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 mu m, 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 mu m spatial resolution. Microsphere-embedded PDMS films as thin as 125 and 155 mu m 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-mu m-thick PDMS was found to decrease from 0.483 +/- 0.034 to 0.473 +/- 0.040 over a period of 20 min. For 125-mu m-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 x 550 mu m(2), 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 mu m 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.