Exposure to buffer solution alters tendon hydration and mechanics

Abstract

A buffer solution is often used to maintain tissue hydration during mechanical testing. The most commonly used buffer solution is a physiological concentration of phosphate buffered saline (PBS); however, PBS increases the tissue’s water content and decreases its tensile stiffness. In addition, solutes from the buffer can diffuse into the tissue and interact with its structure and mechanics. These bathing solution effects can confound the outcome and interpretation of mechanical tests. Potential bathing solution artifacts, including solute diffusion and the effect on mechanical properties, are not well understood. The objective of this study was to measure the effects of long-term exposure of rat tail tendon fascicles to several concentrations (0.9% to 25%) of NaCl, sucrose, polyethylene glycol (PEG), and SPEG (NaCl + PEG) solutions on water content, solute diffusion, and mechanical properties. We found that with an increase in solute concentration the apparent water content decreased for all solution types. Solutes diffused into the tissue for NaCl and sucrose, however, no solute diffusion was observed for PEG or SPEG. The mechanical properties changed for both of NaCl solutions, in particular after long-term (8 hr) incubation the modulus and equilibrium stress decreased compared to short-term (15 min) for 25% NaCl, and the cross sectional area increased for 0.9% NaCl. However, the mechanical properties were unchanged for both PEG and SPEG except for minor alterations in stress relaxation parameters. This study shows that NaCl and sucrose buffer solutions are not suitable for long-term mechanical tests. We therefore propose using PEG or SPEG as alternative buffer solutions that after long-term incubation can maintain tissue hydration without solute diffusion and produce a consistent mechanical response.

Accumulation of collagen molecular unfolding is the mechanism of cyclic fatigue damage and failure in collagenous tissues

“…The biphasic nature of connective tissues provides a demonstrated mechanism in which fluid content and fluid flow relative to the solid tissue structure can both influence quasistatic tissue mechanics and impart rate-dependent behavior ( 19 , 20 ). To investigate whether a fluid flow–based mechanism could explain the observed rate-dependent fatigue resistance, we performed finite-element simulations using an anisotropic, high Poisson’s ratio permeable elastic solid with a mobile fluid phase ( Fig.…”

Water-content related alterations in macro and micro scale tendon biomechanics

“…Similar findings have been reported by Safa, et al . 35 , who found significant alterations in modulus and equilibrium stress of rat tail tendon fascicles following an 8-hour submersion in 0.9% saline solution. Their study consequently recommended against using saline buffer solutions in favor of PEG-based solutions 35 .…”

“…36 and Safa, et al . 35 . Nevertheless, there was no significant change in strain at maximum force at the various water contents investigated in the macro mechanical tests carried out here.…”

Swelling of fiber-reinforced soft tissues is affected by fiber orientation, fiber stiffness, and lamella structure

“…Moreover, collagen fibrils in tendons and ligaments are more aligned and stiffer than fibrils from the AF (Cassidy et al, 1989;Lynch et al, 2003). These differences in tissue composition are related to a limited overall swelling capacity (~10% increase in tendon volume in saline verses~40% increase in AF volume) (Adams, 2004;Amiel et al, 1983;Henninger et al, 2010;Iatridis et al, 2007;Riley et al, 1994;Safa et al, 2017). Our findings showed that tissues with fibers aligned in a single direction (i.e., no cross-ply pattern) have less ability to resist matrix swelling.…”