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.[1]
High ratio of calcium to phosphorus promotes formation of apatite crystals; calcium and silica ions can act as crystallization nuclei.[2]
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.[1] Generally, there are four classes of bioglasses:[2]
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%.[2]
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.[2]
Bioglasses are divided to two categories:[3]
| glass | SiO2 | P2O5 | CaO | Ca(PO3)2 | CaF2 | Na2O | others | properties |
|---|---|---|---|---|---|---|---|---|
| Bioglass 42S5.6[4] | 42.1 | 2.6 | 29.0 | 26.3 | mol.% | |||
| Bioglass 46S5.2[4] | 46.1 | 2.6 | 26.9 | 24.4 | mol.%; best tissue bonding of Bioglass formulas | |||
| Bioglass 49S4.9[4] | 49.1 | 2.6 | 25.3 | 23.8 | mol.% | |||
| Bioglass 52S4.6[4] | 52.1 | 2.6 | 23.8 | 21.5 | mol.% | |||
| Bioglass 55S4.3[4] | 55.1 | 2.6 | 22.2 | 20.1 | mol.% | |||
| Bioglass 60S3.8[4] | 60.1 | 2.6 | 19.6 | 17.7 | mol.%; no phosphate film formed | |||
| Bioglass 45S5[1] | 45 | 6 | 24.5 | 24.5 | the original Bioglass formulation; binds with bone and soft tissues | |||
| Bioglass 45S5F[1] | 45 | 6 | 12.25 | 12.25 | 24.5 | |||
| Bioglass 45S5.4F[1] | 45 | 6 | 14.7 | 9.8 | 24.5 | |||
| Bioglass 40S5B5[1] | 40 | 6 | 24.5 | 24.5 | 5 B2O3 | |||
| Bioglass 52S4.6[1] | 52 | 6 | 21 | 21 | ||||
| Bioglass 55S4.3[1] | 55 | 6 | 19.5 | 19.5 | ||||
| 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[1] | 46.2 | 20.2 | 25.5 | 4.8 | 2.9 MgO, 0.4 K2O | |||
| Ceravital KGS[1] | 46 | 33 | 16 | 5 | ||||
| Ceravital KGy213[1] | 38 | 31 | 13.5 | 4 | 7 Al2O3, 6.5 Ta2O5/TiO2 | |||
| Ceravital bioactive[4] | 40-50 | 10-15 | 30-35 | 5-10 | 2.5-5 MgO, 0.5-3 K2O | |||
| Ceravital nonbioactive[4] | 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)[1] | 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[5] |
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.[2]
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.[1]
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.[2]
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.[1]
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.[5]
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.[6] 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.[7]
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.[6] 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.[8]
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.[9]
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