Gel sensors and actuators

Gels, soft polymeric or composite materials that have a high fraction of water, are often found as structural materials and actuators in nature but have so far not found many uses when fabricated synthetically. We first examine some natural systems such as jellyfish, sea anemones, starfish, legumes, and human tissue, all having interesting ways of moving or otherwise reacting to the surrounding environment. Then we discuss swelling and cross-linking of hydrogels, followed by a look at actuation by electrically, thermally, and chemically stimulated gels, noting that electrical stimulation needs a chemical intermediary to show substantial actuation (comparable to human muscle, for instance). Electroactive gels have great potential as sensors and actuators but their actual uses are mainly restricted to passive drug delivery and matrices for sensors. For most applications as artificial muscles, electrically driven actuators are too weak, but chemically driven actuators look very promising. Better ways of coupling electrical energy to chemically driven gels are needed.