The aliphatic polyesters are widely used in biomedical applications since they are susceptible to hydrolytic and/or enzymatic chain cleavage, leading to α-hydroxyacids, generally metabolized in the human body. This is particularly useful for many biomedical applications, especially, for temporary mechanical supports in regenerative medical devices. Ideally, the degradation should be compatible with the tissue recovering. In this work, the evolution of mechanical properties during degradation is discussed based on experimental data. The decrease of tensile strength of PLA-PCL fibers follows the same trend as the decrease of molecular weight, and so it can also be modeled using a first order equation. For each degradation stage, hyperelastic models such as Neo-Hookean, Mooney-Rivlin and second reduced order, allow a reasonable approximation of the material behavior. Based on this knowledge, constitutive models that describe the mechanical behavior during degradation are proposed and experimentally validated. The proposed theoretical models and methods may be adapted and used in other biodegradable materials, and can be considered fundamental tools in the design of regenerative medical devices where strain energy is an important requirement, such as, for example, ligaments, cartilage and stents.

This bibliographic review is focused on ligament tissue rehabilitation, its anatomy-physiology, and, mainly, on the dimensioning considerations of a composite material solution. The suture strength is problematic during the tissue recovering, implying reduction of mobility for several months. However, early postoperative active mobilization may enable a faster and more effective recovering of tissue biomechanical functions. As the risk of tendon rupture becomes a significant concern, a repair technique must be used to withstand the tensile forces generated by active mobilization. However, to avoid stress shielding effect on ligament tissue, an augmentation device must be designed on stiffness basis, that preferably will decrease. Absorbable biocomposite reinforcements have been used to allow early postoperative active mobilization and avoid the shortcomings of current repair solutions. Tensile strength decrease of the repair, during the initial inflammatory phase, is expected, derived from oedema and tendon degradation. In the fibroblastic phase, stiffness and strength will increase, which will stabilize during the remodeling phase. The reinforcement should be able to carry the dynamic load due to locomotion with a mechanical behavior similar to the undamaged natural tissue, during all rehabilitation process. Moreover, the degradation rate must also be compatible with the ligament tissue recovering. The selection and combination of different biodegradable materials, in order to make the biocomposite reinforcement functionally compatible to the damaged sutured tissue, in terms of mechanical properties and degradation rate, is a major step on the design process. Modelling techniques allow pre-clinical evaluation of the reinforcement functional compatibility, and the optimization by comparison of different composite solutions in terms of biomechanical behavior.

Aliphatic polyesters, such as polylactic acid (PLA), polyglycolic acid (PGA), polycaprolactone (PCL), polydioxone (PDO) and others, have been commonly used in biodegradable products. Hydrolytic and/or enzymatic chain cleavage of these materials leads to α-hydroxyacids, which, in most cases, are ultimately assimilated in human body or in a composting environment. However, each of these has some shortcomings, in terms of mechanical properties and degradation time, which restrict its applications. The combination of these materials, by copolymerization or blending, enables a range of mechanical properties and degradation rates. These are extremely promising approaches which can improve or tune the original properties of the polymers. A composite solution of several materials with different degradation rates also enables tuning the rate of degradation of a device and the mechanical properties. After immersion of an aliphatic polyester device, diffusion occurs very rapidly compared to hydrolysis. Therefore, it is usually considered that hydrolysis of ester bonds starts homogeneously and has traditionally been modelled according to a first order kinetics. In this experimental study, fibres of PLA-PCL, PGA-PCL, PDO and PGA, with two different dimensions, were characterized in terms of their degradation rate under three different environments (water, NaCl and PBS) at constant temperature (37°C). Weights and mechanical properties were measured after six different degradation stages. Stages durations were different depending on materials, according to the predicted degradation times. As other thermoplastics, they are viscoelastic materials. In this experimental study mechanical properties of fibres were compared at different strain rates.

A large range of biodegradable polymers are used in many products with short life cycle. Important applications of these are found in the biomedical field, where biodegradable materials are used to produce scaffolds that temporarily replace the biomechanical functions of a biologic tissue, while it progressively regenerates its capacities. However, the mechanical behavior of biodegradable materials along its degradation time, which is an important aspect of the project, is still an unexplored subject. In this work, hyper elastic constitutive models, such as the Neo-Hookean, the Mooney-Rivlin modified and the second reduced order are discussed. These can be used to predict the mechanical behavior of a blend composed of polylatic acid (PLA) and polycaprolactone (PCL). A numerical approach using ABAQUS® is presented, where the material properties of the model proposal are automatically updated in correspondence to the degradation time, by means of a User Material subroutine (UMAT). The parameterization of the material model proposal for different degradation times were achieved by fitting the theoretical curves with the experimental data of tensile tests. The material model proposal implemented in a subroutine could be used as a design toll for generic biodegradable devices

Several biodegradable polymers are used in many products with short life cycles. The performance of a product is mostly conditioned by the materials selection and dimensioning. Strength, maximum strain and toughness will decrease along its degradation, and it should be enough for the predicted use. Biodegradable plastics can present short-term performances similar to conventional plastics. However, the mechanical behavior of biodegradable materials, along the degradation time, is still an unexplored subject. The maximum strength failure criteria, as a function of degradation time, have traditionally been modeled according to first order kinetics. In this work, hyperelastic constitutive models are discussed. An example of these is shown for a blend composed of poly(L-lactide) acid (PLLA) and polycaprolactone (PCL). A numerical approach using ABAQUS is presented, which can be extended to other 3D geometries. Thus, the material properties of the model proposed are automatically updated in correspondence to the degradation time, by means of a user material subroutine. The parameterization was achieved by fitting the theoretical curves with the experimental data of tensile tests made on a PLLA-PCL blend (90:10) for different degradation times. The results obtained by numerical simulations are compared to experimental data, showing a good correlation between both results.