Control of the cryosurgical process in nonideal materials

A theoretical analysis and experimental results of a controlled cryosurgical process are presented. The analysis is based on the energy equations describing the probe response and the phase change occurring in the medium. The device developed is based on the standard cryosurgical system. consisting of a liquid nitrogen tank, insulated tube, and cryoprobe. To control the process, the probe temperature is controlled by supplying variable amounts of heat directly to the probe tip while a constant valve setting for the flow of liquid nitrogen is maintained. The novelty of the method is in implementing a closed-loop control of the amount of heat supplied to the cryoprobe. thereby controlling temperature variations according to any desired forcing function. The supplied heat is controlled by varying the current through an electrical resistor wound around the probe tip. The developed system was applied to an inanimate solution of mashed potatoes, which freezes over a range of temperatures and has thermophysical properties that depict those of biological tissues.