Subcutaneous tissue mechanical behavior is linear and viscoelastic under uniaxial tension

Subcutaneous tissue is part of a bodywide network of "loose" connective tissue including interstitial connective tissues separating muscles and surrounding all nerves and blood vessels. Despite its ubiquitous presence in the body and its potential importance in a variety of therapies utilizing mechanical stretch, as well as normal movement and exercise, very little is known about loose connective tissue's biomechanical behavior. This study aimed to determine elastic and viscoelastic mechanical properties of ex-vivo rat subcutaneous tissue in uniaxial tension with incremental stress relaxation experiments. The elastic response of the tissue was linear, with instantaneous and equilibrium tensile moduli of 4.77 kPa and 2.75 kPa, respectively. Using a 5 parameter Maxwell solid model, material parameters micro(1) = 0.95 +/- 0.24 Ns/m and micro(2) = 8.49 +/- 2.42 Ns/m defined coefficients of viscosity related to time constants tau(1M) = 3.83 +/- 0.15 sec and tau(2M) = 30.15 +/- 3.16 sec, respectively. Using a continuous relaxation function, parameters C = 0.25 +/- 0.12, tau(1C) = 1.86 +/- 0.34 sec, and tau(2C) = 110.40 +/- 25.59 sec defined the magnitude and frequency limits of the relaxation spectrum. This study provides baseline information for the stress-strain behaviors of subcutaneous connective tissue. Our results underscore the differences in mechanical behaviors between loose and high-load bearing connective tissues and suggest that loose connective tissues may function to transmit mechanical signals to and from the abundant fibroblasts, immune, vascular, and neural cells present within these tissues.