This paper reviews the past, present, and future of the hydroxyapatite (HAp)-based biomaterials from the point of view of preparation of hard tissue replacement implants. Properties of the hard tissues are also described. The mechanical reliability of the pure HAp ceramics is low, therefore it cannot be used as artificial teeth or bones. For these reasons, various HAp-based composites have been fabricated, but only the HAp-coated titanium alloys have found wide application. Among the others, the microstructurally controlled HAp ceramics such as fibers/whiskers-reinforced HAp, fibrous HAp-reinforced polymers, or biomimetically fabricated HAp/collagen composites seem to be the most suitable ceramic materials for the future hard tissue replacement implants.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400042527

Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants

Tissue engineering has shown great promise for creating biological alternatives for implants. In this approach, scaffolding plays a pivotal role. Hydroxyapatite mimics the natural bone mineral and has shown good bone-bonding properties. This paper describes the preparation and morphologies of three-dimensional porous composites from poly(L-lactic acid) (PLLA) or poly(D,L-lactic acid-co-glycolic acid) (PLGA) solution and hydroxyapatite (HAP). A thermally induced phase separation technique was used to create the highly porous composite scaffolds for bone-tissue engineering. Freeze drying of the phase-separated polymer/HAP/solvent mixtures produced hard and tough foams with a co-continuous structure of interconnected pores and a polymer/HAP composite skeleton. The microstructure of the pores and the walls was controlled by varying the polymer concentration, HAP content, quenching temperature, polymer, and solvent utilized. The porosity increased with decreasing polymer concentration and HAP content. Foams with porosity as high as 95% were achieved. Pore sizes ranging from several microns to a few hundred microns were obtained. The composite foams showed a significant improvement in mechanical properties over pure polymer foams. They are promising scaffolds for bone-tissue engineering.

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Poly(α-hydroxyl acids)/hydroxyapatite porous composites for bone-tissue engineering. I. Preparation and morphology

The ultimate values for compressive strength, Young's modulus, and toughness of cylindrical specimens of unitary aspect ratios and uniform grain-size distributions were extrapolated for hydroxyapatite (HAP) to 70 MPa, 9.2 GPa, and 0.36 J cm(-3), and for tricalcium phosphate (TCP), to 315 MPa, 21 GPa, and 2.34 J cm(-3). For total volume porosities of 50%, the corresponding values were determined: for HAP, 9.3 MPa, 1.2 GPa, 0.042 J cm(-3), for TCP, 13 MPa, 1.6 GP, 0.077 J cm(-3). Porosities of HAP specimens ranged from 3%-50%; TCP from 10%-70%. Two pore-size distributions were employed. Exponential dependencies of the mechanical properties were found upon porosity (p < 0.0001). No differences in measured mechanical properties, as determined in compression, could be attributed to pore size. The superiority of TCP increases with density and suggests that a larger or more selective pore-size distribution could be effectively employed in TCP biological implants. This work also suggests the dominant role of secondary calcium phosphates in increasing compressive strengths.

Mechanical properties of sintered hydroxyapatite and tricalcium phosphate ceramic.

Bone is the most implanted tissue after blood. The major solid components of human bone are collagen (a natural polymer, also found in skin and tendons) and a substituted hydroxyapatite (a natural ceramic, also found in teeth). Although these two components when used separately provide a relatively successful mean of augmenting bone growth, the composite of the two natural materials exceeds this success. This paper provides a review of the most common routes to the fabrication of collagen (Col) and hydroxyapatite (HA) composites for bone analogues. The regeneration of diseased or fractured bones is the challenge faced by current technologies in tissue engineering. Hydroxyapatite and collagen composites (Col-HA) have the potential in mimicking and replacing skeletal bones. Both in vivo and in vitro studies show the importance of collagen type, mineralisation conditions, porosity, manufacturing conditions and crosslinking. The results outlined on mechanical properties, cell culturing and de-novo bone growth of these devices relate to the efficiency of these to be used as future bone implants. Solid free form fabrication where a mould can be built up layer by layer, providing shape and internal vascularisation may provide an improved method of creating composite structures.

/pdf/collagen-hydroxyapatite-composites-for-hard-tissue-repair-4lzsdznx4v.pdf

Collagen-hydroxyapatite composites for hard tissue repair.

Biological materials: Functional adaptations and bioinspired designs

Hydroxy apatite ceramic blocks of varying density have been prepared from a commercial powder. The elastic properties, fracture toughness, strength and sub-critical crack growth of these materials have been investigated. Young's modulus for the nearly fully dense material is 112 GPa while the compressive strength is about 800 MPa. For the same material the strength and fracture toughness under dry conditions are 115 MPa and 1.0 MPa m1/2, respectively. Substantial slow crack growth was found under these conditions. Under wet conditions the values for strength and fracture toughness drop to about 75% of their “dry” values. In this case very serious slow crack growth is present.

Preparation, microstructure and mechanical properties of dense polycrystalline hydroxy apatite

Hydrogen is a clean alternative fuel without carbon gas emission. This paper presents a critical evaluation of the different methods available for generating hydrogen from various feedstocks. The advantages and disadvantages of each process are discussed deeply by recent literatures. Steam reforming of fossil fuels (SRF) has been proved as an attractive method and commercialized on the larger scale. However, CO2 emission that produced during the process is critical issue by this method and therefore, CO2 capture, and storage/utilization technology are required. Besides, water splitting can produce ultra-purity hydrogen and oxygen as byproduct, but this method cannot be competed with SRF because of its expensive costs. The only possible to reduce this gap is by using solar energy with low-cost as energy source for water splitting and carbon taxes are imposed by the government to support research and development. Hydrogen production derived from biomass through gasification and pyrolysis currently shown an economic visibility and expected compete with available technology in the future. Moreover, by utilizing membrane reactor and integrated with a cheaper solar energy could significantly improve biomass-to-hydrogen conversion. 

A Critical review of the Hydrogen Production Biomass-based Feedstocks: Challenge, Solution, and Future Prospect

The fracture behaviour of AIN ceramics has been investigated as a function of porosity and temperature. The material was produced by hot-pressing and contained about 1.8 wt % impurities. Only slight anisotropy due to the hot-pressing procedure was present as observed by the ultrasonic pulse-echo technique. For the completely dense material, Young's moduli for directions parallel and perpendicular to the hot-pressing direction are 324 and 342 GPa, respectively. The fracture toughness and strength of the same material are 2.7 MPa m1/2 and 310 MPa, respectively, both values being for fracture planes parallel to the hot-pressing axis. The fracture toughness value decreases by about 10% up to 1500 K, while the strength decreases by about 20%. All three properties show a strong decrease with increasing porosity.

N. Hattu

Papers

Preparation, microstructure and mechanical properties of dense polycrystalline hydroxy apatite

A Critical review of the Hydrogen Production Biomass-based Feedstocks: Challenge, Solution, and Future Prospect

High-temperature fracture of hot-pressed AIN ceramics

The influence of CaO-doping on the fracture toughness of hot-pressed Al2O3

On the use of small specimens in the measurement of the fracture toughness for brittle materials