Shulamith Radin

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.

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.

Sol-gel derived carrier for the controlled release of proteins.

Sol-gel derived porous silica carriers for the controlled release of proteins were synthesized using a room temperature process. The materials are intended to serve as both substrates for bone growth as well as to allow incorporated proteins such as growth factors to diffuse out and stimulate cell function and tissue healing. The data document that the in vitro release of trypsin inhibitor, a model protein of size similar to growth factors with documented effect in bone, was dose and time dependent during immersion up to nine weeks. The release pattern included an initially slow release, with further release occurring at a rate which is proportionate to the square root of time, an indicative of a diffusion-driven process.

In vitro release kinetics of biologically active transforming growth factor-β1 from a novel porous glass carrier

Sol-gel silica-based porous glass (xerogel) was used as a novel carrier material for recombinant human transforming growth factor-beta 1 (TGF-beta 1). Room temperature synthesis procedures included sol preparation, the addition of TGF-beta 1 solution to the sol, subsequent gelation and drying. After determination of optimal synthesis parameters, the material was assayed in vitro for its ability to release biologically active TGF-beta 1 in a controlled manner. Sustained release of TGF-beta 1 over a 7-day period was demonstrated. On the basis of published TGF-beta 1 potency, the amount released is capable of eliciting bone tissue reactivity. These findings suggest that this novel glass-growth factor composite may serve as an effective bone graft material for the repair of osseous defects.

Silica sol-gel for the controlled release of antibiotics. I. Synthesis, characterization, and in vitro release

Room temperature processed silica sol-gel (xerogel) was investigated as a novel controlled release carrier of antibiotics (vancomycin). Xerogel characteristics, in vitro release properties, and bactericidal efficacy of the released antibiotic were determined. The xerogel/vancomycin composite showed a long-term sustained release (up to 6 weeks). In addition, bactericidal efficacy of released vancomycin was retained. The kinetics of release and the amount released were dose dependent. The initial, first-order release was followed by a near-zero-order release. The time to transition from the first- to zero-order release increased with vancomycin load (from 2 to 3 weeks with load increase from 2.2 to 11.1 mg/g). Regardless of the load, about 70% of the original vancomycin content was released by the transitional point, and the cumulative release after 6 weeks of immersion was about 90%. This study, combined with other reports documenting biocompatibility and controlled resorbability of the xerogel/drug composite in vivo, suggests that silica xerogel is a promising controlled release material for the treatment of bone infections.

Silica sol-gel for the controlled release of antibiotics. II. The effect of synthesis parameters on the in vitro release kinetics of vancomycin

Room temperature-processed silica sol-gel (xerogel) was investigated as a novel controlled release carrier of vancomycin for the treatment of osteomyelitis. Vancomycin-loaded xerogels were fabricated with varying water/alkoxysilane molar ratios and vancomycin concentrations. The goal of this study was to determine the effect of varying the aforementioned synthesis parameters on the daily in vitro release kinetics of vancomycin from the xerogel disks. A controlled, load-dependent, long-term release of vancomycin was observed for all of the molar ratios that were used in the study (4, 6, and 10). Variations in the water/alkoxysilane molar ratio affected the release process extensively. A cumulative release of about 90% of the original amount of vancomycin was found for molar ratios 6 and 10 by 21 and 14 days, respectively. Only about 30% was released from xerogels with a molar ratio of 4 after 21 days of immersion. A first-order release stage was followed by a steady release stage for xerogels with molar ratios of 6 and 10, whereas zero-order release was observed for xerogels with a molar ratio of 4. The findings of this study indicate that the release kinetics of vancomycin from xerogel can be tailored by varying the xerogel synthesis parameters.

Si-Ca-P xerogels and bone morphogenetic protein act synergistically on rat stromal marrow cell differentiation in vitro.

This study describes a novel bioactive xerogel glass as a carrier for bone morphogenetic protein (BMP) and the value of this carrier in terms of stimulating osteogenic activity of rat stromal marrow cells in vitro. These cells were seeded onto the surface of xerogel glass disks with BMP either incorporated in the glass, adsorbed to the surface of the glass, or added to the culture media and then compared to cells on glass with no added BMP or to cells on tissue culture plastic (TCP) with and without BMP. Cells were cultured for 6 and 10 days and examined for total DNA, alkaline phosphatase activity, and osteocalcin and total protein production. Stromal cell differentiation, as measured by alkaline phosphatase activity and osteocalcin synthesis was most increased when the BMP was incorporated or adsorbed onto the xerogel glass. Cells on xerogel glass without BMP were more differentiated than cells grown on plastic with BMP, thereby demonstrating the additive effect of a bioactive substrate and BMP on osteoblastic cell differentiation. These data indicate that xerogel glass effects differentiation of cells with osteogenic potential and that it can serve as a delivery vehicle for BMP.

Titanium nitride coatings on surgical titanium alloys produced by a powder immersion reaction assisted coating method: residual stresses and fretting behavior

Abstract Titanium and Ti–6Al–4V alloy samples were coated using a Powder Immersion Reaction Assisted Coating (PIRAC) nitriding method in order to modify their surface properties. Depending on the processing temperature, strongly adherent single(TiN)- or double(Ti2N/TiN)-layer coatings were obtained on both substrates. Several characteristics of PIRAC-coated Ti alloys relevant to their applications in total joint replacements were studied. Residual stresses in PIRAC coatings measured by sin2ψ X-ray diffraction method were found to be compressive in nature and were significantly lower than those reported for PVD TiN layers on similar substrates. In vitro fretting tests of PIRAC nitrided Ti–6Al–4V-to-Ti–6Al–4V couples simulating in vivo conditions at the interface of modular orthopedic implants demonstrated a major reduction in fretted areas, as well as a remarkable reduction of the corrosion potential drop at the initial stages of fretting as compared to the uncoated alloy. In addition, a 25% reduction of fretting-induced dissolved Ti ions concentration in testing solution was measured by EAAS. The results of the research suggest that titanium nitride PIRAC coatings can provide surgical titanium alloys with the longed-for fretting wear and corrosion resistant surface thereby minimizing the ion- and particulate-generating potential of modular orthopedic implants.

In vitro transformation of bioactive glass granules into Ca-P shells

Bioactive glass (BG) granules of narrow size are excavated when implanted in mandibular bone of beagles. Bone tissue forms within these internally hollowed particles without a connection to the bone at the margins of the defect. In this study the internal excavation of BG granules was simulated by in vitro immersion experiments. Postimmersion solutions were analyzed for changes in Si, Ca, and P concentrations. Using scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and Fourier Transform Infrared (FTIR) spectroscopy, granules were analyzed for compositional, morphologic, and structural changes resulting from immersion. Only when the solution was continuously replenished and only if this solution was composed of electrolyte- and protein-containing serum was excavation achieved. Without solution replenishment, that is, under so-called integral immersion conditions, the solution quickly became saturated in silicon, and the silicon no longer dissolved. When the glass was immersed in a solution with serum, a porous surface structure with fine precipitates was formed, in contrast to a dense surface reaction layer with closely packed globular precipitates that was formed in a solution without serum. The combined effect of continuous solution replenishment and the use of a solution containing serum proteins led to the formation of a surface reaction layer that did not impede continued corrosion. As such, all Si was released, and eventually a hollow Ca-P shell was formed. Thus this study supports the hypothesis that there is a physico-chemical mechanism of Si transport through the Ca-P-rich layer followed by Si dissolution. This mechanism may be operative in vivo and thereby may contribute to the observed in vivo excavation.

Initial events at the bioactive glass surface in contact with protein‐containing solutions

Upon implantation, bioactive glass undergoes a series of reactions that leads to the formation of a calcium phosphate-rich layer. Most in vitro studies of the changes that occur on the surface of bioactive glass have employed the use of buffer solutions with compositions reflecting the ionic composition of interstitial fluid. Although these studies have documented the physical and chemical changes associated with bioactive glass immersed in aqueous media, they do not reveal the effect of serum proteins and cells that are present at the implantation site. In the present study, we document, using atomic force microscopy (AFM) and Rutherford backscattering spectrometry (RBS), significant differences in the reaction layer composition, thickness, morphology, and kinetics of formation arising from the presence of serum proteins. The data reveal that the uniform and rapid adsorption of serum proteins on the surface may serve to protect the surface from further direct interaction with the aqueous media, slowing down the transformation reactions. This finding is in agreement with previous studies that have shown that the presence of serum proteins significantly delays the formation of hydroxyapatite at the surface of bioactive glass. These data also support the hypothesis that initial reaction layers in vivo interact with cells in order to produce the tissue-bioactive glass interface typically observed on ex vivo specimens.