Studies on the Conducting Properties of the Human Skin to Direct Current

From the Laboratory for the Theory of Gymnastics, University of Copenhagen.

Studies on the Conducting Properties of the Human Skin to Direct Current.

BY

THOMAS ROSENDAL.

Received 12 October 1942.

Introduction.

GIILIEMEISSTER’S monograph (1928) on the conducting properties of the skin to direct current (d. c.) and alternating current (a. c.), and XCHAEPKR’S book on electroph ysiology (1940) contain comprehensive descriptions of the investigations on this problem. The results are briefly as follows.

The d. c.-resistance decreases after conduction of a constant current for some time through the skin and also with increasing voltage. When the voltage is decreased subsequently, the resistance does not increase to the initial value but to a lower value (hysteresis). The resistance to anodic conduction is different from that to cathodic conductionl. I n the case of short-lasting current impulses, the skin shows properties similar t o a polarization cell or a shunted condenser. The a. c.-resistance of the skin is less than its d. c.-resistance, decreasing with increasing frequency of the a. e. -- Pinally, the skin gives rise to a phase shift hetween alternating voltage and -current (voltage after current) in the same way as a polarization cell or a condenser. The subcutaneous and internal tissue of the human organism conducts a galvanic current alrnost as a low-ohmic resistance.

As the cause of the above mentioned conducting properties, GIL- DEMEISTER (1928) and his school assume polarization of the skin due

Anodic or cathodic conduction is defined as a d. c.-conduction through the respective part of the skin with the anode or the cathode, respectively, as different electrode. The indifferent electrode is immersed into an electrolyte bath into which also it large part of the skin, e. g. one arm, is immersed.

CONDUCTING PROPERTIES OF THE HUMAN SHIN. 131

t o different ion mobility in the cells and the cell membranes with an accumulation of ions on the stratum corneum and, especially, on the cell membranes in the stratum germinativum. The d. c.-resistance of the skin is therefore an apparent resistance, determined by the state of permeability of the cell membranes and varying with their permeability.

MUNK (1873) and GARTNER (1882) explained the decrease in d. c.- resistance of the skin depending on the time of conduction by an increase in electrolyte content of the skin due to electro-endosmosis. GILDEMEISTER (1928), however, assumed that this effect must be due to a reversible increase in the membrane permeability of the living cell layers of the stratum germinativum accompanied by a decrease in skin polarization. EBBECKE (1921, 22, and 23) proposed the same explanation for the decrease in d. c.-resistance after mechanical treat- ment of the skin and aft8er conduction of d. c. through the skin (ER- BECKE’S mechanical and galvanical react>ion); the decrease in d. G. -

resistance during the psycho-galvanical reflex should be due to a decrease in polarization on the surface of the sweat glands (GILDE- MEISTER 1915). As a basis for these assumptions, GILDEMEISTER point- ed out that a sirnultaneous determination of the d. c.- and the a. c.- rcsistctnce of the skin (at frequencies above 840 cycles), before and after conduction of (1. c. through the skin, showed a decrease in the d. c.-resistance, only (GALLER 1912, UILUEMEISTER 1915, GILDE- NEISTICR and KAUFHOLD 1920). Since the a. c.-resistance which GIL- 1)EMEISTER regards as the real resistance of the skin, determined by its elect,rolyte content, remained constant after d. c.-conduction, it must be considered impossible that a change in electrolyte content of the skin occurred in the above described case. MVNK’S and GXRTNER’S interpretation is thereby considered disproved.

However, after d. c.-conduction through the skin, EINTHOVEN and BIJTEL (1923) actually proved the existence of a decrease in the a. c.- resistance of the skin a t frequencies below 840 cycles, but not a t higher frequencies. The same observations were made by ROSENUAL (1940) after moistening of the stratum corneuni with an electrolyte. These results may be explained by the fact that the a. c.-resistance of the skin at frequencies above 500--1,000 cycles is almost completely capacitive and is independent of a change in the electrolyte content. The a. c.-resistance a t frequencies below 500-1,000 cycles, however, has both a capacitive and an ohmic component which latter decreases with increasing electrolyte content of the skin. It must, further, be emphasized that GILDEMEISTER’S high-frequency a. c.-resistance (5,000 cycles) of the skin in series with the internal tissue is predominantly due to the resistance of the internal tissue which is independent of the electrolyte content of the skin, while the low-frequency a. c.-resistance mainly originates froni the skin. A change in the electrolyte content of the skin will therefore not be registered by a determination of the a. c.-resistance a t frequencies above 500-1,000 cycles (GILDE- MEISTER and his school), but exclusively by a determination of the low-frequency a. c.-resistance. Since, however, the last-mentioned re-

132 THOMAS ROBENDAL.

sistance shows a decrease after conduction of d. c. and after moistening wikh electrolyte (EINTHOVEN and HIJTEL, ROSEI~AL) , MUNK’S and G~RTNER’S previously mentioned explanation is further supported and GILDEMEISTER’S objection against i t is invalidated.

As mentioned above, GILDENEISTEK located the electric resistance of the skin to the stratum corneum and especially to all cell layers in the stratum germinativum, while REIN (1930) thought it localized to the stratum lucidum. LEWIS and ZOTTERMAN (1927) demonstrated tha t the decrease in d. c.-resistance of the skin in E n m c m ’ s gal+-anic reaction must be caused by a lesion of the stratum corneum and that the same decrease could he produced by pricking this layer with a needle. These investigations make a localization of t,he d. c.-resistance of the skin t o the stratum ccmieuni probahle. The sanie localization was found by ROSEXDAL (1940) in the case of the a. c.-rcsistaiice a t frequencies betw-eeri 30 a,nd 20,000 cycles1.

I n view of t h e differing interpretations given in the l i terature concerning tlic cause of t,he decrease in skin resistance after eon- tluction of d. c. arid t,he lncaliza,tion of t h e d. c.-resist~ance in t h e skin, n niiniher of nen experiments were per€ornied which niigtit elucitlate t’his qiicstion.

iwe timi. The &ctrotles. The most suit,able electrodes were cliosen in agree-

ment with the conditions stated by GILLXWICISTEE (1915) and on the basis of soiiie investigations coriccrning the d. c.-resistance as a function of e. in. f . o f the following electrode-electrolyte systems. The d. c..- resistaricc of the systems was deterniined ut different potentials in a Wheatstom bridge with a n error of ahout 1 per cent.

1) Mercury-calomel electrode. a) &rcury-calomel electrode with an arca of 1 . 7 cni2 arid a thick-

ness of the calomel layer of 3 mm. b) Mercury-calomel paste electrode with an area of 1 . 7 emz a i d a

thickness of the mercury-calomel paste - saturated with KC1-soh- tion - of 1 cm. The paste was prepared by grinding in a mortar 2 cc mercury, 5 g of calomel and 3 cc saturated KC1-solution.

c) Platinum-mercury-calomel electrode with a n area of 2 cmz. The electrode was prepared as described by BISKUPSKI (1938).

Contact between two of the above described electrodes was obtained by means of a glass tube containing saturated KC1-solution with 2 per cent agar. The resistance of this tube amounted to about 200 ohms.

2) Silver-silver chloride electrode. a) Platinum-silver-silver chloride electrode with an area of n cmZ

prepared according to BROWN (1934). As regards the experimental technique and a number of other points con-

cerning the conducting properties of the skin and the internal tissue t o n. c., cf. ROSEKDAL (1940).

_ _ - - ___

CONDUCTING PROPERTIES O F THE HUMAN SKIN. 133

b) Silver-silver chloride electrode coiisisting of a silver plate 15 x 1 x 0.1 em, which was wound as a helix, 1 cm high and 2-2.5 clli in diameter. The silver helix was fastened to a silver wire, 5 cni long, and was then covered with silver chloride by electrolysis of a N HC1- solution during 10-12 hours a t a current of 10 milliamperes. The silver helix was then mounted iii a cylindrical ebonite vessel with a hasis of 7 cni2 and a height of 2 em. The lead went through the bottom of the vessel. Finally, the silver helix was covered by saturated KC1 solution with 2-3 per cent agar.

Three separate sets of qilver-silver chloride electrodes were investigated. Their resistance was determined for the conduction in both directions (marked in the table: I, 11) after immersion of the electrodes into a saturated KC1-solution.

Table 1 shows the resistances of the abore mentioned electrode- electrolyte systems measured a t various potentials and, furthermore, the potential of the systems themselves.

Table 1.

millivolts. Resis tance values in ohms to direct voltuges between 1.32 a n d 1940

t'lati- num mrr- cury calomel electrode

-

Voltage in mV.

- 1.32 6.65

13.2 64.5

113 57 1

1 940

Voltage in mV of the system

itself

~.

Plati- num

silver AgCl

Mer- cury calomel

I 1

0.4 I

Mer-

omel paste

3840 3 8 4 0 3 8 4 2 3850

- -~

Pilver- AgCl

I

-- 420 420 422 419 419

-

I -

6.4 5.e 5.3 5.3 5.2 5.3

16 14

-

I1 -

5.4 5.4

5 ~

11 12

= Silver- ,4g;C1 I1 -

I -

4.6

4.6 4.4 4.4 -

0.3 1 ~ 1.5

-

I1 -

5

5

Silver- BgCl 111

~

0.4 1

TI1 10 days

later

I -

11 14

9 9 9

I1 -

9 8

8 8 8 -

Table 1 shows the high potential-independent resistance of the calomel electrodes. Their high resistance must be ascribed to the calomel layer and therefore these electrodes are less suited for resistance measurements on the skin. The resistance of the platinum-mercury calomel paste electrodes is also potential-independent, but somewhat lower, The resistance of the platinum-mercury-calomel electrodes is to a great extent dependent on the voltage. Hence, this electrode -

134 THOMAS ROSENDAL.

in the form described by BISKUPSKI (1938) - cannot be considered unpolarizable and is, therefore, unsuitable for biological measurements. Silver-silver chloride electrodes show a very low resistance in both current directions and they are almost potential-independent. Consequently, these electrodes must be regarded as best suited for resistance determinations on the skin. In the course of the investigations, the electrodes have been tested repeatedly; resistance and voltage of the system itself were found to be constant and of the magnitude given in the table. As the resistance of the object of measurements - viz. the skin - has been above 1,000 ohms in almost all experiments, and since the voltages applied were above 100 millivolts, the influence of the electrodes upon the determination of the resistance of the object was less than 1 per cent.

The measurements. Part of the experiments concerning the conducting properties of the skin to direct current were performed as current determinations; the error involved was less than 1 per cent in a circuit consisting of a known source of e. m. f., a galvanometer, and the object of measurements. The object i n the present case was a silver-silver chloride electrode in an ebonite cylinder (7 cm2) - l per cent KC1-solution or saturated KCI-solution as contact electrolyte - the skin on the volar side of a person’s forearm which rests upon the electrode - the internal tissue of the same arm, the thorax, and the other arm - and, finally, the skin of the arm which is immersed until the elbow into a vessel containing 1 per cent KC1-solution into which the other silver-silver chloride electrode is also immersed. Since the immersed region of the skin is of a far larger area than the region which is in contact with one of the polarization-free electrodes (7 an2) , the resistance of the first mentioned will be very low. The resistance of the object of measurements will therefore almost exclusively be determined by the resistance of the 7 cm2 of skin, since the resistance of the electrode and of the contact electrolyte can be disregarded. In agreement with this fact, most experiments showed a resistance of the object of measurements higher than 10,000 ohms a t 2 volts. After removal of the stratum corneum of the 7 emz of skin - where the skin resistance is located - the same object of measurements showed a resistance of about 600 ohms a t 2 volts. The last mentioned resistance, which is exclusively that of the internal tissue and of the immersed area of the skin, is further independent of potential, variations of the direction of current,and of the time of conduction. Therefore, the present method enables us to measure the d. c.-resistance of a known skin area.

In some experiments, the object of measurenients was 2 x 7 cmZ skin area on the volar side of a forearm resting on 2 silver-silver chloride electrodes a t a distance of about 5 em, connected by the internal tissue of the forearm, the resistance of which is 200-300 ohms. The resistance of this object was measured with an error of less than 1 per cent in a Wheatstone bridge.

When studying the time dependence of the current, the d. c . was registered by means of an electrostatic oscillograph (resonance period 1/3,000 see) and a d. c.-amplifier (BUCHTHAL and NIELSEN 1936).

CONDUCTING PROPERTIES O F THE HUMAN SKIN. 135 In another series of experiments, the d. c. was overlaid by a. c.

with a frequency of 200 cycles. The a. c.-amplitude was registered as a function of time by means of an oscillograph and a d. c.-amplifier and was compared with the amplitude of a. c. of the same frequency over a known ohmic resistance. Further, an a. c. with a frequency of 1,000 cycles was overlaid by the d. c. through the above mentioned object of measurements. By means of two d. c.-amplifiers, the alternating current and -voltage were then via an electronic switch (Phi- lips) introduced into a cathode beam oscillograph where the standing picture was photographed. The variation with time of the current amplitude was registered and the phase shift relative to the constant amplitude of the alternating voltage was determined.

Experimental results.

1) Variations from day to day, individual and regional variations of the skin resistance to direct current.

In order t o elucidate the individual and regional variations of the skin resistance, the d. c.-resistance to anodic conduction a t 2 volts was determined on the volar side of the forearm over a period of 6 days on the same region of the skin. It was furthermore determined on 6 different persons and on different regions of the same person. The resistance of 7 cms of the skin at 2 volts and ni KC1-solution as contact electrolyte varies between 70 x lo3 and 485 x lo3 ohms on one person and between 40 x 103 and 200 x lo3 ohms on the other.

I n different individuals the resistance of 7 cm2 of the skin at 2 volts and 1 per cent KC1-solution as contact electrolyte varied between 5 x lo3 and 2.50 >( 103 ohms.

The regional variation is shown in table 2.

Table 2. Reg iona l variation of {he d . c.-resistunce in ohms of 7 cm2 of the s k i n at 2 tolts and 1 per cent KCI-solution as contact electrolyte.

1 K. W. 8 21 years i Proximal Middle of Near the Upper arm I left arm I t o wrist forearm elbow

Resistance in ohms at I 2 Volts . . . . . ~ 2200 1 82000 )I 160000 I 9000 1 The experiments show the great difference in the d. c.-re-

sistance of the skin at 2 volts on the same person from day t o day and on different regions, and moreover the great individual

136 THOMAS KOSENDAL.

variations. The variations in the d. c.-resistance of the skin must, be ascribed to variations in the stratum corneum (cf. section 2) of different persons, of the different regions, and of the same region from day to (lay.

These variations in the skin-resistance make it necessary that studies on the interdependence of skin resistance and voltage, direction of current, and properties of the contact electrolyte are performed on the same region of the skin and are compared only with resistance values determined in the same experiment.

2 ) Localizatiori of the d . c.-rrsistance. I n recent investigations (ROSENUAT, 1940) it was shown that

the a. c.-resistance of the skin a t frequencies betwyeen 30 and 20,000 cycles is exclusively located in the stratum corneum. This was made evident by the decrease in a. c.-resistance of the skin to 0 ohm after abrasion of the skin with emery paper. Histolog- ical investigations of the abraded skin region showed further that the lesion was only located to the stratum corneum and not to the stratum germinativum.

I n the following experiments, the d. c.-resistance was determined before and after abrading the skin with emery paper in the same way as in the above mentioned a. c.-experiments. The lesion is a maceration of the stratum corneuni only, without in- juring the corium and without bleedings. The depth of the lesion has not in this case been determined microscopically; as earlier investigations (ROSENDAL 1940) had shown that the abrasion here applied only injured the stratum corneum, i t is justified to locate the present lesions to the same layer. Table 3 contains the figures obtained on two persons.

These experiments illustrate the decrease in d. c.-resistance after lesion of the stratum corneum to a value which is of the same order of magnitude as that found recently for the a. c.- resistance of a corresponding object (ROSENDAL 1940).

I n agreement with LEWIS and ZOTTERMAN (19271, it, is thus shown that the d. c.-resistance of the skin as well as its a. c.-resist- rmce are located cxclusiwely in. the stratum corneum, while the stratum germinativum conducts an electric current as does the internal tissue. Moreover, the resistance of the internal tissue, either between two skin areas or from one skin area to the other arm or leg immersed into KC1-solution, is independent of the voltage and of the direction of current. As this resistance is also

CONDUCTING PROPERTIES O F THE HUMAN S P I N . 13i

TIIhlC 5.

D. c.-resistance in ohms o f 2 x 7 cm2 o f the s k i n before a n d a f t e r abrasion o/ (1 per cent KC

r , I est-person

A . s. y 24 years old right forearm

34 years old left forearni

T. R. 8

&fore abrasion

Resistance in ohms at

2 volts

115 500

15 000

e s k i n with emery paper. ;oht ion as contact electrolyte.)

Direction of

ciirrc.iit

1

II I

I t

I S f t e r abrasion I

0.154 volts - 614

608 257

220

Resistance in ohms at

0.224 volts - 608

608 354

21s

- 1.14

volts - 5SS

5ss 230

- 1.5

volts

588

592 229

219 , 218

~

2 volts

5so

330

independent of the kind and of the concentration of the contact electrolyte, and as i t is small compared with the resistance of the skin, it may be disr,egarded in the following experiments.

3 ) Dependence of the d . c.-resistun,ce on the moistening of the skiiz

The studies on the influence of moistening of the stratum corneum either with 1 per cent KC1-solution or with saturated KCl-solution were performed on 10 persons of both sexes. The d. c.-resistance was measured a t different potentials between 0 . 1 2 9 and 2 volts. The region investigated was the volar side of the persons’ forearm. The resistance was measured during up to 60 min. of anodic and cathodic conduction. I n some experiments, the direction of current was reversed after 1 min. or after 2 min.; in other experiments, after 5-10 sec or after 2 min., and the current was then interrupted for 5 min. in between every determination.

All experiments revealed a very marked decrease in d. c.-resistance of the skin, most pronounced after the first 10 minutes of moistening. After the lapse of 30 min., the resistunnce approxz- mates a constant value which i s 5 to 10 times as low as the initial vulue and which corresponds to a saturation of the stratum corneum with electrolyte. The decrease in resistance is greatest after moistening of the skin with saturated KC1-solution. Some curves of resistance decrease or current increase, respectively, are given in fig. 1.

with electrolyte.


. d O b r n J r n A 80 -

90 -

' 2 4 6 8 I0 12 14 16 18 20 22 24 26 28 30 32 39

Fig. 1. Dependence of the skin resistance on moistening with electrolyte and

Ordinates: left, resistance in ohms, right, current in mA. Abscissae: Time in min. Curve I: 2 x 7 cmp of skin area, 1 per cent KC1-solution. Curve 11: 2 x 7 emz of skin area, saturated HC1-solution. Curve 111: 2 x 7 cm2 of skin area, 1 per cent KC1-solution. Curve IV: 7 em2 of skin area, 1 per cent KC1-solution. Curve 111 was registered after washing of the skin with distilled water, and

should be compared with curve 11. I n curves I, 11, and 111, the skin resistance to anodic and cathodic conduction for 1 rnin. was determined, however, only the first mentioned is plotted in the curves. I n curve IV, the skin resistance was determined after 5 see. of conduction and a t 5 min. intervals in between every determination.

on conduction of d. c. a t 2 volts.

The decrease in skin resistance is presumably due to an increase in the electrolyte content of the stratum corneum. This is made probable by curve 111 which represents the increasing resistance obtained after the saturated KC1-solution (experimental curve 11) had been washed off from the stratum corneum with distilled water. This explanation is further supported by experiments in which the skin was moistened for 20 min with saturated KCl-solution. I n these experiments, an increase in d. c.-resistance of the skin a t 2 volts from 10,000 ohms to 18,000 ohms and 19,000 ohms, respectively, in the course of 12-14 min. was found when

CONDUCTING PROPERTIES OF THE HUMAN SKIN. 139

the saturated KC1-solution w-as replaced by 1 per cent KCl- solution as contact electrolyte.

When an e. m. f. above 2-4 volts is applied after the resistance has become constant, a further decrease in the skin d. c.-resistance can be observed. The value then obtained approximates the resistance of the internal tissue. However, this decrease niust be due to a short-circuiting of the stratum corneum. as will be discussed in a later section (p. 141).

The experiments described on the foregoing pages indicate that the d. c.-resistance of the skin depends to a high degree on the electrolyte content of the stratum corneum. The investigations of the interdependence between skin resistance and niag- nitude of the direct voltage, direction of current, and time of conduction therefore involve a considerable error, if the nieasure- ments are performed immediately after the electrode is applied. This error can be avoided when the measurements are performed after moistening of the stratum corneum with contact electrolyte for 10 niin.

4) T h e deperzdence of the sk in resistance o n the direct voltage. In order t o elucidate the voltage-dependence of the skin re-

sistance, the d. c.-resistance in ohnis was determined on 7 cm2 of skin on the forearm a t an e. m. f. varying between 0.129 and 2 . 5 7 volts and with a 1 per cent KC1-solution as contact electrolyte (fig. 2). The resistance was determined for anodic conduction and, subsequently, for cathodic conduction, each during 1 min. The resistance determination was performed immediately after application of the electrode and was repeated twice a t different voltages in the course of 26 min.

I n the first experiment (group of curves I), the resistance to anodic and cathodic conduction decreases with increasing e. m. f. from 0.129 to 2 . 5 7 volts, if the resistance is measured immediately after application of the electrode. However, the decrease in resistance is no real expression of the voltage-dependence, but must be ascribed to a moistening of the stratum corneum with 1 per cent KCY-solution (cf. p. 137). On repeating the experiment (group 11)) the resistance is found considerably lower and the voltage dependence disappears in the case of anodic conduction. The reproducible interdependence between voltage and resistance is found after the lapse of 20 min (curves of group 111). With increasing voltage from 0.129 to 2.57 volts, the curve indicates


2

a5 I 4s 2 1.5 Volts

Fig. 2 . 'I'hc dependence of the resistance on the voltage. 7 cm2 of skin and 1 per cent KC1-solution as contact electrolyte.

Ordinates: resistance in ohms in a logarithmic scale. Abscissae: potential in \ olts. 0 anodic conduction. 0 cathodlc conduction. The time of the resistance determination after application of the electrode in min. 1s given above or beneath the curves.

an increasing resistance to anodic conduction from 8,200 ohms to 13,000 ohms while the resistance to cathodic conduction decreases to 4,000 ohms.

The voltage dependence of the ?kin resistance a t higher voltage is represented in fig. 3. After moistening of the skin with saturated KC1-solution for 20 min., the resistance was determined for anodic and cathodic conduction, during 1 min. each, and a t potentials between 0 .2 and 12 volts. The voltage dependence between 0.2 and 2 volts was measured in 3 groups of experiments, in the figure marked I, 11, 111. The voltage dependence up to 12 volts was determined in 2 series of experiments, marked I11 and IV.

The increase in resistance in the case of anodic conduction and the decrease with cathodic conduction are reproducible as long

CONDUCTING PROPERTIES O F THE HUMAN SKIN.

xf~30hms

5

4

3

2

f

141

- O f 2 4 6 8 I0 M Volts

Fig. 3. I-oltage tlrperititmce of thc rcsistarwc of 7 ( ~ 1 1 ~ of skin moistcmctl for 20 miri with s:ituwtc:l KC1-solution which serves also ns contact elcctrolytc.

Ordinate :: resistance in ohms. Abscissav: potential in volts. (1 ;Inodic coii- duction. 0 cathodic conduction. At tlic :trrow, tlic pcrsori felt, pain in thc skin rcgioii irivchgatod.

as the e. ni. f. does not exceed 2 volts (fig. 3, curves 1, 11, H I ) . At further increase of the voltage (curve HI), the resistancc both to anodic and to cathodic conduction decreases. On repeating the experiment (curve IV), the incrrase as wcll as the decrease in resistance alniost ceased, both values now approaching one another and being almost independent of vojtage. The decrease in repistance a t e. m. f. above 2 volts is acconipanied by a marked feeling of pain in the skin, which becomes stronger with increasing e. m. f. Simultaneously, the respective regions become hyperaemic.

The experinients described above reveal a difference in the interdependence of skin resistance and voltage for anodic or cathodic conduction, respectively, a t e. ni. f. up to 2 volts. Above 2 volts and for both directions of current, the difference disappears, since the skin now behaves like a low- ohmic resistance of a magnitude which corresponds to the resistance o€ the internal tissue. This decrease in resistance is probably due to a short-circuiting of the stratum corneuni a t voltages above 2 volts. The feeling of pain and the hyperaemia might be correlat-


ed with the formation of histamin in the skin (cf. ROSENTHAL and MINARD'S (1939) studies on histamin formation and feeling of pain during electrical stiniulation of the skin).

5 ) T h e dependence of the sk in resistance on the direction of current. As shown in fig. 3, the skin resistance is higher during anodic

than during cathodic conduction through the same region of t he skin. The difference depends on the voltage and aniounts to 350 ohms a t 0.2 volts, t o 2,600 ohms a t 2 volts; a t 10 volts, however, the difference is 100 ohms, only.

Fig. 4 exhibits further evidence for this observation. The experiments reveal that the resistance to CknOdiC conduction at 2 voks is about 60-100 p e ~ cent higher than the resistance to cathodic cowbuction, irrespective of the initial direction of conduction, and illdependent of the duration. The difference is greatest in the case of saturated KC1-solution as contact electrolyte. This phenomenon was ohserved on different regions of the forearm and on 10 different persons.

Fig. 5 shows clearly that this phenomenon just as the other (1. c.-conduction phenomena must be located in the stratum corneuw . The skin resistance t o anodic and cathodic conduction was determined before and after abrasion of the stratum corneuni x5ith emery paper, 2nd the difference in resistance was found t o vanish as soon as the stratum corneuni was injured. The sanie applies when the stratum corneuni ii; short-circuited by e. m. f. ahove 2 volts (cf. fig. 3 , p. 141).

Fig. 4. Dependence of the resistance on the direction of conduction a t 2 volts. 7 om2 of skin moistened for 6 min with 1 per cent KC1-solution (I) and with saturated KC1-solution (11) and (111). Experiment 11 begins with anodic conduc-

tion, experiment I11 with cathodic conduction. Ordinates: left, resistance in ohms; right, current in mA. Abscissae: time in

min. 0 anodic conduction. cathodic conduction.

CONDUCTING PROPERTIES .OF -THE HUMAN SKIN.

f0- -

-

2 - -

0 " ' " " " ' " " " " ~ " ~ ~ ~ -

L 2 4 6 8 f O { ? f I V

92

0,4

(0

m A

d 0 2 4 6 8 0 2 4 6 8 m i n .

143

144 TIIOMAS ROSENDAL.

the tl. c.-resist,ance with the time of anodic as well as cathodic conduction. The a. c.-resistance a t 1,000 cycles which is almost exclusively a capacitive resistance did not shorn- any variations.

6) Investiyatlons of the p I 1 of the contuct electrolyte beforc. t r r d after conduction o f d . c. throu:yh the ski l l .

REIN ( I 926) fouricl that anodic conduction of (1. c. through the skin caused an increase in pH of the contact electrolyte. During cathodic conduction, the pH decreases in the case of males and increases in the case of females. REIN lias eiven no details concerning the pH of the electrolyte before the experi- nlent5 or whether application of the electrolyte on the sltin changed its pH.

In order t o elucidate these phenomena, the pH of the contact electrolyte was (let erniinetl before and after application of the electrolyte on the skin and, further, before and after arrotlic and cathodic conduction of t l . c.

The pH was riiensuretl by ineans of n glass electrode and a valve voltrneter; the error of this deternunation did not exceed 0.03 in the pH value. (Only Jena glass was used).

The pH of 4 cc of n 1 per cent IZCII-solution was nieasured be- €ore and after standing on the volar side of a forearm. The liquid nitli a mean p€I of 5 . i 3 \+-as kept in a cylindrical vessel with a contact area of 7 cm2. 14 experinients on 1- persons showed a rilean decrease in pH of 0.76, to a mean constant value of pH 4.07, when the pH was measured up to 45 min aCter standing. The change in pH is most considerable during the first 20 niin of standing.

The decrease in pH of the contact electrolyte is not due to an escape of CO, from the skin into the electrolyte; furthermore, an increase in pH of the contact electrolyte was found after cleaning of the skin with water, soap, and alcohol in two cases. These observations might indicate that the decrease in pH after standing of the electrolyte in contact with the skin is caused by acid substances liberated from the skin. This explanation is in agreement with SCHADE and MARCHIONINI’S (1928) investigations, w-ho found the mean pH of the skin surface to be 3.78.

These observations indicate that the variations of the pH after conduction of d. c. through the skin can first be determined after the pH of the contact electrolyte has reached a constant value in the course of 20-30 min.

CONDUCTING PROPERTIES OF THE HUMAN SKIN. 3 45

Table 4 contains the pH values before and after anodic and cathodic conduction of d. c. a t voltages up t o 10 volts through 7 cm2 of skin on the volar side of 4 persons' forearms. The pH values given in the table were obtained before and after the pH value of the solution had become constant after standing in contact with the skin for 20--45 min. At the highest voltage, the current did not exceed 1.65 mA. The last column of the table shows the maximum change in pH of every individual experiment after conduction of d. c.

4.88

4.88

5.4%

5.63 5.74

Table 4. pH of the electrolyte in contact with the skin before and a f t e r

conduction of d. c.

4.8F

5.4(

1

- Person

T. R. left

forearm

H. F. right

forearm L. L. right

forearm R. R. right

forearm

5.27 4.91 4.89

5.03 4 . 3 i

5.63 5 . i 3

5.61

=

llectro- lyte

-- at. KC 6 cc do. do.

%: KC 4 cc do. do.

do.

no. do.

do.

5.12 4.39

- __

PH iefore xperi. merit

- 4.9; 5.09 4.71

5.48 5.94 5.66

5.30

5.89 5.71

6.67

4.83

= PH atter

miu of stand-

ing

30-45

- 5.22 4.98 4.91

5.03 4.39 4.88

5.49

6.67 5.74

5.68

I / + 0.05 - 0.07 - 0.08

+ 0.09 1 0 1.65 - 0.05

1.35 -0.09

0.55 - 0.04 0.95 - 0.01

I - 0.oi

~~~~ ~

pH after 5 min of d. c.-couduc- 1 Max- , imum I . tion

anodic

4 V i 8 V - 2 v

5.4:

cathodic 1 1 change ' in

PH 0v1 , -1 mA 1

I Mean value + 0.05

It can be seen from the table that the highest variation in pH of the contact electrolyte is 0.09, the mean variation being 0.05. During anodic conduction, the pH value increased in 2 experiments, decreased in 5 experiments, and remained unchanged in one experiment. In the case of cathodic conduction, the pH decreased in 5 experiments and remained unchanged in one experiment.

As a result of the pH determinations it was found that electro- 10-424075. Acla phys. Scandinav. Vol. 5.


lyte in contact with the skin changes its p H from about 5 .73 t o 4.97 in the course of 45 min. Conduction of d. c., however, is of no appreciable effect on the pH of the contact electrolyte.

Discussion.

The experiments described above confirm LEWIS and ZOTTER- MAN’S (1927) localization of the d. c.-resistance of the skin to the stratum corneum and they are also in agreement -with ROSEN- DAL’S (1940) localization of the a. c.-resistance of the skin to the same layer. I n contrast to the view held by most of the previous investigators - GILDEMEISTER and his school -, the various conductivity phenomena must be located in the stratum corneum. The assumption can no longer be maintained that the d. c.-resistance of the skin is an apparent resistance due to polarization corresponding to the cell membranes in the living cell layers of the stratum germinativum.

Presumably, the d. c.-conduction through the stratum corneum occurs through the excretory ducts of the sweat glands or the sebaceous glands and along the hair sacks; this was made probable by REIN’S investigations (1926) concerning the transport of coloured cations and anions (methylen blue and eosin) through the skin. However, the probability exists that an ion migration also takes place directly through interspaces between the horny cells of the stratum corneum. The cause of the large individual and regional difference in the d. c.-resistance of the skin might he found in variations of the length, dimensions, and electrolyte content of these channels through the stratum corneum .

The decrease in d. c.-resistance of a skin area o l 7 cm2 either after a lesion of the stratum corneum by abrasion with emery paper or by short-circuiting a t an e. ni. f. above 2-6 volts is due to the formation of electrolyte-filled low ohmic shunt resistances in the stratum corneum which form a contact with the well-conducting internal tissue through the living cell layers of the stratum germinativum. It is natural to explain the decrease in the d. c.-resistance after EBBECKE’S mechanical and galvanic reaction in the same way. EBBECKE’S galvanic reaction (1921) was accompanied by a dilatation of the vessels, which EBRECKE ascribed to a formation of dilating substances during the passage of d. c. through the skin. On the basis of ROSEN-

CONDUCTING PROPERTIES O F THE HUMAN SKIN. 147

THAL and MINARD‘S (1939) investigations it may be assumed that the dilatation of the vessels and the feeling of pain which appear in the respective skin area a t potentials above 2-6 volts are caused by the formation of histamin during the passage of d. c. through the skin.

Moistening and conduction experiments show the significance of the electrolyte content of the stramturn corneum for the d. c.- resistance of the skin, in agreement with earlier observations when low-frequency a. c. was applied (cf. p. 131).

The increase in electrolyte content of the stratum corneum in the course of the conduction also explains the hysteresis phenomenon (cf. p. 130) which GILDEMEISTER assumed to he caused by a change in polarization of the living cell layers.

Finally, the increasing electrolyte content of the stratum corneum due to moisture and conduction of d. c. may explain the decrease in d. c.-resistance of the skin during anodic and cathodic conduction which appears with increasing voltage up to 2-4 volts. I n the recent literature, even in SCHAEFER’S book on electro- physiology (1940), the decrease in skin resistance with increasing voltage is interpreted as a reduced polarization of the skin a t higher voltages. However, this interpretation is erroneous, and it would also he a misinterpretation to consider the decrease in resistance a t voltages above 2-6 volts, which are due t o a short- circuiting of the skin, as an expression of reduced polarization.

The voltage dependence of the skin resistance determined after moistening of the stratum corneum with 1 per cent KC1- solution or saturated KC1-solution shows - in contrast to earlier investigations - that the resistance to anodic conduction increases with increasing potential up to 2--4 volts, while the resistance to cathodic conduction decreases. The difference between the resistances in both directions approaches zero a t 0.1 volt, while a t 2 volts the difference can exceed 100 per cent of the resistance to cathodic conduction. The difference in resistance to the two directions of current is an expression of a polarity of the stratum corneuin to d. c. The polarity is independent of the initial direction of the current and it increases with time during conduction in both directions, since the resistance decreases or increases, respectively, until a constant value is obtained after 2-3 min of conduction in each direction. A polarity was also found in the case of a. c. of a frequency of 200 cycles where the resistance is partly an ohmic, partly a capacitive re-


sistance; it could, however, not be found a t 1,000 cycles when the conduction is only capacitive.

REIN (1926) found a similar difference in skin resistance to anodic and cathodic conduction when 1jlOO m or 1/10 m KC1- solutions were applied as contact electrolyte. He interpreted the difference as an expression of a change in polarization of the skin after reversion of the current.

According to investigations of the polarity of organic membranes (BETHE and TOROPOFF 1914, 1915, MICHAELIS and cowor- kers 1925-27, FREUNDLICH 1930, and HOBER 1936), an electronegatively charged membrane is mainly permeable to niono- valent anions. The polarity can be reversed by re-charging the membrane. I n the case of an electronegatively charged, water- free collodion membrane, BETHE and TOROPOFF observed an alkali formation on the anode side, and a formation of acid on the cathode side of the membrane after conduction of d. c.

REJX (1927) has put forward the view that the membrane effect of the skin must exclusively be located to the stratum lucidum and that it is due to a negative charge of this layer; the membrane should, further, be more permeable to anions than to cations, and the outward migration of anions should be more difficult than the inward migration. REIN refers to the fact that water permeates the skin cathodically when the anode is placed on the outer side of the epidermis and the cathode on its inner side. However, the opposite direction of conduction has not, been studied. REIN points out further that the living cell layers of the stratum germinativum and the hair follicles are better stained by basic methylen blue with a staining cation than by acid eosin with a staining anion. Since, however, the stratum corneum is not stained by any of these substances, the experiments do not give any information concerning the electric conduction of this layer, but t,hey show only tha t the stratum germinativum is best stained by methylen blue. Finally, REIN mentions the observation previously discussed of an alkali formation in the contact electrolyte beneath the anode after conduction of d. c. through the skin, and this is considered to indicate that the skin is negatively charged; BETHE and TORO- POFB’S investigations of the pH variations on each side of an electronegatively charged collodion membrane are taken to sup- port this view. However, the writer’s investigations do not confirm REIY’s change in pH of the contact electrolyte after conduc-

CONDUCTING PROPERTIES OF THE EUMAN SKIN. 149

tion of d. c. through the skin; therefore, and moreover in view of the above discussed objections, it is the writer’s opinion that the conditions determining the electric charge of the skin are not yet elucidated.

This view-point is supported in a paper by BRAUNER (1930). In the case of an electronegatively charged, non-living, semi- permeable membrane, BRAUNER found the opposite polarity for d. c. t o that in the skin, and he explained this effect by a pH variation in alkaline direction on the anode side and iiz acid direction on the cathode side. ,4s a membrane, he used the seed- coat of the horse-chestnut. The d. c.-resistance of this membrane a t 2.5 volts and during cathodic conduction is 315 per cent of the resistance during anodic conduction, when 1 i 4 N K,SO,-solution is used as an electrolyte on each side of the membrane. Since the polarity of this inembralie to d. c. is opposite to the polarity of the skin, we cannot for them both expect the same change in pH on the anode- and cathode side of the membrane as a cause of their polarity. Consequently, it must be assumed either that this explanation is erroneous, or that the skin is not electronega. tively charged.

Also other facts seem to indicate that the electric charge of the stratum corneum is more complicated than assumed by REIN. SCHADE and MARCHIONINI’S (1928) observation of an acid reaction on the surface of the skin are in favoiir of an electro- positive charge on the outer side of the stratum corneum. The same authors found an allialine reaction (pH 7.44) after lesion of the epidermis which might indicate an electronegative charge on the inner side of the stratum corneuni.

Apart from the electric charge of the stratum corneum, the ion size of the electrolytes on the outer- and inner side of the stratum corneum will be of importance, since the resistance to anodic conduction is determined by the migrating-in of the anions and -out of the cations, while the resistance to cathodic conduction is determined by the opposite transport. This view has been further supported by preliminary experinients in which solutions with large and small anions, respectively, and different cations were used as contact electrolytes. It was found that 10 per cent sodium benzoate solution and 10 per cent sodium citrate solution as contact electrolyte annulled the difference in skin resistance to anodic and cathodic conduction, while 1 per cent NaCl led to a similar difference as 1 per cent KC1-solution.


Finally, also the concentration of the contact electrolyte is of importance, since the difference in skin resistance a t the two directions of current increased from 45 per cent t o 200 per cent when saturated KC1-solution was applied as contact electrolyte instead of 1 per cent KC1-solution.

It may, therefore, be assumed that the polarity of the skin t o d. c. can be ascribed to the electric charge of the stratum cop- neum, however, also the type and the concentration of the contact electrolyte are of significance.

The results from a closer study of these phenomena will be given in a later publication.

Summary.

The resistance to direct current (voltage 0-12 V) of a 7 ern2 skin area on the volar side of the forearm has been determined under different conditions, using polarization-free silver-silver chloride electrodes.

The skin resistance is almost exclusively located in the stratum corneuni and decreases very markedly with increasing electrolyte content of this layer. The conductivity of the stratum germinativum corresponds to that of the internal tissue which is rich in electrolyte, and behaves like a low ohmic resistance t o d. c.

On 7 em2 of skin area, the stratum corneum shows polarity to d. c. a t currents below 1 mA, since the resistance to anodic conduction increases with increasing e. m. f. up to 2--4 volts, while the resistance to cathodic conduction decreases. At voltages above 2-4 volts, the resistance to both directions of current decreases to a value which corresponds to the resistance of the internal tissue. This decrease in resistance is presumably due to a short-circiiiting of the stratum corneum. The polarity of the stratum corneum, which is highest when voltages around 2 volts are applied, is brought into relation to a possible electric charge of the stratum corneuni and to the type and the concentration of the contact electrolyte.

The pH of the contact electrolyte (4 cc I per cent KC1-solution) decreased after standing on the skin for 20-45 min to a mean value of 4,97, determined on 4 persons in 14 experiments. I n contradistinction to REIN’S investigations, no change in the pH

CONDUCTING PROPERTIES OF THE HUMAN SKIN. 151

of the contact electrolyte after anodic and cathodic conduction of d. c. through the skin could be observed.

My heartiest thanks are due t o F. BUCHTHAL, 31. D., for sti- mulating advice and helpful discussions in the course of this work.

References.

BETHE, A., and T. TOROPOFF, Z. phys. Chem. 1914. 88. 686. -, Ibidem 1915. 89. 597. BISRUPSKI, F., Pfliig. Arch. ges. Physiol. 1938. 240. 282. BRAUNER, L., Jahrb. wiss. Botanik 1930. 73. 513. BROWN, A. S., J. Amer. chem. Soc. 1934. 56. 646. BUCHTHAL, F., and I. 0. NIELSEN, Skand. Arch. Physiol. 1936. 74.

EBBECKE, U., Pfliig. Arch. ges. Physiol. 1921. 190. 230. -, Ibidem 1922. 195, 300. -, Ibidem 1923. 199. 197.

202.

EINTROVEN, W., and J. BIJTEL, Pfliig. Arch. ges. Physiol. 1923. 198. 439.

PREUNDLICH, H., Kapillarchemie, Leipzig 1930. GALLER, H., Pfliig. Arch. ges. Physiol. 1913. 149. 156. GARTNER, G., Med. Jahrb. Ges. Arzte, Wien 1882. 519. GILDEMEISTER, M., Pfliig. Arch. ges. Physiol. 1915. 162. 489. -, Z. biol. Techn. Meth. 1915. 3. 28. -, and E. R. KAUFHOLD, Pflug. Arch. ges. Physiol. 1920, 179, 154. -, Handb. norm. pathol. Physiol. 1928. VIII. 2. 657. HOBER, R., Physiol. Rev. 1936. 16. 52. LEWIS, T., and Y. ZOTTERMAN, J. Physiol. 1926-27. 62. 280. MICHAELIS, and others, quoted from FREUNDLICH, H., Kapillarchemie. MUNK, H., Arch. Anat. Physiol., Lpzg. 1873. 505. REIN, H., Z. Biol. 1926. 84. 41. 118. -, Ibidem 1927. 85. 195. 217. -, Handb. Haut- u. Geschlechtskrankheiten 1929. 1: 3. 43. ROSENDAL, T., The conducting properties of the human organism to

ROSENTIIAL, S. R., and D. MINARD, J. exp. Med. 1939. 70. 415. SCHADE, H., and A. MARCHIONINI, Arch. Dermat. Syf. 1928. 154. 690. SCKAEFER, H., Elektrophysiologie, Berlin 1940.

alternating current, Copenhagen 1940.

Documents

Studies on the Conducting Properties of the Human Skin to Direct Current