The challenges in artificial muscle research to treat incontinence.

Sphincters to guarantee continence are in principal the simplest muscles, because only two states (closed and open) seem to be important. The healthy urinary sphincter, however, provides dynamic components. During the filling phase the increase in tonus prevents urinary loss. The sphincter rapidly responds to pressure pulses caused, for example, by coughing. Contemporary artificial sphincters, however, merely generate two states and often induce atrophy and erosion. Hence the success of commercially available, continually improved implants is still limited. This communication reviews two physical principles, shape memory alloys and electrically activated polymer nanostructures, for applications in artificial sphincters which adapt the pressure acting on the urethra and react to stress situations such as coughing. The application of these principles allows intermittent reduction of pressure on the urethra, thus involving significantly less atrophy. The fabrication of reliably working nanostructures, however, is ambitious and will need time-consuming, high-level engineering.