Transparent Polydimethylsiloxane Research Articles

Fabrication of Subretinal 3D Microelectrodes with Hexagonal Arrangement.

This study presents the fabrication of three-dimensional (3D) microelectrodes for subretinal stimulation, to accommodate adjacent return electrodes surrounding a stimulating electrode. For retinal prosthetic devices, the arrangement of return electrodes, the electrode size and spacing should be considered together, to reduce the undesired dissipation of electric currents. Here, we applied the hexagonal arrangement to the microelectrode array for the localized activation of retinal cells and better visual acuity. To provide stimuli more efficiently to non-spiking neurons, a 3D structure was created through a customized pressing process, utilizing the elastic property of the materials used in the fabrication processes. The diameter and pitch of the Pt-coated electrodes were 150 μm and 350 μm, respectively, and the height of the protruded electrodes was around 20 μm. The array consisted of 98 hexagonally arranged electrodes, supported by a flexible and transparent polydimethylsiloxane (PDMS) base, with a thickness of 140 μm. Also, the array was coated with 2 μm-thick parylene-C, except the active electrode sites, for more focused stimulation. Finally, the electrochemical properties of the fabricated microelectrodes were characterized, resulting in the mean impedance of 384.87 kΩ at 1 kHz and the charge storage capacity (CSC) of 2.83 mC·cm−2. The fabricated microelectrodes are to be combined with an integrated circuit (IC) for additional in vitro and in vivo experiments.

Highly stretchable, transparent and room-temperature self-healable polydimethylsiloxane elastomer for bending sensor

Highly stretchable and self-healable elastomers are attractive for a variety of applications in the fields of electrical skin and wearable devices. Herein, we proposed a simple one-pot two-step approach to synthesize room-temperature self-healable polydimethylsiloxane (PDMS) elastomers. Excess aminopropyl terminated polydimethylsiloxane was firstly reacted with isophorone diisocyanate to synthesize amino-terminated PDMS with incorporated ureido groups, followed by further reaction with terephthalaldehyde as chain extender to yield self-healing PDMS elastomers. The obtained elastomer exhibited high stretchability of 1670% and transmittance of 92%. Owing to the dynamic intermolecular hydrogen bonds, reversible imine bonds and highly flexible SiO chains, the elastomer showed excellent self-healing capability with a healing efficiency of 95% after healing at room temperature for 24 h. Even in water and artificial sweat, the healing efficiencies also reached 89% and 78%, respectively. In addition, the elastomer supported triple-layer bending sensor was fabricated with a sandwiched hydroxylated multiwalled carbon nanotubes (MWCNTs-OH) film and successfully applied for detecting human motions. Interestingly, the cut sensor was able to be recovered for working after being irradiated under sunlight for only 10 min. Our method to synthesize highly stretchable, transparent and self-healing elastomers is simple and the reaction can be carried out at room temperature, which is beneficial for the large-scale production and the further practical application in functional electronics.

Silicone-epoxy block hybrid network to achieve high-performance and transparent polydimethylsiloxane materials

The preparation of highly compatible polydimethylsiloxane/epoxy system has been a challenging task. Here, we designed and synthesized a vinyl-containing bifunctional epoxy resin, which was chemically bonded with polydimethylsiloxane by hydrosilylation. A silicone-epoxy block hybrid network with restricted micromorphology and outstanding macro-performance was formed. The phase separation scale of polydimethylsiloxane/epoxy system was strictly restricted by molecular structure design and the modified PDMS was transparent. The mechanical test results showed that the tensile strength, elongation at break and shear strength of the hybrid material were increased by 391%, 78% and 292% respectively. More importantly, the initial thermal decomposition temperature of the hybrid material increased, and its thermal residue at 800 °C was kept at a high level. All these results indicate that we have prepared a transparent, high-performance polydimethylsiloxane material, which is expected to be applied in many prospects such as heat resistant adhesive and transparent surface coating.

Photo‐Curable Ion‐Enhanced Fluorinated Elastomers for Pressure‐Sensitive Textiles

Smart textiles integrated with the function of recording tactile stimulation attract much attention due to its great potential for health monitoring and wearable electronics. However, the highly uneven surface and poor thermal stability of textiles become main challenges for precision measurement and device fabrications. To solve these issues, an ion‐assisted pressure‐sensitive textile using a novel photo‐curing process is demonstrated in this work. Using a photo‐cured fluorinated polymer poly(vinylidene fluoride‐co‐hexafluoropropene) (PVDF‐HFP) and a transparent polydimethylsiloxane (PDMS) mold, a highly sensitive pyramid array of PVDF‐HFP is successfully fabricated on a textile. Furthermore, after doping the ionic additive, tetrabutylammonium perchlorate (TBAP), the sensors show improved sensitivity due to the enhanced dielectric constant. The dielectric constant has a significant increase in the value over 37 after doping a small amount of TBAP (2 wt%). Due to the ion doping, the sensitivity of the textile pressure sensors show significant improvement (0.73 kPa−1) higher than that of the PDMS‐based one (0.19 kPa−1). This work not only opens an efficient approach for the fabrication processes of a high‐k elastomer pyramid array on a textile but also provide a design strategy to improve performance of capacitive pressure sensor for smart textiles.

Durable Perovskite UV Sensor Based on Engineered Size-Tunable Polydimethylsiloxane Microparticles Using a Facile Capillary Microfluidic Device from a High-Viscosity Precursor.

In this work, size-tunable polydimethylsiloxane (PDMS) microparticles are fabricated from a high-viscosity oil phase using a facile coflowing capillary microfluidic device and optimized aqueous phase composition. The dispersity of the microparticle size is tuned by engineering of the viscosity of the continuous phase and flow rate ratio that leads us to achieve monodisperse microparticles. Regarding the high potential of the PDMS microparticles for optical applications, efficient environmentally durable perovskite-based UV sensors are fabricated employing the designed size-tunable microparticles. Surprisingly, the UV sensors comprising CH3NH3PbBr3 perovskite quantum dots as UV-sensitive nanocrystals embedded in transparent PDMS microparticles are water resistant because of the high hydrophobicity of PDMS. Remarkably, the UV sensors show a photoluminescence quantum yield as high as 75% that can be employed effortlessly as reusable leak detectors in different fluidic working systems.

Modified inverted selective plane illumination microscopy for sub-micrometer imaging resolution in polydimethylsiloxane soft lithography devices.

Moldable, transparent polydimethylsiloxane (PDMS) elastomer microdevices enable a broad range of complex studies of three-dimensional cellular networks in their microenvironment in vitro. However, the uneven distribution of refractive index change, external to PDMS devices and internally in the sample chamber, creates a significant optical path difference (OPD) that distorts the light sheet beam and so restricts diffraction limited performance. We experimentally showed that an OPD of 120 μm results in the broadening of the lateral point spread function by over 4-fold. In this paper, we demonstrate steps to adapt a commercial inverted selective plane illumination microscope (iSPIM) and remove the OPD so as to achieve sub-micrometer imaging ranging from 0.6 ± 0.04 μm to 0.91 ± 0.03 μm of a fluorescence biological sample suspended in regular saline (RI ≈1.34) enclosed in 1.2 to 2 mm thick micromolded PDMS microdevices. We have proven that the removal of the OPD from the external PDMS layer by refractive index (RI) matching with a readily accessible, inexpensive sucrose solution is critical to achieve a >3-fold imaging resolution improvement. To monitor the RI matching process, a single-mode fiber (SMF) illuminator was integrated into the iSPIM. To remove the OPD inside the PDMS channel, we used an electrically tunable lens (ETL) that par-focuses the light sheet beam with the detection objective lens and so minimised axial distortions to attain sub-micrometer imaging resolution. We termed this new light sheet imaging protocol as modified inverted selective plane illumination microscopy (m-iSPIM). Using the high spatial-temporal 3D imaging of m-iSPIM, we experimentally captured single platelet (≈2 μm) recruitment to a platelet aggregate (22.5 μm × 22.5 μm × 6 μm) under flow at a 150 μm depth within a microfluidic channel. m-iSPIM paves the way for the application of light sheet imaging to a wide range of 3D biological models in microfluidic devices which recapitulate features of the physiological microenvironment and elucidate subcellular responses.

Transparent Polymer Coatings for Energy-Efficient Daytime Windows Cooling

Sunlight management of transparent windows is important for buildings or vehicle cooling. Functional layers that block near-infrared solar radiation also absorb sunlight and aggravate the interior heat loads. Here, we introduce a daytime radiative cooling strategy that promotes window cooling without affecting the aesthetics or functionalities. This strategy relies on selecting suitable bond vibrations using the typical transparent silicone polydimethylsiloxane (PDMS). The PDMS coating with solar absorption of 0.004 and infrared emissivity of 0.93 across the atmospheric window, achieves a record theoretical cooling power of 103 W/m2. Outdoor experiments show the temperature drop can reach 8.7 °C for a PDMS-coated near-infrared reflective glass. Model evaluation of the annual cooling energy consumption in 12 cities worldwide shows that the PDMS-coated window can save energy between 0.62 and 30.52 kWh/m2, which is most prominent in warm cities. This window cooling strategy provide a different path to simultaneously optimize the daylighting and thermal management of buildings.

Heating of a water droplet on inclined transparent polydimethylsiloxane (PDMS) surface

Heat transfer from replicated hydrophobic and transparent PDMS surface to a water droplet is considered. The laser texturing of alumina surface is carried out to obtain hydrophobic surface and later the textured surface is replicated by polydimethylsiloxane (PDMS) via a solvent casting method. An experiment is carried out to assess the water droplet pinning on the replicated inclined PDMS surface while keeping the replicated surface at uniform temperature (308 K). The flow and temperature fields inside the inclined water droplet are simulated in line with the experimental conditions. The flow velocities inside the droplet are validated through the data obtained from the particle imaging velocimetry (PIV). It is found that the velocity predictions agree well with the PIV data. The droplet heating results in a circulation cell inside the droplet due to the Marangoni and the buoyancy currents. Increasing inclination angle of the PDMS replicated surface enhances the maximum velocity inside the droplet, which is more pronounced for the large size droplets. The Nusselt and the Bond numbers increase with the inclination angle of the PDMS surface.

Near-Field Electrospinning for Three-Dimensional Stacked Nanoarchitectures with High Aspect Ratios.

Near-field electrospinning (NFES) was developed to overcome the intrinsic instability of traditional electrospinning processes and to facilitate the controllable deposition of nanofibers under a reduced electric field. This technique offers a straightforward and versatile method for the precision patterning of two-dimensional (2D) nanofibers. However, three-dimensional (3D) stacked structures built by NFES have been limited to either micron-scale sizes or special shapes. Herein, we report on a direct-write 3D NFES technique to construct self-aligned, template-free, 3D stacked nanoarchitectures by simply adding salt to the polymer solution. Numerical simulations suggested that the electric field could be tuned to achieve self-aligned nanofibers by adjusting the conductivity of the polymer solution. This was confirmed experimentally by using poly(ethylene oxide) (PEO) solutions containing 0.1-1.0 wt% NaCl. Using 0.1 wt% NaCl, nanowalls with a maximum of 80 layers could be built with a width of 92 ± 3 nm, height of 6.6 ± 0.1 μm, and aspect ratio (height/width) of 72. We demonstrate the 3D printing of nanoskyscrapers with various designs, such as curved "nanowall arrays", nano "jungle gyms," and "nanobridges". Further, we present an application of the 3D stacked nanofiber arrays by preparing transparent and flexible polydimethylsiloxane films embedded with Ag-sputtered nanowalls as 3D nanoelectrodes. The conductivity of the nanoelectrodes can be precisely tuned by adjusting the number of 3D printed layers, without sacrificing transmittance (98.5%). The current NFES approach provides a simple, reliable route to build 3D stacked nanoarchitectures with high-aspect ratios for potential application in smart materials, energy devices, and biomedical applications.

A self-healing transparent polydimethylsiloxane elastomer based on imine bonds

Smart materials with self-healing properties that can heal themselves with restored mechanical performance are highly desired. Herein, a transparent healable PDMS (HPDMS) elastomer based on imine bonds is successfully prepared from amino-modified polydimethylsiloxane (PDMS) and 1,4-diformylbenzene (DFB) through a simple method. The concentration of DFB has a great effect on the gelation process. The elastomer features an outstanding optical transmittance (more than 80%) over the full visible light region. The results of TGA and DSC indicate that the HPDMS elastomer is highly flexible at room temperature and can be used in a wide temperature range. The presence of imine bonding and the formation of Schiff base linkages is proved by the FT-IR and Raman spectra. The reversible imine bond can promote the contact of the fracture surfaces and result in quick and repeatable self-healing without any external intervention such as heat or light. Furthermore, the elastomer exhibits excellent recyclability and reprocessability. Besides, the adhesion test reveals the potential of the elastomer in application as adhesives. We anticipate that this strategy can be employed to prepare a transparent PDMS elastomer with mechanical strength, stretchable, and excellent self-healing properties.

Microscale electrodeionization: In situ concentration profiling and flow visualization

Electrodeionization (EDI) is membrane-based desalination utilizing ion exchange membranes and ion exchange resins. By combining Electrodialysis and Ion exchanger, EDI can produce ultrapure water in a continuous-flow manner. Although its theoretical mechanisms are well documented, there is no experimental platform that can provide microscopic details inside of the system. In this paper, we present microscale EDI that can visualize in situ ion concentration, pH, and fluid flows. The platform was fabricated by filling ion exchange resins as a monolayer in a transparent polydimethylsiloxane channel between cation and anion exchange membranes. According to operating voltages (0–15V), distinct behaviors of ion concentration profile, pH shift, and fluid flows were observed in Ohmic, limiting, and overlimiting regimes. It is noteworthy that overlimiting regimes can be sub-categorized as water-splitting and electroconvection regimes. In the early stage (4–8V), water-splitting is dominant with pH change near the membranes and resins; under a higher voltage (8–15V), electroconvection starts to occur even water-splitting tries to suppress the development of the extended space charge layer and corresponding electroconvective instability. Accelerated ionic migration by electroconvection can improve current efficiency up to 80%. This is a clear departure from overlimiting dynamics in electrodialysis (with electroconvection only), ion exchanger (with no distinct regime), and even from that in previous EDI experiments (with water splitting only).

Compact and on-demand 3D-printed optical device based on silicone optical technology (SOT) for on-site measurement: Application to flow injection analysis

The current trend for analytical systems and devices is to be small, portable, and low in cost because of the flexibility required to ensure their availability for on-site measurement applications. Using the concept of silicone optical technology (SOT), we propose a fully flexible analytical system, and digital fabrication of a compact optical module for flow-injection analysis (FIA) is demonstrated. A simple structure composed of a transparent polydimethylsiloxane (PDMS) optical core with a carbon black-dispersed PDMS cladding acts as a tiny but straightforward optical filter and shows an ability to trap light at tilted incidence; this structure is embedded on a silicone 3D-printed optical module via a simple injection and coating method. The coating properties, the optical trapping performance, and the cross talk test performance of the device were evaluated. The SOT-FIA optical module successfully demonstrated a 99.8% undesired light trapping performance; cross talk of less than 0.5% was observed, illustrating the low-noise detection performance of the SOT-FIA optical system. Colorimetric determination of iron in natural water samples demonstrated excellent linearity, a wide chemical analysis range, and good percentage recovery performance. The results of the determination in real water samples using the proposed SOT-FIA optical device agreed well with the results from the conventional spectrophotometric method. With advantages that include simplicity, low cost, flexibility, portability, and high sensitivity, the proposed optical device will be potentially useful for on-site chemical analysis measurements.

Flexible PDMS microparticles to mimic RBCs in blood particulate analogue fluids

Polydimethylsiloxane (PDMS) has a wide variety of commercial and industrial applications due to its mechanical and rheological properties in a range similar to the living tissues. In this study, we demonstrate that PDMS can be used to produce deformable microparticles to be integrated in the development of particulate blood analogue fluids. The difficulties associated with the use of in vitro blood make it necessary to perform in vitro experiments of blood flow with blood analogue fluids. However, an ideal analogue must match the rheology of blood at several points, and for that, blood analogue fluids should be a suspension of microparticles with similar properties (size, shape and flexibility) to blood cells, in particular to the red blood cells (RBCs). The microparticles used in this study were produced from a transparent PDMS with crosslinking ratios of 10:1, 8:2 and 6:4; from a black PDMS with a ratio of 1:1 and from a red-pigmented PDMS. Each PDMS microparticles sample was suspended in Dextran 40 to perform deformability assays and cell-free layer analysis in a hyperbolic-shaped microchannel and steady shear viscosity measurements in a rheometer. The proposed microparticles suspensions show a great potential to mimic the structural and rheological properties of RBC suspensions and consequently to develop blood analogue fluids with rheological properties similar to real blood.

Ionic liquids-filled patterned cavities improve transmittance of transparent and stretchable electronic polydimethylsiloxane films

Transparent electrodes for flexible and wearable devices require considerable stretchability and patterned conductive materials. Transparent and stretchable polydimethylsiloxane (PDMS) films with patterned cavities were prepared by laser processing and transfer printing. Transparent conductive ionic liquids (ILs) and aqueous solution (AS) were injected into the cavities, obtaining electrodes with transmittance in the visible range of 99.94% (relative to blank PDMS films) and 92.80% (relative to air). This transmittance was almost identical to that of a blank PDMS film. The maximum tensile strain applied to the electrodes was 117.23%, and the resistance of the electrodes with network patterns measured by the four-probe method was 3.20 kΩ. After 10,000 stretching/releasing cycles under strain of 0–50%, these electrodes still performed optimally. The ILs and AS showed various transmittance and electrical properties. The electrodes with ILs had higher transmittance, while those with AS showed increased conductivity. These electrodes with five patterns (serpentine, circle, straight line, fold line, and rhombus) were analyzed. The pattern containing curves with a large curvature would result in a severe local stress concentration in the stretching process. Patterns containing parallel curves could optimize electrical conductivity. And patterns containing complex curves had a certain effect on the transmittance. In addition, taking advantage of the fact that the elastomer and the liquid can deform at will, these electrodes were used to fabricate two pressure sensors and two strain sensors, which were employed to demonstrate their sensitivity stability during cycling.

A study of saliva lubrication using a compliant oral mimic

Due to ethical issues and the difficulty in obtaining biological tissues, it is important to find synthetic elastomers that can be used as replacement test media for research purposes. An important example of this is friction testing to understand the mechanisms behind mouthfeel attributes during food consumption (e.g. syrupy, body and clean finish), which requires an oral mimic. In order to assess the suitability of possible materials to mimic oral surfaces, a sliding contact is produced by loading and sliding a hemispherical silica pin against either a polydimethyl siloxane (PDMS), agarose, or porcine tongue sample. Friction is measured and elastohydrodynamic film thickness is calculated based on the elastic modulus of the samples, which is measured using an indentation method. Tests were performed with both saliva and pure water as the lubricating fluid and results compared to unlubricated conditions.PDMS mimics the tongue well in terms of protein adhesion, with both samples showing significant reductions in friction when lubricated with saliva versus water, whereas agarose showed no difference between saliva and water lubricated conditions. This is attributed to PDMS's OSi(CH3)2- group which provides excellent adhesion for the saliva protein molecules, in contrast with the hydrated agarose surface. The measured modulus of the PDMS (2.2 MPa) is however significantly greater than that of tongue (3.5 kPa) and agarose (66–174 kPa). This affects both the surface (boundary) friction, at low sliding speeds, and the entrained elastohydrodynamic film thickness, at high speeds.Utilising the transparent PDMS sample, we also use fluorescence microscopy to monitor the build-up and flow of dyed-tagged saliva proteins within the contact during sliding. Results confirm the lubricous boundary film forming nature of saliva proteins by showing a strong correlation between friction and average protein intensity signals (cross correlation coefficient = 0.87). This demonstrates a powerful method to study mouthfeel mechanisms.

Detecting the thermoplasmonic effect using ellipsometry parameters for self-assembled gold nanoparticles within a polydimethylsiloxane matrix

Light-to-heat conversion using active plasmonic materials is essential in wide-ranging applications, such as sensing, photonics, drug delivery, biomedical imaging, photothermal tumor therapy, and optoelectronics. In this work, we studied the thermoplasmonic effect and performed an optical analysis of different concentrations of self-assembled gold nanoparticles in transparent dielectric polydimethylsiloxane polymer medium under continuous-wave radiations. Composite samples were prepared and investigated experimentally by ellipsometry method and thermal photography. The gold-nanoparticle content of each composite film directly increased the generated temperature. The structure and optical properties of the samples under ambient conditions were obtained efficiently depending on the ellipsometry parameters for each polarized light. Results showed that the prolongation of the incubation time led to a lower phase value for p polarization than that for s polarization. This finding was due to the change in ellipsometry parameters and, thus, the thermoplasmonic effect. This new method of investigating the thermoplasmonic effect can provide new insights into the medical applications of plasmonic media.

Fabrication of a flexible transparent superomniphobic polydimethylsiloxane surface with a micropillar array.

Although superomniphobic surfaces have attracted extensive interest owing to many important applications, successful fabrication of such surfaces still remains a critical challenge. Herein, we present a flexible transparent superomniphobic polydimethylsiloxane (PDMS) surface with a micropillar array using Si nanowires as the mould. The as-obtained PDMS not only exhibits excellent liquid-repellent performance with a static contact angle of over 150° and sliding angle of less than 6° against a wide range of liquids, but also maintains the super-repellency even under acid/base corrosion, mechanical damage, and unidirectional stretching.

Room temperature lasing from microdisk laser in aqueous medium

Lasing of optically pumped semiconductor microdisks immersed in aqueous medium is demonstrated for the first time. Microlasers containing quantum dot active region were placed into the transparent polydimethylsiloxane chamber filled with distilled water at room temperature. The spectral and threshold characteristics of the lasers are compared in both air and aqueous environments. We suppose that such high-Q microlasers can be used as highly sensitive biosensors.

Optical detection of ultrasound by lateral shearing interference of a transparent PDMS thin film.

A lateral shearing interferometric technique combined with an 11.6μm polydimethylsiloxane (PDMS) transparent thin film is proposed and demonstrated for optical detection of ultrasound. We experimentally report the device change of reflectivity with pressure of 5.1×10-7 Pa-1, 9.5 times more sensitive than the critical-angle-based sensor, 31 times more sensitive than the surface-plasmon-based sensor, and comparable to the Fabry-Perot sensor. The objective-lens-based angle scanning characterization setup inspired from a laser scanning system allows direct comparison between the PDMS sensor and critical-angle-based sensor by adjusting the incident angle with a scanning mirror, thereby eliminating optical and electronics system dependence. The sensing element is easily fabricated through spin coating and the detection element incorporated into an existing optical system with minimum modification.

Investigation of Pore-Scale Behaviors of Foam Flow in a Polydimethylsiloxane Micromodel

Foam has great potential in underground fluid displacements in porous media, such as in enhanced oil recovery projects and CO2 geological storage, because of the effectiveness of mobility control. The mechanism of foam flow in porous media is essential to understand the complex process of foam displacement. In this work, pore-scale behaviors of CO2 foam and N2 foam flows have been investigated using a transparent polydimethylsiloxane micromodel. The effects of foam quality and gas type on gas trapping and foam texture were evaluated. Image results gave the phase distribution during foam flow through the porous medium directly. Both trapped gas saturation and trapped gas fraction overall increased with foam quality ranging from 50% to 94%, but foam texture became coarser at foam qualities above 75%. Compared to N2 foam, CO2 foam exhibited lower trapped gas saturation and a wider distribution of bubble size in the porous medium. It was observed that trapped CO2 bubbles within pores shrank with time, which m...