Invited Lecture, Intensive Course in Dermato-cosmetic Sciences, Vrije Universiteit, Brussels, 2007.
After a quarter-century of dominance, the open-chamber method of TEWL measurement is now challenged by new technologies using closed measurement chambers. Whilst the characteristics of the open-chamber method are well recognised, there is still uncertainty about the capabilities of the challengers.
This report provides the background to the oral presentation from the standpoint of measurement science: what are you trying to measure, how can you go about it, how do you interpret the data and what are the measurement errors. The main points from this analysis are:-
|1.||There are no direct methods for measuring TEWL.|
|2.||TEWL is condensed water diffusing through the skin.|
|3.||TEWL methods measure water evaporation from the skin surface.|
|4.||The microclimate of uncovered skin is dominated by air movements.|
|5.||All TEWL measurement chambers perturb the microclimate.|
|6.||SC property change is slow compared with TEWL measurement times.|
|7.||Wet-cup calibration methods are fundamentally flawed.|
|8.||The droplet calibration method is traceable and has been independently verified.|
|9.||Closed-chamber TEWL methods work.|
The report also presents some comparative data, to illustrate the performance of closed-chamber instruments, and a description of a membrane integrity test for in-vitro measurements of percutaneous penetration.
Click here to download the presentation (pdf format, 0.6MByte).