In cryosurgical probes of a type now undergoing development, the flow of coolant (typically, liquid nitrogen) would be regulated by magnetostrictively actuated needle valves controlled by use of superconductive electromagnet coils. In comparison with cryosurgical probes now in use, the developmental probes would be smaller and lighter, and would afford better regulation of temperature. This concept is made feasible by two recent advances:
- Research at NASA's Jet Propulsion Laboratory has shown that reliable magnetostrictive cryogenic actuators can be manufactured relatively inexpensively by making actuator elements from commercial-grade magnetostrictive polycrystallike materials through a simple deformation process.
- Solenoids made from high-temperature superconductors, in the size and actuating-current ranges needed for cryosurgical probes, have recently become available in research quantities and are becoming commercially available.
Cryosurgical probes are used in some abdominal surgery and in prostate surgery to remove tumors. The basic concept in cryosurgery of the prostate is to place a probe in contact with the tumor, then use the probe to kill the tumor by freezing it. To freeze the affected prostate tissue to a fatal temperature without excessively damaging adjacent tissues, it is necessary to have a high cooling rate and to turn the cooling on and off at the proper times. Such thermal control is difficult to achieve in current cryosurgical probes and, as a result, adjacent tissues are damaged.
The main reason for the difficulty of thermal control in a current cryosurgical probe is that the valve that controls the flow of coolant is located near the coolant supply, and not in the probe. The approach taken in the present development effort is to exploit the inherent capability for miniaturization of a magnetostrictively actuated needle valve, placing the valve inside a cryosurgical probe within 1 or 2 cm of the tip. Thus, a small, rapidly controllable valve would be placed close to the location to which the cryogen is required to be delivered, making it possible to achieve better control over the timing and the rate of cooling. Moreover, the placement of the valve near the probe tip would make it possible to use narrower coolant-supply tubes with thinner insulation, so that the probe could be made smaller and lighter.
This work was done by Jennifer Dooley, Christian Lindensmith, and Robert Chave of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.