PDMS Samples Research Articles

Novel elastomeric spiropyran-doped poly(dimethylsiloxane) optical waveguide for UV sensing

Novel poly(dimethylsiloxane) (PDMS) doped with two different spiropyran derivatives (SP) were investigated as potential candidates for the preparation of elastomeric waveguides with UV-dependent optical properties. First, free-standing films were prepared and evaluated with respect to their photochromic response to UV irradiation. Kinetics, reversibility as well as photofatigue and refractive index of the SP-doped PDMS samples were assessed. Second, SP-doped PDMS waveguides were fabricated and tested as UV sensors by monitoring changes in the transmitted optical power of a visible laser (633 nm). UV sensing was successfully demonstrated by doping PDMS using one spiropyran derivative whose propagation loss was measured as 1.04 dB/cm at 633 nm, and sensitivity estimated at 115% change in transmitted optical power per unit change in UV dose. The decay and recovery time constants were measured at 42 and 107 s, respectively, with an average UV saturation dose of 0.4 J/cm2. The prepared waveguides exhibited a reversible and consistent response even under bending. The sensor parameters can be tailored by varying the waveguide length up to 21 cm, and are affected by white light and temperatures up to 70 ℃. This work is relevant to elastomeric optics, smart optical materials, and polymer optical waveguide sensors.Graphical

Real-Time Plasmonic Strain Sensors Based on Surface Relief Diffraction Gratings.

Large-scale diffraction gratings were fabricated in surface relief on azobenzene thin films and transferred to flexible PDMS substrates using soft lift-off lithography. The PDMS gratings were strained along the grating vector axis and the resulting surface topography was analyzed using diffraction angle measurements, AFM imagery and surface plasmon resonance (SPR) spectra. All measurement methods exhibited a linear response in strain indicating the useability of these sensors in real-world applications. For SPR-based strain sensing, an increasing pitch and a decreasing modulation depth were observed with increasing strain. The SPR peak shifted by ~1.0 nm wavelength and the SPR intensity decreased by ~0.3 a.u. per percentage of applied strain. The tested PDMS samples retained their integrity even after multiple cycles of stretching and relaxation, making them a suitable strain sensor.

Ultrasound Characterization of Cortical Bone Using Shannon Entropy.

Ultrasound backscattered signals encompass information on the microstructure of heterogeneous media such as cortical bone, in which pores act as scatterers and result in the scattering and multiple scattering of ultrasound waves. The objective of this study was to investigate whether Shannon entropy can be exploited to characterize cortical porosity. In the study described here, to demonstrate proof of concept, Shannon entropy was used as a quantitative ultrasound parameter to experimentally evaluate microstructural changes in samples with controlled scatterer concentrations made of a highly absorbing polydimethylsiloxane matrix (PDMS). Similar assessment was then performed using numerical simulations on cortical bone structures with varying average pore diameter (Ct.Po.Dm.), density (Ct.Po.Dn.) and porosity (Ct.Po.). The results suggest that an increase in pore diameter and porosity lead to an increase in entropy, indicating increased levels of randomness in the signals as a result of increased scattering. The entropy-versus-scatterer volume fraction in PDMS samples indicates an initial increasing trend that slows down as the scatterer concentration increases. High levels of attenuation cause the signal amplitudes and corresponding entropy values to decrease drastically. The same trend is observed when porosity of the bone samples is increased above 15%. Sensitivity of entropy to microstructural changes in highly scattering and absorbing media can potentially be exploited to diagnose and monitor osteoporosis.

Tailoring mechanical and electrical properties of polydimethylsiloxane nanocomposites With graphene and carbon nanotubes for wearable electronics

We explored the use of soft and flexible graphene (Gr)-reinforced polydimethylsiloxane (PDMS; Gr-PDMS) and Gr-carbon nanotube (CNT)-reinforced PDMS (Gr-CNT-PDMS) as nanocomposites (NCPs) in wearable electronic applications and investigated their mechanical and electrical properties. Specifically, we evaluated the mechanical reinforcement of the NCPs by performing Shore scleroscope hardness and machine-washing tests and evaluated their electrical properties by using a four-point probe. The results revealed that as the Gr or CNT concentrations increased, the NCPs’ hardness and surface resistance increased and decreased, respectively. Scanning electron microscopy revealed the formation of layered polymeric structures in the Gr-PDMS NCPs, contributing to the increase in hardness and decrease in surface resistance. Furthermore, bending tests revealed that changes in resistance were positively correlated with the Gr concentration. PDMS with low Gr concentrations exhibited no substantial changes in surface resistance after bending tests. However, for PDMS, as the Gr concentration increased (>5 wt.%), the cross-sectional morphology observed after 500 bending cycles differed substantially from that before observed bending tests. Additionally, the influence of one-dimensional (CNT) and two-dimensional (Gr) nanomaterials on the mechanical and electrical properties of the flexible PDMS samples was investigated. The aim of this study is to highlight the importance of considering the properties of different nanomaterials in developing high-performance NCPs for wearable electronics.

Biaxial testing system for characterization of mechanical and rupture properties of small samples

The study of damage and rupture of soft tissues using a tensile testing system is essential to understand the limits of mechanical behavior and loss of function in diseased tissues. However, commercial material testing systems are often expensive and may not be fully suitable for rupture tests of small samples. While several research laboratories have developed custom, less expensive, uniaxial or biaxial devices, there is a need for an open source, inexpensive, accurate and easy to customize biaxial material testing system to perform rupture tests in small soft samples.We designed a testing system (BiMaTS) that (a) was shown able to perform uniaxial and biaxial tests, (b) offers a large travel range for rupture tests of small samples, (c) maintains a centered field of view for effective strain mapping using digital image correlation, (d) provides a controlled temperature environment, (e) utilize many off-the-shelve components for easy manufacture and customization, and it is cost effective (∼$15 K).The instrument performance was characterized using 80%-scaled down, ASTM D412-C shaped PDMS samples. Our results demonstrate the ability of this open source, customizable, low-cost, biaxial materials testing system to successfully characterize the mechanical and rupture properties of small samples with high repeatability and accuracy.

Analysis of oxidative degradation and calcification behavior of a silicone polycarbonate polyurethane-polydimethylsiloxane material.

The biocompatibility and chemical stability of implantable devices are crucial for their long-term success. CarboSil® is a silicon polycarbonate polyurethane copolymer with good biocompatibility and biostability properties. Here, we explored the possibility to improve these characteristics by introducing 30% of extra-chain cross-linkable poly(dimethyl siloxane) (PDMS). Patches made of CarboSil and CarboSil-30% PDMS were manufactured by spray, phase-inversion technique and subjected to a heating-pressure treatment. Both materials showed good biocompatibility, either in viability and proliferation of cell-based experiments both with mouse fibroblasts and subcutaneous implant in rats. Fourier-transform infrared spectroscopy showed a significant decrease in soft segment loss in CarboSil-30% PDMS samples with respect to CarboSil in in vitro accelerated oxidative treatments with CoCl2 and 20% H2 O2 at 37°C up to 36 days. Same results were observed in subcutaneous implants up to 90 days. Field-emission scanning electron microscopy on samples exposed to calcification solutions during 80 days highlighted the presence of a homogeneous distribution of calcium deposition over the entire surface of CarboSil samples, while no calcium deposits were observed in CarboSil-30% PDMS samples. Patches subjected to subcutaneous experiments showed no sign of calcification after 90 days, irrespectively of their composition. Thanks to the improved characteristics in terms of degradation and calcification the modified materials described in this work hold great promise for their use in the manufacture of cardiovascular devices.

Modeling Dynamic Surface Tension on Surfactant-Enhanced Polydimethylsiloxane.

Surfactants are often added to aqueous solutions to induce spreading on otherwise unwettable hydrophobic surfaces. Alternatively, they can be introduced directly into solid hydrophobic materials─such as the soft elastomer, polydimethylsiloxane─to induce autonomous wetting without requiring additional surface or liquid modifications. Given the similarity between mechanisms of these two approaches, models that describe wetting by aqueous surfactant solutions should also characterize wetting on surfactant-solid systems. To investigate this theory, multiple surfactants of varying size and chemical composition were added to prepolymerized PDMS samples. After cross-linking, water droplets were placed on the surfaces at set time points, and their contact angles were recorded to track the temporal evolution of the interfacial tension. Multiple nonlinear models were fitted to this data, their parameters were analyzed, and each goodness of fit was compared. An empirical model of dynamic surface tension was found to describe the wetting process better than the single established model found in the literature. The proposed model adapted better to the longer time scales induced by slow molecular diffusivity in PDMS. Siloxane ethoxylate surfactants induced faster and more complete wetting of PDMS by water than oxyoctylphenol ethoxylates did. The generalizability of this model for characterizing nonionic surfactants of a wide range of physiochemical properties was demonstrated.

Impact of the surface modifications and cell culture techniques on the biomechanical properties of PDMS in relation to cell growth behavior

PDMS is the most widely used material in the microfluidic systems because of its advantageous properties including ease of manufacture, low cost, transparency, and gas permeability. Although all biomaterials need to be sterilized for usage for in vitro or in vivo systems to avoid possible contaminants, none of them proved effective in providing the gold standards so far for PDMS sterilization. Therefore we evaluated the influence of both UV and ethanol sterilization of PDMS substrates on the cell behavior via investigating cell proliferation, oxidative stress and nutrient metabolism of 293 T cells growing on the PDMS first time in the literature. Modulus of elasticity and stress relaxation of UV-treated PDMS samples increased compared to the ethanol-treated ones leading to a state of surface stiffness preferable for cell growth and proliferation. Oxidative stress metabolism of cells growing on the UV-treated PDMS was more stable compared to the ethanol-treated samples. Trace element and mineral metabolism are not impaired in the cells growing on the UV-treated PDMS samples. First time in the literature, we showed that UV treatment is more suitable for the PDMS samples used in the cell culture experiments.

Development of a polymer-based antimicrobial coating for efficacious urinary catheter protection

This study reports the development of a polymer-based catheter coating to facilitate controlled release of antimicrobial peptides (AMP) to target both planktonic bacteria and biofilm in the urinary catheter environment. Catheter associated urinary tract infection (CAUTI) is a common nosocomial infection among hospitalized patients and is a major reservoir of antimicrobial resistant pathogens. Although silver- or antibiotics-coated catheters have been deployed to minimise CAUTI, the inconsistency and lack of durability in antibacterial properties of these coatings have limited their clinical use. The incorporation of AMPs in catheter coatings has gained interest due to the effective bacteria killing effects of AMPs, with few reports on bacterial resistance development against peptides. This study aims to deploy a novel and potentially cost-effective technique to coat an anhydrous polymeric coating impregnated with AMPs for silicone-based catheters, to overcome limitations in conventional hydrogel-based coatings. Sustained peptide release was achieved with the development of an Ethyl Cellulose (EC): 1-Palmitoyl-2-oleoylphosphatidylcholine (POPC)-based diffusion layer over an AMP-laden Polycaprolactone (PCL)-based layer to control AMP diffusion into the environment over a clinically relevant duration. The ‘AMP-EC-PCL’ coating showed good anti-bacteria performance against uropathogenic Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa for up to 6 days. The coating also showed excellent anti-biofilm capability against green fluorescent protein (GFP)-tagged UTI E. coli. Fifteen centimeter catheter segments of single layer ‘AMP-EC-PCL’-coated catheters showed sustainable AMP release kinetics up to 7 days, where good antibacterial and anti-biofilm activity against E. coli was observed. The full scale ‘AMP-EC-PCL’-coated catheter showed improved mechanical integrity compared to commercial silicone catheters with preservation of the catheter balloon integrity upon expansion. Wound healing studies of the coated PDMS samples in mice models showed a reduction in bacteria concentration as compared to uncoated PDMS, indicating in vivo efficacy potential of the developed catheter coating platform.

Plasma Polymerized HMDSO Coatings For Syringes To Minimize Protein Adsorption.

Current parenteral containers used for the storage and delivery of protein-based drugs, contain silicone oil which may seep into the protein solution and can result in adsorption, aggregation and denaturation of the protein. Tightly adherent surface coatings prepared by radio frequency glow-discharge (RFGD) plasma polymerization are described in this paper. Using this robust technique, methacrylic acid (MA) (hydrophilic), hexamethyldisiloxane (HMDSO) (hydrophobic), tetraglyme (TG) (hydrophilic) were plasma polymerized onto glass. In addition, HMDSO and MA were copolymerized to create a plasma polymerized HMDSO-MA (hydrophobic) surface. Untreated glass and glass dip-coated in PDMS were used as controls. TG and MA plasma coatings adsorbed the least amount of protein in all pH conditions. Interestingly HMDSO-MA retained significantly lesser protein compared to HMDSO and dip-coated PDMS samples. In the presence of Polysorbate 80 (PS80) all plasma polymerized coatings adsorbed and retained negligible amounts of protein, compared to controls. Furthermore, the peak glide force of plasma coated syringes did not significantly increase compared to syringes without plasma coating. Due to the versatility of RFGD plasma, this process is scalable and could potentially be used for the treatment of hypodermic syringes used for the storage and delivery of protein-based therapeutics.

Probing local nonlinear viscoelastic properties in soft materials

Minimally invasive experimental methods that can measure local rate dependent mechanical properties are essential in understanding the behaviour of soft and biological materials in a wide range of applications. Needle based measurement techniques such as Cavitation Rheology (Zimberlin et al., 2007) and Volume Controlled Cavity Expansion (VCCE, Raayai-Ardakani et al. (2019a)), allow for minimally invasive local mechanical testing, but have been limited to measuring the elastic material properties. Here, we propose several enhancements to the VCCE technique to adapt it for characterisation of viscoelastic response at low to medium stretch rates (10−2 - 1 s−1). Through a carefully designed loading protocol, the proposed technique performs several cycles of expansion–relaxation at controlled stretch rates in a cavity expansion setting and then employs a large deformation viscoelastic model to capture the measured material response. Application of the technique to soft PDMS rubber reveals significant rate dependent material response with high precision and repeatability, while isolating equilibrated states that are used to directly infer the quasistatic elastic modulus. The technique is further established by demonstrating its ability to capture changes in the rate dependent material response of a tuneable PDMS system. The measured viscoelastic properties of soft PDMS samples are used to explain earlier reports of rate insensitive material response by needle based methods: it is demonstrated that the conventional use of constant volumetric rate cavity expansion can induce high stretch rates that lead to viscoelastic stiffening and an illusion of rate insensitive material response. We thus conclude with a cautionary note on possible overestimation of the quasistatic elastic modulus in previous studies and suggest that the stretch rate controlled expansion protocol, proposed in this work, is essential for accurate estimation of both quasistatic and dynamic material parameters.

Soft lithography with liquid marbles

The soft lithography process with water marbles immersed into the PDMS matrix is demonstrated. Near-surface cavities, decorated with the toroidal residues, built of the colloidal particles, initially coating the marbles, are formed in the course of evaporation of water filling the liquid marbles. Water liquid marbles coated with lycopodium, polyethylene and polytetrafluoroethylene particles remained attached to the PDMS/air interface from below due to the capillary bridges. The analysis of the equilibrium of a liquid marble supported by the capillary bridge is suggested. Thermal fields arising from cross-linking of PDMS and evaporation of water from liquid marbles are explored. The migration of colloidal particles coating the marbles towards the contact line in the course of water evaporation is reported and analyzed. Migration of particles is related to the “coffee-stain effect”. Mechanical properties of the PDMS samples stamped with the liquid marbles are reported.

PDMS samples characterization with variations of synthesis parameters for tunable optics applications

PDMS has become a frequently used material in the elaboration of optical components such as: variable focal length liquid lenses, optical waveguides, solid elastic lenses, etc. In this work we describe the elaboration of PDMS samples, and we present the physical and optical properties of the material when a variation on its synthesis parameters (mixture ratio of base: curing agent, curing temperature and curing time) is implemented during their elaboration. Tensile and compressive tests were carried out to obtain the corresponding stress-strain curves of the material, and UV-Vis spectroscopy was applied to obtain transmittance and absorbance curves of the samples. A variation of the refractive index of the samples was observed and homogeneity of the samples was studied with the Raman spectra obtained from the samples. Results of the characterization determined the appropriate synthesis parameters for the elaboration of a tunable refractive surface for potential applications in artificial vision.

Modulation of Foreign Body Reaction against PDMS Implant by Grafting Topographically Different Poly(acrylic acid) Micropatterns.

The surface of poly(dimethylsiloxane) (PDMS) is grafted with poly(acrylic acid) (PAA) layers via surface-initiated photopolymerization to suppress the capsular contracture resulting from a foreign body reaction. Owing to the nature of photo-induced polymerization, various PAA micropatterns can be fabricated using photolithography. Hole and stripe micropatterns ≈100-µm wide and 3-µm thick are grafted onto the PDMS surface without delamination. The incorporation of PAA micropatterns provides not only chemical cues by hydrophilic PAA microdomains but also topographical cues by hole or stripe micropatterns. In vitro studies reveal that a PAA-grafted PDMS surface has a lower proliferation of both macrophages (Raw 264.7) and fibroblasts (NIH 3T3) regardless of the pattern presence. However, PDMS with PAA micropatterns, especially stripe micropatterns, minimizes the aggregation of fibroblasts and their subsequent differentiation into myofibroblasts. An in vivo study also shows that PDMS samples with stripe micropatterns polarized macrophages into anti-inflammatory M2 macrophages and most effectively inhibits capsular contracture, which is demonstrated by investigation of inflammation score, transforming-growth-factor-β expression, number of macrophages, and myofibroblasts as well as the collagen density and capsule thickness.

Capturing strain stiffening using Volume Controlled Cavity Expansion

Strain-stiffening is a well-documented behavior in soft biological materials such as liver and brain tissue. Measuring and characterizing this nonlinear response, which is commonly considered as a mechanism for damage prevention, is of great interest to engineers for design of better biomimetic materials, and to physicians for diagnostic purposes. However, probing the elastic response of soft or biological materials at large deformation in their natural habitat, is an arduous task. Here, we present the Volume Controlled Cavity Expansion (VCCE) technique as a measurement method that offers the ability of characterizing the local stiffening response of materials in addition to identifying their shear modulus. By employing minimal constitutive representations involving only two constants (Mooney–Rivlin, Gent, and Ogden) we show that for the conventional PDMS samples, this technique and an accompanying data analysis method capture the shear modulus, as well as providing reliable measures of the stiffening behavior of the samples.

Simple Surface Modification of Poly(dimethylsiloxane) via Surface Segregating Smart Polymers for Biomicrofluidics

Poly(dimethylsiloxane) (PDMS) is likely the most popular material for microfluidic devices in lab-on-a-chip and other biomedical applications. However, the hydrophobicity of PDMS leads to non-specific adsorption of proteins and other molecules such as therapeutic drugs, limiting its broader use. Here, we introduce a simple method for preparing PDMS materials to improve hydrophilicity and decrease non-specific protein adsorption while retaining cellular biocompatibility, transparency, and good mechanical properties without the need for any post-cure surface treatment. This approach utilizes smart copolymers comprised of poly(ethylene glycol) (PEG) and PDMS segments (PDMS-PEG) that, when blended with PDMS during device manufacture, spontaneously segregate to surfaces in contact with aqueous solutions and reduce the hydrophobicity without any added manufacturing steps. PDMS-PEG-modified PDMS samples showed contact angles as low as 23.6° ± 1° and retained this hydrophilicity for at least twenty months. Their improved wettability was confirmed using capillary flow experiments. Modified devices exhibited considerably reduced non-specific adsorption of albumin, lysozyme, and immunoglobulin G. The modified PDMS was biocompatible, displaying no adverse effects when used in a simple liver-on-a-chip model using primary rat hepatocytes. This PDMS modification method can be further applied in analytical separations, biosensing, cell studies, and drug-related studies.

Contributions of network topological structures to the mechanical properties of PDMS elastomers

Silicone elastomers (poly dimethyl siloxane, PDMS) are widely used in the fabrication of MEMS devices and in mechanobiological studies. Mechanical properties of the elastomers may be varied by changing the crosslinking density and physical entanglements in the networks. Other factors like dangling chains and uncrosslinked sol fractions also influence the network mechanics. We investigated the role of crosslinks, entanglements and sol fraction in the mechanical properties of PDMS prepared with different ratios of base to crosslinker. Results from compression tests show greater elastic modulus for the highly crosslinked networks. We removed uncrosslinked sol, altered the entanglement density with xylene extraction, and used data from mechanical tests to assess predictions from the Frenkel-Flory-Rehner model. These studies show that the polymer-solvent interaction parameter, χ, varied linearly with the equilibrium volume fraction. Dynamic mechanical analyses of extracted PDMS samples showed that crosslinking density, solvent presence, and sol fraction affect their overall viscoelasticity properties. We also compared experimental data with scaling laws and demonstrate that trapped entanglements in the networks contribute to the mechanical properties of PDMS. Finally, we show from these studies that swelling behaviors of PDMS in xylene follow affine deformation.

Dual surface modification of PDMS-based silicone implants to suppress capsular contracture

In this study, we report a new physicochemical surface on poly(dimethylsiloxane) (PDMS)-based silicone implants in an effort to minimize capsular contracture. Two different surface modification strategies, namely, microtexturing as a physical cue and multilayer coating as a chemical cue, were combined to achieve synergistic effects. The deposition of uniformly sized microparticles onto uncured PDMS surfaces and the subsequent removal after curing generated microtextured surfaces with concave hemisphere micropatterns. The size of the individual micropattern was controlled by the microparticle size. Micropatterns of three different sizes (37.16, 70.22, and 97.64 μm) smaller than 100 μm were produced for potential application to smooth and round-shaped breast implants. The PDMS surface was further chemically modified by layer-by-layer (LbL) deposition of poly-l-lysine and hyaluronic acid. Short-term in vitro experiments demonstrated that all the PDMS samples were cytocompatible. However, lower expression of TGF-β and α-SMA, the major profibrotic cytokine and myofibroblast marker, respectively, was observed in only multilayer-coated PDMS samples with larger size micropatterns (70.22 and 97.64 μm), thereby confirming the synergistic effects of physical and chemical cues. An in vivo study conducted for 8 weeks after implantation in rats also indicated that PDMS samples with larger size micropatterns and multilayer coating most effectively inhibited capsular contracture based on analyses of tissue inflammation, number of macrophage, fibroblast and myofibroblast, TGF-β expression, collagen density, and capsule thickness. Statement of SignificanceAlthough poly(dimethylsiloxane) (PDMS)-based silicone implants have been widely used for various applications including breast implants, they usually cause typical side effects called as capsular contracture. Prior studies have shown that microtexturing and surface coating could reduce capsular contracture. However, previous methods are limited in their scope for application, and it is difficult to obtain FDA approval because of the large and nonuniform size of the microtexture as well as the use of toxic chemical components. Herein, those issues could be addressed by creating a microtexture of size less than 100 m, with a narrow size distribution and using layer-by-layer deposition of a biocompatible polymer without using any toxic compounds. Furthermore, this is the first attempt to combine microtexture with multilayer coating to obtain synergetic effects in minimizing the capsular contracture.

Temporal Changes in Nucleus Morphology, Lamin A/C and Histone Methylation During Nanotopography-Induced Neuronal Differentiation of Stem Cells.

Stem cell differentiation can be regulated by biophysical cues such as nanotopography. It involves sensing and integration of these biophysical cues into their transcriptome with a mechanism that is yet to be discovered. In addition to the cytoskeletal and focal adhesion remodeling, nanotopography has also been shown to modulate nucleus morphology. Here, we studied the effect of nanotopography on the temporal changes in nuclei of human embryonic stem cells (hESCs) and human mesenchymal stem cells (hMSCs). Using a high throughput Multi-architecture (MARC) chip analysis, the circularity of the stem cell nuclei changed significantly on different patterns. Human ESCs and MSCs showed different temporal changes in nucleus morphology, lamin A/C expression and histone methylation during topography-induced neuronal differentiation. In hESCs, the expression of nuclear matrix protein, lamin A/C during neuronal differentiation of hESCs on PDMS samples were weakly detected in the first 7 days of differentiation. The histone 3 trimethylation on Lysine 9 (H3K9me3) decreased after differentiation initiated and showed temporal changes in their expression and organization during neuronal differentiation. In hMSCs, the expression of lamin A/C was significantly increased after the first 24 h of cell culture. The quantitative analysis of histone methylation also showed a significant increase in hMSCs histone methylation on 250 nm anisotropic nanogratings within the first 24 h of seeding. This reiterates the importance of cell-substrate sensing within the first 24 h for adult stem cells. The lamin A/C expression and histone methylation shows a correlation of epigenetic changes in early events of differentiation, giving an insight on how extracellular nanotopographical cues are transduced into nuclear biochemical signals. Collectively, these results provide more understanding into the nuclear regulation of the mechanotransduction of nanotopographical cues in stem cell differentiation.

Poisson's ratio of PDMS thin films

Despite the fact that a small uncertainty in PDMS Poisson's ratio leads to significant errors in traction force microscopy, there is a clear lack of data for PDMS films at the scale of 100 μm, a relevant size scale frequently employed in cell mechanics studies. Equally important is the need for consideration of the viscoelastic nature of PDMS, as no mechanical property - including Poisson's ratio - can be taken as a time-independent constant. The foremost challenge for addressing these issues is the difficulty of carrying out stress relaxation tests on miniature PDMS samples accompanied by non-contact strain measurement with a very high spatiotemporal resolution. This study introduces such a stress relaxation platform incorporating i) the proper means for the application of necessary boundary conditions, ii) a high-precision in load measurement, and iii) a non-contact, local strain measurement technique based on single particle tracking. During stretching, images were recorded at a rate of 18 Hz with a 40 μm spatial resolution. Microsphere-embedded PDMS films as thin as 125 and 155 μm were prepared to study the Poisson's ratio by a local strain microscope. After tracing the displacement of microspheres by a single particle tracking method and using a strain mapping, Poisson's ratio for 155-μm-thick PDMS was found to decrease from 0.483 ± 0.034 to 0.473 ± 0.040 over a period of 20 min. For 125-μm-thick PDMS, this reduction took place from 0.482 ± 0.041 to 0.468 ± 0.038. Moreover, a non-monotonic reduction was observed in both cases. This negative correlation between Poisson's ratio and relaxation time was found to be statistically significant for both thicknesses with p < 0.001. The viscoelastic behavior was further characterized through the Burgers model. With a measurement field of 597 × 550 μm2, this study emphasizes the importance of the local investigation of mechanical properties. Furthermore, the dependence of transverse strain on a film thickness difference of 30 μm was measured to determine the sensitivity of local strain tracking. The inherent high resolution of the proposed approach enables one to measure deformations more precisely and to observe the temporal evolution of the Poisson's ratio that has not been observed before. In addition to the high-precision determination of PDMS Poisson's ratio, this work also offers a promising pathway for the accurate and time-dependent determination of the mechanical properties of other soft materials, where similar ambiguities exist regarding the mechanical behavior.