Adrian J. Wright

Preparation of macroporous calcium phosphate cement tissue engineering scaffold

Unlike sintered hydroxyapatite there is evidence to suggest that calcium phosphate cement (CPC) is actively remodelled in vivo and because CPC is formed by a low-temperature process, thermally unstable compounds such as proteins may be incorporated into the matrix of the cement which can then be released after implantation. The efficacy of a macroporous CPC as a bone tissue engineering scaffold has been reported; however, there have been few previous studies on the effect of macroporosity on the mechanical properties of the CPC. This study reports a novel method for the formation of macroporous CPC scaffolds, which has two main advantages over the previously reported manufacturing route: the cement matrix is considerably denser than CPC formed from slurry systems and the scaffold is formed at temperatures below room temperature. A mixture of frozen sodium phosphate solution particles and CPC powder were compacted at 106 MPa and the sodium phosphate was allowed to melt and simultaneously set the cement. The effect of the amount of porogen used during processing on the porosity, pore size distribution and compressive strength of the scaffold was investigated. It was found that macroporous CPC could reliably be fabricated using cement:ice ratios as low as 5:2.

Biocompatibility of magnesium phosphate minerals and their stability under physiological conditions

Magnesium phosphates such as newberyite (MgHPO(4)·3H(2)O) are formed in vivo and are known to be biodegradable and nontoxic after implantation. Indeed, magnesium apatites have been shown to support osteoblast differentiation and function, and bone formation can occur around metallic magnesium implants. However, very little is known regarding the precipitation and stability of magnesium phosphates in physiological environments. In order to address this, the aqueous formation of magnesium phosphate as a function of pH, temperature and ion concentration is reported. Physicochemical characterization of the precipitates was carried out; additionally, biocompatibility and gene expression of osteoblast differentiation markers for bone formation via an in vitro cell culture assay were determined. Precipitation conditions for newberyite, tribasic magnesium phosphate pentahydrate, holtedahlite, bobierrite and cattiite were determined. Under physiological conditions of pH, temperature and magnesium phosphate concentration, no precipitates were formed. However, at concentrations 10-100 times higher than physiological, magnesium phosphate precipitates of cattiite and newberyite were formed. These two minerals demonstrated biocompatibility with osteoblast cultures and induced osteoblast adhesion and differentiation. The pattern of expression of OCN and CollA1 genes in the presence of newberyite crystals was comparable to that of calcium phosphate bioceramics. In our experiments, we have shown that certain magnesium phosphate phases such as newberyite and cattiite are able to promote in vivo osteogenic activity in a similar way to calcium phosphates such as hydroxyapatite and brushite. This confirms the great potential of magnesium phosphate ceramics in the development of new biomaterials for bone regeneration.

Temperature dependent setting kinetics and mechanical properties of β-TCP–pyrophosphoric acid bone cement

Brushite (CaHPO4·2H2O) is the product of acidic calcium phosphate cement forming reactions. It has a higher solubility than hydroxyapatite under physiological conditions and is a promising resorbable bone cement candidate. It reacts extremely fast so dilute mixes are required to form cements having setting times with surgical utility, which in turn compromises strength and produces a highly porous material. Pyrophosphate ions have a strong affinity for calcium orthophosphate surfaces, can inhibit their mineralisation and are thought to hydrolyze, forming orthophosphate during bone formation. The effect of replacing the acid orthophosphate component of a brushite cement with pyrophosphoric acid on the cement setting reaction time at temperatures of between 24 °C and 70 °C was determined. The substitution of pyrophosphate for orthophosphate retarded setting and improved mechanical performance of the cement. Pyrophosphate did not inhibit the extent of reaction but did influence microstructure of the brushite crystals. Temperature was found to have a significant (p < 0.01) influence on mechanical performance, and this was attributed to the formation of monetite (CaHPO4) rather than brushite at temperatures of ≥55 °C.

The effect of amorphous pyrophosphate on calcium phosphate cement resorption and bone generation.

Pyrophosphate ions are both inhibitors of HA formation and substrates for phosphatase enzymes. Unlike polyphosphates their hydrolysis results simultaneously in the complete loss of mineral formation inhibition and a localised elevation in orthophosphate ion concentration. Despite recent advances in our knowledge of the role of the pyrophosphate ion, very little is known about the effects of pyrophosphate on bone formation and even less is known about its local delivery. In this work we first developed a self setting pyrophosphate based calcium cement system with appropriate handling properties and then compared its in vivo degradation properties with those of a non-pyrophosphate containing control. Contrary to expectation, the presence of the pyrophosphate phase in the cement matrix did not inhibit mineralisation of the healing bone around the implant, but actually appeared to stimulate it. In vitro evidence suggested that enzymatic action accelerated dissolution of the inorganic pyrophosphate ions, causing a simultaneous loss of their mineralisation inhibition and a localised rise in supersaturation with respect to HA. This is thought to be a rare example of a biologically responsive inorganic material and these materials seem to be worthy of further investigation. Bioceramics to date have mainly been limited to orthophosphate, silicate and carbonate salts of calcium, here we report the successful application of a pyrophosphate material as a degradable osteoconductive bone repair cement.

Structures and magnetic ordering in the brownmillerite phases, Sr2MnGaO5 and Ca2MnAlO5

The crystal and magnetic structures of the two related phases, Sr2MnGaO5 and Ca2MnAlO5 are reported. Rietveld analysis of neutron powder diffraction has revealed that both phases adopt the brownmillerite structure. Subtle differences in structure lead to the structure of Sr2MnGaO5 being best described by the space group Icmm (a = 5.4888(2) A, b = 16.2256(6) A, c = 5.35450(2) A at 2 K) while that of Ca2MnAlO5 is best described by Ibm2 (a = 5.46258(9) A, b = 14.9532(3) A, c = 5.23135(8) A at 2 K). Low temperature neutron powder diffraction data show that both phases have a simple antiferromagnetic structure. However, magnetisation data suggest a more complex picture of the magnetic order within these phases.

Formation of translucent hydroxyapatite ceramics by sintering in carbon dioxide atmospheres

Hydroxyapatite is used in a variety of clinical applications as a result of the apparent adherence to and mild reaction of bone and soft tissue to it owing to its structural similarity with bone mineral. Transparent hydroxyapatite has previously been fabricated by either or both of two methods; namely the application of pressure during sintering and/or the use of fine particle sized apatite prepared by either a sol-gel process or aqueous precipitation. Recently it has been shown that translucent carbonate hydroxyapatite may be formed by sintering nanocrystalline gels of carbonate hydroxyapatite in a wet carbon dioxide atmosphere. In this study we report for the first time that this atmosphere can be used to sinter microcrystalline powder compacts of hydroxyapatite to form translucent ceramics at ambient pressure. The effect of water partial pressure and sintering time at 1300°C on the optical transmission and microstructure of the ceramic was investigated. It was found that translucent ceramics were formed in all carbon dioxide atmospheres and that optical transmission varied with sintering time. Maximum transmission (∼13%) of 2 mm thick ceramic was obtained in materials sintered for four hours at 1300°C in a mixture of carbon dioxide containing water at a partial pressure of 4.6 kPa.

Brushite cement additives inhibit attachment to cell culture beads

Brushite‐forming calcium phosphate cements are of great interest as bone replacement materials because they are resorbable in physiological conditions. Cell‐attached culture beads formed from this material could be of great use for cell therapy. Despite a significant amount of work on optimizing the physicochemical properties of these materials, there are very few studies that have evaluated the capacity of the materials to facilitate cell adhesion. In this study, we have formed resorbable calcium phosphate (brushite) culture beads and for the first time we showed that cell attachment to the surface of the brushite cement (BC) could be inhibited by the presence of an intermediate dicalcium phosphate–citrate complex, formed in the cement as a result of using citric acid, a retardant and viscosity modifier used in many cement formulations. The BC beads formed from the mixture of β‐TCP/orthophosphoric acid using citric acid did not allow cell attachment without further treatment. Ageing of BC beads in serum‐free Dulbeccos Modified Eagles Medium (DMEM) solution at 37°C for 1 week greatly enhanced the cell adhesion capacity of the material. Scanning electron microscopy, X‐ray diffraction (XRD), and confocal Raman microspectrometry indicated the increased capacity for cell adhesion was due to the changes in phase composition of BC. XRD patterns collected before and after ageing in aqueous solution and a high initial mass loss, suggest the formation of a dicalcium phosphate–citrate complex within the matrix. Since compacts formed from brushite powder supported cell attachment, it was hypothesized that the dicalcium phosphate–citrate complex prevented attachment to the cement surface. Biotechnol. Bioeng. 2013; 110: 1487–1494.

A neutron diffraction study of structural distortions in the Ruddlesden–Popper phase Na2La2Ti3O10

The structure of Na2La2Ti3O10(tetragonal, space group I4/mmm) has been re-examined using time-of-flight powder neutron diffraction in order to clarify uncertainties remaining from an X-ray diffraction study. One of the oxygen atoms is shown to be displaced significantly from its ideal position, the shift being consistent with the rotation of one of the TiO6 octahedra by ca. 12.5° around the [001] direction. This rotation allows for a 2.5% increase in four of the Ti—O bond lengths and a corresponding decrease in the bond valence sum from 4.9 to 4.1. The structural distortion is similar to that observed previously at overbonded octahedral sites in related structures.

The effects of partial substitutions of Zn and Fe on the Cu sites in LaBa2Cu3O7±x, and a comparison with similar substitutions in YBa2Cu3O7−x

Abstract The structural effects of Zn and Fe substitutions in LaBa2Cu3O7−x have been examined using time-of-flight neutron powder diffraction, and compared with previous data relating to similar studies of YBa2Cu3O7−x. Although the Fe and Zn ions display a similar site preference (for Cu(1) and Cu(2), respectively) in both systems, significant structural differences have been found. In particular, higher concentrations of dopant are possible in LaBa2Cu3O7±x (up to 25% Cu replacement by Zn and 66% replacement by Fe), and superconductivity is rapidly destroyed due to the incorporationof additional structural defects. For Zn substitutions, a significant concentration of vacancies on the O(4) sites is introduced, whereas Fe substitutions result in a gradual increase in structural disoder of a more general type. Evidence for this disorder is found on both the oxygen sublattice, e.g. partial filling of sites in the La plane at z = 1 2 , and the cation sublattice, with some intermixing of the La and Ba ions.

Enhanced stability and local structure in biologically relevant amorphous materials containing pyrophosphate

There is increasing evidence that amorphous inorganic materials play a key role in biomineralisation in many organisms, however the inherent instability of synthetic analogues in the absence of the complex in vivo matrix limits their study and clinical exploitation. To address this, we report here an approach that enhances long-term stability to >1 year of biologically relevant amorphous metal phosphates, in the absence of any complex stabilisers, by utilising pyrophosphates (P2O74−); species themselves ubiquitous in vivo. Ambient temperature precipitation reactions were employed to synthesise amorphous Ca2P2O7.nH2O and Sr2P2O7.nH2O (3.8 < n < 4.2) and their stability and structure were investigated. Pair distribution functions (PDF) derived from synchrotron X-ray data indicated a lack of structural order beyond ∼8 A in both phases, with this local order found to resemble crystalline analogues. Further studies, including 1H and 31P solid state NMR, suggest the unusually high stability of these purely inorganic amorphous phases is partly due to disorder in the P–O–P bond angles within the P2O7 units, which impede crystallization, and to water molecules, which are involved in H-bonds of various strengths within the structures and hamper the formation of an ordered network. In situ high temperature powder X-ray diffraction data indicated that the amorphous nature of both phases surprisingly persisted to ∼450 °C. Further NMR and TGA studies found that above ambient temperature some water molecules reacted with P2O7 anions, leading to the hydrolysis of some P–O–P linkages and the formation of HPO42− anions within the amorphous matrix. The latter anions then recombined into P2O7 ions at higher temperatures prior to crystallization. Together, these findings provide important new materials with unexplored potential for enzyme-assisted resorption and establish factors crucial to isolate further stable amorphous inorganic materials.