Thermal control of shape memory alloy artificial anal sphincters for completeimplantation

Abstract

This paper presents an approach for the thermal control of an artificial anal sphincterusing shape memory alloys. An artificial anal sphincter has been proposed by theauthors to resolve problems of severe fecal incontinence in patients. The basicdesign of the artificial sphincter consists of two all-round shape memory alloyplates as the main functional parts, and heaters that are attached to the SMAplates for generating the thermal cycles required for the phase transformationaccompanied shape changes of the plates. The SMA artificial sphincter could be fittedaround intestines, performing an occlusion function at body temperature and arelease function upon heating. Thermal compatibility of such prostheses is mostimportant and is critical for practical use. Since a temperature rise of approximately20 °C from body temperature is needed to activate a complete transformation of SMA plates, anearlier model of ours allowed only a short period of heating, resulting in incompleteevacuation. In this work, a thermal control approach using a temperature-responsive reedswitch has been incorporated into the device to prevent the SMA plates fromoverheating. Then, with thermal insulation the artificial anal sphincter is expected toallow a long enough opening period for fecal continence; without any thermalimpact to the surrounding tissues that would be in contact with the artificialsphincter. Thermal control was confirmed in both in vitro and in vivo experiments,suggesting the effectiveness of the present approach. The modified SMA artificialanal sphincter has been implanted into animal models for chronic experimentsof up to 4 weeks, and has exhibited good performance by maintaining occlusionand release functions. At autopsy, no anomaly due to thermal impact was foundon the surfaces of intestines that had been in contact with the artificial analsphincter.