Date of Award

2015

Document Type

Master's Thesis

Degree Name

Doctor of Philosophy in Biomedical Engineering (PhD)

College, School or Department Name

Department of Biomedical Engineering

Advisor

Bruce P. Lee

Abstract

Smart hydrogel adhesives with tunable properties consist of adhesive moieties in the polymer network that respond to external stimuli like pH, temperature, etc. Responsiveness of smart adhesives to pH, in particular, is important because of the simple actuation mechanism and the ability to achieve facile bonding and debonding upon command. Covalently crosslinked hydrogel adhesives were prepared by employing an N-HEAA (hydroxyethyl acrylamide) backbone embedded with dopamine methacrylamide (DMA), a marine mussel inspired adhesive protein and 3-acrylamido phenylboronic acid (AAPBA), to determine the effect of pH on the interfacial binding properties of the hydrogel adhesive with a borosilicate glass substrate. Swelling tests were performed to determine the response of the synthesized hydrogels to changes in pH values. These tests revealed that in a pH 3 buffered solution, hydrogels containing DMA and AAPBA showed a shrinking trend, while at pH 9, a swelling phenomenon was observed. The evidence from oscillatory rheometry tests exhibited elevated loss moduli (Gʹ) for hydrogels with DMA and AAPBA at pH 9, when compared to the relevant controls. In conjunction, the data from swelling tests and rheometry explained the unusual swelling of the hydrogels and formation of the catechol-boronate complex at pH 9, which caused more than an order of magnitude of increase in the Gʺ owing to the viscous dissipation of energy at that pH as compared to the control gels. The interfacial binding properties were tested by performing contact mechanics tests, in the presence of an acidic/basic medium. The maximum work of adhesion values of 0.59mJ/m2 were obtained for hydrogels with 2.5mol% DMA and 10mol%AAPBA in the polymer network, when tested against a borosilicate glass surface wetted with 250μL of the pH 3 solution. At pH 9, this value reduced to as much as 1/5th of its value at pH 3. Earlier works have proposed that the oxidation of the catecholic groups that are chiefly responsible for adhesion with an inorganic substrate, is a deterrent to the adhesive properties of a hydrogel. We have accomplished the development of a model adhesive system in which we utilized the pH responsiveness of the hydrogels to demonstrate the elevated and reduced works of adhesion at acidic and basic pHs respectively. We believe that the catechol- boronic acid complex at pH 9 will allow for the reversible DOPA- facilitated adhesion. Reversibility studies performed in this direction revealed that while the hydrogels could recover their shape in terms of the measured diameters, further testing and analysis is required for understanding the ideal composition of the hydrogel and environmental trigger to actuate reversibility.

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