**3. Conclusion**

292 Biomaterials – Physics and Chemistry

Fig. 13. Simulated IR spectra of the four models of glasses at increasing %P2O5 content.

The natural subsequent step in bioactive glass simulation deals with the modeling of surfaces. Indeed, each process of the Hench mechanism that leads to the implant integration typically occurs at the interface between the inorganic material and the biological fluid. Thus, the knowledge of surface properties, such as electrostatic potential and adsorptive behavior towards simple molecules as water, becomes essential in the investigation of

Modeling surfaces is generally not a trivial task, particularly when the bulk material is amorphous. For an amorphous material the identification of a particular face by crystallographic indexes is rather arbitrary as the atomic density is statistically distributed in space in a rather uniform way. A second difficulty is the need of breaking both ionic and

In Figure 14, the model of one of the many possible bioglass surfaces extracted from the P2.5 bulk of Figure 8b is presented. The surface was cut out from the bulk as a real 2D slab (infinite in the two dimensions), dangling bonds were saturated with hydrogen atoms and a full optimization run was performed. The resulting surface is very interesting per se, but much more considering its behaviour when hydrated, since water molecules are ubiquitously present in the biological fluids where the material is immersed. In particular, a key issue is to see whether H2O will chemisorb by dissociating on the exposed Na+ and Ca2+

In our laboratory a systematic study of the several possible surfaces of the structure with the 45S5 composition is on-going. The application of different methodologies, such as *ab initio* molecular dynamics, already used in the literature (Tilocca, 2010), will be considered to fully characterize the adsorption processes of water and even collagen occurring at the interface

covalent bonds during the slab definition which may render the system non-neutral.

**2.2.4 Future perspectives: Surface modelling** 

cations, a step essential in the Hench mechanism.

between bioactive material and the biological tissue.

bioglasses (Tilocca & Cormack, 2009).

In the present Chapter it has been explained how crucial the computational techniques are when applied together with experimentalist measurements in the understanding of biological complex systems and mechanisms dealing with biomaterials for a large number of reasons. Indeed, computational methods are extremely powerfully applied to predict structure formation and crystal growth as well as to describe at a molecular level the real interactions responsible of the attachment of the inorganic biomaterial to the organic tissue. In the investigation of phenomena related to a complex system such as the human body, many approximations are required, so a reductionist approach is employed also in the computational analysis.

In this Chapter, the approach has been explained for two typical biomaterials: hydroxyapatite and Bioglass® 45S5. In particular, for the first material, the aim was to describe the study of its (010) non-stoichiometric surfaces in interaction with water and carbon monoxide. For the latter, the adopted strategy has been analyzed and then a specific example has been reported, dealing with the spectroscopic characterization of computed vibrational features with the increasing amount of phosphorous in a sufficiently large unit cell starting from the well-know 45S5 Bioglass® composition.

The general knowledge gained in recent years through the use of computational techniques such as those described in this chapter is great, but not enough to fully understand the peculiar characteristics of the materials that make up the musculo-skeletal system and to provide appropriate care for important illnesses such as osteoporosis or degenerative and metabolic diseases, benign and malignant tumors and trauma.

*In Silico* Study of Hydroxyapatite and Bioglass®: How Computational Science Sheds Light on Biomaterials

pp.1099-1111, ISSN 1463-9084.

No.3, pp.195-202, ISSN 0022-3093.

No.6, pp.399-411, ISSN 2041-3033

*Discussions,* Vol.134, 195-214, ISSN 1364-5498.

*Materials,* Vol.2, No.2, pp.399-498, ISSN 1996-1944.

2614.

Vol.111, No.10, pp.4027-4035, ISSN 1932-7447.

Astala, R., Stott, M. J. & Calderin, L. (2006) Ab Initio Simulation of Si-Doped Hydroxyapatite. *Chemistry of Materials,* Vol.18, No.2, pp.413-422, ISSN 1520-5002. Becke, A. D. (1993) Density-functional thermochemistry. III. The role of exact exchange. *The Journal of Chemical Physics,* Vol.98, No.7, pp.5648-5652, ISSN 0021-9606. Bertinetti, L., Tampieri, A., Landi, E., Ducati, C., Midgley, P. A., Coluccia, S. & Martra, G.

Canepa, P., Chiatti, F., Corno, M., Sakhno, Y., Martra, G. & Ugliengo, P. (2011a) Affinity of

Canepa, P., Hanson, R. M., Ugliengo, P. & Alfredsson, M. (2011b) J-ICE: a new Jmol interface

Christie, J. K., Pedone, A., Menziani, M. C. & Tilocca, A. (2011) Fluorine Environment in

Clayden, N. J., Pernice, P. & Aronne, A. (2005) Multinuclear NMR study of phosphosilicate

Corno, M., Busco, C., Bolis, V., Tosoni, S. & Ugliengo, P. (2009) Water Adsorption on the

Corno, M., Busco, C., Civalleri, B. & Ugliengo, P. (2006) Periodic *ab initio* study of structural

Corno, M. & Pedone, A. (2009) Vibrational features of phospho-silicate glasses: Periodic

Corno, M., Pedone, A., Dovesi, R. & Ugliengo, P. (2008) B3LYP Simulation of the Full

Corno, M., Rimola, A., Bolis, V. & Ugliengo, P. (2010) Hydroxyapatite as a key biomaterial:

de Leeuw, N. H., Bowe, J. R. & Rabone, J. A. L. (2007) A computational investigation of

Dorozhkin, S. V. (2009) Calcium Orthophosphates in Nature, Biology and Medicine.

Dovesi, R., Civalleri, B., Orlando, R., Roetti, C. & Saunders, V. R. (2005a). Ab Initio Quantum

*Chemistry of Materials,* Vol.20, No.17, pp.5610-5621, ISSN 1520-5002.

*Chemistry Chemical Physics,* Vol.8, No.21, pp.2464-2472, ISSN 1463-9084. Corno, M., Orlando, R., Civalleri, B. & Ugliengo, P. (2007) Periodic B3LYP study of

*Applied Crystallography,* Vol.44, No.1, pp.225-229, ISSN 0021-8898.

*Chemistry B,* Vol.115, No.9, pp.2038-2045, ISSN 1520-6106.

Study. *Langmuir,* Vol.25, No.4, pp.2188-2198, ISSN 1520-5827.

*Mineralogy,* Vol.19, No.5, pp.757-767, ISSN 0935-1221.

(2007) Surface Structure, Hydration, and Cationic Sites of Nanohydroxyapatite: UHR-TEM, IR and Microgravimetric Studies. *The Journal of Physical Chemistry C,*

hydroxyapatite (001) and (010) surfaces to formic and alendronic acids: a quantummechanical and infrared study. *Physical Chemistry Chemical Physics,* Vol.13, No.3,

for handling and visualizing crystallographic and electronic properties. *Journal of* 

Bioactive Glasses: Ab Initio Molecular Dynamics Simulations. *The Journal of Physical* 

gels derived from POCl3 and Si(OC2H5)4. *Journal of Non-Crystalline Solids,* Vol.351,

Stoichiometric (001) and (010) Surfaces of Hydroxyapatite: A Periodic B3LYP

and vibrational features of hexagonal hydroxyapatite Ca10(PO4)6(OH)2. *Physical* 

hydroxyapatite (001) surface modelled by thin layer slab. *European Journal of* 

B3LYP simulations. *Chemical Physics Letters,* Vol.476, No.4-6, pp.218-222, ISSN 0009-

Vibrational Spectrum of 45S5 Bioactive Silicate Glass Compared to v-Silica.

quantum-mechanical simulation of its surfaces in interaction with biomolecules. *Physical Chemistry Chemical Physics,* Vol.12, No.24, pp.6309-6329, ISSN 1463-9084. Currey, J. D. (1998) Mechanical properties of vertebrate hard tissues. *Proceedings of the* 

*Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine,* Vol.212,

stoichiometric and calcium-deficient oxy- and hydroxy-apatites. *Faraday* 

Simulation in Solid State Chemistry. In *Review in Computational Chemistry, Vol. 21,*

295

To achieve this goal it is fundamental to understand the structure and properties of natural bone at a molecular level and to investigate the chemical-physical interaction between collagen and mineral phase comprising the bone composite.

This can only be achieved through the development and use of multiscale computational methods that combine quantum, classical and continuum approaches enabling to study chemical-physical-biological phenomena on large-scales both in space and time.

Regarding the study of the human bone, we believe that the key issues to be addressed by computational science researchers in the coming years will be the study of the structure and assembly of the collagen protein, the interaction at the molecular level of collagen with the mineral apatite, and finally the structure and mechanical properties of collagen-apatite composite.

As for the study of bioactive glasses, an important line of research that is developing in different research groups located in different nations involves the characterization of the chemical and physical properties and reactivity of the 45S5 Bioglass® surface. However, it will be wise not to neglect the study of the effect of composition on the structure and bioactivity of different systems and the study of the thermodynamics and crystallization kinetics of crystalline phases that are well-known to affect the bioactivity of the glass. Finally, the design of new bioactive glasses will also rely on a deep understanding of their fracture mechanism and the prediction of important properties such as brittleness and toughness, which determine the final use of glass.
