Dry vs. wet: Properties and performance of collagen films. Part I. Mechanical behaviour and strain-rate effect

Collagen forms one-third of the body proteome and has emerged as an important biomaterial for tissue engineering and wound healing. Collagen films are used in tissue regeneration, wound treatment, dural substitute etc. as well as in flexible electronics. Thus, the mechanical behaviour of collagen should be studied under different environmental conditions and strain rates relevant for potential applications. This study's aim is to assess the mechanical behaviour of collagen films under different environmental conditions (hydration, submersion and physiological temperature (37 °C)) and strain rates. The combination of all three environment factors (hydration, submersion and physiological temperature (37 °C)) resulted in a drop of tensile strength of the collagen film by some 90% compared to that of dry samples, while the strain at failure increased to about 145%. For the first time, collagen films were subjected to different strain rates ranging from quasi-static (0.0001 s^-1) to intermediate (0.001 s^-1, 0.01 s^-1) to dynamic (0.1 s^-1, 1 s^-1) conditions, with the strain-rate-sensitivity exponent (m) reported. It was found that collagen exhibited a strain-rate-sensitive hardening behaviour with increasing strain rate. The exponent m ranged from 0.02-0.2, with a tendency to approach zero at intermediate strain rate (0.01 s^-1), indicating that collagen may be strain-rate insensitive in this regime. From the identification of hyperelastic parameter of collagen film, it was found that the Ogden Model provides realistic results for future simulations.