Characterization of Thermomechanical Behaviour of Skin Tissue II. Viscoelastic Behaviour

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1 Proceedings of the World Congress on Engineering 7 Vol II WCE 7, July -, 7, London, U.K. Chrcteriztion of Thermomechnicl Behviour of Skin Tissue II. Viscoelstic Behviour F. Xu, T. Wen, K. A. Seffen, nd T. J. Lu Abstrct This pper presents the viscoelstic behviour of skin tissue under different therml lodings nd discusses the effect of temperture nd corresponding derml collgen denturtion on the mechnicl viscoelstic properties of skin tissue. Differentil scnning clorimetry (DSC) hs been used to detect derml collgen denturtion nd to ssess its therml stbility. The DSC results under vrious heting rtes re used to derive the Arrhenius prmeters ( E, A ) in the burn dmge integrtion, which re then used to clculte the degree of dentured collgen in the skin. Temperture tests hve been performed using dynmic mechnicl nlyzer (DMA) to evlute chnges in the skin viscoelstic properties s function of collgen dmge, specificlly, to ssess the chnges in the storge modulus ( E ' ), nd loss fctor ( tnδ ). The results show remrkble chnges in E ', which is mybe due to the relese of wter, but there is no significnt effect from tnδ. These results suggest tht t constnt frequency the denturtion of collgen molecules hs little effect on the viscoelsticity. Index Terms Skin tissue, thermomechnicl behviour, therml denturtion, viscoelsticity. I. INTRODUCTION Skin is viscoelstic mteril. Previous studies of the viscoelsticity of skin tissue revel tht its stress-strin reltionship depends on strin rte, loding rte, the period of loding nd on the preconditioning stress history, nd tht it exhibits considerble hysteresis in cyclic tests, s well s stress relxtion under constnt strin [1]-[3]. Most of these studies were performed under qusi-sttic experimentl conditions; however, few reserchers hve described the dynmic viscoelstic properties of skin tissue [], []. Mnuscript received Mrch 13, 7. This work is supported by the Overses Reserch Studentship (ORS) nd Overses Trust Scholrship of Cmbridge University, the Ntionl Nturl Science Foundtion of Chin (33, 7111, 3), the Ntionl 111 Project of Chin (B), nd the Ntionl Bsic Reserch Progrm of Chin (CB). F. Xu is with the Engineering Deprtment, Cmbridge University, Cmbridge, CB 1PZ UK (e-mil: fx1@cm.c.uk). T. Wen is with the Engineering Deprtment, Cmbridge University, Cmbridge, CB 1PZ UK (e-mil: tw9@cm.c.uk). K. A. Seffen is with the Engineering Deprtment, Cmbridge University, Cmbridge, CB 1PZ UK (e-mil: ks1@cm.c.uk). T. J. Lu is with the MOE Key Lbortory of Strength nd Vibrtion, School of Aerospce, Xi n Jiotong University, Xi n 79 P.R. Chin (phone: -9-; fx: ; e-mil: tjlu@mil.xjtu.edu.cn) As the skin temperture increses, denturtion of collgen tkes plce. There re not only structure chnges, but the hydrtion of collgen lso chnges with denturtion, which my involve libertion of wter initilly nd, n bsorption of wter lter. Not surprisingly, therml denturtion of collgenous tissue cn led to mrked chnges in its viscoelstic properties. To the uthors knowledge, there is no published study on temperture-dependent skin viscoelsticity, lthough other collgenous tissues hve been studied, for exmple, crtilge [], [7] nd bone [], [9]. The objective of this investigtion is to test the hypothesis tht collgen is significnt determinnt of skin viscoelsticity, nd prticulr ttention hs been pid here to the thermomechnicl properties of skin under dynmic loding. For this purpose, collgen in skin tissue is thermlly dentured to different degrees by pplying het. Differentil scnning clorimetry (DSC) mesurement is used to detect the denturtion of collgen nd to mesure its therml stbility. The integrity of the collgen network is then nlyzed using the therml dmge integrtion. In ddition, dynmic mechnicl nlysis (DMA) is crried out to chrcterize the viscoelstic properties of skin tissue s function of temperture nd collgen dmge. II. QUANTIFICATION OF THERMAL DENATURATION AND THERMAL DAMAGE Therml dmge nd therml denturtion cn be quntified by the following clcultions: C() t E () t ln Aexp Ω = = dt Ct () RT (1) C() C( t) Deg() t = = 1 exp ( Ω () t ) () C() where t is the time fter strting, Ω is the dimensionless indictor of therml dmge, Deg is the degree of therml dmge (concentrtion of dentured collgen), C () nd Ct () re the initil nd current concentrtions of un-dentured collgen, A is mteril prmeter (frequency fctor), E is ISBN: WCE 7

2 Proceedings of the World Congress on Engineering 7 Vol II WCE 7, July -, 7, London, U.K. the ctivtion energy, nd R =.31 J/mol K is the universl gs constnt. The Arrhenius prmeters ( E, A ) re derived experimentlly from the DSC results in Section.1. III. MATERIALS AND METHODS A. Smple nd smple preprtion The smple nd its preprtion hve been described in detil in our first pper, see []. B. Experimentl procedure Differentil Scnning Clorimetry (DSC) Differentil scnning clorimetry hs been used extensively to chrcterize the therml behviour of collgenous tissues [11], [1]. DSC detects thermodynmic chnges by mesuring the flow of het between smple nd reference, from which the effect of therml history on the het cpcity cn be determined, enbling different prmeters to be interrogted, such s the therml denturtion tempertures of skin collgen, rection kinetics, etc. In this study, the therml stbility of collgen in skin tissue ws ssessed with TA Instruments DSC of type Q T zero, scnning from to C t four different heting rtes (,,, C/min). Dynmic Mechnicl Anlysis (DMA) The viscoelstic properties of the skin tissue s function of temperture were mesured with Dynmic Mechnicl Anlyzer (TA Instruments DMA Q). The mesurements re crried out by bending single cntilever t frequency of 1 Hz, over temperture rnge from to C, subject to different heting rtes (,,, C/min). The flexurl moduli re mesured within the liner elstic region. The deformtion mode produces combintion of both flexurl nd sher forces where, in short thick specimen, sher forces re dominnt nd vice vers in long thin specimens. In our tests, the length to thickness rtio of skin smples is greter thn, which fvors flexure [7]. Note tht ll the tests re performed in test chmber filled with ir. Specific cpcity c p (kj/kg O C) Smple: Pig flnk skin; r = O C/min T mx =.3 O C Temperture T ( O C) Fig. 1 Chrcteristic DSC thermogrm of skin tissue smple ln(r/t mx ) (K-1 min -1 ) - Smple: Pig flnk skin ln(r/t ) = (1/T ) mx mx -9 - R = /T mx ( -3 K -1 ) Fig. Chrcteristic plot of ( mx ) ln rt vs. 1 T mx for skin tissue IV. RESULTS AND DISCUSSION A. DSC results A typicl DSC thermogrm of pig flnk skin smple is shown in Fig. 1. The mesured specific het cpcity, c p, compres well with published results [11], [13]. For stress-free skin smple, denturtion is chrcterized by sudden increse in the energy bsorption endotherm, strting t bout C: with further heting, the endotherm reches its mximum vlue t the denturtion temperture,.3 C nd then decreses. It is lso observed tht the denturtion endotherm chrcteristics re heting rte dependent but re not shown here for they offer similr trend. E ( J/mol) 7 Eq.() Bck Belly Er Fce Flnk 3 3 ln(a) (ln(s -1 )) Fig. 3 Comprison of our results of Arrhenius prmeters ( A, E ) with literture results ISBN: WCE 7

3 Proceedings of the World Congress on Engineering 7 Vol II WCE 7, July -, 7, London, U.K. Storge modulus E' (MP) Loss fctor tnδ Storge modulus E' (MP) Smple: pig flnk skin 3 7 Temperture T( O C) Fig. Vrition of storge modulus with temperture Smple: pig flnk skin. 3 7 Temperture T ( O C) Fig. Vrition of loss fctor with temperture Smple: pig flnk skin Thermld dmge Ω Fig. Vrition of storge modulus with therml dmge Loss fctor tnδ Storge modulus E' (MP) Smple: pig flnk skin Therml dmge Ω Fig. 7 Vrition of loss fctor with therml dmge Smple: pig flnk skin Therml dmge degree Deg Fig. Vrition of storge modulus with therml dmge degree Loss fctor tnδ Smple: pig flnk skin Therml dmge degree Deg Fig. 9 Vrition of loss fctor with therml dmge degree ISBN: WCE 7

4 Proceedings of the World Congress on Engineering 7 Vol II WCE 7, July -, 7, London, U.K. Tble I Experimentl results of Arrhenius prmeters ( E, A ) Smple Slope Intercept -1 E ( J mol) A ( s ) Bck skin Belly skin Er skin Fce skin Flnk skin In this study, the collgenous tissue is heted from n initil vlue t constnt heting rte, r. According to the thermodynmic nd Arrhenius eqution, the following reltion between the prmeters, E nd A, nd the pek temperture of therml denturtion, T mx, cn be obtined s [1]-[1]: re ART mx E = exp RT mx Thus, the following eqution becomes pprent, re E ln = ln exp ART mx RT mx r E 1 E ln ln ln = + Tmx RT mx R The vlue of ctivtion energy slope of the plot of ( mx ) ( A) (3) () E cn be obtined from the ln rt versus 1 T mx, while the vlue of A cn be derived from the intercept vlue. A chrcteristic plot for flnk skin smple is given in Fig. nd for seprte tests with belly, bck, er nd fce skin, the Arrhenius prmeters re presented in Tble I. As shown in our erly study [17], there is liner reltionship between ln ( A ) nd E given by, fter curve fitting: E = ln( A) () The present results compre fvourbly with this eqution. Accordingly, the therml dmge nd the degree of therml denturtion of skin collgen for given heting history cn be found using Eq.(1) nd Eq.(). B. DMA results The viscoelstic properties of skin re obtined from the DMA tests. During ech DMA experiment, the storge modulus ( E ' ), loss modulus ( E " ), nd the loss fctor ( tnδ ), re mesured s function of the temperture history. The storge modulus is relted to the stored elstic energy in the viscoelstic cycle, nd cn be considered nlogous to stiffness mesurement of monotonic test; loss fctor yields the rtio between the mechnicl energy lost nd stored during given cycle, tn δ = E" E', effectively mesuring the dmping performnce of the smple. The storge modulus nd the loss fctor re given in Fig., nd Fig., respectively s function of temperture. The storge modulus increses with incresing temperture: the skin gets stiffer t higher tempertures nd the incresing rte increses with heting rtes. The loss fctor, however, is independent of temperture but increses mrginlly with the rtes of heting: temperture hs little effect on the viscous property of skin or the dmping of the smple is not ffected by temperture. The vlue of loss fctor in our results is bout.1, similr to the result mesured by the method of surfce wve propgtion t room temperture [1]. It hs been generlly ccepted tht, under qusi-sttic conditions, collgen is responsible for the viscoelstic properties nd the stiffness of skin tissue [19], suggesting tht collgen plys n importnt role in determining the overll mechnicl properties of skin tissue. Thus, it seems resonble to ssume tht collgen my ply mjor role in the viscoelstic nture of skin tissue under therml lodings. In order to investigte this hypothesis, the storge modulus nd loss fctor re plotted s function of therml dmge nd therml denturtion degree, s given in Fig. -9. These results show tht the effects of therml dmge nd therml denturtion degree re similr to tht of temperture bove; in other words, the chnge of the intervl structure of collgen during therml denturtion does not ffect the storge modulus nd the loss fctor t the test frequency used in this study. It hs been shown elsewhere tht the mechnicl chnges observed using dynmic mechnicl nlysis re the result of wter loss nd of chnges in the intrinsic moleculr structure of the mteril with heting [7]. Furthermore, weight loss of skin smples hs lso been observed during the test. Thus, the vritions in our tests my be due to due to the relese of wter by the denturtion of collgen moleculr during heting. V. CONCLUSION To our knowledge, this study first chrcterizes the effect of temperture nd corresponding therml denturtion of skin collgen on skin viscoelstic properties. The results show tht the storge modulus is temperture sensitive, which is possibly due to dehydrtion. However, the loss fctor does not show significnt dependency on either temperture or therml denturtion. These results suggest tht t constnt frequency, the denturtion of collgen hs little effect on skin viscoelsticity. Further study should focus on identifying the reltive ISBN: WCE 7

5 Proceedings of the World Congress on Engineering 7 Vol II WCE 7, July -, 7, London, U.K. contribution of wter loss nd therml denturtion. Although not explored in this study, it is expected tht the test frequency plys role. In future, more extensive studies using wide rnge of frequencies should be performed. REFERENCES [1] Y. C. Fung, Biomechnics: Mechnicl Properties of Living Tissues. New York: Springer-Verlg, 1993, [] Y. Lnir, "Bixil stress relxtion in skin," Ann. Biomed. Eng., vol., 197, pp. -7. [3] T. Hermnns-Le, I. Uhod, S. Smitz, nd G. E. Pierrd, "Skin tensile properties revisited during geing. Where now, where next?," J. Cosmet. Dermtol., vol. 3,, pp. 3-. [] E. K. Dwes-Higgs, M. V. Swin, R. J. E. D. Higgs, R. C. Appleyrd, nd S. Kossrd, "Accurcy nd relibility of dynmic biomechnicl skin mesurement probe for the nlysis of stiffness nd viscoelsticity," Physiol. Mes., vol.,, pp [] C. Jcquemoud, K. Bruyere-Grnier, nd M. Coret, "Methodology to determine filure chrcteristics of plnr soft tissues using dynmic tensile test," Journl of Biomechnics, vol., 7, pp. -7. [] K. K. H. Cho, M. A. Burden, nd B. J. F. Wong, "Dynmic chnges in the elstic modulus of lgomorph nsl septl crtilge during Nd:YAG (&lmbd;=1.3 μm) lser irrdition," Sn Jose, CA, 1, pp. 7-. [7] Y. Che, G. Aguilr, E. J. Lverni, nd B. J. Wong, "Chrcteriztion of temperture dependent mechnicl behviour of crtilge," Lsers Surg. Med., vol. 3, 3, pp [] J. Ymshit, B. R. Furmn, H. R. Rwls, X. Wng, nd C. M. Agrwl, "Collgen nd bone ciscoelsticity: dynmic mechnicl nlysis," J. Biomed. Mter. Res. (Appl. Biomter.), vol. 3,, pp [9] J. F. Mno, "Viscoelstic properties of bone: Mechnicl spectroscopy studies on chicken model," Mterils Science nd Engineering C, vol.,, pp [] F. Xu, T. Wen, K. A. Seffen, nd T. J. Lu, "Chrcteriztion of Thermomechnicl Behviour of Skin Tissue I. Tensile nd Compressive behviours," in World Conference on Engineering London, UK, 7. [11] G. S. Young, "Thermodynmic Chrcteriztion of Skin, Hide nd Similr Mterils Composed of Fibrous Collgen," Studies in Conservtion, vol. 3, 199, pp [1] R. Schiller, A. P. Funke, nd C. Gunther, "DSC mesurements on full thickness mice skin: An dditionl tool to investigte permetion enhncement of highly lipophilic drugs," Journl of Therml Anlysis nd Clorimetry, vol. 77,, pp [13] F. C. Henriques nd A. R. Moritz, "Studies of therml injury, 1. The conduction of het to nd through skin nd the tempertures ttined therein. A theoreticl nd n experimentl investigtion," A. J. Pthol., vol. 3, 197, pp [1] T. Ozw, "Kinetic nlysis of derivtives curves in therml nlysis," Journl of Therml Anlysis nd Clorimetry, vol., 197, pp [1] C. A. Miles, "Kinetics of collgen denturtion in mmmlin lens cpsules studied by differentil scnning clorimetry," Int. J. Biol. Mcromol., vol. 1, 1993, pp [1] C. A. Miles, T. V. Burjndze, nd A. J. Biley, "The kinetics of the therml denturtion of collgen in unrestrined rt til tendon determined by differentil scnning clorimetry," J.Mol.Biol., vol., 199, pp [17] F. Xu, T. Wen, K. A. Seffen, nd T. J. Lu, "Biothermomechnics of Skin Tissue," Journl of the Mechnics nd Physics of Solids, 7, [1] S. J. Kirkptrick, D. D. Duncn, nd L. Fng, "Low-frequency surfce wve propgtion nd the viscoelstic behviour of porcine skin," Proceedings of SPIE vol. 7,, p. 17 [19] F. H. Silver, J. W. Freemn, nd D. DeVore, "Viscoelstic properties of humn skin nd processed dermis," Skin Res.Technol., vol. 7, 1, pp ISBN: WCE 7