Heimo Ylänen

Influence of the non-bridging oxygen groups on the bioactivity of silicate glasses

The effect of the composition and bonding configuration of the bioactive silica-based glasses on the initial stage in vitro bioactivity is presented. Information of the IR active Si–O groups of glass in the system SiO2–P2O5–CaO–Na2O–K2O–MgO–B2O3 was obtained by fourier transform Infrared (FTIR) spectroscopy. Two different bands associated to non-bridging oxygen stretching vibrations (Si–O–1NBO and Si–O–2NBO) and a gradual shifting of the bridging oxygen stretching vibration (Si–O) have been observed and evaluated. Both effects are attributed to a decrease of the local symmetry originating from the incorporation of alkali ions into the vitreous silica network. The Si–O–NBO(s)/Si–O(s) absorbance intensity ratio increases with a gradual incorporation of the alkali ions (diminution of SiO2 content) following a linear dependence up to values close to 50 wt % of SiO2. In vitro test analysis by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA) showed a correlation between the amount and type of the non-bridging oxygen functional groups and the growth of the silica-rich and CaP layers. It was found that a minimum concentration of Si–O–NBO bonds in the glass network is required in order to have an efficient ion exchange and dissolution of the silica network. Finally, the bioactivity of the glass is favored by the presence of the Si–O–2NBO groups in the glassy network. The role of these functional groups in the dissolution of the silica network through the formation of silanol groups and the adsorption of water is discussed.

Silica-based bioactive glasses modulate expression of bone morphogenetic protein-2 mRNA in Saos-2 osteoblasts in vitro

A chemical exchange of the silica gel layer forming on the surface of bioactive glasses is thought to be the principal reaction for bone-bioactive glass bonding. The contribution of biological molecules on cell-bioactive glass interaction is largely unknown. To further analyze the mechanisms involved in efficient bone bonding to bioactive glass, Saos-2 osteoblastic cells with proven osteogenic phenotype were cultured for 4, 7 and 14 days on two bioactive glasses with different Si contents. Culture plates and dishes made of bioactive (BAG, 53 % SiO2), biocompatible (BCG, 58% SiO2) and control (GO) glasses were extensively conditioned with phosphate buffer and DMEM medium before seeding the cells. Northern hybridization was used for analysis of mRNA levels of collagen type I (Col-I), alkaline phosphatase (ALP) and bone morphogenetic protein-2 (BMP-2). A significant increase was observed in Col-I mRNA levels in cells grown on the two bioactive glasses when compared with those grown on controls at 4 and 7 days (p < 0.04). The mRNA level for ALP in the cultures of bioactive glasses-made plates and dishes was also increased over control at 7 days (p < 0.02) and remained this way between BAG and G0 at 14 days. Striking differences in BMP-2 mRNA levels existed between BAG and G0 plates and dishes at 7 days (p < 0.05). BMP-2 mRNA level in BAG group was higher than in BCG group at 4, 7 and 14 days, but without statistical significance. Saos-2 osteoblastic cells with strong ALP staining were mostly seen on BAG plates under a light microscope. In confocal microscopy, a bright FITC-stained F-actin ring was present in the cytoplasm of cells grown on BAG dish, demonstrating an active functional status. Stimulation of the expression of BMP-2 and other bone mRNAs by bioactive glasses in osteoblastic cells suggests biological involvement of bone related growth factors, peptides and cytokines in bone-bioactive glass bonding.

Antibacterial effects and dissolution behavior of six bioactive glasses

Dissolution behavior of six bioactive glasses was correlated with the antibacterial effects of the same glasses against sixteen clinically important bacterial species. Powdered glasses (<45 microm) were immersed in simulated body fluid (SBF) for 48 h. The pH in the solution inside the glass powder was measured in situ with a microelectrode. After 2, 4, 27, and 48 h, the pH and concentration of ions after removing the particles and mixing the SBF were measured with a normal glass pH electrode and ICP-OES. The bacteria were cultured in broth with the glass powder for up to 4 days, after which the viability of the bacteria was determined. The antibacterial effect of the glasses increased with increasing pH and concentration of alkali ions and thus with increased dissolution tendency of the glasses, but it also depended on the bacterium type. The changes in the concentrations of Si, Ca, Mg, P, and B ions in SBF did not show statistically significant influence on the antibacterial property. Bioactive glasses showed strong antibacterial effects for a wide selection of aerobic bacteria at a high sample concentration (100 mg/mL). The antibacterial effects increased with glass concentration and a concentration of 50 mg/mL (SA/V 185 cm(-1)) was required to generate the bactericidal effects. Understanding the dissolution mechanisms of bioactive glasses is essential when assessing their antibacterial effects.

Effect of immersion in SBF on porous bioactive bodies made by sintering bioactive glass microspheres

Abstract Since the mid 90s it has been possible to draw fibers and manufacture microspheres of novel bioactive glasses. Thus, by sintering bioactive glass microspheres it is now possible to form porous textures, in which the bioactive surface area is increased manifold compared with non-porous bodies. Four different types of porous bodies were made by sintering glass microspheres of diameter 250–300 μm. Two of the body types contained only one kind of spheres; either highly bioactive glass spheres or spheres made of glass having a low bioactivity (biocompatible glass). Two additional types of test bodies were obtained by sintering mixtures of bioactive and biocompatible spheres (composites). The dissolution of silica and calcium into simulated body fluid (SBF) was determined at different time intervals using a direct current plasma atomic emission spectrometer (DCPAES). The influence of immersion on the mechanical strength of the porous structures was studied by means of a compression test. Further, the thickness of the silica-rich gel formation on the surface of bioactive glass spheres was measured at each time interval using back-scattered electron imaging of scanning electron microscopy (BEI-SEM). A non-porous glass rod made from the same bioactive glass was used as the control. The results showed that dissolution of silica and calcium into SBF from the porous glass texture was inversely related to the silica content of the glass. The rate of silica gel formation on the sintered bioactive microspheres was significantly higher than on a rod made from the same glass. The initial mechanical strength of porous bodies consisting of only one kind of glass was 17–20 MPa. However, these bodies lost their mechanical strength at an early stage of the immersion showing compression strength of only 7–8 MPa at 14 days of immersion. The initial strength of composite glass bodies (7–11 MPa) was lower compared with bodies containing only one kind of glass but the bodies showed no notable mechanical weakening during the test. Softening of the surface of smooth bioactive glass plates correlated well with the formation of the silica-rich layer on the plate. Interestingly, the study also showed that in porous glass structures containing both bioactive and biocompatible glass the biocompatible glass can act as a site for calcium phosphate precipitation.

Characterisation of bioactive glass coatings on titanium substrates produced using a CO2 laser.

Titanium and its alloys are widely used in load-bearing bioinert implants. Bioactive glasses (BAGs) form a chemical bond with bone, but they are not suitable for load-bearing applications. Creating a BAG coating on a titanium implant could combine the best properties of both materials. The results tend to be poor when conventional firing methods are applied to coat titanium with BAG. A local application of heat to melt the glass can be achieved by a CO2 laser. A new method is introduced to create BAG coatings on titanium locally in a controlled manner, with a focused CO2 laser beam. The coatings produced by this method precipitate calcium phosphate in vitro. Processing parameters (number of coated layers, laser power, and processing atmosphere) providing a firm attachment of the glass and good in vitro bioactivity were identified. XRD analysis showed no crystallisation of the glass due to processing with the laser. EDXA indicated the formation of a calcium phosphate layer, which FTIR suggested to be a hydroxyapatite. The results show CO2 laser processing to be a promising technique for the manufacture of 30–40 μm BAG coatings on titanium.

Creation of microrough surface on sintered bioactive glass microspheres

Bioactive glasses are surface-active, generally silica-based, synthetic materials that form a firm chemical bond to bone. The aim of this study was to further enhance the bioactivity of glasses by creating a microroughness on their surface. Microroughness increases potential surface area for cell attachment and biomaterial-cell interactions. Three bioactive glasses of different composition were studied. Each material was flame-sprayed into microspheres, and a selected fraction of the spheres (250-300 microm) was sintered to form porous bioactive glass specimens. To create microrough surfaces, different acid etching techniques were tested. Atomic force microscopy (AFM) and back-scattered electron imaging of scanning electron microscopy (BEI-SEM) were used to characterize surface roughness. The degree of roughness was measured by AFM. A novel chemical-etching method, developed through intensive screening of different options, was found consistently to create the desired microroughness, with an average roughness value (R(a)) of 0.35-0.52 microm and a root mean-square roughness value (R(rms)) of 0.42-0.64 microm. Microroughening of the glass surface was obtained even in the internal parts of the porous glass matrices. Measured by BEI-SEM, the etching of a bioactive glass surface did not interfere with the formation of the characteristic surface reactions of bioactive glasses. This was confirmed by immersing the etched and control glass bodies in a simulated body fluid and tris(hydroxymethyl) aminomethane/HCl. The etching process did not significantly affect the mechanical strength of the sintered bioactive glass structures. Based on these experiments, it seems possible to create a reproducible microroughness of appropriate size on the surface of porous bioactive glass. The biologic benefits of such a surface treatment need to be validated with in vivo experiments.

Porous bone implants

Porous conical implants made by sintering micro-spheres made from bioactive and bio-compatible glasses have been tested mechanically as well as in vivo by inserting the implant through the cortical bone into the bone marrow. The behaviour is compared to a reference implant made by sintering micro-spheres of metallic titanium. Due to capillary forces the implant pores were filled with bone marrow fluid when inserted. An extended bone ingrowth occurred in the cones of bioactive glass, in the titanium ones only in the cortical area. Further, the factors influencing the formation of a chemical bond between glass and living tissue are discussed.

Factors Controlling Antibacterial Properties of Bioactive Glasses

Factors controlling the antibacterial ability of three bioactive glasses were studied by comparing the changes in the SBF during immersion of the glasses with their response to four microorganisms. After immersion of 100 mg/ml fine powder (<45μm) of the glasses in the SBF for 1, 2, 4, 8, 16, 27 and 48 hours, the immersion solutions were filtered and the pH in the bulk solution was measured. Ionic concentrations of Na, K, Ca, Mg, P and Si ions in the immersion solutions were determined by ICP-OES. The antibacterial activity of the glasses showed good correlation with the changes of the pH values in the SBF solutions. No correlation was found between the ionic concentration and the antibacterial ability of the three glasses against the four tested microorganisms. The results suggest that, the antimicrobial effect of the glass powder against the microorganisms tested is mostly dependent on the increase of the pH in the solution to values detrimental for the bacteria growth.

Influence of the Network Modifier Content on the Bioactivity of Silicate Glasses

The influence of the substitution of calcium oxide for sodium oxide in the composition of silica-based glasses on the in vitro bioactivity is presented. Valuable information on the active Si-O groups present in the glasses is obtained by Fourier Transform Ra man and Infrared spectroscopies. In vitro test analysis by Scanning Electron Microscopy and Energy Dis persive X-ray Analysis show a correlation between the network disruption induced by the modifier type and the bioactive process. It is demonstrated that glasses with high SiO 2 content can be bioactive depending on the alkali/alkali-earth modifiers ratio included into the vitreous silica network. Introduction Bioactive glasses are interesting materials for medical pur poses due to their ability to bond chemically to living bone and soft tissues when soaked in physiologica l fluids [1, 2]. The clinical applications of bioactive glasses are numerous, especially in maxi llofacial reconstruction, otorhinolaryngology, oral surgery and periodontal repair [1]. The bioactive silica-based glass network is basically the same of vitreous silica, where the structural units consist of slightly distorted SiO 4 tetrahedra. This structure enables the accommodation of alkali and alkali-earth cations which create non-bridging oxygen s it s (Si-O-NBO) throughout the glass network [3]. When the bioactive glasses are soaked in human plasma or an analogous solution, it is known that a partial dissolution of the glass surface oc urs leading to the formation of a silica-rich gel layer and, subsequently, the precipitation of a c alcium phosphate film on the bioactive material takes place. The formation rate of this lay er is a critical parameter, which is directly related to the type and content of the network modifiers [1]. Several spectroscopic techniques, such as Infrared and Raman spectrosc opies, are sensitive to changes in the composition and the bonding configuration of the glasses, and they provide valuable information on the local structure of the silicate glasses. Thus, the aim of this work is to evaluate, through spectroscopic tec hniques and in vitro tests, the role of content and type of network modifiers on the bioactivity of silica-based gla sses. Materials and methods Glasses of different compositions in the quaternary system Na 2O-C O-P2O5-SiO2, with a systematic substitution of CaO and Na 2O concentrations, have been investigated (Table 1). Glasses were obtained by melting the appropriate quantities of analytical grade CaCO3, Na2CO3, CaHPO4·2H2O and commercial Belgian quartz sand in a Pt-crucible at 1360 oC for 3 h. The glasses were cast, annealed, crushed, and remelted to improve homogeneity. In the final casting, a graphite mould of 20 mm diameter and 100 mm long was used. The test pieces were obtai ned by sawing discs of 2 mm thick. The discs were washed and stored in ethanol. The structure of the glasses was studied by X-ray Diffract ion (XRD). Information on the active SiO groups present in the glasses was obtained by Fourier Transfor m Infrared (FTIR) and Raman spectroscopies. Key Engineering Materials Online: 2003-12-15 ISSN: 1662-9795, Vols. 254-256, pp 23-26 doi:10.4028/www.scientific.net/KEM.254-256.23

Comparison of Antibacterial Effect of Three Bioactive Glasses

Antibacterial effects of three powdered bioactive glasses were compared by measuring the pH of the simulated body fluid in immediate contact with the glass powder particles and by cultivating four microorganisms in the powder-solution mixtures. After individual cultivation with the microorganisms the bioactive glasses showed ability to kill bacteria, but the effects were very dependent on glass composition and on the microorganism as well as on cultivation time. The results indicated that antimicrobial property of bioactive glasses correlates with their ability to change the pH of the body fluid in immediate contact with the glass. However, the increase in pH is not the only factor influencing the antimicrobial properties.