Hard Polydimethylsiloxane Research Articles

Improvement in Strain Sensor Stability by Adapting the Metal Contact Layer.

Research on stretchable strain sensors is actively conducted due to increasing interest in wearable devices. However, typical studies have focused on improving the elasticity of the electrode. Therefore, methods of directly connecting wire or attaching conductive tape to materials to detect deformation have been used to evaluate the performance of strain sensors. Polyaniline (PANI), a p-type semiconductive polymer, has been widely used for stretchable electrodes. However, conventional procedures have limitations in determining an appropriate metal for ohmic contact with PANI. Materials that are generally used for connection with PANI form an undesirable metal-semiconductor junction and have significant contact resistance. Hence, they degrade sensor performance. This study secured ohmic contact by adapting Au thin film as the metal contact layer (the MCL), with lower contact resistance and a larger work function than PANI. Additionally, we presented a buffer layer using hard polydimethylsiloxane (PDMS) and structured it into a dumbbell shape to protect the metal from deformation. As a result, we enhanced steadiness and repeatability up to 50% strain by comparing the gauge factors and the relative resistance changes. Consequently, adapting structural methods (the MCL and the dumbbell shape) to a device can result in strain sensors with promising stability, as well as high stretchability.

Polyelectrolyte adsorption in single small nanochannel by layer-by-layer method

Nanochannel size control by using layer-by-layer (LBL) deposition of polyelectrolytes is experimentally studied in single hard Polydimethylsiloxane (PDMS) nanochannel. Electroosmotic flow (EOF) velocity is measured by using the current-slope method in the modified nanochannel with various numbers of the deposition layers. The results reveal that the growth of polyelectrolyte layer in nanochannels is quite different from that on flat substrates. In small nanochannels, the increment of the multilayer thickness will be limited unlike that on flat surfaces. As the number of the deposition layers increases, the nanochannel size will decrease initially and reach a saturation value finally where the nanochannel cannot be filled with the polyelectrolytes. It is observed that both the decrement and the saturation value of the nanochannel size strongly depend on the concentration of the salt addition in polyelectrolyte solutions. At low ionic concentration, increasing the salt addition increases the average layer thickness within the nanochannels and decreases the saturation value of nanochannel size. However, the effects of salt addition can demonstrate opposite behaviors at high ionic concentration. The EOF can be alternatively reversed in the modified nanochannels by repeatedly coating oppositely charged polyelectrolytes, and the EOF velocity will gradually decrease with the increasing deposition cycles.

Penetrating Microindentation of Hyper-soft, Conductive Silicone Neural Interfaces in Vivo Reveals Significantly Lower Mechanical Stresses

ABSTRACTThere is growing evidence that minimizing the mechanical mismatch between neural implants and brain tissue mitigates inflammatory, biological responses at the interface under long-term implant conditions. The goal of this study is to develop a brain-like soft, conductive neural interface and use an improvised, penetrating microindentation technique reported by us earlier to quantitatively assess the (a) elastic modulus of the neural interface after implantation, (b) mechanical stresses during penetration of the probe, and (c) periodic stresses at steady-state due to tissue micromotion around the probe. We fabricated poly- dimethylsiloxane (PDMS) matrices with multi-walled carbon nanotubes (MWCNTs) using two distinct but carefully calibrated cross-linking ratios, resulting in hard (elastic modulus∼484 kPa) or soft, brain-like (elastic modulus∼5.7 kPa) matrices, the latter being at least 2 orders of magnitude softer than soft neural interfaces reported so far. Subsequent loading of the hard and soft silicone based matrices with (100% w/w) low-molecular weight PDMS siloxanes resulted in further decrease in the elastic modulus of both matrices. Carbon probes with soft PDMS coating show significantly less maximum axial forces (-587 ± 51.5 µN) imposed on the brain than hard PDMS coated probes (-1,253 ± 252 µN) during and after insertion. Steady-state, physiological micromotion related stresses were also significantly less for soft- PDMS coated probes (55.5 ± 17.4 Pa) compared to hard-PDMS coated probes (141.0 ± 21.7 Pa). The penetrating microindentation technique is valuable to quantitatively assess the mechanical properties of neural interfaces in both acute and chronic conditions.

Fabrication of textured substrates for dye-sensitized solar cells using polydimethylsiloxane nanoimprint lithography

Abstract We proposed a fabrication of nanoimprinted textures on a front glass/transparent conductive oxide interface for dye-sensitized solar cells (DSSCs). These textures were fabricated through polydimethylsiloxane (PDMS) nanoimprint lithography on organosilsesquioxane solution. The texture structures were estimated via optical simulation. Master molds were anodic aluminum oxide templates with nano-texture (N-Tx) and micro-nano double texture (D-Tx). Meanwhile, replicate molds used a hard PDMS. Fluorine-doped tin oxide and titanium dioxide were deposited on textured glass substrates to generate electrodes for DSSCs. Unlike the DSSCs without texture, textured DSSCs realized 11.4% (N-Tx) and 10% (D-Tx) improvement in conversion efficiency.

Simultaneous formation of multiscale hierarchical surface morphologies through sequential wrinkling and folding

Highly organized hierarchical surface morphologies possess various intriguing properties that could find important potential applications. In this paper, we demonstrate a facile approach to simultaneously form multiscale hierarchical surface morphologies through sequential wrinkling. This method combines surface wrinkling induced by thermal expansion and mechanical strain on a three-layer structure composed of an aluminum film, a hard Polydimethylsiloxane (PDMS) film, and a soft PDMS substrate. Deposition of the aluminum film on hard PDMS induces biaxial wrinkling due to thermal expansion mismatch, and recovering the prestrain in the soft PDMS substrate leads to wrinkling of the hard PDMS film. In total, three orders of wrinkling patterns form in this process, with wavelength and amplitude spanning 3 orders of magnitude in length scale. By increasing the prestrain in the soft PDMS substrate, a hierarchical wrinkling-folding structure was also obtained. This approach can be easily extended to other thin films for fabrication of multiscale hierarchical surface morphologies with potential applications in different areas.

Effect of viscoelasticity on the soft-wall transition and turbulence in a microchannel

The modification of soft-wall turbulence in a microchannel due to small amounts of polymer dissolved in water is experimentally studied. The microchannels are of rectangular cross-section with height ${\sim}$160 $\unicode[STIX]{x03BC}\text{m}$, width ${\sim}$1.5 mm and length ${\sim}$3 cm, with three walls made of hard polydimethylsiloxane (PDMS) gel, and one wall made of soft PDMS gel with an elasticity modulus of ${\sim}$18 kPa. Solutions of polyacrylamide of molecular weight $5\times 10^{6}$ and mass fraction up to 50 ppm, and of molecular weight $4\times 10^{4}$ and mass fraction up to 1500 ppm, are used in the experiments. In all cases, the solutions are in the dilute limit below the critical overlap concentration, and the solution viscosity does not exceed that of water by more than 10 %. Two distinct types of flow modifications are observed below and above a threshold mass fraction for the polymer, $w_{t}$, which is ${\sim}$1 ppm and 500 ppm for the solutions of polyacrylamide with molecular weights $5\times 10^{6}$ and $4\times 10^{4}$, respectively. At or below $w_{t}$, there is no change in the transition Reynolds number, but there is significant turbulence attenuation, by up to a factor of 2 in the root-mean-square velocities and a factor of 4 in the Reynolds stress. When the polymer concentration increases beyond $w_{t}$, there is a decrease in the transition Reynolds number and in the intensity of the turbulent fluctuations. The lowest transition Reynolds number is ${\sim}$35 for the solution of polyacrylamide with molecular weight $5\times 10^{6}$ and mass fraction 50 ppm (in contrast to 260–290 for pure water). The fluctuating velocities in the streamwise and cross-stream directions are lower by a factor of 5, and the Reynolds stress is lower by a factor of 10, in comparison to pure water.

Investigation of the anti-adhesion layers for nanoimprint molding

To realize the anti-adhesion issue between silicon mold and nanostructures of hard polydimethylsiloxane (H-PDMS), anti-adhesion layers were coated on molds to improve the H-PDMS grating structure with half-pitch below 200 nm. Various deposition methods such as liquid immersion method, vapor deposition method, and plasma-enhanced chemical vapor deposition were used to coat various anti-adhesion layers, respectively. Three anti-adhesion layers including diamond-like carbon (DLC) film, fluorine-doped diamond-like carbon (F-DLC) film, and perfluorooctyl-trichlorosilane (PFOTCS) were investigated. PFOTCS was functioned as a mold-releasing agent using the liquid immersion and vapor deposition methods, respectively. The effect of those anti-adhesion layers on the molding process was characterized. The results show that the PFOTCS anti-adhesion layer coated by vapor deposition method exhibits the best separability among all of the anti-adhesion layers. The maximum contact angle using PFOTCS is about 115° under 0.5 h of heating time. Compared to DLC, the F-DLC anti-adhesion layer shows good separability when the line width exceeds 100 nm, and does not require recoating again after each use. In addition, atomic force microscopy (AFM) was adopted to measure the adhesion force between the H-PDMS and the anti-adhesion layers.

Locally-tailored structure of an elastomeric substrate for stretchable circuits

We demonstrate a new process for fabricating a hybrid elastomeric polydimethylsiloxane (PDMS) substrate, which can provide a high ratio (as large as ∼50) of the elastic modulus between the active device region and the interconnect area, as well as a locally tailored surface profile for each region. For this process, a Si master mold with a dual surface profile is prepared, where locally flat regions are distributed within a wavy-surfaced area. The stiffer elastomeric islands for active devices are formed on the flat regions by photolithography of a photo-patternable and hard PDMS layer (E ∼ 160 MPa), over which a soft PDMS layer (E ∼ 2 to 3 MPa) is casted. By releasing the whole PDMS layer from the mold, a hybrid silicone substrate with stiff and flat islands embedded within a soft and wavy matrix is obtained. In this hybrid structure, active devices located on the stiff regions can provide high reliability under stretched conditions, while most strain is accommodated by wavy interconnects within the soft area. Such beneficial effects are demonstrated by organic thin film transistors produced on the hybrid substrate.

Study on Anti-Adhesion Layers Using AFM for Nanoimprint Process

This study investigates how to improve the anti-adhesion issues between Silicon mold and nanostructures of hard polydimethylsiloxane (H-PDMS). A Silicon mold with different depths and widths was made using a focused ion beam (FIB). During the soft-lithography molding process, anti-adhesion layers were needed between the Silicon mold and H-PDMS samples to prevent the de-molding failure caused by the adhesion issues between the interfaces. This study adopts three methods to deposit anti-adhesion layers, such as liquid immersion, vapor deposition, and fluorine-doped diamond-like carbon (F-DLC) film. Perfluorooctyl-trichlorosilane (PFOTCS) was used as a mold-releasing agent for the liquid immersion and vapor deposition methods. The contact angles between each film were measured to determine the effect of anti-adhesion on the molding process. In addition, atomic force microscopy (AFM) was used to measure the adhesion force between the H-PDMS and anti-adhesion layers. The results show that the coatings of anti-adhesion layers are an effective approach to improve the formability of molding.

A New Mold Structure to Replicate Patterns Over 1 <I>μ</I>m in Depth for Substrate Conformal Imprint Lithography

This paper shows an improved mold replication process that uses polyurethane acrylate (PUA) and polyethylene terephthalate (PET) for the fabrication of an ultraviolet (UV) imprinting mold used in substrate conformal imprint lithography (SCIL). With the conventional replication process, which uses hard polydimethylsiloxane (h-PDMS) as a pattern layer, it is difficult to detach the mold from a silicon master for metal oxide semiconductor field effect transistor (MOSFET) that has patterns with over 1-micron depth. However, the method proposed in this paper allows us to easily replicate patterns that have more than 1-micron depth. The key idea of this method is to use PET film as a bonding layer to attach the PUA layer to the polydimethylsiloxane (PDMS) cushion layer to overcome the weak the adhesion force between the PUA and PDMS layer. We demonstrate how to make the modified replica mold and present imprinting results obtained using this replica mold in the SCIL process.

Effect of mold treatment by solvent on PDMS molding into nanoholes

Polydimethylsiloxane (PDMS) is the most popular and versatile material for soft lithography due to its flexibility and easy fabrication by molding process. However, for nanoscale patterns, it is challenging to fill uncured PDMS into the holes or trenches on the master mold that is coated with a silane anti-adhesion layer needed for clean demolding. PDMS filling was previously found to be facilitated by diluting it with toluene or hexane, which was attributed to the great reduction of viscosity for diluted PDMS. Here, we suggest that the reason behind the improved filling for diluted PDMS is that the diluent solvent increases in situ the surface energy of the silane-treated mold and thus the wetting of PDMS to the mold surface. We treated the master mold surface (that was already coated with a silane anti-adhesion monolayer) with toluene or hexane, and found that the filling by undiluted PMDS into the nanoscale holes on the master mold was improved despite the high viscosity of the undiluted PDMS. A simple estimation based on capillary filing into a channel also gives a filling time on the millisecond scale, which implies that the viscosity of PMDS should not be the limiting factor. We achieved a hole filling down to sub-200-nm diameter that is smaller than those of the previous studies using regular Sylgard PDMS (not hard PDMS, Dow Corning Corporation, Midland, MI, USA). However, we are not able to explain using a simple argument based on wetting property why smaller, e.g., sub-100-nm holes, cannot be filled, for which we suggested a few possible factors for its explanation.

To Fabricate Bragg Grating on D-Shaped Optical Fibre by Micro-Pressing Method

In this paper, we proposed a method to fabricate a D-shaped optical fiber of Bragg surface grating filter component by D-shaped fiber polishing along with photolithography、holographic interference lithography technologies and micro-molding process. The optical fiber was side-polished until it reaches the core of fiber. The master Bragg grating was first fabricated on the positive photoresister by using the holography interference lithography, then the patterned resist was used as a mother mold to transfer the pattern onto a hard polydimethylsiloxane (hPDMS)/ polydimethylsiloxane (PDMS) mold. The gratings pattern on hPDMS/PDMS was then transferred onto the UV polymer on the surface of a D-shaped fiber using a UV replication process. The transmission spectra of the resulting gratings were measured, with test results showing the transmission dip of -17dB , and 3-dB-transmission bandwidth of 9nm.

Using Micro-Pressing Method to Fabricate Bragg Grating on D-Shaped Optical Fiber

In this paper, we proposed a method to fabricate a D-shaped optical fiber of Bragg surface grating filter component by D-shaped fiber polishing along with photolithography、holographic interference lithography technologies and micro-molding process. The optical fiber was side-polished until it reaches the core of fiber. The master Bragg grating was first fabricated on the positive photoresister by using the holography interference lithography, then the patterned resist was used as a mother mold to transfer the pattern onto a hard polydimethylsiloxane (hPDMS)/ polydimethylsiloxane (PDMS) mold. The gratings pattern on hPDMS/PDMS was then transferred onto the UV polymer on the surface of a D-shaped fiber using a UV replication process. The transmission spectra of the resulting gratings were measured, with test results showing the transmission dip of -17dB , and 3-dB-transmission bandwidth of 9nm.

SU-8 rib waveguide Bragg grating filter using composite stamp and solvent-assisted microcontact molding technique

We present a simple fabrication technique using a hydrophobic composite elastomer for patterning an SU-8-based rib waveguide Bragg grating filter via solvent-assisted microcontact molding (SAMIM). The proposed technique utilizes a two-layered composite stamp made of polydimethylsiloxane and hard polydimethylsiloxane, which enables both the rib waveguide and surface relief grating structure to be patterned simultaneously. The SAMIM method realizes patterning at ambient temperatures with the weight of the mold applying all the pressure necessary. By allowing the mold to absorb solvent when in contact with the polymer, the solvent dissolves or swells the polymer's surface, creating the desired pattern. This composite mold successfully demonstrates the SAMIM creation of the SU-8 rib waveguide and gratings, paving the way for a facile and efficient reproduction of a polymeric rib waveguide grating filter. Based on experimental results, the stamp demonstrates an operational life span in excess of 10 successful replications, utilizing a Bragg grating test pattern with a grating period of ~492 nm, a depth of ~250 nm, and a total length of 12 mm. The resulting waveguides show a band-rejection gain of −17 dB at the center wavelength of 1545 nm and a 3-dB bandwidth of 2.5 nm.

Gold nanoparticles by soft UV nanoimprint lithography coupled to a lift-off process for plasmonic sensing of antibodies

In this paper, we present the results of gold nanoparticles fabrication on large area by soft UV nanoimprint lithography (UV-NIL) and lift-off process for antibodies sensing. For this fabrication, the hard polydimethylsiloxane (H-PDMS) is used as flexible mold material. A simple AMONIL/PMMA bilayer is used to transfer patterns by etching the substrate with a suitable RIE process. The UV-NIL process has enabled to obtain gold nanoparticles with a diameter about 160 nm on a zone of 1 mm 2. For bioplasmonic applications, the sensing of Anti-Biotin was studied.

Fabrication of an elastomeric rib waveguide Bragg grating filter

We report the design and fabrication of a Bragg grating device on a silicone-based elastomer rib waveguide, constructed using solvent-assisted microcontact molding and polymer casting techniques. The waveguide design utilizes polydimethylsiloxane (PDMS) and hard polydimethylsiloxane (hPDMS) polymers as core and cladding materials, with a period of ~0.55 μm, a depth of ~350 nm, and a grating length of 6 mm. Fabrication is relatively simple, with the hPDMS/PDMS waveguide and gratings cast from a two-layer mold made of a phenolic resin-based positive photoresist and an epoxy Novolak resin-based negative photoresist. The mold we have designed has a demonstrated operational lifespan in excess of ten successful waveguide fabrications. The transmission spectra of the resulting gratings were measured, with test results showing a band-rejection gain of −17 dB and a 3 dB bandwidth of ~3 nm at transmission.