Mechanical characterization and constitutive modelling of commercial biopolymers and their blends for biomedical applications

Nowadays, biopolymers like Poly(lactic acid) (PLA) and Polycaprolactone (PCL) are commonly adopted in several fields of medicine, from orthopaedics to pharmacology. When dealing with medical applications like prostheses or scaffolds, it is crucial to have a deep knowledge of the mechanical properties of such biopolymers. Both biopolymers show a viscoplastic behaviour, namely, their mechanical response depends on the temperature and the velocity at which the loading or the deformation is applied. Currently, several companies commercialise a large variety of PCL and PLA blends with different ratios classified as “medical grade”, indicating that such blends are suitable for manufacturing medical devices. The information about the mechanical behaviour of these blends remains unclear, since the datasheets available report information about the Young's Modulus, a limited amount of data considering their full mechanical behaviour. Most of these commercially available biopolymers have not been investigated thoroughly in the past. In this paper two commercially available biopolymers, Resomer®️ LR 704 S and LC 703 S, from Evonik were investigated. Specifically, the original polymers and the following blend combinations were tested: 60:40, 40:60, and 50:50. The original biopolymers and their blend combinations were considered to explore the application of developing two innovative devices for soft tissues repair, T-REMEDIE for tendon repair (Tendon Repair Medical DevIcE) [patent ID: IT202000006967A1](“Device and assembly for the repair of soft tissues, such as tendons and ligaments,” 2020) and T-SURE for abdominal hernia repair (Tissue Surgical REpair), under development in the BIOMAST Lab (BIO-MAterials and STructures Laboratory) at the Politecnico di Torino. Experimental tensile tests on dog bone specimens manufactured by compression and injection moulding were evaluated. Based on the experimental results, the constitutive three network model (TNM), the three network viscoplastic (TNV) model and the Flow Evolution Network (FEN) model were implemented in MATLAB and calibrated. This work represents the first time these constitutive laws have been applied to biopolymers. All the models are suitable for biopolymer constitutive modelling, showing promising results. The constitutive material parameters for all the models are reported in the paper.