Bioglass Structure

Bioglass 45S5 or calcium sodium phosphosilicate, is a bioactive glass specifically composed of 45 wt% SiO₂, 24.5 wt% CaO, 24.5 wt% Na₂O, and 6.0 wt% P₂O₅. Typical applications of Bioglass 45S5 include: bone grafting biomaterials, repair of periodontal defects, cranial and maxillofacial repair, wound care, blood loss control, stimulation of vascular regeneration, and nerve repair.

The name "Bioglass^®" was trademarked by the University of Florida as a name for the original 45S5 composition. It should therefore only be used in reference to the 45S5 composition and not as a general term for bioactive glasses. Bioglass 45S5 is available commercially under the registered trade name NovaMin, which is owned by the pharmaceutical company GlaxoSmithKline. NovaMin is bioactive glass that has been ground into a fine particulate with a median size of less than 20 microns. It can reduce dentin hypersensitivity by blocking open dentinal tubules and by supplying calcium (Ca²⁺) and phosphate (PO₄³⁻) ions to form hydroxycarbonate apatite (HCA), the principal mineral component of bone tissue in mammals. NovaMin is the active ingredient in Sensodyne "Repair & Protect" toothpaste, except when sold in the United States, containing stannous fluoride instead.

Characteristics

45S5 bioactive glass is white in color and is in powder form, with particulates with a median size of less than 20 microns. Its chemical composition by weight is: silica (SiO₂) 43-47%, calcium oxide (CaO) 22.5-26.5%, phosphorus pentoxide (P₂O₅) 5-7% and sodium oxide (Na₂O) 22.5-26.5%>

Glasses are non-crystalline amorphous solids that are commonly composed of silica-based materials with other minor additives. Compared to soda-lime glass (commonly used, as in windows or bottles), Bioglass 45S5 contains less silica and higher amounts of calcium and phosphorus.  The 45S5 name signifies glass with 45 weight % of SiO₂ and 5:1 molar ratio of calcium to phosphorus. This high ratio of calcium to phosphorus promotes formation of apatite crystals; calcium and silica ions can act as crystallization nuclei. Lower Ca:P ratios do not bond to bone. Bioglass 45S5's specific composition is optimal in biomedical applications because of its similar composition to that of hydroxyapatite, the mineral component of bone. This similarity provides Bioglass 45S5's ability to be integrated with living bone.

This composition of bioactive glass is comparatively soft in comparison to other glasses. It can be machined, preferably with diamond tools, or ground to powder. Bioglass 45S5 has to be stored in a dry environment, as it readily absorbs moisture and reacts with it. Bioglass 45S5 is the first formulation of an artificial material that was found to chemically bond with bone, and its discovery led to a series of other bioactive glasses. One of its main medical advantages is its biocompatibility, seen in its ability to avoid an immune reaction and fibrous encapsulation. Its primary application is the repair of bone injuries or defects too large to be regenerated by the natural process.

History

Bioglass 45S5 is important to the field of biomimetic materials as one of the first completely synthetic materials that seamlessly bonds to bone. It was developed by Larry L. Hench in the late 1960s. The idea for the material came to him during a bus ride in 1967. While working as an assistant professor at the University of Florida, Dr. Hench decided to attend the U.S. Army Materials Research Conference held in Sagamore, New York, where he planned to talk about radiation resistant electronic materials. He began discussing his research with a fellow traveller on the bus, Colonel Klinker, who had recently returned to the United States after serving as an Army medical supply officer in Vietnam.

After listening to Dr. Hench's description of his research, the Colonel asked, “If you can make a material that will survive exposure to high energy radiation can you make a material that will survive exposure to the human body?” Klinker then went on to describe the amputations that he had witnessed in Vietnam, which resulted from the body's rejection of metal and plastic implants. Hench realized that there was a need for a novel material that could form a living bond with tissues in the body.

When Hench returned to Florida after the conference, he submitted a proposal to the U.S. Army Medical Research and Design Command. He received funding in 1968, and in November 1969 Hench began to synthesize small rectangles of what he called 45S5 glass. 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 Hench asking, "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."

With this first successful experiment, Bioglass was born and the first compositions studied. Hench published his first paper on the subject in 1971 in the Journal of Biomedical Materials Research, and his lab continued to work on the project for the next 10 years with continued funding from the U.S. Army. By 2006, there were over 500 papers published on the topic of bioactive glasses from different laboratories and institutions around the world. The first successful surgical use of Bioglass 45S5 was in replacement of ossicles in middle ear as a treatment of conductive hearing loss, and the material continues to be used in bone reconstruction applications today.

Other uses include 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. Further research is being conducted for the development of new processing techniques to allow for more applications of Bioglass.

Applications

Bioglass 45S5 is used in jaw and orthopedics applications, in this way it dissolves and can stimulate the natural bone to repair itself. Bioactive glass offers good osteoconductivity and bioactivity, it can deliver cells and is biodegradable. This makes it an excellent candidate to be used in tissue engineering applications. Although this material is known to be brittle, it is still used extensively to enhance the growth of bone since new forms of bioactive glasses are based on borate and borosilicate compositions. Bioglass can also be doped with varying quantities of elements like copper, zinc, or strontium which can allow the growth and formation of healthy bone. The formation of neocartilage can also be induced with bioactive glass by using an ''in vitro'' culture of chondrocyte-seeded hydrogels and can serve as a subchondral substrate for tissue-engineered osteochondral constructs.

The borate-based bioactive glass has controllable degradation rates in order to match the rate at which actual bone is formed. Bone formation has been shown to enhance when using this type of material. When implanted into rabbit femurs, the 45S5 bioactive glass showed that it could induce bone proliferation at a much quicker rate than synthetic hydroxyapatite (HA). 45S5 glass can also be osteoconductive and osteoinductive