Liquid PDMS Research Articles

Preparation of flexible and stretchable circuit via printing method based on silver nanowires

Flexible and stretchable electronics attract much more attention in recent years due to their excellent conformability to non-planar environment and potential applications in wearable electronics, flexible display and bioengineering. In this work, we report the use of printing strategy applied to the fabrication of flexible and stretchable circuit. The prepared Ag NWs paste was printed on glass substrate to form conductive circuit via screen printing, and liquid PDMS was casted on top of the printed Ag NWs circuit and cured to peel off the substrate. The randomly oriented Ag NWs were buried in PDMS to form conductive and stretchable circuit. We studied the influence of the diameter of Ag NWs on the surface morphology of Ag NWs/PDMS circuit during strain process and further analyzed the relationship between the surface micro-structure and electrical property of Ag NWs/PDMS conductor. The results showed that the cracks happened on the surface of circuit with Ag NWs of smaller diameter in stretching process, while the buckling structure appeared after pre-tension strain of 100% with Ag NWs of larger diameter. The buckling of Ag NWs/PDMS layer resulted in the stable resistance of stretchable circuit in the strain range of 0–50%. We also demonstrated the printed stretchable circuit integrated with LED array under bending, twisting and even stretching deformation. The printing method may provide a new step towards the development of stretchable strategies for conductive circuit and other wearable electronics.

Direct formation of high aspect ratio multiple tilted micropillar array in liquid phase PDMS by proton beam writing

High aspect ratio (HAR), straight and tilted micropillars were fabricated in PDMS polymer by proton beam writing (PBW) method. The polymer was applied as a negative tone resist material, in its liquid and high viscosity form. The microstructures were created by irradiating the polymer with 2MeV focused proton beam, by which, the irradiated area became cross-linked and solid. The formed micropillars had very smooth side walls and they were stable against the capillary and adhesion forces, above a certain applied proton fluence. The rate of the solidification strongly depends on the deposited ion fluence. Furthermore, some physicochemical properties, such as elastic modulus also significantly change due to ion irradiation induced processes. This liquid PDMS resist with PBW makes the possibility to create high aspect ratio, straight or even tilted micropillar arrays with tunable properties, which can be useful in many potential applications, like lab-on-a-chip devices, high surface required applications, catalysis, biomedical applications, cell separation, and directional adhesive surfaces.

Proton beam lithography in negative tone liquid phase PDMS polymer resist

In this work we investigated the applicability of liquid PDMS polymer as a negative resist material for direct proton beam writing technique. We irradiated the polymer in liquid phase, spin-coated on different substrate materials creating various microstructures. PDMS pre-polymer was cross-linked just by PBW. As the cross-linking process increases, the irradiated area becomes more solid. The rate of the solidification strongly depends on the deposited ion dose. The effects of fluence, beam current, substrate type and developer solvent was investigated. Furthermore, at the irradiated areas the adhesion, the wettability and Young’s modulus also changes due to the chemical change of the PDMS polymer. This effect makes the possibility to form microstructures in PDMS with tunable adhesion and wettability properties. In practical viewpoint, the PDMS resist can also have some advantages compared to other resists such as easy stripping, very fast developing (as the un-cross-linked PDMS is soluble in many organic solvents), not sensitive to light, high current or high fluence.

Networked neuro-spheres formed by topological attractants for engineering of 3-dimensional nervous system

The nervous systems including central and peripheral nervous system have an important role in transmitting signals from brain to organs and tissues. Due to such critical function of nervous system, considerable effort has been tried to establish in vitro nervous model. In this paper, the neuro-spheres networked by the nerve-like structure were created using concave well arrays connected by the hemicylindrical channels. The concave microwells and the hemicylindrical channels were fabricated using the surface tension of viscose liquid PDMS prepolymer for the creation of neuro-spheres-networking (NSN). To investigate the topological effect of the concave-well-hemicylindrical-channel-networking (CWHCN), comparative experiments were conducted on a conventional cylindrical-wells-rectangular-channel-networking (CWRCN). Neuro-progenitor cells from the rat were seeded on the concave well arrays connected by the hemicylindrical channels and cultured for 10 days. Small neuro-pehroids consisting of neurons and glia cells were autonomously formed in the concave microwell arrays. These neuro-spheres were networked by the nerve-like structures formed along the CWHCN. In order to confirm the interconnection of the neurites in the NSN, a calcium imaging experiment was performed to measure the calcium flux along the nerve-like networking. To demonstrate further use of the networked neuro-networking for brain regeneration, we transferred the NSN onto hydrogel maintaining approximately 90% viability, and this model is expected to be used for the regeneration of a damaged brain.

Fabrication of a Wrinkled Surface on Poly(dimethylsiloxane) Using a Thermal Curing Process

We describe the fabrication of a spontaneous wrinkled surface on poly(dimethylsiloxane) (PDMS). The wrinkled surface on an elastic PDMS substrate was fabricated by thermally shrinking liquid PDMS coated on a cured PDMS layer during a simple thermal curing process. Optical and atomic force microscopy, deflections of PDMS cantilever beams on the wrinkled surface, and contact angles of water droplets on the wrinkled surface were used to quantitatively investigate the wrinkled patterns on the PDMS layer. The morphology of cells seeded on the fabricated wrinkled surface was observed.

Carbon Nanotube Fiber Based Stretchable Conductor

AbstractCarbon nanotube (CNT) based continuous fiber, a CNT assembly that could potentially retain the superb properties of individual CNTs on a macroscopic scale, belongs to a fascinating new class of electronic materials with potential applications in electronics, sensing, and conducting wires. Here, the fabrication of CNT fiber based stretchable conductors by a simple prestraining‐then‐buckling approach is reported. To enhance the interfacial bonding between the fibers and the poly(dimethylsiloxane) (PDMS) substrate and thus facilitate the buckling formation, CNT fibers are first coated with a thin layer of liquid PDMS before being transferred to the prestrained substrate. The CNT fibers are deformed into massive buckles, resulting from the compressive force generated upon releasing the fiber/substrate assembly from prestrain. This buckling shape is quite different from the sinusoidal shape observed previously in otherwise analogous systems. Similar experiments performed on carbon fiber/PDMS composite film, on the other hand, result in extensive fiber fracture due to the higher fiber flexural modulus. Furthermore, the CNT fiber/PDMS composite film shows very little variation in resistance (≈1%) under multiple stretching‐and‐releasing cycles up to a prestrain level of 40%, indicating the outstanding stability and repeatability in performance as stretchable conductors.

Modification of Polydimethylsiloxane Coatings by H2 RF Plasma, Xe2* Excimer VUV Radiation, and Low‐Energy Electron Beams

AbstractThe crosslinking of thin liquid PDMS layers by three different technically relevant processes, H2 radio‐frequency plasma treatment, Xe2* excimer VUV irradiation, and low‐energy electron beam processing is investigated. The modifications to the layers due to the processing are monitored by means of RAIRS. Plasma processing of liquid PDMS leads to a direct conversion to a SiOx‐like material of the topmost layers, whereas a gradual transition from PDMS to the same product is observed upon VUV irradiation. Electron exposure does not induce oxidation. The initiating steps of the conversion induced by the interaction with VUV photons, low‐energy electrons, or their combined effect with ions and H atoms in the plasma are discussed. The latter creates a high density of damage sites. magnified image

Meniscus induced self organization of multiple deep concave wells in a microchannel for embryoid bodies generation

Embryonic stem cells (ESCs) have attracted great interest in the fields of tissue engineering, regenerative medicine, and organogenesis for their pluripotency and ability to self-renew. ESC aggregation, which produces an embryoid body (EB), has been widely utilized as a trigger of in vitro directed differentiation. In this paper, we propose a novel method for constructing large numbers of deep concave wells in PDMS microfluidic chips using the meniscus induced by the surface tension of a liquid PDMS prepolymer, and applied this chip for the mass production of uniform sized EBs. To investigate if the microenvironment in the deep concave well is suitable for ES cells, the oxygen diffusion to the deep concave well was analyzed by CFD simulation. Murine EBs were successfully formed in the deep concave wells without loss of cells and laborious careful intervention to refresh culture media. The size of the EBs was uniform, and retrieving of EBs was done just by flipping over the chip. All the processes including EB formation and harvest are easy and safe to cells, and their viability after completion of all processes was over 95%. The basic properties of the EBs were generated and their capacity to differentiate into 3 germ layers was investigated by analyzing the gene expression profile. The harvested EBs were found to differentiate into cardiac cells and neurons, and neurofilaments formed branches of elongated extensions more than 1.0 mm in length.

Soft lithography replication based on PDMS partial curing

In this paper, a partial curing technique is presented. Its aim is to enable the soft lithography replication process for the case requiring usage of PDMS mold. Through controlling the curing time during molding step, the liquid PDMS prepolymer in original status, which will later constitute the device substrate, can be partially polymerized as well as solidified. As a result, not only can the structural pattern in the mold be successfully transferred into the device layer, but also the spontaneous adhesive interaction between these two PDMS parts happening during curing can be effectively limited within very low level, thus largely facilitating the demoulding process. Based on this process, several devices have been successfully developed and high fidelity pattern transfer capability has also been demonstrated. Comparing with the most commonly used treatments such as silanization, the presented method demonstrates easier operation and better user-friendly interface advantages.

Tailoring the structure of S-PEEK/PDMS proton conductive membranes through applied electric fields

Composite membranes were formed composed of proton conductive sulfonated poly(ether ether ketone) (S-PEEK) particles dispersed in a non-proton conductive polymeric matrix, a cross-linked poly(dimethyl siloxane) (PDMS). The structure of the composites was controlled by applying electric fields to suspensions of S-PEEK particles in the liquid PDMS precursor, followed by thermally initiated cross-linking polymerization to fix the field-induced structure. The effects of the electric field on membrane structure, proton conductivity, methanol permeability, and water swelling were examined. Under certain conditions, the applied electric field induced the S-PEEK particles to form long chains across the liquid PDMS prepolymers. The degree of particle chaining was a function of the electric field frequency, magnitude, and application time. The S-PEEK particle chaining resulted in an improvement of the membrane conductivity, water uptake ability, and dimensional stability in comparison to membranes containing randomly distributed particles. The particle chaining also increased the methanol permeation across the composite membranes, but the selectivity of the membranes for protons over methanol increased sharply because the increase in proton conductivity was much larger relative to the methanol permeability increase. The membranes also display anisotropic swelling behavior in water that may prove advantageous for enhancing mechanical stability in fuel cells undergoing humidity cycling. The present study demonstrates a novel fabrication approach that can be used to control the structure of a variety of types of composite membranes to enhance performance for fuel cell applications.

Fabrication of round channels using the surface tension of PDMS and its application to a 3D serpentine mixer

A novel fabrication technique was developed to fabricate round microchannels and applied to a micro mixer having a barrier structure using surface tension of PDMS. When the solidified PDMS layer (channel layer) contacts the liquid PDMS film (meniscus layer), a meniscus is formed around the sidewall in the microchannel due to surface tension. The external load pressure and contact area of the channel layer were adjusted to form various cross-sectional shapes such as a U shape, ellipse, semi-circle and circle. Since the width of the channel also determines the depth formed by the difference in capillary height, a multi-depth channel can be fabricated using a one-step process. It was applied to a dual-depth serpentine mixer, eliminating the aligned bonding of conventional soft lithography. The 3D-structure mixer enhanced mixing performance in the range of Re > 10 compared with a 2D-structure. It could fully mix phenolphthalein and sodium hydroxide when the Reynolds number was 80. The suggested fabrication method could be very useful in various microfluidic devices that need round corners and multi-depth channels.

Rapid fabrication of microchannels using microscale plasma activated templating (μPLAT) generated water molds

Poly(dimethylsiloxane) (PDMS) is a common material used in fabricating microfluidic devices. The predominant PDMS fabrication method, soft lithography, relies on photolithography for fabrication of micropatterned molds. In this technical note, we report an alternative molding technique using microscale PLasma Activated Templating (microPLAT). The use of photoresist in soft lithography is replaced by patterned water droplets created using microPLAT. When liquid PDMS encapsulates patterned water and then solidifies, the cavities occupied by water become structures such as microchannels. Using this method, device fabrication is less time consuming, more cost efficient and flexible, and ideal for rapid prototyping. An additional important feature of the water-molding process is that it yields structural profiles that are difficult to achieve using photolithography.

Rapid fabrication of a microfluidic device with integrated optical waveguides for DNA fragment analysis

The fabrication and performance of a microfluidic device with integrated liquid-core optical waveguides for laser induced fluorescence DNA fragment analysis is presented. The device was fabricated through poly(dimethylsiloxane) (PDMS) soft lithography and waveguides are formed in dedicated channels through the addition of a liquid PDMS pre-polymer of higher refractive index. Once a master has been fabricated, microfluidic chips can be produced in less than 3 h without the requirement for a cleanroom, yet this method provides an optical system that has higher performance than a conventional confocal optical assembly. Optical coupling was achieved through the insertion of optical fibers into fiber-to-waveguide couplers at the edge of the chip and the liquid-fiber interface results in low reflection and scattering losses. Waveguide propagation losses are measured to be 1.8 dB cm(-1) (532 nm) and 1.0 dB cm(-1) (633 nm). The chip displays an average total coupling loss of 7.6 dB due to losses at the optical fiber interfaces. In the electrophoretic separation and detection of a BK virus PCR product, the waveguide system achieves an average signal-to-noise ratio of 570 +/- 30 whereas a commercial confocal benchtop electrophoresis system achieves an average SNR of 330 +/- 30. To our knowledge, this is the first time that a waveguide-based system has been demonstrated to have a SNR comparable to a commercially available confocal-based system for microchip capillary electrophoresis.

Wetting by polymer solutions

Very small droplets of polystyrene dissolved in toluene were placed on a glass slide and studied under a goniometer microscope. The drops were allowed to spread and dry and their extent of spreading is plotted as a variation of the concentration of the polymer in the solution. It was observed that drops of polystyrene in a volatile solvent such as toluene, spread to some extent and then gets dried up thus preventing the contact line from moving further. When liquid PDMS in non-volatile solvent, HMDS, and in volatile solvent, toluene, was used, the droplets continued to spread. One conclusion among others, is that even if the droplet dries but leaves behind a mobile contact line then the solution spreads out, on the other hand if the droplet dries and the contact line loses its mobility, the spreading automatically comes to a stop.

Nanoscale Hydrophobic Recovery: A Chemical Force Microscopy Study of UV/Ozone-Treated Cross-Linked Poly(dimethylsiloxane)

Chemical force microscopy (CFM) in water was used to map the surface hydrophobicity of UV/ozone-treated poly(dimethylsiloxane) (PDMS; Sylgard 184) as a function of the storage/recovery time. In addition to CFM pull-off force mapping, we applied indentation mapping to probe the changes in the normalized modulus. These experiments were complemented by results on surface properties assessed on the micrometer scale by X-ray photoelectron spectroscopy and water contact-angle measurements. Exposure times of < or = 30 min resulted in laterally homogeneously oxidized surfaces, which are characterized by an increased modulus and a high segmental mobility of PDMS. As detected on a sub-50-nm level, the subsequent "hydrophobic recovery" was characterized by a gradual increase in the pull-off forces and a decrease in the normalized modulus, approaching the values of unexposed PDMS after 8-50 days. Lateral imaging on briefly exposed PDMS showed the appearance of liquid PDMS in the form of droplets with an increasing recovery time. Longer exposure times (60 min) led to the formation of a hydrophilic silica-like surface layer. Under these conditions, a gradual surface reconstruction within the silica-like layer occurred with time after exposure, where a hydrophilic SiOx-enriched phase formed < 100-nm-sized domains, surrounded by a more hydrophobic matrix with lower normalized modulus. These results provide new insights into the lateral homogeneity of oxidized PDMS with a resolution in the sub-50-nm range.

Adsorption of Hydrophobic Silica Nanoparticles at the PDMS Droplet−Water Interface

The adsorption of hydrophobic silica nanoparticles at the poly(dimethylsiloxane) (PDMS) droplet−water interface has been investigated through particle adsorption isotherms, with complementary studies of the adsorbed layer structure by freeze-fracture SEM. The influences of pH and electrolyte concentration as well as droplet cross-linking (deformability) are reported. Adsorption isotherms for liquid droplets are sigmoidal, whereas those for cross-linked droplets are of “low affinity” or “affinity increase” type. The surface coverage reaches values that correspond to multiple close-packed layers and are significantly influenced by droplet cross-linking, conferring extensive interfacial penetration, as confirmed by SEM. Densely packed adsorbed particle layers with interfacial aggregation are observed over a wide range of solution conditions. Both liquid and cross-linked PDMS droplets show pH-dependent adsorption, in agreement with DLVO theory; this is in contrast to hydrophilic silica adsorption [Langmuir 2003, 19, 3785]. Interfacial particle saturation occurred at a salt concentration 2 orders of magnitude less than the critical coagulation concentration (ccc) for hydrophobic silica in water. This phenomenon was independent of droplet cross-linking and indicates that particle interaction through the water phase plays a decisive role in particle packing at the interface. SEM indicated the presence of a rigid interfacial crust layer at salt concentrations corresponding to interfacial saturation and a multilayered interfacial particle wall at salt concentrations ≥ ccc.

Glass Transition Behavior of Polystyrene Blocks in the Cores of Collapsed Dry Micelles Tethered by Poly(Dimethylsiloxane) Coronae in a PS-b-PDMS Diblock Copolymer

The micellization of a polystyrene-b-poly(dimethylsiloxane) (PS-b-PDMS) diblock copolymer with the number-average molecular weights (Mn) of 193 000 g/mol for PS block and 39 000 g/mol for PDMS block and the glass transition behavior of the PS blocks in the cores of collapsed dry micelles with PDMS tethered coronae were studied. A range of micelle morphologies was observed by varying the micellization conditions and varying the ratio of the methylene chloride (MC) solvent and the n-dodecane (D) selective nonsolvent. MC is a solvent for both of the blocks, but D is a selective solvent for only the PDMS block. After completely removing the solvents below the glass transition temperature (Tg) of the initially solvated PS blocks in MC via extraction and vacuum-drying, the collapsed dry micelles, which were originally spheres and cylinders, became “tablets” or “ribbons”. The diameters of the “tablets” and widths of the “ribbons” were approximately 50 and 100 nm, respectively. The thicknesses of these textures ranged from 20 to 40 nm. The PS block cores of the dried micelles exhibited a Tg very close to that of the homo-PS bulk samples with the same Mn. However, for a broad temperature range below the TgPS, the collapsed dry micelles possessed heat capacity (Cp) values higher than the addition scheme calculated Cp based on PS and PDMS in their solid and liquid bulk states; whereas the Cp values of the liquid state above the TgPS fit well with the calculated data. These observations support the idea of PS interface layers with mobility induced by the liquid PDMS coronae such that only the inner cores of the PS blocks were in the glassy state. In a temperature region between 0 and 50 °C, the lower limit of the interface thickness (i.e., assuming that the mobile PS layers possessed the same mobility as the PS liquid) was estimated to be approximately 2 nm for the collapsed dry micelles. The interface thickness slightly increased with increasing temperature.

Spreading and surface tension gradient driven phenomena during heating of alumina-supported palladium crystallites in oxygen

Very small droplets of polystyrene dissolved in toluene were placed on a glass slide and studied under a goniometer microscope. The drops were allowed to spread and dry and their extent of spreading is plotted as a variation of the concentration of the polymer in the solution. It was observed that drops of polystyrene in a volatile solvent such as toluene, spread to some extent and then gets dried up thus preventing the contact line from moving further. When liquid PDMS in non-volatile solvent, HMDS, and in volatile solvent, toluene, was used, the droplets continued to spread. One conclusion among others, is that even if the droplet dries but leaves behind a mobile contact line then the solution spreads out, on the other hand if the droplet dries and the contact line loses its mobility, the spreading automatically comes to a stop.