Polymers that respond to environmental conditions, other than an applied electric current, have also been a large part of this area of study. In 1949 Katchalsky "et al." demonstrated that when collagen filaments are dipped in acid or alkali solutions, they would respond with a change in volume. The collagen filaments were found to expand in an acidic solution and contract in an alkali solution. Although other stimuli (such as pH) have been investigated, due to its ease and practicality most research has been devoted to developing polymers that respond to electrical stimuli in order to mimic biological systems.