Glasses are amorphous materials, therefore lacking short-range order, have a disordered atomic structure that can be likened to that of a subcooled liquid, that is, as if the disordered structure of a liquid had been frozen. 

Types and roles of the main constituents of glasses, so oxide-based glasses:

• forming oxides, (e.g. silicon oxide), which can give rise to a glassy lattice.
• modifiers oxides, alkali or alkaline earth metal oxides, which in themselves cannot give rise to a glass but can lower the melting temperature, play an important role in some biological properties of glass.
• intermediate oxides or Conditional Glass Formers , such as aluminum oxide, which are useful in increasing the mechanical properties of glass.

The modifiers oxides essentially fragment the glassy lattice consisting of silicon ions and oxygen, then break the pontant oxygens, which bind two silica tetrahedra, resulting in 2 non-pontant oxygens, then 2 terminal oxygens, because the bond has been broken.

Glasses, prior to melting, progressively soften, then as the temperature increases they go from a rigid solid state to a honey-like state and their viscosity decreases greatly and below a conventional viscosity value. Glass at this stage can actually be considered melted.

Let's enter the world of bioactive glasses or bioglasses. Bioglasses are special glasses that can be used as implant materials in the biomedical field, in particular they are used in bone repair or regeneration interventions. Bone is a biocomposite material, composed of a mineral matrix of hydroxyapatite combined with collagen fibers while a bioglass is a completely inorganic material, with physical-mechanical properties very similar to those of the bone mineral matrix.

Some particular glass compositions have the ability to bond to living bone.

The first bioactive glass was devised by a researcher at the University of Florida, Larry Hench in 1969, and it is also interesting to narrate the genesis of this material: during a train trip with a U.S. Army colonel during the Vietnam War period, the colonel knew that Hench was studying glass capable of resisting nuclear radiation and asked him a question: 'If you are able to develop such resistant glass, wouldn't you also be able to develop glass capable of resisting a biological environment, which is aggressive anyway? " (an aggressive environment because biological fluids are salt fluids that can attack metallic materials that are implanted, promoting corrosion reactions). Hench was tickled by this challenge and developed between 1967 and 1969 some glassy compositions that might be suitable for replacing bone with an implant in an organism. Among other things, the colonel who asked him the question proposed an interesting idea for that period, given the high number of war veterans returning home with severe limb damage, for which prostheses were necessary.

Hench developed a glass system consisting of four oxides. 

• silicon oxide, the main forming oxide; 
• phosphorus oxide, another forming oxide but present in a minority quantity; 
• two oxides of alkali and alkaline earth metals, namely calcium oxide and sodium oxide.

The composition of this glass, which is called bioglass, a patented glass marketed under that name, was as follows: 

• 45% by weight silicon oxide
• 6% by weight phosphorous oxide
• 24.5% by weight calcium oxide
• 24.5% by weight sodium oxide

All obtained through casting, thus using a classic production method. The design of this glass composition was based on a few considerations.

First, Hench designed a glass with a high calcium oxide/phosphorus oxide ratio, a ratio that was similar to the calcium/phosphorus ratio present in the bioapatites of the mineral phase of bone. He then decided to introduce a high content of sodium oxide and calcium oxide into this glass, this was to achieve a highly reactive glass, in fact these oxides are modifying oxides, which break up the glass lattice, and allow it to be reactive in contact with biological fluids.  Hench also noted that a silicon oxide content of no more than 60% was required for these glasses, which is a bit of a threshold for the material's bioactivity. Glasses containing more than 60% silicon oxide are inert.

From this base glass many other derivatives were born, for example for the most recent compositions there is a partial substitution of silicon oxide with boron oxide, which is always a forming oxide, to increase the reactivity of the glass. Thus, boron-silicate glasses are more reactive and more soluble in contact with biological fluids than purely silicate glasses. Basically, boron-silicate glasses are somewhere between bioactive and bioresorbable materials .

To obtain high strength glass, for example for structural applications such as replacing a cortical bone portion, aluminum oxide can be incorporated within the glass composition. Aluminum oxide is an intermediate oxide whose function is to increase the mechanical properties of the material. Therefore, by playing on the composition it is possible to obtain glasses with different properties: higher or lower mechanical properties, higher or lower solubility and bioactivity. Often it is necessary to find a balance, because if you add aluminum oxide in amounts greater than 3% the glass becomes inert, then the glass network becomes chemically very stable and, in contact with biological fluids, behaves like a bioinert material.

Hench wanted to obtain a bioactive material that could bind to the surrounding bone and developed a hypothesis about the mechanism of bioactivity of silicate glasses that you see summarized in 5 steps in the slide below.

Once implanted, the bioglass can release cations of modifying oxides, such as sodium oxide, into the biological environment. An ionic exchange is established between glass and biological fluids: the glass releases sodium ions and acquires hydrogen ions from the biological solution. As a result (step 2) silanol formation occurs on the surface of the bioglass, which is then hydrated and exposes hydroxyl groups. It is as if Si-OH groups were formed on the surface of the glass.

In step 3 there is a sort of condensation of these Si-OH groups, and there is the formation of a silica gel layer, then the glass starts to absorb calcium ions and phosphorus ions from the solution, so that a thin film of inorganic calcium phosphate is formed on the surface of this silica gel layer, which (step 5) progressively crystallizes into nanocrystalline hydroxyapatite, very similar to the hydroxyapatite of the mineral phase of natural bone. This is referred to as biomimetic hydroxyapatite, which is very similar to the hydroxyapatite of bone; however, the commercial hydroxyapatite that can be used clinically as a bone filler typically has larger crystals, microcrystalline hydroxyapatite, which has crystals in the orine of the micrometer. In the image below you see these 5 steps summarized graphically.

In the image you can see what is happening: 

on the left the bioactive glass implant, on the right the bone, and in the middle what are called "reaction layers"; above the situation before the new bone is formed, below the situation after the new bone is formed. In contact with the surface of the bioglass implant there is a layer rich in silica gel, then, at the interface between this silica gel layer and the surrounding bone, there is a layer of calcium phosphate, or better yet, nanocrystalline hydroxyapatite. The bond formed between bioglass and living bone is not only a mechanical bond, but it is really a chemical bond, the two materials (bioglass and bone) are intimately bonded with a stable and continuous interface, which therefore leaves no room for empty areas.  It is interesting to note that the formation of this layer of nanocrystalline hydroxyapatite takes place certainly in vivo but also in vitro, i.e. appropriate simulated physiological solutions, acellular, reproducing the component of blood plasma, in which the bioactivity of the material can be evaluated. By "bioactivity" we mean the ability of a material to bind to bone, but at the same time the ability of a material to coat itself with nanocrystalline hydroxyapatite in contact with biological fluids.

There is an ISO standard that regulates these tests, how to assess the bioactivity of materials and there are a whole series of prescriptions, it is suggested how to produce the samples, what geometry, the volume of this simulated physiological solution to be used, its composition etc. .. and after how long to go to assess whether the hydroxyapatite is formed.

This diagram clarifies how the various elements of bioglass and bone vary as a function of position along the interface.

Calcium and phosphorus grow in the hydroxyapatite layer until they reach a maximum inside the bone, while silicon, which is the typical element of bioglass, decreases along the interface until it cancels out inside the bone.