Journal of Biomechanics 37 (2004) 1491–1497 Viscoelastic properties of skin in Mov-13 and Tsk mice Z. Del Prete a , S. Antoniucci a , A.H. Hoffman b , P. Grigg c, * a Department of Mechanical Engineering, University of Rome ‘La Sapienza’, Rome, Italy b Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA, USA c Department of Physiology, University of Massachusetts Medical School, 55 Lake Avenue N, Worcester MA 01655, USA Accepted 21 January 2004 Abstract Viscoelastic properties of skin samples were measured in three types of mice (tight skin, Tsk, control and Mov-13), that are known to differ with regard to content of type I collagen. The experimental design used uniaxial stretching and measured the creep response and the complex compliance. The creep response was measured directly. The complex compliance was determined using a Wiener– Volterra constitutive model for each sample. The models were calculated from data obtained by applying a stress input having a pseudo-Gaussian waveform and measuring the strain response. The storage compliance of Mov-13 and control skin were similar and were greater than Tsk (po0.001). The loss compliance of each group was significantly different (po0.001) from each other group; Tsk had the lowest and control had the highest loss compliance. The phase angle of the Mov-13 and Tsk were similar and were less than the controls (po0.001). The creep response was fit with a linear viscoelastic model. None of the parameters in the creep model differed between groups. The results indicate that gene-targeted and mutant animals have soft tissue mechanical phenotypes that differ in complex ways. Caution should be exercised when using such animals as models to explore the role of specific constituents on tissue properties. r 2004 Elsevier Ltd. All rights reserved. Keywords: Viscoelasticity; Skin; Tsk; Mov-13 1. Introduction A number of investigators have used gene-targeted and mutant animals as experimental models to explore the role of individual constituents in determining the material behavior of soft tissues. For example studies centered on tail tendons have used C1M8, Mov-13 and decorin knockout mice, as well as control mice of two ages, to explore the potential effect of variables such as fibril size and content of collagen and glucoseaminogly- can (GAGs), on a variety of material parameters (Derwin and Soslowsky, 1999; Derwin et al., 2001; Elliott et al., 2003; Robinson et al., 2003). In addition Clark et al. (2001) and Mikic et al. (2001) have shown that GDF-5 deficient mice have reduced collagen content, alterations in tendon morphology, and tendons that are softer and weaker than controls (Clark et al., 2001; Mikic et al., 2001). Osborn et al. (1983) reported that tight-skin (Tsk) mice, whose skin is stiffer than controls (Menton and Hess, 1980; Menton et al., 1978) have increased collagen in skin as well as altered dermal morphology. In this communication we had several goals. First, the role of collagen in determining the viscoelasticity of skin was explored using several transgenic and mutant mouse strains known to differ with regard to collagen content. The mouse strains were Tsk mice that have skin with approximately double the collagen content of C57BL/6 controls (Osborn et al., 1983), and Mov-13 mice that have approximately half the collagen content of controls (Bonadio et al., 1990). Second, since soft tissues are viscoelastic, it is important that material characterizations should include a comprehensive determination of these properties. Elliot et al. (2003) determined the quasi-linear viscoe- lastic properties of tail tendons, using creep tests. All of the other studies cited above used quasi-static measures of material properties. We used both transient (creep response) and dynamic (complex compliance) measures of viscoelasticity of samples. The complex compliance ARTICLE IN PRESS *Corresponding author. Tel.: +1-508-856-2457; fax: +1-508-856- 5997. E-mail address: peter.grigg@umassmed.edu (P. Grigg). 0021-9290/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2004.01.015