Functional Fiber Based Materials for Microsystem Applications

Doctoral thesis, 2014

This thesis explores the integration of functional electrospun micro- and nanofiber based materials in two fields of microsystem technology: fibrous scaffolds as cell culture substrates and fiber-reinforced metal matrix composites as thermal interface materials (TIMs). Electrospun structures have been found to carry great potential as biomimicking microenvironments for future tissue engineering and cell culture applications. In the first part of this thesis, electrospun polyurethane fiber architectures have been fabricated, characterized and studied as cell culture scaffolds. The effects of plasma treatment, as a physical surface modification of the electrospun scaffold interface, are characterized and demonstrated to improve the possibility for human embryonic stem cell culture expansion. Electrospun scaffolds with specific fiber diameters are shown to allow maintenance of astrocytes with complex morphologies, limit up-regulation of protein expression related to astrocyte stress and activation, and to facilitate the formation of three-dimensional neuronal networks. Taken together, these findings indicate that electrospun scaffolds can be used to complement or improve traditional in vitro culture methods. To enable detailed studies of the interplay between physical and chemical cues for cells in controlled microenvironments, a method to integrate electrospun structures with microfluidic systems was devised. Thermal interface materials have been identified by the semiconductor industry as one of the major bottlenecks in the heat dissipation for high power density devices. The second part of this thesis presents the development of a novel metal matrix composite TIM technology utilizing fiber reinforcements formed through electrospinning. Employed as a TIM, the composite structure relies on heat conduction through its continuous metal phase, while the fiber network modifies the thermo-mechanical properties. Three specific implementations have been developed: Polyimide fibers with an indium matrix and a tin-silver-copper (SnAgCu) alloy matrix, and carbonized mesophase-pitch based fibers with a SnAgCu alloy matrix. To facilitate liquid phase infiltration of the indium matrix into the polyimide fibers, a chemical reduction based method for coating Ag nanoparticles on the fiber surface was developed. Using the same Ag coating, the SnAgCu composite was fabricated and demonstrated to exhibit a reduced elastic modulus, which indicates the potential to lower the thermally induced stress when bonding materials with thermal expansion mismatch. The expected increase in thermal conductivity when switching from polymer to carbonized fibers is found to be limited, and improved methods to reduce the thermal contact resistance at the matrix-fiber interface are needed. Measurements of low thermal contact resistances (<1 Kmm2/W) and high thermal conductivities (~20 W/mK) show that the composite architectures are promising candidates for future applications when compared to both existing and other emerging TIM technologies.