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Bioglass is a commercially available family of bioactive glasses, composed of SiO2, Na2O, CaO and P2O5 in specific proportions. The proportions differ from the traditional soda-lime glasses in low amount of silica (less than 60 mol.%), high amount of sodium and calcium, and high calcium/phosphorus ratio.〔''(Biomaterials and tissue engineering )'' by Donglu Shi p. 27, Springer, 2004 ISBN 3-540-22203-0〕 High ratio of calcium to phosphorus promotes formation of apatite crystals; calcium and silica ions can act as crystallization nuclei.〔''(The chemistry of medical and dental materials )'' by John W. Nicholson, p. 92, Royal Society of Chemistry, 2002 ISBN 0-85404-572-4〕 Bioglasses have different formulations. Some bind to soft tissues and bone (e.g. 45S5), some only to bone (e.g. 5S4.3 or Ceravital), some do not form a bond at all and after implantation get encapsulated with nonadhering fibrous tissue, and others are completely resorbed within few weeks. Fine powders resorb faster than bulk materials. A thin layer of apatite forms on the glass-tissue interface, facilitating strong bond to the bone. Some formulations can facilitate growth of osteoblasts through the material.〔 Generally, there are four classes of bioglasses:〔 *35-60 mol.% SiO2, 10-50 mol.% CaO, 5-40 mol.% Na2O: bioactive, bonds to bone, some formulations bond to soft tissues *<35 mol.% SiO2: non glass-forming *>50 mol.% SiO2, <10 mol.% CaO, <35 mol.% Na2O: bioactive, resorption within 10–30 days *>65 mol.% SiO2: non-bioactive, nearly inert, gets encapsulated with fibrous tissue Some CaO can be replaced with MgO and some Na2O with K2O without much effect to bone bonding. Some CaO can be replaced with CaF2 without altering bone bonding, this however modifies the dissolution rate of the glass. B2O3 or Al2O3 may be added for easier material processing, however these influence the bone bonding; alumina inhibits bonding and its content is therefore restricted to small levels of about 1-1.5%.〔 Phosphate-free glasses also exhibit bioactivity. The role of the phosphate is only in aiding of nucleation of apatite on the surface; phosphate ions adsorbed from the organism itself can play the same role.〔 Bioglasses are divided to two categories:〔''(Processing and Fabrication of Advanced Materials, XVII: Part 8: Polymer-based composites and nano composites )'' Volume 2 of ''Processing and Fabrication of Advanced Materials'', XVII, N. Bhatnagar, ISBN 81-907770-2-5〕 *Class A bioglasses are osteoproductive. They bind with both soft tissues and bone. The HCA layer forms within several hours. *Class B bioglasses are osteoconductive. Bond to soft tissues is not facilitated. The HCA layer takes one to several days to form. ==Bioglass 45S5== Bioglass 45S5, one of the most important formulations, is composed of SiO2, Na2O, CaO and P2O5. Professor Larry Hench developed Bioglass at the University of Florida in the late 1960s. He was challenged by a MASH army officer to develop a material to help regenerate bone, as many Vietnam war veterans suffered badly from bone damage, such that most of them injured in this way lost their limbs. The composition was originally selected because of being roughly eutectic.〔 The 45S5 name signifies glass with 45 wt.% of SiO2 and 5:1 ratio of CaO to P2O5. Lower Ca/P ratios do not bond to bone.〔 The key composition features of Bioglass is that it contains less than 60 mol% SiO2, high Na2O and CaO contents, high CaO/P2O5 ratio, which makes Bioglass highly reactive to aqueous medium and bioactive. High bioactivity is the main advantage of Bioglass, while its disadvantages includes mechanical weakness, low fracture resistance due to amorphous 2-dimensional glass network. The bending strength of most Bioglass is in the range of 40–60 MPa, which is not enough for load-bearing application. Its Young's modulus is 30–35 GPa, very close to that of cortical bone, which can be an advantage. Bioglass implants can be used in non-load-bearing applications, for buried implants loaded slightly or compressively. Bioglass can be also used as a bioactive component in composite materials or as powder. The first successful surgical use of Bioglass 45S5 was in replacement of ossicles in middle ear, as a treatment of conductive hearing loss. The advantage of 45S5 is in no tendency to form fibrous tissue. Other uses are in cones for implantation into the jaw following a tooth extraction. Composite materials made of Bioglass 45S5 and patient's own bone can be used for bone reconstruction.〔 Bioglass is comparatively soft in comparison to other glasses. It can be machined, preferably with diamond tools, or ground to powder. Bioglass has to be stored in a dry environment, as it readily absorbs moisture and reacts with it.〔 Bioglass 45S5 is manufactured by conventional glass-making technology, using platinum or platinum alloy crucibles to avoid contamination. Contaminants would interfere with the chemical reactivity in organism. Annealing is a crucial step in forming bulk parts, due to high thermal expansion of the material. Heat treatment of Bioglass reduces the volatile alkali metal oxide content and precipitates apatite crystals in the glass matrix. The resulting glass-ceramic material, named Ceravital, has higher mechanical strength and lower bioactivity.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Bioglass」の詳細全文を読む スポンサード リンク
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