Polydimethylsiloxane Thin Films Research Articles

Muscular Thin Films for Label-Free Mapping of Excitation Propagation in Cardiac Tissue.

Muscular thin films (MTFs), have already found a variety of applications in cardiac tissue engineering and in building of lab-on-a-chip systems. Here we present a novel approach to label-free mapping of excitation waves in the cardiomyocyte cell cultures with the use of MTFs. Neonatal rat ventricular cardiomyocytes were cultured on polydimethylsiloxane (PDMS) thin films and observed by means of off-axis illumination. Inflexions of the membrane created by the contraction of cardiomyocytes led to formation of patterns of bright and dark areas on the surface of the membrane. These patterns were recorded and analyzed for the monitoring of the contraction propagation. The method was compared with a standard optical mapping technique based on the use of a Ca2+-sensitive fluorescent dye. A good consistency of the results obtained by these two methods was demonstrated. The proposed method is non-toxic and might be of particular interest for the purpose of continuous monitoring in test systems based on human induced pluripotent stem cells.

Comment on "Bioinspired Reversible Switch between Underwater Superoleophobicity/Superaerophobicity and Oleophilicity/Aerophilicity and Improved Antireflective Property on the Nanosecond Laser-Ablated Superhydrophobic Titanium Surfaces".

Laser-textured surfaces enabling reversible wettability switching and improved optical properties are gaining importance in cutting-edge applications, including self-cleaning interfaces, tunable optical lenses, microfluidics, and lab-on-chip systems. Fabrication of such surfaces by combining nanosecond-laser texturing and low-temperature annealing of titanium Ti-6Al-4V alloy was demonstrated by Lian et al. in ACS Appl. Mater. Inter. 2020, 12 (5), 6573–6580. However, it is difficult to agree with (i) their contradictory explanation of the wettability transition due to low-temperature annealing and (ii) their theoretical description of the optical behavior of the laser-textured titanium surface. This comment provides an alternative view—supported by both experimental results and theoretical investigation—on how the results by Lian et al. could be interpreted more correctly. The annealing experiments clarify that controlled contamination is crucial in obtaining consistent surface wettability alterations after low-temperature annealing. Annealing of laser-textured titanium at 100 °C in contaminated and contaminant-free furnaces leads to completely different wettability transitions. Analysis of the surface chemistry by XPS and ToF-SIMS reveals that (usually overlooked) contamination with hydrophobic polydimethylsiloxane (PDMS) may arise from the silicone components of the furnace. In this case, a homogeneous thin PDMS film over the entire surface results in water repellency (contact angle of 161° and roll-off angle of 15°). In contrast, annealing under the same conditions but in a contaminant-free furnace preserves the initial superhydrophilicity, whereas the annealing at 350 °C turns the hydrophobicity “off”. The theoretical calculations of optical properties demonstrate that the laser-induced oxide layer formed during the laser texturing significantly influences the surface optical behavior. Consequently, the interference of light reflected by the air–oxide and the oxide–metal interfaces should not be neglected and enables several advanced approaches to exploit such optical properties.

Qualitative theoretical modeling to study the possibility of detecting multi-virus in blood flow using Nano-quartz crystal microbalance.

Methods for testing the presence of a virus in the blood are of interest to researchers and doctors because they determine how rapidly a virus is detected. In general, virus detection is a major scientific problem due to the serious effects of viruses on the human body. At present, only one virus can be detected in a single test. This potentially costs the medical establishment more time and money that could be saved if blood testing was more efficient. This study presents a qualitative method to enable doctors and researchers to detect more than one virus simultaneously. This was performed using quartz nanoparticles. Using polymer thin films of polydimethylsiloxane (PDMS), each chip emits a different frequency for each specific type of virus on the chip. The multiplicity of these chips allows for the detection of a number of viruses with the same number of nanoscale chips simultaneously. Blood flow around quartz nanoparticles was modelled. In this model, several conventional Quartz Crystal Microbalance (QCM) with nanostructures (Nano-QCM) particles are inserted into the three main types of blood vessels. The results showed that the best location for the Nano-QCM is the large artery and that it is possible to test for a number of viruses in all types of blood vessels.

3D Printing Customized Optical Lens in Minutes

AbstractSynergizing grayscale photopolymerization and meniscus coating processes, rapid 3D printing of optical lenses is reported previously using projection microstereolithography (PµSL) process. Despite its 14 000‐fold‐improved printing speed over the femtosecond 3D printing process, PµSL still consumes significant amount of the fabrication time for precise recoating 5 µm thick fresh resin layers. At the reported speed of 24.54 mm3 h−1, 3D printing of the millimeter‐size lenses still takes hours. To further improve the printing speed, the microcontinuous liquid interface production process is implemented to eliminate the time‐consuming resin recoating step. However, the micrometer‐size pores in the Teflon membrane needed for oxygen transportation are found to completely spoil the surface smoothness. The use of polydimethylsiloxane thin film possessing much refined nanoscopic porosities as the functional substitute of Teflon membrane is reported to significantly reduce the surface roughness to 13.7 nm. 3D printing of 3 mm high aspherical lens in ≈2 min at a 200‐fold‐improved speed at 4.85 × 103 mm3 h−1 is demonstrated. The 3D printed aspherical lens has the demonstrated imaging resolution of 3.10 µm. This work represents a significant step in tackling the speed‐accuracy trade‐off of 3D printing process and thus enables rapid fabrication of customized optical components.

Reusable polystyrene wafer coating as an antiadhesive layer for PDMS thin film production

Polydimethylsiloxane (PDMS) is a commonly used material in many fields such as microfluidics and sensor technology. Despite its abundant usage, there is still need for easier and less repetitive methods of PDMS thin film fabrication. In this work we present a wafer preparation technique which allows multiple PDMS castings onto the substrate without the need of retreatment. A polystyrene thin film was spin coated onto the wafer which serves as a PDMS antiadhesive layer. The coating was characterized before and after PDMS preparation, remaining unchanged throughout processing. Furthermore, a PDMS thin film fabricated on the coated wafer was bonded to a thicker PDMS base yielding a membrane device. The device was demonstrated as an optical pressure sensor.

Eddy current pressure sensor based on planar coil

The traditional pressure sensors are mostly based on piezoresistive, piezoelectric effect, and strain gauge, which are easily affected by ambient temperature. The structure of some sensors is rather complex to effectively acquire the signal or measure distribution force. Therefore, a pressure sensor based on the planar coil and elastic element was designed to solve this problem. The newly designed sensor has a spiral planar coil made by a printed circuit board and a polydimethylsiloxane (PDMS) thin film as the elastic element. The conductive layer is attached on the upper surface of PDMS to realise the eddy current effect. Meanwhile, a simulation model was established to find the relationship between deformation displacement and the pressure of the elastic element. By generating the experimental curves of the deformation displacement of the elastic element and the output frequency of the eddy current sensor, the pressure measurement on the surface of the elastic element can be identified. The preliminary experimental results showed that the eddy current pressure sensor based on the plane coil has good linearity and repeatability, and it can realise large area distributed pressure measurement, which is also suitable for accurate measurement of distribution force.

Controlled Gold Nanoparticle Placement into Patterned Polydimethylsiloxane Thin Films via Directed Self-Assembly

An economically scalable and reproducible method to assemble nanoparticles (NPs) into ordered arrays with high fidelity remains a fundamental roadblock. Methods like directed self-assembly have shown the highest promise resulting in >85% density of NP-filled prepatterned polymer cavities. This work refines directed self-assembly by controlling the evaporation rate, substrate velocity (deposition rate), and NP diameter resulting in reproducible fabrication of ordered arrays with areas >2 mm×2 mm and ~100% density of filled cavities. Measured optical spectra showed a blueshift in the localized surface plasmon resonance (LSPR) and surface lattice resonance (SLR) peaks with increasing NP density for both 100 and 150 nm gold (Au) NPs. Discrete dipole approximation (DDA), coupled dipole approximation (CDA), rapid semi-analytical CDA (rsa-CDA), and Mie theory simulations closely matched extinction per nanoparticle (extinction/NP) calculations for measured extinction spectra. An ordered array containing 150 nm AuNPs was used for comparison with rsa-CDA estimates using both polydimethylsiloxane (PDMS) and glass refractive indices (RI) resulting in peak location estimates within 1.7% and comparable relative increases in peak heights. Both the measured and simulated SLR peak heights were shown to significantly increase when the array was on glass as opposed to within PDMS.

Stretchability enhancement of electronics according to the surface patterns fabricated by thermal reflow process.

Fabricating flexible and stretchable substrate for circuits is very important to make biomedical devices like biosensors or neural prosthesis. In this paper, the resistance changes of the polydimethylsiloxane (PDMS) film are observed when it is stretched. And the stretchability is measured and classified according to the shape of the engraved patterns. Patterns are fabricated on Si wafer by lithography. Thermal reflow process is conducted to modify the shape of the tips and we used spin coating method to make thin PDMS film. PDMS film is cut into uniform small pieces and stretched at a constant rate by the syringe pump. The resistance of the film is measured while film is stretched. According to the curvature and depth of the groove on the surface, the degree of the improvement varies. The deeper and round patterns enhance the stretchability of the film but sharp and shallow patterns degrade it.

Increasing surface-enhanced Raman scattering density using gold-coated magnetic nanoparticles controlled via a magnetic field for sensitive and efficient biomarker detection

Early detection of various diseases is expected using surface-enhanced Raman scattering (SERS). For example, a method of labeling an antibody of a disease-related molecule on metal nanoparticles and detecting the SERS signals of the particles bound to the antigen is a promising approach. However, the problems of a slow antigen–antibody reaction and low sensitivity remain unsolved. In this study, we fabricated nanoparticles that can be freely moved using an external magnetic field for rapid antigen–antibody reaction and also nanoengineered the substrate to increase the density of hotspots required for SERS. Gold-coated magnetic nanoparticles (Au-MNPs) with a core–shell structure were prepared by applying multiple coatings of gold onto magnetic iron(II,III) oxide nanoparticles, which were used as the core. A neodymium magnet easily moved and converged the Au-MNPs in the solution within a few seconds. In addition, a silver nanoparticle substrate (Ag-NS) with a hexagonal close-packed structure fixed on a polydimethylsiloxane thin film was prepared, and the stable generation of SERS was confirmed over the entire substrate. Upon aggregation of the Au-MNPs onto Ag-NS using a neodymium magnet, the total SERS strength per unit area drastically increased, suggesting that the combination of Au-MNPs and Ag-NS increased the density of the generated hotspots. In future work, with the labeling of antibodies onto Au-MNPs, we expect the proposed method to be applied in the sensitive measurement of biomarkers associated with diseases.

Stretchable interconnections based on horseshoe shape metal nanowires

In this paper, we presented the fabrication and testing of a new type of stretchable interconnections based on horseshoe shape metal nanowires. The stretchable interconnections could serve for stretchable electronics which are widely used in the field of tactile sensor, surgical devices, intelligent manufacturing and so on. The significant advantage of the stretchable interconnections is that excellent stretchability and outstanding conductivity are obtained simultaneously. The proposed horseshoe shape stretchable interconnections were fabricated by embedding silver nanowires (AgNWs) into the surface of a polydimethylsiloxane (PDMS) thin film. The results from the stretching tests have demonstrated that the stretchable interconnections has a stable resistance (around 15[Formula: see text]) after repeated stretching and release. Moreover, the resistance increment in the 30% stretched state after fifty stretching cycles is only 138% of the initial value. With the good conductivity and stability during stretching, the stretchable interconnections developed in this paper are applicable to stretchable electronic devices.

A study of the heat transfer properties of CIP doped magnetorheological elastomers

This work investigates the enhancement of thermal transport in polydimethylsiloxane (PDMS) thin films with aligned carbonyl iron particles (CIP) embedded, also known as magnetorheological elastomers (MREs). The flash method is used to measure the effective thermal conductivity of PDMS/CIP thin films in the direction parallel to the magnetic field. Under an external magnetic field, the CIPs form chain-like structures, which become effective thermal paths inside the composites. Hence, a significant increase in the thermal conductivity of aligned MREs has been achieved compared with the composites prepared without a magnetic field. The effects of the volume fraction of CIP and pre-structured magnetic flux density have also been studied. The increase in the CIP volume fraction from 5% to 20% brings about a twofold improvement in the thermal conductivity of the aligned MREs. For 2.5 μm CIP doped MREs with a volume fraction of 10%, when the pre-structured magnetic flux density changes from 0 mT to 100 mT, the thermal conductivity k increases by approximately 50%. However, a further increase in the magnetic field intensity only leads to a slight increase in k. Scanning electron microscopy (SEM) inspections demonstrate that the chain length keeps growing with the strength of the pre-structured magnetic field while the structure transition turns into lateral congregation as the strength of the pre-structured magnetic field reaches 100 mT. A finite element method (FEM) model is proposed to investigate in detail the relationship between the MREs’ thermal conductivity and inner chains. This work provides an effective way to improve thermal conductivity of MREs used for electromagnetic radio-absorbers and builds a connection between the thermal properties of MREs and their inner microstructures.

Fabrication of ionic polymer metal composite for bio-actuation application: sputtering and electroless plating methods

In this study, Ionic polymer metal composites were fabricated for bio-actuation applications using chemical electroless plating (CEP) and physical sputtering (PS) methods. To use IPMC in biomedical devices, it was encapsulated by a polydimethylsiloxane (PDMS) thin film which not only makes it biocompatible but also decreases the water loss from the IPMC channels resulting in longer IPMC actuation. The fabricated IPMCs in both methods were characterized and compared to each other in each stage of fabrication and actuation performance. CEP IPMC exhibited superior properties in all examinations. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay test showed a 92% cell viability by coating PDMS on the CEP IPMC. The CEP IPMC presented a 48% higher bending degree at the same voltages compared to the PS kind. This may be due to the low adhesion of the sputtered deposited electrode. Surprisingly, due to its rigidity, the PS IPMC disclosed a 33% higher output force than the CEP one at the same voltage.

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.

Confinement-Induced Stiffening of Elastomer Thin Films.

While entropy is understood to dictate the elastic mechanics of elastomers, a consensus has not been reached regarding why they stiffen when confined to thin films or even the magnitude of this effect. Here, we present a combined computational and experimental approach to measure the confinement-induced stiffening of thin elastomer films using nanoindentation. First, we use finite element computations to modify the adhesion-free contact model to include the presence of a rigid substrate and verify this correction using indentation experiments on macroscopic polydimethylsiloxane (PDMS) films. Next, we validate this correction factor in indentation experiments where adhesion plays a major role through a series of experiments on micrometer-scale PDMS films. With this correction factor in hand, we perform an extensive series of nanoindentation experiments on submicrometer thickness films of varying cross-link density using a variety of probe radii and probe surface energies. We observe a consistent stiffening of all PDMS thin films that, interestingly, converges to a consistent elastic modulus at thicknesses ∼100 nm regardless of the cross-link density. To explain these results, we propose a surface cross-linking model where the density of crosslinks between chains increases near the surface of the film because of oxidation-induced bonds between chains. Importantly, the apparent moduli predicted by this entropic model are in good agreement with all experimental results. Collectively, these results shed new light on the nanomechanics of elastomers and provide a general method for predicting the size-dependent effective moduli of soft polymers.

Investigating the mechanical and optical properties of thin PDMS film by flat-punched indentation

We investigated mechanical and optical properties of a thin polydimethylsiloxane (PDMS) film through a flat-punched indentation experiment and a finite element simulation. A microscopic imaging method was used to measure the compressive strain of thin PDMS films, and its corresponding bulk refractive index (RI) was estimated using the relation between density and refractive index. A total internal reflection (TIR) experiment was conducted to estimate the local RI values near the bottom surface of PDMS film. We obtained the correlations between force and indentation displacement in thin PDMS films (10–70 micron). Stress-strain curves present the non-linear deformation of PDMS film with the instantaneous modulus depending on the load and the thickness. Poisson's ratio of PDMS film is estimated by fitting with Yang's asymptotic solution and is inversely proportional to the aspect ratio between the radius of the flat punch probe and the film thickness. RI of PDMS film increases with the decreasing film thickness as well as the increasing load. The bulk RI was increased up to 1.476 by the compressing load while the bottom-surface RI was increased only up to 1.435. This difference is explained qualitatively through the localized stress-strain distribution from finite element analysis. An inhomogeneous stress-strain distribution is observed in the simulation with lower strain at the bottom of the film, corresponding to experiment. A lower strain leads to lower local density resulting in a lower refractive index. Our research provides fundamental insights into the correlation between mechanical and optical properties of thin polymer films.

Highly Sensitive Flexible Pressure Sensor by the Integration of Microstructured PDMS Film With a-IGZO TFTs

The development of flexible and highly sensitive pressure sensors is critical for the realization of wearable health monitors and electronic skins. However, the sensitivity, flexibility, and large-area fabrication of pressure sensors remain the key challenges for their practical applications. In this letter, we present a novel pressure sensing system in an amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistor (TFT) configuration comprised of a microstructured polydimethylsiloxane (PDMS) capacitor fabricated on a 200 mm $\times200$ mm flexible substrate. The microstructured PDMS thin film shows the average sensitivity of 0.1 kPa−1 and response time of <190 ms for the pressure of 12 kPa. Through the amplification effect of the a-IGZO TFT, the device shows a high sensitivity in the subthreshold region for ultra-low-pressure regime (≤1.5 kPa) detection and good linearity in the linear region for low-pressure regime (≤7 kPa) detection. These results demonstrate a novel approach to pressure-sensing intelligent products.

Rational Design of Ultrasensitive Pressure Sensors by Tailoring Microscopic Features

AbstractWearable sensors are increasingly used in a wide range of applications such as tactile sensors and artificial skins for soft robotics, monitoring human motions for wellbeing and sports performance, and pressure control of compression garments for wound healing. In this work, an ultrasensitive resistive pressure sensor based on conductive polydimethylsiloxane (PDMS) thin films with different microstructures is presented. These microscopic features include micropyramids, micro‐semispheres, and micro‐semicylinders which are created by soft lithography replication of 3D printing templates. To enable piezoresistivity, a thin layer of carbon nanofibers (CNFs) is spray‐coated on the textured PDMS film. The resistance changes of the three microstructure designs under compression loading show that the micro‐semicylinder‐based sensor has the highest sensitivity of −3.6 kPa−1. Finite element modeling reveals that among the three designs, the micro‐semicylinders show the largest change in contact area under the same pressure, consistent with the experimental results that the largest resistance change under the same pressure. This sensor is capable of detecting pressure as low as 1.0 Pa. This 3D printing technology is a promising fabrication technique to design microstructured piezoresistive layers, paving the way to tailor sensor performance by engineering their microstructures and to produce ultrasensitive pressure sensors at low cost.

Effect of substrate curvature on thickness distribution of polydimethylsiloxane thin film in spin coating process**Project supported by the National Natural Science Foundation of China (Grant Nos. 51605079 and 51475076), the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (Grant No. 51621064), and the China Postdoctoral Science Foundation (Grant No. 2016M591424).

The polymer spin coating is critical in flexible electronic manufaction and micro-electro-mechanical system (MEMS) devices due to its simple operation, and uniformly coated layers. Some researchers focus on the effects of spin coating parameters such as wafer rotating speed, the viscosity of the coating liquid and solvent evaporation on final film thickness. In this work, the influence of substrate curvature on film thickness distribution is considered. A new parameter which represents the edge bead effect ratio (re) is proposed to investigate the influence factor of edge bead effect. Several operation parameters including the curvature of the substrate and the wafer-spin speed are taken into account to study the effects on the film thickness uniformity and edge-bead ratio. The morphologies and film thickness values of the spin-coated PDMS films under various substrate curvatures and coating speeds are measured with laser confocal microscopy. According to the results, both the convex and concave substrate will help to reduce the edge-bead effect significantly and thin film with better surface morphology can be obtained at high spin speed. Additionally, the relationship between the edge-bead ratio and the thin film thickness is like parabolic curve instead of linear dependence. This work may contribute to the mass production of flexible electronic devices.

A Flexible Optical Sensor for Microalbuminuria Spectroscopy

A flexible optical sensor unit was developed for the detection of albumin level in human urine. It consists of polydimethylsiloxane thinfilm with the microscopic glass as the substrate. A gradual increase of albumin level may indicate signs of health problem such as diabetes, hypertension, and endothelial dysfunction. The experimental results showed there are a spectral shift and an increase in absorbed light increase in intensity of light with sodium urate at different concentration as a substitute of urine sample. The transmittance and reflection of PDMS was determined in the 400–1400 nm wavelength range using halogen-detrium as the excitation light source to predict the sensor sensitivity based on the Beer–Lambert law. The results exhibited that the light intensity increases as the concentration of the sodium urate increases. This performance of prototype sensor ignites an alternative for albumin detection using flexible structure.

A novel approach for PDMS thin films production towards application as substrate for flexible biosensors

PDMS (Polydimethylsiloxane) thin films are widely used in stretchable electronics. However, it still remains challenging to fabricate PDMS thin films in an easy and convenient manner. To address this issue, here we novelly present an approach to ease the fabrication of PDMS membranes using lift-off method. For application as substrates in flexible biosensors, a gold thin film has been deposited on the PDMS thin film. The reliability of electric conductivity has been investigated. Moreover, biocompatibility of the PDMS thin film has been tested by culturing HeLa live cells it. The results demonstrate that the PDMS thin films fabricated by lift-off method has a high prospect towards application as substrate in flexible biosensors.