Mechanical properties and performance of collagen film: Effect of environment

Protein-based polymers are becoming increasingly popular for in-vivo applications thanks to their outstanding cellular interactions and ability to mimic the mechanical behaviour of the neighbouring tissues. One of the most important proteins – collagen – comprises of one-third of the total human body protein, thus, contributing to the mechanical stability and structural integrity. There are as many as 29 types of collagens reported with Type I being the most prominent. It possesses a complex hierarchical structure having collagen molecules lowest in the hierarchy. The molecules combine to form the fibrils, followed by collagen fibres which ultimately contribute to the formation of the tissues. The mechanical properties of various collagen-rich mineralised and non-mineralised tissues have been investigated extensively under various physiological conditions. However, they consist of other components such as hydroxyapatite (bone) and proteoglycans (skin) and do not necessarily represent the mechanical response of pure collagen. Though the investigation of the mechanical behaviour of collagen at lower hierarchy (molecules and fibrils) were estimated, the mechanical properties of pure collagen at macroscale has remained relatively unexplored. Additionally, the application-relevant conditions should be considered during experiments. Hence, the mechanical response and long-term performance of pure collagen film was estimated and compared in both in-air (dry) and in-aqua (wet) conditions. In this PhD work, the mechanical properties and performance were investigated through various experimental techniques while the modelling of collagen for biomedical applications was performed with finite-element software. The tensile behaviour of the film under hydration demonstrated a drastic drop in the mechanical strength and modulus while the strain at failure was more than twice that of the dry specimens signifying that water molecules act as plasticizing agent, disrupting the interfibrillar bonds. The studied collagen film showed a strain-rate dependent hardening behaviour with increasing strain rate. Cyclic testing revealed that the unloading modulus of dry collagen was almost insensitive to the number of cycles after the first cycle, while in aqua it demonstrated an increase in the unloading modulus (cyclic stiffening) with increase in the number of cycles. Collagen film showed a time-dependent behaviour for both the environmental conditions with increased viscosity in wet environment. The hyperelastic and Prony-series material constants were extracted from the experiments and later used in the simulations. Long-term storage in ambient laboratory conditions (dry) revealed an increase in mechanical strength and modulus with a decrease in the ductility of the specimens with increasing exposure time. Contrary, the long-term in-aqua exposure showed a deterioration in mechanical properties from day 0 to 1, followed by an increase till day 3 and finally, decrease from day 3 to 14. Collagen films were modelled in Abaqus as a substrate for flexible electronics. The interfacial damage evolution and delaminated area for wet collagen was comparatively lower than dry collagen. Other important factors such as device geometry and orientation were also considered to determine the level of damage evolution. The major contribution of this thesis is that for the first time, the mechanical quantification of pure collagen film was investigated under different environmental conditions which is useful to produce better collagen-based composites. The long-term mechanical performance of collagen in various environments and modelling the collagen substrate is useful for applications such as tissue regeneration and flexible electronics.