A simple, non-invasive and inexpensive method for evaluating the displacement of local tissue surfaces: from vascular changes to muscle contraction.

This paper describes a non-invasive, very inexpensive method of estimating tissue displacements of various origin that is easy and fast to set up. This technique utilizes an inductive proximity sensor (IPS), which is a non-contact length transducer measuring the distance between its probe and a metal target. Its working principle is based on the electromagnetic coupling originating between the sensor probe, a source of high-frequency magnetic field, and the metal target where parasitic currents take place. The linear working range of the IPS model used here is 0.1 to 6 mm probe-target distance, its resolution is about 2 microns. The IPS has been employed on rabbits and humans to measure the displacement of a target glued to the skin of various body areas with respect to the fixed probe of the sensor. Its high resolution, together with an extensive working range, allows the evaluation of numerous physiological events which produce displacements ranging from 2 microns -- to 9 mm, reflecting either tissue volume changes or movements. In particular, an interesting application is to monitor, through volume variations, the extent and the time course of local vascular modifications induced by manoeuvres which elicit changes in vasomotor tone; vascular filling, tissue swelling etc. Therefore, this measure may be considered a 'surface plethysmography' record. In addition, the contractions of skeletal muscles, under either isotonic or isometric conditions, can be estimated through this sensor. This system may therefore find applications for research purposes and practical demonstrations to students.