ELECTROCHEMICAL APPROACHES TO CONTROL CATECHOL-BASED ADHESION

Catechol molecules in the adhesive proteins secreted by marine mussels are often used to design external stimulus (e.g., pH, temperature, or light)-responsive smart adhesives. Here, we are introducing electricity as an external stimulus to tune the adhesion of these adhesives. Controlling the extent of oxidation of catechol with electricity was the mechanism to tune adhesion. At first, tunable adhesion of a catechol adhesive with 1 - 9 V was obtained in water by utilizing a two-electrode-based (anode and cathode) Johnson–Kendall–Roberts (JKR) contact mechanics test setup. The electricity initiated water electrolysis, elevated pH near the cathode, and oxidized catechol exposed to the cathode to its poorly adhesive quinone form. We established that the applied voltage and current levels, exposure time, and salt concentration of the water were the controlling parameters to tune adhesive properties. However, the rapid deactivation needed elevated voltage due to the poorly conductive adhesive. Moreover, the adhesive was irreversible. Thus, in our second approach, a thin catechol-containing adhesive coating was coated on an aluminum disc, and its conductivity was increased by adding the conductive pyrene monomer. Increased conductivity enabled the deactivation of adhesive with only 1 V. In addition, phenylboronic acid (APBA) was added into the coating to impart reversibility. This coating was reversible after deactivation for up to five cycles utilizing pH-responsive catechol-boronate complexation chemistry. The deactivation process discussed in the last two approaches depended on the conductive surface. Therefore, in our final approach, a catechol adhesive was deactivated by a peripheral silver cathode while in direct contact with a nonconductive glass surface. Based on lap shear tests, deactivation was achieved within 4 min while applying 20 V. The deactivation rate was dependent on the applied voltage level, exposure time, area of the adhesive interface, and conductivity of the surface material. Additionally, electrochemical reversibility of the adhesive up to 3 cycles was obtained by changing electrode polarity and utilizing catechol-boronate complexation chemistry. Overall, this dissertation provides approaches to combine oft-used adhesion testing methods with electrical circuitry to tune the adhesion of catechol adhesives of different formulations and architectures utilizing novel chemistries.