Random copolymer composed of poly(acrylic acid) and poly(vinyl sulfonic acid) have been prepared, and their swelling ratios at various temperatures and pH, and their deformation ratio in an electrical field have been measured. The polymer hydrogels that exhibited contraction are good candidates for biomaterials that can be stimulated by an applied electrical field. The contraction and expansion behavior was similar to natural muscle. Hydrogels can be utilized as biosensors and as artificial muscle when employing an electrical stimulus.