Date of Award

2018

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Biomedical Engineering (PhD)

Administrative Home Department

Department of Biomedical Engineering

Advisor 1

Bruce P. Lee

Committee Member 1

Megan C. Frost

Committee Member 2

Smitha Malalur Nagaraja Rao

Committee Member 3

Patricia A. Heiden

Committee Member 4

Kenneth R. Shull

Abstract

Catecholic groups in mussel adhesive proteins transition from being strongly adhesive in a reduced state under acidic conditions to being weakly adhesive in an oxidized state under basic conditions. Here, we exploit this pH responsive behavior of catechol and demonstrate that its oxidation state can be manipulated by incorporation of boronic acid to facilitate reversible transitions between strong and weak adhesion. Our first approach involved the addition of 3- acrylamido phenylboronic acid (APBA) to dopamine methacrylamide (DMA) containing adhesives. The synthesized adhesives showed strong adhesion to quartz surface in an acidic medium (pH 3), while weak adhesion was observed on raising the pH to a basic value (pH 9), due to unavailability of catechol and boronic acid because of the formation of a reversible catechol-boronate complex. Boronic acid not only contributed to adhesion at an acidic pH, but also allowed the catechol to reversibly interact with the surface in response to changing pH. In our second study, we demonstrated that addition of an anionic monomer, acrylic acid (AAc), preserved the reduced and adhesive state of catechol even at a neutral to mildly basic pH, while the addition of a cationic monomer, N-(aminopropyl) methacrylamide hydrochloride, led to the oxidized and weak adhesive state at higher basic pH values. This was due to the buffering of local pH offered by the incorporation of the ionic species, which affected the oxidation state of catechol. Although the ideal pH for formation of the complex is 9, it readily forms at neutral to mildly basic pH, leading to decreased adhesion and limiting the adhesive’s application in physiological and marine pH environments. In our third 2 approach, adding elevated amounts of AAc to smart adhesives consisting of DMA and APBA led to strong adhesion to quartz substrate at neutral to mildly basic pH. Moreover, the complex formed at pH 9 remained reversible and the interfacial binding could be tuned by changing the pH during successive contact cycles. pH 3 was required to break the complex and recover the strong adhesive property. Bulk adhesives analyzed in our first three approaches needed extended periods of incubation (up to 30 min) to switch between their adhesive and non-adhesive states. This is because infiltration of the pH media into the bulk polymer is limited by the slow process of diffusion. Finally, we fabricated a hybrid adhesive which was composed of gecko-inspired microstructured PDMS pillars (aspect ratios of 0.4-2) coated with the smart adhesive that we developed in our first approach. By tuning the aspect ratio of the bare templates, hybrid structures that showed strong, elevated adhesion at pH 3, were obtained. The increased adhesion was attributed to contact-splitting effects due to the micropatterning combined with the interfacial binding of the smart adhesive. On the other hand, formation of the complex, and the associated swelling of the adhesive together contributed to a significant decrease in adhesion at pH 9. Additionally, the adhesive properties could be recovered appreciably at pH 3. Further, we also demonstrated that the hybrid structures could rapidly and repeatedly switch adhesion states in response to alternating the pH value between 3 and 9 at 1 min intervals. This dissertation describes various strategies used to tune the oxidation state of catechol to control its reversibly switching adhesion to different substrates under varying pH conditions, and the morphological modifications to enhance adhesion and pH responsivity.

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