Mechanical, Biological and Electrochemical Investigations of Advanced Micro/Nano Materials for Tissue Engineering and Energy Storage

Various micro/nano materials have been extensively studied for applications in tissue/nengineering and energy storage. Tissue engineering seeks to repair or replace damaged/ntissue by integrating approaches from cellular/molecular biology and material/nchemistry/engineering. A major challenge is the consistent design of three-dimensional/n(3D) scaffolds that mimic the structure and biological functions of extracellular matrix/n(ECM), guide cell migration, provide mechanical support, and regulate cell activity./nElectrospun micro/nanofibers have been investigated as promising tissue engineering/nscaffolds because they resemble native ECM and possess tunable surface morphologies./nSupercapacitors, one of the energy storage devices, bridge the performance gap between/nrechargeable batteries and conventional capacitors. Active electrode materials of/nsupercapacitors must possess high specific surface area, high conductivity, and good/nelectrochemical properties. Carbon-based micro/nano-particles, such as graphene,/nactivated carbon (AC), and carbon nanotubes, are commonly used as active electrode/nmaterials for storing charge in supercapacitors by the electrical double layer mechanism/ndue to their high specific surface area and excellent conductivity./nIn this thesis, the mechanical properties of electrospun bilayer microfibrous membranes/nwere investigated for potential applications in tissue engineering. Bilayer microfibrous/nmembranes of poly(l-lactic acid) (PLLA) were fabricated by electrospinning using a/nparallel-disk mandrel configuration, which resulted in the sequential deposition of a layer/nwith aligned fibers (AFL) across the two parallel disks and a layer with random fibers/n(RFL), both deposited by a single process step. The membrane structure and fiber/nalignment were characterized by scanning electron microscopy and two-dimensional fast/nFourier transform. Because of the intricacies of the generated electric field, the bilayer/nmembranes exhibited higher porosity than the membranes fabricated with a single drum/ncollector. However, despite their higher porosity, the bilayer membranes exhibited/ngenerally higher elastic modulus, yield strength, and toughness than single-layer/nmembranes consisting of random fibers. Bilayer deformation at relatively high strain rates/ncomprised multiple abrupt microfracture events comprising discontinuous fiber breakage./nBilayer membrane elongation yielded excessive necking of the RFL and remarkable fiber/nstretching (on the order of 400%) of the AFL. In both layers, however, the fibers exhibited/nmultiple localized necking, attributed to the nonuniform distribution of crystalline phases/nin the fibrillar structure. The high membrane porosity and good mechanical properties of/nthe electrospun bilayer membranes and good biocompatibility and biodegradability of/nPLLA demonstrated in this work make these bilayer membranes good scaffold candidates/nfor various tissue engineering applications./nFurthermore, the bilayer PLLA scaffolds showed gradual variation in through-thickness/nporosity and fiber alignment and an average porosity much higher than that of/nconventionally electrospun scaffolds (controls) with randomly distributed fibers. The/nbiocompatibility and biological performance of the bilayer fibrous scaffolds was evaluated/nby in vivo experiments involving subcutaneous scaffold implantation in Sprague-Dawley/nrats, followed by histology and immunohistochemistry studies. The results illustrate the/npotential of bilayer scaffolds to overcome major limitations of conventionally electrospun/nscaffolds associated with intrinsically small pores, low porosity and, consequently, poor/ncell infiltration. The significantly higher porosity and larger pore size of the RFL enhanced/ncell motility through the scaffold thickness, whereas the relatively dense structure of the/nAFL provided adequate mechanical strength. The bilayer scaffolds showed more than two/ntimes higher cell infiltration than controls during implantation in vivo. Moreover, the/nunique structure of bilayer scaffolds promoted collagen fiber deposition, cell proliferation,/nand ingrowth of smooth muscle cells and endothelial cells in vivo. The results of this work/nreveal the high potential of the fabricated bilayer fibrous scaffolds for tissue engineering/nand regeneration./nNovel all-solid-state microsupercapacitors (MSCs) with 3D electrodes consisting of active/nmaterials (i.e., graphene or AC particles) and a polymer electrolyte (PE) designed for high-/nenergy-density storage applications were fabricated and tested. The incorporation of a PE/nin the electrode material enhanced the accessibility of the surface of active materials by/nelectrolyte ions and decreased the ion diffusion path during electrochemical/ncharging/discharging. For a scan rate of 5 mV s –1 , the MSCs with graphene/PE and AC/PE/ncomposite electrodes demonstrated a very high areal capacitance of 95 and 134 mF cm –2 ,/nrespectively, comparable with that of 3D MSCs having a liquid electrolyte. In addition, the/ngraphene/PE MSCs showed ~70% increase in specific capacitance after 10,000/ncharge/discharge cycles, attributed to an electro-activation process resulting from ion/nintercalation between the graphene nanosheets. The AC/PE MSCs also demonstrated/nexcellent stability. The obtained results illustrate the suitability of the present 3D MSCs for/nvarious high-density solid-state energy storage applications./nSingle-walled carbon nanotube (SWCNT) networks were deposited on an ultrathin/npolyimide substrate using the spray-deposition technique and patterned into interdigital/nelectrodes to construct ultrahigh-power, extremely flexible, and foldable MSCs capable of/noperating at an ultrahigh scan rate (up to 1000 V s –1 ) and delivering a stack capacitance of/n18 F cm –3 and an energy density of 1.6 mWh cm –3 , which is comparable with that of lithium/nthin-film batteries. An ultrahigh power density of 1125 W cm –3 and extremely small time/nconstant of 1 ms were obtained with SWCNT MSCs, comparable with aluminum/nelectrolytic capacitors. The present MSCs showed superior electrochemical stability, with/n96% capacity retention after 100,000 cycles. Furthermore, these microdevices could be/nreversibly and elastically bent, folded, and rolled without undergoing significant/nperformance degradation. The developed SWCNT MSCs demonstrate high potential for/nintegration in flexible and wearable electronic systems for high-rate energy storage or ac/nline filtering./nA honeycomb polydimethylsiloxane substrate was introduced for stretchable MSC arrays/nbased on SWNCT interdigital electrodes, which enables facile integration in flexible or/nwearable electronics. The honeycomb structure accommodates large deformation without/ngenerating large strains in the MSCs and interconnects. The results show that such/nstretchable MSC arrays with SWCNT electrodes demonstrate excellent rate capability and/npower performance as well as electrochemical stability up to 150% (zero prestrain) or 275%/n(–50% prestrain) stretching and under excessive bending or twisting. The present/nstretchable MSC arrays with honeycomb structures show high potential for integration with/nother electronics, such as energy harvesters, power management circuits, wireless charging/ncircuits, and various sensors, encompassing a wide range of wearable, bio-implantable/nelectronic systems.

Advisor: Komvopoulos, Kyriakos