Papers by Author: I.G. Turner

Abstract: The production of nano-calcium phosphate powders, such as HA (hydroxyapatite), from synthetic chemicals can be expensive and time consuming. The skeleton or shells of sea creatures (e.g. sea urchins, shells, corals) could be an alternative source of materials to produce very fine and even nano-structured calcium phosphate biomaterial powders. Ηydrothermal conversion under very high pressures or methods such as hot-plating (chemical) or ultrasonication (mechano-chemical), have been proposed to transform naturally derived CaCO₃, e.g. aragonite, into apatite based materials. The aim of the present work was to prepare inexpensive nano-sized HA and TCP bioceramics powders from a local sea snail shells as a possible raw material for HA/TCP bioceramics. Empty shells of a local sea snail (Nassarius hinia reticulatus) from Marmara Sea, Turkey were collected from a beach near Istanbul. The collected shells were ground to a particle size <75µm. Thermal analyses (DTA/TGA) were performed to determine the exact CaCO₃ content and thermal behavior. The raw powder was suspended in an aqueous media which was placed in an ultrasonic bath. The temperature was set at 80°C for 15min. Then, an equivalent (to CaO content) amount of H₃PO₄ was added drop by drop very gently into the solution. The reaction continued for 8h, following which the liquid component was evaporated off in an incubator at 100°C for 24h. The dried sediment was collected and heat treated at two different temperatures, 400 and 800°C. The morphology of the powders produced was examined using SEM. The crystalline phases were indentified using X-ray analysis. X-ray diffractograms indicated the presence of two calcium phosphate phases, namely HA and whitlockite. SEM observations showed that the powder produced comprised nano-sized particles. FTIR results also indicated the presence of HA and whitlockite structures. The experimental results suggest that Nassarius hinia reticulatus shells could be an alternative source for the production of various mono or biphasic calcium phosphates. In this study, local sea snail shells were successfully converted to HA and whitlockite with a simple mechano-chemical (ultrasonic) conversion method without the use of complex hydrothermal methods.

Abstract: This paper studies the effects of porosity, sintering atmosphere and composition on the polarisation behaviour of calcium phosphate (CaP) ceramics. Hydroxyapatite (HA) and α- tricalcium phosphate (α-TCP) materials were prepared as both dense discs and as porous samples with interconnecting porosity. Materials were sintered in either air or water vapour at 1300°C and poled at a variety of temperatures, applied electric fields and time periods to optimise the polarisation properties of the materials. Thermally Stimulated Depolarisation Current (TSDC) measurements were used to investigate the degree of polarisation of the HA and α-TCP ceramics. A high depolarisation current was observed for both the porous HA and α-TCP ceramics. Sintering in water vapour also increased the depolarisation current due to reduced dehydration reactions, which result in a greater concentration of hydroxyl ions.

Abstract: The demand in the medical industry for load bearing materials is ever increasing. The techniques currently used for the manufacture of such materials are not optimized in terms of porosity and mechanical strength. This study adopts a microstructural shape design approach to the production of open porous materials, which utilizes spatial periodicity as a simple way to generate the models. A set of triply periodic surfaces expressed via trigonometric functions in the implicit form are presented. A geometric description of the topology of the microstructure is necessary when macroscopic properties such as mechanical strength, stiffness and isotropy are required to be optimised for a given value of volume fraction. A distinction between the families of structures produced is made on the basis of topology. The models generated have been used successfully to manufacture both a range of structures with different volume fractions of pores and samples of functional gradient material using rapid prototyping.

Abstract: Several novel methods for the production of calcium phosphate based functional gradient materials have been explored. The processes involved the use of polyurethane foams with a different number of pores per inch which were joined together in a variety of ways to form unique templates prior to vacuum impregnation with a ceramic slip. Before processing, rectangular blocks of foam were joined by stitching or trapezoidal blocks were compressed into rectangular shapes to produce a gradient of porosity along the length of the template. Four-point bend testing of the sintered samples which combined two porous structures showed them to have comparable mechanical properties to homogeneous ceramics based on foam templates with uniform pore sizes, with no evidence of weakness at the interface. The method was further developed to make a cylindrical sample with two diverse porous structures which more closely mimic the natural bone morphology. The two very different areas, which represented cortical and cancellous bone, had good structural integrity at the interface.

Abstract: Hydroxyapatite (HA) and barium titanate (BT) powders were mixed and sintered to form hydroxyapatite – barium titanate (HABT) ceramics. These materials were then poled and their piezoelectric properties were measured. The microstructure of unpoled samples was examined using scanning electron microscopy (SEM).The piezoelectric constants (d33 and d31) of the ceramics were found to be dependent on the proportion of BT in the ceramic In materials containing less than 70% BT, no piezoelectric effect was found. Above this value, the piezoelectric constant increased with the addition of BT up to a value of 108pCN-1 for pure BT. Values of d33 for ceramics containing more than 80% BT are above values previously shown to have a positive influence on bone growth in vivo. SEM analysis indicated that the grain size within the materials decreased as the proportion of BT in the material was reduced. Examination of the microstructure of the ceramics indicated the presence of electrical domains in the 100% BT and 95% BT ceramics. Domains were not visible below 95% BT. The reduction in grain size may influence the reduction in piezoelectric activity within the materials but cannot be considered to be the only cause.

Abstract: Ceramic slips with powder loadings in the range of 80-140 wt% were used to investigate the effect of slip loading on the physical and mechanical properties of open pore HA/TCP bioceramics. The results indicated that increasing the slip loading had an effect on the properties of the samples. The average apparent density, the work of fracture and compressive strength all increased with slip loading. In contrast, the effect of increasing slip loading on the four-point bending strength was not significant.

Abstract: Calcium phosphate (CaP) ceramics possessing an interconnecting porosity network in the appropriate size range for vascularisation offer the possibility of providing a structural matrix for replacement of diseased or damaged bone. Such bioceramics must possess sufficient mechanical strength to avoid failure whilst offering a bioactive surface for bone regeneration. The objective of the current study was to produce a hydroxyapatite/tricalcium phosphate (HA/TCP) bioceramic that imitated the orientated trabecular structure found in cancellous bone. The structure-property relationship of these bioceramics was then analysed. It was hypothesised that the mechanical properties would be linked to the shape of the pore structure due to the orientation of the open porous scaffolds (OPS) produced. OPS bioceramics possessed an interconnected macroporosity network of 40-70% by volume with bending strengths of 0.30MPa ± 0.01MPa and apparent densities of 0.35g/cm3 ± 0.05g/cm3. Typically, pore sizes in the range of 150-300µm were produced. The fabrication of CaP OPS resulted in a wide range of macroporosity in the correct size range for osseointegration to occur. Elongating the pore structure did not affect the total porosity of the bioceramics. Strengths were low due to microcrack formation on sintering and not due to the shape of the pores present in the scaffold as initially hypothesised.

Abstract: The aim of this study was to fabricate porous Hydroxyapatite/Tricalcium phosphate (HA/TCP) bioceramics with an adequate degree of interconnected porosity combined with optimal mechanical properties. Porous HA/TCP bioceramics with interconnected porosity and the controlled pore sizes necessary to simulate natural bone tissue morphology were fabricated by a novel technique of vacuum impregnation of reticulated polymeric foams with ceramic slip. By varying the characteristics of the slips and using foams of different pores per inch (ppi), samples of porous HA/TCP, blocks and granules, with a wide range of pore sizes were successfully manufactured. Functionally gradient materials (FGM) with porosity gradients were also made and no weakness was found at the interface. The pore size of the HA/TCP bioceramics was in the range of 197 – 254 µm (for 20 ppi foam), 143 – 182 µm (for 30 ppi foam) and 105 – 127 µm (for 45 ppi foam). The compressive strengths and the apparent densities of the HA/TCP samples were in the range of 30 –170 MPa and 2.34 – 2.76 g/cm3 respectively. These results indicate that it is possible to manufacture open pore HA/TCP bioceramics with compressive strengths comparable to human bone which could be of clinical interest.