1. What is TEWL and how do you measure it ?
(a) What is TEWL ? |
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TEWL is the flux of (liquid) water diffusing through the Stratum Corneum (SC), from the Viable Epidermis (wet) to the surface (dry). The skin surface remains dry because the TEWL water evaporates into the adjacent air. | |
(b) How do you measure TEWL ? |
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You cannot measure TEWL directly because that would imply measuring water flux inside the SC. Instead, TEWL is measured indirectly, by measuring the evaporation flux in the adjacent air. Of course, this only works as long as TEWL is the ONLY source of evaporation flux, ie skin surface must be dry and there must be no sweating.
Illustrated on the left are three instruments (evaporimeters) that are commonly used for measuring TEWL. They use three different measuring methods as described below. |
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(i) Open-chamber Method |
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| The measurement chamber comprises a hollow cylinder open at both ends, one end of which is placed into contact with the skin. The other end acts as an exhaust allowing the water vapour from the skin to escape into the ambient atmosphere. Under ideal conditions, the air inside the chamber is perfectly still and water evaporating from the skin surface diffuses through the chamber and into the ambient atmosphere. When water evaporates from the skin surface, the humidity of the air next to the skin increases above ambient humidity while the humidity at the exhaust opening remains close to that of ambient air. Sensors for relative humidity and temperature are positioned on-axis at two locations close to the skin surface, to measure the humidity gradient. According to Nilsson’s diffusion gradient measurement principle [1], this humidity gradient can be used to calculate the flux density (i.e. the amount of water diffusing through the SC per square metre of skin surface per hour). | ||
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(ii) Condenser-chamber Method |
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| The measurement chamber is in the form of a cylinder about the size of a thimble. One end of the chamber is closed by a condenser maintained at a controlled temperature several degrees below the freezing point of water. The other end of the chamber is open and acts as the measurement orifice that is placed into contact with the skin. The condenser continually removes water vapour originating from the skin, storing it as ice. This maintains a low humidity at the condenser, whereas the humidity at the skin surface increases with increasing water evaporation rate. The resulting humidity gradient is calculated from two humidity values measured at two spatially separated points. Relative humidity and temperature sensors mounted in the chamber wall provide one value. A second value comes from the condenser, where the humidity can be calculated from its temperature without the need for a second humidity sensor. This humidity gradient is used to calculate flux density using the same diffusion gradient measurement principle as the open-chamber. | ||
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(iii) Unventilated-chamber Method |
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| The measurement chamber is in the form of a cylinder about the size of a thimble. One end of the chamber is closed, the other end has a measurement orifice that can be placed in contact with the skin. The chamber is equipped with sensors for relative humidity and temperature. Water vapour from the skin surface collects in the chamber from which it cannot escape. This causes the humidity to rise with time, slowly at first but linearly thereafter. The flux density is calculated from the slope of the linearly rising part of the curve. After the measurement is complete, the chamber needs to be lifted from the skin to allow the accumulated water vapour to escape, otherwise the humidity would rise towards saturation level. | ||
References |
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| [1] | GE Nilsson. Measurement of Water Exchange through Skin. Med Biol Comput. 15: 209-18 1977. | |
| [2] | RE Imhof, EP Berg, RP Chilcott, LI Ciortea and FC Pascut. New instrument for measuring water vapour flux density from arbitrary surfaces. IFSCC Magazine. 5(4): 297-301, 2002. Click here to download in pdf format. | |
2. How do AquaFlux™ TEWL measurements compare with conventional open-chamber measurements ?
| The AquaFlux™ out-performs all its competitors in terms of accuracy, sensitivity, repeatability, reproducibility and versatility. Below, we present data on three aspects, namely (a) how do they correlate, (b) how do the readings compare and (c) where’s the difference.
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(a) How do they correlate ? |
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| In ideal conditions, open-chamber measurements correlate well with condenser-chamber measurements. This was illustrated in a side-by-side comparison using Tewameter TM300 and AquaFlux™ AF200 instruments. The aim of the study was to assess skin damage and subsequent recovery caused by a 2% SLS solution applied occlusively for 24 hours on mid-volar forearm skin [1]. The experiments were performed on 22 healthy volunteers, with measurements taken at baseline, 2.5 hours after patch removal and repeated 24, 48, 72 and 96 hours thereafter. In all, over 650 same-site TEWL measurements, ranging in value from 4 to 83 g/(sq.m h) were performed with each instrument. The raw data are shown below, where the correlation between their readings was found to be r=0.98. | ||
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(b) How do the readings compare ? |
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Although the measurements correlate well, the readings generally differ. This is due to calibration differences, with no agreement among manufacturers about best practice. The calibration disagreement in the above study was found to be quite small, ~14%, but disagreements by a factor 2 or more are not uncommon. Such disagreements can be resolved by the traceable droplet calibration method developed in collaboration with the UK National Physical Laboratory.
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(c) Where’s the difference ? |
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| The main difference between AquaFlux™ and open-chamber methods is in the consistently high quality of AquaFlux™ measurements, especially in less than ideal conditions. This is illustrated below, where an AquaFlux™ signal is compared with those from two open-chamber instruments under well-controlled laboratory conditions using procedures recommended by the manufacturer, but without a shielding box. Ambient air movements do not affect closed chamber AquaFlux™ signals, but cause additional disturbance to open-chamber signals. | ||
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| Signal fluctuations cause scatter in TEWL measurements. One comparative test, for example, found AquaFlux™ TEWL repeatability to be more than ten times better (CV=1.4%) than comparable Evaporimeter repeatability (CV=16%) [2].
How this translates into practice is illustrated in the figure below, where two skin sites of the same volunteer were repeatedly tested in alternation. |
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| The somewhat higher scatter in the measurements on the palm of the hand may be due to skin heterogeneity and imprecise probe placement. Note that no air-conditioning or other ambient environment controls are needed for such measurements. The above data were obtained at home, watching TV. | ||
References |
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| [1] | I Angelova-Fischer, TW Fischer & D Zillikens. Die Kondensator-Kammer-Methode zur nicht-invasiven Beurteilung von irritativen Hautschäden und deren Regeneration: eine Pilotstudie. Dermatol Beruf Umwelt. 57(3):125 (2009). Click here to download the poster in pdf format. | |
| [2] | RE Imhof, EP Berg, RP Chilcott, LI Ciortea and FC Pascut. New instrument for measuring water vapour flux density from arbitrary surfaces. IFSCC Magazine. 5(4): 297-301, 2002. Click here to download in pdf format. | |
3. How do AquaFlux™ TEWL measurements compare with Delfin VapoMeter measurements ?
| The AquaFlux™ and the VapoMeter have one thing in common: they both use closed-chamber measurement methods. In every other respect, they are very different, right down to the measurement method itself. If it’s shirt pocket convenience you want, then forget about the AquaFlux™. If it’s performance you need, then forget about the VapoMeter.
A comparison between an AquaFlux™ Model AF102 and a Delfin VapoMeter was published as a poster at the ISBS Meeting in Philadelphia in 2005 [1]. Here we show a similar comparison with updates to illustrate the performance of our latest AquaFlux™ Model AF200.
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(a) In-vitro repeatability |
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| An in-vitro test can give you an important insight into how well an instrument is performing. Are the readings constant when the flux is constant ?
Below are the results of an experiment consisting of 200 repeat measurements using an upside-down wet-cup source with each instrument. More… |
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| These data are characterised by a coefficient of variation of CV=0.93% for the AquaFlux™ and CV=10.3% for the VapoMeter. This gives a measure of what each instrument contributes to the scatter in any experiment, in-vivo or in-vitro.The bigger the instrumental scatter, the less reliance you can place on a single reading. According to Gaussian statistics, you would need to average ~100 VapoMeter readings to match the precision of a single AquaFlux™ reading.
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(b) In-vivo repeatability |
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| For in-vivo measurements, skin variability and instrumental scatter combine to produce an observed scatter that is larger than either one alone. The figure below illustrates how this works in Gaussian statistics. | ||
Below is a table summarising the scatter of readings observed in an experiment consisting of 12 repeat measurements in rapid succession on 7 untreated sites of the volar forearm of an elderly volunteer. More… |
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| The AquaFlux™ scatter shows a clear pattern across the 7 sites: lowest in the middle part of the forearm and rising towards either end. There is no such pattern in the VapoMeter data. There is also a significant difference in the magnitude of the observed scatter. The mean over all 84 measurements works out to CV=3.8% for the AquaFlux™ and CV=10.2% for the VapoMeter.From the above it is clear that the much lower instrumental scatter of the AquaFlux™ enables it to resolve small differences in skin properties, heterogeneity in this case. With the Vapometer, the measurements are dominated by instrumental scatter.
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(c) TEWL Measurement speed |
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| The two instruments work very differently.
The AquaFlux™ records ~2 flux readings per second. TEWL measurement speed depends on skin condition and software settings. Flux readings settle quicker with dry, well acclimatised skin than with moist skin. Software settings determine how precisely the flux readings need to settle before a TEWL measurement terminates. The default criterion is a standard deviation of 0.075 g/(sq.m h) in a running average over the last 10 flux readings. You can adjust these values to trade off precision for speed. There is no waiting time between measurements – you can site-hop. The VapoMeter measures vapour accumulation rate rather than flux. A typical contact time of ~10 seconds is required to produce a reading. This is followed by a recovery period of up to 90 seconds, where the measurement chamber needs to be voided of accumulated water vapour before the next measurement can begin. AquaFlux™ and VapoMeter measurement times were compared in an experiment consisting of 12 repeat measurements in rapid succession on 7 untreated sites of the volar forearm of an elderly volunteer. Average repeat-times worked out to ~47 seconds for an AquaFlux™ Model AF200 and 38 seconds for a VapoMeter.
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(d) Quality control |
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| How do you know that you are measuring TEWL and not a momentary sweat gland emission or surface evaporation from a minute quantity of superficial moisture? The VapoMeter just gives you a number. The AquaFlux™ gives you detailed information in its recorded flux curves.
You’ve finished a study and are pondering the results. You spot something unexpected and are not sure how to interpret it. With the AquaFlux™ you can inspect the recorded flux curves and other supporting data. With the VapoMeter you’re stuck. Which one gives you more insight into what went on? Which one may save you having to do a repeat-study?
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(e) Versatility |
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| The VapoMeter is limited to two spot readings per minute or less. It is therefore difficult to make measurements of changing properties, such as recovery after occlusion or the evaporation of formulation water. By contrast, the AquaFlux™ measures continuous flux curves, sampled about twice per second. This, together with its controlled microclimate makes the AquaFlux™ uniquely versatile, providing detailed information about sample property changes with time. | ||
References |
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| [1] | RE Imhof, P Xiao, EP Berg & LI Ciortea. Rapid Measurement of TEWL with a Condenser-chamber Instrument. Poster, ISBS World Congress on Non-Invasive Studies of the Skin, Philadelphia, September 2005. Click here to download in pdf format. | |
4. What is the TEWL measurement speed of the AquaFlux™ ?
| TEWL measurement speed depends on several factors, including instrumental characteristics, skin condition and the accuracy you want.
The AquaFlux™ determines TEWL from water vapour flux density measurements performed at a rate of ~2 per second. When you first place the measurement head onto the skin, the conditions in the measurement chamber are perturbed and it takes some time for the instrumental readings to settle. From then on, the software is in control, looking for the end-point that meets your accuracy requirements.
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Normal TEWL MeasurementNormal TEWL measurements on acclimatised, healthy skin are quick, because there is no skin surface water (SSWL) to slow things down. According to the TEWL guidelines for open-chamber instruments [1, 2], you need to wait for the probe to recover after every TEWL measurement before starting the next measurement. This is not necessary with the AquaFlux™. You can go from site to site without pause and get the job done with maximum efficiency.This in-vivo site-hopping technique was first tested with a now obsolete Model AF102 AquaFlux™ in a series of measurements on 7 sites of a volar forearm [3]. The updated measurement times using a Model AF200 AquaFlux™ were found to be as follows:- |
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Accuracy & Measurement TimeAs indicated above, measurement time can be traded for accuracy. If speed is important, then you can set less stringent TEWL endpoint settings in the software. The effect of this is illustrated in the three screenshots below. |
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| The main endpoint criterion used in the software is a user-specified Standard Deviation (StDev) over a user-specified number of the most recent flux density readings. This is indicated in the screenshots above by the cyan-coloured average bars, where the vertical length is ±1 StDev and the horizontal length indicates the data included in the average. The default settings for the AquaFlux™ are:-
StDev = 0.075 g/(sq.m h) over the 10 most recent flux density readings. How this looks in practice is indicated in the uppermost screenshot. These settings were used for determining the measurement times in the table above. If you want quicker measurements, then you can use a larger StDev setting. This is illustrated in the two lower screenshots with StDev settings of 0.3 and 0.5 g/(sq.m h). Clearly, the flux readings have not settled, but if you use the same settings throughout a study, then the relative TEWL values will be meaningful. These StDev values are typical for open-chamber instruments. The higher sensitivity of the AquaFlux™ makes them look inappropriate. |
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References
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| [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] | RE Imhof, P Xiao, EP Berg & LI Ciortea. Rapid Measurement of TEWL with a Condenser-chamber Instrument. Poster, ISBS World Congress on Non-Invasive Studies of the Skin, Philadelphia, September 2005. Click here to download in pdf format. | |||||||
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 |
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| 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.
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(b) Contact Pressure |
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| 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.
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(c) Atmospheric Pressure |
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| 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.
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(e) Probe Temperature |
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| 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].
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(g) Skin cooling |
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| 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.
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(f) Skin drying |
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| 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 |
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| [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. | |
6. How often do you need to remove ice from the condenser ?
| This is an end-of-the-day job, except for the wettest or most demanding applications.
Captured water vapour forms a layer of ice on the condenser. The thickness of this layer grows at the rate of ~26microns for every milligram (mg) of captured water vapour. To put this into context, with a typical volar forearm TEWL of 10g/(sq.m h), it would take over 2.5 hours of continuous measurement to accumulate 1mg of ice. The main effect of ice build-up is a gradual change of calibration, attributable mainly to the reduction of measurement chamber length. This is illustrated by the measurements below, where ice was accumulated by repeatedly calibrating an AF200 instrument by the droplet method |
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| The blue correlation line indicates a 1% change of calibration for every ~2.5mg of accumulated ice. This translates into ~500 volar forearm TEWL measurements at 10g/(sq.m h).In practice, you need to keep an eye on the Ice Monitor of the AquaFlux™ software. You normally leave the instrument running all day and get rid of the ice when the job is done. Before going home, you switch the instrument to stand-by mode and allow the ice to melt and the water to evaporate
If you do a lot of high-flux measurements, then you may need to stop for ~10 minutes to get rid of the ice. Switch to stand-by mode for a few seconds while you dab off the melt-water on the condenser using one of the fibre-free cleanroom swabs supplied with the instrument. When done, switch back on, close the measurement orifice and wait for the baseline to settle. |
7. AquaFlux™ calibration - why, how often, by whom & how ?
Why ? |
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| Calibration is crucial. Use multiple instruments, share data with collaborators, relate what you did yesterday with what you plan for tomorrow, defend your claims in court – the list is endless. The alternative is normalisation, but that wastes information.
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How Often ? |
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| You should check the flux density calibration at 3-6 month intervals, or before a major study. You can do this yourself in about half an hour using the calibration accessories supplied with the instrument.
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By Whom ? |
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| By you of course! You can also get it done as part of a Passport Service, but that involves cost, shipping and delay. Calibration accessories (calibrated micro-syringe & calibration caps) are supplied as standard with the instrument, so what’s to stop you? The software handles all the calculations and you’re done in less than half an hour. At the end you have the choice to use the new calibration or to discard it, so nothing is lost if you don’t quite get it right.
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How ? |
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| The AquaFlux™ uses the Droplet Method of calibration [1, 2], which has been verified by an independent Standards Laboratory (National Physical Laboratory, UK). Not only can this calibration be traced to fundamental measures, it has also been shown to bring condenser-chamber AquaFlux™ and open-chamber Tewameter readings into closer agreement. | ||||
| Calibration by the droplet method ensures that you get consistent measurements from instrument to instrument and from time to time. There is an additional calibration that cannot be performed by users because it needs specialised equipment. This is the calibration of the sensors in the measurement head. This calibration does not affect the flux density readings directly, but it is nevertheless important. We recommend that you have this done annually, as part of a Passport Service. | ||||
References |
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| [1] | RE Imhof & the TEWL Calibration Consortium. Towards a Traceable Calibration for Trans-Epidermal Water Loss. Post-deadline Poster, Stratum Corneum IV Congress, Paris, June 2004. Click here to download in pdf format. | |||
| [2] | P Xiao, RE Imhof, MEP de Jesus, Y Cui & the TEWL Calibration Consortium. A New Calibration Method for TEWL with Traceability to Measurement Standards. Contributed Talk, US-Regional ISBS Meeting, Orlando, October 2004. Click here to download in pdf format. | |||
8. What is the AquaFlux™ Passport Service and why do I need it ?
Biox offers a Passport Service that aims to return your instrument to as near new condition as possible. During the service we repeat all the checks and calibrations performed on new instruments and take remedial action where necessary.
Of course, you can perform your own checks, so there is an argument that a passport service may be unnecessary. Judge for yourself. Take a look at the tests we performed on your instrument when new, as listed in its Instrument Passport. Are you satisfied that your instrument would still pass those tests? Is the flux density calibration that you measure today still close to the current factory calibration?
We recommend a passport service be carried out annually, and before major studies where you may need independent proof of correct functioning. The passport service checks every aspect of performance, including all calibrations, speed of response and temperature sensitivity. On some older instruments we also retro-fit some hardware and firmware updates that bring performance up to modern standards.
9. Are there any published Guidelines for TEWL measurement, and how do they apply to the AquaFlux™ ?
| The quick answer is yes and no. Yes, much of what they recommend is valid for all measurement methods. No, they were written for open-chamber instruments and the AquaFlux™ is different. If you want the long answer, read on.
There are two published guidelines for TEWL measurement in general [1, 2], and one for TEWL measurement in non-clinical settings [3]. Guidelines [1, 2] consider open-chamber instruments only. Guidelines [3] consider both open-chamber and unventillated-chamber instruments. But none of these publications included any AquaFlux™ measurements. Guidelines [1, 2] discuss (a) person-linked variables, (b) environmental variables and (c) instrumental variables in some depth before presenting their recommendations for best practice. Most of the considerations of sections (a) and (b) are valid for all methods of measurement. The exceptions are section (a) of [2], where the discussion of skin surface temperature includes consideration of how this may affect the measurement probe, and section (b) of both both publications, which include discussions of air circulation and how this may cause fluctuations in open-chamber measurements. Neither of these are concerns for the AquaFlux™. Section (c) and elsewhere is instrument-specific and not applicable to the AquaFlux™. The table below presents a side-by side comparison of the recommendations of these guidelines with what we recommend for the AquaFlux™. |
References |
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| [1] | J Pinnagoda, RA Tupker, J Agner and 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 and the EEMCO Group. EEMCO Guidance for the Assessment of Transepidermal Water Loss in Cosmetic Sciences. Skin Pharmacol Appl Skin Physiol. 14, | |||||||||||||||||||||||||||
| [3] | J du Plessis, A Stefaniak, F Eloff, S John, T Agner, T-C Chou, R Nixon, M Steiner, A Franken, I Kudla & L Holness. International Guidelines for the In-vivo Assessment of Skin Properties in Non-clinical Settings: Part 2. Transepidermal Water Loss and Skin Hydration. 19(3), 265-78: 2013. | |||||||||||||||||||||||||||
10. Why should TEWL measure skin barrier function ?
| This question appeared in the title of a poster presentation at the US Symposium of the International Society for Bioengineering and the Skin in Orlando, October 2004 [1]. It concerned in-vitro membrane integrity testing with Franz cells, but the question is equally valid for in-vivo TEWL.
Our answer is that TEWL does indeed give a useful measure of barrier function because a good skin barrier will let through less water than a poor skin barrier. The difficulty arises because TEWL instruments do not measure TEWL directly. In fact, TEWL instruments are evaporimeters and they measure the flux of water vapour coming from the skin surface. You can interpret this measured water vapour flux as TEWL, if two conditions are satisfied, as follows:
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TEWL and nothing but TEWL.TEWL may not be the only source of water vapour flux. Other possible contributions include perspiration, evaporation of free surface water and leaks in the apparatus. The measured vapour flux can be related to the skin barrier only when these non-TEWL sources have been eliminated.
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All must evaporate.If you’ve satisfied that (1) above is OK, then TEWL will be your only source of water vapour. But remember that TEWL is the migration of condensed water through the epidermis, whereas TEWL instruments measure vapour flux from the surface. All the water arriving at the skin surface must evaporate for the vapour flux to be equal to TEWL. If the humidity in the measurement chamber is too high, then this won’t happen. |
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The AquaFlux™ is ideal for TEWL measurement, because it maintains a low microclimate humidity at the skin surface, irrespective of ambient conditions. This satisfies condition (2) above better than any other instrument on the market. It also helps with condition (1) by (a) providing the low humidity conditions necessary to get rid of any free surface water quickly, and (b) providing reliable, leak-free means to couple the measurement head to the surface of interest, including Franz cells [2]. |
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References |
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| [1] | RP Chilcott. Why should TEWL measure skin barrier function? Skin Res Tech 10(4). Abstracts p15, 2004. | ||
| [2] | RE Imhof, P Xiao, EP Berg & LI Ciortea. Franz Cell Barrier Integrity Assessment using a Condenser-chamber TEWL Instrument. Podium Prsentation, US Symposium of the International Society for Biophysics and Skin Imaging, Atlanta October 2006. Click here to download in pdf format. | ||
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Do you have a question? We’d like to hear from you.
1. What is contact imaging ?
| Contact imaging is a method for acquiring an image by direct contact between the object of interest and the sensor(s). Of course, this is a very wide definition and includes ultrasonic imaging, atomic force microscopy, microradiography, etc.
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The contact imaging device used in the Epsilon™ is a semiconductor fingerprint sensor. Its contact surface measures 12.8mm x 15mm and contains 76800 individual sensors, arranged in a rectangular array of 256 columns and 300 rows. These sensors respond to changes of capacitance and therefore to the dielectric permittivity (dielectric constant) of any electrically insulating materials that touch its surface. In terms of skin science, the capacitance response provides information about hydration, because water has an unusually high dielectric permittivity. |
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With contact imaging, there is no light, no optics, no focusing, no colours and no shadows. The “intensity” of the pixels is determined not by illumination, but by the property to which the sensor responds. The capacitance response of the Epsilon™ sensor provides a powerful means for studying skin hydration. |
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2. What is dielectric permittivity and why is it relevant ?
| Dielectric permittivity, also known as dielectric constant, is a property of insulating materials that characterises their interaction with an electric field. It is relevant because the Epsilon™ sensor responds to capacitance and capacitance depends on dielectric permittivity. The reason we emphasise dielectric permittivity rather than capacitance is calibration. Dielectric permittivity is a material property that can be used for calibration, whereas capacitance is a device property for which no calibration standards are available. The common symbol used for dielectric permittivity is the Greek letter epsilon (ε) and this is where the Epsilon™ got its name from.
The dielectric permittivity of some common materials is listed in the table below. |
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As can be seen in the table above, water has a particularly high dielectric permittivity and this is what capacitance-sensing instruments such as the Corneometer® and the Epsilon™ rely on for measuring hydration. |
3. What kind of sensor does the Epsilon™ use ?
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The Epsilon™ uses a Fujitsu MFB200 fingerprint sensor (Fujitsu Ltd, Japan), which has 76800 sensing elements arranged in a rectangular array measuring 12.8mm x 15mm. The contact surface is coated with a 2µm thick protective layer of SiO2.
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Each sensing element measures 50µm x 50µm and responds to the dielectric permittivity (dielectric constant) of the sample in contact with it. The native sensor response is digitised with 8-bit (0-255) resolution. |
4. What is the image resolution and measurement depth of the Epsilon™ ?
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The lateral resolution is 50µm, determined by the geometry of the sensing elements. | |
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The measurement depth is <5µm, determined by the penetration of the electric field generated by the sensing elements into the contacting material. |
5. Are there any patent restriction on my use of the Epsilon™ ?
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No, there are no longer any restrictions. Since April 2016, Biox has an unrestricted, non-exclusive, world-wide licence from L’Oréal to exploit their SkinChip patents EP1438922 & EP1177766 relating to non-optical imaging on non-dermatoglyphic skin, hair and mucous membranes. Under this licence, all Epsilon™ images and measurements claimed in these patents can be used for all purposes, including commercial purposes such as claims support and advertising. |
6. Can the Epsilon™ be used for skin hydration measurement ?
| Yes, the Epsilon™ can measure skin hydration with greater accuracy and flexibility than conventional single-sensor probes such as the Corneometer®. Both the Epsilon™ and the Corneometer use the same capacitance measurement principle. Both the Epsilon™ and the Corneometer have a sensing depth that confines the measurement predominantly to the Stratum Corneum. However, the Epsilon™ has 76800 sensors whereas the Corneometer has one. That’s a game-changer because skin is heterogeneous and skin-sensor contact is variable. The reason why the Epsilon™ can be used for hydration measurement is its unique calibration technology that converts the highly non-linear sensor output into a linear and calibrated response. By contrast, the MoistureMap (CK Technology sprl, Belgium) can only visualise hydration and needs a Corneometer for measurement. Our research shows that Epsilon™ and Corneometer measurements correlate well. The Epsilon™ measures hydration using its calibrated dielectric permittivity (dielectric constant, ε), scale rather than some arbitrary scale such as Corneometer Units. This works because the dielectric permittivity of water is much higher (ε~80) than that of other constituents of skin.
There are (at least) four reasons why Epsilon™ hydration measurements are superior to Corneometer hydration measurements, as illustrated below.
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1. Hydration Heterogeneity Assessment |
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| Epsilon™ images provide a revealing visualisation of hydration distribution in the vicinity of the site of interest. These images can be processed to give quantitative measures of heterogeneity in terms of Standard Deviation, Coefficient of Variation (CV%), or visually as images or histograms. This is illustrated in the volar forearm image shown below.
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The two images left are the same, but with the 2.6mm diameter green Region of Interest (RoI) circle displaced by a horizontal distance of 3.8mm. The mean hydration within the left RoI is close to that of the whole image, with a dielectric permittivity of ε~15 and a CV~90%.
The mean hydration within the right RoI is clearly higher than that of the whole image, with a dielectric permittivity of ε~24.4 and a CV~70%. From these data, the hydration of the two RoIs differ by more than 60%! |
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The Epsilon™ software displays colour-co-ordinated histograms that indicate hydration distribution for both the whole image (red) and the RoI (green). In this case, the hydration distribution within the left RoI is similar to that of the whole image, whereas there is a distinct shift towards higher hydration for the right RoI. | |
2. Correction for Skin-Sensor Contact |
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| The Epsilon™ software has a powerful ε-filter to correct for bad contact between the sensor and skin. Bad contact may be due to microrelief or wrinkles, hair or other obstructions. The example below illustrates how this works. | ||
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The image on the left is of an area of male ventral forearm, where the skin-sensor contact is impeded mainly by hair and the protruding sensor surround. The bad contact shows up as black because of the low hydration/ε of hair and air. The associated histogram shows the bad contact by a prominent peak at low values of ε. Note that the histogram uses a logarithmic scale, which makes the peak look less prominant than it is. | |
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The ε filter controls allow you to remove both low and high ε pixels from the image. | |
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The image and its histogram show the effect of filtering pixels with ε-values below 3.5. Pixels removed by the filter are shown in grey in both the image and the histogram. Clearly, the ~43% of pixels that remain give more accurate skin hydration information (ε = 18.1) than the unfiltered image (ε = 8.3). Note that you can also use the Region of Interest (RoI) controls to focus on specific features within the filtered image. | |
3. Correction for Skin Surface Water |
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| Skin surface water can be a problem when measuring hydration (i) in the vicinity of mucous membranes, occluded or damaged skin, (ii) in the presence of water-containing topical products, or (iii) in the presence of insensible perspiration or sweat. Skin surface water is not hydration, but its presence will increase hydration readings of conventional instruments. The example below illustrates how the ε-filter described in 2. above can deal with skin surface water. | ||
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The image on the left is of the second joint of a male left thumb. Its main feature is a size mismatch between the skin contact area and the sensor. However, the non-contacting pixels (76% of the total) have been removed by the ε-filter described in 2. above, as indicated by the grey colouring. For the remaining pixels, the mean hydration is ε~21.1 with a CV~60%.
The yellow/white spots are surface water from perspiration. The 2mm diameter RoI encloses an area where surface water predominates. Within this RoI, the mean permittivity is considerably higher (ε~27.8, CV~78%) than for the whole area of contact. |
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In the left image, the surface water was filtered out for ε-values above 60, as indicated by the light grey colour in the image and histogram. For the whole image, this filter changes the mean ε from 21.1 to 20.1. For the RoI, the change is from 27.8 to 21.3.
The effect of the surface water alone was a ~5% measurement error for the whole contact area and a ~23% measurement error for the RoI. |
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4. Time-dependent Measurementss |
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| The Epsilon™ system can be used to study time-dependent phenomena by recording short bursts or long videos. This is illustrated below with a study of occlusive surface water accumulation in a tape stripping experiment. Scotch tape was used on a volar forearm skin site, repeatedly stripping the same site. Epsilon™ bursts (60 second total duration @ 1 frame per second) were recorded on the intact site and after every second strip.
Occlusion by contact between the skin and the Epsilon™ sensor during the 60 second burst measurements causes TEWL to be trapped as skin surface water. Trapped skin surface water causes the dielectric permittivity to increase, which shows up in the images as a colour change from dark red through to yellow. |
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Shown here are example images from the recorded bursts. Note that significant barrier damage becomes visible after just 2 strips, as indicated by the increased rate of surface water accumulation compared with intact skin. Also, the barrier damage after 14 strips is highly heterogeneous, with the bright yellow areas indicating greater than average damage. | |
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These occlusion plots were calculated as whole-image averages. They clearly show (i) an almost unchanged hydration at t=0, irrespective of the number of strips removed, and (ii) increased barrier damage with number of strips removed. | |
These examples illustrate the capabilities of the Epsilon™ for static and dynamic skin hydration measurement on previously inaccessible sites such as hairy skin, scalp and skeletal joints, and under less than ideal conditions such as when sweat glands are active. |
7. Can the Epsilon™ be used for characterising Skin Micro-Relief ?
This facility is available in the the current release of the Epsilon™ software (Version 3), which you can download from the Supportsection of this website.
8. Can the Epsilon™ be used for characterising hair ?
Yes, both static and dynamic measurements can be performed. More soon.
9. How does the Epsilon™ compare with the MoistureMap ?
| Both the Epsilon™ and the MoistureMap (CK Technology sprl, Belgium) are contact imaging devices. Both the Epsilon™ and the MoistureMap use semiconductor fingerprint sensors. Both the Epsilon™ and the MoistureMap are based on L’Oréal SkinChip research, eg [1-2].
The big difference between the Epsilon™ and the MoistureMap is its linear and calibrated response. This uniquely enables the Epsilon™ to make quantitative capacitance-related measurements such as skin or hair hydration. By contrast, CK recommend that the MoistureMap should be used for visualisation only and that a Corneometer® should be used alongside the MoistureMap for quantitative measurement. As for the rest, see below. |
Attribute |
Epsilon™ Model E100 |
MoistureMap Model MM 100 |
| L’Oréal Patent Licence | Yes | Yes |
| Linear Response to Capacitance | Yes | No |
| Calibrated Response | Yes | No |
| Hydration Histogram | 256 levels of discrimination | 5 levels of discrimination |
| Hydration Measurement | Yes, 256 levels of discrimination | No, visualisation only |
| Skin Micro-Relief Analysis | Yes | Yes |
| Fingerprint Sensor | Fujitsu MFB200 | Upek TCS1CT |
| Number of Pixels | 256 x 300 = 76800 | 256 x 360 = 92160 |
| Sensor Area | 12.8mm x 15mm | 12.8mm x 18mm |
| Pixel Spacing | 50µm | 50µm |
| Measurement Head Dimensions | 27mm x 30mm | 30mm x 43mm |
| Spring-loaded Sensor | Yes | Yes |
| Live streaming | Yes | Yes |
| Image Format | Lossless TIFF | Jpeg |
| Video Format | Lossless AVI | AVI |
| Video Analysis | Hydration, Area | None |
| Event Trigger | Yes | No |
| Footswitch Trigger | Option | Option |
| In-vitro Adaptor | Included as standard | Option at extra cost |
| Interface | USB2 | Proprietary interface box |
| Power | USB2 | Separate 12V, 4A Supply |
| Warranty | 1 Year | 1 Year |
References |
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| [1] | JL Lévêque & B Querleux. SkinChip, a New Tool for Investigating the Skin Surface in-vivo. Skin Res Technol 9(4): 343-7, 2003. | |
| [2] | E Xhauflaire-Uhoda & G Piérard. Skin Capacitance Imaging. Chapter 13, Handbook of Cosmetic Science and Technology, 3rd Edition, 2009. ISBN 9781841847436. | |
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