Material	Polymer gels
	Polymer gels consist of a cross-linked polymer network inflated with a solvent such as water. They have the ability to reversibly swell or shrink (up to 1000 times in volume) due to small changes in their environment (pH, temperature, electric field). Micro sized gel fibres contract in milliseconds, while thick polymers layers require minutes to react (up to 2 hours or even days). They have high strength and can deliver sizeable stress (approximately equal to that of human muscles). The most common are polyvinylalcohol (PVA), polyacrylicacid (PAA) and polyacrylonitrile (PAN). Many potential applications (e.g. artificial muscles, robot actuators, adsorbers of toxic chamicals), but presently, few of them have a commercial diffusion.
Category	Responsive (smart) materials, Mooving materials
Similar materials	Conducting polymers Dielectric elastomers
References	Intelligent gels MIT, Physics Department MIT, Artificial Intelligence Laboratory UNM, Artificial Muscles Research Institute
Environmen- tal notes	Use: The majority are biocompatible.
Additional Info	Response time is still not fast enough for artificial muscles. Lifetime of a gel actuator is very short. Their structure gradually degrades and they become unusable. Commercial reality is still far away.
Photo	Thomas Nissen (Computer graphics)
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