In the new work, Emre Kizilkan of Kiel University and his colleagues used previously developed surfaces with mushroom-shaped polydimethylsiloxane microstructures to produce geckolike adhesion. But for easy manipulation, they desired a system that could attach and detach its adhesive hold at will. “We wanted to control the adhesion by light,” Kizilkan says. To do so, the

Unveiling the wet chemical etching characteristics of polydimethylsiloxane film for soft micromachining applications

Micromachining of a polydimethylsiloxane (PDMS) microstructure by wet chemical etching is explored for microelectromechanical systems (MEMS) and microfluidic applications. A 100 µm thick PDMS film was patterned with different microstructure designs by wet chemical etching using a N-methyl-2-pyrrolidone (C16H36FN) and tetra-n-butylammonium fluoride (C5H9NO) mixture solution with 3:1 volume ratio after lithography for studying etching characteristics. The patterning parameters, such as etch rate, surface roughness, pH of etchant solution with time, were thoroughly investigated. A detailed study of surface morphology with etching time revealed nonlinear behaviour of the PDMS surface roughness and etch rate. A maximum rate of 1.45 µm min−1 for 10 min etching with surface roughness of 360 nm was achieved. A new approach of wet chemical etching with pH controlled doped etchant was introduced for lower surface roughness of etched microstructures, and a constant etch rate during etching. Variation of the etching rate and surface roughness by pH controlled etching was performed by doping 5–15 gm l−1 of silicic acid (SiO2xH2O) into the traditional etchant solution. PDMS etching by silicic acid doped etchant solution showed a reduction in surface roughness from 400 nm to 220 nm for the same 15 µm etching. This study is beneficial for micromachining of various MEMS and microfluidic structures such as micropillars, microchannels, and other PDMS microstructures.

Micro-structured PDMS piezoelectric enhancement through charging conditions

Micro-structured cellular polydimethylsiloxane (PDMS) materials were prepared by a low-cost molding process allowing us to control geometry and sample size. Cellular structures are charged with a triangular quasi-static voltage with amplitudes between 1 kV and 4 kV and a frequency of 0.5 Hz fixed after having evaluated the conditions enhancing the piezoelectric response of the cellular PDMS. The piezo-electret PDMS material charged at room temperature has a piezoelectric coefficient d33 of 350 pC/N, which is ten times larger than that of polyvinylidene fluoride. The high piezoelectric coefficient with a very low elastic modulus of 300 kPa makes these materials very useful for wearable device applications. The piezoelectric coefficient d33 of the samples poled at high temperatures improves thermal stability but reduces PDMS piezo-electret piezoelectricity, which is explained by the structure’s stiffness. These results are useful and allow us to set the conditions for the preparation of the piezo-electret materials according to desired applications.

Fabrication and integration of microscale permanent magnets for particle separation in microfluidics

Microfluidic magnetophoresis is an effective technique to separate magnetically labeled bioconjugates in lab-on-a-chip applications. However, it is challenging and expensive to fabricate and integrate microscale permanent magnets into microfluidic devices with conventional methods that use thin-film deposition and lithography. Here, we propose and demonstrate a simple and low-cost technique to fabricate microscale permanent magnetic microstructures and integrate them into microfluidic devices. In this method, microstructure channels were fabricated next to a microfluidic channel and were injected with a liquid mixture of neodymium (NdFeB) powders and polydimethylsiloxane (PDMS). After the mixture was cured, the resulted solid NdFeB–PDMS microstructure was permanently magnetized to form microscale magnets. The microscale magnets generate strong magnetic forces capable of separating magnetic particles in microfluidic channels. Systematic experiments and numerical simulations were conducted to study the geometric effects of the microscale magnets. It was found that rectangular microscale magnets generate larger \(({\mathbf {H}}\cdot \nabla ) {\mathbf {H}}\) which is proportional to magnetic force and have a wider range of influence than the semicircle or triangle magnets. For multiple connected rectangular microscale magnet, additional geometric parameters, including separation distance, height and width of the individual elements, further influence the particle separation and were characterized experimentally. With an optimal size combination, complete separation of yeast cells and magnetic microparticles of similar sizes (\(4\;\upmu \hbox {m}\)) was demonstrated with the multi-rectangular magnet microfluidic device.

A Two-Dimensional (2D) Distributed-Deflection Sensor for Tissue Palpation With Correction Mechanism for Its Performance Variation

This paper presents a proof-of-concept study on a two-dimensional distributed-deflection sensor for tissue palpation and the associated correction mechanism for its performance variation. The core of the sensor is one whole polydimethylsiloxane microstructure embedded with a $3\times 3$ sensing-plate/transducer array. Upon pressing the sensor against a tissue, the sensing-plate array translates its elasticity distribution to the deflection distribution, which registers as resistance changes by the transducer array and is further converted into the stiffness distribution. The connection of the sensing-plate/transducer array into one piece allows the sensor interacting with a tissue in a continuous manner and, thus, unifies misalignment errors from non-ideal normal contact between the sensor and the tissue. A correction mechanism is developed to remove the effect of performance variation among the sensing-plate/transducer array on the measured stiffness distribution of a tissue. The stiffness distribution normalized to the minimum stiffness across a tissue is utilized to identify the existence and location of a tumor. The related measurement errors are both theoretically and experimentally examined.

Microfluidic separation of magnetic particles with soft magnetic microstructures

This paper demonstrates simple and cost-effective microfluidic devices for enhanced separation of magnetic particles by using soft magnetic microstructures. By injecting a mixture of iron powder and polydimethylsiloxane (PDMS) into a prefabricated channel, an iron–PDMS microstructure was fabricated next to a microfluidic channel. Placed between two external permanent magnets, the magnetized iron–PDMS microstructure induces localized and strong forces on the magnetic particles in the direction perpendicular to the fluid flow. Due to the small distance between the microstructure and the fluid channel, the localized large magnetic field gradients result a vertical force on the magnetic particles, leading to enhanced separation of the particles. Numerical simulations were developed to compute the particle trajectories and agreed well with experimental data. Systematic experiments and numerical simulation were conducted to study the effect of relevant factors on the transport of superparamagnetic particles, including the shape of iron–PDMS microstructure, mass ratio of iron–PDMS composite, width of the microfluidic channel, and average flow velocity.

Template assisted surface microstructuring of flowable dental composites and its effect on microbial adhesion properties

ObjectivesDespite their various advantages, such as good esthetic properties, absence of mercury and adhesive bonding to teeth, modern dental composites still have some drawbacks, e.g., a relatively high rate of secondary caries on teeth filled with composite materials. Recent research suggests that microstructured biomaterials surfaces may reduce microbial adhesion to materials due to unfavorable physical material–microbe interactions. The objectives of this study were, therefore, to test the hypotheses that (i) different surface microstructures can be created on composites by a novel straightforward approach potentially suitable for clinical application and (ii) that these surface structures have a statistically significant effect on microbial adhesion properties. MethodsSix different dental composites were initially tested for their suitability for microstructuring by polydimethylsiloxane (PDMS) templates. Each composite was light-cured between a glass slide and a microstructured PDMS template. The nano-hybrid composite Grandio Flow was the only tested composite with satisfying structurability, and was therefore used for the bacterial adhesion tests. Composites samples were structured with four different microstructures (flat, cubes, linear trapezoid structures, flat pyramids) and incubated for 4h in centrifuged saliva. The bacterial adherence was then characterized by colony forming units (CFUs) and scanning electron microscopy (SEM). ResultsAll four microstructures were successfully transferred from the PDMS templates to the composite Grandio Flow. The CFU-test as well as the quantitative analysis of the SEM images showed the lowest bacterial adhesion on the flat composite samples. The highest bacterial adhesion was observed on the composite samples with linear trapezoid structures, followed by flat pyramids and cubes. The microstructure of dental composite surfaces statistically significantly influenced the adhesion of oral bacteria. SignificanceModifying the composite surface structure may be a clinically suitable approach to control the microbial adhesion and thus, to reduce the risk of secondary caries at dental composite restorations. Smaller composite surface structures may be useful for accomplishing this.

Generation of Dynamic Free-Form Temperature Gradients in a Disposable Microchip.

Temperature gradients (TGs) provide an effective approach to controlling solvated molecules and creating spatiotemporally varying thermal stimuli for biochemical research. Methods developed to date for generating TGs can only create a limited set of static temperature profiles. This article describes a method for establishing dynamic free-form TGs in polydimethylsiloxane (PDMS) as well as in gases and liquids in contact with the PDMS. The heating mechanism relies on the efficient acoustic absorption by the PDMS of high-frequency (5-200 MHz) surface acoustic waves (SAWs). MATLAB-aided actuation of a transducer enabled the generation and propagation of SAWs in a controlled fashion, which permitted spatiotemporal control over the temperature in the PDMS microstructures. This technique is exploited to perform one-shot high-resolution melting (HRM) analysis to detect single nucleotide polymorphisms (SNPs) in DNA. The experimental results displayed a 10-fold higher resolution and an enhanced signal-to-noise ratio compared to the results obtained using a conventional real-time PCR machine.

Piezoelectric cellular micro-structured PDMS material for micro-sensors and energy harvesting

This paper reports a novel low-cost fabrication process of a charged cellular microstructured polydimethylsiloxane (PDMS) material referred as piezo-electret or ferro-electret for micro-sensors applications. The dielectric spectra reached on these structures exhibit a high piezoelectric longitudinal coefficient d33 of 350pC/N. A mechanical characterization method proves the reliability of this material for low-frequencies applications around 100Hz.

Shaping living tissues using microfabricated structures

In this paper, we present a technique for shaping the morphology of living Multi Cellular Tumour Spheroids (MCTS) by using micro-engineered structures. To that aim we created biocompatible, high aspect ratio, polydimethylsiloxane (PDMS) microstructures, that are compatible with the size of MCTS. We show that these microstructures can conform MCTS into pre-determined shapes by confining them, under cell culture conditions. This moulding of a living growing tissue by PDMS microstructures needs optimization of surface conditions, in order to favour the wetting of the mould by the cells, while preventing their attachment to the confining structures. An adequate treatment of PDMS with non-adhesive coating turned out to be mandatory for combining a successful shaping and a soft unmoulding of the engineered tissue. After unmoulding, the tissues are still viable and can be further cultured. Our work demonstrates that very sharp corners can be moulded on originally spherical aggregates of cells and that freestanding cell architectures of arbitrary shapes can be generated. This bio-moulding process can therefore be used for the investigation of the rheological properties of tissues as well as morphogenesis mechanisms by measuring the post-moulding evolution of the engineered shape.

Investigation of contact electrification based broadband energy harvesting mechanism using elastic PDMS microstructures

Triboelectric energy harvesting has recently garnered a lot of interest because of its easy fabrication and high power output. Contact electrification depends on the chemical properties of contacting materials. Another important factor in contact electrification mechanism is surfaces’ elastic and topographical characteristics. One of the biggest limitations of resonant mechanism based devices is their narrow operating bandwidth. This paper presents a broadband mechanism which utilizes stiffness induced in the cantilever motion due to contact between two triboelectric surfaces. We have conducted experiments using polydimethylsiloxane (PDMS) micropad patterns to study the effect of micropad array configuration on the performance of triboelectric energy harvesting devices. The maximum power output measured from the device was observed to be 0.69 μW at an acceleration of 1 g. Due to the non-linearity introduced by contact separation mechanism, the bandwidth of the triboelectric energy harvester was observed to be increased by 63% at an acceleration level of 1 g. A hybrid energy harvesting mechanism has also been demonstrated by compounding the triboelectric energy harvester with a piezoelectric bimorph.

Fabrication of three-dimensional PDMS microstructures by selective bonding and cohesive mechanical failure

This paper presents a novel patterning method of polydimethylsiloxane (PDMS) microstructures on solid surfaces by selective bonding and cohesive mechanical failure of PDMS. Generally, when the gold film is laid between PDMS and a Si substrate, Au film with PDMS structure is easily peeled off from a Si substrate because of weak adhesion between Au and the Si substrate. However, when the Au is patterned on the Si substrate locally, PDMS bonds selectively to the Si substrate where Au patterns are not covered. The selectivity of bonding is promoted by treatment with (3-mercaptopropyl)trimethoxysilane on Au patterns and Si substrates. Then, three-dimensional PDMS microstructures are fabricated by a cohesive mechanical failure of PDMS during a dry peel-off process due to enhanced adhesion of PDMS and the Si substrate.

Surface Tension Flows inside Surfactant-Added Poly(dimethylsiloxane) Microstructures with Velocity-Dependent Contact Angles

Filling of liquid samples is realized in a microfluidic device with applications including analytical systems, biomedical devices, and systems for fundamental research. The filling of a disk-shaped polydimethylsiloxane (PDMS) microchamber by liquid is analyzed with reference to microstructures with inlets and outlets. The microstructures are fabricated using a PDMS molding process with an SU-8 mold. During the filling, the motion of the gas-liquid interface is determined by the competition among inertia, adhesion, and surface tension. A single ramp model with velocity-dependent contact angles is implemented for the accurate calculation of surface tension forces in a three-dimensional volume-of-fluid based model. The effects of the parameters of this functional form are investigated. The influences of non-dimensional parameters, such as the Reynolds number and the Weber number, both determined by the inlet velocity, on the flow characteristics are also examined. An oxygen-plasma-treated PDMS substrate is utilized, and the microstructure is modified to be hydrophilic. Flow experiments are conducted into both hydrophilic and hydrophobic PDMS microstructures. Under a hydrophobic wall condition, numerical simulations with imposed boundary conditions of static and dynamic contact angles can successfully predict the moving of the meniscus compared with experimental measurements. However, for a hydrophilic wall, accurate agreement between numerical and experimental results is obvious as the dynamic contact angles were implemented.

Flexible-textured polydimethylsiloxane antireflection structure for enhancing omnidirectional photovoltaic performance of Cu(In,Ga)Se_2 solar cells

Because of the Sun's movement across the sky, broadband and omnidirectional light harvesting is a major development in photovoltaic technology. This study reports the fabrication and characterization of flexible-textured polydimethylsiloxane (PDMS) film on Cu(In,Ga)Se2 (CIGS) solar cells, which is one of the simplest and cheapest peel-off processes for fabricating a three-dimensional structure. A cell containing a textured PDMS film enhanced the short-circuit current density from 22.12 to 23.93 mA/cm2 in a simulated one-sun scenario. The omnidirectional antireflection of CIGS solar cells containing various PDMS films is also investigated. This study uses an angle-resolved reflectance spectroscope to investigate the omnidirectional and broadband optical properties of the proposed PDMS film. This improvement in light harvesting is attributable to the scattering of the PDMS film and the gradual refractive index profile between the PDMS microstructures and air. The flexible-textured PDMS film is suitable for creating an antireflective coating for a diverse range of photovoltaic devices.

Fabrication of Freestanding 1-D PDMS Microstructures Using Capillary Micromolding

A method to create freestanding, biocompatible polydimethylsiloxane (PDMS) microstructures is presented. First, capillary flow through micro channels on silicon (Si) substrates is used to create high fidelity PDMS structures that are a few μm wide and deep but several mm long (length to width/depth ≈ 500:1). Next, an improvised procedure is employed to remove the cured PDMS microstructures from the Si substrate without damaging them. The method is used to create extremely sensitive cantilever beams with stiffness less than 0.1 pN/μm, and micro platforms for cell biology studies. The PDMS microstructures created using this method have applications in cell mechanobiology as force and mass sensors.

Migration speed and directionality switch of normal epithelial cells after TGF-β1-induced EMT (tEMT) on micro-structured polydimethylsiloxane (PDMS) substrates with variations in stiffness and topographic patterning

The epithelial to mesenchymal transition (EMT) involves several physiological and pathological phenomena and endows cells with invasive and migratory properties. However, the effects of substrate stiffness and topography on the migration of cells before or after transforming growth factor-β1 (TGF-β1)-induced EMT (tEMT) are unknown. Herein, we seed control or tEMT NMuMG cells on the 2D patterns consisted of 1 μm or 5 μm line-widths and groove or cone patterns on either 2 MPa (1.96 ± 0.48 MPa) or 4 MPa (3.70 ± 0.74 MPa) polydimethylsiloxane (PDMS) substrates. After tEMT, the increased expression of α-SMA with vinculin in focal adhesion (FA) sites led to an acceleration of tEMT cell motility. On the 2 MPa substrate, the most influenced substrate was the 1 μm, cone-patterned substrate, where the tEMT cells' motility decelerated by 0.13 μm/min (36% slower than the cells on groove pattern). However, on the 5 μm, groove-patterned substrate, where the tEMT cells demonstrated the most rapid motility relative to the control cells, with an increment of 0.18 μm/min (100%). Among the different physical cues from substrate, the cone pattern could impede the migration speed of tEMT cells. Furthermore, we recommend the groove-patterned with a 5 μm line-width substrate as a useful tool to differentiate control and tEMT cells by migration speed.

Thermomechanical noise measurement of polydimethylsiloxane microcantilevers fabricated by multilayer soft lithography

Polydimethylsiloxane microstructures such as cantilevers are useful for nanomechanical measurements of biomolecules, living cells, and soft matter. As a fundamental characterization, we report thermomechanical noise measurements of the flexural modes of polydimethylsiloxane cantilevers, which were fabricated by multilayer soft lithography. From the resonance frequency data, we extracted the values of Young's moduli for different cross-linking ratios. Our devices have measured resonance frequencies and quality factors ranging from 405 Hz to 1.63 kHz and from 5 to 10, respectively. Our analysis shows that these cantilevers will be able to perform nanomechanical measurements with piconewton force resolution.