Surface Cracking and Degradation of Dense Hydroxyapatite through Vickers Microindentation Testing

Surface degradation and cracking of dense hydroxyapatite were evaluated through Vickers micro indentation using indentation loads ranged from 25 gf to 2000 gf. Crack lengths, imprint diameters and the number of lateral cracks and chips were measured using SEM. The crack length-indentation load data were analyzed with regard to the specific relations of Palmqvist and fully developed radial cracks. Crack type transition load from Palmqvist to median crack was experimentally assessed through serial sectioning technique. The analytical estimated transition load, based on the theoretical relation of the indentation load and crack lengths showed a good agreement with one obtained from experimental itinerary. Palmqvist and median cracks were identified in low and medium indentation loads, respectively. High indentation load could also lead to the formation of lateral cracks and chips. The tendency for lateral cracking was evaluated taking into account the number of lateral cracks and chips. The chips were found to be appeared just after test in higher indentation load, whereas in medium loads they could be detectable only after several weeks

Experimental and computational analysis of the mechanical behavior of poly(lactic-co-glycolic acid) during degradation for medical implant applications

Biodegradable polymers such as PLGA have been used in a wide range of biomedical applications due to their hydrolytic degradation and biocompatibility. The mechanical performance of biodegradable polymers during degradation is strongly influenced by the degradation rate. The size dependent degradation mechanisms in these polymers lead to different degradation rates which make significant differences in the mechanical properties of different sized medical device components. Therefore, a clear understanding of the degradation behaviour and the mechanical performance of PLGA material during degradation is required. In this thesis, the mechanical properties of PLGA materials prepared by solvent casting and compression moulding are evaluated using the nanoindentation technique. The measured elastic modulus and hardness are strongly depth dependent for both forms of the material, for indentations less than 3000 nm. The mechanical properties of PLGA material are significantly influenced by the material processing method. The solvent-cast material is more elastically compliant and plastically softer than the compression-moulded material and it also shows lower work hardening characteristics. Changes in the mechanical properties of PLGA material under simulated physiological degradation conditions are evaluated. The relationship between the changes in the molecular weight and the Young’s modulus of PLGA material is established. It is shown that the PLGA mechanical properties during degradation do not decrease until the number average molecular weight of the polymer chains reaches a critical molecular weight of 1500 g mol-1. The experimental observations demonstrate that there is no significant difference in the degradation rate and the changes in the mechanical properties of PLGA thicker than 120 μm. It is shown that the material processing methods considered here do not have a significant effect on the degradation rate and the mechanical performance of PLGA material. A computational modelling framework is developed to predict the degradation behaviour and the changes in the mechanical properties of PLGA material during degradation. The degradation behaviour is predicted using both analytical and numerical solutions. When applied to PLGA films, semi-analytical solution cannot capture the differences in degradation rates for films of thickness above 25 μm. When applied to PLGA scaffolds, architecture of the scaffold does not have a significant influence on the degradation rate, but it determines the initial stiffness of the scaffold. The size of the scaffold strut controls the degradation rate and the mechanical collapse. A critical length scale due to competition between diffusion of degradation products and autocatalytic degradation is determined to be in the range 2-100 μm. Below this range, slower homogenous degradation occurs; however, for larger samples faster autocatalytic degradation occurs. The experimental observations for the degradation rate and the mechanical performance of PLGA materials support the computational modelling predictions. In conclusion, the use of experimental testing methods and computational modelling in this thesis has led to an improved understanding of the mechanical performance of solvent-cast and compression-moulded PLGA materials with different thicknesses during degradation.2017-04-2

An investigation of children’s musical collaborations: the effect of friendship and age

Scaffolding plays a critical rule in tissue engineering and an appropriate degradation rate and sufficient mechanical integrity are required during degradation and healing of tissue. This paper presents a computational investigation of the molecular weight degradation and the mechanical performance of poly(lactic-co-glycolic acid) (PLGA) films and tissue engineering scaffolds. A reaction-diffusion model which predicts the degradation behaviour is coupled with an entropy based mechanical model which relates Young's modulus and the molecular weight. The model parameters are determined based on experimental data for in-vitro degradation of a PLGA film. Microstructural models of three different scaffold architectures are used to investigate the degradation and mechanical behaviour of each scaffold. Although the architecture of the scaffold does not have a significant influence on the degradation rate, it determines the initial stiffness of the scaffold. It is revealed that the size of the scaffold strut controls the degradation rate and the mechanical collapse. A critical length scale due to competition between diffusion of degradation products and autocatalytic degradation is determined to be in the range 2-100 mu m. Below this range, slower homogenous degradation occurs; however, for larger samples monomers are trapped inside the sample and faster autocatalytic degradation occurs. (C) 2015 Elsevier Ltd. All rights reserved.Funding support was provided by the Structured PhD Programme in Biomedical Engineering and Regenerative Medicine (BMERM). Funded under the Programme for Research in Third-Level Institutions (PRTLI) Cycle 5 (Strand 2) and co-funded under the European Regional Development Fund (ERDF).peer-reviewed2017-09-0

Surface Cracking and Degradation of Dense Hydroxyapatite through Vickers Microindentation Testing

International audienceSurface degradation and cracking of dense hydroxyapatite were evaluated through Vickers micro indentation using indentation loads ranged from 25 gf to 2000 gf. Crack lengths, imprint diameters and the number of lateral cracks and chips were measured using SEM. The crack length-indentation load data were analyzed with regard to the specific relations of Palmqvist and fully developed radial cracks. Crack type transition load from Palmqvist to median crack was experimentally assessed through serial sectioning technique. The analytical estimated transition load, based on the theoretical relation of the indentation load and crack lengths showed a good agreement with one obtained from experimental itinerary. Palmqvist and median cracks were identified in low and medium indentation loads, respectively. High indentation load could also lead to the formation of lateral cracks and chips. The tendency for lateral cracking was evaluated taking into account the number of lateral cracks and chips. The chips were found to be appeared just after test in higher indentation load, whereas in medium loads they could be detectable only after several weeks

Experimental mechanical testing of Poly (l-Lactide) (PLLA) to facilitate pre-degradation characteristics for application in cardiovascular stenting

Next-generation stents made from Biodegradable Polymers (BPs) aim to address the long-term risks (i.e. late restenosis and in-stent thrombosis) associated with both Bare Metal Stents and Drug Eluting Stents, whilst aiming to reduce the healthcare costs associated with secondary care. However, the true potential of BPs for cardiovascular load bearing applications does not appear to be fully realised. While the literature provides data on stiffness and strength of BPs, it is lacking pre-degradation experimental data on the recovery behaviour and temperature and strain rate dependency. In this paper, an experimental study is undertaken to address this knowledge gap using Poly (L-Lactide) (PLLA) samples, subjected to tensile testing. Stress-strain characteristics, recovery, relaxation and creep data at body temperature are reported and considered in the context of real-life stent deployment. The experimental data herein reveal a strong temperature and strain rate dependency, whilst demonstrating associated plasticity within the material. The work provides a physical evaluation of PLLA's pre-degradation behaviour, establishing key data points to allow the assessment of PLLA as a viable material in the wider context of stent deployment and load carrying capacity. (C) 2016 Elsevier Ltd. All rights reserved.The authors like to acknowledge the funding of this project through a Hardiman Scholarship at NUI Galway.peer-reviewed2018-07-1

Experimental mechanical testing of Poly (l-Lactide) (PLLA) to facilitate pre-degradation characteristics for application in cardiovascular stenting

Next-generation stents made from Biodegradable Polymers (BPs) aim to address the long-term risks (i.e. late restenosis and in-stent thrombosis) associated with both Bare Metal Stents and Drug Eluting Stents, whilst aiming to reduce the healthcare costs associated with secondary care. However, the true potential of BPs for cardiovascular load bearing applications does not appear to be fully realised. While the literature provides data on stiffness and strength of BPs, it is lacking pre-degradation experimental data on the recovery behaviour and temperature and strain rate dependency. In this paper, an experimental study is undertaken to address this knowledge gap using Poly (L-Lactide) (PLLA) samples, subjected to tensile testing. Stress-strain characteristics, recovery, relaxation and creep data at body temperature are reported and considered in the context of real-life stent deployment. The experimental data herein reveal a strong temperature and strain rate dependency, whilst demonstrating associated plasticity within the material. The work provides a physical evaluation of PLLA\u27s pre-degradation behaviour, establishing key data points to allow the assessment of PLLA as a viable material in the wider context of stent deployment and load carrying capacity. (C) 2016 Elsevier Ltd. All rights reserved.The authors like to acknowledge the funding of this project through a Hardiman Scholarship at NUI Galway.2018-07-1

Nanomechanical properties of poly(lactic-co-glycolic) acid film during degradation

Despite the potential applications of poly(lactic-co-glycolic) acid (PLGA) coatings in medical devices, the mechanical properties of this material during degradation are poorly understood. In the present work, the nanomechanical properties and degradation of PLGA film were investigated. Hydrolysis of solvent-cast PLGA film was studied in buffer solution at 37 °C. The mass loss, water uptake, molecular weight, crystallinity and surface morphology of the film were tracked during degradation over 20 days. Characterization of the surface hardness and Young’s modulus was performed using the nanoindentation technique for different indentation loads. The initially amorphous films were found to remain amorphous during degradation. The molecular weight of the film decreased quickly during the initial days of degradation. Diffusion of water into the film resulted in a reduction in surface hardness during the first few days, followed by an increase that was due to the surface roughness. There was a significant delay between the decrease in the mechanical properties of the film and the decrease in the molecular weight. A sudden decline in mechanical properties indicated that significant bulk degradation had occurred