High-frequency radio energy is applied to tissue therapeutically in a number of different medical applications. The ability to model the effects of RF energy on the collagen, elastin, and liquid content of the target tissue would allow for the refinement of the control of the energy in order to improve outcomes and reduce negative side-effects. In this paper, we study the time-varying impedance spectra of the circuit. It is expected that the collagen/elastin ratio does not change over time such that the time-varying impedance is a function of the liquid content. We apply a non-parametric model in which we characterize the measured impedance spectra by its frequency response function. The measurements indicate that the changing impedance as a function of time exhibit a polynomial shift which we characterize by a polynomial regression. Finally, we quantify the uncertainty to obtain prediction intervals for the estimated polynomial describing the time variation of the impedance spectra.