Nonresonant and Resonant Frequency-Selectable Induction-Heating Targets

This paper examines a scheme for developing frequency-selectable induction-heating targets for stimulating temperature-sensitive polymer gels. The phrase “frequency selectable” implies that each target has a frequency at which it heats preferentially in the presence of other targets. Targets using both nonresonant and resonant designs are discussed. In the case of nonresonant targets, single-turn conductors whose critical dimensions are small compared to their associated skin depth (over the frequency range of interest) are examined. One way to achieve frequency selectivity with these nonresonant targets is by designing each to have the same self-inductance while forcing the resistance of each target to differ from the previous one by a specified factor (α). In this way, a target driven at its R/L break-point frequency will heat by at least a factor of (α² + 1)/(2 α) more than the remaining targets. In the resonant-target case, RLC circuits that are inductively coupled to a primary induction coil are examined. Frequency selectivity in resonant targets is achieved by designing each target to have a different resonant frequency. When such a target is driven at its resonant frequency, it will heat preferentially compared to the remaining targets.