Poster Title: A Practical Guide to Computerized Evaporimetry

Poster Abstract: G. Grove, ]. Damia & C. Zerweck

Our group has nearly 30 years of experience with a variety of devicesthat allows one to measure evaporative water loss from human skinnon-invasively. These include both closed-chamber and openchamber devices that were either laboratory constructed prototypesor commercially available instruments such as the ServoMedEvaporimeter and the DermaLab TEWL Probe. Many of our studieshave monitored the response of the stratum corneum barrier to avariety of mechanical, physical and chemical insults by measuringTEWL rates under steady-state conditions at appropriate times afterthe insult has been delivered and the test products administeredeither prophylactically or therapeutically. Other studies such as thePost Occlusive Stress Test and evaluating the absorbance of diapershave required these assessments to be made under dynamicconditions. As a result of our considerable experience, we have agood appreciation of the various problems and pitfalls that one canencounter in these types of investigation. In this poster we willdiscuss different design considerations, various calibrationprocedures, performance tests and other quality assurance concernsrelated to evaporative water loss rate measurements

Conflict of Interest Disclos1u·e Statenlent

Dr. Gary Grove and his group at cyberDERM, inc. in Media, PAare the North American agents for Cortex Technology, Hadsund,Denmark who are the manufacturers of the DermaLab TEWL Probe.

cyberDERM, inc. also custom builds computerized systems formeasuring EWL based on either closed chamber or open chamberdesigns. These include the cyberDERM RG-l and CCM-l modelEvaporimeters. cyberDERM, inc. also provides world-wide technicaland service support including calibration for these types of devices.

INTRODUCTION

Our group has nearly 30 years of experience in measuringevaporative water loss from human skin. While with AI Kligman'sgroup at Penn, we utilized various closed chamber systems that wererather crude in that they used gravimetric assessments of the amountwater pick up over various times. Even with the advent of electronichumidity sensors, we realized that closed chamber systems shouldonly be considered as being a "quick and dirty" measurement device.Although we had better success with ventilated chamber systems, itwas very difficult to establish what flow rates would provide the mostreliable values. It was only when we began to use the Servo-MedEvaporimeter based on Gert Nilsson's Vapor Pressure GradientEstimation Method [1] that we began to feel comfortable with thesetypes of assessments. We soon learned that we could greatlyincrease the reproducibility and reliability of EWL measurements bydeveloping a computerized system. Computerized evaporimetry notonly made it much easier to determine when steady-state conditionswere reached but also allowed measurements to be taken in real timeunder dynamic conditions needed to evaluate the performance ofbaby diapers [2]. Some of our design elements were incorporated intolater versions of the ServoMed Evaporimeter and more recently intothe DermaLab TEWL probe manufactured by Cortex Technology[3,4]. Our latest effort is the cyberDERM RG-1 Evaporimeter which isa research grade instrument that utilizes the same TEWL probes asthe DermaLab but with advanced electronics in a smaller footprintbox that is USB interfaced to a laptop.

As a result of our considerable experience in offering technicaland service support for both the ServoMed and DermaLabinstruments, we have become very familiar with the typical problemsand pitfalls that our customers have encountered over the years. Wehave also learned quite a bit about evaporative water loss whiledeveloping our SSS bioassay which is based on shed snake skinspecimens exposed to various toxic agents [5,6].

In this poster, we hope to provide some guidelines on how tomaximize the performance of any open chambered device based onNilsson's Vapor Pressure Gradient Estimation Method.

THE OPEN CHAMBER EVAPORIMETER

The key to understanding the open chamber devices based onNilsson's Vapor Pressure Gradient Estimation Method is the pairedsensors that are located at fixed distances above the surface fromwhich the water is evaporating as diagrammatically shown below:

Thermistors

EV APORA TIONCylindricalCapsule

Relative- Humidity

Sensors

In order to achieve maximum performance, three major questionsneed to be answered. They are ...

• Are the sensors properly calibrated for both temperature and relativehumidity?

• Are the sensors, properly positioned?

• Do the sensors have a rapid and equal response when either temperatureand/or RH changes?

How to best answer these 3 questionswin be discussed in this poster.

Are the sensors properly calibratedfor both temperature and relative humidity?

For sure this is the first question and in fact mistakenly often theonly question that the user may consider worth addressing. To makematters worse, the Factory Calibration Certificates generally only dealwith whether or not the sensors provide the proper temperature andrelative humidity readings when the unit left the factory.

For both temperature and RH one should always use referencestandards that are traceable to NIST. In the case of RH this involvesa series of saturated salt solutions that are known to generatedifferent RH levels in the head space just above the liquid level.There was some debate when the OermaLab was first introducedover whether a 2 point or 3 point calibration check was best but it isnow generally agreed that a 2 point approach based on saturatedsolutions of Lithum Chloride and Sodium Chloride is sufficient.

Although kits do exist for calibrating in the field, we believe it best thatthe factory calibrate the probes since they have not only moreexperience but also much more sophisticated electronic equipmentand truly traceable references standards that are required for thesecritical adjustments.

Routine Performance Check

Although we don't recommend that the user attempt to adjustthe calibration of their unit, we do strongly urge them to run RoutinePerformance Checks immediately before and after a measurementsession. This simple test will reveal how well the upper and lowersensors are matched.

It just requires placing the EWLprobe on its side on top of a drysurface. In this position, both theupper and lower sensors are beinglicked by the same air.

This means that the temperature and relative humidity readings of theupper and lower sensors should be nearly identical and correspond tothat of the ambient room conditions. This can easily be checked withthe cyberDERM diagnostic software as shown below:

If the temperature and the relative humidly of the ambient airare known as would be the case for an environmental chamber, thenone can easily establish tolerance limits for assessing the calibrationstatus of that EWL probe. Indeed, it is a simple matter to create asource document that would allow the Routine Performance Checks

to be logged and documented for QA purposes. Discrepancies meanthat either the EWL probe and / or the environmental room conditionsare out of spec.

Are the paired sensors, properly positioned?

It is generally assumed that if the Open Chamber Evaporimeterpasses the Routine Performance Check that the device is in perfectworking order. However, this only establishes that the paired sensorsare properly calibrated as to temperature and RH. What is oftenforgotten is that the EWL calculations are based on those 2 sensorsbeing located at fixed distances not only from one another but alsorelative to the surface being measured. If this geometry shouldaccidentally change, then the sensors would no longer be samplingthe vapor pressure gradient at the proper points and the calculatedEWL would certainly be in error.

In order to verify that the sensors are indeed properlypositioned, we run another test based on a model system in whichthe relative evaporative water loss rates are known and held constantover time. This is not a new concept and in fact was discussed insome detail in the original studies by Gert Nilsson [1].

free evapora! ion

vapour permeableframe

saline __ ----=-

moistened ai r

reduced evaporation ~specimen_ f1' I I.screen

evapo;ation"i :"

/water permeable

membrane

A simplified but temporary version based on covering a dish of waterwith a Op-site menbrane was proposed by the Standardization Groupof the European Society of Contact Dermatitis [7]. In our comparativemetrology study [5], we used a reference substrate based on an openfoam material which was fully loaded with a standard amount of waterand covered with polymeric films that differed in water vaportransmission rates. A similar system is utilized in the SSS-bioassayexcept that the shed snake skin specimens [5,6] take the place of thepolymeric films.

The Wet Substrate Test

Regardless of which artificial wet surface is chosen, theunderlying principle of this test is that if the paired sensors areproperly positioned and the test conditions remain constant over time,then the calculated EWL will also remain constant. Not only that butthe paired sensor outputs for both RH and temperature although nolonger equivalent will show values that should vary little from day today. Again it is a simple matter to create a source document thatwould allow results from the Wet Substrate Test to be logged anddocumented for QA purposes. We also include the results from thistype of test when we calibrate a probe to cyberDERM specs.

Do the sensors have a rapid and equal responsewhen either temperature and/or RH changes?

With dynamic assessments such as a Post Occlusion StressTest or for evaluating the performance of diapers immediately afterremoval, it is extremely important that both sensors respond rapidlyand equally to these changes. In such critical studies, we alsorecommend that a dual probe instrument be used and their outputspooled to compensate for variations in probe performance. You cansee how well matched they are by simultaneously placing both on awet substrate and plotting the EWL over time as shown below:

Seconds

For steady-state measurements such as determining theintegrity of the stratum corneum barrier, how fast the sensors respondis less critical. The only real consequence is that a probe with a slowresponding sensor will take longer to achieve steady-state but thevalue finally obtained will be a reliable indicator of EWL rates. Forthis reason, we generally advocate using interactive programs wherethe operator determines when steady-state conditions are meetrather than having the program run for a fixed duration and thenautomatically stops.

cyberDERM Training Course

In this poster we have tried to stress various Qualatity AssuranceConcerns realted to the used of Open Chamber Devices for measuringEvaporative Water Loss based on Gert Nilsson's Vapor Pressure GradientEstimation Method. As you can see at the top of our list shown above is highlvtrained operators. As part of our Certified Operator Program, cyberDERM hascreated a CD with a series of training courses in the form of PowerPointpresentations with voice overlays that are available for free upon request.

Training includes practical tips of how to take the measurements and yes wehave found it necessary to show some operators which side of the probe shouldbe placed on the skin, hence the pointer arrows.

References:

1. Nilsson GE. On the measurement of evaporative water loss.Methods and clinical applications. Thesis. Linkoping: LinkopingMedical university Dissertation #48, 1977.

2. Davis JA, Leyden JJ, Grove GL, Raynor. Comparisons ofdisposable diapers with fluff absorbent and fluff plus absorbentpolymers: effects of skin hydration, skin Ph and diaperdermatitis. Pediatric Dermatol1989: 6:102-108.

3. Grove GL, Grove MJ, Zerweck C, Pierce E. Comparativemetrology of the evaporimeter and the DermaLab TEWL probe.Skin Res Technol1999: 5:1-8.

4. Grove GL, Grove MJ, Zerweck C, Pierce E. Computerizedevaporimetry using the DermaLab TEWL probe. Skin ResTechnol1999: 5:9-134.

5. Grove GL, Grove MJ , Damia J. Snake exuviate as an in vitromodel of stratum corneum: a novel approach for evaluatingenvironmental insults presented at Stratum Corneum IV, BaselSwitzerland, 2002.

6. Grove GL, Grove MJ , Damia J. The Shed Snake Skin or S3Bioassay: a novel approach for evaluating environmentalinsults presented at the 2004 ISBS Regional Meeting, Orlando.

7. Piinagoada J, Tupker RA, Agner T, Serup J. Guidelines fortransepidermal water lossd (TEWL) measurements. A report ofthe Standardization Group of the European Society of ContactDermatitis. Contact Dermatitis 1990: 22: 164-178.