Softening of elastic and viscoelastic properties is independent of overstretch rate in cerebral arteries

Collagenous soft tissues are frequently injured by supraphysiologic mechanical deformation, leading to measurable changes in both extra-cellular matrix (ECM) structure and mechanical properties. While each of these alterations has been well studied following quasi-static deformation, little is known about the influence of high strain rate. Previous investigations of high-rate ECM alterations found tropocollagen denaturation and fibrillar kinking to be rate dependent. Given these observations of rate dependence in microstructure alterations, the present work evaluated if the rate and magnitude of overstretch affect the baseline viscoelastic properties of porcine middle cerebral arteries (MCAs). Changes in tissue response were assessed using a series of harmonic oscillations before and after sub-failure overstretches across a large range of rates and magnitudes. We used collagen-hybridizing peptide (CHP) to evaluate the role of tropocollagen denaturation in mechanical softening. Experiments show that softening is dependent on overstretch magnitude but is independent of overstretch rate. We also note that softening progresses at the same rate for both equilibrium (quasi-static) and non-equilibrium (high-rate) properties. Finally, results suggest that tropocollagen denaturation is not the source of the observed sub-yield softening behavior. This study expands fundamental knowledge on the form-function relationship of constituents in collagen fibrils and clarifies material behavior following sub-failure overstretch across a range of strain rates.