Electrogelation for Protein Adhesives

Abstract

Adhesives are common in biology as critical elements in motility, adhesion, and survival for many land and sea creatures.[1] 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. [2] 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, mediated most often by solvents and mechanical 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, [3] 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 V DC is applied over a 3 min 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; ...

Injectable silk-polyethylene glycol hydrogels

“…Silk biomaterials, due to their desirable mechanical strength, biocompatibility, degradation and controlled drug release, have been widely used for tissue engineering and drug delivery. Silk hydrogels, unlike many other polymeric hydrogels, can be prepared via physical treatments of the silk fibroin solution, such as via sonication, vortexing and electrical fields [4][5][6]. Since no harmful solvents or compounds are used in these processes, the silk hydrogel is a useful carrier for encapsulating and delivering cells as well as bioactive molecules, towards tissue repairs or therapeutics.…”

Programming function into mechanical forms by directed assembly of silk bulk materials

“…assembly by leaving the suspension unperturbed for several weeks, (ii) water evaporation-driven assembly achieved with slow drying of the silk fibroin suspension for several days, (iii) directed assembly of silk fibroin molecules through electrogelation (23), which accelerates the process to few hours. The three methods yield silk fibroin gels of different nanoscale and microscale characteristics, as previously reported, which ultimately affect the structural properties of the silk solids (24).…”

Physically Crosslinked Biocompatible Silk‐Fibroin‐Based Hydrogels with High Mechanical Performance

“…[20][21][22][23][24] Many approaches were employed to fabricate the physically crosslinked silk based hydrogels, such as sonication, vortex, addition of ethanol, and electrical fi eld. [25][26][27][28][29] However, most of the SF based hydrogels show poor mechanical performance which is a major stumbling block for applications. Recently, though a new covalently crosslinked SF based hydrogel with signifi cant elasticity was engineered via a kind of bio-crosslink reagent, i.e., HRP (horseradish peroxide), [ 7 ] it still could not escape the fate of poor mechanical properties in terms of the softness.…”