Development of a spectrally matched TENS stimulation waveform

A modified transcutaneous electric nerve stimulator (TENS) stimulation waveform was implemented in an experimental prototype unit. The new stimulation waveform was developed to increase electrical power transfer to human tissue. This waveform differs from those generated in traditional TENS in both shape and spectral content. A pilot study of skin impedance using TENS electrodes was performed. The gain and phase of skin impedance were measured, and the results were consistent with a first order system across the entire subject population. Individual skin impedance profiles did not vary significantly with respect to skin preparation, texture, or moisture. Bode plots were made of data from inter-electrode measurements on subjects with both prepared and unprepared skin. Results revealed that electrode-to-electrode surface resistance can be modeled as a first order low pass filter. The new stimulation waveform was developed to provide more energy within the resistive portion of the skin impedance profile. This spectral shaping increases real power transfer to the skin by spectrally matching the waveform to the resistive frequency range of the skin. The distributed spectral energies for the new prototype waveform and a traditional TENS device stimulation waveform were multiplied by the skin impedance to provide a theoretical measurement of effective power transfer. This effective power transfer ratio was calculated for the two waveforms. The prototype waveform showed a 80% theoretical effective power transfer increase over the traditional waveform. An assessment of TENS function was done in a pilot test with chronic TENS users. The results showed that a spectrally matched waveform is more electrically efficient, allows more stimulation amplitude without causing irritation, is more comfortable in the long term, causes minimal nerve stimulation, and does not induce erratic skeletal muscle quiver.