UC Berkeley

Various micro/nano materials have been extensively studied for applications in tissue

engineering and energy storage. Tissue engineering seeks to repair or replace damaged

tissue by integrating approaches from cellular/molecular biology and material

chemistry/engineering. A major challenge is the consistent design of three-dimensional

(3D) scaffolds that mimic the structure and biological functions of extracellular matrix

(ECM), guide cell migration, provide mechanical support, and regulate cell activity.

Electrospun micro/nanofibers have been investigated as promising tissue engineering

scaffolds because they resemble native ECM and possess tunable surface morphologies.

Supercapacitors, one of the energy storage devices, bridge the performance gap between

rechargeable batteries and conventional capacitors. Active electrode materials of

supercapacitors must possess high specific surface area, high conductivity, and good

electrochemical properties. Carbon-based micro/nano-particles, such as graphene,

activated carbon (AC), and carbon nanotubes, are commonly used as active electrode

materials for storing charge in supercapacitors by the electrical double layer mechanism

due to their high specific surface area and excellent conductivity.

In this thesis, the mechanical properties of electrospun bilayer microfibrous membranes

were investigated for potential applications in tissue engineering. Bilayer microfibrous

membranes of poly(l-lactic acid) (PLLA) were fabricated by electrospinning using a

parallel-disk mandrel configuration, which resulted in the sequential deposition of a layer

with aligned fibers (AFL) across the two parallel disks and a layer with random fibers

(RFL), both deposited by a single process step. The membrane structure and fiber

alignment were characterized by scanning electron microscopy and two-dimensional fast

Fourier transform. Because of the intricacies of the generated electric field, the bilayer

membranes exhibited higher porosity than the membranes fabricated with a single drum

collector. However, despite their higher porosity, the bilayer membranes exhibited

generally higher elastic modulus, yield strength, and toughness than single-layer

membranes consisting of random fibers. Bilayer deformation at relatively high strain rates

comprised multiple abrupt microfracture events comprising discontinuous fiber breakage.

Bilayer membrane elongation yielded excessive necking of the RFL and remarkable fiber

stretching (on the order of 400%) of the AFL. In both layers, however, the fibers exhibited

multiple localized necking, attributed to the nonuniform distribution of crystalline phases

in the fibrillar structure. The high membrane porosity and good mechanical properties of

the electrospun bilayer membranes and good biocompatibility and biodegradability of

PLLA demonstrated in this work make these bilayer membranes good scaffold candidates

for various tissue engineering applications.

Furthermore, the bilayer PLLA scaffolds showed gradual variation in through-thickness

porosity and fiber alignment and an average porosity much higher than that of

conventionally electrospun scaffolds (controls) with randomly distributed fibers. The

biocompatibility and biological performance of the bilayer fibrous scaffolds was evaluated

by in vivo experiments involving subcutaneous scaffold implantation in Sprague-Dawley

rats, followed by histology and immunohistochemistry studies. The results illustrate the

potential of bilayer scaffolds to overcome major limitations of conventionally electrospun

scaffolds associated with intrinsically small pores, low porosity and, consequently, poor

cell infiltration. The significantly higher porosity and larger pore size of the RFL enhanced

cell motility through the scaffold thickness, whereas the relatively dense structure of the

AFL provided adequate mechanical strength. The bilayer scaffolds showed more than two

times higher cell infiltration than controls during implantation in vivo. Moreover, the

unique structure of bilayer scaffolds promoted collagen fiber deposition, cell proliferation,

and ingrowth of smooth muscle cells and endothelial cells in vivo. The results of this work

reveal the high potential of the fabricated bilayer fibrous scaffolds for tissue engineering

and regeneration.

Novel all-solid-state microsupercapacitors (MSCs) with 3D electrodes consisting of active

materials (i.e., graphene or AC particles) and a polymer electrolyte (PE) designed for high-

energy-density storage applications were fabricated and tested. The incorporation of a PE

in the electrode material enhanced the accessibility of the surface of active materials by

electrolyte ions and decreased the ion diffusion path during electrochemical

charging/discharging. For a scan rate of 5 mV s –1 , the MSCs with graphene/PE and AC/PE

composite electrodes demonstrated a very high areal capacitance of 95 and 134 mF cm –2 ,

respectively, comparable with that of 3D MSCs having a liquid electrolyte. In addition, the

graphene/PE MSCs showed ~70% increase in specific capacitance after 10,000

charge/discharge cycles, attributed to an electro-activation process resulting from ion

intercalation between the graphene nanosheets. The AC/PE MSCs also demonstrated

excellent stability. The obtained results illustrate the suitability of the present 3D MSCs for

various high-density solid-state energy storage applications.

Single-walled carbon nanotube (SWCNT) networks were deposited on an ultrathin

polyimide substrate using the spray-deposition technique and patterned into interdigital

electrodes to construct ultrahigh-power, extremely flexible, and foldable MSCs capable of

operating at an ultrahigh scan rate (up to 1000 V s –1 ) and delivering a stack capacitance of

18 F cm –3 and an energy density of 1.6 mWh cm –3 , which is comparable with that of lithium

thin-film batteries. An ultrahigh power density of 1125 W cm –3 and extremely small time

constant of 1 ms were obtained with SWCNT MSCs, comparable with aluminum

electrolytic capacitors. The present MSCs showed superior electrochemical stability, with

96% capacity retention after 100,000 cycles. Furthermore, these microdevices could be

reversibly and elastically bent, folded, and rolled without undergoing significant

performance degradation. The developed SWCNT MSCs demonstrate high potential for

integration in flexible and wearable electronic systems for high-rate energy storage or ac

line filtering.

A honeycomb polydimethylsiloxane substrate was introduced for stretchable MSC arrays

based on SWNCT interdigital electrodes, which enables facile integration in flexible or

wearable electronics. The honeycomb structure accommodates large deformation without

generating large strains in the MSCs and interconnects. The results show that such

stretchable MSC arrays with SWCNT electrodes demonstrate excellent rate capability and

power performance as well as electrochemical stability up to 150% (zero prestrain) or 275%

(–50% prestrain) stretching and under excessive bending or twisting. The present

stretchable MSC arrays with honeycomb structures show high potential for integration with

other electronics, such as energy harvesters, power management circuits, wireless charging

circuits, and various sensors, encompassing a wide range of wearable, bio-implantable

electronic systems.