Paul Ducheyne

Bioactive ceramics: the effect of surface reactivity on bone formation and bone cell function

Surface reactivity is one of the common characteristics of bone bioactive ceramics. It contributes to their bone bonding ability and their enhancing effect on bone tissue formation. During implantation, reactions occur at the material-tissue interface that lead to time-dependent changes in the surface characteristics of the implant material and the tissues at the interface. This review describes some of the current concepts regarding the surface reactivity of bone bioactive materials and its effect on attachment, proliferation, differentiation and mineralization of bone cells.

The effect of calcium phosphate ceramic composition and structure on in vitro behavior. II. Precipitation.

The formation of a biologically equivalent carbonate-containing apatite on the surface of synthetic calcium phosphate ceramics (CPC) may be an important step leading to bonding with bone. Reactions of several single phases CPCs upon immersion into a simulated physiologic solution (SPS) with an electrolyte composition of human plasma were determined. The CPCs covered a wide range of solution stabilities from low-soluble hydroxyapatites (HA) to metastable tricalcium phosphates (TCP) and tetracalcium phosphate (TTCP). Changes in chemical compositions of SPS and infrared spectral features after CPC immersion were analyzed. New phase formation was observed on all the CPCs. However, kinetics, compositions, and structures of the new phases were significantly different. The studied CPCs can be characterized by the time to new phase formation in vitro; the minimum time for measurable precipitate formation was found to increase in the order: not-well-crystallized HAs < well-crystallized HAs < alpha-TCP, TTCP < beta-TCP. Among the CPCs only not-well-crystallized HAs led to immediate new phase formation. The metastable CPCs, beta-TCP, alpha-TCP, and TTCP required an induction time during which dissolution occurred. beta-TCP showed the longest induction time and the lowest lattice ion uptake rate of all the CPCs tested. Only the not-well-crystallized HAs elicited immediate formation of carbonated HA. The well-crystallized HAs and beta-TCP did not elicit carbonated apatite formation within the time frame of the experiment. Instead, intermediate phases were formed. On alpha-TCP amorphous calcium phosphate (ACP) with a relatively low carbonate content was formed. TTCP was found to transform extensively to poorly crystallized carbonated apatite after 2 days of immersion.

Structural analysis of hydroxyapatite coatings on titanium

Hydroxyapatite from two sources was electrophoretically deposited onto flat titanium plate material. Depending upon the deposition conditions various changes in the structure of the ceramic were identified. A well-adhering Ti-P compound was present at the interface. Hydroxyapatite oxygenated to various degrees and tetracalcium phosphate were reproducibly formed in the coating.

Calcium phosphate ceramic coatings on porous titanium: effect of structure and composition on electrophoretic deposition, vacuum sintering and in vitro dissolution

Bioactive calcium phosphate ceramics (CPC) guide bone formation along their surface. This property is conceptually attractive from the viewpoint of enhancing early bone tissue formation in porous metal coatings. The various studies conducted to exploit this idea, however, reveal a considerable variability of the effect. This suggests material- and processing-induced parametric influences. Thus this study focuses on the formulation of model porous metal-CPC materials for use in one-parametric analyses of material factors. Easily reproducible, porous metals with a uniform porous structure and CPC coating are made with orderly oriented wire mesh (OOWM) porous metal coatings and electrophoretically deposited CPC films. The deposition of the ceramic can be hampered by adsorbed water. Subsequent vacuum sintering leads to several phase transformations: hydroxyapatite is transformed to a mixture of oxyhydroxyapatite and tetracalcium phosphate; the underlying titanium promotes the beta- to alpha-tricalcium phosphate transformation; and Ca-deficient hydroxyapatite is transformed to a mixture containing oxyhydroxyapatite and alpha- and beta-tricalcium phosphate. These phase transformations provoke a considerable increase of in vitro dissolution in 0.05 M tris buffered physiological solution.

Plasma spraying induced changes of calcium phosphate ceramic characteristics and the effect onin vitro stability

Plasma spraying is a commonly used technique to apply thin calcium phosphate ceramic coatings. Special consideration is given to retaining the original structure of CPC particles. However, changes are possible. Thus this study focused on plasma spraying induced changes in material characteristics of commercial coatings and their influence onin vitro dissolution. All analysed coatings were found to undergo significant plasma spraying induced changes in phase composition, crystal structure, and specific surface area. The phase transformations depended on the starting particle characteristics. Specifically, β-TCP transformed to α-TCP. HA was dehydroxylated and transformed to oxyhydroxyapatite (OHA), and partly decomposed to α-TCP and tetra calcium phosphate. These transformations lead to a considerable increase ofin vitro dissolution rates at physiological pH.

Hydration and preferential molecular adsorption on titanium in vitro.

Surface sensitive spectroscopies, Auger electron and X-ray photoelectron (XPS), were used to determine changes in titanium oxide composition, oxide stoichiometry, and adsorbed surface species as a function of exposure to human serum in a balanced electrolyte (serum/SIE) and 8.0 mM ethylenediaminetetraacetic acid in a balanced electrolyte (EDTA/SIE) at 37 degrees C. Before immersion, the oxide was near ideal TiO2, covered by two types of hydroxyl groups: acidic OH(s) with oxygens doubly coordinated to titanium, and basic Ti-OH groups singly coordinated. After extended exposure to both solutions, up to 5000 h (ca. 208 d), the surface concentration of OH groups increased and non-elemental P appeared. The P LVV Auger transition and P 2p spectra indicated the peak positions were similar to reference phosphate compounds. The adsorbed phosphate species were presumed to be either Ti-H2PO4 or Ti-HPO4-. The XPS data suggested that a lipoprotein and/or glycolipid film was adsorbed to the specimens exposed to serum/SIE. Analysis of the preferential lipoprotein/glycolipid adsorption using electrostatic bonding concepts contributed to the refinement of the hierarchical model for the Ti-tissue interface. The salient features are that Ti metal is not in direct contact with the biological milieu, rather there is a gradual transition from the bulk metal, near-stoichiometric oxide, Ca and P substituted hydrated oxide, adsorbed lipoproteins and glycolipids, proteoglycans, collagen filaments and bundles to cells.

Formation of surface reaction products on bioactive glass and their effects on the expression of the osteoblastic phenotype and the deposition of mineralized extracellular matrix

The objective of the study was to examine the effect of alkali ion release, pH control and buffer capacity on the expression of the osteoblastic phenotype. In addition we determined the importance of modifications of the surface of porous bioactive glass (BG) on the activity of rat calvaria osteoblasts in vitro. We found that at a low tissue culture medium (TCM) volume to BG surface area (Vol/SA) ratio, the products of glass corrosion elevated the pH of the TCM to a value that adversely affected cellular activity; thus, the matrix synthesized by the cells was non-mineralized. On the other hand, when the Vol/SA was high and the buffer capacity of the medium was not exceeded, the cells generated a mineralized extracellular matrix. Addressing the second issue, we observed that modification of the composition of the BG surface markedly influenced osteoblast activity. BG that was coated with either a calcium phosphate-rich layer only or a serum protein layer changed the phenotypic characteristics of the osteoblasts. The presence of either of these surfaces lowered the alkaline phosphatase activity of the attached cells; this finding indicated that the osteoblast phenotype was not conserved. However, when the BG was coated with a bilayer of calcium phosphate and serum proteins, the alkaline phosphatase (AP) activity was elevated and the extracellular matrix contained characteristic bone markers. Our findings indicate that the calcium phosphate-rich layer promotes adsorption and concentration of proteins from the TCM, and it is utilized by the osteoblasts to form the mineralized extracellular matrix.

Quantification of cell adhesion using a spinning disc device and application to surface-reactive materials

Quantitative analysis of cell adhesion is essential in understanding physiological phenomena and developing biotechnological applications. Electrochemical measurements demonstrated that the transport patterns associated with a spinning disc device approximate the fluid flow and mass transport fields for a disc spinning in an infinite fluid. Therefore, this device applies a linear range of forces to attached cells under uniform and constant chemical conditions at the interface. The application of this apparatus for examining cell adhesion to surface-active materials was illustrated by investigating the attachment of osteoblast-like cells to fibronectin adsorbed onto bioactive and non-reactive glasses for different chemical environments. Cells were seeded on fibronectin-coated substrates for 15 min and then subjected to detachment forces for 10 min. The number of adherent cells decreased non-linearly with applied force and the detachment profile was accurately described by a sigmoidal curve fit, as expected for a cell population with normally distributed adhesion properties.

The inhibition of Staphylococcus epidermidis biofilm formation by vancomycin-modified titanium alloy and implications for the treatment of periprosthetic infection.

Peri-prosthetic infections are notoriously difficult to treat as the biomaterial implant is ideal for bacterial adhesion and biofilm formation, resulting in decreased antibiotic sensitivity. Previously, we reported that vancomycin covalently attached to a Ti alloy surface (Vanc-Ti) could prevent bacterial colonization. Herein we examine the effect of this Vanc-Ti surface on Staphylococci epidermidis, a Gram-positive organism prevalent in orthopaedic infections. By direct colony counting and fluorescent visualization of live bacteria, S. epidermidis colonization was significantly inhibited on Vanc-Ti implants. In contrast, the gram-negative organism Escherichia coli readily colonized the Vanc-Ti rod, suggesting retention of antibiotic specificity. By histochemical and SEM analysis, Vanc-Ti prevented S. epidermidis biofilm formation, even in the presence of serum. Furthermore, when challenged multiple times with S. epidermidis, Vanc-Ti rods resisted bacterial colonization. Finally, when S. epidermidis was continuously cultured in the presence of Vanc-Ti, the bacteria maintained a Vanc sensitivity equivalent to the parent strain. These findings indicate that antibiotic derivatization of implants can result in a surface that can resist bacterial colonization. This technology holds great promise for the prevention and treatment of periprosthetic infections.

Calcium phosphate ceramic coatings as carriers of vancomycin.

Infection in the setting of total joint arthroplasty remains a challenging problem. Attention has turned to developing methods of local delivery of antibiotics for prophylaxis. Vancomycin loaded into calcium phosphate ceramic coatings on titanium alloy substrates is a clinically relevant concept in the setting of total joint arthroplasty. Drug loading was accomplished by immersion of ceramic-coated discs in vancomycin-containing simulated physiological solution; in some experiments drug loading by immersion was followed by lipid coating in egg phosphatidylcholine solutions. The kinetics of vancomycin release and the efficacy of drug inhibition of Staphylococcus aureus were determined in vitro in comparison to the release from currently used antibiotic-laden poly(methyl methacrylate) (PMMA). The loading by immersion provided effective release and inhibition at early time points (up to 24 h); however, the lipid-coated samples demonstrated significant release and effective bacterial inhibition up to 72 h. The two-step procedure, i.e. drug loading followed by lipid coating in order to slow antibiotic elution, is more effective than the conventional one-step loading. The study indicated that the osteoconductive calcium phosphate coatings have the potential to serve as drug carriers to prevent infection in the setting of total joint arthroplasty.