| The importance of TEWL as a measure
of the skin barrier is well recognised. Currently, the open-chamber
method is dominant, but it is increasingly challenged by newer
closed-chamber technologies. Whilst there is familiarity with open-chamber
characteristics, there is uncertainty about the capabilities of
the challengers. The main issues are related to how microclimate
affects TEWL measurements.
The aim of this paper is to provide a framework for understanding
the effects of microclimate on TEWL measurement. Part of the
problem is that TEWL measurement is indirect. TEWL is the diffusion
of condensed water through the stratum corneum (SC), whereas
TEWL methods measure water vapour flux in the air above the SC.
This vapour flux depends on (i) the rate of supply of water to
the skin surface and (ii) the rate of evaporation of water from
the skin surface. Rate (i) is a skin property (TEWL), rate (ii)
is a microclimate property. The controlling rate for the combined
process is the lower of the above two rates. Therefore, TEWL
instruments measure TEWL only when TEWL is the rate-limiting
process.
Another problem is that SC barrier property and SC hydration
are affected by the microclimate adjacent to the skin surface.
This is discussed insofar as it affects the measurement of TEWL.
The conclusion is that such changes occur on a timescale that
is long compared with TEWL measurement times.
An important aspect of TEWL measurement is calibration. We present
an analysis of the traditional wet-cup method and a new droplet
method that is traceable and has been independently verified
by a standards laboratory.
Finally, we review performance indicators of commercial closed-chamber
instruments with reference to open-chamber instruments. The main
findings are that TEWL readings correlate well, but there are
significant differences in the other aspects of performance. |
