Using Electric Fields to Modulate Polymeric Materials: Electro-adhesion, Electro-gelation and Electro-carving

Abstract

This dissertation concerns the effects of electric fields on aqueous polyelectrolytes (solutions and gels), including those of polysaccharides and proteins. Electrical effects on such polymeric systems have not been studied in detail thus far. In this work, we apply electric fields as stimuli to trigger responses in these materials. We have discovered three novel responses: electro-adhesion of a gel to a solid electrode; electro-gelation of a polymer solution, which allows gels to be made in 3D, and localized electro-disruption of gels, which allows gels to be carved or sculpted. In our first study, we show that it is possible to adhere a soft ionic conductor (like a polymeric hydrogel) to a hard, electronically conductive electrode using a low DC voltage without any adhesive. When 5 to 10 V DC is applied between a pair of electrodes (e.g., graphite, copper, etc.) spanning a cylindrical hydrogel (e.g., acrylamide, gelatin, etc.), in 3 to 15 min, the gel strongly adheres to either or both electrodes. The ultimate adhesion strength can exceed 150 kPa and is only limited by the strength of the soft material. This hard-soft electro-adhesion applies to not only lab-synthesized hydrogels but also animal or plant tissues, such as beef, pork, apples, bananas, etc. We show that this adhesion results from electrochemical reactions that form chemical bonds between the polymers in the gel backbone and the electrode surface. Hard-soft electro-adhesion can be used to assemble hybrid materials with hard and soft compartments, which could be useful in robotics, energy storage, underwater adhesion etc. Next, we demonstrate how an electric field can be used to gel a polymer solution with spatial control  thereby, we can ‘print’ gels in 3D. When a solution of alginate (an anionic biopolymer) is subjected to a DC electric field (~ 10 V) using a platinum (Pt) needle as the anode, a gel is formed right around the anode within seconds. By using a mobile anode, gel “voxels” can be formed sequentially and these merge into 3D structures. Similar electro-gelation can also be done with the cationic biopolymer chitosan, but at the cathode instead of the anode. The mechanism for gelation with both alginate and chitosan involves the polymer chains losing their charge next to the electrode. A loss of charge leads to insolubility, and insoluble domains act as crosslinks and connect the chains into networks. We have built a prototype for a 3D-printer that can translate a 3D design into a robust biopolymer gel formed by electro-gelation. Lastly, we show that an electric field applied by an electrode can be used like a knife to carve or sculpt hydrogels into 3D shapes. When we apply a DC electric field across certain gels, the gel shrinks near the anode, while water is expelled out of the gel near the cathode. Ultimately the gel shrinks by more than 50% of its original size. Such shrinkage is observed with a range of anionic gels, including both physical gels of biopolymers like agar and alginate as well as covalent gels such as sodium acrylate. If the ionic strength of the gel is high, the shrinkage does not occur. The origin of this effect lies in a combination of electroosmosis as well as pH changes near the electrodes. Finally, we show that with a focused electric field, the shrinkage can be limited to a specific location in a gel, thereby allowing us to electro-carve gels in 3D.