Electrically Induced Gelation, Rupture, and Adhesion of Polymeric Materials
Raghavan, Srinivasa R
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There has been considerable interest in developing stimuli-responsive soft materials for applications in drug delivery, biosensing, and tissue engineering. A variety of stimuli have been studied so far, including temperature, pH, light, and magnetic fields. In this dissertation, we explore the use of electric fields as a stimulus for either creating new soft materials or for rupturing existing ones. Our materials are based typically on biocompatible polymers such as the polysaccharides alginate, chitosan, and agarose. We also discuss the advantages and disadvantages of electric fields over other stimuli. First, we describe the use of electric fields to form transparent and robust alginate gels around an initial mold made of agarose. Moreover, we can melt away the agarose by heat, leaving us with hollow alginate tubes. In our technique, a tubular agarose mold with dissolved calcium chloride (CaCl2) is placed in a solution of sodium alginate. A voltage of ~ 10 V is then applied, with the mold as the anode and the container as the cathode. As the Ca2+ ions migrate from the mold towards the cathode, they contact the alginate chains at the mold surface. In turn, the Ca2+ crosslinks the alginate chains into a gel, and the gel grows outward with time. The technique can be used to grow multiple layers of alginate, each with a different content, and it is also safe for encapsulation of biological species. Complex tubular structures with multiple branches and specific patterns can be created. Next, we report that electric fields can be used to rupture particles formed by ionic complexation. The particles under study are typically in the microscale (~ 200 µm radius) and are either uniformly crosslinked microbeads (e.g., alginate/Cu2+) or microcapsules formed by complexation of oppositely charged polymers (alginate and chitosan). When these particles are placed in aqueous solution and subjected to an electric field of about 10 V/cm (applied remotely, i.e., electrodes not in contact), the particles rupture within about 5 min. A possible mechanism for the electric-field-induced disruption is discussed. We also use the above particles to create electrically actuatable valves, where the flow of a liquid occurs only when the particle blocking the flow is disrupted by the field. In our final study, we show that polyelectrolyte gels and beads can be rapidly induced to adhere by an electric field. We typically work with crosslinked acrylate hydrogels made with cationic co-monomers, and anionic beads made by contacting alginate with Ca2+. When the cationic gel (connected to an anode) is contacted for just a few seconds with the anionic bead (connected to a cathode) under a voltage of ~ 10 V, the two form a strong adhesive bond. When the polarity of the electrodes is reversed, the phenomenon is reversed, i.e., the gel and bead can be easily detached. We suggest that the adhesion is due to electrophoretic migration of polyelectrolyte chains, resulting in the formation of polyion complexes. Applications of this reversible adhesion are discussed for the pick-up and drop-off of soft cargo, and for the sorting of beads.