Key Engineering Materials Vols. 361-363

Abstract: Apatite-mullite glass-ceramics have been developed as an alternative to hydroxyapatite for use in vivo as a bioactive, osseoconductive biomedical alloy coating. In the cerammed state, they present a number of advantages including control over dissolution rates and mechanical properties by altering the composition of the parent glass or heat treatment regime. In the present study, a simple sedimentation route was used to coat a biomedical titanium alloy, commonly used for orthopaedic applications. The material was deposited as a glass and cerammed in situ to create a well adheared coating that resisted delamination or cracking. To investigate the nature of the coating-substrate reaction, a number of characterization techniques were used to examine the crystallization behaviour of the glass, the glass-ceramic microstructure, and the interfacial reaction region composition. The presence of products such as titanium silicides and unexpected pores are explained by proposed reaction routes between the titanium and glass coating.

Abstract: Lithium disilicate glass and glass with addition of P2O5, CaO and CaF2 (in relative ratios corresponding to 10 wt. % of „apparent fluoroapatite”) were prepared by quenching their melts obtained at 1400 °C. Then, the parent glasses were thermally treated at 550°C and 750°C to obtain glass ceramics. The bioactivity test in vitro and the Wicker hardness in relation to „apparent fluoroapatite“ presence and heat treatment were investigated. The presence of fluoroapatite in samples promotes mineralization of new phase apatite-like on the surface of glass and glass ceramics after 6-week immersion in SBF as proved by SEM and EPMA. The bioactivity decreases with thermal treatment of parent glass. However, whole surface of glass-ceramics was covered with apatite phase after long-term immersion in SBF. The Vickers hardness of samples increases with increasing temperature treatment and with the presence of fluoroapatite.

Abstract: Pulsed laser deposition was used to obtain functionally graded bioactive glass coatings on titanium substrates. An UV KrF* (λ=248 nm, τ>7 ns) excimer laser was used for the multi-pulse irradiation of the targets. The depositions were performed in oxygen while keeping substrate temperature at 400°C. We used sintered glass targets in the system SiO2-Na2O-K2O-CaO-MgOP2O5 that differed in SiO2 content, which was either 57 wt.% (6P57) or 61 wt.% (6P61). A glass 6P61 was used as the first layer in direct contact with the metallic substrate, while the outer bioactive layer was made of glass 6P57. Both the bioactive coatings and the bulk glasses were analyzed by Fourier transform infrared spectrometry (FTIR), grazing incidence X-ray diffraction (GIXRD), and scanning electron microscopy (SEM). The FTIR spectra of the glass powders and glass coatings showed the main vibration modes of the Si-O-Si groups. GIXRD analysis confirmed that the glass coatings had an amorphous structure. The SEM micrographs of the glass coatings showed the films to consist of droplets with diameters ranging from 0.2 to 5 μm. SEM was used to determine the rate of apatite formation on the coating when exposed to simulated body fluid (SBF) solution for 7 days. We demonstrated that pulsed laser deposition leads to good glass-metal adhesion on the substrate and well attached bioactive particles on the surface. We consider therefore this method appropriate for forming implants that can develop an apatite layer after immersion in SBF.

Abstract: Sodium-doped CPP was synthesized using three dopant sources (sodium carbonate, sodium hydroxide and sodium phosphate). These materials were analyzed by XRD to determine phase composition and by differential thermal analysis to identify phase transition temperatures. Sintering of resulting glass powders showed that both dopant source and dopant concentration affects sinter neck formation and crystallinity. The open porosity of sodium phosphate and sodium carbonate doped samples at 0.1 Na2O/CaO sintered at different temperatures changed significantly. Crystallization of the construct during sintering was noted at temperatures lower than expected.

Abstract: Hierarchically 3D porous bioactive glasses (BGs) with various combination of both pore sizes and pore structures have been produced by multi-polymer templating, such as amphiphilic block copolymers, poly urethane (PU) forms, poly styrene (PS) beads, or methyl cellulose (MC), sol-gel method, evaporation-induced self-assembly process, and rapid prototyping technique. The amphiphilic block copolymers used for producing the meso pores into the BGs, which induces large specific surface area and subsequently carries with good bone-forming bioactivity of BGs. Each poly urethane form, poly styrene bead, and methyl cellulose adapted for the fabrication of macro pores. The rapid prototyping (RP) techniques introduced to produce 3D BGs scaffolds with giant pores.

Abstract: Porous scaffolds have been developed in many forms and materials, but few have reached the combination of adequate physical, biological and mechanical properties. In previous works hybrid foams bioactive glass polyvinyl alcohol (PVA) were prepared by the sol-gel process for application as scaffold for bone tissue engineering. We observed that synthesis parameters such as PVA hydrolysis grade, PVA solution concentration, and PVA content in the hybrids affected both synthesis results and structural characteristics of the obtained foams. A marked change in foaming behavior occurs for PVA contents around 60%. In this work we analyze the effect of different compositions and synthesis parameters on the mechanical behavior of PVA-bioative glass foams. The compression tests showed that an increase of PVA fraction changes the mechanical behavior due to different mechanisms leading to cell collapse. For hybrids with lower PVA contents (20 to 30%) the cell collapse is due to brittle crushing. For intermediate polymer content (40-60%) the contribution of plastic yielding in the plateau region increases and it becomes the predominant mechanism of cell collapse for samples with higher polymer content (70-80%).

Abstract: The effect of composition on the reactivity of a calcium phosphate cement (CPC) made of tricalcium phosphate (TCP) – water mixtures was investigated by isothermal calorimetry at 37°C. The parameters of interest were the mean particle size of the powder, the use of small amounts of nanosized hydroxyapatite powder, and the phosphate concentration and the pH of the aqueous solution. The results could be well explained by theoretical considerations. The main parameter controlling CPC reactivity was TCP particle size.

Abstract: The first generation of synthetic bone substitute materials (BSM) was initially investigated in the mid 1970s using hydroxyapatite (HA) as a biomaterial for remodeling of bone defects. The concepts established by CPC pioneers in the early 1980s were used as a platform to initiate a second generation of BSM for commercialization. Since then, advances have been made in composition, performance and manufacturing. A self-setting and injectable calcium phosphate cement (CPC) based on amorphous calcium phosphate (ACP) with calcium to phosphate (Ca/P) atomic ratio less than 1.5, combined with dicalcium phosphate dihydrate (DCPD or brushite, seeded with apatite), is proposed. Amorphization of raw material was observed following high energy mechano-chemical processing. Upon hydration, the cement hardened in less than 3 minutes at 37°C and reached a maximum compressive strength of about 50 MPa. The final product was a low crystalline calcium deficient carbonated apatite similar to the composition and structure of bone mineral. In vivo performance of this cement in mediating bone healing was compared to α-BSM® in a rabbit femoral defect model. Performance characteristics of some commercially available CPC products are compared. The concerns of CPC designers and the needs of product users (surgeons) are discussed.

Abstract: The effect of mechanical mixing on compressive strength, relative porosity and reliability of strength data of a brushite forming cement at different powder to liquid ratios (PLRs) was investigated. Mean compressive strengths were measured, associated reliability (Weibull moduli) and survival probability distributions of the data sets were analysed. Relative porosities were determined using helium pycnometry. For low PLR (2.2g/ml), no significant differences in compressive strength were observed for either mechanical or hand mixed samples, although reliability of the former was significantly increased. At high PLR (3.4g/ml), mechanically mixed cements exhibited approximately twice the mean compressive strength compared with hand mixing, although Weibull moduli remained statistically similar. At medium PLR (2.8g/ml) strength and reliability of cements were similar and independent of mixing regime. For all PLRs, a significant decrease in porosity of mechanical- compared with hand-mixed cements was observed. Mechanical mixing of a brushite cement can provide lower porosity, increased reliability and higher strength.