Biox Systems Ltd click to contact us
AquaFlux Questions >  Answer 5

AquaFlux Answer 5

Are AquaFlux TEWL measurements affected by (a) probe angle, (b) contact pressure, (c) atmospheric pressure, (d) probe temperature, (e) skin cooling, or (f) skin drying ?

(a) Probe angle

Angular dependence may be an issue with TEWL instruments, because natural convection air movements arising out of the temperature difference between the skin and the ambient air may disturb the measurement. However, different instruments using different measurement methods are affected differently, as follows:-

According to published TEWL Guidelines [1, 2] open-chamber instruments such as the C+K Tewameter can only be used reliably on horizontal surfaces.

Unventilated-chamber instruments such as the Delfin VapoMeter have also been found to be affected by probe angle [3], although the manufacturer claims orientation-independence.

For the condenser-chamber AquaFlux, detailed measurements show that the probe has an orientation-dependence that is non-symmetrical, depending on whether the RH and T sensors in the wall of the measurement chamber are above or below the chamber axis [4]. For negative angles, where these sensors are below the chamber axis, probe sensitivity was found to decrease by as much as ~6% relative to the calibrated value. For positive angles, where these sensors are above the chamber axis, probe sensitivity was found to remain within about ±1% of the calibrated value. The probe can therefore be used with all surface orientations with little effect on sensitivity, as long as its inclination is confined to the positive semi-circle.

(b) Contact Pressure

Increases of contact pressure between the skin and a TEWL measurement head may produce changes to the skin and changes to the measurement geometry. Changes to the skin include compression of the underlying tissues and surface stretching, with the skin around the periphery of the measurement chamber depressed and the skin within the measurement orifice raised into a convex spherical shape. Changes to the measurement geometry include changes in the relative positions of the sensors and a decreased separation between the sensors and the skin surface. Decreases of contact pressure may compromise the seal between the skin and a measurement head and cause leakage and positional drift.

In practice, relatively large contact pressure effects have been reported for open-chamber instruments [6, 7]. Guidelines [1, 2] recommend that the contact pressure of the probe onto the skin should be kept low and constant, with measurements within a series preferably performed by the same operator.

For the unventilated-chamber VapoMeter, De Paepe et al [5] observed no statistically significant change of TEWL readings (from 7±2 to 8±3[g/(m2h)]) when the contact pressure was increased.

The effect of contact pressure on condenser-chamber AquaFlux TEWL measurements was assessed in an experiment using a spring balance to measure contact force. Measurements of mid-volar forearm TEWL were repeated 18 times on the same site with contact forces progressively increased then reduced in the range 0.1 to 2kg. The average TEWL over all 18 measurements worked out to 8.23[g/(m2h)] with a CV of 1.7% [4]. It appears, therefore, that the effect of contact pressure on TEWL measurement has more to do with measurement head design than skin barrier property change.

(c) Atmospheric Pressure

TEWL measurement methods all rely on evaporimetry, where TEWL is inferred from the water evaporation flux in the air immediately adjacent to the skin surface. All the commonly used methods involve the diffusion water vapour through air, from the skin surface to the sensor(s) and beyond. The associated mass diffusion coefficient, according to gas theory, changes with temperature and pressure and this affects the calibration of TEWL instruments.

The effect on open-chamber TEWL measurements was first discussed by Nilsson [6]. He concluded that, at a given location, weather-related changes of atmospheric pressure could affect TEWL readings by as much as ±6%. This was deemed to be too small for further consideration.

The effect on condenser-chamber AquaFlux and unventilated-chamber VapoMeter instruments was investigated by Kramer er al [8]. For the AquaFlux, measurements showed a clear trend of increasing TEWL readings with increasing atmospheric pressure (gradient = [150±22]x10-4 [g/(m2h)]/hPa). No trend with pressure was apparent in the VapoMeter measurements (gradient = [13±44]x10-4 [g/(m2h)]/hPa), although a weak dependence cannot be ruled out, given the relatively large standard deviation of the gradient.

(e) Probe Temperature

Probe temperature may be affected by hand heat as well as by ambient temperature. The guidelines for open-chamber instruments [1, 2] recognise probe temperature as an essential variable, with instrumental readings increasing with temperature until the probe and skin temperatures are similar. Their recommendation is not to touch the probe before and during measurements. Instead, they recommend to handle the probe with a burette clamp, its cable, a coating or by wearing gloves.

Unventilated-chamber VapoMeter readings have also been found to be strongly temperature dependent. By holding the VapoMeter between both hands, De Paepe et al [5] caused its temperature to increase by ~6°C and observed an increase of volar forearm TEWL readings from a baseline of 7±2[g/(m2h)] to 15±6[g/(m2h)], which works out to a temperature coefficient of ~1.3±1.1[g/(m2h)] per °C temperature change.

By contrast, the condenser-chamber AquaFlux probe shows little effect upon heating. A typical figure, calculated as the root-mean-square (RMS) average over 10 instruments, is ~0.05±0.06[g/(m2h)] per °C temperature change [4].

(g) Skin cooling

The heat flux caused by the low condenser temperature (conduction through the air & radiation) is too small to cause the skin surface temperature to change significantly. We went to some lengths to try to measure an effect, using highly sensitive thermocouples and superglue (PhD thesis, Don O'Driscoll, London South Bank University, 2001). The main finding was that skin cooling was dominated by conduction between the skin and the measurement head and any effect from the condenser was masked by this. No effect on measured TEWL values was found. Don has made a full recovery from his superglue ordeal.

(f) Skin drying

There is a measurable effect from prolonged contact with a condenser-chamber measurement head [9]. The main finding was that the TEWL decreased at a rate of ~0.1% per minute of contact. Given that a typical TEWL measurement requires less than 1 minute of skin contact, the effect on accuracy is negligible. These experiments used an AquaFlux Model AF100 instrument with a condenser temperature of -13.4°C. The current Model AF200 AquaFlux uses a condenser temperature of -7.6°C, which has even less effect on TEWL.

References

[1]   J Pinnagoda, RA Tupker, J Agner & J Serup. Guidelines for Transepidermal Water Loss (TEWL) Measurement. A Report from the Standardization Group of the European Society of Contact Dermatitis. Contact Dermatitis 22: 164-78, 1990.

[2]   V Rogiers & the EEMCO Group. EEMCO Guidance for the Assessment of Transepidermal Water Loss in Cosmetic Sciences. Skin Pharmacol Appl Skin Physiol 14: 117-28, 2001.

[3]   JC Cohen, DG Hartman, MJ Garofalo, A Basehoar, B Raynor, E Ashbrenner & FJ Akin. Comparison of closed chamber and open chamber evaporimetry. Skin Res Tech 15: 51-4, 2009.

[4]   RE Imhof, MEP De Jesus, P Xiao, LI Ciortea & EP Berg. Closed-chamber TEWL measurement:- microclimate, calibration and performance. Int J Cosmet Sci 31: 97-118, 2009.

[5]   K De Paepe, E Houben, R Adam, F Wiesemann & V Rogiers. Validation of the VapoMeter, a closed unventilated chamber system to assess transepidermal water loss vs. the open chamber Tewameter. Skin Res Tech. 11: 61-9, 2005.

[6]   GE Nilsson. Measurement of Water Exchange through Skin. Med Biol Comput. 15: 209-18 1977.

[7]   AO Barel, & P Clarys. Comparison of methods for measurement of transepidermal water loss. Handbook of non-invasive methods and the skin. (J. Serup, G.B.E. Jemec, eds), pp. 179-84. CRC Press Inc, Boca Raton 1995.

[8]   G Kramer, P Xiao, J Crowther & RE Imhof. Multi-location Clinical Trials: Do Tewl Readings Change with Altitude ? Poster Presentation, SCC Annual Scientific Meeting & Technology Showcase, New York 2015. Click here to download in pdf format.

[9]   LI Ciortea, E Fonseca, J Sarramagnan and RE Imhof. TEWL and Stratum Corneum Hydration Changes Caused by Prolonged Contact with a new TEWL Measurement Head. The Essential Stratum Corneum (R Marks, JL Lévêque & R Vögeli Eds) Martin Dunitz Ltd, London, UK, 2002, pp255-7. Click here to download in pdf format.


Home Page Top webmaster © Biox Systems Ltd Top