A High End Computing project investigating the dissolution of bio-active phosphate glasses

Lead Research Organisation: University College London
Department Name: Chemistry


Phosphate glasses are an increasingly important class of biomedical materials, finding an expanding role in implant design, as replacements for bone and other hard tissue. Most recently their potential in tissue engineering is being explored because of their chemical composition, which is close to that of natural bone tissue, and their active degradability in the body. One of the main advantages of these phosphate glasses is their solubility, which can be tailored to suit the end application by controlling the chemistry. In this way, the dissolution rates of the phosphate glasses can be varied by several orders of magnitude. From a fundamental point of view, the solubility must be linked to atomic-scale structure, but little is really known in this regard. Thus, if we are going to optimize the exploitation of these materials, it is clearly necessary to understand their dissolution behaviour, which forms the central theme of the present proposal. We plan to conduct a detailed computational study designed to elucidate the atomic-scale mechanisms underlying the dissolution processes. In outline, the computational study will produce atomic-scale structural models of phosphate glasses, where identification of the key features will be related to existing experimental observations of the dissolution products. This will entail a detailed analysis and characterization of the model structures.The project will exploit a range of state-of-the-art computational techniques and the latest High End Computing (HEC) facilities, which are essential for the successful completion of the research programme.
Description The project has used two approaches to investigate the dissolution behaviour of phosphate bioglasses (PBG) and interpret the experimental observations on the dissolution products:

(i) Modelling structural and dynamical properties of mixed Ca-Na-phosphate bioglasses as a function of composition

"Third-generation" biomedical materials are a class of implant materials, which rather than passive and inert materials, can play a role in tissue regeneration and degrade after the tissue has healed. Phosphate-based bio-active glasses (PBGs), containing phosphorous pentoxide (P2O5) as a network former and sodium oxide (Na2O) and calcium oxide (CaO) as network modifiers, have many unique properties: (i) their composition is chemically related to the surrounding tissue; (ii) they dissolve completely in the aqueous media; (ii) their dissolution rate can be controlled by changing the composition of the phosphate-sodium-calcium glasses. Experimental evidence shows, for example, an inverse relation between CaO content and dissolution rate, which suggests that the interaction of the Ca2+ ions with the glass networks controls the glass degradation. The solubility can therefore be tailored to suit the end application. As the solubility of a glass is linked to its structure, insights into the effect of calcium and sodium ions to the P2O5 network is of fundamental interest to tailor the composition of the phosphate glasses for applications in tissue engineering.
First principles molecular dynamics simulations of ternary phosphate-based glasses P2O5-CaO-Na2O have been carried out in order to provide an accurate description of the local structure of these important materials for biomedical applications. The structures of PBGs with compositions (x = 30, 35 and 40) were generated using a full ab initio molecular dynamics melt-and-quench procedure. The analysis of the structure of the glasses at 300 K shows the prevalence of the metaphosphate Q2 and pyrophosphate Q1 species, whereas the number of Q3 units, which constitute the three-dimensional phosphate network, significantly decreases with the increase of calcium content in the glass. The rigidity of the phosphate interconnectivity increases with the concentration of calcium, an observation which is interpreted in terms of the property of Ca2+ to be a stronger "coordinator" than sodium.

(ii) Modelling the hydration behaviour of phosphate glass dissolution products

Extensive Car-Parrinello molecular dynamics simulations of a number of phosphate species in explicit water show that the process of proton transfer from Hn(PO4)3-n to the surrounding water molecules is very fast, less than 1 ps, and indicate that the dehydrogenation occurs through a concerted proton hopping mechanism, which involves Hn(PO4)3-n and three water molecules. Analysis of the intermolecular Hn(PO4)3-n/water structure show that the [PO4]3- anions have a significant effect on the H-bonding network of bulk water and the presence of [P-O]- moieties induce the formation of new types of H-H interactions around this orthophosphate. These phosphate species display a flexible first coordination shell of water of between 7 and 13 molecules, and the flexibility increases on going from [PO4]3- to [H2PO4]-. The H-bond interactions between the oxygen atoms of the phosphates and the surrounding water molecules, which decrease on going from [PO4]3- to the hydrogenated [H2PO4]- species, explain the diminished effect on the structure of water with the increasing hydrogenation of the orthophosphate anions.

The project has led already to 3 publications in peer-reviewed scientific journals, with another paper in preparation for submission later this year. The student has presented her work in 2 posters and an oral presentation at international conferences; she has obtained an MSc in High Performance Computing and has submitted her doctoral thesis for examination next month.

A comprehensive final report was submitted to the EPSRC upon completion of the grant.
Exploitation Route Peer-reviewed publications in scientific journals

Conference presentations
Sectors Healthcare

Description Peer-reviewed scientific publications, presentations at international conferences
First Year Of Impact 2009
Sector Other