Arthur Brandwood

Sintering effects on the strength of hydroxyapatite.

Mechanisms underlying temperature-strength interrelations for dense (> 95% dense, pores closed) hydroxyapatite (HAp) were investigated by comparative assessment of temperature effects on tensile strength, Weibull modulus, apparent density, decomposition (HAp:tricalcium phosphate ratio), dehydroxylation and microstructure. Significant dehydroxylation occurred above approximately 800 degrees C. Strength peaked at approximately 80 MPa just before the attainment of closed porosity (approximately 95% dense). For higher temperatures (closed porosity), the strength dropped sharply to approximately 60 MPa due to the closure of dehydroxylation pathways, and then stabilized at approximately 60 MPa. At very high temperatures (> 1350 degrees C), the strength dropped catastrophically to approximately 10 MPa corresponding to the decomposition of HAp to tricalcium phosphate and the associated sudden release of the remaining bonded water.

Osteogenic induction of human bone marrow-derived mesenchymal progenitor cells in novel synthetic polymer-hydrogel matrices.

The aim of this project was to investigate the in vitro osteogenic potential of human mesenchymal progenitor cells in novel matrix architectures built by means of a three-dimensional bioresorbable synthetic framework in combination with a hydrogel. Human mesenchymal progenitor cells (hMPCs) were isolated from a human bone marrow aspirate by gradient centrifugation. Before in vitro engineering of scaffold-hMPC constructs, the adipogenic and osteogenic differentiation potential was demonstrated by staining of neutral lipids and induction of bone-specific proteins, respectively. After expansion in monolayer cultures, the cells were enzymatically detached and then seeded in combination with a hydrogel into polycaprolactone (PCL) and polycaprolactone-hydroxyapatite (PCL-HA) frameworks. This scaffold design concept is characterized by novel matrix architecture, good mechanical properties, and slow degradation kinetics of the framework and a biomimetic milieu for cell delivery and proliferation. To induce osteogenic differentiation, the specimens were cultured in an osteogenic cell culture medium and were maintained in vitro for 6 weeks. Cellular distribution and viability within three-dimensional hMPC bone grafts were documented by scanning electron microscopy, cell metabolism assays, and confocal laser microscopy. Secretion of the osteogenic marker molecules type I procollagen and osteocalcin was analyzed by semiquantitative immunocytochemistry assays. Alkaline phosphatase activity was visualized by p-nitrophenyl phosphate substrate reaction. During osteogenic stimulation, hMPCs proliferated toward and onto the PCL and PCL-HA scaffold surfaces and metabolic activity increased, reaching a plateau by day 15. The temporal pattern of bone-related marker molecules produced by in vitro tissue-engineered scaffold-cell constructs revealed that hMPCs differentiated better within the biomimetic matrix architecture along the osteogenic lineage.

Phagocytosis of carbon particles by macrophages In vitro

Particles of known size ranges of carbon fibre-reinforced carbon were presented to in vitro cultures of murine macrophages. Particles of up to 20 microns diameter were phagocytosed. Larger particles were not phagocytosed but became surrounded by aggregations of macrophages, some of which migrated on to the particle surfaces. Mean rates of phagocytosis up to 2.5 particles per hour were observed. Cells presented with a large excess of particles became rounded, detached from the substrate and some underwent lysis. The implications of these findings for the fate of particulates released from implanted medical devices is discussed. It is argued that a mechanism exists where particles in the size range 8-20 microns, released from medical devices, are small enough to be phagocytosed by macrophages and transported to the lymphatics and subsequently to the vascular circulation but large enough to lodge in capillary beds of tissues remote from the implant site.

The effects of sintering atmosphere on the chemical compatibility of hydroxyapatite and particulate additives at 1200°C

According to Le Chateliers principle, dehydration and the associated decomposition of hydroxyapatite (HAP) to biodegradable unhydrated calcium phosphates during sintering may be suppressed under a moist sintering atmosphere (thermodynamic effect), or possibly under a pressurized sintering atmosphere (physical effect), by opposing the release of water. The present study explored this possibility. High-purity powdered additives were used to minimize impurity and morphological effects. Al2O3, C, SiC, SiO2, ZrO2, and 316L stainless steel were all trialled at an addition level of 20 vol%. Heat treatment was at 1200°C for 1 h under two experimental atmospheres and two corresponding control atmospheres: flowing H2O/O2 mix—ambient air as a control; pressurized (1 MPa) argon—ambient argon (0.1 MPa) as a control. Specimens were analysed for decomposition by X-ray diffraction (XRD), for densification by porosity measurement, and for microstructural uniformity by energy dispersive spectroscopy (EDS) and image analysis. Significant decomposition occurred under all atmospheres with the exception of flowing H2O/O2 which eliminated decomposition in the HAP-Al2O3, HAP-ZrO2, and HAP-316L systems, and reduced the decomposition levels from near completion to ∼50% in the HAP-SiC and HAP-SiO2 systems. Moistureless pressurization had little effect. Microstructural uniformity was confirmed. No generalized atmosphere-densification interrelationships were observed.

Investigation of microstructural features in regenerating bone using micro computed tomography.

We illustrate some of the uses of micro-computed tomography (micro-CT) to study tissue-engineered bone using a micro-CT facility for imaging and visualizing biomaterials in three dimensions (3-D). The micro-CT is capable of acquiring 3D X-ray CT images made up of 20003 voxels on specimens up to 5 cm in extent with resolutions down to 2 μm. This allows the 3-D structure of tissue-engineered materials to be imaged across orders of magnitude in resolution. This capability is used to examine an explanted, tissue-engineered bone material based on a polycaprolactone scaffold and autologous bone marrow cells. Imaging of the tissue-engineered bone at a scale of 1 cm and resolutions of 10 μm allows one to visualize the complex ingrowth of bone into the polymer scaffold. From a theoretical viewpoint the voxel data may also be used to calculate expected mechanical properties of the tissue-engineered implant. These observations illustrate the benefits of tomography over traditional techniques for the characterization of bone morphology and interconnectivity. As the method is nondestructive it can perform a complimentary role to current histomorphometric techniques.

In vivo evaluation of polyurethanes based on novel macrodiols and MDI

A series of novel polyurethane elastomers based on methylenediphenyl diisocyanate, 1,4-butanediol and the macrodiols, poly(hexamethylene oxide), poly(octamethylene oxide), and poly(decamethylene oxide) were implanted subcutaneously in sheep for periods of 3 and 6 months. The specimens that were subjected to 3 months of implantation were strained to 250% of their resting length, while those implanted for 6 months had no applied external strain. SEM examination of the explanted specimens revealed that the novel materials displayed resistance to environmental stress cracking. Proprietary materials, Pellethane 2363-80A, Biomer and Tecoflex EG-80A, which had been implanted under identical conditions, showed evidence of significant stress cracking. The extent of stress cracking in the 3-month strained experiment was similar to that from the 6-month unstrained experiment. Stress cracking was also observed in Pellethane 2363-55D, when implanted for 6 months (unstrained). Neither changes in molecular weight nor in tensile properties provided a clear indication of early susceptibility to degradation by environmental stress cracking.

Improved Polymers for Medical Implants-Polyurethanes

Polyurethane elastomers, prepared from: (i) polyether macrodiols that contain a reduced number of ether linkages compared with PTMO [poly(tetramethylene oxide)], (ii) the diisocyanate MDI [4,4’-diphenylmethanediisocyanate], and (iii) the chain extender BDO [1,4-butanediol] offer enhanced stability towards oxidation and hydrolysis over their PTMO-based counterparts. Polyurethane-ureas prepared from the diisocyanate TMXDI (m-tetramethylxylene diisocyanate), however, show decreased stability. In vivo subcutaneous implant experiments (sheep; 90 days and 180 days), show that the new MDI-based ether-reduced polyurethanes do not undergo stress cracking while the PTMO-based materials do. The TMXDI materials performed poorly when implanted.

A Comparison of In Vivo Degradation of Novel Polyurethanes with Performance in In Vitro Accelerated Tests

In in vitro accelerated tests, both novel and commercially produced polyurethanes were subjected to severe hydrolysing and oxidising conditions for 24 hours. Degradation in these tests was compared to degradation of the same materials in an ovine in vivo accelerated test in which polyurethane dumbbells were stretched over poly(methyl methacrylate) holders to a constant strain and implanted for 90 days subcutaneously. Further samples of the same materials were implanted unstressed for 6 months. Degradation in both in vitro and in vivo accelerated tests was correlated with degradation observed in the 6 month in vivo study. The in vitro test is suitable for initial Screening of novel polyurethanes. The accelerated in vivo model generated environmental stress cracking in 90 days and was able to discriminate between materials of differing biostability. Three of the novel materials studied were more biostable than the commercial materials. The accelerated in vivo model provides a technique to identify materials which are more biostable than existing commercial polyurethanes.