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- Strategy to improve the mechanical properties of bioabsorbable materials based on chitosan for orthopedic fixation applicationsPublication . Figueiredo, Lígia; Fonseca, Rita; Pinto, Luís F. V.; Ferreira, Frederico Castelo; Almeida, Amélia; Rodrigues, AlexandraBioabsorbable polymeric fixation devices have been used as an alternative to metallic implants in orthopedics, preventing the stress shielding effect and avoiding a second surgery for implant removal. However, several problems are still associated with current bioabsorbable implants, including the limited mechanical stiffness and strength, and the adverse tissue reactions generated. To minimize or even eliminate the problems associated with these implants, strategies have been developed to synthesize new implant materials based on chitosan. To overcome the brittle behavior of most 3D chitosan-based structures, glycerol and sorbitol were blended to chitosan and the effect of these plasticizers in the produced specimens was analyzed by flexural tests, Berkovich tests, scanning electron microscopy (SEM) and micro-CT analyzes. The improvement of the mechanical properties was also tested by adding ceramics, namely hydroxyapatite powder and biphasic mixtures of hydroxyapatite (HA) and beta-tricalcium phosphate (beta-TCP). In the plasticizers group, the best combination of the measured properties was obtained for chitosan with 10% glycerol (flexural strength of 53.8 MPa and indentation hardness of 19.4 kgf/mm(2)), while in the ceramics group the best mechanical behavior was obtained for chitosan with 10% HA+beta-TCP powder (flexural strength of 67.5 MPa and indentation hardness 28.2 kgf/mm(2)). All the tested material compositions were dense and homogeneous, fundamental condition for a good implant performance. These are encouraging results, which support the continued development of chitosan-based materials for orthopedic fixation applications.
- Influence of strain rate on the mechanical behavior of dry and hydrated chitosan-based dense materials for bioabsorbable implant applicationsPublication . Figueiredo, Lígia; Gregório, Afonso V. L.; Rodrigues, Alexandra; Rosa, P.A.R.Chitosan has generated enormous interest in the scientific community because of its distinctive biological and physicochemical properties, which allow new advanced structures and applications. Porous chitosan scaffolds have been extensively studied and explored in bone generation, however it is still a challenge to obtain bio-absorbable orthopedic implants that involves pure 3D dense chitosan geometries due to the inherent difficulties in producing and shaping such structures. In this work, chitosan was blended with 10% glycerol and 10% glycerol + 10% biphasic mixture of calcium phosphate (70% hydroxyapatite with 30% beta-tricalcium phosphate) to produce dense chitosan-based blocks, which were then shaped into rods. The introduction of plasticizer aimed to improve the materials ductility while the ceramic particles were used to increase stiffness and strength. The mechanical behavior of the two chitosan-based compositions was evaluated by uniaxial compression tests using a customized split-Hopkinson pressure bar (SHPB). The specimens were analysed in quasi-static conditions (less than 0.1 s(-1)) and medium strain rate conditions (200-800 s(-1)), both in dry state and in different hydrated conditions, in the latter case to approximate the in vivo implant conditions. The results showed promising results for the intended application. The chitosan blends present excellent ductility with an elastic perfect-plastic behavior in quasi-static conditions, with yield stresses around 40 MPa for the dry state, with a decay for 3 MPa after 48h hydration. An empirical model was proposed to describe the flow stress curves, with a good agreement with the experimental data, allowing future modelling of this material behavior.