Torsional Elasticity of Human Skin in vivo

Transcription

1 Pfliigers Arch. 342, (1973) 9 by Springer-Verlag 1973 Trsinal Elasticity f Human Skin in viv 1~. Sanders Develpment Labratries, N.V. Philips'Gleflampenfabrieken, Drachten, The Netherlands Received May 16, 1973 Summary. Measurements f physical prperties f the intact human skin are described. An analysis f the measurements yields parameters that characterize elastic, vise-elastic and plastic prperties. Between 6 and 61 years the elastic cmpnent f the extensibility f the skin under trsinal cnditins increases with age. The visc-elastic and plastic cmpnents are cnstant up t an age f furty; at higher ages they shw a small tendency t increase. Mduli f elasticity f the intact human skin derived frm the trsin measurements yield values between abut N/m ~ and llp N/m 2, the highest values crrespnding t the yungest age. Frm these values the mduli f elasticity f the elastic fibres in the living skin are estimated between abut N/m ~ and l0 T N/m S. Key wrds: Skin -- Elasticity -- Aging. Elastic prperties f human skin have ften been determined by using skin strips excised frm cadavers. The cnclusins drawn frm such investigatins d nt necessarily hld fr skin under physilgical circumstances. Experiments n skin in viv have hithert been scarce in literature as far as measurements n large grups f nrmal subjects are cncerned. Uni-axial lading was applied by Gibsn, Stark, and Evans (i969), a suctin methd was used by Grahame and Hlt (i969) and Finlay (1970, i97i) measured trsinal characteristics f the human skin. A trque apparatus was als designed by V]asblm (1967). We have used this instrument fr the present investigatin. Methd The trque apparatus and its perfrmance have been described in extens by Vlasblm (1967). Via a thin bar the twisting mment (8.3 x 10 -~ Nm) f a cil mving in a magnetic field under the influence f an electric current is transmitted t a circular disk (diameter 8.7 ram) that is attached t the skin f the frearm by a piece f adhesive tape sticky n bth sides. The rtatin f the skin results in an ppsed mment and the cil mves until an equilibrium psitin is reached. At given times the trsin angle is recrded by means f a light beam which is reflected by a mirrr attached t the abve -mentined bar. At a cnstant twisting mment the rtatin f the skin depends n time. One f the results is shwn in Fig. 1, where the deflectin f the light beam alng a scale at a distance f 20 cm frm the bar (and cncentric with it) is given against time. The

2 - exp(--t/~)] 256 R. Sanders um 150 z _me K3~ N ~ 100 z 50 llj I1_ llj I I I O TIME IS) 2&O Fig. 1. Trsin f skin as a functin f time at cnstant twisting mment. At t = 120 secnds the lad is remved initial quick respnse f the skin t the applied lad can be described as a truly elastic cmpnent. This is fllwed by a viscus damped elastic cmpnent. After remval f the lad the skin des nt return t its riginal psitin, but the deflectin decreases t a nn-zer rest value, that remains during several rain. This permanent defrmatin depends lineariy n the lading time (Vlasblm, 1967). Using Us fr the initial respnse, and Ur fr the amplitude f the viscus cmpnent with a time cnstant 7, the curve f Fig. 1 can during the lad be represented by U(t) = UB + Ur [1 - + At. At describes the linear dependence n time f the plastic defrmatin. Vlasblm (1967) shwed that this dependence is nt a prprtinality. This means that a cnstant term shuld be added t At. As this term is small cmpared t UE we neglect it here. By fitting the experimental pints with the mathematical expressin the parameters UE, Ur ~ and A are btained. The magnitudes f these parameters depend n the distance frm the rtating bar t the scale. The measurements were perfrmed at fhe drsal side f the subject's left frearm abut 40 cm frm the tip f the middle finger. Nineteen healthy persns, cmprising five females and furteen males, varying in age frm 6 t 61 years, were repeatedly examined n differen~ days. The temperature and humidity f the air were maintained cnstant thrughut the experimental perid. Results In Fig. 2 the deflectin f the light spt n a scale at 20 cm frm the rtating bar after 2 rain f lading (after which the deflectin des nt increase cnsiderably) is pltted against age. These experiments perfrmed under physilgical circumstances clearly shw that the extensibility f the intact skin increases with age. In rder t examine which cmpnent is respnsible fr this behavlut, the results f several measurements were fitted t the mathe-

3 Trsinal Elasticity f Human Skin in viv 257 z r,~ 20( 15[ -- ~,J,..-:., 10[ MALES, FEMALES I I I I I AGE tyears] Fig. 2. Deflectin f light spt at 20 em scale distance after 2 min f lading as a functin f age. The vertical bars represent standard deviatins mathieal expressin given abve with the aid f a cmputer prgramme using a least squares prcedure. The resulting parameters are given in Fig. 3, again as a fuctin f age. The elastic cmpnent UE shws a clear dependence n age, while the time cnstant T f the visc-elastic cmpnent des nt. The parameters Uv and A d nt shw a dependence n age up t abut 40 years. At higher ages they seem t increase. The mdulus f elasticity can nw be derived frm the frmula (see Vlasblm, 1967). y : 2M(1 + v) 4 ar2~9 in which M = twisting mment, v ---- Pissn's rati, a = skin thickness, R = radius f disk, ~0 -- angle f rtatin.

4 258 R. Sanders ~ UECmm} "U(s) 50 1 ~176 ~ I 150 l! :'~ I I ~ 5~ I Uv{mm} 9! '~ 9 s ~ t,,, I,, A (ram/s) 0 I l l l [ i 50 ~ ~ ~ I # 9 9 ~ : i~ ~ ~ 1,, 5O AGE (years} Fig. 3. The parameters UE, Uv, ~ and A at different ages Deriving this frmula the skin was assumed t be a hmgeneus and istrpic elastic plate f unifrm thickness a with infinite bundaries. The bnd between skin and subcutaneus tissue was neglected. Vlasblm (1967) shwed that the influence f this bnd was indeed small at a tangential lading f the skin. Values fr M and R are given abve. A gd apprximatin fr Pissn's rati may be v By taking a cm we will nt intrduce a large errr. The angle ~0 is d/2l, where d is the distance cvered by the light spt n the scale at distance 1. In the equipment used, l was 20 cm. Cnsidering the elastic cmpnent nly, we take d--~ UE. Substituting all these values in the frmula given abve, we find a mdulus f elasticity Y fr each UE. The mduli fr the skin as a whle thus derived frm ur measurements are given in Table 1. Over the age range cvered we btain values frm abut N/m 2 t abut 105 N/m 2, the highest values crrespnding t the yungest age.

5 Trsinal Elasticity f Human Skin in viv 259 Table 1. Mduli f elasticity f the intact human skin calculated frm trsin measurements Subject Age (years) Sex 35dulus f elasticity (10 a N/m 2) , F F ~ ~ ~ N F , M 3, M 3, , F 2,8 2, lv[ F Discussin Grahame and Hlt (1969) fund values fr the mdulus f elasticity frm N/m ~ t l0 s N/m ~ frm in viv measurements f human frearm skin by a suctin methd. These values are cmparable t that fund frm pure cllagen. In enstrast t ur results they find a slight increase in the mdulus f elasticity at higher ages. This stiffening up

6 260 R. Sanders in a directin perpendicular t the skin surface can be understd, as the authrs mentin, as being due t the increase f crss-linkages between peptide chains f the cllagenus netwrk. It is well knwn that the elastic fibres shw fraying and fragmentatin at advancing age. It is therefre tempting t ascribe the lss f elasticity in a directin parallel t the skin surface that we bserved, t this phenmenn. This becmes still mre likely if we divide the mduli we fund by the vlume fractin (0.01) f the elastic fibres in human skin (Tregear, 1966). Fr the mduli f elasticity f the elastic fibres in the living skin we then btain values between abut 2 N/m S and 107 N/m s, in agreement with the value 5 )<106 N/m S, mentined by Tregear (1966) fr pure elastic tissue. In additin t bserving a tendency fr the elastic mdulus t increase with age, Grahame and Hlt (1969) btained significantly higher values fr females than fr males. Althugh the number f females examined in the present wrk was very small, there may be a slight indicatin f smaller values fr females, while the mduli f elasticity decrease with age. Frm a mechanical pint f view the skin f females seems t be smewhat lder than the skin f males. Cmparisn f ur results with literature shws that the mdulus f elasticity f skin is nt simply a material cnstant. Because skin is nt a hmgeneus and istrpic medium, quite different mduli can be fund under different experimental cnditins. Acknwledgements. Thanks are due t Dr. D. C. Vlasblm (St. Elisabeth Ziekenhuis, Tilburg) and t Prfessr J. J. Denier van der Gn (Labratrium veer Medische en Fysilgische Fysica der Rijksuniversiteit, U~recht) fr placing the trque apparatus at ur dispsal. References Finlay, B. : Dynamical mechanical testing f human skin "in rive". J. Bimechanics 3, (1970) Finlay, B. : The ~rsinal characteristics f human skin in rive. Bi-med. Eng. 6, (1971) Gibsn, T., Stark, H., Evans, J. H. : Directinal variatin in extensibility f human skin in viv. J. Bimechanics ~., (1969) Grahame, R., Hlt, P. L. J. : The influence f ageing n the in rive elasticity f human skin. Gerntlgia (Basel)15, (1969) Tregear, g. T. : Physical functins f skin. Lndn-New Yrk: Academic Press, 1966 u D. C. : Skin elasticity. Thesis, Utrecht 1967 Dr. R. Sanders Labratriumschl Bedumerweg 129 Grningen The Netherlands