Cavitation Rheology as a Potential Method for In Vivo Assessment of Skin Biomechanics (2013)

Sir:FigureEvaluation of the biomechanical properties of skin is essential for characterization of wound healing kinetics and cutaneous diseases. Available techniques require tissue excision and preclude serial tissue assessment, which limit in vivo monitoring of biomechanical changes. Cavitation rheology is a method for focal assessment of the mechanical properties of materials.1 An air cavity is created at the tip of a needle inserted into a material and pressurized until the material yields by rapid elastic deformation (cavitation) or irreversible fracture. An incisional wound model was used to determine whether this technique could discern differences in the mechanical responses of skin in various states. In accordance with our institutional animal care and use committee guidelines, a full-thickness incision was created in the dorsal midline of six Sprague-Dawley rats (The Jackson Laboratory, Bar Harbor, Me.), and then closed with a subcuticular suture (4-0 Monocryl; Ethicon, Inc., Somerville, N.J.). Rats were killed 28 days after wounding and skin was harvested. Cavitation was achieved with a 34-gauge needle (PY2-72-0081; Harvard Apparatus, Holliston, Mass.) connected to a syringe pump (Nexus 6000; Chemyx, Stafford, Texas) and real-time pressure sensor (Cajon Transducer 300psi; Swagelok, Solon, Ohio) (Fig. 1). Air was injected in multiple incisional and surrounding unwounded areas, and yielding pressures were recorded. The preferred pathway for yield is a product of material properties and length scale, which can be determined as described previously.1 Cavitated tissues were excised and stained with hematoxylin and eosin in standard fashion. Statistical comparison of yielding pressures was made using a two-tailed t test. Statistical significance was assumed for values of p < 0.05.Fig. 1: A schematic representation of the experimental apparatus used for cavitation rheology. The inset demonstrates the expanding cavity of air (blue circle) within the dermis. The deformational force generated by this cavity (blue arrows) is opposed by the intrinsic elastic properties of the dermis (brown arrows).Incisional skin yielded at a lower pressure than unwounded skin (Fig. 2). Sampled sites (n = 36, 18 incisional and 18 unwounded) demonstrated a significant difference (p < 0.01) in the mean yielding pressure of 69 kPa and 79 kPa for incisional and unwounded skin, respectively. Histologic analysis demonstrated a reliable cavitation plane at the dermal-subcutaneous junction, consistent with the real-time observation of maintenance in skin integrity.Fig. 2: Box plot demonstrating yielding pressures for incisional (left) and unwounded (right) rat skin 28 days after wounding. Error bars represent ±SD.Nondestructive methods previously developed to measure cutaneous biomechanical properties have not achieved widespread application.2,3 The criterion standard of tensiometry has several limitations.3 Tissue must be excised and sectioned, leading to shear stress on samples. Accurate measurements may also be confounded by tissue slippage in tensiometer clamps and imprecise timing of wound breakage. In addition, methods that rely on stretch cannot isolate individual layers of cutaneous tissues and their different biomechanical properties. Cavitation rheology has been used to describe the ex vivo biomechanical properties of other animal tissues.4 We have demonstrated the ability to reproducibly measure the biomechanical properties of skin in various states, with results consistent with previous findings.5 Ex vivo measurements of the biomechanical properties of skin may not accurately reflect in vivo properties,3 and our future studies will use in vivo models to compare the ex vivo and in vivo biomechanical properties of skin as assessed by cavitation rheology. We propose cavitation rheology as a nondestructive method of assessing cutaneous biomechanical properties. Cavitation rheology may provide accurate in vivo evaluation of biomechanical changes during cutaneous wound healing or disease evolution. Michael S. Chin, M.D. Brian B. Freniere, B.A. Division of Plastic Surgery, University of Massachusetts Medical School, Worcester, Mass. Sami Fakhouri, B.S. Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Mass. John E. Harris, M.D., Ph.D. Division of Dermatology Janice F. Lalikos, M.D. Division of Plastic Surgery, University of Massachusetts Medical School, Worcester, Mass. Alfred J. Crosby, Ph.D. Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Mass. DISCLOSURE The authors have no financial interests to report. ACKNOWLEDGMENTS Sami Fakhouri, B.S., and Alfred J. Crosby, Ph.D., gratefully acknowledge the financial support of the Army Research Laboratory through Cluster M of the Center for UMass/Industry Research on Polymers.