Ambulatory sensor for the monitoring of the edema circumference in lower limbs

The monitoring of geometrical variations in segments of the human body is currently not widely used due to the lack of suitable devices. For lower limbs, for example, a simple measuring tape is generally used to periodically measure key dimensions. This measurement approach, and the associated lack of reproducibility, is not compatible with ambulatory monitoring and it is further recognized that precise, standardized reference points are required for the more meaningful monitoring of the lower leg. The paper presents the design and realization of a new sensor system dedicated to the measurement of geometrical variations in the lower limbs, enabling the ambulatory monitoring of leg edemic swelling during daily activity. The sensor is based on an inductive loop directly integrated into a textile band and connected to a miniaturized electronic system which sends, via wireless transmission, the perimeter value of the leg (assuming that its shape is circular) to a laptop and in the near future, to a mobile phone. The inductive loop has been modeled using Matlab to predict the inductance value and determine the necessary parameters of the electronic circuit. The portable, ambulatory device enables the measurement of perimeters ranging from 25 cm to 33 cm, with an accuracy of 0.3 cm. Stability of the sensor over the time is very good (3.4% of the full scale). Hysteresis occurs when the textile is stretched between two extreme values which can be minimized by regular calibration of the system. The presented results are significantly better than those currently obtained using the traditional measuring tape.