Electrogelation for Protein Adhesives

Adv. Mater. 2010, 22, 711–715 2010 WILEY-VCH Verlag Gm IC A T IO N Adhesives are common in biology as critical elements in motility, adhesion, and survival for many land and sea creatures. Despite many attempts to mimic such features with natural or synthetic polymers, this has proven to be challenging due to the subtle and metastable state of the polymeric material properties that are required to control the functional attributes of such systems including during storage, processing, adhesion, and release. The viscoelastic behavior also limits the types of material systems that can be exploited for biomimetic approaches to this important material behavior. Most often, modified polysaccharides are found associated with mucoadhesives from biological systems, due to their hydration and charge density. We report the discovery of a novel, electrically mediated adhesive formed from silkworm silk. This process, termed electrogelation provides a protein-based adhesive that offers biomimetic features when used in conjunction with devices. Further, we report on the solution behavior, morphology, and structural features of electrogels (e-gels), to demonstrate the mechanisms involved in the process. The adhesion can be controlled via electrical inputs. Most importantly, and quite unexpectedly, this is a reversible process, depending on voltage, time, and conditions used. This finding is very novel, as silkbased protein systems in particular are usually considered irreversible in terms of polymer transitions from the solution to solid state, mediatedmost often by solvents andmechanical shear forces. The basis for the current discovery comes from recent observations where aqueous solutions of silkworm silk were exposed to direct current (DC). Under certain electric fields, the solution began to gel on the positive electrode (Fig. 1). This observation prompted further investigation into the conditions and responses of the solution under different electric fields. While electrospinning of polymers, including silks, is performed at voltage potentials as high as>30 kV, the utilization of low DC voltages to generate a controlled volume of silk gel is novel. In the basic setup (Fig. 1), electrodes are immersed in an aqueous solution of silk protein and 25 VDC is applied over a 3min period to a pair of mechanical pencil leads. As the process progresses, bubbles evolve on both electrodes. Since the silk solution has a high water content, electrolysis occurs during electrogelation. The bubbles reflect the generation of oxygen gas at the positive electrode and hydrogen gas at the negative electrode during electrolysis. Within seconds of the application of the voltage, a visible gel forms at the positive electrode, locking in some oxygen bubbles at the electrode surface as the gel emanates outward. While the gel appears to have formed symmetrically about the electrode in Figure 1, it is typical that the forming gel front is directed toward the negative electrode. When silk electrogelation is executed in a voltage-controlled format, the current draw in the process follows a repeated trend; initially high current draw drops exponentially to a minimal milliampere level. The actual current amplitudes depend on many factors, including applied voltage level, electrode area and spacing, and conductivity of the silk solution. The decay in current draw is likely related to the electrical insulating effect of the growing volume of silk gel and the bubbles that become trapped near the positive electrode surface. Silk gel formed through electrogelation has a highly viscous (soft) consistency and is very tacky, bearing a resemblance to thick mucus. Remarkably, we observed that after the electric field was turned off, the adhesive gel state was retained, thus, the structural state of the protein formed under e-gel conditions was sufficiently stable to retain material functions in the absence of the applied electric field. Yet, the gel formed can be returned to the solution state through a reverse electrical process (Fig. 1). If the electrode polarity is reversed and 25 VDC reapplied, the gel disappears, while fresh gel is formed on the newly created positive electrode. Electrogelation and reversal back to silk solution can be cycled many times. If an electrogelation process is performed for an extended period of time, or at high voltage, the gel nearest the positive electrode persists and reversal of the electric field no longer drives the gel back to solution form. Alternatively, after intermediate electrogelation times, heating the gel up to ca. 60 8C led to the disappearance of the gel-like material and transitioning back to the solution state. When cooled back to room temperature, e-gel reformed as a highly viscous and tacky material, thus, the gelation is reversible over several cycles. Consequently, the process is controllable in terms of gel features, reversibility, or permanency, such as by temperature. The changes in mechanical characteristics due to electrogelation of silk solutions were investigated by dynamic oscillatory shear rheology. For silk solutions, liquidlike, viscous behavior measured by the loss modulus (G00) dominated the mechanical response within the probed frequency range (v) with G00 v (Fig. 2A). On the other hand, the mechanical response of e-gels resembled that of a soft–solidlike, physical gel. There was a significant increase in the elastic response, measured by the storage modulus (G0). The frequency dependence of G0 was weak but finite (G0 v), while the apparent minimum in G00 suggested a possible G0, G00 crossover at even lower frequencies due to eventual relaxation of temporary, physical crosslinks. To investigate the significance of increased proton concentration in the mechanism of e-gel formation at the positive electrode, we titrated the solution to control the pH, termed pH-gels (Fig. 2A).