J. R. de Wijn

Fast precipitation of calcium phosphate layers on titanium induced by simple chemical treatments.

A simple two-step chemical treatment, i.e. etching with HCl and H2SO4 followed by immersion in boiling dilute NaOH solution, has been developed by our group to prepare bioactive microporous titanium surfaces allowing fast deposition of a calcium phosphate layer (CPL) from an in vitro supersaturated calcification solution (SCS). In this work, a precalcification (Pre-Ca) procedure was applied by soaking the two-step treated titanium in Na2HPO4 and then saturated Ca(OH)2 solution before immersion in SCS to accelerate further the CPL precipitation. The treated titanium surfaces with Pre-Ca were characterized after 1, 2, 4, 8 and 16 h of immersion in SCS by means of scanning electron microscopy together with energy dispersive X-ray analysis, X-ray diffraction and infrared absorption analysis. It was observed that the CPL precipitation rate with Pre-Ca averaged 1 microm h-1, twice as fast as without Pre-Ca. No precipitation was observed on untreated titanium with Pre-Ca up to day 14 of immersion in the SCS.

PREPARATION OF BIOACTIVE MICROPOROUS TITANIUM SURFACE BY A NEW TWO-STEP CHEMICAL TREATMENT

Microporous oxide layers allowing fast deposition of calcium phosphate layers (CPLs) were formed on commercially pure titanium (c.p.Ti) after the application of a newly developed two-step chemical treatment. The micropores were of submicrometre size. The two-step treatment was carried out by etching c.p.Ti samples with HCl and H2SO4 first and then treating them in boiling 0.2 N NaOH solution at 140 °C for 5 h. Conformal CPLs, about 20 μm thick, were deposited on the two-step treated c.p.Ti surface by means of a two-day immersion in an in vitro supersaturated calcification solution. The CPL was characterized to be mainly composed of two sublayers, i.e. an outside loose octacalcium phosphate crystal sublayer and an inside dense carbonated apatite sublayer. A scratching test indicated that the apatite sublayer was strongly bonded to the c.p.Ti substrate. Moreover, it was observed that the untreated or single-step treated c.p.Ti surfaces are not only morphologically different from one another but significantly different from the two-step treated one, in that no precipitation was observed on them up to 14 d immersion in the same calcification solution. It is indicated that the two-step chemical treatment is a simple and easily controllable method to prepare bioactive titanium surfaces and subsequently to induce the rapid precipitation of conformal and adherent CPL from in vitro supersaturated calcification solutions.

Incorporation of bovine serum albumin in calcium phosphate coating on titanium.

Calcium phosphate (Ca-P) and bovine serum albumin (BSA) were coprecipitated as a coating on commercially pure titanium (cpTi) with a high protein loading (15 wt %) by employing a recently developed wet-chemistry technique. It was observed that the incorporation of BSA significantly modified the morphology, composition, and crystallinity of the Ca-P coating. The Ca-P coating without BSA is a mixture of hydroxyapatite (HA) and octacalcium phosphate (OCP) with sharp-edged thin OCP crystal plates on the top layer, whereas only an HA phase was detected in the Ca-P/BSA coating. The crystal plates in the latter had a more rounded appearance. The Ca-P/BSA coatings were immersed respectively in neutral (pH 7.4) and acidic (starting pH 4.0) phosphate-buffered saline (PBS) at 37 degrees C over a 14-day period. No protein release was detected in the neutral PBS during the immersion; however, a continuous release of BSA was measured in the acidic PBS, subsequently leading to the formation of a very dense and well-adherent composite coating of BSA and Ca-P on cpTi. The present study provides the possibility to achieve a long-term effective release of biologically active proteins from a Ca-P-coated metallic implant.

Nano-apatite/polymer composites: mechanical and physicochemical characteristics

Hydrothermally synthesized acicular nano-apatite (Nap) was used as filler to make composites with a polyethylene glycol/poly(butylene terephthalate) (PEG/PBT) block copolymer (Polyactive 70:30). The Nap had a particle diameter of 9-25 nm and a length of 80-200 nm. The mechanical properties and the physiochemical characteristics of the composites, such as Youngs modulus, swelling degree in water and the calcification behaviour, have been determined. It was found that Nap had a strong ability to promote the calcification of composites when incorporated into Polyactive 70:30, while poly(acrylic acid) (PAA) coating of Nap had an adverse effect on the calcification of composites, presumably due to the formation of complexes between PAA and PEG segments. Nap had a prominent stiffening effect for Polyactive 70:30 in the dry state, but had a poor stiffening effect for composites in an aqueous environment due to the hygroscopic nature and/or the formation of aggregates. PAA coating on Nap had almost no additional effect on the mechanical properties of composites either in the dry state or in an aqueous environment. To reinforce the polymer by Nap, achieving a more homogeneous dispersion of Nap in the polymer matrix and surface modifications to render the powders less hygroscopic appear to be necessary.

Preparation of bioactive Ti6Al4V surfaces by a simple method.

Boiling diluted alkali incubation was found to be an effective way to prepare bioactive Ti6Al4V surfaces, whether polished or not, as indicated in vitro after immersion in two different supersaturated calcification solutions (SCSs). The induction of calcium phosphate (Ca-P) precipitation from the SCSs is most probably made possible by the formation of a new TiO2 surface layer and a large number of submicron-scaled etched pits therein. The morphologies and composition of the Ca-P deposited from different SCSs are entirely different from each other. The processes on Ti6Al4V surfaces during treatment and immersion were investigated in detail by means of scanning electron microscopy combined with energy dispersive X-ray analysis, X-ray photoelectron spectroscopy and X-ray diffraction.

Morphology and composition of nanograde calcium phosphate needle-like crystals formed by simple hydrothermal treatment

Nanograde calcium phosphate needle-like crystals are prepared from wet synthesized Ca−P precipitates by simple hydrothermal treatment at 140°C and 0.3 MPa for 2 h. The morphology of these crystals is observed by transmission electron microscopy (TEM). The phase composition is tested through X-ray diffractometer (XRD) and infrared spectroscopy (IR). It is found that the morphology of these crystals is related to the activity or fresh degree of the starting Ca−P precipitates and the added fluorine ions, but is not greatly influenced by the Ca/P ratio of the precipitates. These crystals with a Ca/P ratio between 1.67 and 1.5 show a poorly crystallized apatite structure at room temperature and a biphasic (HA+β−TCP) structure at 1100°C, corresponding to their Ca/P ratio. It is demonstrated that these nonstoichiometric apatite crystals contain lattice-bound water which could play an important role in the formation of bone apatite. The similarity in morphology and composition between these needle-like crystals and the apatite crystals in bone provides a possibility to make a bone-like implant consisting of these needle-like crystals and collagen, etc.

Preparation and characterization of nanograde osteoapatite-like rod crystals

In this paper, nanograde osteoapatite-like rod crystals are made from wet synthesized calcium phosphate precipitates by hydrothermal treatment at 140°C under 0.3 MPa pressure for 2 h. The morphology, crystal structure, crystallinity and phase composition of these nanograde rod crystals are similar to those of thin apatite crystals in bony tissues of the body. This analogy provides an opportunity in the near future to build bone-like substitutes which consist of the nanograde rod crystals and special organic matrices and cells.

Covalent bonding of PMMA, PBMA, and poly(HEMA) to hydroxyapatite particles

In our earlier study, we showed that the surface hydroxyl groups of hydroxyapatite have the ability to react with organic isocyanate groups. In this study, the feasibility of grafting poly(methyl methacrylate) (PMMA), poly(n-butyl methacrylate) (PBMA), and Poly(hydroxyethyl methacrylate) [poly(HEMA)] by using the reaction of isocyanate groups with the hydroxyl groups on the surface of HA was investigated. Double bonds were introduced to the surface of HA via the coupling reaction of isocyanateoethyl methacrylate (ICEM) with HA, or through hexamethylene diisocyanate (HMDI) with hydroxyethyl methacrylate (HEMA) and HA, followed by radical polymerization in MMA, BMA, or HEMA. Infrared spectra indicated the existence of polymers on the surfaces of HA. Thermogravimetric analysis also confirmed the presence of grafted polymers on the surface of HA powder particles (20-26 wt%). The polymers gave typical PMMA, PBMA, or poly(HEMA) infrared spectra, with the exception of amide bands, a result of the coupling reaction of ICEM or HMDI with hydroxy groups of HA or HEMA. Therefore it is concluded that the polymers were chemically bonded to the surface of HA through the isocyanate groups of ICEM or HMDI.

Surface modification of hydroxyapatite to introduce interfacial bonding with polyactiveTM 70/30 in a biodegradable composite

A method was developed to improve the interfacial bonding between hydroxyapatite and a biodegradable copolymer PolyactiveTM 70/30. Hydroxyapatite was first surface modified by the polyelectrolytes polyacrylic acid or poly(ethylene-co-maleic acid) in aqueous solutions. Subsequently the surface-modified hydroxyapatite was used as filler in composites with PolyactiveTM 70/30. The strength, elongation at break and elastic modulus of the composite in aqueous environment were significantly improved by this method. Based on these experimental results, it is believed that the interface improvement is due to hydrogen bonding and/or dipole interactions formed between polyelectrolyte molecules and polyethylene glycol segments in the polymer matrix. Due to the introduction of interfacial bonding by using such method, a new biodegradable bone-bonding composite can be made.

A study on the grafting reaction of isocyanates with hydroxyapatite particles

The surface grafting reactions of a series of isocyanates with hydroxyapatite particles at different temperatures were studied by Infrared spectrophotometry (IR) and thermal gravimetric analysis (TGA). The study results show that both hexamethylene diisocyanate (HMDI) and isocyanatoethyl methacrylate (ICEM) react readily with HA while ethyl isocyanate acetate (EIA) and butyl isocyanate (BIC) have lower reactivity towards HA particles. It also has been found that the reaction of ICEM with HA follows a second-order reaction mechanism, despite the heterogeneous nature of the reaction, while the reaction of HMDI with HA does not due to the complexity of the reaction. Based on this study, it is concluded that ICEM and HMDI are suitable agents for the coupling of polymers due to their reactivity towards HA.