Mobile Measurement system for clinical diagnosis: A Transepidermal Water Loss Probe

Transepidermal water loss (TEWL) is commonly used as a surrogate marker for skin barrier function. Although current devices enable TEWL to be measured with ease and precision under controlled laboratory conditions, they do not yield reliable readings in an uncontrolled environment. This project aimed to develop a new TEWL device that is mobile, i.e. that would fit in the pocket and measure TEWL anywhere. For this, the prototype had to be portable, wireless and to have good autonomy. In addition the prototype performances were evaluated in terms of repeatability and sensitivity. The primary design is based on a closed-chamber to limit the effect of the environment. Moreover, the use of a fan to remove surface water and homogenise the micro-climate in the chamber was introduced. Mathematical modelling and measurements of water vapour and heat flux in closed TEWL chambers of a range of geometries was investigated to establish ideal sensors locations and blower flow rates. It was found that the fan decreased repeatability but increased sensitivity. In addition, it decreased coefficient of variation of measurements on moist skin, contrary to other TEWL chamber methods. Furthermore, lower fan speed yielded better precision. Finally, the gradient calculation is the crucial part of a TEWL measurement and Least Square Fitting was used to stabilise the readings and improve repeatability at the expense of computational speed. The work suggests that it should be possible to develop a viable new design that can generate measurements in the field that are easy to make and reliable but further work is needed to establish accuracy evaluation from a calibration with a fixed quantity of water and to develop a full commercial product