Electric-sensitive Hydrogels Based On Natural Polymers

Electric-sensitive hydrogel is one kind of intelligent hydrogels which can swell, shrink or bend under electric stimulus.This kind of hydrogels can transform electrical energy into mechanical work,therefore they have potential application in the field of robots,sensors,controlled drug delivery system,artificial muscles etc.However,most of the electric-sensitive hydrogels developed recently contain synthetic polymers.Due to the poor biocompatibility and latent toxic effect of synthetic polymers,the application of those hydrogels in biological and pharmaceutical fields is quite limited. In addition,most of the electric-sensitive hydrogels reported were either polycationic or polyanionic,which only showed an electrical response in a specific pH range.This also limits their application in many fields.To overcome these limitations and enlarge the application fields of electric-sensitive hydrogels,we used natural polymers to prepare four kinds of amphoteric hydrogels,they are chitosan/carboxymethyl cellulose, chitosan/carboxymethyl chitosan,soybean protein and silk fibroin hydrogels.On one hand,natural polymers are nontoxic and biocompatible,these characters provide possibilities for electric-sensitive hydrogels to be used in biomedical field;On the other hand,amphoteric hydrogels have the characteristic of both cationic and anionic polymers.In acidic conditions,they are positively charged,so they bend toward the anode under electric stimulus;while in basic conditions,they are negatively charged and bend toward the cathode.Therefore,amphoteric hydrogels show electric-sensitivity over a wide pH range,and can change bending directions according to the pH value of electrolyte solutions.For chitosan/carboxymethyl cellulose and chitosan/carboxymethyl chitosan hydrogels,in pH＜7 buffer solutions,the amino groups(-NH₂) on chitosan are protonated to become -NH₃⁺,the hydrogels behave as polycation and bend toward the anode in electric field;while in pH＞7 buffer solutions,the carboxyl groups(-COOH) on carboxymethyl cellulose or carboxymethyl chitosan are ionized to be -COO^-,the hydrogels behave as polyanion and bend toward the cathode.Besides pH of electrolyte solutions,other factors such as ionic strength,electric voltage,thickness of hydrogel films and content of cross-linking agent also influence the electric-sensitive properties of these hydrogels.By adjusting these influential factors,we found that the equilibrium bending angle of the hydrogels reached a maximum at about 90°in pH=6 buffer solution with an ionic strength of 0.2 M under 15 V electric stimulus.We also found that increasing electric voltage or reducing the thickness of films increased the bending angle of hydrogels;a certain amount of cross-linking agent improved the mechanical properties of hydrogels but had some negative effects on their bending behavior.For soybean protein and silk fibroin hydrogels,which are polyampholyte by themselves,if the pH of electrolyte solutions is lower than their isoelectric point,they are positively charged and bend toward the anode under electric stimulus;if the pH is higher than their isoelectric point,they are negatively charged and bend toward the cathode.Because soybean protein hydrogel contains more acidic and basic amino acid residues,it has better electric sensitivity than silk fibroin hydrogel.To sum up,four kinds of natural amphoteric hydrogels were prepared and they all showed bending behavior in electric field.In a specific condition,the bending angle, bending rate and mechanical properties of these four hydrogels are different.This diversity enlarges the application filed of electric-sensitive hydrogels,we can select the most suitable hydrogel to meet different practical requirements.By changing pH of electrolyte solutions,ionic strength,voltage,thickness of films and concentration of cross-linking agent,the responsive behavior of these hydrogels can be controlled. Furthermore,we also designed several experiments to show the potential application of electric-sensitive hydrogels as energy-transforming equipments,such as artificial muscles and chemical valve etc.