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Research Papers

In Vivo and ex Vivo Approaches to Studying the Biomechanical Properties of Healing Wounds in Rat Skin

[+] Author and Article Information
Clare Y. L. Chao

Department of Rehabilitation Sciences,
The Hong Kong Polytechnic University,
Hong Kong SAR, China;
Physiotherapy Department,
Queen Elizabeth Hospital,
Hong Kong SAR, China

Kwok-Kuen Cheung

Department of Rehabilitation Sciences,
The Hong Kong Polytechnic University,
Hong Kong SAR, China

Li-Ke Wang

Interdisciplinary Division of
Biomedical Engineering,
The Hong Kong Polytechnic University,
Hong Kong SAR, China

Gladys L. Y. Cheing

Department of Rehabilitation Sciences,
The Hong Kong Polytechnic University,
Hong Kong SAR, China
e-mail: Gladys.Cheing@polyu.edu.hk

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical 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.

J Biomech Eng 135(10), 101009 (Sep 20, 2013) (8 pages) Paper No: BIO-13-1027; doi: 10.1115/1.4025109 History: Received January 19, 2013; Revised July 12, 2013; Accepted July 29, 2013

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 Sprague-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, and the biomechanical properties of the wound tissues were assessed ex vivo using a tensiometer. At the central wound bed region, the stiffness measured by the air-jet system increased significantly from day 0 (17.2%), peaked at day 7 (208.3%), and 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 tensiometer showed a marked reduction upon wounding, then increased with time (all p < 0.05). On day 21, the ultimate tensile strength of the skin wound tissue approached 50% of the normal skin; while the stiffness 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 stiffness of wound tissues measured by air-jet could be an indicator for monitoring wound healing and contraction.

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Figures

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Fig. 1

The ex vivo biomechanical evaluation of healing rat skin wound tissues using the material testing system. (A) The skin wound specimen mounted on the material testing system; (B) skin wound specimen breaks at failure point of maximum load.

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Fig. 2

Overhead photographs of a 6 mm circular incisional wound on the hind limb of a rat (A) at the time of wounding (post-wounding day 0), and on (B) day 3, (C) day 7, (D) day 10, (E) day 14, and (F) day 21 post-wounding (Arrow indicates cephalad, *: wound central, ♦: margin a, ×: margin b, •: margin c, ▽: margin d)

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Fig. 3

Load-depth curve was plotted by using data obtained from the airjet indentation system on healing wound tissues. (A) A scatter plot for data obtained in three indentation cycles. (B) The data in the loading phases at the second to third cycle were selected for estimation of stiffness of the healing wound tissues. The relationship between the force measured within the air-jet and the depth of the deformed wound tissues is shown in the corresponding regression curve.

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Fig. 4

The changes in the stiffness of the healing wound over a 3-week period as measured by the air-jet indentation system in vivo

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Fig. 5

Load-deformation curve of the healing rat skin wound was plotted using data obtained by the material testing system. The slope of the steepest part represents the stiffness, and the shaped area under the curve until the break point represents the energy absorption capacity. The extent of the strain-stiffening region (toe-in) was approximated using a measure of the limit displacement.

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Fig. 6

Comparisons on the recovery rate on various biomechanical parameters over time obtained in the ex vivo test with reference to the value of prewounding status

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Fig. 7

The correlation curve of stiffness data collected via in vivo airjet test versus limit displacement as obtained via ex vivo tensiometer test

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