Clare Y. L. Chao Department of Rehabilitation Sciences, Ttie Hong Kong Poiytechnic University, Hong Kong SAR, Ciiina; Physiotherapy Department, Queen Eiizabeth iHospitai, Hong Kong SAR, China Gabriel Y. F. Ng Kwok-Kuen Cheung Department of Rehabilitation Sciences, The Hong Kong Poiytechnic university. Hong Kong SAR, China Yong-Ping Zheng Li-Ke Wang Interdiscipilnary Division of Biomedicai Engineering, The Hong Kong Poiytechnic University, Hong Kong SAR, China Gladys L Y. Cheing^ Department of Rehabiiitation Sciences, The iHong Kong Polytechnic University, Hong Kong SAR, China e-mail: Cladys.Cheing@polyu.edu.hk In Vivo and ex Vivo Approaches to Studying the Biomechanicai Properties of Heaiing Wounds in Rat Skin An evaluation of wound mechanics is crucial in reflecting the wound healing status. The present study examined the biomechanical properties of healing rat skin wounds in vivo and ex vivo. Thirty male Spragtie-Dawley rats, each with a 6 mm full-thickness circular punch biopsied wound at both posterior hind limbs were used. The mechanical stiffness at both the central and margins of the wound was measured repeatedly in five rats over the same wound sites to monitor the longitudinal changes over time of before wounding, and on days 0, 3, 7, 10,14, and 21 after wounding in vivo by using an optical coherence tomography-based air-jet indentation system. Five rats were euthanized at each time point, atid the biomechanical properties ofthe wound tissues were assessed ex vivo using a tensiometer. At the central wound bed region, the stiffness measured by the air-jet sys- tem increased significantly fivm day 0 (17.2%), peaked at day 7 (208.3%), attd then decreased progressively until day 21 (40.2%) as compared with baseline prewounding status. The biomechanical parameters of the skin wound samples measured by the tensi- ometer showed a marked reduction upon wotinding, then increased with time (all p<0.05). On day 21, tire ultimate tensile strength of the skin wowid tissue approached 50% ofthe twrmal skin; while the stifftiess of tissue recovered at a faster rate, reaching 97% of its prewounded state. Our results suggested that it took less time for healing wound tissues to recover their stiffness than their maximal strength in rat skin. The stiff- ness of wound tissues measured by air-jet could be an indicator for monitoring wound healing attd contraction. [DOI: 10.1115/1.4025109] Keywords: biomechanical properties, wound, skin, ultimate tensile strength, stiffness, optical coherence tomography Introduction Human skin acts as the body's biological barrier, shielding it from mechanical trauma. The biomechanical properties of skin are important to its normal functions, by allowing repeated revers- ible extensions and compressions during activities of daily living. Cutaneous wounds can break the integrity of the skin and the res- toration of both the structural and functional properties of wounded tissue back to its prewounding state are good indicators of effective wound healing [1]. Therefore, an evaluation of wound mechanics is crucial in assessing the progress made in the wound healing process. Wound healing can be divided into defined but overlapping phases of inflammation, proliferation, and maturation (or remodel- ing) [2]. It involves a series of complex cascading events that gen- erate the resurfacing, reconstitution, and proportionate restoration of the mechanical strength of injured tissue [3]. Both the inflam- matory and proliferative phases are critical for restoring the bar- rier function of the wound and for contraction, while the final maturation or remodeling phase involves continued strengthening and contraction of the wound. Wound closure involves changes in the physical and mechanical properties of the wound bed and wound margins. Skin is composed of collagen fibers and elastic fiber bundles that demonstrate viscoelastic behavior [4,5]. The viscous compo- nent is associated with the dissipation of energy and the elastic 'Corresponding author. Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANTCAL ENGINEERING. Manuscript received January 19. 2013; final manuscript received July 12, 2013; accepted manuscript posted July 29, 2013; published online September 20, 2013. Assoc. Editor: Carlijn V. C. Bouten. component is associated with energy storage and is important for ensuring the recovery of shape after defonnation [6]. Such me- chanical properties are mainly contributed by the dermis and are related to the structural properties of collagen and elastin fibers [7]. Collagen fibers are arranged in different configurations at dif- ferent anatomic sites. The orientation of the collagen fibers of skin tissue is multidirectional with elastin twisted filaments interwoven into a ropelike structure that provides the distinct nature of skin [6]. It is highly organized in the plane of dermis with collagen fibers arranged according to the dominant fiber direction known as the Langer's Line. This natural tensional line is generally per- pendicular to the orientation of the underlying muscle fibers, mak- ing the skin least extensible in this direction and can influence the final appearance of a scar [8]. It is generally accepted that inci- sions made parallel to Langer's lines may heal better and produce less scaring than those that cut across. Therefore, any damage in skin may significantly interrupt its mechanical function, and the biomechanical properties of skin can reflect the organization of the constituent of tissues making up the skin [9]. The biomechanical properties of the healing tissue can be eval- uated by a standard load defonnation test using a material testing machine in vitro [9-15]. A typical load-deformation curve for a biological tissue can be divided into two major portions, the initial toe-in region (viscous) followed by the later linear response (elas- tic) [6,16]. At low-load, the contribution ofthe undulated collagen fiber can be neglected and elastin is responsible for the skin stretching by straightening out of crimped collagen fibers, elonga- tion of fibers occurs without appreciable force in this region. As the force continue to increase (at high-load), there is increasing fraction of collagen fibers straightening that causes an increasing stiffness. This linear stiffness can be determined from the slope of Journal of Biomechanical Engineering Copyright © 2013 by ASiME OCTOBER2013, Vol. 135 / 101009-1