TRANSEPIDERMAL WATER LOSS (TEWL) MEASUREMENTS WITH TWO
NOVEL SENSORS BASED ON DIFFERENT SENSING PRINCIPLES
M. Mündlein1, B. Valentin1, R. Chabicovsky1,*, J. Nicolics1, J. Weremczuk2, G. Tarapata2,
Vienna University of Technology, Institute of Sensor and Actuator Systems, A-1040 Vienna, Austria
Warsaw University of Technology, Institute of Electronic Systems, 00-665 Warsaw, Poland
*Corresponding author: Rupert Chabicovsky, Phone +43-1-58801-36651, Fax +43-1-58801-36699,
e-mail [email protected]
Abstract: In this paper we compare results of TEWL measurements carried out with two different instruments
recently developed in our institutes. One sensor is based on a dew point hygrometer, while the other one uses
a conductance type humidity sensor. In both cases the closed chamber measuring method is used.
Furthermore, we have applied the same principle of calibration to both instruments.
Keywords: TEWL, dew point hygrometer, conductance type humidity sensor
1. INTRODUCTION arise when this barrier function does not work
Measurement of the transepidermal water loss
(TEWL, expressed in grams per squaremeter and Several techniques have been developed to
per hour) is used for studying the water barrier measure the skin properties that are influenced by
function of the human skin . The more perfect the water content. One possibility is the
the skin protective coat, the higher the water measurement of the transepidermal water loss of
content and the lower the TEWL (Fig.1). the skin.
TEWL measurements allow to discover
exogenic factors disturbances in the skin protective function in an
(germs, bacteria) early stage, even before they are visible. Normal
skin allows water loss only in small amounts. In the
case of atopic skin the water loss is much higher.
The determination of the TEWL is an important
support to investigate the skin irritation that occurs
by various physical and chemical influences.
corneum Typical fields of application are allergic tests,
occupational medicine, observation of the newborn,
supervision the healing process of skin damages
and burns or testing the effectiveness and
low TEWL a) biocompatibility of cosmetic products. Different
methods for TEWL measurement from local skin
exogenic factors sites have been described: Closed chamber
(germs, bacteria) methods and open chamber methods .
2. OPERATING PRINCIPLES
In this paper we report on two novel TEWL
instruments based on the closed chamber method.
corneum The microsensor is placed in a housing which
forms a closed measuring chamber after touching
the skin. The water vapour emitted from the skin
fills the small measuring chamber and causes an
high TEWL b) increasing relative humidity inside the chamber.
The growing rate of the humidity is a measure for
Figure 1. Schematic illustration of the barrier function of
the TEWL value of the skin. Some recovery time is
the stratum corneum. a) healthy skin , b) disturbed skin. necessary after each measurement. The sensor
chip must have enough time to try up and to reach
The outer part of the skin is the stratum corneum its initial condition before starting a new
which forms a barrier against diffusion of water and measurement.
is also an effective barrier for microbes and
chemical substances. The stratum corneum The first instrument (later on called instrument A,
contains much water and is flexible in the healthy Fig.2) is based on a ceramic chip carrying an
state, but it becomes hard and brittle when interdigital electrode structure which is covered by
dehydrated. Disorders such as atopic dermatitis a hygroscopic anorganic salt film.
experimentally determined function based on Ts
and tm. The algorithm optimizes Peltier couple
current and energy injected into the heater to
achieve fast detections and to follow humidity
changes in the surrounding environment. The
hygrometer can measure air humidity in the range
from 0 to 30 °C of dew point temperatures with
resolution 0.1 °C and accuracy 0.4 °C (with
detections of every 0.2 – 0.3 s).
In the case of instrument A a ceramic chip with the
dimensions of 5 mm x 5 mm x 0.6 mm is used. The
Figure 2. Schematic cross section of the measuring head chip is mounted in a distance of about 1.4 mm
of the instrument A (conductivity method). away from the skin surface. The lead-in wires are
guided through funnel-shaped holes to the rear
The main sensing effect used in this instrument is substrate surface (Fig.2). They are bonded to the
the conductance change of the hygroscopic film contact pads of the chip by using an isotropically
represented by the real part of the admittance [3-5]. conductive adhesive. The width of the electrodes is
The measuring frequency is 500 kHz. The about 55 µm, the gap between interdigital
admittance Y is measured by using a Precision electrodes is approximately 15 µm (Fig.4). The
LCR Meter. Furthermore, it is necessary to electrodes are made of a double layer of
measure the relative humidity RH of the ambient molybdenum (0.2 µm) and gold (8 µm). The
air. After a time interval tm (30 s) starting at the molybdenum film is deposited by RF-sputtering.
moment of touching the skin the change of The gold film must be deposited in such a way that
admittance (Ys at the starting time, Ym after the it completely and safely covers the underlying
measuring interval tm) is recorded. The TEWL value molybdenum electrode including side walls.
is calculated from tm, (Ym – Ys) and RH by using an Therefore, the gold film is produced by
experimentally determined function. electroplating. It has an important protective
function against chemical degradation. The active
The second instrument described in this paper moisture sensing area is 1.75 mm x 3.15 mm and
(later on called instrument B, Fig.3) is based on a is covered with a hygroscopic anorganic salt film.
silicon chip, which is mounted on a Peltier couple.
The sensing effect of this instrument is the change
of the dew point temperature by the emission of
water from the skin [6,7].
molecules Peltier module
Figure 4. Schematic cross sectional view of the electrode
Human skin system of the instrument A (conductivity method).
Sensing area covered by an anorganic salt film.
In the case of instrument B the hygrometer is
Figure 3. Schematic cross section of the measuring head based on a silicon semiconductor structure, which
of the instrument B (dew point method).
contains a dew point interdigital impedance
detector (on level II), and thermistor and heater (on
level I). The chip face is positioned in a distance of
At the moment of touching the skin the actual dew
8 mm away from the skin surface. On the silicon
point recording process is started. Depending on
the humidity value the hygrometer takes about 5 substrate (385 µm thick) the following layers were
formed and patterned: thermal silicon dioxide (0.3
readings (detections) per second. After the time
µm), gold (0.25 µm as thermoresistor and heater),
interval tm (5 s) the dew point temperature Ts is
recorded. A value of TEWL is calculated with an silicon dioxide (0.6 µm) and gold (0.15 µm) as
impedance detector. The impedance detector
electrodes pitch was set to 6 µm (Fig.5). The
sensitive detector area opened in the flexible PCB
is approximately 2 mm x 2 mm. Finally a flip-chip
technology was used for electrical structure measurement
input/output bonding into flexible PCB ribbon. head
I level (thermometer + heater)
II level (dew point detector)
6µm 6µm contact pad
Figure 5. Schematic cross sectional view of the electrode
system of the instrument B (dew point method).
The experimental methods used for the calibration
of the instruments A and B are practically the same Figure 6. Schematic illustration of a TEWL generator
and differ only in marginal details. To generate a used for the calibration of the instruments.
certain TEWL value we have used a small and light
vessel containing some water and covered by a
semipermeable diaphragm. The water evaporation 5. MEASUREMENTS AND RESULTS
rate (ER) can be calculated from the formula
Measurements with the two different instruments
∆m have been carried out in an office room at a
ER = (1) temperature of 25°C and a relative humidity of
A ⋅ ∆t
with the mass loss ∆m in grams, the time interval
∆t in hours and the area of the diaphragm A in
squaremeter. To determine the water loss the
vessel is arranged on a precision balance. First a
stable value of ER is generated by using a proper
diaphragm. Then the TEWL sensor to be tested is
placed on top of the diaphragm whereby the sensor
head touches the diaphragm forming a closed
measuring chamber (Fig.6). The evaporation rate
defined by (1) corresponds to the TEWL in case of
a measurement on the human skin. Hence the
evaporation rate of the calibration configuration is
also called TEWL. It is supposed that the
evaporation rate is uniformly distributed over the
surface of the diaphragm. Different TEWL values Figure 7. Comparison of measurement results. The
can be adjusted either by applying a different upper curve refers to instrument A (conductivity
number of membrane layers or different types of method), the lower curve belongs to instrument B (dew
membrane materials (methods used at WUT). point method).
Different TEWL values can also be achieved during
a long term drying process of the vessel (method We have investigated five persons in the age
used at VUT). between 19 and 65 years. The measurements were
carried out in the crook of the left arm. The test
The instrument A (developed at VUT) has been persons have been asked to rest at least 30
calibrated at different values of relative humidity of minutes before starting the measurement in order
the ambient air in the range from 31 % to 64 %. to avoid errors caused by perspiration. One
characteristic result is shown in Fig.7. The persons humidity sensor by Prof. Dr. T. Komeda, Shibaura
(all male) are ordered in their age: person 1: 19 y, Institute of Technology, Japan. They are also very
person 2: 27 y, person 3: 27 y, person 4: 32 y, grateful to Prof. Dr. F. Paschke, Vienna University
person 5: 65 y. of Technology, for financial support. The authors
from WUT acknowledge the support of the TEWL
6. DISCUSSION sensor research project through COMBAT (EU FP5
Project QoL/Growth/EESD-2001-INTEGR). They
also would like to thank very much Mrs. H.
The results presented in Fig.7 show that both Wrzesinska, Mrs. M. Gorska and their co-workers
instruments indicate the individual differences in from Institute of Electron Technology, Warsaw,
the TEWL value of the investigated persons. It can Poland for the sensors structures fabrication and
be clearly seen that the TEWL of subject 3 differs very helpful discussions.
significantly from the other subjects. Furthermore,
we find that the old person 5 has a lower TEWL
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