Wear mechanisms of chemically crosslinked hydrogels under mild abrasion

The structural and mechanical resemblance of hydrogels to cartilage makes them promising candidates for cartilage replacement in load bearing joints. These biphasic materials are composed of an elastic network that retains a large volume of water allowing them to efficiently maintain smooth sliding while under high compressive loads. As with any sliding interface under high loads, the hydrogels in this application would be highly susceptible to wear. While the fluid component of these materials mediates low friction sliding, the hydrophilic polymer network of hydrogels can endure only a limited number of sliding cycles. For hydrogels to be viable candidates to replace osteoarthritic cartilage, a study of their robust long-term use and surface failure mechanism is necessary. In this dissertation, we evaluate different aspects of the wear behavior of hydrogels to develop tools for predicting their wear resistance as well as to improve future hydrogel surface design to withstand the severe environment of the body. Pristine hydrogel surfaces typically have low friction, which is controlled by composition, slip speeds, and immediate slip history. The stiffness of such samples is typically measured with bulk techniques and is assumed to be homogeneous at the surface. While the surface properties of homogeneous hydrogel samples are generally controlled by composition, the surface also interfaces with the open bath, which distinguishes it from the bulk. Because structure dictates function, the surface structural inhomogeneity inherent of hydrogels influences its wear behavior. Through the disruption of polyacrylamide hydrogel surfaces with abrasive wear, we connect the effects of the surface structure to the mechanical performance using microtribometry with a rough probe to apply wear. Through our measurements of surface stiffness and lubrication of worn and unworn hydrogels, we found that wear increases their surface stiffness and decreases their frictional speed dependence by revealing a stiffer bulk. As this newly revealed bulk interacts with the water bath, it regenerates its soft, swollen surface with its properties resembling that of the unworn surface. Because the magnitude of wear and the resulting surface features depend on the wear application parameters, we wore the surface of hydrogels under different sliding conditions. The load, hydrophilicity of the surface mold, stroke length, and experimental duration influenced the degree of microplowing and microcutting whereas the tested sliding speeds had negligible effects. Considering that wear is a microfracture process, by relating the wear rate of hydrogels with varying concentrations of polymer and crosslinker to their corresponding threshold fatigue fracture, an expression for predicting wear rate for single network chemically crosslinked hydrogels was developed. Incorporating a strengthening agent into the hydrogel network did not affect its wear behavior; however, increasing the toughness of hydrogels through a double network architecture significantly increased their wear resistance. While the double network hydrogels maintained incredibly low wear rates, their wear behavior and features were markedly different from that of porcine cartilage. Cartilage has a strong ability to swell their surface locally within the wear scar reducing the appearance of surface damage, which is a valuable property to integrate into future double network hydrogel designs. This dissertation provides a foundation to better predict the lifespan of hydrogels in different sliding applications as well as improve their design to be more resilient.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2021

Author dc:creator

• Bonyadi, Shabnam Zahra

Contributors dc:contributor

• Dunn, Alison C
• Jasiuk, Iwona M
• Wagoner Johnson, Amy J
• Elbanna, Ahmed E

• wear
• hydrogels
• friction
• tribology
• fracture
• fatigue

Statement dc:rights

• Copyright 2020 Shabnam Bonyadi

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/109592

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/109592