Development of a Biocompatible Conductive Platform to Measure Cell Traction Forces

Changes in the mechanical properties of mammalian cells are indicators for cell health and can provide better understanding towards disease diagnosis. While routine cancer cell detection methods rely mainly on diagnosis through morphological analysis, it can be inconclusive or unreliable for certain cell lines. Mechanical factors such as cell exertion of traction forces on a surface are instrumental in biological processes such as cell migration, extracellular matrix formation, and cell signaling. Several researchers have investigated methods to calculate the traction forces exerted by cells, which may provide a more accurate method to diagnose cancer cells. While some of these methods, which involve micropillars, microneedles, and nanowire arrays, are able to isolate the mechanical force of a cell, the process to make these platforms is laborious and exhibits an interface which is not biocompatible or characteristic of a native cell environment. In this work, we investigated the use of single-walled carbon nanotubes (SWNTs), and by utilizing various deposition techniques, we developed a facile technique to manufacture a platform that is both biocompatible and a conductive source for quantitative analysis of the mechanical behavior of cell traction forces. Due to their excellent electrical characteristics for uses in organic electronic applications including thin film transistors, conducting electrodes, and biosensors, SWNTs were chosen as the material to provide a conductive platform. Previous studies were able to adsorb SWNTs for film deposition by drop casting, airbrush spray coating, spin coating, vacuum filtration, electrophoretic deposition, and Langmuir-Blodgett deposition. Furthermore, researchers have found that when surfaces are covalently functionalized with primary amines, they selectively adsorb semiconducting SWNTs. However, this and similar techniques are dependent upon environmentally sensitive surface modification techniques. To address this issue, we explored a range of substrates modified with physisorbed polymers, poly(L-lysine) (PLL), as an alternative methodology. In this work, we detail a number of methods for depositing SWNTs onto various substrate materials using amine-rich PLL and other methods of covalently functionalizing the surface with primary amines. We constructed devices using these methods and used them to observe if cell movement on the surface would elicit changes in the device performance. SWNT adsorption and alignment were characterized by atomic force microscopy (AFM), which revealed that the surface density was strongly dependent upon the adsorbed concentration of PLL on the surface, spin coating speed, and SWNT solution concentration. We also demonstrated the biocompatibility of PLL as an adhesion layer with cells. Results from examining mitochondrial hydrogenase activity and Live/Dead fluorescence assay suggest that the PLL SWNTs spin-coat devices exhibited higher biocompatibility with NIH-3T3 fibroblast cells than the drop-cast SWNTs devices, possibly due to differences of substrate surface roughness. To further elucidate the effect of SWNT roughness on biocompatibility, cell morphology was observed on substrate surfaces of varying SWNT network density using a spray coating method. We envision these conducting biocompatible SWNT networks could be used as biosensors to investigate cell adhesion mechanics or disease diagnosis.