Temperature control of SMA artificial anal sphincter

The implanted device discussed here is an artificial sphincter that utilizes shape memory alloy (SMA). To provide an independent evacuation for the patients using artificial anus, this device is implanted into their body. It receives driving energy by a transcutaneous energy transmission system using an electromagnetic coupling of a coil pair. This artificial sphincter operates according to the phase transformation of the SMA. Temperature control in the living body is necessary in order to heat the SMA to its phase transformation completion temperature. Taking advantage of the needless of a power supply for the temperature control circuit, we examined the way of controlling electric power to be transmitted by the transformer consisting of thermosensitive magnetic ferrite core. The SMA temperature is controlled by using the Curie-point of the ferrite attached to SMA as a reference temperature. Phase transformation completion temperature of the SMA that we used is 55°C. The purpose of this system is to automatically prevent overheating of the SMA above 55°C. Actually, because the electric power transmitted through the transformer decreases smoothly as the temperature rises to the Curie-point, the temperature of the SMA becomes stable to a point at which the heat conduction of the SMA balances the heat generated by the heater.