Question

What is the primary molecular level difference between a ceramic and a glass material?

How does increasing porosity of a bioceramic material influence the biodegradation rate?

The bioactivity of Bioglass 45S5 is due to what three chemical species?

Show the first five reaction steps in the formation of the hydroxyl-carbonate-apatite (HCA) layer upon dissolution of Bioglass 45S5 in biological media.

Answer 1

Ceramic materials are usually oxides, nitrides or carbides. The bonding is very strong- either ionic or network covalent. Ceramics can withstand high temperatures, are good thermal insulators and do not expand greatly when heated. Ceramics are stong , hard and durable . Ceramics are but brittle, oxidation resistant, wear resistant, refractory, non-magnetic, chemically stable and prone to thermal shock. Where as about Glass if we talk, glass is hard, amorphous, inert , biologically inactive, fragile and transparent. Glass is non-crystalline, ceramics may be crystalline or partlly crystalline. Glass can be a type of ceramic as it is an amorphous solid , which means it has no long range order of positioning of its molecules.

The increasing porosity of a bio-ceramic material would result in a higher surface area which may eventually cause the higher dissolution rate or the rate of biodegaradtion .

The chemicals that are responsible for the 45S5 glass are its low SiO2 content , High Na₂O and CaO (glass network modifiers) and high CaO/P₂O₅ ratio.

The machanism of dissolution of biogalss 45S5 involves following five steps in the formation of the HCA( hydroxyl-carbonate-apatite) layer :

1 step: Rapid ion exchange reaction between the glass network modifiers(Na⁺ and Ca⁺⁺) with H⁺(or H3O+) ions from the solution , leads to the hydrolysis of silica group and creaction of Si-OH groups on the glass surface.

Si-- O-- Na⁺   + H⁺ ------> Si--OH + Na⁺(aq)

2 step: The increase in pH (OH^- conc) leads to the attack of SiO2 glass network and the dissolution of Si, in the form of Si(OH)₄, into the solution and the continued formation of Si-OH groups on the the glass surface.

Si--O--Si +H2O-----> Si--OH +OH--Si

3 step: Condensation and polymerisation of an amorphous SiO2 rich layer on the surface of glass depleted in Na⁺ and Ca⁺⁺.

4 step: Further, dissolution of the glass ,coupled with migration of Ca⁺⁺ and PO₄^3- ions from the glass through the SiO2 rich layer and from the solution, leading to the formation of an aphormous Calcium phosphate(ACP) layer on the surface of SiO2 -rich layer.

5 step: The glass continues to dissolve, as the ACP layer incorporates OH^- and CO3^- from the solution and crystallizes as an HCA layer.