Uncoated Hydroxyapatite Research Articles

Electroless tin coated hydroxyapatite reinforced Mg–Sn alloy composite for enhanced bio corrosion resistance and bioactivity

The present study focuses on the influence of tin coated hydroxyapatite (HA) particles reinforced in Mg-5 wt.% Sn matrix on corrosion resistance. Electroless coating process has been used to coat hydroxyapatite particles with tin. From the results, it was understood that the electroless coating is a potential technique to successfully coat Sn on HA particles which helps in refinement of grains and uniform distribution of phases. It also prevents the dissociation of HA, thereby formation of unwanted phases such as CaSn which could be detrimental for corrosion properties due to galvanic coupling. The biodegradation was reduced by the addition of Sn coated HA in comparison with reinforcement of uncoated HA particles.

Morphogen Delivery by Osteoconductive Nanoparticles Instructs Stromal Cell Spheroid Phenotype.

Mesenchymal stem/stromal cells (MSCs) exhibit a rapid loss in osteogenic phenotype upon removal of osteoinductive cues, as commonly occurs during transplantation. Osteogenic differentiation can be more effectively but not fully maintained by aggregating MSCs into spheroids. Therefore, the development of effective strategies that prolong the efficacy of inductive growth factors would be advantageous for advancing cell-based therapies. To address this challenge, osteoinductive bone morphogenetic protein-2 (BMP-2) was adsorbed to osteoconductive hydroxyapatite (HA) nanoparticles for incorporation into MSC spheroids. MSC induction was evaluated in osteogenic conditions and retention of the osteogenic phenotype in the absence of other osteogenic cues. HA was more uniformly incorporated into spheroids at lower concentrations, while BMP-2 dosage was dependent upon initial morphogen concentration. MSC spheroids containing BMP-2-loaded HA nanoparticles exhibited greater alkaline phosphatase (ALP) activity and more uniform spatial expression of osteocalcin compared to spheroids with uncoated HA nanoparticles. Spheroids cultured in media containing soluble BMP-2 demonstrated differentiation only at the spheroid periphery. Furthermore, the osteogenic phenotype of MSC spheroids was better retained with BMP-2-laden HA upon the removal of soluble osteogenic cues. These findings represent a promising strategy for simultaneous delivery of osteoconductive and osteoinductive signals for enhancing MSC participation in bone formation.

Effect of alginate on the properties of hydroxyapatite scaffold and cell migration assay study

Abstract This study investigates the effect of alginate concentration and immersion time towards the mechanical properties of hydroxyapatite (HAp) scaffold. Sodium alginate was extracted via neutral extraction from local brown seaweed, Sargassum polycystum. HAp scaffolds were produced from polymer reticulate method and were sintered at 1250°C. HAp scaffolds were immersed in sodium alginate solution at 1%, 3% and 5% concentration under vacuum condition and immersion time of 1, 6, 12 and 24 hours respectively. FTIR spectra confirms the present of functional groups similar to commercially available sodium alginate. Compression test on the HAp scaffolds of different concentration and immersion time shows a significant increase in compression strength of the scaffold of 86.9 kPa for 24 hours of immersion time with 5% concentration of sodium alginate compared to 2.8 kPa of uncoated HAp scaffold. Morphology observation on the surface of HAp scaffolds reveal that the microcracks on the struts were covered by alginate and the coverage was improved as immersion time increased. The increased in compressive strength of scaffold is due to the filled microcrack and struts by sodium alginate. In vitro migration assay revealed that HAp coated alginate enhanced cell migration whereby the cells were able to migrate to fully close the wound however, no significant effect was observed for different alginate concentration used.

Integrated molecular design of melt-processable bioresorbable engineering nanocomposites for healthcare (BENcH)

Event Abstract Back to Event Integrated molecular design of melt-processable bioresorbable engineering nanocomposites for healthcare (BENcH) Andrew J. Parsons1*, Kirsty Walton1, Fredric Hild1, Alexander Ilchev1, Miquel Gimeno-Fabra1, Magdalena M. Tomczynska1, Michael Ward1, Alessia Canciani1, Gabriel Y. Choong1, Davide S. De Focatiis1*, Edward H. Lester1*, David M. Grant1* and Derek J. Irvine1* 1 The University of Nottingham, Faculty of Engineering, United Kingdom Introduction: The aim of the BENcH project is to develop novel, integrated methodologies for the design and scalable manufacture of next generation resorbable polymer nanocomposites. The long term objectives are the production of highly tailored health-care materials in applications ranging from bone fracture fixation to drug delivery. Novel plate-shaped hydroxyapatite (HA) nanoparticles have been simultaneously formed and coated with specifically designed molecular dispersants using a hydrothermal process, to produce a feedstock for compounding with medical grade polylactic acid (PLA). The end-products are formed using traditional plastics processing techniques and their properties investigated both post-moulding and during degradation. Close contact with industrial partners has ensured that all the processes in use are commercially scalable and a demonstrator product is under development. Materials and Methods: Ring-opening polymerisation has produced a series of low molecular weight dispersant/coupling agents. These have functional head groups linking to the surface of the HA nanoparticles and PLA chain structure tails to interact with bulk PLA[1]. Dispersants are applied to the surface of the nanoparticles during a one-step hydrothermal synthesis process to produce coated feedstock[2], processed using a micro-compounder and compression- or injection-moulded to form nanocomposite components[3][4]. Extensive characterisation of both the feedstocks and the compounded products was undertaken in terms of thermal, physical, mechanical, imaging and rheological studies[5][6]. Results and Discussion: The synthetic dispersants have been key to achieving not only improved dispersion of the nanoparticles in the polymer (fig. 1) but also to enhance the interaction of the polymer with the particles. Experimental data shows that the addition of nanoplates by themselves, or together with an uncoated dispersant, provide only limited improvements to material characteristics. Figure 1. TEM images showing dispersion of 2.5 w% uncoated HA platelets (top) and 2.5 wt% equivalent of dispersant coated HA platelets (bottom) in HA/PLA nanocomposites The addition of coated nanoHA increases the viscosity during compounding, suggesting that energy is being dissipated in nanoparticle dispersion. A custom micro-mechanical test rig has allowed measurement of increases in modulus and yield strength (creep resistance) in the nanocomposites. Dramatic effects have also been observed on the rate of water uptake with nanoparticles , allowing an added element of control to the degradation. Rheological studies have monitored molecular weight changes during processing as well as the effect of nanoparticles on material structure. Imaging techniques such as TEM and microCT have contributed to a greater understanding of the dispersion and of the process of degradation in PLA materials. Conclusion: The BENcH project has been successful in producing HA platelet/medical grade PLA nanocomposite materials. This was achieved through the use of bespoke dispersant materials in commercially applicable processes. The nanocomposites display desirable characteristics for a resorbable medical device and a prototype is in development. This work was supported by grant EP/J017272/1 from the UK Engineering and Physical Sciences Research Council; The authors would also like to acknowledge the valuable input of TESco Associates, Inc., Evonik Industries, ThermoFisher Scientific and Promethean Particles

PCL-coated hydroxyapatite scaffold derived from cuttlefish bone: In vitro cell culture studies

In the present study, we examined the potential of using highly porous poly(ε-caprolactone) (PCL)-coated hydroxyapatite (HAp) scaffold derived from cuttlefish bone for bone tissue engineering applications. The cell culture studies were performed in vitro with preosteoblastic MC3T3-E1 cells in static culture conditions. Comparisons were made with uncoated HAp scaffold. The attachment and spreading of preosteoblasts on scaffolds were observed by Live/Dead staining Kit. The cells grown on the HAp/PCL composite scaffold exhibited greater spreading than cells grown on the HAp scaffold. DNA quantification and scanning electron microscopy (SEM) confirmed a good proliferation of cells on the scaffolds. DNA content on the HAp/PCL scaffold was significantly higher compared to porous HAp scaffolds. The amount of collagen synthesis was determined using a hydroxyproline assay. The osteoblastic differentiation of the cells was evaluated by determining alkaline phosphatase (ALP) activity and collagen type I secretion. Furthermore, cell spreading and cell proliferation within scaffolds were observed using a fluorescence microscope.

Pamidronate Enhances Bacterial Adhesion to Bone Hydroxyapatite. Another Puzzle in the Pathology of Bisphosphonate-Related Osteonecrosis of the Jaw?

Bacterial colonization of the denuded bone in bisphosphonate-related osteonecrosis of the jaw suggests that bisphosphonates increase bacterial adhesion and biofilm formation. This study evaluated the adhesion of gram-positive and gram-negative bacteria on hydroxyapatite coated with pamidronate, one of the most potent bisphosphonates. Twenty-five Staphylococcus aureus and 25 Pseudomonas aeruginosa strains were cultured on pamidronate-coated and uncoated hydroxyapatite discs. After incubation, nonadherent bacteria were removed by rinsing and centrifugation. Formation of a biofilm was confirmed by confocal laser 3-dimensional and scanning electron microscopy. The number of bacterial colonies was counted using quantitative cultures and mean numbers were compared using the Mann-Whitney rank sum test (statistical significance defined as P ≤ .05). The Hartree-Fock method was used for the calculation of electron interactions between hydroxyapatite ions and pamidronate. Fold increases in the number of colonies formed by S aureus and P aeruginosa in the presence of pamidronate compared with controls were 7.19 ± 4.127 and 2.87 ± 0.622, respectively. Hartree-Fock analysis showed that the reactive NH3(+) group of pamidronate may act as a steric factor, facilitating anchoring of bacteria to the hydroxyapatite surface. Alternatively, the NH3(+) group may attract bacteria by direct electrostatic interaction. Increased bacterial adhesion in the presence of bisphosphonates can promote osteomyelitis in patients with bisphosphonate-related osteonecrosis of the jaw. There may be increased infection rates when bisphosphonates are used for stabilization of prostheses in joint arthroplasty and in osteotomies and open fractures in patients treated with bisphosphonates.

BMP‐2 tethered hydroxyapatite for bone tissue regeneration: Coating chemistry and osteoblast attachment

The goal of this study was to determine the effectiveness of using polyethyleneimine (PEI) and a polyethylene glycol (PEG) tether to bind human recombinant bone morphogenetic protein-2 (rhBMP-2) to hydroxyapatite (HAp) to enhance rhBMP-2 loading, alter its release properties, and enhance cellular interaction with the material. By using a branched PEI that was derived to express free thiols, rhBMP-2 was coated onto dense HAp surfaces at ~43 ng/cm(2). Using this novel attachment methodology, it was observed that the PEI-SH coating did not change the morphology of the HAp surfaces and that the amount of rhBMP-2 loaded was comparable to a direct adsorption method. In addition, it was also observed that the PEI and PEG tether significantly retained the rhBMP-2 to the HAp surface, inhibiting the burst release effect. Using human fetal osteoblast cells, the PEI- and PEG-tethered BMP-2 was also observed to increase cellular attachment by 10-fold when compared with uncoated HAp and adsorbed rhBMP-2. It was concluded from this study that PEI and PEG tether significantly reduce the initial burst release effect of rhBMP-2. It was also concluded that the rhBMP-2 conjugation to PEI and PEG tether promoted an increase in cellular attachment to the HAp surface.

A composite material model for improved bone formation

The combination of synthetic polymers and calcium phosphates represent an improvement in the development of scaffolds for bone-tissue regeneration. Ideally, these composites provide both mechanically and architecturally enhanced performances; however, they often lack properties such as osteoconductivity and cell bioactivation. In this study we attempted to generate a composite bone substitute maximizing the available osteoconductive surface for cell adhesion and activity. Highly porous scaffolds were prepared through a particulate leaching method, combining poly-ε-caprolactone (PCL) and hydroxyapatite (HA) particles, previously coated with a sucrose layer, to minimize their embedding by the polymer solution. Composite performances were evaluated both in vitro and in vivo. In PCL-sucrose-coated HA samples, the HA particles were almost completely exposed and physically distinct from the polymer mesh, while uncoated control samples showed ceramic granules massively covered by the polymer. In vivo results revealed a significant extent of bone deposition around all sucrose-coated HA granules, while only parts of the control uncoated HA granules were surrounded by bone matrix. These findings highlight the possibility of generating enhanced osteoconductive materials, basing the scaffold design on physiological and cellular concepts.

Evaluation of the Effect of Two Chlorhexidine Preparations on Biofilm Bacteria In Vitro: A Three-Dimensional Quantitative Analysis

Introduction Microorganisms are essential in the development of periradicular diseases and are the major causative factors associated with endodontic treatment failures. Microbial biofilms are communities of bacteria that attach to surfaces and form heterogeneous three-dimensional structures. The purpose of this study was to develop a biofilm model that closely mimicked in vivo biofilm and to determine its susceptibility to endodontic antimicrobial irrigants by three-dimensional quantitative analysis. Methods Collagen-coated hydroxyapatite (C-HA) and uncoated hydroxyapatite (HA) disks were inoculated with dispersed subgingival plaque for 3 weeks. Thick biofilms rich in spirochetes were formed on both substrates. Biofilms were subjected to 1-, 3-, and 10-minute exposures to CHX-Plus (Vista Dental Products, Racine, WI) and 2% chlorhexidine (CHX). After treatment, the volume ratio of dead bacteria to all bacteria in biofilms, indicated by the ratio of red and (red + green) fluorescence, was analyzed by confocal laser scanning microscopy for each medicament. Results The proportion of killed bacteria was depended on the type of irrigant and the time of exposure in both C-HA and HA biofilm models (p = 0.00). CHX-Plus showed higher levels of bactericidal activity at all exposure times than 2% CHX (p < 0.001). The C-HA biofilm was thicker than the HA biofilm. Less bacteria were killed in C-HA biofilm than in the HA model. Conclusions This multispecies biofilm model and quantitative analysis methodology may be useful for the evaluation of the antimicrobial effectiveness of endodontic disinfecting agents.

The effect of collagen I mimetic peptides on mesenchymal stem cell adhesion and differentiation, and on bone formation at hydroxyapatite surfaces

Integrin-binding peptides increase cell adhesion to naive hydroxyapatite (HA), however, in the body, HA becomes rapidly modified by protein adsorption. Previously we reported that, when combined with an adsorbed protein layer, RGD peptides interfered with cell adhesion to HA. In the current study we evaluated mesenchymal stem cell (MSC) interactions with HA disks coated with the collagen-mimetic peptides, DGEA, P15 and GFOGER. MSCs adhered equally well to disks coated with DGEA, P15, or collagen I, and all three substrates, but not GFOGER, supported greater cell adhesion than uncoated HA. When peptide-coated disks were overcoated with proteins from serum or the tibial microenvironment, collagen mimetics did not inhibit MSC adhesion, as was observed with RGD, however neither did they enhance adhesion. Given that activation of collagen-selective integrins stimulates osteoblastic differentiation, we monitored osteocalcin secretion and alkaline phosphatase activity from MSCs adherent to DGEA or P15-coated disks. Both of these osteoblastic markers were upregulated by DGEA and P15, in the presence and absence of differentiation-inducing media. Finally, bone formation on HA tibial implants was increased by the collagen mimetics. Collectively these results suggest that collagen-mimetic peptides improve osseointegration of HA, most probably by stimulating osteoblastic differentiation, rather than adhesion, of MSCs.

Enhancement of bone formation in hydroxyapatite implants by rhBMP‐2 coating

The combination of hydroxyapatite (HA) implants serving as osteoconductive scaffold with growth factors is an interesting approach for the improvement of bone defect healing. The purpose of this study was to test whether recombinant human bone morphogenetic protein-2 (rhBMP-2) coating of solid HA-implants improves bone formation in a cortical bone defect. Cylindrical trephine mill defects (diameter: 9.8 mm, depth: 10 mm) were created into the cortical tibia shaft of minipigs and subsequently filled either by plain HA cylinders (Endobon) or by rhBMP-2-coated HA cylinders. Fluorochrome labeling for the evaluation of time-dependent bone formation was done on days 8, 9, and 10 postsurgery with tetracyclin-100, at days 25 and 30 with alizarin-komplexon, and finally on days 32, 37, 73, and 79 with calcein green. Twelve weeks after implantation, the tibiae were harvested and were prepared for standard histological staining, fluorochrome analysis, and histomorphometry. Coating of HA implants with rhBMP-2 led to significant enhanced new bone formation of 84.7% (+/-4.6%) of the implant area with almost complete bony incorporation compared with only 27.7% (+/-8.5%) in the uncoated HA implants (p = 0.028). In both types of implants, osteoconduction of HA led to bone ingrowth of the surrounding host bone into the implants. However, only rhBMP-2-coated implants showed multitopic de novo bone formation reflecting the osteoinductive properties of rhBMP-2 in all areas of the HA implant. This study showed that the coating of HA ceramic implants with rhBMP-2 can significantly enhance new bone formation attributable to its osteoinductive effects.

Immediate Erosive Potential of Cola Drinks and Orange Juices

Little is known about the erosive potential of soft drinks within the first minutes of exposure to teeth, and about the potentially protective role of salivary proteins. We hypothesized that the erosive potential is determined primarily by pH and decreases in the presence of salivary proteins. To investigate this, we first added uncoated hydroxyapatite crystals and, second, salivary-protein-coated hydroxyapatite crystals to 20 commercially available cola drinks and orange juices simultaneously, with pH recordings every 15 sec for 3 min. The amount of apatite lost per liter of soft drink per sec was calculated from titratable acidity values to each pH obtained by crystal addition. The erosive potential within the first minutes of exposure was determined solely by the pH of the drink, and the erosive potential was ten-fold higher in cola drinks compared with juices. However, salivary proteins reduced the erosive potential of cola drinks by up to 50%.

The mechanical properties of calcium phospate ceramics modified by collagen coating and populated by osteoblasts

Bioceramics containing hydroxyapatite (HA), tricalcium phosphate (TCP) or composites which combine the best properties of both materials are among the principal candidates for bone replacement grafts. In this study we have investigated the mechanical strength of HA, TCP and composites of the two in the ratios 75;25 (H75), 50:50 (H50) and 25:75 (H25). The strength of each material was investigated in the presence and absence of collagen coating, and the influence of osteoblast culture for up to 28 days on strength was determined. TCP, H25 and H75 were significantly weakened by collagen coating, the strengths of the other materials were either not affected (HA) or increased (H50). Culture with osteoblasts significantly increased the strength of uncoated HA and H50, but this effect was not observed when the materials were coated with collagen. Our results indicate that ceramic composition affects the interactions between collagen coating, culture with osteoblasts and mechanical strength of the material. Although collagen coating has been found to increase the proliferation of osteoblasts into these ceramic materials, it may be necessary to stabilise and optimise the coating process to minimise effects on mechanical strength.

Effects of bone morphogenetic protein‐2 and hyaluronic acid on the osseointegration of hydroxyapatite‐coated implants: An experimental study in sheep

Research efforts aim at enhancing early osseointegration of cementless implants to improve early fixation and, thus, reduce the risk of loosening. The aim of the present study was to investigate whether bone morphogenetic protein (BMP) 2 had a positive effect on the osseointegration of hydroxyapatite-coated implants. Hydroxyapatite (HA) implants (perforated hollow cylinders and solid rods) were coated with BMP-2 and hyaluronic acid (HY) as the carrier or with HY alone. Uncoated HA implants served as controls. The osseointegration of the implants was evaluated either by light microscopy or by pullout tests after 1, 2, and 4 weeks of unloaded implantation in the cancellous bone of 24 sheep. The BMP-2 coating significantly increased bone growth into the implant perforations compared with HA-coated implants at 2 and 4 weeks. Bone-implant contact and interface shear strength of BMP-2 implants were lower than HA implants at 2 weeks. At 4 weeks, there was no significant difference in bone-implant contact and shear strength between BMP-2 and HA-coated implants. The BMP-2 coating enhanced gap healing but had no positive or even an inhibitory effect (at 2 weeks) on bone-implant contact and interface shear strength. In the clinical situation, a perfect press-fit implantation cannot be achieved, and BMP-2 may be beneficial for enhancing bone growth into gaps around cementless implants.

Bacterial adhesion to bisphosphonate coated hydroxyapatite

Staphylococcus aureus (S. aureus) is commonly associated with microbial infection of orthopaedic implants. Such infections often lead to osteomyelitis, which may result in failure of the implant due to localised bone destruction. Bacterial adhesion and subsequent colonisation of the device may occur as a consequence of contamination during surgery, or by seeding from a distant site through the blood circulation. Coating of the hydroxyapatite (HA) ceramic component of artificial hip joints with the bisphosphonates clodronate (C) and pamidronate (P) has been proposed as a means to minimise osteolysis and thereby prevent loosening of the implant. However, the effect of the bisphosphonate coating on bacterial adhesion to the HA materials must be determined before this approach can be implemented. In this study coated HA materials were incubated with the S. aureus and the number of adherent bacteria determined using the Modified Vortex Device (MVD) method. The number of bacteria adherent to the P coated HA material was significantly greater than that adherent to uncoated HA (60-fold increase) or to the C coated HA (90-fold increase). Therefore, even though earlier studies suggested that P bound to HA may improve osseointegration, the results presented would suggest that the use of this coating may be limited by the potential increased susceptibility of the coated device to infection.

Adsorption of serum fetuin to hydroxylapatite does not contribute to osteoblast phenotype modifications

Osteoblasts exhibit enhanced differentiation and altered gene profiles when cultured on hydroxyapatite (HA) compared to plastic surfaces. To begin determining mechanisms for this response, we used proteomics to identify proteins predominantly found in osteoblasts on HA but not plastic surfaces. Two-dimensional gel electrophoresis and Western analyses indicate that fetuin is abundant in extracts from HA, but not plastic surfaces. Incubation of HA and plastic surfaces with cell culture medium (containing 10% serum) under cell-free conditions shows that fetuin is predominantly derived from the culture medium serum and readily adsorbs to the HA surface. However, we did detect low levels of fetuin B mRNA in osteoblasts. Serum albumin, actin-beta, apolipoprotein-AI, and vimentin also adsorbed to HA. To determine the role of fetuin in the HA-induced osteoblast phenotype changes, osteoblasts were seeded onto fetuin-coated or uncoated HA under serum-free conditions. Osteoblast morphology was similar on both HA surfaces, suggesting that HA alone (without adsorbed serum proteins) is sufficient for cell attachment and spreading. Similarly, genes previously reported to be modulated by HA (glvr-1, DMP-1, osteoglycin, and proliferin 3) were modulated even in the absence of fetuin or other serum proteins. These data show that HA surface can be enriched selectively with fetuin from serum; however, neither fetuin or other serum proteins are required to mediate HA-induced osteoblast attachment, spreading, or changes in expression of genes examined. This finding suggests that factors intrinsic to HA are required for the response.

Bone ingrowth in bFGF-coated hydroxyapatite ceramic implants

This experimental study was performed to evaluate angiogenesis, bone formation, and bone ingrowth in response to osteoinductive implants of bovine-derived hydroxyapatite (HA) ceramics either uncoated or coated with basic fibroblast growth factor (bFGF) in miniature pigs. A cylindrical bone defect was created in both femur condyles of 24 miniature pigs using a saline coated trephine. Sixteen of the 48 defects were filled with HA cylinders coated with 50 μg rhbFG, uncoated HA cylinders, and with autogenous transplants, respectively. Fluorochrome labelled histological analysis, histomorphometry, and scanning electron microscopy were performed to study angiogenesis, bone formation and bone ingrowth. Complete bone ingrowth into bFGF-coated HA implants and autografts was seen after 34 days compared to 80 days in the uncoated HA group. Active ring-shaped areas of fluorochrome labelled bone deposition with dynamic bone remodelling were found in all cylinders. New vessels could be found in all cylinders. Histomorphometric analysis showed no difference in bone ingrowth over time between autogenous transplants and bFGF-coated HA implants. The current experimental study revealed comparable results of bFGF-coated HA implants and autogenous grafts regarding angiogenesis, bone synthesis and bone ingrowth.

Surface modification of hydroxyapatite. Part II. Silica

Nanophase hydroxyapatite (HAP) particles were coated with varying amounts of silica (5–75 wt%) via the hydrolysis of tetraethyl orthosilicate. The nanocomposite particles were characterized by transmission electron spectroscopy, X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), BET N 2 gas adsorption, sedimentation time studies, acid dissolution, and zeta potential ( ζ). A sequential decrease in infrared spectral features characteristic of HAP was accompanied by an increase in features characteristic of silica as revealed by DRIFTS. The specific surface area of the silica-coated HAP particles showed a non-systematic increase. In comparison to the uncoated HAP (50 m 2/g), silica coatings of 5, 25, 50 and 75 wt% yielded specific surface areas of 55, 93, 70, and 138 m 2/g, respectively. This behavior can be explained based on a heterocoagulation coating mechanism in which silica clusters of approximately 14 nm in diameter adsorb onto the HAP particle surface. The decrease in specific surface area at 50 wt% silica corresponded to the attainment of a complete surface coating. This conclusion was substantiated by the observed resistance of these particles to dissolution in 1 m HCl. However, the acid treatment transformed the silica-coated HAP core particles to CaCl 2·Ca(H 2PO 4) 2·2H 2O (calcium chloride phosphate hydrate) based on XRD analysis.

Effect of low-molecular-weight chitosans on the adhesive properties of oral streptococci.

It was previously shown that a low-molecular-weight chitosan and its derivatives N-carboxymethyl chitosan and imidazolyl chitosan inhibit Streptococcus mutans adsorption to hydroxyapatite. The ability of the same molecules to interfere with adhesive properties of other oral streptococci (Streptococcus sanguis, Streptococcus gordonii, Streptococcus constellatus, Streptococcus anginosus, Streptococcus intermedius, Streptococcus oralis, Streptococcus salivarius, Streptococcus vestibularis) was tested. When saliva-coated or -uncoated hydroxyapatite beads were treated with N-carboxymethyl chitosan, a reduction varying from 60% to 98% depending on strains was observed. Low-molecular-weight chitosans and imidazolyl chitosan did not have any effect. Growth in N-carboxymethyl chitosan-supplemented medium (final concentrations ranging from 20 to 500 micrograms.ml-1) caused a dose related reduction in the adsorption of all strains to hydroxyapatite and in their affinity towards xylene. No effect was observed with low-molecular-weight chitosans and imidazolyl chitosan. In contrast to what observed with S. mutans, the three polysaccharides did not affect detachment from hydroxyapatite beads and adherence to cheek epithelial cells of the other streptococci. These results suggest that low-molecular-weight chitosans and/or imidazolyl chitosan, selectively affecting S. mutans adsorption to hydroxyapatite, may be very interesting as potential anti-dental caries agents.

Dissolution of lysozyme-coated hydroxyapatite

The effect of the adsorption of chicken egg-white lysozyme, a basic protein, on the dissolution kinetics of synthetic hydroxyapatite (HAP) is studied at a constant pH of 5.0. It is confirmed that lysozyme adsorption occurs in spite of the positive net charge of the HAP surface and is favoured by high concentrations of phosphate ions. This implies that adsorption sites for the protein are phosphate ions adsorbed onto HAP. The dissolution kinetics performed with lysozyme-coated HAP in the presence of the protein in solution show no significant difference from kinetics obtained with uncoated HAP surfaces. This absence of effect is explained by considering the limited coverage of the HAP by the protein, whose low molecular weight, globular shape, rigidity and stability lead to a small number of anchoring points.