Polydimethylsiloxane Sheet Research Articles

Transfer printing via a PAA sacrificial layer for wrinkle-free PDMS metallization

Polydimethylsiloxane (PDMS) metallization has been extensively used in varieties of micro- and nanosystem technologies. However, the deposited metal film on the PDMS surface is prone to wrinkling because of a large thermal mismatch stress, which may cause difficulties in some applications. In this work, a simple transfer printing method via a poly(acrylic acid) (PAA) sacrificial layer for PDMS metallization is presented. A PAA film was spin-coated on the silicon substrate, metal patterns were fabricated on the PAA film based on photolithography techniques, and a PDMS sheet was put on the surface of metal patterns and separated with metal patterns from the silicon substrate by dissolving the PAA film in room-temperature water. Au patterns with a variety of shapes and sizes were successfully transferred on the PDMS sheet, and there were no wrinkles in Au patterns. As a demonstration, a PDMS-based tunable diffraction grating with a line width of 2 µm and an initial period of 4 µm was fabricated. The grating displayed excellent diffraction efficiencies, and the period could be tuned by simply stretching the grating.

Smart Stretchable Janus Membranes with Tunable Collection Rate for Fog Harvesting

AbstractWater scarcity is one of the severest global resource shortages because freshwater plays a vital role in human existence. Fog harvesting is a promising approach to alleviate freshwater shortage, but most existing strategies based on rigid materials suffer from the shortage of nontunable fog collection rate. Herein, a smart Janus membrane with a tunable fog collection rate via uniaxially stretching a flexible polydimethylsiloxane sheet fabricated by ultrafast laser drilling is reported. Compared to its original form, the maximum strain of 200% shows up to 67% enhancement in its fog collection efficiency. Further theoretical calculation demonstrates that the increase of fog collection rate along with uniaxially stretching is estimated to be ≈79% for 200% strain sample based on the cooperation between the wetting driving force and Laplace pressure, which is in well agreement with the experimental result. It is believed that this smart Janus membrane would operate not only in the field of fog harvesting, but also in specific scenarios, such as dynamic fog‐flux regulators.

All-Optical Cryogenic Thermometry Based on Nitrogen-Vacancy Centers in Nanodiamonds

The nitrogen-vacancy (NV) center in diamond has been recognized as a high-sensitivity nanometer-scale metrology platform. Thermometry has been a recent focus, with attention largely confined to room temperature applications. Thermometry has been a recent focus, with attention largely confined to room temperature applications. Temperature sensing at low temperatures, however, remains challenging as the sensitivity decreases for many commonly used techniques which rely on a temperature dependent frequency shift of the NV centers spin resonance and its control with microwaves. Here we use an alternative approach that does not require microwaves, ratiometric all-optical thermometry, and demonstrate that it may be utilized to liquid nitrogen temperatures without deterioration of the sensitivity. The use of an array of nanodiamonds embedded within a portably polydimethylsiloxane (PDMS) sheet provides a versatile temperature sensing platform that can probe a wide variety of systems without the configurational restrictions needed for applying microwaves. With this device, we observe a temperature gradient over tens of microns in a ferromagnetic-insulator substrate (YIG) under local heating by a resistive heater. This thermometry technique provides a cryogenically compatible, microwave-free, minimally invasive approach capable of probing local temperatures with few restrictions on the substrate materials.

Porous Polydimethylsiloxane–Silver Nanowire Devices for Wearable Pressure Sensors

We demonstrate a simple, nonlithographic method for fabricating piezoresistive pressure sensors with a broad range of working pressures and low detection limit. Our wearable pressure sensor is fabricated by using two metallized, porous polymer layers which undergo a change in resistance as a function of pressure. This sensor has a sandwich structure composed of top and bottom sheets of porous polydimethylsiloxane (PDMS) fabricated by using a simple templating method. The inner face of each of these layers is coated with a layer of conductive silver nanowires (AgNWs). The AgNW layers are initially in light contact, and the device undergoes a change in resistance when pressure is applied perpendicular to the plane of the sheets. In these devices, the size of the pores is an important determinant of the device sensitivity and range. Powder templates of KCl, NaCl, and sugar are used to create different sized pores (60–90, 200–275, and 400–550 μm, respectively) in the elastomeric PDMS layers, which in turn affects the mechanical deformability of the devices. The sensors fabricated with KCl porous layers (which had the smallest pores) demonstrate the best performance, with a sensitivity of 14.1 kPa–1 (up to 3.5 kPa), 4.8 kPa–1 (up to 10 kPa), and 1.84 kPa–1 (up to 40 kPa) and good stability over 1000 loading and unloading cycles. Pressure sensors are also fabricated by decreasing the ratio of PDMS base to curing agent from the standard 10:1 formulation, which increases the stiffness of the viscoelastic PDMS layer and thus shortens the response time. The stiffest formulation (5:1 (base:curing agent)) sample gives the shortest response time of ∼47 ms. Wearable electronic applications of these devices, including pulse measurement, facial movements, and sound tracking, are demonstrated in this work.

Characterization of four functional biocompatible pressure-sensitive adhesives for rapid prototyping of cell-based lab-on-a-chip and organ-on-a-chip systems

In the advent of affordable photo- and soft-lithography using polydimethylsiloxane (PDMS), low cost multi-step microfabrication methods have become available to a broad scientific community today. Although these methods are frequently applied for microfluidic prototype production in academic and industrial settings, fast design iterations and rapid prototyping within a few minutes with a high degree of flexibility are nearly impossible. To reduce microfluidic concept-to-chip time and costs, a number of alternative rapid prototyping techniques have recently been introduced including CNC micromachining, 3D printing and plotting out of numeric CAD designs as well as micro-structuring of thin PDMS sheets and pressure sensitive adhesives. Although micro-structuring of pressure sensitive adhesives promises high design flexibility, rapid fabrication and simple biochip assembly, most adhesives are toxic for living biological systems. Since an appropriate bio-interface and proper biology-material interaction is key for any cell chip and organ-on-a-chip system, only a limited number of medical-grade materials are available for microfluidic prototyping. In this study, we have characterized four functional biomedical-grade pressure sensitive adhesives for rapid prototyping (e.g. less than 1 hour) applications including structuring precision, physical and optical properties as well as biocompatibilities. While similar biocompatibility was found for all four adhesives, significant differences in cutting behavior, bonding strength to glass and polymers as well as gas permeability was observed. Practical applications included stability testing of multilayered, membrane-integrated organ-on-a-chip devices under standard cell culture conditions (e.g. 2–3 weeks at 37 °C and 100% humidity) and a shear-impact up to 5 dynes/cm2. Additionally, time- and shear-dependent uptake of non-toxic fluorescently labelled nanoparticles on human endothelial cells are demonstrated using micro-structured adhesive-bonded devices. Our results show that (a) both simple and complex microdevices can be designed, fabricated and tested in less than 1 hour, (b) these microdevices are stable for weeks even under physiological shear force conditions and (c) can be used to maintain cell monolayers as well as 3D cell culture systems.

A flexible tactile sensor integrated with carbon black/carbon nanotube composite film and flexible printed circuit

In this study, two kinds of composite films, carbon black (CB) and carbon nanotube (CNT) composite films, were fabricated and integrated with flexible printed circuit (FPC) for the flexible tactile sensor. CB and CNT with high compressive strength and low resistivity were immersed into the tissue paper as the sensing materials. The flexible tactile sensors were fabricated by sandwiching the CB/CNT composite film with FPC between two thin polydimethylsiloxane sheets. The tactile sensor is operated by the contact resistance change between CB/CNT composite film and FPC electrode by the different applied force. The measurements of the sensitivity, response time and stability of the fabricated tactile sensors were carried out. The fabricated CB composite film tactile sensor exhibits the sensitivity of 5.05 × 10−2 kPa−1 in the range below 20 kN m−2, the response time of 10 ms, and with the high stability of the loading and unloading pressure.

Measurement of Frictional Properties of Aortic Stent Grafts and Their Delivery Systems

Stent grafts are medical devices used to treat abdominal aortic aneurysms (AAAs) in endovascular aneurysm repair (EVAR). Computational and experimental models have been developed to study stent graft delivery and deployment during EVAR; however, frictional properties have not been taken into account in most previous studies. The objective of this study was to determine the coefficients of friction of three commercially available stent grafts (Cook Zenith, Medtronic Endurant, and Vascutek Anaconda), their delivery sheaths, a porcine aorta, and two mock arterial materials. Stent grafts were obtained and separated into stents, graft fabric, and sheaths. Using a custom-made friction measurement apparatus, the coefficients of friction were measured between five material pairs: (i) the stents and inner surface of the sheath, (ii) the graft fabric and inner surface of the sheath, (iii) the outer surface of the sheath and a porcine aorta, (iv) the outer surface of the sheath and three different polyvinyl alcohol (PVA) cryogels, and (v) the outer surface of the sheath and a polydimethylsiloxane (PDMS) sheet. The results show that the coefficients of friction between the graft fabric and the sheath were higher than those between the stents and the sheath. The PVA cryogels showed more comparable frictional properties to the porcine aorta than did the PDMS sheet, suggesting that PVA cryogels provide a more accurate approximation for the in vivo frictional properties. These results can be used to improve the accuracy of computational models for stent graft delivery and deployment and to select appropriate materials for vascular phantoms.

Patterning defects in high-speed reverse offset printing: lessons from contact dynamics

Printing speed limitation is a critical issue for the high-throughput production in printed electronics. In this study, the formation mechanism of severe rash-type defects in a reverse-offset printing system operating at a high speed is discussed. A systematic investigation showed that the size and density of the defects were significantly influenced by the printing speed, indentation and by the material properties, such as thickness and elasticity of the polydimethylsiloxane (PDMS) sheets. The observation via a high-speed camera of the nip formations between a curved PDMS sheet and a flat glass revealed that, at the very initial stage of contact formation, parts of contacts were concurrently and unstably formed then widened; however, during the later stage, the coalescence of these spots led to the generation of air bubbles trapped in the contact regions, which lasted for several seconds. Based on complementary experiments on the extinction behaviours of the trapped air, we propose a new defect formation model for the rash-like transfer defects observed in the printing tests due to the trapped air regions resulting from the contact instability of PDMS.

Establishing Liposome-Immobilized Dexamethasone-Releasing PDMS Membrane for the Cultivation of Retinal Pigment Epithelial Cells and Suppression of Neovascularization.

Age-related macular degeneration (AMD) is the eye disease with the highest epidemic incidence, and has great impact on the aged population. Wet-type AMD commonly has the feature of neovascularization, which destroys the normal retinal structure and visual function. So far, effective therapy options for rescuing visual function in advanced AMD patients are highly limited, especially in wet-type AMD, in which the retinal pigmented epithelium and Bruch’s membrane structure (RPE-BM) are destroyed by abnormal angiogenesis. Anti-VEGF treatment is an effective remedy for the latter type of AMD; however, it is not a curative therapy. Therefore, reconstruction of the complex structure of RPE-BM and controlled release of angiogenesis inhibitors are strongly required for sustained therapy. The major purpose of this study was to develop a dual function biomimetic material, which could mimic the RPE-BM structure and ensure slow release of angiogenesis inhibitor as a novel therapeutic strategy for wet AMD. We herein utilized plasma-modified polydimethylsiloxane (PDMS) sheet to create a biomimetic scaffold mimicking subretinal BM. This dual-surface biomimetic scaffold was coated with laminin and dexamethasone-loaded liposomes. The top surface of PDMS was covalently grafted with laminin and used for cultivation of the retinal pigment epithelial cells differentiated from human induced pluripotent stem cells (hiPSC-RPE). To reach the objective of inhibiting angiogenesis required for treatment of wet AMD, the bottom surface of modified PDMS membrane was further loaded with dexamethasone-containing liposomes via biotin-streptavidin linkage. We demonstrated that hiPSC-RPE cells could proliferate, express normal RPE-specific genes and maintain their phenotype on laminin-coated PDMS membrane, including phagocytosis ability, and secretion of anti-angiogenesis factor PEDF. By using in vitro HUVEC angiogenesis assay, we showed that application of our membrane could suppress oxidative stress-induced angiogenesis, which was manifested in decreased secretion of VEGF by RPE cells and suppression of vascularization. In conclusion, we propose modified biomimetic material for dual delivery of RPE cells and liposome-enveloped dexamethasone, which can be potentially applied for AMD therapy.

A novel method for quantifying traction forces on hexagonal micropatterned protein features on deformable poly-dimethyl siloxane sheets

Many methods exist for quantifying cellular traction forces, including traction force microscopy and microfabricated post arrays. However, these methodologies have limitations, including a requirement to remove cells to determine undeflected particle locations and the inability to quantify forces of cells with low cytoskeletal stiffness, respectively. Here we present a novel method of traction force quantification that eliminates both of these limitations. Through the use of a hexagonal pattern of microcontact-printed protein spots, a novel computational algorithm, and thin surfaces of polydimethyl siloxane (PDMS) blends, we demonstrate a system that:•quantifies cellular forces on a homogeneous surface that is stable and easily manufactured.•utilizes hexagonal patterns of protein spots and computational geometry to quantify cellular forces without need for cell removal.•quantifies cellular forces in cells with low cytoskeletal rigidity.

Perfect blackbody sheets from nano-precision microtextured elastomers for light and thermal radiation management

Microtextured polydimethylsiloxane sheets exhibit an exceptionally low reflectance of ≲0.0005 across the entire thermal infrared wavelengths while maintaining high resilience.

Perfusion microbioreactor system with permeable membranes to monitor bacterial growth

AbstractBACKGROUNDMicrofluidics offers itself as a potential platform for optimizing microbial growth in different substrate concentrations using a single assay. The aim of this study was to construct a reversible microfluidic device for monitoring the bacterial growth in chambers with different substrate concentrations with its respective technical triplicates. The convective concentration gradient generator (CCG) system was constructed with three inlets for solutions with different concentrations that distributed them to the sequential cultivation chambers (CCs). A perfusion microbioreactor system (PM) was constructed using a commercial polydimethylsiloxane (PDMS) sheet and glass, with an adapted semi‐permeable membrane system to constrain the cells within the CC. Cell growth of a fluorescent Escherichia coli JM109 was monitored by fluorescence microscope with time‐lapse technique.RESULTSThe growth profile of E. coli with different streptomycin concentrations and its specific growth rates (µx) in the microfluidics device (µx of 0.0088 and 0.0092 h−1) were compared in batch cultivations, which show no significant difference between the two methods. In addition, the half maximal inhibitory concentration (IC50) values indicate that continuous perfusion surpasses consumption of the tested drug.CONCLUSIONThe results demonstrate the efficiency of the microfluidic device for cell cultivation and its applicability in industrial biotechnology, allowing rapid screening of multiple parameters. © 2018 Society of Chemical Industry

Integrated Silicon-on-Insulator Spectrometer With Single Pixel Readout for Mid-Infrared Spectroscopy

Mid-infrared spectroscopy in the 2–4 $\mu$ m wavelength range is of cardinal value for many sensing applications. Current solutions involve bulky and expensive systems to operate. Silicon-on-Insulator (SOI) waveguide technology offers means to miniaturize the different parts of the spectrometer. However, the development of on-chip detectors for the mid-infrared wavelength range is in its infancy and the characteristics are not on par with their discrete (cooled) counterparts. In this work, a compact and cheap mid-infrared spectrometer is realized by integrating a SOI arrayed waveguide grating (AWG) spectrometer operating in the 2.3 $\mu$ m wavelength range with a high performance mid-infrared photodiode. The AWG has 12 output channels with a spacing of 225 GHz (4 nm) and a free spectral range of 3150 GHz (56 nm), which are simultaneously collected by a single, transistor outline (TO)-packaged extended InGaAs PIN photodiode. The response of each AWG channel is discerned by time-sequentially modulating the optical power in each output channel using integrated Mach-Zehnder based thermo-optic modulators with a $\pi$ -phase shift power consumption of $\approx$ 50 mW. As an example, the absorption spectrum of a 0.5 mm thick polydimethylsiloxane sheet is sampled and compared to a benchtop spectrometer to good agreement.

Surface modification of silicone with colloidal polysaccharides formulations for the development of antimicrobial urethral catheters

In this work, surfaces of casted silicone sheets and silicone tubes were functionalized with colloidal polysaccharide complexes (chitosan, carboxymethyl chitosan, and hyaluronic acid in combination with a lysine-based surfactant) in order to introduce antimicrobial and antifouling surface properties. The surface chemistry and morphology, as well as the chemical and mechanical stability of the coatings were characterized. For this purpose, different microscopic and spectroscopic methods, pH-potentiometric titrations and standard methods for evaluation of mechanical properties were used. Finally, the antimicrobial and antifouling properties of functionalized silicone materials were evaluated in vitro. The findings of the physicochemical characterization showed that a discontinuous 3-step dip-coating process can be successfully implemented to coat casted polydimethylsiloxane sheets and tubes with colloidal polysaccharide complexes. These coatings exhibit slow-release leaching in aqueous environment at pH 4.5, 7, and 8 and show very good stability against mechanical abrasion, thus displaying high stability potential during catheter insertion. The antimicrobial properties against gram-positive bacteria, gram-negative bacteria and fungi, showed inhibition of bacterial growth of up to 86%. Furthermore, the bacterial biofilm formation tests have revealed that the hyaluronic acid-surfactant coating exhibits high biofilm growth reduction.

A Laser-Engraving Technique for Portable Micropneumatic Oscillators.

Microfluidic automation technology is at a stage where the complexity and cost of external hardware control often impose severe limitations on the size and functionality of microfluidic systems. Developments in autonomous microfluidics are intended to eliminate off-chip controls to enable scalable systems. Timing is a fundamental component of the digital logic required to manipulate fluidic flow. The authors present a self-driven pneumatic ring oscillator manufactured by assembling an elastomeric sheet of polydimethylsiloxane (PDMS) between two laser-engraved polymethylmethacrylate (PMMA) layers via surface activation through treatment with 3-aminopropyltriethoxysilane (APTES). The frequency of the fabricated oscillators is in the range of 3–7.5 Hz with a maximum of 14 min constant frequency syringe-powered operation. The control of a fluidic channel with the oscillator stages is demonstrated. The fabrication process represents an improvement in manufacturability compared to previous molding or etching approaches, and the resulting devices are inexpensive and portable, making the technology potentially applicable for wider use.

Elastomer thin-film pressure sensor based on embedded photonic tunnel-junction arrays

We propose an elastomer thin-film pressure sensor enabled by pressure-sensitive optical signals through vertical photonic tunnel-junction couplers. We provide the operation principle, design, fabrication, and test results from a 50μm thick polydimethylsiloxane sheet accommodating embedded vertical photonic tunnel-junction couplers. The result with a 5mm long device shows a differential optical power change that is ∼140% of the incident power under moderate external pressure of ∼40 kPa, thereby clearly demonstrating a robust pressure-sensing capability realized in a highly flexible, lightweight, transferrable, optically transparent, and bio-compatible thin-film material. Therefore, the proposed approach potentially enables versatile pressure and touch sensors for many applications in practice.

Microfluidic-based flexible reflective multicolor display

This paper describes a microfluidic-based flexible reflective display constructed using dyed water droplets and air gaps as pixel elements. Our display is composed of a flexible polydimethylsiloxane sheet with a connected pixel-patterned microchannel. Several types of dyed water droplets and air gaps are sequentially introduced to the microchannel through a suction process to display a multicolor image. The displayed image is stable and can be retained without an energy supply. To ensure that images are displayed correctly, the geometric parameters of the dot pixel design and minimum differential pressure necessary to drive the water droplets are evaluated. As a demonstration, we successfully display three-color dot-matrix reflective images and bitmap characters in the microchannel. Our proposed method can be applied to energy-less and color-changeable displays for use in future daily-life accessories, such as bags, shoes, and clothes, and can change the surface color and pattern of these accessories.

Synthesis and Characterization of Thermoplastic PDMS

Polydimethylsiloxane (PDMS), a silicone based thermosetting elastomer, has been widely used because of its biocompatibility, low cost, low toxicity, high oxidative and thermal stability, optical transparent, low permeability to water, low electrical conductivity, and ease of micropatterning [1, 2]. Various nanocomposites have been fabricated employing PDMS, that are electrically conductive [3] magnetic [4, 5, 6] in nature and can be easily micromolded using conventional hybrid soft lithography techniques [7, 8]. With the emerge of 3D printing, we present synthesis of thermosplastic PDMS, which can be 3D printed and is recyclable. The crosslinked recyclable PDMS was prepared using the reaction between PDMS-diglycidyl ether（17.6mmol） and pripol 1017 (17.6mmol) with Zn(OAc) (0.88mmol)2 as the catalyst. The crosslinking process completed after 12-hour at 130 °C [8]. The prepared PDMS film and sheets were cut to small pieces using a palletizer and passed through a filament extruder to make PDMS filament. Their mechanical, thermal and solvent resistance properties were characterized and appropriate conditions for 3D printing has been optimized.

2D Optical Gratings Based on Hexagonal Voids on Transparent Elastomeric Substrate.

A chromatic vectorial strain sensor constituted by hexagonal voids on transparent elastomeric substrate has been successfully fabricated via soft colloidal lithography. Initially a highly ordered 1.6 microns polystyrene spheres monolayer colloidal crystal has been realized by wedge-shaped cell method and used as a suitable mold to replicate the periodic structure on a polydimethylsiloxane sheet. The replicated 2D array is characterized by high periodicity and regularity over a large area, as evidenced by morphological and optical properties obtained by means of SEM, absorption and reflectance spectroscopy. In particular, the optical features of the nanostructured elastomer have been investigated in respect to uniaxial deformation up to 10% of its initial length, demonstrating a linear, tunable and reversible response, with a sensitivity of 4.5 ± 0.1 nm/%. Finally, it has been demonstrated that the specific geometrical configuration allows determining simultaneously the vectorial strain-stress information in the x and y directions.

Reverse Offset Printing of Semidried Metal Acetylacetonate Layers and Its Application to a Solution-Processed IGZO TFT Fabrication.

The submicrometer resolution printing of various metal acetylacetonate complex inks including Fe, V, Mn, Co, Ni, Zn, Zr, Mo, and In was enabled by a robust ink formulation scheme which adopted a ternary solvent system where solubility, surface wettability, and drying as well as absorption behavior on a polydimethylsiloxane sheet were optimized. Hydrogen plasma in heated conditions resulted in bombarded, resistive, or conductive state depending on the temperature and the metal species. With a conductivity-bestowed layer of MoO x and a plasma-protecting layer of ZrO x situated on the top of an IGZO layer, a solution-processed TFT exhibiting an average mobility of 0.17 cm2/(V s) is demonstrated.