An energy efficient wearable tissue monitor

Abstract

Tissue transplantation is often crucial for victims of accidents. Monitoring the healing process in tissue transplants during the first hours and days is a key factor to successful reconstructive surgery. Visual inspection is the preferred method today with two serious flaws - lack of qualified personnel, and uncertainty in interpretation of observations, which could lead to late discovery of problems in transplanted muscle flaps. Tissue impedance reflects these changes in tissue state as the level of reperfusion and development of edema, which characterize the revivability or resuscitation of the tissue after transplantation. A multi-channel device for monitoring of the tissue impedance at different frequencies is proposed. By definition, bioimpedance is complex parameter of the tissue under examination, and a measure of opposition to the sinusoidal alternating electric current. Correct sinusoidal excitation is computationally and energetically inefficient to use. Limits to available energy resources and processing power in wearable electronics require more robust means, while preserving accuracy of the signal. Also the speed of conventional discrete Fourier transform based algorithms need to be increased. In the designed and prototyped device, a modified square wave excitation is used together with numerical synchronous demodulation, employing multiplication with only +1, 0, and -1. This allows the use of simple sample sorting and summing algorithms. © Springer-Verlag 2007.