Smart Hybrid Graphene Hydrogels: A Study of the Different Responses to Mechanical Stretching Stimulus.

Polymer-based hydrogels, in particular those containing nanoscale fillers, are currently regarded as promising candidates for a plethora of applications. With respect to graphene, the vast majority of publications concern chemical derivatives, and, as a consequence, knowledge of the potential of pristine graphene within these systems is lacking. In this study, novel graphene-based hydrogels containing nonoxidized graphene have been prepared by a mild aqueous process. The mechanical and electrical properties of these hybrid materials were characterized. In the compositions studied, maximum improvements of Young's modulus, ultimate tensile strength, and toughness of 30, 100, and 70% were obtained, respectively. In addition to obtaining an improved hybrid material in terms of mechanical and electrical properties, the response experienced by these systems on applying mechanical stretching was evaluated and stimuli-response behavior is generated by the presence of graphene. Two different kinds of responses were found. A significant change in electrical resistance was observed with a strain gauge effect and with an average gauge factor of ∼9 (for 30% strain). The electromechanical performance of these hybrid hydrogels was tested for a range of mechanical strains and graphene contents, and the stability of these materials was assessed with successive stretching cycles. It was also observed that upon stretching these hybrid hydrogels were able to release the internalized water more efficiently in the presence of graphene, and, as a result, a second possible stimulus response was studied in the form of controlled drug delivery as a proof of concept. The release of a loaded aqueous solution of ibuprofen stimulated by controlled stretching and aided by wet capillary was studied. Much more efficient delivery was achieved in the presence of graphene. These novel systems can be used in the future for sensing or drug-delivery applications.