Ceramics used for the repair and reconstruction of diseased or damaged parts of the musculo-skeletal system, termed bioceramics, may be bioinert (alumina, zirconia), resorbable (tricalcium phosphate), bioactive (hydroxyapatite, bioactive glasses, and glass-ceramics), or porous for tissue ingrowth (hydroxyapatite-coated metals, alumina). Applications include replacements for hips, knees, teeth, tendons, and ligaments and repair for periodontal disease, maxillofacial reconstruction, augmentation and stabilization of the jaw bone, spinal fusion, and bone fillers after tumor surgery. Carbon coatings are thromboresistant and are used for prosthetic heart valves. The mechanisms of tissue bonding to bioactive ceramics are beginning to be understood, which can result in the molecular design of bioceramics for interfacial bonding with hard and soft tissues. Composites are being developed with high toughness and elastic modulus match with bone. Therapeutic treatment of cancer has been achieved by localized delivery of radioactive isotopes via glass beads. Development of standard test methods for prediction of long-term (20-year) mechanical reliability under load is still needed.

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Bioceramics: From Concept to Clinic

Bioceramics : from concept to clinic

High-strength bioactive glass-ceramic A-W was soaked in various acellular aqueous solutions different in ion concentrations and pH. After soaking for 7 and 30 days, surface structural changes of the glass-ceramic were investigated by means of Fourier transform infrared reflection spectroscopy, thin-film x-ray diffraction, and scanning electronmicroscopic observations, in comparison with in vivo surface structural changes. So-called Tris buffer solution, pure water buffered with trishydroxymethyl-aminomethane, which had been used by various workers as a "simulated body fluid," did not reproduce the in vivo surface structural changes, i.e., apatite formation on the surface. A solution, ion concentrations and pH of which are almost equal to those of the human blood plasma--i.e., Na+ 142.0, K+ 5.0, Mg2+ 1.5, Ca2+ 2.5, Cl- 148.8, HCO3- 4.2 and PO4(2-) 1.0 mM and buffered at pH 7.25 with the trishydroxymethyl-aminomethane--most precisely reproduced in vivo surface structure change. This shows that careful selection of simulated body fluid is required for in vitro experiments. The results also support the concept that the apatite phase on the surface of glass-ceramic A-W is formed by a chemical reaction of the glass-ceramic with the Ca2+, HPO4(2-), and OH- ions in the body fluid.

Solutions able to reproduce in vivo surface-structure changes in bioactive glass-ceramic A-W.

Biodegradable and bioactive porous polymer/inorganic composite scaffolds for bone tissue engineering

Historically the function of biomaterials has been to replace diseased or damaged tissues. First generation biomaterials were selected to be as bio-inert as possible and thereby minimize formation of scar tissue at the interface with host tissues. Bioactive glasses were discovered in 1969 and provided for the first time an alternative; second generation, interfacial bonding of an implant with host tissues. Tissue regeneration and repair using the gene activation properties of Bioglass® provide a third generation of biomaterials. This article reviews the 40 year history of the development of bioactive glasses, with emphasis on the first composition, 45S5 Bioglass®, that has been in clinical use since 1985. The steps of discovery, characterization, in vivo and in vitro evaluation, clinical studies and product development are summarized along with the technology transfer processes.

The story of Bioglass

Bioactive glass powders, with a composition of SiO2-CaO-P2O5,have been successfully synthesized via a sol-gel process at considerably loer temperatures than required for conventional melting method. Bioactive glass powders made via conventional methods form an interfacial bond wih none when they are implanted. Bonding is correlated with the formation of a surface hydroxyapatite layer. This study examinded the formation of a hydroxyapatite layer in Tris-buffered solution as a function of SiO2 content of sol-gel derived powders. A FT-IRRS technique was used to monitor the formation of the hydroxyapatite on the surface of the powders. X-ray diffraction analysis and BET were also used to characterize the chemical and physical properties of the sol-gel derived bioactive powders. It was discovered that: (a) the rate of hydroxyapatite formation decreased wih increasing SiO2 content for powders whose SiO2 content was less than 90 mol%; (b) a hydroxyapatite film does not form for the powders whose SiO2 content is more than 90 mol%; (c) the SiO2 limit, beyond which the powders lost their bioactivity, was much higher for bioactive glass powders mad through sol-gel process (90%) than those made by conventional melting methods (60%). These results indicate that it is possible to significantly expand the bioactive composition range through microstructural control made possible by sol-gel processing techniques.

An investigation of bioactive glass powders by sol‐gel processing

When soluble salts are used in the sol-gel processing of bioactive glasses, the resulting materials are not homogeneous. To produce homogeneous gel-glasses, a sol-gel method using all alkoxide precursors was developed. In this work the in vitro bioactivity of the new all-alkoxide gel-glasses is investigated. Three compositions in the system, CaO-P2O5-SiO2, were studied. Solid samples were soaked in Tris buffer and simulated body fluid (SBF), and the rate of formation of hydroxy carbonate apatite (HCA) on the surface of the glasses was measured by Fourier transform infrared spectroscopy. The solutions were analyzed before and after reaction by inductively coupled plasma. All compositions studied formed a hydroxy carbonate apatite (HCA) layer within 8 h in both test solutions. The HCA layer grew rapidly when SBF was used.

Calcium phosphate formation on sol‐gel‐derived bioactive glasses in vitro

Bioglasses with and without fluoride have been shown to be surface-active biomaterials. In the present in-vitro work, apatite formation on the surface of fluoride Bioglass® and non-fluoride Bioglass® is studied at the early stages of reaction. Fourier transform infrared reflection spectroscopy (FT-IRRS) analysis is used to monitor apatite-layer and silica-rich-layer formation on Bioglass® surfaces after they are reacted in a tris-buffer solution. Ion concentrations in the reacted solutions, and elemental depth profiles are obtained by induced coupled plasma atomic emission spectrometry (ICP) and Auger electron spectroscopy (AES) respectively. At a very early stage of reaction, the formation of SiO bonds with one non-bridging oxygen and SiOSi bridging bonds produced a silica-rich layer. Simultaneously, an amorphous calcium-phosphate layer forms and crystallizes after 2 h of reaction. Fluorine ions are incorporated within the apatite crystals in the fluoride Bioglass®. The release rates for Na and Ca ions are higher for fluoride Bioglass®. The formation of HPO 4 2− ions was found in fluoride Bioglass® reaction layers, but not in the non-fluorid Bioglass® when reacted up to 120 h.

Early stages of calcium-phosphate layer formation in bioglasses

An apatite layer can be formed on pure gel-silica soaked in simulated body fluid. The rate of formation depends on solution parameters and sintering temperature of the gelsilica. In this study, the effect of the texture of the gel-silica on the rate of hydroxyapatite formation was investigated. The apatite formation was monitored by means of Fourier-transform infrared reflection spectroscopy as well as by the measurement of changes in the ion concentration of the fluid. The induction time for apatite nucleation on the gel silica decreased as pore size and pore volume increased. The substrate parameters that affect nucleation are discussed and a mechanism that assumes pores as nucleation sites for hydroxyapatite is proposed.

Effect of Texture on the Rate of Hydroxyapatite Formation on Gel-Silica Surface

A theory is proposed stating that an ideal implant material must have a dynamic surface chemistry that induces histological changes at the implant interface which would normally occur if the implant were not present. Evidence for the validity of this theory is provided with a series of bone-implant studies which result in stable interfacial osteogenesis under specific implant surface chemistry conditions. Insufficient or excess surface ion concentrations produce negative osteogenesis and fixation results. Implantation of osteogenic implants in soft tissues also produces undesirable histological responses as proposed in the theory. A variety of surface chemical analyses of the implant are reviewed which provide a scientific basis for the implant surface theory.

Arthur E. Clark

Papers

An investigation of bioactive glass powders by sol‐gel processing

Calcium phosphate formation on sol‐gel‐derived bioactive glasses in vitro

Early stages of calcium-phosphate layer formation in bioglasses

Effect of Texture on the Rate of Hydroxyapatite Formation on Gel-Silica Surface

The influence of surface chemistry on implant interface histology: a theoretical basis for implant materials selection.