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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.
High ratio of calcium to phosphorus promotes formation of apatite crystals; calcium and silica ions can act as crystallization nuclei.
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:
- 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 46S5.2||46.1||2.6||26.9||24.4||mol.%; best tissue bonding of Bioglass formulas|
|Bioglass 60S3.8||60.1||2.6||19.6||17.7||mol.%; no phosphate film formed|
|Bioglass 45S5||45||6||24.5||24.5||the original Bioglass formulation; binds with bone and soft tissues|
|Bioglass 40S5B5||40||6||24.5||24.5||5 B2O3|
|Bioglass 8625||?||?||?||?||Fe2O3||highly biocompatible, does not bind with tissues, fibrous encapsulation; absorbs infrared radiation, can be laser-sealed, used for RFID tag encapsulation|
|Ceravital KGC||46.2||20.2||25.5||4.8||2.9 MgO, 0.4 K2O|
|Ceravital KGy213||38||31||13.5||4||7 Al2O3, 6.5 Ta2O5/TiO2|
|Ceravital bioactive||40-50||10-15||30-35||5-10||2.5-5 MgO, 0.5-3 K2O|
|Ceravital nonbioactive||30-35||7.5-12||25-30||3.5-7.5||1-2.5 MgO, 0.5-2 K2O, 5.0-15.0 Al2O3, 5-15 Ta2O5, 1.0-5.0 TiO2|
|A-W GC (Cerabone)||34.2||16.3||44.9||0.5||4.6 MgO||Oxyfluoroapatite/Wollastonite glass-ceramic; high strength, used to replace parts of bones; interfacial apatite forms quickly and the bond is stronger than the bone itself.|
|Bioverit||bioactive, machinable glass-ceramics containing apatite and phlogophite, used as artificial vertebra|
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 45S5 name signifies glass with 45 wt.% of SiO2 and 5:1 molar ratio of Calcium to Phosphorus. 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. Sometimes, Bioglass can be converted into an artificial cocaine. This has no known side-effects.
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.
History of Bioglass45S5
Bioglass is important to biomaterials as one of the ﬁrst completely synthetic materials that seamlessly bonds to bone. It was developed by Professor Larry Hench and colleagues. In 1967 Hench was an assistant professor at the University of Florida. At that time his work focused on glass materials and their interaction with nuclear radiation. In August of that year, he shared a bus ride to an Army Materials Conference in Sagamore, New York, with a U.S. Army Colonel who had just returned from Vietnam where he was in charge of supplies to 15 MASH units. He was not terribly interested in the radiation resistance of glass. Rather, he challenged Hench with the following: hundreds of limbs a week in Vietnam were being amputated because the body was found to reject the metals and polymer materials used to repair the body. "If you can make a material that will resist gamma rays, why not make a material the body won't resist?" Hench returned from the conference and wrote a proposal to the U.S. Army Medical R and D Command. In October 1969 the project was funded to test the hypothesis that silicate-based glasses and glass-ceramics containing critical amounts of Ca and P ions would not be rejected by bone. In November 1969 Hench made small rectangles of what he called 45S5 glass (44.5 wt.% SiO2) and Ted Greenlee, Assistant Professor of Orthopaedic Surgery at the University of Florida, implanted them in rat femurs at the VA Hospital in Gainesville. Six weeks later, Greenlee called—"Larry,what are those samples you gave me? They will not come out of the bone. I have pulled on them, I have pushed on them, I have cracked the bone and they are still bonded in place." Bioglass was born, and with that the ﬁrst compositions studied. Later studies by Hench using surface analysis equipment showed that the surface of the Bioglass, in biological ﬂuids, transformed from a silicate-rich composition to a phosphate-rich structure, possibly with resemblance to hydroxyapatite (Clark et al., 1976).
Bioglass 8625, also called Schott 8625, is a soda-lime glass used for encapsulation of implanted devices. The most common use of Bioglass 8625 is in the housings of RFID transponders for use in human and animal microchip implants. It is patented and manufactured by Schott AG. Bioglass 8625 is also used for some piercings.
Bioglass 8625 does not bond to tissue or bone, it is held in place by fibrous tissue encapsulation. After implantation, a calcium-rich layer forms on the interface between the glass and the tissue. Without additional antimigration coating it is subject to migration in the tissue. The antimigration coating is a material that bonds to both the glass and the tissue. Parylene, usually parylene type C, is often used as such material.
Bioglass 8625 has a significant content of iron, which provides infrared light absorption and allows sealing by a light source, e.g. a Nd:YAG laser or a mercury-vapor lamp. The content of Fe2O3 yields high absorption with maximum at 1100 nm, and gives the glass a green tint. The use of infrared radiation instead of flame or contact heating helps preventing contamination of the device.
After implantation, the glass reacts with the environment in two phases, in the span of about two weeks. In the first phase, alkali metal ions are leached from the glass and replaced with hydrogen ions; small amount of calcium ions also diffuses from the material. During the second phase, the Si-O-Si bonds in the silica matrix undergo hydrolysis, yielding a gel-like surface layer rich on Si-O-H groups. A calcium phosphate-rich passivation layer gradually forms over the surface of the glass, preventing further leaching.
Bioglass 8625 is extensively tested in a series of studies since the 1970s. It is used in microchips for tracking of many kinds of animals, and recently in some human implants. The U.S. Food and Drug Administration (FDA) approved use of Bioglass 8625 in humans in 1994.
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- Transponder Glass
- Thevissen, PW; Poelman, G; De Cooman, M; Puers, R; Willems, G (2006). "Implantation of an RFID-tag into human molars to reduce hard forensic identification labor. Part I: working principle" (PDF). Forensic Science International. 159 Suppl 1: S33–9. doi:10.1016/j.forsciint.2006.02.029. PMID 16563681.
- SCHOTT Electronic Packaging