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Polydimethylsiloxane (PDMS) is an important polymeric material widely used in bio-MEMS devices such as micropillar arrays for cellular mechanical force measurements. The accuracy of such a measurement relies on choosing an appropriate material constitutive model for converting the measured structural deformations into corresponding reaction forces. However, although PDMS is a well-known viscoelastic material, many researchers in the past have treated it as a linear elastic material, which could result in errors of cellular traction force interpretation. In this paper, the mechanical properties of PDMS were characterized by using uniaxial compression, dynamic mechanical analysis, and nanoindentation tests, as well as finite element analysis (FEA). A generalized Maxwell model with the use of two exponential terms was used to emulate the mechanical behavior of PDMS at room temperature. After we found the viscoelastic constitutive law of PDMS, we used it to develop a more accurate model for converting deflection data to cellular traction forces. Moreover, in situ cellular traction force evolutions of cardiac myocytes were demonstrated by using this new conversion model. The results presented by this paper are believed to be useful for biologists who are interpreting similar physiological processes. [ABSTRACT FROM AUTHOR]

Polydimethylsiloxane (PDMS) microcantilevers have been used as force sensors for studying cellular mechanics by converting their displacements to cellular mechanical forces. However, PDMS is an inherently viscoelastic material and its elastic modulus changes with loading rates and elapsed time. Therefore, the traditional approach to calculating cellular mechanical forces based on elastic mechanics can result in errors. This letter reports a more in-depth method for viscoelastic characterization, modeling, and analysis associated with the bending behavior of the PDMS microcantilevers. A viscoelastic force conversion model was developed and validated by proof-of-principle bending tests. [ABSTRACT FROM AUTHOR]

Polymeric deformable sensor arrays have been employed to measure cellular forces and offered insights into the study of cellular mechanics. Previous studies have been focused on using transducers in static domain and assumed elastic beam theory as the force conversion model. Neglecting the inherent viscoelastic behavior of polydimethylsiloxane and low aspect ratios of the sensor arrays compromised the accuracy of these devices. In this work, a more in-depth viscoelastic Timoshenko beam model was developed incorporating dynamic cellular forces. We studied chemically stimulated contractions of cardiac myocytes and found that the loading rate has a considerable influence on the sensitivity of the sensor arrays. [ABSTRACT FROM AUTHOR]

A wafer topography measurement system has been designed and demonstrated based on shadow Moiré. Three-step phase-stepping and phase unwarping techniques are also incorporated to enhance the system resolution. Wafer curvatures or bows can be achieved by analyzing the Moiré fringe patterns and film stress can be obtained subsequently by transforming this wafer curvature using a conversion equation such as Stoney's formula. Wafer bow of plasma enhanced chemical vapor deposition nitride and oxide coated wafers are measured by this shadow Moiré system and are subsequently verified by the KLA-Tencor FLX 2320 system. The discrepancy between both bow measurements is within 2 μm, regardless of the magnitude of the measurement. Therefore, this system is especially suitable for stress characterization of thicker, stiffer, or highly stressed films. In comparison with the traditional laser scanning method, wafer curvature obtained by shadow Moiré is based on full-field information and it would have a better accuracy. By integrating this system with a more accurate wafer curvature to film stress conversion formula, this system should also provide a better film stress characterization. [ABSTRACT FROM AUTHOR]