Polydimethylsiloxane Gel Research Articles

A super-stretchable and anti-fatigue silicone gel by regulating chain entanglement and cross-link density

Applications of silicone gel in flexible devices and soft machines require it to maintain excellent stretchability and antifatigue. However, the reported fatigue threshold of traditional silicone gels is in the range of 1–100 J m−2 and the stretchability below 2000 %. Herein, we report a simple strategy for constructing soft polydimethylsiloxane gels, which possesses high stretchability up to 8300 %, and high fatigue resistance (321 J m−2). This is achieved by controlling the ratio of chemical crosslinking and physical entanglement in the polydimethylsiloxane gels. The symbiotic relationship of this combination includes: extending the polymer chains helps them unfold to increase softness and intrinsic toughness; controlling the physical entanglement and chemical cross-linking helps to further increase the tensile rate and fatigue threshold, and adding small-molecule dimethyl PDMS as a solvent further increases fatigue resistance and tensile properties. This work provides a facile approach for developing super-stretchable and anti-fatigue silicone gel, which promises wide range of applicant in flexible devices and soft machines.

Increased liver stiffness promotes hepatitis B progression by impairing innate immunity in CCl4-induced fibrotic HBV+ transgenic mice.

Hepatitis B virus (HBV) infection develops as an acute or chronic liver disease, which progresses from steatosis, hepatitis, and fibrosis to end-stage liver diseases such as cirrhosis and hepatocellular carcinoma (HCC). An increased stromal stiffness accompanies fibrosis in chronic liver diseases and is considered a strong predictor for disease progression. The goal of this study was to establish the mechanisms by which enhanced liver stiffness regulates HBV infectivity in the fibrotic liver tissue. For in vitro studies, HBV-transfected HepG2.2.15 cells were cultured on polydimethylsiloxane gels coated by polyelectrolyte multilayer films of 2 kPa (soft) or 24 kPa (stiff) rigidity mimicking the stiffness of the healthy or fibrotic liver. For in vivo studies, hepatic fibrosis was induced in C57Bl/6 parental and HBV+ transgenic (HBVTg) mice by injecting CCl4 twice a week for 6 weeks. We found higher levels of HBV markers in stiff gel-attached hepatocytes accompanied by up-regulated OPN content in cell supernatants as well as suppression of anti-viral interferon-stimulated genes (ISGs). This indicates that pre-requisite "fibrotic" stiffness increases osteopontin (OPN) content and releases and suppresses anti-viral innate immunity, causing a subsequent rise in HBV markers expression in hepatocytes. In vitro results were corroborated by data from HBVTg mice administered CCl4 (HBVTg CCl4). These mice showed higher HBV RNA, DNA, HBV core antigen (HBcAg), and HBV surface antigen (HBsAg) levels after liver fibrosis induction as judged by a rise in Col1a1, SMA, MMPs, and TIMPs mRNAs and by increased liver stiffness. Importantly, CCl4-induced the pro-fibrotic activation of liver cells, and liver stiffness was higher in HBVTg mice compared with control mice. Elevation of HBV markers and OPN levels corresponded to decreased ISG activation in HBVTg CCl4 mice vs HBVTg control mice. Based on our data, we conclude that liver stiffness enhances OPN levels to limit anti-viral ISG activation in hepatocytes and promote an increase in HBV infectivity, thereby contributing to end-stage liver disease progression.

Superslippery Long‐Chain Entangled Polydimethylsiloxane Gel with Sustainable Self‐Replenishment

A superslippery gel coating with sustainable self-replenishment is developed by Sang Joon Lee and co-workers using a simple fabrication method, in article number 2201530. Thanks to the low-viscosity oil layer formed on the surface, the developed coating is not contaminated by bacteria and biofoulants, and exhibits stable slippery properties even when exposed to harsh environments.

Roll to roll triboelectric fiber manufacturing for smart-textile self-powered sensor and harvester

The integration of a multifunctional electronic textile-based triboelectric nanogenerator (TENG) with miniaturized devices has opened up a new horizon in the field of wearable electronics. Unfortunately, the development of textile based TENG has enormous shortcomings, including shortcomings in durability, stability, washability, and compatibility with mass production. Thus, this work reports a cost-effective, simple fabrication, and industrially scalable production method of producing fiber TENG using silver conductor fiber encapsulated by polydimethylsiloxane (PDMS). To produce continuous PDMS-coated silver fiber, a home-made coating technology was used, in which silver fiber was fed up through a nozzle filled with PDMS gel solution using a roll-to-roll continuous fabrication process. The feasibilities of TENG fiber production were demonstrated by the manufacturing of over 300-meter-long fibers and therefrom a half-meter-long TENG woven fabric of 20 cm width including the integration technique into wearable garments for self-powered sensors and harvesters. The maximum output open circuit voltage and short circuit current of the PDMS-coated silver triboelectric nanogenerator (PS-TENG) were ∼50 V and ∼8.0 μA from 50.0 cm-long and 0.55 mm diameter fiber. A PS-TENG coil harvester out of this fiber can easily light up 160 LEDs with simple hand tapping. We also demonstrated the PS-TENG integration on the heel of a shoe insole, which was also able to light up 25 LEDs in serial connection during walking. Furthermore, since the PS-TENG provides flexibility and excellent durability against washing, repeatable uses, rubbing, and stability in harsh chemical environments, we have also integrated the self-powered sensor fiber into various healthcare objects and wearable sports items, including hand gloves for cardiopulmonary resuscitation sensing (CPRS), a belt for respiration sensing, and a belt for football shoes and boxing gloves to evaluate the impact of kicking and boxing. All these advantages of PS-TENG not only enable small wearable electronics to be powered but also accelerate the development of industrially scalable energy harvesting through engineering design.

Negative and Positive Energetic Elasticity of Polydimethylsiloxane Gels.

We investigated the thermoelasticity of polydimethylsiloxane (PDMS) gels containing six types of solvents with different solubilities. The contribution of energetic elasticity to the total stress (σE/σ) ranges from +0.20 to -0.20 depending on the solvent species. The σE/σ values are positive for the solvents with low molecular mass. By contrast, it is negative for oligodimethylsiloxane (ODMS) or PDMS solvents acting as athermal solvents, each of which has the same chemical structure as the network strands. The investigation using a PDMS rubber without a solvent and the PDMS gels with various ODMS contents reveal a crossover of the σE/σ value from positive to negative with increasing ODMS content. The pronounced dependence of σE/σ on the solvent species and the negative energetic elasticity specific to the high contents of ODMS and PDMS unveil previously unknown aspects of thermoelasticity of polymer gels. The orientation coupling between the segments of the free polymeric chains and network strands is one of the possible scenarios to explain the negative energetic elasticity specific to the ODMS and PDMS solvents, because it stabilizes the aligned state, reducing the elastic energy.

Atomic Force Microscopy Micro-Indentation Methods for Determining the Elastic Modulus of Murine Articular Cartilage.

The mechanical properties of biological tissues influence their function and can predict degenerative conditions before gross histological or physiological changes are detectable. This is especially true for structural tissues such as articular cartilage, which has a primarily mechanical function that declines after injury and in the early stages of osteoarthritis. While atomic force microscopy (AFM) has been used to test the elastic modulus of articular cartilage before, there is no agreement or consistency in methodologies reported. For murine articular cartilage, methods differ in two major ways: experimental parameter selection and sample preparation. Experimental parameters that affect AFM results include indentation force and cantilever stiffness; these are dependent on the tip, sample, and instrument used. The aim of this project was to optimize these experimental parameters to measure murine articular cartilage elastic modulus by AFM micro-indentation. We first investigated the effects of experimental parameters on a control material, polydimethylsiloxane gel (PDMS), which has an elastic modulus on the same order of magnitude as articular cartilage. Experimental parameters were narrowed on this control material, and then finalized on wildtype C57BL/6J murine articular cartilage samples that were prepared with a novel technique that allows for cryosectioning of epiphyseal segments of articular cartilage and long bones without decalcification. This technique facilitates precise localization of AFM measurements on the murine articular cartilage matrix and eliminates the need to separate cartilage from underlying bone tissues, which can be challenging in murine bones because of their small size. Together, the new sample preparation method and optimized experimental parameters provide a reliable standard operating procedure to measure microscale variations in the elastic modulus of murine articular cartilage.

Superslippery Long‐Chain Entangled Polydimethylsiloxane Gel with Sustainable Self‐Replenishment

Inspired by mucus‐secreting organisms, biomimetic slippery surfaces have been studied in various engineering fields. The liquid‐infused polymer surface (LIPS) has received considerable interest because of its ability to store lubricants inside the polymer itself, facile fabrication, and high scalability. However, the conventional LIPS easily loses its slippery property owing to its inability to secrete lubricants to the surface. In this study, a long‐chain entangled polydimethylsiloxane (LEP) gel is proposed as a superslippery functional surface with sustainable self‐replenishment. The developed LEP gel has large lubricant storage spaces and exhibits an extremely low sliding angle close to 0° because of the low‐viscosity oil layer formed on the surface. In addition, although conventional LIPSs easily undergo lubricant drought on their surfaces, the proposed LEP gel continuously secretes low‐viscosity oil to the surface. The LEP gel with a superslippery surface shows nearly perfect antifouling performance and reduces 99.97% of bacteria compared with pure polydimethylsiloxane surface. It maintains slippery performance without deterioration even after exposure to harsh conditions, such as high‐pressure and high‐speed shear flow. The outstanding slippery performance of the proposed LEP gel would be usefully utilized in various engineering fields after further improvement in the future.

Contactless Rheology of Soft Gels Over a Broad Frequency Range

We report contactless measurements of the viscoelastic rheological properties of soft gels. The experiments are performed using a colloidal-probe atomic force microscope in a liquid environment and in dynamic mode. The mechanical response is measured as a function of the liquid gap thickness for different oscillation frequencies. Our measurements reveal an elastohydrodynamic coupling between the flow induced by the probe oscillation and the viscoelastic deformation of the gels. The data are quantitatively described by a viscoelastic lubrication model. The frequency-dependent storage and loss moduli of the polydimethylsiloxane gels are extracted from fits of the data to the model and are in good agreement with the Chasset-Thirion law. Our results demonstrate that contactless colloidal-probe methods are powerful tools that can be used for probing soft interfaces finely over a wide range of frequencies.

Influence of laser processing conditions for the manufacture of microchannels on ultrahigh molecular weight polyethylene coated with PDMS and PAA

<abstract> <p>Ultrahigh molecular weight polyethene (UHMWPE) is employed as a bearing material in a range of applications due to its improved elasticity, compatibility, and impact resistance, processing conditions for a suitable surface texture are necessary. Surface texture processing on microchannels using lasers is always associated with the effect of heat damage on the polymer specimen surface. This study aims to explore the use of polydimethylsiloxane (PDMS) and polyacrylic acid (PAA) in the form of liquid gel coatings in order to reduce heat damage to surfaces during the laser processing of ultrahigh molecular weight polyethene (UHMWPE). First, PDMS and PAA were coated on the surface of the UHMWPE material specimen, and then texturing was performed using a laser diode and cleaned using the ultrasonic method. Second, the dimensions and texture profiles of all the samples from this study were measured using a confocal microscope and open source software. In addition, the effect of adding liquid gel on the surface at 150 µm thickness and laser power parameters was determined. The results show that the PDMS and PAA liquid gel layers help regulate the dimensional bulge of the fabricated microchannels at laser powers below 6 watts, compared to those produced without the coating.</p> </abstract>

Synthesis of Polydimethylsiloxane with hydrolysis and condensation methods using monomer of Dichlorodimethylsilane as vitreous humour substitute

Vitreous humour is a fluid in the eye socket, that can be disturbed due to changes in its chemical or physical structure. Polydimethylsiloxane (PDMS) that also known as a silicone oil is a liquid that is commonly used as a substitute for vitreous humour in vitreoretinal surgery. PDMS can be synthesized from the monomer of Octamethylcyclotetrasiloxane (D4). However, in Indonesia, it is still difficult to obtain D4 monomer, so an alternative is needed. In this study, a synthesis of PDMS was carried out using the monomer precursor Dichlorodimethylsilane (DCMS) through a hydrolysis method under alkaline conditions using potassium hydroxide (KOH). Then the monomer obtained from the hydrolysis process was self-polymerized by aging to form a polymer gel. The concentration of KOH was varied at 0.5, 0.6 and 1 molar. The synthesized PDMS gel have viscosity values of 0.57, 1.53 and 4.49 Pa.s, density values of 0.986, 1.004, and 1.005 g/mL, refractive index in the range of 1.4001-1.4012, and surface tension in the range 19-21 mN/m. Those values of synthesized PDMS are similar to properties of commercial PDMS. We obtained that PDMS sample that synthesized using 1 M KOH shows the fastest polymerization time.

An In Vitro Mat‐on‐Gel Construct for Engineering Cardiac Tissue

AbstractTopographical and mechanical features are two key factors for cell development. Here, an easy‐to‐make Mat‐on‐Gel construct is introduced to study the co‐effects of nanogeometry and stiffness on cardiomyocyte (CM) functional development in vitro. The construct consists of electrospun fibers and polydimethylsiloxane (PDMS) gel, providing both nanofibrillar structure and adjustable stiffness. Comparison studies of three different stiffness ranging from 19 to 52 kPa are conducted using a CM cell line: HL‐1 cells. Physical characterization shows that Mat‐on‐Gel constructs possess improved hydrophilicity than PDMS gel and reduced Young's modulus than electrospun mat. It results in better cell adhesion and enhanced cell metabolic activities. CMs on the Mat‐on‐Gel constructs also present more synchronous electrophysiological activities than those cultured on electrospun fibers or PDMS gel alone. In particular, lower gel stiffness results in more rapid calcium waves. This study collectively demonstrates the feasibility of this Mat‐on‐Gel construct and its potential usage as an in vitro platform for pharmaceutical studies.

A Double-Deck Self-Digitization Microfluidic Chip for Digital PCR.

In this work, a double-deck microfluidic chip was presented for digital PCR application. This chip consists of two reverse-placed micro-patterned polydimethylsiloxane (PDMS) layers between the top and bottom glass substrates. Each micropatterned PDMS layer contains more than 20,000 cylindrical micro-chambers to hold the partitioned droplets. The double-deck designs can double the number of chambers and reagent capacity without changing the planar area of the chip. In addition, carbon black was mixed into the pure PDMS gel to obstruct the passage of fluorescence from the positive chambers between the two layers of the chip. Thus, the fluorescence signal of micro-chambers in different layers of the chip after PCR can be collected without mutual interference. The quantitative capability of the proposed chip was evaluated by measuring a 10-fold serial dilution of the DNA template. A high accuracy of the absolute quantification for nucleic acid with a dynamic range of 105 was demonstrated by this chip in this work. Owing to its characteristics of small planar area, large capacity, and sensitivity, the double-deck microfluidic chip is expected to further promote the extensive applications of digital PCR.

Bioelectronics: Ultrasoft Silicone Gel as a Biomimetic Passivation Layer in Inkjet‐Printed 3D MEA Devices (Adv. Biosys. 9/2019)

In article number 1900130, Hideaki Yamamoto and co-workers describe a novel multielectrode (MEA) array device with a biomimetic, ultrasoft interface. Here, inkjet printing is used to fabricate 3D MEA structures, which are then passivated with a polydimethylsiloxane gel. Primary cortical neurons are grown on the device, and effective stimulation of the culture confirms functional cell-device coupling.

Ultrasoft Silicone Gel as a Biomimetic Passivation Layer in Inkjet-Printed 3D MEA Devices.

Multielectrode arrays (MEAs) are versatile tools that are used for chronic recording and stimulation of neural cells and tissues. Driven by the recent progress in understanding of how neuronal growth and function respond to scaffold stiffness, development of MEAs with a soft cell-to-device interface has gained importance not only for in vivo but also for in vitro applications. However, the passivation layer, which constitutes the majority of the cell-device interface, is typically prepared with stiff materials. Herein, a fabrication of an MEA device with an ultrasoft passivation layer is described, which takes advantage of inkjet printing and a polydimethylsiloxane (PDMS) gel with a stiffness comparable to that of the brain. The major challenge in using the PDMS gel is that it cannot be patterned to expose the sensing area of the electrode. This issue is resolved by printing 3D micropillars at the electrode tip. Primary cortical neurons are grown on the fabricated device, and effective stimulation of the culture confirms functional cell-device coupling. The 3D MEA device with an ultrasoft interface provides a novel platform for investigating evoked activity and drug responses of living neuronal networks cultured in a biomimetic environment for both fundamental research and pharmaceutical applications.

A flexible rheometer design to measure the visco-elastic response of soft solids over a wide range of frequency.

We present a flexible setup for determining the rheology of visco-elastic materials which is based on the mechanical response of a magnet deposited at the surface of a slab of material and excited electromagnetically. An interferometric measurement of the magnet displacement allows one to reach an excellent accuracy over a wide range of frequency. Except for the magnet, there is no contact between the material under investigation and the apparatus. At low frequency, inertial effects are negligible so that the mechanical response, obtained through a lock-in amplifier, directly gives the material complex modulus. At high frequency, damped waves are emitted and the rheology must be extracted numerically from a theoretical model. To validate the design, the instrument was used to measure the rheology of a test polydimethylsiloxane gel which presents an almost perfect scale free response at high frequency.

Perfluoroalkane wax infused gels for effective, regenerating, anti-icing surfaces.

Infusion of solid perfluoroalkanes into polydimethylsiloxane gels provides a simple route to regenerating deicing surfaces, with low adhesion strength from the lower inherent cohesive energy of the perfluoroalkanes. Further, these surfaces are more hydrophobic and environmentally stable than their alkane analogues. The result is a robust, regenerating surface which demonstrates low energy ice adhesion (19.6 kPa), hydrophobicity (water contact angle, CA, >100°), and high environmental stability.

Effect of KOH concentration on characteristics of polydimethylsiloxane synthesized by ring opening polymerization method

Polydimethylsiloxane (PDMS) that widely known as silicone oil is commonly used as vitreous humour substitutes in vitreoretinal surgery. PDMS is needed to be produced in Indonesia and should be provided with sufficient quantity and quality for domestic needs. We synthesized PDMS using ring-opening polymerization method from octamethylcyclotetra-siloxane as monomer, hexamethyldisiloxane as chain terminator, and KOH as catalyst. KOH concentrations are varied from 0.5 M to 2.5 M to study the effect of KOH concentration on characteristics of PDMS. PDMS We successfully synthesized PDMS with a yield in between 39.73% and 65.30% and various viscosities in the range between 0.58 Pa.s and 9.36 Pa.s. From FTIR spectroscopy, it is found that all synthesized samples have structure and functional groups similiar with that of commercial PDMS. From UV-Vis and refractometer measurements, all samples have transparency almost 100% and refractive indexes in the range of 1.3989 to 1.4039. The effect of KOH concentration on the synthesized PDMS gel is on its viscosity, the higher concentration of KOH, the higher viscosity of the obtained PDMS.

Electrothermally Triggered Broadband Optical Switch Films with Extremely Low Power Consumption

Smart films with transmittance switching capabilities based on thermal stimuli are widely used in many optoelectronic applications. Despite the development of stably switchable materials, transition temperature control and broadband stepwise transmittance switching remain challenging topics. Additionally, reduction of the energy consumption during switching is also required. Here, we introduce an electrothermally driven film with switchable transmittance produced by stacking paraffin-immobilized polydimethylsiloxane gel on a transparent heater based on an aligned Cu/Ni network. The film shows stepwise transmittance switching capability with extremely low power consumption because of the controlled melting point of paraffin and the high-efficiency transparent heater.

Integrin Molecular Tensions in Live Cells are Altered by Substrate Rigidity

Mounting evidence suggests that mammalian cells are capable of sensing the rigidity of their local environment and responding to it. However, it remains unclear whether cells sense cellular force alteration caused by matrix compliance (stress sensing) or sense cytoskeleton remodeling caused by matrix deformation (strain sensing). Because integrins are the major mechano-sensitive receptors that transmit cellular force at the cell-matrix interface, the tension transmitted by integrin molecules (integrin tension) may serve as fundamental mechanical signal in cell mechanotransduction and rigidity sensing. Therefore, measuring and mapping integrin molecular tension in live cells on elastic substrates will shed light to the mechanism of cell rigidity sensing. In this work, we applied integrative tension sensor (ITS), a sensor converting molecular tension above a threshold to fluorescent signal, to calibrate and map integrin tension directly by fluorescence imaging. In experiments, ITS with 54 pN tension threshold was grafted on PDMS (Polydimethylsiloxane) substrates with Young's modulus ranging from 1kPa to 1.8 MPa. Integrin tension above 54 pN in live cells was mapped with high resolution (0.4 µm) and sensitivity on these substrates. The results show that cell spreading area and integrin tension activity both increased with increasing PDMS gel stiffness for CHO-K1, NIH-3T3 and skin fibroblast cell lines. Also, keratocytes migrated faster on softer PDMS substrates and integrin tension activity was the highest on the stiffest PDMS sample. These experiments for the first time monitored integrin tension in live cells on elastic substrates, and clearly demonstrated that the substrate rigidity affects integrin tension activity, providing evidence in favor of stress sensing hypothesis.

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.