Functional Fibers for Biomedical and Thermal Management Applications

Licentiate thesis, 2012

This thesis explores the integration of electrospun polymeric structures in two fields; fibrous scaffolds as cell culture substrates for biomedical applications and fiber carrier networks in metal matrix polymer composites as thermal interface materials in microsystems. Electrospinning has been found to carry great potential for a range of future biomedical engineering applications. To exploit possible benefits of using electrospun scaffolds further investigations of how imposed physical cues emanating from fibrous extrinsic cellular microenvironments affects cell behaviour are needed. In this thesis, electrospun polyurethane (PU) fiber architectures have been fabricated, characterized and studied as cell culture scaffolds. Previous studies have shown that differentiation of human embryonic stem cells (hESC) towards neuronal lineage can be induced from electrospun PU fibrous topographies. Following this the effects of oxygen plasma treatment as a physical surface modification on the fiber network is characterized and demonstrated to improve hESC attachment and growth. To enable in-depth analysis of cell-scaffold interactions a route through direct photolithography to integrate patterned electrospun topographies within microfluidic systems allowing formation of complex microenvironments is devised. Further, protein coated electrospun scaffolds with specific dimensions are shown to render complex astrocyte morphologies, resembling in vivo appearance, that exhibit significantly reduced stress associated protein expression. The findings indicate that the fabricated electrospun structures may provide a useful platform to complement or improve traditional in vitro culture methods. Thermal interface materials (TIMs) have been identified by the semiconductor industry as one of the major bottlenecks in heat dissipation for high power density devices. New interface material technologies with improved thermomechanical properties are required to proceed towards higher integration and packaging densities. Aiming to address these needs, this thesis presents contributions to the development of a novel composite technology formed through infiltration of a metal matrix into a carrier network of electrospun fibers. Employed as a TIM, the composite relies on heat conduction through its continuous metal phase and is shown to have promising performance compared to conventional materials with heat transfer through particle-particle contacts. To facilitate liquid phase infiltration of the matrix during fabrication, a surface confined chemical reduction based synthesis of nano particles on the fiber surface is investigated. Competitive effective thermal conductivity up to 25 W/mK is attained at bond line thickness determined by the carrier network, showing that the fabricated composite has potential to meet current and future demands in microsystems.