Production and characterization of bioactive lithium disilicate (Li2Si2O5) - wollastonite (CaSiO3) glass-ceramics

Glass-ceramics are polycrystalline materials obtained through controlled heat treatments of glass compositions suitable for crystallization. The final product may contain one or more crystalline phases precipitated within an amorphous matrix; however, the structure is not entirely crystalline, and the degree of crystallinity is generally in the range of 30-70 vol%. Among various glass-ceramic systems, lithium disilicate (Li2Si2O5) based glass-ceramics produced from the Li2O-SiO2 system have gained an increasingly wide range of applications in the field of dental restoratives due to their properties such as biocompatibility, chemical stability, machinability, high flexural strength, tooth-like translucency and appearance, and excellent color compatibility. Wollastonite (CaSiO3) is a calcium silicate mineral, chemically composed of approximately 51.75% SiO2 and 48.25% CaO by weight. Wollastonite stands out with its high whiteness, low coefficient of thermal expansion, low shrinkage, low dielectric constant and loss, and thermal stability. In recent years, wollastonite-based materials have been extensively investigated in the field of orthopedics owing to their biodegradability, excellent bioactivity, non-toxicity, and biocompatibility. Bioactivity is defined as the ability of biomaterials to support the formation of a hydroxyapatite layer in physiological fluids. The structure and composition of this layer closely resemble to natural apatite in bone. The presence of such a layer prevents the formation of fibrous tissue that could hinder the integration between the material and the host tissue. Glass-ceramic materials, combining the formability of glass with the superior mechanical and biological properties of crystalline materials, offer significant potential in dental restorations, bone reconstruction and regeneration in orthopedic applications, bone filling materials, and dental implantology. Although comprehensive studies exist in the literature on the development of bioactive glass-ceramics, investigations on lithium disilicate-based systems in the context of biological activity have remained relatively limited. Lithium disilicate based glass-ceramics have long been successfully employed in dental applications due to their high mechanical strength, machinability, and aesthetic properties. Therefore, imparting bioactive properties to these materials through the incorporation of wollastonite phase carries remarkable potential for the development of new-generation glass-ceramics that can establish biological interactions with surrounding tissues in addition to their existing superior mechanical performance. This study aimed to produce bioactive lithium disilicate-wollastonite-based glass-ceramics from the Li2O-SiO2-CaO-Al2O3-K2O-P2O5 system. For this purpose, four base glasses, coded as LSW-A (80 mol% lithium disilicate, 20 mol% wollastonite), LSW-B (70 mol% lithium disilicate, 30 mol% wollastonite), LSW-C (60 mol% lithium disilicate, 40 mol% wollastonite), and LSW-D (50 mol% lithium disilicate, 50 mol% wollastonite), with compositions calculated to provide the targeted lithium disilicate and wollastonite phase contents to be precipitated in the glass-ceramics, were produced by melting followed by water quenching. The glass frits were subsequently crushed, milled and sieved to obtain powders with a particle size below 45 µm, and X-ray diffraction (XRD) analyses confirmed that they were completely amorphous. In order to determine the heat treatment conditions required for the glass-to-glass-ceramic transformation and to investigate crystallization kinetics, Differential Thermal Analysis (DTA) was performed on the glass powders at heating rates of 10, 15, 20, and 25°C min–1. Glass powder compacts obtained by uniaxial hydraulic pressing were subsequently subjected to a four-stage heat treatment schedule based on the characteristic temperatures determined from the DTA curves, thereby yielding LSW-coded glass-ceramics. The crystalline phases present in the produced glass-ceramic specimens were identified by XRD analysis, while the microstructural features of these specimens, as well as the morphology and elemental compositions of the crystals precipitated in their glassy matrix, were examined in detail using scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). Furthermore, the apparent density was determined by the Archimedes method, while the mechanical properties were evaluated by microhardness measurements and three-point bending tests. Bioactivity tests were carried out in vitro by immersing the specimens in simulated body fluid (SBF) for 3, 7, 14 and 28 days, and their bioactivity after immersion was characterized by thin-film XRD, SEM-EDS analyses, pH measurements and mass change analysis. In the crystallization kinetics study, the activation energy values calculated from the first crystallization peaks attrributed to the crystallization of the lithium metasilicate phase were in the range of 231-302 kJ mol–1 for the LSW glasses, depending on the method used, and exhibited a gradual increase from LSW-A to LSW-D. The crystallization activation energy of the lithium disilicate phase was calculated to be in the range of 304-430 kJ mol–1 for the LSW-A, LSW-B, and LSW-C glasses. The crystallization activation energy of the wollastonite phase could be determined only for the LSW-C and LSW-D glasses and was in the range of 211-256 kJ mol–1. XRD analyses performed after sintering reveal that in LSW glass-ceramics prepared with different compositions and sintering conditions, lithium disilicate and wollastonite are the main crystalline phases precipitated from the glassy matrix. However, in the LSW-A glass-ceramics, in addition to these two phases, the lithium metasilicate phase, which is the precursor of the lithium disilicate, was also found to remain present in the structure at low sintering temperatures. For the LSW-B and LSW-C glass-ceramics, only lithium disilicate and wollastonite phases were identified under all sintering parameters. For the LSW-D specimens sintered at 750°C, diffraction peaks belonging to the α-quartz and lithium metasilicate phases were also detected in addition to these two phases. Increasing the sintering temperature and duration led to the dissolution of these phases in the structure and to the lithium disilicate and wollastonite crystalline phases becoming predominant. SEM analyses revealed the side-by-side growth of lithium disilicate and wollastonite crystals, while increasing sintering temperature and duration led to an enlargement of crystal sizes. In general, the lithium disilicate crystals precipitated within the microstructures of the LSW glass-ceramics exhibited a rod-like morphology and formed an interlocked structure. The apparent densities of the produced glass-ceramic samples were determined to be in the range of 2.09-2.55 g/cm3, while their porosity values were found to vary between 1.0% and 13.80%. In LSW-coded glass-ceramics, increasing porosity was accompanied by a decrease in apparent density. This behavior was attributed to the inability of the glassy phase to fill the pores that developed and grew within the microstructure by a viscous flow mechanism, despite the increase in crystal size as crystallization progressed. The microhardness values of the LSW glass-ceramic specimens ranged from 514 to 708 HV0.05 and were significantly influenced by the amount of residual porosity in the samples after sintering. The highest average hardness value was obtained for the LSW-B specimen sintered at 750°C for 5 h. Three-point bending test results showed that the flexural strength of the glass-ceramic specimens ranged from 77 to 219 MPa, and that both the morphology of the precipitated crystals in the microstructure and the aspect ratio of the lithium disilicate crystals had a significant influence on the flexural strength. The highest flexural strength was measured for the LSW-C specimen sintered at 800°C for 1 h. Bioactivity tests carried out in simulated body fluid (SBF) demonstrated that, after 28 days of immersion, all LSW-A and LSW-D glass-ceramic specimens developed a characteristic cauliflower-like surface morphology consisting of spherical agglomerates of nanoscale worm-like crystals, indicative of hydroxyapatite formation. In contrast, after 28 days no Ca-P layer was observed on the surfaces of LSW-B specimens sintered at 750 or 800°C, nor on those of LSW-C specimens sintered at 750°C, whereas hydroxyapatite formation was detected for the same compositions under the other sintering conditions. Thin-film XRD analyses performed on the surfaces of the bioactive specimens revealed distinct diffraction peaks at 2θ=25.87° and 31.97°, corresponding to the (002) and (211) crystallographic planes of the hydroxyapatite phase, confirming that these formations observed on the surface were hydroxyapatite. pH analyses of the test solutions revealed a rapid increase in pH during the first three days, which was attributed to ion leaching from the specimen surfaces into the solution. As the test duration increased, the rate of increase in the solution pH of the bioactive samples decreased, although the pH continued to rise. In contrast, the pH values of non-bioactive samples remained stable after the third day, exhibiting a plateau behavior. Bioactivity tests conducted in SBF over 28 days revealed that bioactive glass-ceramic samples exhibited a net mass gain after 28 days, while all non-bioactive samples experienced a net mass loss throughout the test period. This mass increase observed for the bioactive specimens was attributed to the rapid Ca-P enrichment of the material surface, leading to the formation of a hydroxyapatite layer and the subsequent thickening of this layer over time.