Viscoelastic characterization and modeling of PDMS micropillars for cellular force measurement applications
Highly compliant, biocompatible polydimethylsiloxane (PDMS) micropillar (micro-cantilever) arrays have been used as a force transducer to measure the cellular contraction forces. The basic sensing principle of these devices relies on converting lateral deflections to corresponding reaction forces using appropriate beam bending models. While these micropillar-based force transducers have many benefits over previous methods, the time/frequency dependent modulus of PDMS materials and the low aspect ratio of micropillars greatly compromise accuracy in calculation of cellular forces and understanding of cellular response mechanism. In addition, recently there is an increasing demand to explore the multifunctionality of PDMS materials, such as electrical properties, besides the role of pure mechanical structural material. Adding conducting fillers into the PDMS matrix will enable the unique tunability in both mechanical and electrical properties for the composites. However, the phase separation and high volume fraction are two major issues that limit the practical applications. This dissertation addresses three critical technical issues for next-generation, high accuracy and tunable polymeric bio-transducers: 1) viscoelastic properties of PDMS, 2) force conversion models for micro-cantilever beams, and 3) mechanical and electrical properties characterization of PDMS and conducting polymer nanowire composites. This research provides a comprehensive characterization of the viscoelastic properties of PDMS, in the time domain (relaxation modulus) and frequency domain (complex modulus), using advanced nanoindentation techniques. Both theoretical models and experimental methodologies are presented to improve the extraction accuracy of viscoelastic properties. The viscoelastic moduli of PDMS are incorporated into analytical and numerical models to convert the micropillar deflections into corresponding forces. The models are ultimately utilized to calculate the loading rate and beating frequency dependent cellular contraction forces with improved accuracy. In addition, the electrical and mechanical properties of PDMS composites are systematically studied using impedance spectroscopy and nanoindentation techniques. The effect of conducting polymer nanowire concentration on the dielectric constant and elastic modulus of the composites is analyzed by appropriate model. This dissertation demonstrates a comprehensive method for viscoelastic characterization, modeling, and analysis associated with the bending behavior of PDMS micropillar arrays, providing a more in-depth and physically accurate conversion model for force measurement applications. The scientific insights from the in-depth study of the viscoelasticity of PDMS will contribute to the analysis of many other soft polymer materials at micro/nano scales commonly used in biomedical research.
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