Hydrogels have attracted a great deal of interest as responsive materials for applications such as controlled release, injectable biomaterials, smart actuators, and sensors. A major challenge in the engineering of these responsive materials is to improve their selectivity, moving towards responses that more closely resemble selective molecular recognition. One area where this presents a particular challenge is the development of porous barrier membranes that can selectively close in response to a specific toxic chemical threat. Not only must the threat be detected, but the membrane must convert the sensing event into a sufficiently large mechanical response to reduce transport rates.<br/><br/>Here, we report the ability to generate such a polymer membrane based on acrylamide hydrogels that can selectively detect and respond to certain organophosphate agents, toxic agents of wide concern. In these materials, oximes are used to selectively degrade an organophosphate compound, producing a thiol degradation product. The acrylamide gel is engineered with disulfide crosslinks, such that thiol exchange leads to a decrease in the total crosslink density and consequently swelling of the gel. This enables simultaneous decontamination, detection, and actuation of the membrane to perform mechanical work such as pore closing. Here, we describe the quantitative response of the materials to model organophosphates and demonstrate the fabrication of the gels into useful structures.<br/><br/>A second example is the response of hydrogels to oxidative stresses, enabling tunable mechanical responses when exposed to air or to selective oxidizing/reducing reagents. When thiols are incorporated at both ends of an associating polymer, mild oxidative conditions can lead to polycondensation of the thiols, resulting in the formation of high molar mass polymers. Such thiol incorporation is easily achieved in artificially engineered protein hydrogels, producing materials that demonstrate remarkable plastic draw to very high strain at break. Stress-optics experiments are used to understand the molecular rearrangements underlying this phenomenon.

Selective response and actuation in polymer hydrogels

251st National Meeting (2016)

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