Rui Miranda Guedes

Mechanical study of PLA–PCL fibers during in vitro degradation

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.

Development of ligament tissue biodegradable devices: a review.

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.

Analytical and Experimental Evaluation of Nonlinear Viscoelastic-Viscoplastic Composite Laminates under Creep, Creep-Recovery, Relaxation and Ramp Loading

A numerical procedure to predict long-term laminate properties of fibre reinforced composite materials was developed. In the procedure, we extended the classical laminate theory to include time related response of composite materials for membrane and flexural loading. The material response, dependent on the stress history, was modelled using the Schapery single integral equation. The integrals were handled by an approximate method that uses the Pronys series and only requires the storing of the current stress and some internal strain components. An efficient semi-direct time-integration scheme, providing a stable integration process, was derived to be included in the numerical procedure. Comparisons of theoretical results were made with experiments conducted on composite materials under creep-creep recovery, relaxation and ramp loading.

Creep behaviour of FRP-reinforced polymer concrete

Polymer concrete is a kind of concrete where natural aggregates such as silica sand or gravel are binded together with a thermoset resin, such as epoxy. Although polymer concretes are stronger in compression than cementitious concrete, its tension behaviour is still weak. The reinforcement of polymer concrete beams in the tension zone with pultruded profiles made of epoxy resin and glass fibers are a good compromise between stiffness and strength. In this paper it is reported an investigation of the creep behaviour of polymer concrete beams reinforced with fiber-reinforced plastics (pultruded) rebars. Four-point bending creep test were performed. An analytical model was applied to verify the experimental results.

Creep and fatigue in polymer matrix composites

Part 1 Viscoelastic and viscoplastic modelling: Viscoelastic constitutive modeling of creep and stress relaxation in polymers and polymer matrix composites Time-temperature-age superposition principle for predicting long-term response of linear viscoelastic materials Time-dependent behaviour of active/intelligent polymer matrix composites incorporating piezoceramic fibers Predicting the elastic-viscoplastic and creep behaviour of polymer matrix composites using the homogenization theory Measuring fiber strain and creep behaviour in polymer matrix composites using Raman spectroscopy Predicting the viscoelastic behaviour of polymer nanocomposites Constitutive modelling of viscoplastic deformation of polymer matrix composites Creep analysis of polymer matrix composites using viscoplastic models Micromechanical modeling of viscoelastic behaviour of polymer matrix composites undergoing large deformations. Part 2 Creep rupture: Fiber bundle models for creep rupture analysis of polymer matrix composites Micromechanical modelling of time-dependent failure in off-axis polymer matrix composites Time-dependent failure criteria for lifetime prediction of polymer matrix composite structures. Part 3 Fatigue modelling, characterisation and monitoring: Testing the fatigue strength of fibers used in fiber-reinforced composites using fiber bundle tests Continuum damage mechanical modelling of creep damage and fatigue in polymer matrix composites Accelerated testing methodology for predicting long-term creep and fatigue in polymer matrix composites Fatigue testing methods for polymer matrix composites The effect of viscoelasticity on fatigue behavior of polymer matrix composites Characterization of vicoelasticity, viscoplasticity and damage in composites Structural health monitoring of composite structures for durability.

Prediction of long-term behaviour of composite materials

Abstract After a brief introduction, where the question of durability related to composite materials is raised concerning the type of applications and the constituents of the composite material, some considerations are given about the interaction between physical and chemical ageing and its effect on the long-term behaviour of those materials. A comparison of different methodologies to define models for the constitutive laws of the material is made, together with a reference to methodologies used for real structures in order to predict long-term behaviour. An algorithm to predict long-term laminate properties of fibre reinforced composites is presented. In the procedure, the classical laminate plate theory was extended to include time-related response of composite materials for membrane and bending load. An efficient semi-direct time integration scheme, providing a stable integration process, was derived to be included in the numerical procedure. The present formulation enables solving problems related with creep, stress relaxation and rate dependent stress–strain behaviour for in-plane and bending loads. The results concerning studies related to the influence of water and temperature on the long-term behaviour of fibre reinforced composite, hence on the constitutive models, are presented. The experimental results shows that a combined time–temperature–moisture superposition principle (TTMSP) is not valid. This principle simply states that the effects of temperature and moisture on the viscoelastic properties can be uncoupled.

Mathematical analysis of energies for viscoelastic materials and energy based failure criteria for creep loading

In this work energy based failure criteria are applied and assessed usingpublished experimental data. Reiner and Weissenberg (R–W) developed anenergy failure criterion based on deviatoric free energy. In thistheoretical model, failure is part of the complete constitutive descriptionof the material. Some considerations are given to the energy forviscoelastic materials characterized by single-integrals. Some studies showthat for materials with memory there is more than one free energy compatiblewith the thermodynamical principles. The free energy is defined and relatedto the viscoelastic model. Failure criteria are defined and tested for somepolymers and FRP. The experimental data of some materials, at room andelevated temperatures, reveals some discrepancies with R–W criterion and acriterion modification is proposed. It is concluded that the R–W criterionis not universal.

Antimicrobial Approaches for Textiles: From Research to Market

The large surface area and ability to retain moisture of textile structures enable microorganisms’ growth, which causes a range of undesirable effects, not only on the textile itself, but also on the user. Due to the public health awareness of the pathogenic effects on personal hygiene and associated health risks, over the last few years, intensive research has been promoted in order to minimize microbes’ growth on textiles. Therefore, to impart an antimicrobial ability to textiles, different approaches have been studied, being mainly divided into the inclusion of antimicrobial agents in the textile polymeric fibers or their grafting onto the polymer surface. Regarding the antimicrobial agents, different types have been used, such as quaternary ammonium compounds, triclosan, metal salts, polybiguanides or even natural polymers. Any antimicrobial treatment performed on a textile, besides being efficient against microorganisms, must be non-toxic to the consumer and to the environment. This review mainly intends to provide an overview of antimicrobial agents and treatments that can be performed to produce antimicrobial textiles, using chemical or physical approaches, which are under development or already commercially available in the form of isolated agents or textile fibers or fabrics.

Degradation and viscoelastic properties of PLA-PCL, PGA-PCL, PDO and PGA fibres

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.

Considerations for the design of polymeric biodegradable products

Abstract 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.