ELECTRICAL CHARACTERISTICS OF THE SKIN

THE IMPEDANCE OF THE SURFACE SHEATH AND DEEP TISSUES*

CAPTAIN JAMES C. LAWLER, MC, USA, MAJOR MICHAEL J. DAVIS, MC. USAt AND EVERTONC. GRIFFITH, B.S.

There is a need for an accurate method of de-termining when the skin is normal and when itis not. Except for histopathological examinationour judgment of the condition of the skin islimited to what can be seen by the naked eyeor felt by the examining finger. The response ofdiseased skin to various forms of treatment mustlikewise be judged by what our senses tell us orby the subjective observations of our patients.

it is felt that an understanding of the electricalcharacteristics of the skin may offer a partialsolution of this problem. These characteristicscan be accurately determined and changes fromnormal related to the condition of the skin. Thisreport presents a method for determining theelectrical impedance and phase angle of the skin,and a simplified technic for the separation of theimpedance of the skin from that of the deepertissues.

HJ5TOEICAL REVIEW

Many investigators have attempted to utilizethe electrical properties of the skin as an indica-tion of the condition of the skin or its underlyingtissues. Earliest attempts utilizing direct currentencountered the insurmountable problem ofpolarization currents set up within the bodywhich mask the true electrical resistance of the

Glossary—impedance: the total electrical op-position of a circuit to the passage of an alternatingcurrent through it.

Phase Angle: the relationship between currentand voltage in an alternating current circuitexpressed as an angle of lead or lag.

Capacitance: the property of a capacitor tostore a charge and to oppose any voltage changeacross it.

Reactance: the opposition to flow of an alternat-ing current resulting from the electromotive forcebuilt up by a capacitor or an inductor.

Capacitive Reactance: the opposition to flowof an alternating current resulting from theelectromotive force built up by a capacitor.

* From the Department of Dermatology, WalterReed Army Institute of Research, Walter ReedArmy Medical Center, Washington 12, D. C.

t Present address: 2325 University Avenue,New York 68, N. Y.

Received for publication July 13, 1959.

tissues. Gildemeister (1) was among the first toovercome this difficulty by using an alternatingcurrent and measuring the total opposition to itspassage through tissue. This total electrical op-position is called impedance and in the skin ithas two components, resistance and capacitivereactance.

Many studies (2, 3, 4) have been done relatingthe impedance of the body to thyroid function.In general it was found that the impedance andthe basal metabolic rate were in qualitativeagreement in hyperthyroidism but bore no rela-tion to each other in hypothyroidism. Duringthese studies many arrangements were used inan attempt to separate the impedance of theskin from that of the deeper tissues. Brazier (5)in England using immersion electrodes attributedthe impedance of the body almost entirely tothe deep tissues. Horton and VanRavenswaay(6) and Barnett and Byron (3) developed themultiple electrode technic and were able to showa significant contribution by the skin to theimpedance of the body. Gerstner (7) used carbonpowder-saline electrodes to separate skin im-pedance from deep tissue impedance. He foundlower values of skin impedance for women thanfor men, and felt that skin impedance was acharacteristic trait of the individual. Gerstnerand Gerbstadt (5) attempted to localize the siteof the impedance of the skin by measurementsbefore and after burning and scarification. Theyfound that the tops of blisters behaved as thewhole skin but that denuded skin had almost noimpedance. They also demonstrated that areasof skin with a thick keratin layer had a higherimpedance than those areas in which the keratinlayer was thin.

In addition to impedance, attention has beengiven to various other expressions of electricalbehavior as used in electrical engineering. Amongthese have been phase angle, impedance angle,and Q factor, all of which express a relation be-tween pure resistance, capacitive reactance, andinductive reactance in an alternating current

301

302 THE JOURNAL OF INVESTIGATIVE DERMATOLOGY

Fin. 1

circuit. The most commonly used of these hasbeen the phase angle, the angle between theresistive component and the final impedance.

Gougerot (9) was the first to study impedanceand phase angle in various dermatoses. Using analternating current at a frequency of 4,000 cyclesper second and lead electrodes 7.0 by 8.5 cm. hefound that the impedance of normal skin wasalways greater than 300 ohms, and that thephase angle was greater than 55°. In 23 patientswith active eezematous lesions the values werepathologically lower, impedance 90 ohms, phaseangle 26°. In psoriasis Gougerot found normalvalues in dry lesions but abnormally high valuesin uninvolved skin.

EXPERIMENTAL METHOD

The electrical impedance or total oppositionof a circuit to the passage of an alternating currentcannot be measured in terms of simple ohmicresistance alone. In addition to ohmic resistance,if the circuit contains an inductance such as acoil, the alternating current induces a counterelectromotive force across the coil causing thecurrent to lag behind the applied voltage. Thislag or opposition to the passage of current throughthe coil by virtue of this induced electromotiveforce is called the inductive reactance of the coil.If the circoit contains a capacitor the alternatingcurrent charges and discharges the capacitor, the

capacitor opposes these voltage changes, and thevoltage lags behind the current. This oppositionof voltage by a capacitor is called capacitive re-actance. Early investigations have shown theskin to have capacitive properties, that is, toexert a capacitive reactance to the passage of analternating current. Inductive properties havenot been attributed to the skin.

Although many methods have been used tomeasure the impedance of the skin most haveshared one basic principle: the construction ofan equivalent circuit containing variable resistorsand capacitors against which the skin is balancedelectrically. It is assumed that at balance theimpedance of the skin is the same as that of theequivalent circuit.

For this study a circuit containing variableresistors and capacitors in parallel was con-structed. The diagram of this circuit is shown inFigure 1. An audiooscillator provided an alternat-ing current of variable frequency, with an outputof 2 volts and 0.1 milliamps. An oscilloscopeserved as a balancing device. In essence thisrepresents a bridge arrangement with one arm ofthe bridge being the equivalent circuit and theother arm the circuit through the body. Theelectrodes were stainless steel disks 2 cm. indiameter. This size was selected for ease of ap-plication to comparable areas of skin in differentpatients and because it permitted easy applica-tion to many skin lesions. The electrodes wereplaced on the skin with a fixed distance of 2 cm.

CIRCUIT FOR MEASUREMENT OF IMPEDANCE

ELECTRICAL CHARACTERISTICS OF SKIN 303

between their rims. In order to obtain values forcomparable areas of skin they were always appliedto the ventral surface of the forearm midwaybetween the wrist and elbow in the normal subj ectstested. The electrodes were applied with rubberstraps as used in electrocardiography, allowingapplication with about the same pressure eachtime. The electrical connection with the skinwas made through filter paper moistened withnormal saline, the skin being in parallel with theequivalent circuit. With the electrodes in positionthe alternating current was impressed acrossboth the skin and the equivalent circuit and thetwo balanced by varying the resistance andcapacitance of the equivalent circuit. At balance,readings were obtained in terms of resistance andcapacitance, and the impedance and phase anglewere calculated from the following formulae:

(B)

+ ()21?=

tan'—j—2orf C

whereZ = Impedance in ohmsB = Resistance in ohmsf = Frequency in cycles per secondc = Capacitance in farads

= Phase Angle.Using this technic the impedance of the normal

skin of 104 subj eets was measured at frequenciesof 1,000, 4,000, 10,000 and 20,000 cycles per second.This was done to assess the effect of variation ofthe frequency on skin impedance and to encompasssome of the frequencies used by other investi-gators. In addition to normal skin, a study wasmade of sites from which the stratum corneumand barrier zone had been removed by cellulosetape stripping. The effects of gradual removalof the stratum corneum were also determined.

EXPERIMENTAL RESULTS

A. Impedance and Phase Angle of Normal Skin

The mean value of impedance of normal skinat the four frequencies used in this study isshown in Table I. Increasing the frequency ofthe alternating current results in a decrease ofthe impedance reflecting a decrease in both theresistance and the capacitance. There is no par-ticular advantage of one frequency over anotherexcept that at very high frequencies there is a

TABLE IMean impedance of normal skin in 104 subjects

Frequency Impedance Standard Deviation

cycles per second ohms ohms

1,000 6487 1733

4,000 1882 468

10,000 861 187

20,000 507 111

tendency for the current to travel along thesurface of the skin rather than through it. Wehave selected 4,000 cycles per second as a satis-factory frequency for routine use, and detailedanalysis of the data will be limited to this fre-quency. At 4,000 cycles per second the meanskin impedance of the 104 subjects tested is 1882ohms with a standard deviation of 468 ohms.97% of the results fall within two standarddeviations of the mean, indicating a normaldistribution pattern. This gives a range, withintwo standard deviations, of 946 ohms to 2818ohms for the impedance of the skin on the ventralsurface of the forearm in this group of normalsubjects. The graphic presentation of these re-sults is shown in Figure 2.

If the values for impedance at 4,000 cycles persecond are analyzed according to sex (Table II)it is found that for 59 males the mean impedanceis 1862 ohms with a standard deviation of 456ohms. The mean impedance for 45 females at thesame frequency is 1910 ohms with a standarddeviation of 474 ohms. Within two standarddeviations of the mean the range is 950 ohms to2774 ohms for normal males and 962 ohms to2858 ohms for normal females. The difference inthe mean impedance of the skin of males andfemales is not statistically significant.

The mean value and standard deviation of thephase angle at the four frequencies used in thisstudy is shown in Table III. As is expected in acircuit containing resistance and capacitive reac-tance when the frequency is increased the phaseangle is decreased. At a frequency of 4,000 cyclesper second it is 73.5° for the group of 104 normalsubjects. The standard deviation of the mean is3.6°. There is no significant sex difference in thephase angle of the skin. (Table II).

B. Effects of Altering the Stratum Corneumand Barrier Zone of the Skin

Complete removal of the stratum corneumand barrier zone has a profound effect on the

U)F-z

impedance and phase angle of the skin. Table IVshows the effect of removing the stratum corneumby cellulose tape stripping in 23 subjects. Themean impedance of such sites at a frequency of4,000 cycles per second is 804 obms with astandard deviation of 54 ohms. This gives arange within two standard deviations of the meanof 196 ohms to 412 ohms for the impedance ofskin from which the stratum eorneum and bar-rier zone have been removed. The mean phaseangle of such sites at a frequency of 4,000 cyclesper second is 100 with a standard deviation of1.80. For both impedance and phase angle 95.5%of our results falls within two standard deviationsof the mean.

At low frequencies the capacitance of thestripped sites is higher than that of intact skinwhile at high frequencies (above 10,000 cyclesper second) the capacitance of the stripped sitesis lower than that of the intact skin. At all fre-quencies used the resistance is much lower inareas from which the keratin has been removed.

In six patients the resistance and capacitancewere measured at 4,000 cycles per second duringthe gradual removal of the stratum eorneum bycellulose tape stripping. (Figure 4). The removal

TABLE IIMean impedance and phase angle in 59 males and

45 females

Frequency Impedance Phase Angle Sex

cycles per second s5ms degrees

4,000 1862 73.6 Male

4,000 1910 73.4 Female

TABLE IIIMean phase angle of normal skin in 104 subjects

Fcequency Mean Phase Angle Standard Deviation

cycles per second

1,0004,000

10,000

20,000

degrees

7573.5

66

57

degrees

5.03.6

4.8

5.9

of a single strip of the superficial keratin mayresult in an increase in the resistance probablydue to the removal of surface electrolytes whichare good conductors. As the stripping is con-tinued there is a decrease in resistance and anincrease in capacitance. As small shiny areas

304 THE JOURNAL OF INVESTIGATIVE DERMATOLOGY

IMPEDANCE OF 104 NORMAL SUBJECTS AT 4000 CYCLES PERSECOND

45

40

35

30

25

20

IS

I0

5

0500 1000 1500 2000 2500 3000 3500

IMPEDANCE IN OHMS

Fsa. 2

ELECTRICAL CHARACTERISTICS OF SKIN 305

appear in the stripped sites indicating completeremoval of the keratin the changes become moremarked. When the procedure is continued sothat the entire area under the electrode is strippedof keratin the values are the same as those shownin Table IV, and continued stripping producesno appreciable change.

Washing the skin with ether for one minuteresults in a decrease in resistance attributable tothe removal of poorly conducted lipid substancesfrom the surface of the skin.

C. Impedance, Resistance, and Capacitanceat Various Body Sites

Measurements were made in the usual manneron the dorsal forearm, ventral forearm, dorsalarm, ventral arm, and on the palms. The im-pedance and resistance are highest on the palms,greater on the dorsal forearm than on the ventralforearm, and greater on the dorsal arm than onthe ventral arm. This would indicate that theresistance and impedance of normal skin arehighest in areas having a thick stratum corneum.The capacitance however is lowest on the palms,

TABLE IVImpedance and phase angle after removal of surface

sheath

Impedance (ohms) Phase Angle (degrees)Frequency(cycles per

second) Mean StandardDeviation Mean Standaed

Deviation

1,000 338 64.9 7 I 1.3

4,000 304 53.9 10 1.8

10,000 270 43.2 40 2.1

20,000 242 37.5 54 2.7

ower on the dorsal forearm than on the ventrallforearm and lower on the dorsal arm than on theventral arm. This would indicate that in areaswhere the stratum corneum is thick the capaci-tance of intact skin is low.

DISCUSSION

The measurement of the electrical impedanceor total opposition of the skin to the passage ofan alternating current has been accomplishedby many technics based upon the use of an equiv-alent circuit against which the skin is balanced.This makes necessary the assumption that atbalance the resistance and capacitance of theequivalent circuit reflect the resistance andcapacitance of the skin. There are several factswhich make this assumption a reasonable one.Of primary importance is the fact that the twocircuits can be balanced in terms of resistanceand capacitance. We have also found that resultsare reproducible within 10% in the same indi-vidual for periods of as long as six months. Noseasonal variation in impedance has been ob-served.

There are several factors in the design of theexperiment which influence the values obtainedfor impedance and phase angle. The frequencyof the alternating current and the size of theelectrodes are among the most important. Theimpedance and phase angle both decrease as thefrequency of the alternating current is increased.(Tables I and III). As pointed out by Barnctt(4) if frequency and impedance are plotted on alog-log scale they are found to have a linearrelationship. (Figure 3). This makes possiblethe rough comparison of impedance values ob-tained by using various known frequencies. A

Ava IMPEDANCE -OHMS

60cm2 15cm2 3.14cm2 .78cm2

ELECTRODE SIZEFia. 3

I 10 100 1000 10000

Cz0C)hiCl,

hi0.C',hi-i0>-0>-0ahiahi

20,000

10,000 -

4000 -

1000 —

100,000

linear relationship of frequency and impedanceis usually found in homogenous substances, andits appearance in a tissue as heterogenous asskin is somewhat surprising.

The selection of electrode material is somewhatarbitrary. Other investigators have found leadand monel metal satisfactory; we have usedstainless steel. The pressure with which the elec-trodes are applied is not of critical importanceas long as they make good electrical contact withthe skin. The application of pressure up to 300gm. 1cm2 to our electrodes had no effect on thevalues of resistance and capacitance which weremeasured. The size of the electrodes is of con-siderable importance since this determines theactual area of the skin which is included in thecircuit. We have found that as the size of theelectrodes is increased the values for impedancedecrease. (Figure 3). If the impedance measuredwith various sizes of electrodes is plotted againstfrequency on a log-log scale the curves for thevarious electrodes demonstrate a parallelism,nnd the position on the graph of the curve for anelectrode of a given size is predictable. (Figure

3). This permits comparison of results obtainedwhen using electrodes of various sizes.

Using lead electrodes of the same size as thoseused by Gougerot (9) we obtained values forimpedance in the same range as those he re-ported. In the equivalent circuit used by Gou-gerot the resistors and capacitors were in a seriesarrangement, while in our equivalent circuit theyare arranged in parallel. (Figure 1). Our abilityto duplicate his results confirms the observationof Horton and Van Ravenswaay (6) that in theequivalent circuit the resistors and capacitorsmay be placed in either a series or a parallelarrangement to produce the same final results.

Gerstner (7) reported that females have lowerimpedance values than males. Determinations on45 females and 59 males by our technic reveal nosignificant sex difference in impedance. (Table II).

Separation of the superficial or surface sheathimpedance from that of the deeper tissues hasbeen accomplished previously by the use ofspecial electrodes or by using multiple electrodesand calculating the impedance of the currentpathways between the varions electrodes. These

306

Cl)tnzUIC-)zCl)Cl)UIIt

THE JOURNAL OF INVESTIGATIVE DERMATOLOGY

EFFECT

ON RESISTANCE AND CAPACITANCE OF GRADUAL REMOVAL OF STRATUMCORNEUM

.04007500 I

1000 -

/ I/ I

6000 . I .03505500-

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NUMBER OF STRIPS OF KERATIN REMOVEO

FIG. 4

Cl)CCIt

0ItC)

zUI0z

0C0

ELECTRICAL CHARACTERISTICS OF SKIN 307

methods are cumbersome and some are based onassumptions which are open to criticism. Theuse of cellulose tape stripping provides a rapid,direct, and histologically controllable method ofremoving the superficial portion of the skin fromthe circuit. It should not be assumed howeverthat one can isolate a barrier zone in terms of asingle anatomical area by stripping. The skinsurface with its system of ridges and furrowspresents an uneven surface which precludes re-moval of solid sheets of keratin. Simple observa-tion of the tape after stripping confirms this. Asan area is stripped very small, shiny spots appearindicating exposure of the moist living epidermis.With continued stripping these areas enlargeand coalesce until the entire area is devoid ofkeratin. Effects produced by a limited number ofstrippings may erroneously be attributed solelyto the removal of loose keratin, when in fact,very small areas of the barrier zone have alsobeen removed. We have seen small moist areasappear after 5 to 7 strippings, and if vasoactivesubstances such as histamine are applied at thistime reactions in every way comparable to thoseproduced by intradermal injection are seen.Measurements of potential difference made atthis time show that between normal skin and thepartially stripped sites there is a potential dif-ference of 15 to 25 millivolts in contrast to apotential difference of 0 to 5 millivolts betweentwo areas of intact skin. This offers further evi-dence that a limited number of strippings withcellulose tape may produce changes in the barrierzone of the skin.

We consider the keratin layers and the epi-dermal barrier zone, whatever its precise location,to comprise the surface sheath. This area hasdifferent electrical properties than the livingepidermal cells which can be considered to bepart of the deep tissues. Biopsy studies were takenof the stripped sites when electrical measurementsindicated that the surface sheath had been re-moved from the circuit. These showed that inmost areas the keratin layers had been completelyremoved and the granulosa cell layer remained.In some areas it was possible to find portions ofthe compact keratin remaining. These biopsyspecimens demonstrate that cellulose tape strip-ping results in uneven removal of the keratinlayers and offer little help in determining theexact site of the epidermal barrier zone.

The removal of the surface sheath by strippingresults in a decrease in impedance and resistance

at all frequencies used. The effect of stripping oncapacitance and capacitive reactance however isfrequency dependent. The decrease in resistanceas the keratin is removed (Figure 4) indicatesthat the surface sheath is the site of the highohmic resistance of the skin. The changes ob-served in the capacitance are somewhat moredifficult to interpret. At frequencies of 4,000cycles per second and below, the removal of thekeratin and barrier zone results in a highercapacitance (Figure 4) than that of intact skin.The exact explanation of this cannot be offeredsince the precise composition of the electricalcircuit of the skin is not known. There are how-ever two ways in which stripping could alterthe circuit and produce the observed increase incapacitance: (1) removal of the surface sheath,may in effect, remove a capacitance from thecircuit which is in series arrangement with thecapacitance of the deeper tissues, thus causingan increase in the total capacitance of the circuit;(2) the removal of the surface sheath may alsobe thought of as bringing the plates of the capaci-tive element of the circuit closer together and inthis way the capacitance of the circuit increases.The finding of a high capacitance in normal skinareas where the keratin is thin and a lowercapacitance where the keratin is thick will sup-port either of these explanations. It should beemphasized that these are possible explanationsof the observed phenomenon, and, lacking knowl-edge of the exact structure of the circuit in theskin, no proof can be offered for either one.

In contrast to the results obtained at 4,000cycles per second, when the capacitance of thestripped sites is measured at frequencies of 10,000cycles per second or higher it is found to belower than the capacitance of intact skin. Thiscan be understood if the capacitance is thoughtof as being due to ionic displacement within the

jc

IFIG. 5. Relationship of phase angle (0) and im-

pedance (z) to resistance (R) and capacitance re-actance (Xe).

R

V

308 THE JOURNAL OF INVESTIGATIVE DERMATOLOGY

dielectric of the capacitor element of the circuit.At high frequencies movement of the ions is re-stricted by the frequency with which the chargeon the plates of the capacitor changes sign, andthe circuit has a lower capacitance. At highfrequencies this effect outweighs the effects pro-duced by removal of the surface sheath, and thestripped areas have a lower capacitance than theintact skin.

Removal of the surface sheath also results ina marked decrease in the phase angle. (Table IV).This may seem unreasonable in the face of anincrease in capacitance, as is obtained at lowfrequencies, since ordinarily in a capacitive cir-cuit a small phase angle indicates a low capaci-tance. Phase angle however is a function of bothcapacitive reactance and resistance, and, as canbe seen from the vector diagram (Figure 5), ifthe resistance decreases more than or at a fasterrate than the capacitive reactance a smallerphase angle will result.

Preliminary studies with this technic supportthe finding of Gougerot that patients with atopicdermatitis have a characteristically low im-pedance both at the sites of lesions and in unin-volved skin, and that patients who have psoriasishave an abnormally high impedance in unin-volved skin. It is planned to expand these studies.Measurement of the electrical characteristics ofthe skin may also prove useful in quantitatingimprovement in diseased skin during therapy,and in the study of genetically determined derma-toses by allowing one to detect abnormal mem-bers of a family who do not have clinically evi-dent disease.

SUMMAEY

A method for the determination of the im-pedance and phase angle of human skin and theresults of measurements in 104 normal subjectsare presented.

The use of cellulose tape stripping to separatethe electrical behavior of the surface sheathfrom that of the deeper tissues is described, andthe effects of this separation are discussed.

Possible uses of this technic in the study ofdiseased skin arc briefly outlined.

We wish to thank Mr. Robert E. RobinsonIII, Division of Neuropsychiatry and I)r. IrvinLevin, Division of Instrumentation, Walter ReedArmy Institute of Research, for their many help-ful suggestions during the course of this study.

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