Biphasic Calcium Phosphates

Due to the chemical similarity to the inorganic constituents of calcified tissues of mammals, biologically relevant calcium orthophosphates (CaPO4) have been applied as artificial bioceramics suitable for reconstruction of various types of bone defects. Since none of the known individual types of CaPO4 appears to be able to mimic both the composition and the properties of natural bones, various attempts have been sought to overcome this problem and a multiphasic (polyphasic) concept is one of the reasonable solutions. In general, this approach is determined by advantageous formulations consisting of homogeneous blends of two (biphasic), three (triphasic) or more (multiphasic) individual CaPO4 phases possessing diverse solubility and, therefore, bioresorbability, while the optimum ratios among the phases depend on the definite applications. Therefore, all currently known multiphasic CaPO4 formulations are sparingly soluble in water and, thus, after being implanted they are gradually resorbed inside the body, releasing calcium and orthophosphate ions into the biological medium and, hence, seeding a new bone formation. They have already demonstrated a proven biocompatibility, osteoconductivity, safety and predictability in vitro, in vivo, as well as in clinical trials. More recently, in vitro and in vivo studies have shown that some of them might possess osteoinductive properties. Hence, in tissue engineering, multiphasic CaPO4 bioceramics represent promising formulations to construct various scaffolds capable of carrying and/or modulating the behavior of cells. This review summarizes the available information on biphasic, triphasic and multiphasic CaPO4 bioceramics including their biomedical applications. New formulations have been proposed as well.

Biphasic calcium phosphate powders doped with zinc (Zn-doped BCP) were synthesized via sol-gel technique. Different concentrations of Zn have been successfully incorporated into biphasic calcium (BCP) phases namely: 1%, 2%, 3%, 5%, 7%, 10% and 15%. The synthesized powders were calcined at temperatures of 700-900ºC. The calcined Zn-doped BCP powders were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential and thermogravimetric analysis (TG/DTA) and field-emission scanning electron microscopy (FESEM). X-ray diffraction analysis revealed that the phases present in Zn-doped are hydroxyapatite, β-TCP and parascholzite. Moreover, FTIR analysis of the synthesized powders depicted that the bands of HPO 4 increased meanwhile O-H decreased with an increase in the calcination temperature. Field emission scanning electron microscopy (FESEM) results showed the agglomeration of particles into microscale aggregates with size of the agglomerates tending to increase with an increase in the dopant concentration.

Manganese-doped biphasic calcium phosphate (Mn-doped BCP) powders have been synthesized via solgel technique. The Mn-doped BCP powders were studied for their phase behaviour at different concentrations of Mn (0, 0.01, 2, 5 and 15 mol%) and calcination temperatures (500-1200 • C). For all the Mn concentrations, the X-ray diffraction (XRD) revealed that only hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) phases were present. It was shown that the HA crystallinity increased with increase in Mn content. Moreover, the weight loss at 700-850 • C detected by thermogravimetric analysis confirmed the presence of biphasic mixtures in the BCP powders. The study on effect of calcination temperature was done on the pure BCP and 5 mol% Mn-BCP powder. The XRD revealed that the decomposition of HA to TCP took place at the temperature range of 700-800 • C. Fourier transform infrared spectroscopy and XRD measurements found that HA peaks increased with the increasing Mn content and temperature. This was also accompanied by the enlarging particles at higher calcination temperatures and Mn contents due to fusion of agglomerated particles. These findings have led to the conclusion that Mn has acted as a calcination additive for the BCP powder. 

The development, fabrication, and characterization 1 of a novel miniature MEMS anemometer-nanoscale thermal 2 anemometry probe (NSTAP) for measuring velocity fluctuations 3 in turbulent flows are described. The sensor consists of a 4 freestanding 30 or 60 × 1 × 0.1 µm platinum filament with 5 electrically conductive pads and a silicon structure. A novel 6 deep reactive ion etching lag-based process for fabricating a 7 3-D silicon support structure is described together with other 8 microfabrication steps. The sensors behave similarly to conven-9 tional hot-wire anemometers, with an order of magnitude with 10 better spatial and temporal resolution. Batch fabrication allows 11 a relatively low-cost high-yield process, which together with its 12 superior frequency response and size, make it an attractive ther-13 mal anemometry sensor for velocity measurements in turbulent 14 flows. [2013-0313] 15 Index Terms-Microelectromechanical devices, anemometers, 16 velocity measurement. 17 I. INTRODUCTION 18 M EASURING turbulent velocity fluctuations continues 19 to be one of the great challenges to scientists and 20 engineers who study turbulence. Turbulent flows are composed 21 of recurrent, sometimes coherent flow structures or eddying 22 motions, that exhibit a range of length scales spanning several 23 orders of magnitude, all interacting with one another [1]. 24 To characterize turbulence fully, it is necessary to measure 25 the velocity fluctuations at all scales, including the smallest 26 eddies. In many flows, the smallest eddies can be of order 27 of micrometers, so there is a pressing need for probes that 28 can measure velocity fluctuations with sufficiently small spa-29 tial resolution and sufficiently high frequency response. The 30 widest range of length scales among turbulent eddies is found 31 in so-called high Reynolds number flows, which include flows 32 over large vehicles, inside industrial piping systems, and in the 33 earth's atmosphere. The Reynolds number is determined by the 34 relative size of the inertial forces to the viscous forces acting 35 on the flow, but it can also be interpreted as the ratio of the 36 Manuscript I E E E P r o o f 2 IEEE JOURNAL OF MICROELECTROMECHANICAL SYSTEMS the smallest of the available wires, with diameter of 0.625 μm, 84 but with the restriction of /d > 200 the smallest wire lengths 85 were limited to 125 μm. 86 Integrated circuit fabrication and MEMS fabrication tech-87 niques have introduced new possibilities for developing 88 anemometry sensors with considerably reduced physical sizes. 89 Lofdahl et al. [11] developed single and dual component 90 velocity sensors, which gave results that compared well to con-91 ventional hot-wire probes, and Ebefors et al. [12] introduced 92 micro-joints for fabricating 3D poly-silicon hot-wires. Unfor-93 tunately, the largest dimensions of the sensing elements of 94 these probes offered only a slight improvement in spatial res-95 olution compared to conventional techniques. Jiang et al. [13] 96 used MEMS fabrication techniques to manufacture a poly-97 silicon thermal anemometry probe with a geometry simi-98 lar to conventional hot-wires. These probes were a great 99 improvement in terms of spatial dimensions, with the smallest 100 having a length of 10 μm, but end-conduction was again an 101 important limitation. This was also a problem for the multi-102 component hot-wire probes fabricated by Chen [14], with 103 sensor dimensions 50 × 6 × 2.7 μm. Moreover, these sensors 104 were not suitable for conventional turbulence measurements, 105 being fixed at one wall-normal location. Wang et al. [15] had 106 a different approach, using a micro-cantilever structure for 107 measuring air flow, but these sensors were even larger than 108 conventional hot-wire anemometers. 109 At Princeton, we have been working for a number 110 of years to develop a Nano-Scale Thermal Anemometry 111 Probe (NSTAP) that will have improved spatial and tem-112 poral resolution for small-scale turbulence measurements. 113 NSTAP is a measurement device similar to conventional 114 hot-wire anemometer, having an order of magnitude smaller 115 sensing element compared to current commercial hot-wire 116 anemometers, and it is manufactured using MEMS fabrication 117 techniques. 118 The first successful effort to fabricate an NSTAP sensor was 119 reported by Bailey et al. [16]. The probe consisted of a plat-120 inum film supported on a silicon structure, with a free-standing 121 filament (the sensor) measuring about 60 × 2 × 0.1 μm 122

Hydroxyapatite (Ca10(PO4)6(OH)2 – HAp) is a common material for bone replacement in artificial implants and prostheses, due to its high biocompatibility. In this work we report about the preparation of HAp-based materials of marine origin with antibacterial properties. Bones from cod fish (Gadhus morua) were treated with a dilute AgNO3 solution, to obtain HAp-containing samples with silver as a minor component. The samples were characterised by XRD, FT-IR spectroscopy and SEM, indicating the presence of HAp, β–TCP and some metallic silver. The determination of unit cell parameters, however, indicated that the majority of silver is substituted in the β-TCP lattice in ionic form. The antibacterial activity of these materials was tested towards both Gram-positive and Gram-negative bacterias; results showed that the material is very effective with Gram-negative strains such as Escherichia coli (inactivation rates of 99.82% and 99.999% after 2 and 5 h, respectively) and had a smaller but still significant effect on Gram-positive MRSA (91% inactivation rate). These results show the potentials of these samples as infection-resistant bone replacement materials.

The biphasic calcium phosphate (BCP) concept was introduced to overcome disadvantages of single phase biomaterials. In this study, we prepared BCP from nano HA and β-TCP that were synthesized via a solid state reaction. Three different ratios of pure BCP and collagen-based BCP scaffolds (%HA/%β-TCP; 30/70, 40/60 and 50/50) were produced using a polymeric sponge method. Physical and mechanical properties of all materials and scaffolds were
investigated. XRD pattern proved the purity of each HA, β-TCP and BCP. SEM showed overall distribution of macropores (80-200 μm) with appropriate interconnected porosities. Total porosity of pure BCP (93% ± 2) was found to be higher than collagen-based BCP (85%± 3). It was observed that dimensional shrinkage of larger scaffold (39% ± 4) is lower than smaller one (42% ± 5) and scaffolds with higher HA (50%) ratio experienced greater shrinkage than those with higher β-TCP (70%) ratio (45% ±3 and 36% ±1 respectively). Mechanical properties of both groups tend to be very low and collagen coating had no influence on mechanical behavior. Further studies may improve the physical properties of these composite BCP.

Dense manganese-doped biphasic calcium phosphate (Mn-BCP) ceramics were fabricated via uniaxial pressing using the sol-gel derived powders. The compacted discs were sintered in ambient atmosphere with temperatures ranging from 800C to 1400C. Manganese (Mn) level was varied in the range of 0.6, 1.9, 4.3, and 11.9mol%, and its effect on physical and mechanical properties of the dense samples were investigated. All dense samples have been proved to show HA and -TCP phases only. Mn doping has shifted the onset of the sintering temperature of the BCP, leading to the improved densification of BCP ceramics. The relative density also increased with sintering temperature. Considerable grain growth has been observed for Mn-doped BCP samples when compared to the undoped BCP. Furthermore, 11.9mol% Mn-doped BCP dense samples showed the maximum hardness of 6.66GPa compared to 2.89GPa for the undoped BCP. The incorporation of Mn was also found to be beneficial in enhancing the fracture toughness of BCP throughout the temperature range employed. This study has shown that Mn doping was effective in improving the sintering properties of BCP without affecting the phase stability.

http://www.tandfonline.com/doi/abs/10.1080/10426914.2011.557285#.Uf79787vPgg

As the inorganic constituents of skeletons, dentine and the enamel of teeth in all vertebrates, as well as antlers of male deer, calcium orthophosphates (CaPO4) appear to be the key materials to sustain all life on Earth. Therefore, biologically relevant CaPO4 possess all the necessary features of the biomaterials, such as biocompatibility, bioactivity, bioresorbability, osteoconductivity, osteoinductivity, and appear to be non-toxic, non-inflammatory and non-immunogenic. In this book, the author presents current state-of-the-art applications on three popular types of CaPO4: nano-scaled (nano-dimensional), multiphasic (polyphasic) and amorphous CaPO4. Topics discussed include the preparation, structure, composition, properties and biomedical applications of these types of CaPO4 combined with the specific information for each type. Namely, occurrence in the calcified tissue of mammals is discussed for both nano-scaled and amorphous types of CaPO4; both known and potential formulations, as well as their stability are discussed for multiphasic CaPO4, while both the morphology and the available knowledge on amorphous-to-crystalline transformations are discussed for amorphous CaPO4.

Dense manganese-doped biphasic calcium phosphate (Mn-BCP) ceramics were fabricated via uniaxial pressing using the sol-gel derived powders. The compacted discs were sintered in ambient atmosphere with temperatures ranging from 800C to 1400C. Manganese (Mn) level was varied in the range of 0.6, 1.9, 4.3, and 11.9mol%, and its effect on physical and mechanical properties of the dense samples were investigated. All dense samples have been proved to show HA and -TCP phases only. Mn doping has shifted the onset of the sintering temperature of the BCP, leading to the improved densification of BCP ceramics. The relative density also increased with sintering temperature. Considerable grain growth has been observed for Mn-doped BCP samples when compared to the undoped BCP. Furthermore, 11.9mol% Mn-doped BCP dense samples showed the maximum hardness of 6.66GPa compared to 2.89GPa for the undoped BCP. The incorporation of Mn was also found to be beneficial in enhancing the fracture toughness of BCP throughout the temperature range employed. This study has shown that Mn doping was effective in improving the sintering properties of BCP without affecting the phase stability.

http://www.tandfonline.com/doi/pdf/10.1080/10426914.2011.557285

Open and interconnected porous scaffolds were prepared with various ratios of hydroxyapatite (HA)/β-tricalcium phosphate by a combination of gel casting and polymer sponge methods to improve the mechanical properties and structure. The scaffolds were prepared using slurries containing 50 vol.-% of ceramic powders and sintered at 1100°C for 2 h. Thermogravimetric analysis result shows that the proper temperature to burn out organic materials and polyurethane foams is 600°C. The compressive strength was between 5·3 and 8·4 MPa. Field emission scanning electron microscope shows an open, relatively uniform and large interconnected porous structure with pore size ranging between 150 and 400 μm. X-ray diffraction and Brunauer- Emmett-Teller methods were employed to determine the microstructural crystallite and surface area respectively. The results show that the compressive strength of scaffolds increased with the increase in HA concentration. The reason can be explained by the increasing pore wall thickness and density in scaffolds.

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Biphasic calcium phosphate powders doped with zinc (Zn-doped BCP) were synthesized via sol-gel technique. Different concentrations of Zn have been successfully incorporated into biphasic calcium (BCP) phases namely: 1%, 2%, 3%, 5%, 7%, 10% and 15%. The synthesized powders were calcined at temperatures of 700-900°C. The calcined Zn-doped BCP powders were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential and thermogravimetric analysis (TG/DTA) and field-emission scanning electron microscopy (FESEM). X-ray diffraction analysis revealed that the phases present in Zn-doped are hydroxyapatite, β- TCP and parascholzite. Moreover, FTIR analysis of the synthesized powders depicted that the bands of HPO4 increased meanwhile O-H decreased with an increase in the calcination temperature. Field emission scanning electron microscopy (FESEM) results showed the agglomeration of particles into microscale aggregates with size of the agglomerates tending to increase with an increase in the dopant concentration.

http://www.scientific.net/KEM.531-532.614