Julian R. Jones

Review of bioactive glass: From Hench to hybrids

Bioactive glasses are reported to be able to stimulate more bone regeneration than other bioactive ceramics but they lag behind other bioactive ceramics in terms of commercial success. Bioactive glass has not yet reached its potential but research activity is growing. This paper reviews the current state of the art, starting with current products and moving onto recent developments. Larry Henchs 45S5 Bioglass® was the first artificial material that was found to form a chemical bond with bone, launching the field of bioactive ceramics. In vivo studies have shown that bioactive glasses bond with bone more rapidly than other bioceramics, and in vitro studies indicate that their osteogenic properties are due to their dissolution products stimulating osteoprogenitor cells at the genetic level. However, calcium phosphates such as tricalcium phosphate and synthetic hydroxyapatite are more widely used in the clinic. Some of the reasons are commercial, but others are due to the scientific limitations of the original Bioglass 45S5. An example is that it is difficult to produce porous bioactive glass templates (scaffolds) for bone regeneration from Bioglass 45S5 because it crystallizes during sintering. Recently, this has been overcome by understanding how the glass composition can be tailored to prevent crystallization. The sintering problems can also be avoided by synthesizing sol-gel glass, where the silica network is assembled at room temperature. Process developments in foaming, solid freeform fabrication and nanofibre spinning have now allowed the production of porous bioactive glass scaffolds from both melt- and sol-gel-derived glasses. An ideal scaffold for bone regeneration would share load with bone. Bioceramics cannot do this when the bone defect is subjected to cyclic loads, as they are brittle. To overcome this, bioactive glass polymer hybrids are being synthesized that have the potential to be tough, with congruent degradation of the bioactive inorganic and the polymer components. Key to this is creating nanoscale interpenetrating networks, the organic and inorganic components of which have covalent coupling between them, which involves careful control of the chemistry of the sol-gel process. Bioactive nanoparticles can also now be synthesized and their fate tracked as they are internalized in cells. This paper reviews the main developments in the field of bioactive glass and its variants, covering the importance of control of hierarchical structure, synthesis, processing and cellular response in the quest for new regenerative synthetic bone grafts. The paper takes the reader from Henchs Bioglass 45S5 to new hybrid materials that have tailorable mechanical properties and degradation rates.

Differentiation of fetal osteoblasts and formation of mineralized bone nodules by 45S5 Bioglass?? conditioned medium in the absence of osteogenic supplements

Bioactive glasses bond strongly to bone in vivo and their ionic dissolution products have previously been shown to have stimulatory properties on adult and fetal osteoblasts and to induce the differentiation of embryonic stem cells towards the osteoblastic lineage in vitro. In the present study, the effect of 45S5 Bioglass conditioned medium with two different Si concentrations (15 microg/ml (BGCM/15) and 20 microg/ml (BGCM/20)) on human fetal osteoblast growth, differentiation and extracellular matrix production and mineralization was investigated. In the first instance, primary fetal osteoblasts were examined for the osteoblast phenotypic markers alkaline phosphatase (ALP), collagen type I (Col I) and OB Cadherin (Cadherin 11) (OB Cad) as well as for the mesenchymal stem cell markers CD105 and CD166. At passage 0 more than 50% of the population was positive for Col I and ALP, but at passage 2, the proportion of cells expressing ALP increased. In addition at passage 0 more than 50% of the fetal osteoblasts expressed the mesenchymal stem cell surface markers CD105 and CD166. Treatment with BGCM/15 and BGCM/20 in the absence of osteogenic supplements increased the gene expression of the bone extracellular matrix proteins alkaline phosphatase, osteonectin and bone sialoprotein as determined by quantitative real time reverse transcriptase-polymerase chain reaction (rt RT-PCR) analysis. Extracellular matrix production was also enhanced in the absence of osteogenic supplements by the 45S5 Bioglass conditioned medium as demonstrated by ALP enzymatic activity, osteocalcin and Col I protein synthesis. Furthermore, BGCM/15 and BGCM/20 significantly enhanced the formation of mineralized nodules, based on alizarin red histochemical staining, without necessitating the addition of beta-glycerophosphate, l-ascorbate-2-phosphate or dexamethasone (commonly used osteogenic supplements).

Nanostructure evolution and calcium distribution in sol–gel derived bioactive glass

Sol–gel derived bioactive glasses (70 mol% SiO2 and 30 mol% CaO) have high potential as materials for bone regeneration and devices for sustained drug delivery. They bond to bone and have a controllable degradation rate. They have a unique tailorable nanoporosity, which enhances their surface area and exposes hydroxyl groups and affects protein adsorption and cellular response. This study aims to fully characterise the evolution of the nanoporous structure of sol–gel derived bioactive glass for the first time, to fully understand its nanostructure evolution and control, so that materials with specific nanoporous networks can be produced to further enhance effects on tissue regeneration. It was confirmed that nanopores of sol–gel derived bioactive glass are interstitial spaces between nanoparticles. Nanoparticles, approximately 5 nm in diameter that were produced early in the process, agglomerated into larger particles during the gelation process (10–30 nm in diameter) during stabilisation via heat treatment. Inductive coupled plasma (ICP) analysis of the pore liquor after ageing revealed that calcium nitrate (the calcium precursor) dissolves in pore liquor before drying. Thermal real time X-ray diffraction and MAS-NMR data revealed that calcium nitrate coated the silica nanoparticles during drying and calcium did not enter the silica network until the material was heated to 400 °C. This has implications for ensuring a homogeneous calcium distribution in bioactive glasses made by the sol–gel route.

Hierarchical porous materials for tissue engineering

Biological organisms have evolved to produce hierarchical three-dimensional structures with dimensions ranging from nanometres to metres. Replicating these complex living hierarchical structures for the purpose of repair or replacement of degenerating tissues is one of the great challenges of chemistry, physics, biology and materials science. This paper describes how the use of hierarchical porous materials in tissue engineering applications has the potential to shift treatments from tissue replacement to tissue regeneration. The criteria that a porous material must fulfil to be considered ideal for bone tissue engineering applications are listed. Bioactive glass foam scaffolds have the potential to fulfil all the criteria, as they have a hierarchical porous structure similar to that of trabecular bone, they can bond to bone and soft tissue and they release silicon and calcium ions that have been found to up-regulate seven families of genes in osteogenic cells. Their hierarchical structure can be tailored for the required rate of tissue bonding, resorption and delivery of dissolution products. This paper describes how the structure and properties of the scaffolds are being optimized with respect to cell response and that tissue culture techniques must be optimized to enable growth of new bone in vitro.

Spherical bioactive glass particles and their interaction with human mesenchymal stem cells in vitro

Sub-micron particles of bioactive glass (SMBGs) with composition 85 mol% SiO(2) and 15 mol% CaO were synthesised and characterised. Bioactivity was demonstrated by the formation of calcium apatite following 5 days immersion in simulated body fluid (SBF). The effect of a 24 h exposure of SMBGs (100 μg/ml, 150 μg/ml, 200 μg/ml) to human mesenchymal stem cells (hMSCs) on cell viability, metabolic activity and proliferation were determined using the LIVE/DEAD, MTT, total DNA and LDH assays after 1, 4 and 7 days of culture. None of the SMBG concentrations caused significant cytotoxicity at 1 and 4 days, but the doses of 150 and 200 μg/ml significantly decreased hMSC metabolic activity after 7 days of culture. Cell proliferation decreased as SMBG concentration increased; however none of the SMBGs tested had a significant effect on DNA quantity compared to the control. Confocal microscopy confirmed cellular uptake and localisation of the SMBGs in the hMSC cytoskeleton. Transmission electron microscopy revealed that the SMBGs localised inside the cell cytoplasm and cell endosomes. These findings are important for assessing the toxicity of sub-micron particles that may either be used as injectables for bone regeneration or generated by wear or degradation of bioactive glass scaffolds.

Bioactive glass and hybrid scaffolds prepared by sol–gel method for bone tissue engineering

Abstract Tissue engineering is an important technique for regenerating diseased or damaged tissues. In tissue engineering, a highly porous artificial extracellular matrix or scaffold is required to accommodate cells and guide their growth and tissue regeneration in three dimensions. The choice of scaffolding material is crucial to the success of the technique. Bioactive glasses are an option as scaffold material for bone tissue engineering owing to their recognised osteoconductive and osteoinductive properties and controllable degradation rate. Resorbable 3D macroporous bioactive scaffolds have been produced by foaming sol–gel derived bioactive glasses with the aid of a surfactant. The foams exhibit a hierarchical structure, with interconnected macropores (10–600 μm) and mesopores (2–50 nm). The effects of processing variables on the structure and properties of the obtained bioactive glass foams are discussed in the present paper. The method is then applied to produce bioactive glass–polymer (polyvinyl alcohol) hybrid scaffolds with improved mechanical properties.

Isothermal grain coarsening of spray formed alloys in the semi-solid state

Grain coarsening of alloys in the semi-solid state is important in controlling grain sizes in spray forming and in other processes in which an alloy is formed or cast in the semi-solid state. Until now, this coarsening has been analysed in terms of classic LSW theory or in terms of the migration of grain boundary liquid films. These analyses suggest that the coarsening rate should increase with increasing solid fraction, fS, but this suggestion conflicts with previous experimental results which show that coarsening rate decreases with increasing fS for fS>0.7. This paper shows, for the first time, that coarsening rate does increase with fS for fS less than approximately 0.75 in agreement with the liquid film migration model, and then decreases again with further increasing fS for fS greater than approximately 0.75. A modified model of liquid film migration is proposed which takes into account the reducing area of the liquid film as fS increases for high fS. The formation of intragranular liquid droplets, and the pinning of grain boundary liquid films by dispersoids during coarsening are also discussed.

Effect of surfactant concentration and composition on the structure and properties of sol-gel-derived bioactive glass foam scaffolds for tissue engineering

Bioactive glasses are known to have the ability to regenerate bone, and to release ionic biological stimuli that promote bone cell proliferation by gene activation, but their use has been restricted mainly to the form of powder, granules or small monoliths. Resorbable 3D macroporous bioactive scaffolds have been produced for tissue engineering applications by foaming sol-gel-derived bioactive glasses. The foams exhibit a hierarchical structure, with interconnected macropores (10–500 μm), which provide the potential for tissue ingrowth and mesopores (2–50 nm), which enhance bioactivity and release of ionic products. The macroporous matrices were produced by the foaming of sol-gel glasses with the use of a surfactant. Three glass systems SiO2, SiO2-CaO and SiO2-CaO-P2O5were foamed using various concentrations of surfactant, in order to investigate the effect of surfactant concentration and composition on the structure and properties of the hierarchical construct.

Melt-derived bioactive glass scaffolds produced by a gel-cast foaming technique

Porous melt-derived bioactive glass scaffolds with interconnected pore networks suitable for bone regeneration were produced without the glass crystallizing. ICIE 16 (49.46% SiO(2), 36.27% CaO, 6.6% Na(2)O, 1.07% P(2)O(5) and 6.6% K(2)O, in mol.%) was used as it is a composition designed not to crystallize during sintering. Glass powder was made into porous scaffolds by using the gel-cast foaming technique. All variables in the process were investigated systematically to devise an optimal process. Interconnect size was quantified using mercury porosimetry and X-ray microtomography (μCT). The reagents, their relative quantities and thermal processing protocols were all critical to obtain a successful scaffold. Particularly important were particle size (a modal size of 8 μm was optimal); water and catalyst content; initiator vitality and content; as well as the thermal processing protocol. Once an optimal process was chosen, the scaffolds were tested in simulated body fluid (SBF) solution. Amorphous calcium phosphate formed in 8h and crystallized hydroxycarbonate apatite (HCA) formed in 3 days. The compressive strength was approximately 2 MPa for a mean interconnect size of 140 μm between the pores with a mean diameter of 379 μm, which is thought to be a suitable porous network for vascularized bone regeneration. This material has the potential to bond to bone more rapidly and stimulate more bone growth than current porous artificial bone grafts.

Bioactive Glass Scaffolds for Bone Regeneration

There is a need for new materials that can stimulate the bodys own regenerative mechanisms and heal tissues. Porous templates (scaffolds) are thought to be required for three-dimensional tissue growth. This article discusses bone regeneration and the specifications of an ideal scaffold and the materials that may be suitable. Bioactive glasses have high potential as scaffold materials as they stimulate bone cells to produce new bone, they are degradable in the body and they bond to bone. The two types of bioactive glasses, their mechanisms for bioactivity and their potential for scaffold production are reviewed. Examples of their current clinical use are highlighted.