- Email: [email protected]
Construction, in vitro and in vivo evaluation of an in-house conductance meter for measurement of skin hydration
Medical Engineering & Physics 34 (2012) 1471–1476
Contents lists available at SciVerse ScienceDirect
Medical Engineering & Physics journal homepage: www.elsevier.com/locate/medengphy
Construction, in vitro and in vivo evaluation of an in-house conductance meter for measurement of skin hydration Saja H. Hamed a,∗ , Bilal Altrabsheh b , Tareq Assa’d b , Said Jaradat b , Mohammad Alshra’ah b , Abdulfattah Aljamal b , Hatim S. Alkhatib c , Abdul-Majeed Almalty a a
Faculty of Allied Health Sciences, Hashemite University, Zarqa, Jordan Faculty of Engineering, Hashemite University, Zarqa, Jordan c Faculty of Pharmacy, University of Jordan, Amman, Jordan b
a r t i c l e
i n f o
Article history: Received 11 May 2011 Received in revised form 6 December 2011 Accepted 20 February 2012 Keywords: Skin bioengineering Non-invasive measurement In-house conductance meter Skin hydration
a b s t r a c t Different probes are used in dermato-cosmetic research to measure the electrical properties of the skin. The principle governing the choice of the geometry and material of the measuring probe is not well defined in the literature and some device’s measuring principles are not accessible for the scientific community. The purpose of this work was to develop a simple inexpensive conductance meter for the objective in vivo evaluation of skin hydration. The conductance meter probe was designed using the basic equation governing wave propagation along Transverse Electromagnetic transmission lines. It consisted of two concentric copper circular electrodes printed on FR4 dielectric material. The performance of the probe was validated by evaluating its measurement depth, its ability to monitor in vitro water sorption–desorption and in vivo skin hydration effect in comparison to that of the Corneometer CM 825. The measurement depth of the probe, 15 m, was comparable to that of CM 825. The in vitro readings of the probe correlated strongly with the amount of water adsorbed on filter paper. Skin hydration after application of a moisturizer was monitored effectively by the new probe with good correlation to the results of CM 825. In conclusion, a simple probe for evaluating skin hydration was made from off-the-shelf materials and its performance was validated in comparison to a commercially available probe. © 2012 IPEM. Published by Elsevier Ltd. All rights reserved.
1. Introduction The water content of the skin, especially the stratum corneum (SC), is responsible for keeping the skin smooth and soft. Water content below 10% level is associated with visible appearance of dry flaky skin and impairment of its barrier integrity [1]. The measurement of hydration of the upper layer of the skin is an important tool for assessing the cutaneous effects of skin care products for their effectiveness and for monitoring the state of various skin diseases [2,3]. Different non-invasive devices for monitoring skin hydration are commercially available. These devices vary in their measurement modes which include electrical, microwave, and spectroscopic ones [4]. Each device offers a unique possibility of
∗ Corresponding author at: Faculty of Allied Health Sciences, Hashemite University, P.O. Box 150459, Zarqa 13115, Jordan. Tel.: +962 5 390 3333; fax: +962 5 390 3368. E-mail address: [email protected] (S.H. Hamed).
studying a specific phenomenon in intact skin that may correlate to skin hydration level. Magnetic Resonance (MR) spectroscopy and imaging method involves non-invasive measurement of proton density in the skin. The protons of water respond to the magnetic environment and useful parameters are determined that provide information about the concentration and distribution of water molecules in the epidermis [5]. Infrared (IR) spectroscopy relies on the characteristic of water molecule as an intense absorber of infrared spectroscopy. The infrared absorbance spectrum of water in the skin can be obtained with appropriate instrumentation and used to determine amount of water in the stratum corneum [6]. In addition to the abovementioned non-invasive techniques, a minimally invasive methods called squamometry is used to sample stratum corneum by adhesive coated discs to objectively assess the extent of surface scaliness that is correlated with extent of skin hydration [7]. More and more methods are available and it is quite difficult to have an overview of all of them. However, the most widely used devices are those operating by measuring the electrical properties
1350-4533/$ – see front matter © 2012 IPEM. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.medengphy.2012.02.008
1472
S.H. Hamed et al. / Medical Engineering & Physics 34 (2012) 1471–1476
Fig. 1. A cross section of a coaxial transmission line with inner conductor of radius a and outer conductor of radius b where the electric field lines are in radial direction between the inner and outer conductors (A) and an image of the new probe sensor designed based on coaxial transmission line basics, with inner conductor (exciter electrode) of radius a = 6 mm and outer conductor (sensing electrode) of radius b = 10 mm separated by 4 mm distance (B). The conductors were made of high conductivity material (copper) and were printed on an FR4 dielectric material. The separation distance, the radii of the sensing and exciting electrode, and the material of the two electrodes were chosen to design a probe with negligible resistance measured by its capacitance component (4.5 × 10−5 nF) in order to measure mainly the resistance of the skin.
of the skin, namely conductance and capacitance, as indirect indicators of stratum corneum water content [4]. Standard, commercially available devices for evaluating skin hydration include the conductance-meter Skicon 200 and the capacitance-meter Corneometer CM 825. Skin capacitance and conductance are correlated and, in general, show similar behavior [3]. When either probe is applied to the skin, the capacitance or conductance values recorded by the instruments correlate directly with the moisture content of the skin. Increasing the moisture content of the skin will increase its dielectric constant leading to increased capacitance and conductance. The Corneometer operates at a mean frequency of 1 MHz. The measuring probe consists of a 7 mm × 7 mm ceramic tile with many gold lines that are close to each other. The lines are 75 m wide with an interdigital spacing of 75 m. The parallel gold lines have such a small distance between them that they are only visible under a magnifying glass. The even and odd gold lines are connected to each other to form two conductor tracks separated by an insulating material to form a capacitor. The physical basis of capacitancebased devices is the difference of the dielectric constant of water and other substances brought in the electric measurement field of the sensor (i.e. capacitor). Thus, when the skin is hydrated a significant change in the dielectric properties of this medium is observed with a change in the total measured capacitance that is given in arbitrary units ranging from 0 to 120 AU. Further, the measuring surface of the probe is sealed with a thin special glass cover to protect the conductor track and ensures that there is no galvanic contact between the probe and the skin and that no current will flow to the skin. Hence the influence of ions and salts residues on conductance has no or little influence on the results which is an important advantage of this method as opposed to conductancebased method [8,9]. The Skicon measures the conductance of a single high frequency current of 3.5 MHz. The measuring probe consists of two concentric gold-covered electrodes of 2 and 5 mm diameters with a direct galvanic contact between the electrodes and the skin. The distance between the inner and the outer electrode is 1 mm. The two concentric electrodes are separated by a dielectric and the current flows when the probe is placed in the measurement area as soon as a galvanic contact of the probe with the skin surface is established. The measured electrical conductivity is dependent on skin moisture and is given in microsiemens (S) units ranging from 0 to 1999 S [8,9]. The reproducibility and reliability of the different electrically based devices available commercially in measuring skin hydration under conditions ranging from increased, normal, and low
hydration levels have been investigated and validated [10]. However, the principle governing the choice of the geometry and material of the measuring probe which affect measurement of the electric field in the skin is not well defined in the literature. Furthermore, the measuring principles are not described in sufficient details in the literature as some device’s measuring principles are considered proprietary and not accessible for the scientific community [9]. In the present study, we developed a simple and inexpensive probe to measure skin hydration level from material available off-the-shelf at the Biomedical Engineering Department in the Hashemite University. The probe measures skin conductance and is designed based on basic equation governing wave propagation along Transverse Electromagnetic (TEM) transmission lines. Transmission lines encompass all structures and media that serve to transfer energy between two points. When waves propagate along these lines are characterized by electric and magnetic fields that are entirely transverse to the direction of propagation then the transmission lines are called Transverse Electromagnetic (TEM) transmission lines. An example of TEM-mode transmission lines is the flow of current between outer and inner conductors that is made possible by the material conductivity of the insulator. If the material separating the inner and outer conductors is a perfect dielectric with conductivity = 0 then current flow between the outer and inner conductors equal zero. Transmission lines are called coaxial line when the electric field lines are in the radial direction between the inner and outer conductors [11]. The usefulness of the new probe in dynamic measurement of skin hydration parameters was validated in comparison with the Corneometer® CM 825 under standardized conditions. 2. Materials and methods 2.1. Construction of the probe The practical guidance for designing and constructing the measuring probe is based on the equation governing transmission in Transverse Electromagnetic (TEM) transmission lines. A common feature among TEM lines is that they consist of two parallel conducting surfaces. TEM transmission lines consist of two conductors that are transverse to the direction of propagation and act to support the propagation of electromagnetic waves [11]. The coaxial transmission line is one type of TEM that consists of an inner conductor (excitation electrode) of radius a separated from an outer conductor (sensing electrode) of radius b by an insulating material (Fig. 1).
S.H. Hamed et al. / Medical Engineering & Physics 34 (2012) 1471–1476
Our measuring probe, designed based on coaxial transmission line basics, has two circular electrodes, an excitation electrode and a sensing one. The conductors (i.e. electrodes) were made of high conductivity material (copper) and were printed on an FR4 dielectric material. The separation between the sensing and exciting electrodes was 4 mm which represents the signal propagation distance in the skin, while the radii of the sensing and exciting electrodes were 10 mm and 6 mm respectively as shown in Fig. 1. The circular shape of the two electrodes gives the advantage that the exciter electrode passes the signal in all directions, while the sensing electrode picks up the propagating signal throughout the skin in a closed area from all direction, thus, the propagating signal suffers no leakage in theory. A low frequency sinusoidal wave of 5 KHz with constant amplitude of 5Vp-p was generated by a signal generator (TG101010MHz-DDS function generator, Thurlby Thander Instrument, UK) and fed into the excitation electrode. Then the output voltage signal was sensed by the sensing electrode and the voltage (V) of the sensed current is revealed in a digital recorder (Oscilloscope DX 8000, Metrix, France). The frequency of the applied signal affects the measured conductance of the skin; a dielectric (i.e. skin) with permittivity reduces the power at all frequencies but especially the higher ones [12]. We were interested in measuring the conductance over the entire thickness of the stratum corneum thus we used a low frequency current which is reported previously to measure the stratum corneum over its entire thickness [13]. The low frequency method was proposed early in 1974 for the study of moisturization of the stratum corneum [13]. The 5 KHz is unlikely to be universal; it depends on the probe geometry and construction material. The response of the skin to the change in frequency across the frequency range of 1–10 KHz of the applied current was determined by applying a sinusoidal signal of 5Vp-p to the skin and the frequency at which the measured signal is maximum was determined. In the range of 1–5 KHz, the detected signal increases as we increase the frequency of the applied current after which the detected signal started to decline. 2.2. Performance of the new probe in comparison to that of Corneometer® CM 825 The Corneometer CM® 825 was used as a reference device to study the performance reliability of our in-house conductance measuring probe. The Corneometer® (CK electronic GmbH, Köln, Germany) is a commonly used, commercially available instrument for measurement of the hydration level of the skin. All the measurements were performed by both instruments under controlled conditions in an acclimatized room with a temperature of 24 ± 1 ◦ C and relative humidity of 48 ± 5% for the in vitro studies and 23 ± 1 ◦ C and 28 ± 1% relative humidity for the in vivo study. 2.2.1. In vitro sorption–desorption test The in vitro sorption–desorption test was performed on a cellulose filter paper (Cat No. 1003-125, Whatman International Ltd., Maidstone, England) as previously described [14] with minor modifications. For the water sorption test, distilled water was pipetted into the filter paper with a dose of 100 l each up to a maximum of 400 l. Measurements using both instruments were taken 30 s after adding the specific volume of water to ensure sorption. For the desorption test, filter paper was saturated with 300 l of distilled water and measurements were performed before, 30 and 60 min post water application using both instruments. Simultaneously, the weight of the water content in the filter paper during the desorption process was determined using a precision balance.
1473
2.2.2. Detection of measurement depth The experiments were modified from a protocol described previously [14]. Measurements by both instruments were performed on cellulose filter paper (Cat No. 1003-125, Whatman International Ltd., Maidstone, England) saturated with 300 l of distilled water at the surface of the filter paper and respectively through increasing layers of a low dielectric plastic foil (CK electronic GmbH, Köln, Germany) of 15 m thickness placed on the surface of the filter. 2.2.3. Measurements on filter paper with different liquids Measurements were performed on filter papers saturated with liquids with different dielectric constant values and covered with one sheet of a low dielectric plastic foil (CK electronic GmbH, Köln, Germany) using protocols previously described with minor modifications [15]. The liquids used were mineral oil, glycerol, and distilled water, their dielectric constants are 2–3, 46.5, and 80.1 respectively [15]. 2.2.4. Correlation of in vivo measurements of the new probe and Corneometer® CM 825 The short term effect of a single application of an o/w over-thecounter moisturizer was studied using both the newly developed probe and the Corneometer CM 825 as per the previously described procedure [16] with some modifications. The study was conducted in compliance with the ethical principles of the Declaration of Helsinki and a written informed consent was obtained prior to participation. Two circular skin areas (3.5 cm in diameter) were selected on the mid-volar aspect of each forearm of 4 healthy male volunteers. Volunteers were not allowed to use any skin care products on their forearms 3 days before the study. For each forearm, one area was treated with 0.2 ml of over-the-counter moisturizer and gently rubbed with a gloved finger and the other area served as an untreated control. The untreated and treated areas were randomized among subjects. For each subject, the design was the same on the right and left forearms. Measurements by both instruments were taken at baseline, 60 and 90 min after product’s application. All measurements were conducted according to the guidelines for standardized hydration measurements [17]. The volunteers were acclimatized to a controlled temperature and humidity conditions (23 ± 1 ◦ C and 28 ± 1% relative humidity) for 30 min before the measurements. For each instrument, 3 independent measurements were performed at each time point for each tested area. 2.3. Statistical analysis The data analysis was performed using Sigma plot v11.0 (Systat Software, Inc., Chicago, USA). For each instrument, the values obtained are mean values of 3 individual measurements. The Pearson correlation coefficient (r) was calculated to analyze the relationship between the different instruments. To compare the time dependent change in hydration states in the in vivo study, one way ANOVA was used and subsequent Bonferroni’s test was performed to compare changes in hydration states at 60 and 90 time points versus baseline value for each treated forearm. A level of p < 0.05 was considered statistically significant. 3. Results and discussion The objective of this work was to design an inexpensive and simple probe that can be constructed conveniently in-house to measure stratum corneum hydration levels. The developed probe was intended to evaluate the skin hydration by allowing direct galvanic contact between the electrodes and the skin surface and measuring skin conductance or the ease of the flow of the electric current between the electrodes and through the skin in terms of electrical units as shown in Fig. 2. The conductance measured
1474
S.H. Hamed et al. / Medical Engineering & Physics 34 (2012) 1471–1476
Fig. 2. Measuring principle of the in house conductance meter. The probe makes direct galvanic contact with the skin. When applying the probe to the skin, the inner electrode will pass a low frequency current into the skin. The current will propagate for a distance in the skin equal to the separation between the two electrodes until it is finally sensed by the sensing electrode. The ease of flow of the current (i.e. skin conductivity) is directly related to the hydration of skin surface where conduction increases with hydration.
2ε ln(b/a)
(1)
where ε = εr εo , εr = relative permittivity, εo = permittivity of free space (8.854 × 10−12 F/m), a is the outer radius of the inner conductor (exciter electrode), and b is the inner radius of the outer conductor (sensing electrode) that is separated from the inner conductor by FR4 dielectric material with a relative permittivity (εr ) of 4.1 [11]. As described earlier, the separation between the sensing and exciting electrodes was 4 mm, while the radii of the sensing and exciting electrodes were 10 mm and 6 mm respectively as shown in Fig. 1. With these values the capacitance of the designed probe was calculated to be 0.45 nF/m. and by taking the thickness of the copper plates to be approximately 0.1 mm, then the capacitance of the probe is approximately 4.5 × 10−5 nF. This capacitance value is negligible compared to the published skin capacitance values which are between 48 nF and 55 nF [18]. Thus, the designed probe will not overload the measuring electrical signal from the skin. The ability of the designed probe to assess skin hydration level was evaluated in vivo and in vitro and its results were compared with the Corneometer CM 825. Fig. 3 shows the gradual increase of both the capacitance arbitrary units (AU) and the conductance values (V) measured at the surface of the filter paper as a function of the amount of water added
6
140
5
120 100
4
80 3 60 2
40
1
Arbitrary Unit
C=
onto the filter paper. A gradual increase was observed between the capacitance and conductance readings and the quantity of water impregnating the filter paper until a steady state of conductance and capacitance is reached after 300 l, corresponding to a maximal conductance value of 4.9 V and maximal capacitance value of 106–110 arbitrary units. High correlations (r = 0.902, p = 0.036 and r = 0.963, p = 0.008) were observed between the amount of water adsorbed on the filter paper and the measurement values of the in house probe and CM 825, respectively. Comparable correlation between capacitance readings and quantity of water (r = 0.96) [15] and conductance readings and quantity of water (r = 0.818) was previously reported [14]. The desorption of a filter paper saturated with water due to water evaporation was also followed by measurements of the capacitance and conductance as a function of time using the two devices. The measurements of the desorption test were performed up to 90 min until a plateau was achieved in the readings of the CM 825 as shown in Fig. 4. The two probes measured a declining conductance and capacitance as the result of water desorption from the wetted filter paper. While the capacitance measurements of the CM 825 declined rapidly reaching a steady state after 60 min (93% reduction after 90 min), the decline in the conductance measurements of the inhouse probe was much slower (19% reduction after 90 min) and did not show the same plateau. However, a high correlation coefficient (r = 0.972, p = 0.028) was observed between the two desorption curves which is in line with previous reports showing a similar
Volt
is related to skin hydration [17] as dry skin is a medium of weak electrical conduction and its electrical conductivity increases with hydration. Our probe consisted of two concentric copper circular electrodes printed on FR4 dielectric material as shown in Fig. 1B. It was of great importance to eliminate the impact of the internal capacitance formed between the two non-contacting electrodes from interrupting the current propagation between them thus allowing the measurement of skin conductance. Accordingly, the separation distance, the radii of the sensing and exciting electrode, as well as the material of the two electrodes were chosen to design a probe with negligible resistance measured by its capacitance component in order to measure mainly the resistance of the skin. The capacitance (C) of the two non-contacting conductors (i.e. electrodes) per unit length of the designed probe was calculated as the capacitance of a cross section of coaxial transmission line [11] (Fig. 1A) using the following equation:
In-house conductance meter CM 825
20
0 0
100
200
300
400
0 500
µl
Fig. 3. In vitro sorption test results with distilled water. Each value represents the mean of 3 measurements ± SD. The values for the CM 825 are given in arbitrary units (AU) and those for the in house meter are given in volts (V).
S.H. Hamed et al. / Medical Engineering & Physics 34 (2012) 1471–1476
6
140
1.2
5
120
1
100
0.8
60 2
40
1
0
4 3
In-house conductance meter CM 825
2 0.2
1
0
0 100
50
5
0.6 0.4
20
0
7 Arbitrary Unit
3
In-house conductance meter CM 825
Volt
80
8
6
Arbitrary Unit
Volt
4
1475
0
20
40
60
80
0 100
Dielectric Constant
Minute
140
5
120 100
Volt
4
80 3 60 2
40
1
In-house conductance meter CM 825
20
0
0 0
20
40
60
dielectric constant ranging from 2–3 for mineral oil to 80.1 for water were recorded. Measurements were performed directly on one layer of a low dielectric foil covering the filter paper. The instruments readings are plotted against the dielectric constant for each tested liquid in Fig. 6. The CM 825 readings were directly related to the dielectric constant of the tested liquids. Fig. 6 shows the high correlation (r = 0.992) between the capacitance measurement of the saturated filter papers and the dielectric constant of the tested liquids. A similar high correlation (r = 0.92) between the capacitance values and the dielectric constants of different liquids was previously obtained [15]. However, with the conductance measurement it was difficult to show such a relation and relatively weaker correlation was found between the curves of the two devices (r = 0.737, p = 0.472). The new probe recorded higher values for distilled water than either mineral oil or glycerol. However it showed no increase in the reading between mineral oil and glycerol. Taking into account the very low conductance values of nonionic liquids and the high conductance value obtained for pure water, a lower correlation (r = 0.815) was found between the conductance values of the designed probe and the dielectric constant of the tested liquids in comparison to that obtained with the CM 825. A similar weak correlation between the conductance values and the dielectric constant of the solvents were previously reported [15] with a commercial conductance meter. The applicability of the designed probe in measuring the short time hydration effects of a commercially available moisturizing lotion was evaluated by carrying out measurements as a function of time to see whether the new probe is able to differentiate between different hydration states of the stratum corneum after the application of the moisturizer. The hydration measurements results after the treatment of the right forearms of the volunteers with a hydrating moisturizer are shown in Fig. 7 which shows that the increase
80
µm Fig. 5. Results of in vitro measurement depth test performed on filter paper saturated with 300 l distilled water covered with increasing number of foils (15 m thickness each). Each value represents the mean of 3 measurements ± SD. The values for the CM 825 are given in arbitrary units (AU) and those for the in house meter are given in volts (V).
Volt
6
Arbitrary Unit
high correlation between the capacitance and conductance desorption curves [15]. In addition, a gravimetric determination of the water content of the filter was also carried out and a high correlation was obtained between the amount of water present and respectively the capacitance (r = 0.946, p = 0.054) and conductance (r = 0.975, p = 0.025) readings. It was previously reported that although both the conductancebased Skicon® and the capacitance-based Corneometer correlate; the Skicon is better suited when the skin surface is hydrated experimentally and sorption–desorption dynamics are recorded [19] and that the conductance measurements are more sensitive and suitable for the evaluation of the hydrated skin conditions [10]. To evaluate the penetration depth of the electrical field of the two devices, capacitance and conductance measurements were carried out on filter paper saturated with distilled water respectively at the surface and then through increasing layers of a low dielectric plastic foil of 15 m thickness. The readings of both instruments are plotted against the thickness of the foil layers in Fig. 5. A significant high correlation coefficient was shown between the curves of both devices (r = 0.997, p = 0.0002). The 0 reading was not reached for both instruments; however, a 94% and 80% reduction of the initial values was reached with only one layer of low dielectric plastic foil for both the CM 825 and the new probe respectively. Thus, the new designed probe measures mainly superficial hydration (around 15 m) with a measurement depth similar to that previously reported for the commercially available CM 825 and the conductance based Skicon-200 [14]. The conductance and capacitance measurements for filter papers saturated with water and with solvents of different
Fig. 6. Relationship between instrument’s readings and dielectric constant of the applied liquids. Each value represents the mean of 3 measurements ± SD. The values for the CM 825 are given in arbitrary units (AU) and those for the in house meter are given in volts (V).
2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
18 16 14 12 10 8 6 4 2 0 -2 0
20
40
60
80
Arbitrary Unit
Fig. 4. Results of desorption test performed on filter paper saturated with 300 l distilled water. Each value represents the mean of 3 measurements ± SD. The values for the CM 825 are given in arbitrary units (AU) and those for the in house meter are given in volts (V).
In-house conductance meter (Right forearm) CM 825 (Right forearm)
100
Minute
Fig. 7. Changes in hydration values of the right volar forearms from the untreated control measured by the in house-conductance probe and CM 825 represented as mean ± SD (n = 4) at different times post application.
1476
S.H. Hamed et al. / Medical Engineering & Physics 34 (2012) 1471–1476
in skin hydration level after the application of the moisturizer was easily measured with our probe and the CM 825. It is known that moisturizer products result in an increase in skin hydration with a maximum effect on hydration between 30 and 60 min after application followed by a constant decline in hydration thereafter [20]. The two devices showed a significant increase at 60 min post application for both the left and right forearms followed by slight decrease at 90 min post application which remained for both devices significantly higher than the untreated control skin sites. The usefulness of both the CM 825 and the conductance based Skicon 200 to differentiate between different hydration states of the skin was previously reported [9,10]. The new designed probe also delivers readings that are related to different hydration states of the skin and can be considered suitable for the evaluation of the efficacy of moisturizers. 4. Conclusions The results showed that a simple, inexpensive probe for skin conductance measurement can be constructed through the proper choice of the geometric parameters and the construction materials which is based on equation governing transmission in TEM lines. The main requirement that such a probe should have negligible electrical resistance in order to measure mainly conductance contribution of the measured skin site rather than its own. The performance of the constructed probe was compared in vitro and in vivo to that of the commercially available and widely used Corneometer CM® 825, it was found to be comparable to the performance of the CM® 825 and to the performance of commercially available conductance meters reported in the literature. The constructed probe can be used as a tool in dermato-cosmetic research for evaluating the skin hydration for diagnostic and treatment purposes and for claim substantiation of functional cosmetic products. Conflict of interest There are no conflicts of interest. References [1] Verdier-Sévrain S, Bonté F. Skin hydration: a review on its molecular mechanisms. J Cosmet Dermatol 2007;6:75–82.
[2] Rim JH, Jo SJ, Park JY, Park BD, Youn JI. Electrical measurement of moisturizing effect on skin hydration and barrier function in psoriasis patients. Clin Exp Dermatol 2005;30:409–13. [3] Berardesca E, Borroni G. Instrumental evaluation of cutaneous hydration. Clin Dermatol 1995;13:323–7. [4] Darlenski R, Sassning S, Tsankov N, Fluhr JW. Non-invasive in vivo methods for investigation of the skin barrier physical properties. Eur J Pharm Biopharm 2009;72:295–303. [5] Byrne AJ. Bioengineering and subjective approaches to the clinical evaluation of dry skin. Int J Cosmet Sci 2010;32:410–21. [6] Lodén M, Lindberg M. In: Fluhr J, Elsner P, Berardesca E, Maibach H, editors. Bioengineering of the skin: water and the stratum corneum. 2nd ed. Boca Raton, Washington, DC: CRC Press LLC; 2005. p. 387–407. [7] De Paepe K, Janssens K, Hachem J-P, Roseeuw D, Rogiers V. Squamometry as a screening method for the evaluation of hydrating products. Skin Res Technol 2001;7:184–92. [8] Khazaka G. In: Fluhr J, Elsner P, Berardesca E, Maibach H, editors. Bioengineering of the skin: water and the stratum corneum. 2nd ed. Boca Raton, Washington, DC: CRC Press LLC; 2005. p. 249–61. [9] Clarys P, Barel AO, Gabard B. Non-invasive electrical measurements for the evaluation of the hydration state of the skin: comparison between three conventional instruments – the Corneometer® , the Skicon® , and the Nova DPM® . Skin Res Technol 1999;5:14–20. [10] Fluhr JW, Gloor M, Lazzerini S, Kleesz P, Grieshaber R, Berardesca E. Comparative study of five instruments measuring stratum corneum hydration (Corneometer CM 820 and CM 825, Skicon 200, Nova DPM 9003, DermaLab). Part II. In vivo. Skin Res Technol 1999;5:171–8. [11] Ulaby FT. Fundamentals of applied electromagnetics. 5th ed. NJ: Pearson Education; 2007. p. 40–100. [12] Gabriel S, Lau RW, Gabriel C. The dielectric properties of biological tissues: II. Measurements in the frequency range 10 Hz to 20 GHz. Phys Med Biol 1996;41:2251–69. [13] Leveque JL, De Rigal J. Impedance methods for studying skin moisturization. J Cosmet Sci 1983;34:419–28. [14] Fluhr JW, Gloor M, Lazzerini S, Kleesz P, Grieshaber R, Berardesca E. Comparative study of five instruments measuring stratum corneum hydration (Corneometer CM 820 and CM 825, Skicon 200, Nova DPM 9003, DermaLab). Part I. In vitro. Skin Res Technol 1999;5:161–70. [15] Barel AO, Clarys P. In vitro calibration of the capacitance method (Corneometer CM 825) and conductance method (Skicon-200) for the evaluation of the hydration state of the skin. Skin Res Technol 1997;3:107–13. [16] Li F, Conroy E, Visscher M, Wickett RR. The ability of electrical measurements to predict skin moisturization. I. Effects of NaCl and glycerin on short-term measurements. J Cosmet Sci 2001;52:13–22. [17] Berardesca E. EEMCO guidance for the assessment of stratum corneum hydration: electrical methods. Skin Res Technol 1997;3:126–32. [18] Colbert AP, Yun J, Larsen A, Edinger T, Greqory WL, Thong T. Skin impedance measurements for acupuncture research: development of a continuous recording system. Evid Based Complement Alternat Med 2008;5:443–50. [19] Serup J. In: Fluhr J, Elsner P, Berardesca E, Maibach H, editors. Bioengineering of the skin: water and the stratum corneum. 2nd ed. Boca Raton, Washington, DC: CRC Press LLC; 2005. p. 351–8. [20] Wilhelm KP. In: Elsner P, Barel AO, Berardesca E, Gabard B, Serup J, editors. Skin bioengineering techniques and applications in dermatology and cosmetology, vol. 26. Basel: Karger; 1998. p. 223–34.
Contents lists available at SciVerse ScienceDirect
Medical Engineering & Physics journal homepage: www.elsevier.com/locate/medengphy
Construction, in vitro and in vivo evaluation of an in-house conductance meter for measurement of skin hydration Saja H. Hamed a,∗ , Bilal Altrabsheh b , Tareq Assa’d b , Said Jaradat b , Mohammad Alshra’ah b , Abdulfattah Aljamal b , Hatim S. Alkhatib c , Abdul-Majeed Almalty a a
Faculty of Allied Health Sciences, Hashemite University, Zarqa, Jordan Faculty of Engineering, Hashemite University, Zarqa, Jordan c Faculty of Pharmacy, University of Jordan, Amman, Jordan b
a r t i c l e
i n f o
Article history: Received 11 May 2011 Received in revised form 6 December 2011 Accepted 20 February 2012 Keywords: Skin bioengineering Non-invasive measurement In-house conductance meter Skin hydration
a b s t r a c t Different probes are used in dermato-cosmetic research to measure the electrical properties of the skin. The principle governing the choice of the geometry and material of the measuring probe is not well defined in the literature and some device’s measuring principles are not accessible for the scientific community. The purpose of this work was to develop a simple inexpensive conductance meter for the objective in vivo evaluation of skin hydration. The conductance meter probe was designed using the basic equation governing wave propagation along Transverse Electromagnetic transmission lines. It consisted of two concentric copper circular electrodes printed on FR4 dielectric material. The performance of the probe was validated by evaluating its measurement depth, its ability to monitor in vitro water sorption–desorption and in vivo skin hydration effect in comparison to that of the Corneometer CM 825. The measurement depth of the probe, 15 m, was comparable to that of CM 825. The in vitro readings of the probe correlated strongly with the amount of water adsorbed on filter paper. Skin hydration after application of a moisturizer was monitored effectively by the new probe with good correlation to the results of CM 825. In conclusion, a simple probe for evaluating skin hydration was made from off-the-shelf materials and its performance was validated in comparison to a commercially available probe. © 2012 IPEM. Published by Elsevier Ltd. All rights reserved.
1. Introduction The water content of the skin, especially the stratum corneum (SC), is responsible for keeping the skin smooth and soft. Water content below 10% level is associated with visible appearance of dry flaky skin and impairment of its barrier integrity [1]. The measurement of hydration of the upper layer of the skin is an important tool for assessing the cutaneous effects of skin care products for their effectiveness and for monitoring the state of various skin diseases [2,3]. Different non-invasive devices for monitoring skin hydration are commercially available. These devices vary in their measurement modes which include electrical, microwave, and spectroscopic ones [4]. Each device offers a unique possibility of
∗ Corresponding author at: Faculty of Allied Health Sciences, Hashemite University, P.O. Box 150459, Zarqa 13115, Jordan. Tel.: +962 5 390 3333; fax: +962 5 390 3368. E-mail address: [email protected] (S.H. Hamed).
studying a specific phenomenon in intact skin that may correlate to skin hydration level. Magnetic Resonance (MR) spectroscopy and imaging method involves non-invasive measurement of proton density in the skin. The protons of water respond to the magnetic environment and useful parameters are determined that provide information about the concentration and distribution of water molecules in the epidermis [5]. Infrared (IR) spectroscopy relies on the characteristic of water molecule as an intense absorber of infrared spectroscopy. The infrared absorbance spectrum of water in the skin can be obtained with appropriate instrumentation and used to determine amount of water in the stratum corneum [6]. In addition to the abovementioned non-invasive techniques, a minimally invasive methods called squamometry is used to sample stratum corneum by adhesive coated discs to objectively assess the extent of surface scaliness that is correlated with extent of skin hydration [7]. More and more methods are available and it is quite difficult to have an overview of all of them. However, the most widely used devices are those operating by measuring the electrical properties
1350-4533/$ – see front matter © 2012 IPEM. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.medengphy.2012.02.008
1472
S.H. Hamed et al. / Medical Engineering & Physics 34 (2012) 1471–1476
Fig. 1. A cross section of a coaxial transmission line with inner conductor of radius a and outer conductor of radius b where the electric field lines are in radial direction between the inner and outer conductors (A) and an image of the new probe sensor designed based on coaxial transmission line basics, with inner conductor (exciter electrode) of radius a = 6 mm and outer conductor (sensing electrode) of radius b = 10 mm separated by 4 mm distance (B). The conductors were made of high conductivity material (copper) and were printed on an FR4 dielectric material. The separation distance, the radii of the sensing and exciting electrode, and the material of the two electrodes were chosen to design a probe with negligible resistance measured by its capacitance component (4.5 × 10−5 nF) in order to measure mainly the resistance of the skin.
of the skin, namely conductance and capacitance, as indirect indicators of stratum corneum water content [4]. Standard, commercially available devices for evaluating skin hydration include the conductance-meter Skicon 200 and the capacitance-meter Corneometer CM 825. Skin capacitance and conductance are correlated and, in general, show similar behavior [3]. When either probe is applied to the skin, the capacitance or conductance values recorded by the instruments correlate directly with the moisture content of the skin. Increasing the moisture content of the skin will increase its dielectric constant leading to increased capacitance and conductance. The Corneometer operates at a mean frequency of 1 MHz. The measuring probe consists of a 7 mm × 7 mm ceramic tile with many gold lines that are close to each other. The lines are 75 m wide with an interdigital spacing of 75 m. The parallel gold lines have such a small distance between them that they are only visible under a magnifying glass. The even and odd gold lines are connected to each other to form two conductor tracks separated by an insulating material to form a capacitor. The physical basis of capacitancebased devices is the difference of the dielectric constant of water and other substances brought in the electric measurement field of the sensor (i.e. capacitor). Thus, when the skin is hydrated a significant change in the dielectric properties of this medium is observed with a change in the total measured capacitance that is given in arbitrary units ranging from 0 to 120 AU. Further, the measuring surface of the probe is sealed with a thin special glass cover to protect the conductor track and ensures that there is no galvanic contact between the probe and the skin and that no current will flow to the skin. Hence the influence of ions and salts residues on conductance has no or little influence on the results which is an important advantage of this method as opposed to conductancebased method [8,9]. The Skicon measures the conductance of a single high frequency current of 3.5 MHz. The measuring probe consists of two concentric gold-covered electrodes of 2 and 5 mm diameters with a direct galvanic contact between the electrodes and the skin. The distance between the inner and the outer electrode is 1 mm. The two concentric electrodes are separated by a dielectric and the current flows when the probe is placed in the measurement area as soon as a galvanic contact of the probe with the skin surface is established. The measured electrical conductivity is dependent on skin moisture and is given in microsiemens (S) units ranging from 0 to 1999 S [8,9]. The reproducibility and reliability of the different electrically based devices available commercially in measuring skin hydration under conditions ranging from increased, normal, and low
hydration levels have been investigated and validated [10]. However, the principle governing the choice of the geometry and material of the measuring probe which affect measurement of the electric field in the skin is not well defined in the literature. Furthermore, the measuring principles are not described in sufficient details in the literature as some device’s measuring principles are considered proprietary and not accessible for the scientific community [9]. In the present study, we developed a simple and inexpensive probe to measure skin hydration level from material available off-the-shelf at the Biomedical Engineering Department in the Hashemite University. The probe measures skin conductance and is designed based on basic equation governing wave propagation along Transverse Electromagnetic (TEM) transmission lines. Transmission lines encompass all structures and media that serve to transfer energy between two points. When waves propagate along these lines are characterized by electric and magnetic fields that are entirely transverse to the direction of propagation then the transmission lines are called Transverse Electromagnetic (TEM) transmission lines. An example of TEM-mode transmission lines is the flow of current between outer and inner conductors that is made possible by the material conductivity of the insulator. If the material separating the inner and outer conductors is a perfect dielectric with conductivity = 0 then current flow between the outer and inner conductors equal zero. Transmission lines are called coaxial line when the electric field lines are in the radial direction between the inner and outer conductors [11]. The usefulness of the new probe in dynamic measurement of skin hydration parameters was validated in comparison with the Corneometer® CM 825 under standardized conditions. 2. Materials and methods 2.1. Construction of the probe The practical guidance for designing and constructing the measuring probe is based on the equation governing transmission in Transverse Electromagnetic (TEM) transmission lines. A common feature among TEM lines is that they consist of two parallel conducting surfaces. TEM transmission lines consist of two conductors that are transverse to the direction of propagation and act to support the propagation of electromagnetic waves [11]. The coaxial transmission line is one type of TEM that consists of an inner conductor (excitation electrode) of radius a separated from an outer conductor (sensing electrode) of radius b by an insulating material (Fig. 1).
S.H. Hamed et al. / Medical Engineering & Physics 34 (2012) 1471–1476
Our measuring probe, designed based on coaxial transmission line basics, has two circular electrodes, an excitation electrode and a sensing one. The conductors (i.e. electrodes) were made of high conductivity material (copper) and were printed on an FR4 dielectric material. The separation between the sensing and exciting electrodes was 4 mm which represents the signal propagation distance in the skin, while the radii of the sensing and exciting electrodes were 10 mm and 6 mm respectively as shown in Fig. 1. The circular shape of the two electrodes gives the advantage that the exciter electrode passes the signal in all directions, while the sensing electrode picks up the propagating signal throughout the skin in a closed area from all direction, thus, the propagating signal suffers no leakage in theory. A low frequency sinusoidal wave of 5 KHz with constant amplitude of 5Vp-p was generated by a signal generator (TG101010MHz-DDS function generator, Thurlby Thander Instrument, UK) and fed into the excitation electrode. Then the output voltage signal was sensed by the sensing electrode and the voltage (V) of the sensed current is revealed in a digital recorder (Oscilloscope DX 8000, Metrix, France). The frequency of the applied signal affects the measured conductance of the skin; a dielectric (i.e. skin) with permittivity reduces the power at all frequencies but especially the higher ones [12]. We were interested in measuring the conductance over the entire thickness of the stratum corneum thus we used a low frequency current which is reported previously to measure the stratum corneum over its entire thickness [13]. The low frequency method was proposed early in 1974 for the study of moisturization of the stratum corneum [13]. The 5 KHz is unlikely to be universal; it depends on the probe geometry and construction material. The response of the skin to the change in frequency across the frequency range of 1–10 KHz of the applied current was determined by applying a sinusoidal signal of 5Vp-p to the skin and the frequency at which the measured signal is maximum was determined. In the range of 1–5 KHz, the detected signal increases as we increase the frequency of the applied current after which the detected signal started to decline. 2.2. Performance of the new probe in comparison to that of Corneometer® CM 825 The Corneometer CM® 825 was used as a reference device to study the performance reliability of our in-house conductance measuring probe. The Corneometer® (CK electronic GmbH, Köln, Germany) is a commonly used, commercially available instrument for measurement of the hydration level of the skin. All the measurements were performed by both instruments under controlled conditions in an acclimatized room with a temperature of 24 ± 1 ◦ C and relative humidity of 48 ± 5% for the in vitro studies and 23 ± 1 ◦ C and 28 ± 1% relative humidity for the in vivo study. 2.2.1. In vitro sorption–desorption test The in vitro sorption–desorption test was performed on a cellulose filter paper (Cat No. 1003-125, Whatman International Ltd., Maidstone, England) as previously described [14] with minor modifications. For the water sorption test, distilled water was pipetted into the filter paper with a dose of 100 l each up to a maximum of 400 l. Measurements using both instruments were taken 30 s after adding the specific volume of water to ensure sorption. For the desorption test, filter paper was saturated with 300 l of distilled water and measurements were performed before, 30 and 60 min post water application using both instruments. Simultaneously, the weight of the water content in the filter paper during the desorption process was determined using a precision balance.
1473
2.2.2. Detection of measurement depth The experiments were modified from a protocol described previously [14]. Measurements by both instruments were performed on cellulose filter paper (Cat No. 1003-125, Whatman International Ltd., Maidstone, England) saturated with 300 l of distilled water at the surface of the filter paper and respectively through increasing layers of a low dielectric plastic foil (CK electronic GmbH, Köln, Germany) of 15 m thickness placed on the surface of the filter. 2.2.3. Measurements on filter paper with different liquids Measurements were performed on filter papers saturated with liquids with different dielectric constant values and covered with one sheet of a low dielectric plastic foil (CK electronic GmbH, Köln, Germany) using protocols previously described with minor modifications [15]. The liquids used were mineral oil, glycerol, and distilled water, their dielectric constants are 2–3, 46.5, and 80.1 respectively [15]. 2.2.4. Correlation of in vivo measurements of the new probe and Corneometer® CM 825 The short term effect of a single application of an o/w over-thecounter moisturizer was studied using both the newly developed probe and the Corneometer CM 825 as per the previously described procedure [16] with some modifications. The study was conducted in compliance with the ethical principles of the Declaration of Helsinki and a written informed consent was obtained prior to participation. Two circular skin areas (3.5 cm in diameter) were selected on the mid-volar aspect of each forearm of 4 healthy male volunteers. Volunteers were not allowed to use any skin care products on their forearms 3 days before the study. For each forearm, one area was treated with 0.2 ml of over-the-counter moisturizer and gently rubbed with a gloved finger and the other area served as an untreated control. The untreated and treated areas were randomized among subjects. For each subject, the design was the same on the right and left forearms. Measurements by both instruments were taken at baseline, 60 and 90 min after product’s application. All measurements were conducted according to the guidelines for standardized hydration measurements [17]. The volunteers were acclimatized to a controlled temperature and humidity conditions (23 ± 1 ◦ C and 28 ± 1% relative humidity) for 30 min before the measurements. For each instrument, 3 independent measurements were performed at each time point for each tested area. 2.3. Statistical analysis The data analysis was performed using Sigma plot v11.0 (Systat Software, Inc., Chicago, USA). For each instrument, the values obtained are mean values of 3 individual measurements. The Pearson correlation coefficient (r) was calculated to analyze the relationship between the different instruments. To compare the time dependent change in hydration states in the in vivo study, one way ANOVA was used and subsequent Bonferroni’s test was performed to compare changes in hydration states at 60 and 90 time points versus baseline value for each treated forearm. A level of p < 0.05 was considered statistically significant. 3. Results and discussion The objective of this work was to design an inexpensive and simple probe that can be constructed conveniently in-house to measure stratum corneum hydration levels. The developed probe was intended to evaluate the skin hydration by allowing direct galvanic contact between the electrodes and the skin surface and measuring skin conductance or the ease of the flow of the electric current between the electrodes and through the skin in terms of electrical units as shown in Fig. 2. The conductance measured
1474
S.H. Hamed et al. / Medical Engineering & Physics 34 (2012) 1471–1476
Fig. 2. Measuring principle of the in house conductance meter. The probe makes direct galvanic contact with the skin. When applying the probe to the skin, the inner electrode will pass a low frequency current into the skin. The current will propagate for a distance in the skin equal to the separation between the two electrodes until it is finally sensed by the sensing electrode. The ease of flow of the current (i.e. skin conductivity) is directly related to the hydration of skin surface where conduction increases with hydration.
2ε ln(b/a)
(1)
where ε = εr εo , εr = relative permittivity, εo = permittivity of free space (8.854 × 10−12 F/m), a is the outer radius of the inner conductor (exciter electrode), and b is the inner radius of the outer conductor (sensing electrode) that is separated from the inner conductor by FR4 dielectric material with a relative permittivity (εr ) of 4.1 [11]. As described earlier, the separation between the sensing and exciting electrodes was 4 mm, while the radii of the sensing and exciting electrodes were 10 mm and 6 mm respectively as shown in Fig. 1. With these values the capacitance of the designed probe was calculated to be 0.45 nF/m. and by taking the thickness of the copper plates to be approximately 0.1 mm, then the capacitance of the probe is approximately 4.5 × 10−5 nF. This capacitance value is negligible compared to the published skin capacitance values which are between 48 nF and 55 nF [18]. Thus, the designed probe will not overload the measuring electrical signal from the skin. The ability of the designed probe to assess skin hydration level was evaluated in vivo and in vitro and its results were compared with the Corneometer CM 825. Fig. 3 shows the gradual increase of both the capacitance arbitrary units (AU) and the conductance values (V) measured at the surface of the filter paper as a function of the amount of water added
6
140
5
120 100
4
80 3 60 2
40
1
Arbitrary Unit
C=
onto the filter paper. A gradual increase was observed between the capacitance and conductance readings and the quantity of water impregnating the filter paper until a steady state of conductance and capacitance is reached after 300 l, corresponding to a maximal conductance value of 4.9 V and maximal capacitance value of 106–110 arbitrary units. High correlations (r = 0.902, p = 0.036 and r = 0.963, p = 0.008) were observed between the amount of water adsorbed on the filter paper and the measurement values of the in house probe and CM 825, respectively. Comparable correlation between capacitance readings and quantity of water (r = 0.96) [15] and conductance readings and quantity of water (r = 0.818) was previously reported [14]. The desorption of a filter paper saturated with water due to water evaporation was also followed by measurements of the capacitance and conductance as a function of time using the two devices. The measurements of the desorption test were performed up to 90 min until a plateau was achieved in the readings of the CM 825 as shown in Fig. 4. The two probes measured a declining conductance and capacitance as the result of water desorption from the wetted filter paper. While the capacitance measurements of the CM 825 declined rapidly reaching a steady state after 60 min (93% reduction after 90 min), the decline in the conductance measurements of the inhouse probe was much slower (19% reduction after 90 min) and did not show the same plateau. However, a high correlation coefficient (r = 0.972, p = 0.028) was observed between the two desorption curves which is in line with previous reports showing a similar
Volt
is related to skin hydration [17] as dry skin is a medium of weak electrical conduction and its electrical conductivity increases with hydration. Our probe consisted of two concentric copper circular electrodes printed on FR4 dielectric material as shown in Fig. 1B. It was of great importance to eliminate the impact of the internal capacitance formed between the two non-contacting electrodes from interrupting the current propagation between them thus allowing the measurement of skin conductance. Accordingly, the separation distance, the radii of the sensing and exciting electrode, as well as the material of the two electrodes were chosen to design a probe with negligible resistance measured by its capacitance component in order to measure mainly the resistance of the skin. The capacitance (C) of the two non-contacting conductors (i.e. electrodes) per unit length of the designed probe was calculated as the capacitance of a cross section of coaxial transmission line [11] (Fig. 1A) using the following equation:
In-house conductance meter CM 825
20
0 0
100
200
300
400
0 500
µl
Fig. 3. In vitro sorption test results with distilled water. Each value represents the mean of 3 measurements ± SD. The values for the CM 825 are given in arbitrary units (AU) and those for the in house meter are given in volts (V).
S.H. Hamed et al. / Medical Engineering & Physics 34 (2012) 1471–1476
6
140
1.2
5
120
1
100
0.8
60 2
40
1
0
4 3
In-house conductance meter CM 825
2 0.2
1
0
0 100
50
5
0.6 0.4
20
0
7 Arbitrary Unit
3
In-house conductance meter CM 825
Volt
80
8
6
Arbitrary Unit
Volt
4
1475
0
20
40
60
80
0 100
Dielectric Constant
Minute
140
5
120 100
Volt
4
80 3 60 2
40
1
In-house conductance meter CM 825
20
0
0 0
20
40
60
dielectric constant ranging from 2–3 for mineral oil to 80.1 for water were recorded. Measurements were performed directly on one layer of a low dielectric foil covering the filter paper. The instruments readings are plotted against the dielectric constant for each tested liquid in Fig. 6. The CM 825 readings were directly related to the dielectric constant of the tested liquids. Fig. 6 shows the high correlation (r = 0.992) between the capacitance measurement of the saturated filter papers and the dielectric constant of the tested liquids. A similar high correlation (r = 0.92) between the capacitance values and the dielectric constants of different liquids was previously obtained [15]. However, with the conductance measurement it was difficult to show such a relation and relatively weaker correlation was found between the curves of the two devices (r = 0.737, p = 0.472). The new probe recorded higher values for distilled water than either mineral oil or glycerol. However it showed no increase in the reading between mineral oil and glycerol. Taking into account the very low conductance values of nonionic liquids and the high conductance value obtained for pure water, a lower correlation (r = 0.815) was found between the conductance values of the designed probe and the dielectric constant of the tested liquids in comparison to that obtained with the CM 825. A similar weak correlation between the conductance values and the dielectric constant of the solvents were previously reported [15] with a commercial conductance meter. The applicability of the designed probe in measuring the short time hydration effects of a commercially available moisturizing lotion was evaluated by carrying out measurements as a function of time to see whether the new probe is able to differentiate between different hydration states of the stratum corneum after the application of the moisturizer. The hydration measurements results after the treatment of the right forearms of the volunteers with a hydrating moisturizer are shown in Fig. 7 which shows that the increase
80
µm Fig. 5. Results of in vitro measurement depth test performed on filter paper saturated with 300 l distilled water covered with increasing number of foils (15 m thickness each). Each value represents the mean of 3 measurements ± SD. The values for the CM 825 are given in arbitrary units (AU) and those for the in house meter are given in volts (V).
Volt
6
Arbitrary Unit
high correlation between the capacitance and conductance desorption curves [15]. In addition, a gravimetric determination of the water content of the filter was also carried out and a high correlation was obtained between the amount of water present and respectively the capacitance (r = 0.946, p = 0.054) and conductance (r = 0.975, p = 0.025) readings. It was previously reported that although both the conductancebased Skicon® and the capacitance-based Corneometer correlate; the Skicon is better suited when the skin surface is hydrated experimentally and sorption–desorption dynamics are recorded [19] and that the conductance measurements are more sensitive and suitable for the evaluation of the hydrated skin conditions [10]. To evaluate the penetration depth of the electrical field of the two devices, capacitance and conductance measurements were carried out on filter paper saturated with distilled water respectively at the surface and then through increasing layers of a low dielectric plastic foil of 15 m thickness. The readings of both instruments are plotted against the thickness of the foil layers in Fig. 5. A significant high correlation coefficient was shown between the curves of both devices (r = 0.997, p = 0.0002). The 0 reading was not reached for both instruments; however, a 94% and 80% reduction of the initial values was reached with only one layer of low dielectric plastic foil for both the CM 825 and the new probe respectively. Thus, the new designed probe measures mainly superficial hydration (around 15 m) with a measurement depth similar to that previously reported for the commercially available CM 825 and the conductance based Skicon-200 [14]. The conductance and capacitance measurements for filter papers saturated with water and with solvents of different
Fig. 6. Relationship between instrument’s readings and dielectric constant of the applied liquids. Each value represents the mean of 3 measurements ± SD. The values for the CM 825 are given in arbitrary units (AU) and those for the in house meter are given in volts (V).
2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
18 16 14 12 10 8 6 4 2 0 -2 0
20
40
60
80
Arbitrary Unit
Fig. 4. Results of desorption test performed on filter paper saturated with 300 l distilled water. Each value represents the mean of 3 measurements ± SD. The values for the CM 825 are given in arbitrary units (AU) and those for the in house meter are given in volts (V).
In-house conductance meter (Right forearm) CM 825 (Right forearm)
100
Minute
Fig. 7. Changes in hydration values of the right volar forearms from the untreated control measured by the in house-conductance probe and CM 825 represented as mean ± SD (n = 4) at different times post application.
1476
S.H. Hamed et al. / Medical Engineering & Physics 34 (2012) 1471–1476
in skin hydration level after the application of the moisturizer was easily measured with our probe and the CM 825. It is known that moisturizer products result in an increase in skin hydration with a maximum effect on hydration between 30 and 60 min after application followed by a constant decline in hydration thereafter [20]. The two devices showed a significant increase at 60 min post application for both the left and right forearms followed by slight decrease at 90 min post application which remained for both devices significantly higher than the untreated control skin sites. The usefulness of both the CM 825 and the conductance based Skicon 200 to differentiate between different hydration states of the skin was previously reported [9,10]. The new designed probe also delivers readings that are related to different hydration states of the skin and can be considered suitable for the evaluation of the efficacy of moisturizers. 4. Conclusions The results showed that a simple, inexpensive probe for skin conductance measurement can be constructed through the proper choice of the geometric parameters and the construction materials which is based on equation governing transmission in TEM lines. The main requirement that such a probe should have negligible electrical resistance in order to measure mainly conductance contribution of the measured skin site rather than its own. The performance of the constructed probe was compared in vitro and in vivo to that of the commercially available and widely used Corneometer CM® 825, it was found to be comparable to the performance of the CM® 825 and to the performance of commercially available conductance meters reported in the literature. The constructed probe can be used as a tool in dermato-cosmetic research for evaluating the skin hydration for diagnostic and treatment purposes and for claim substantiation of functional cosmetic products. Conflict of interest There are no conflicts of interest. References [1] Verdier-Sévrain S, Bonté F. Skin hydration: a review on its molecular mechanisms. J Cosmet Dermatol 2007;6:75–82.
[2] Rim JH, Jo SJ, Park JY, Park BD, Youn JI. Electrical measurement of moisturizing effect on skin hydration and barrier function in psoriasis patients. Clin Exp Dermatol 2005;30:409–13. [3] Berardesca E, Borroni G. Instrumental evaluation of cutaneous hydration. Clin Dermatol 1995;13:323–7. [4] Darlenski R, Sassning S, Tsankov N, Fluhr JW. Non-invasive in vivo methods for investigation of the skin barrier physical properties. Eur J Pharm Biopharm 2009;72:295–303. [5] Byrne AJ. Bioengineering and subjective approaches to the clinical evaluation of dry skin. Int J Cosmet Sci 2010;32:410–21. [6] Lodén M, Lindberg M. In: Fluhr J, Elsner P, Berardesca E, Maibach H, editors. Bioengineering of the skin: water and the stratum corneum. 2nd ed. Boca Raton, Washington, DC: CRC Press LLC; 2005. p. 387–407. [7] De Paepe K, Janssens K, Hachem J-P, Roseeuw D, Rogiers V. Squamometry as a screening method for the evaluation of hydrating products. Skin Res Technol 2001;7:184–92. [8] Khazaka G. In: Fluhr J, Elsner P, Berardesca E, Maibach H, editors. Bioengineering of the skin: water and the stratum corneum. 2nd ed. Boca Raton, Washington, DC: CRC Press LLC; 2005. p. 249–61. [9] Clarys P, Barel AO, Gabard B. Non-invasive electrical measurements for the evaluation of the hydration state of the skin: comparison between three conventional instruments – the Corneometer® , the Skicon® , and the Nova DPM® . Skin Res Technol 1999;5:14–20. [10] Fluhr JW, Gloor M, Lazzerini S, Kleesz P, Grieshaber R, Berardesca E. Comparative study of five instruments measuring stratum corneum hydration (Corneometer CM 820 and CM 825, Skicon 200, Nova DPM 9003, DermaLab). Part II. In vivo. Skin Res Technol 1999;5:171–8. [11] Ulaby FT. Fundamentals of applied electromagnetics. 5th ed. NJ: Pearson Education; 2007. p. 40–100. [12] Gabriel S, Lau RW, Gabriel C. The dielectric properties of biological tissues: II. Measurements in the frequency range 10 Hz to 20 GHz. Phys Med Biol 1996;41:2251–69. [13] Leveque JL, De Rigal J. Impedance methods for studying skin moisturization. J Cosmet Sci 1983;34:419–28. [14] Fluhr JW, Gloor M, Lazzerini S, Kleesz P, Grieshaber R, Berardesca E. Comparative study of five instruments measuring stratum corneum hydration (Corneometer CM 820 and CM 825, Skicon 200, Nova DPM 9003, DermaLab). Part I. In vitro. Skin Res Technol 1999;5:161–70. [15] Barel AO, Clarys P. In vitro calibration of the capacitance method (Corneometer CM 825) and conductance method (Skicon-200) for the evaluation of the hydration state of the skin. Skin Res Technol 1997;3:107–13. [16] Li F, Conroy E, Visscher M, Wickett RR. The ability of electrical measurements to predict skin moisturization. I. Effects of NaCl and glycerin on short-term measurements. J Cosmet Sci 2001;52:13–22. [17] Berardesca E. EEMCO guidance for the assessment of stratum corneum hydration: electrical methods. Skin Res Technol 1997;3:126–32. [18] Colbert AP, Yun J, Larsen A, Edinger T, Greqory WL, Thong T. Skin impedance measurements for acupuncture research: development of a continuous recording system. Evid Based Complement Alternat Med 2008;5:443–50. [19] Serup J. In: Fluhr J, Elsner P, Berardesca E, Maibach H, editors. Bioengineering of the skin: water and the stratum corneum. 2nd ed. Boca Raton, Washington, DC: CRC Press LLC; 2005. p. 351–8. [20] Wilhelm KP. In: Elsner P, Barel AO, Berardesca E, Gabard B, Serup J, editors. Skin bioengineering techniques and applications in dermatology and cosmetology, vol. 26. Basel: Karger; 1998. p. 223–34.