Microfibrous and Nanofibrous Materials for Cartilage Repair and Energy Storage
This thesis explores the application of nanofibrous and microfibrous materials in the fields of cartilage repair and water electrolysis.
Articular cartilage lesions have a limited intrinsic ability to heal and are associated with joint pain and disability. The current treatment options suffer from high failure rates, prolonged rehabilitation times, and can be very costly. Therefore, an ideal solution is a low cost, mechanically strong, biocompatible replacement material with long lifetime.
To develop a cartilage replacement material, I first developed a two-step method to 3D print double network hydrogels at room temperature with a low-cost ($300) 3D printer. A first network precursor solution was made 3D printable via extrusion from a nozzle by adding a layered silicate to make it shear-thinning. After printing and UV curing, objects were soaked in a second network precursor solution and UV-cured again to create interpenetrating networks of poly(2-acrylamido-2-methylpropanesulfonate) and polyacrylamide. By varying the ratio of polyacrylamide to cross-linker, the trade-off between stiffness and maximum elongation of the gel can be tuned to yield a compression strength and elastic modulus of 61.9 and 0.44 MPa, respectively, values that are greater than those reported for bovine cartilage. The maximum compressive (93.5 MPa) and tensile (1.4 MPa) strengths of the gel are twice that of previous 3D printed gels, and the gel does not deform after it is soaked in water. By 3D printing a synthetic meniscus from an X-ray computed tomography image of an anatomical model, I demonstrate the potential to customize hydrogel implants based on 3D images of a patient’s anatomy.
On the basis of the previous work, I developed the first hydrogel with the strength and modulus of cartilage in both tension and compression, and the first to exhibit cartilage-equivalent tensile fatigue at 100,000 cycles. These properties were achieved by infiltrating a bacterial cellulose nanofiber network with a PVA-PAMPS double network hydrogel. The bacterial cellulose provided tensile strength in a manner analogous to collagen in cartilage, while the PAMPS provided a fixed negative charge and osmotic restoring force similar to the role of aggrecan in cartilage. The hydrogel has the same aggregate modulus and permeability as cartilage, resulting in the same time-dependent deformation under confined compression. The hydrogel is not cytotoxic, has a coefficient of friction 45% lower than cartilage, and is 4.4 times more wear-resistant than a polyvinyl alcohol hydrogel. The properties of this hydrogel make it an excellent candidate material for replacement of damaged cartilage.
In the field of water electrolysis, I studied the effect of fiber dimensions to their performance in water electrolysis. Water electrolysis is a good way to convert excess renewable energy to hydrogen. The generation of renewable electricity is variable, leading to periodic oversupply. Excess power can be converted to hydrogen via water electrolysis, but the conversion cost is currently too high. One way to decrease the cost of electrolysis is to increase the maximum productivity of electrolyzers. I investigated how nano- and microstructured porous electrodes could improve the productivity of hydrogen generation in a zero-gap, flow-through alkaline water electrolyzer. Three nickel electrodes—foam, microfiber felt, and nanowire felt—were studied to examine the tradeoff between surface area and pore structure on the performance of alkaline electrolyzers. Although the nanowire felt with the highest surface area initially provided the highest performance, this performance quickly decreased as gas bubbles were trapped within the electrode. The open structure of the foam facilitated bubble removal, but its small surface area limited its maximum performance. The microfiber felt exhibited the best performance because it balanced high surface area with the ability to remove bubbles. The microfiber felt maintained a maximum current density of 25,000 mA cm-2 over 100 hrs without degradation, which corresponds to a hydrogen production rate 12.5- and 50-times greater than conventional proton-exchange membrane and alkaline electrolyzers, respectively.
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