Computational Insight into the Effect of CaO/MgO Substitution on the Structural Properties of Phospho-Silicate Bioactive Glasses Alfonso Pedone, Gianluca Malavasi, and M. Cristina Menziani* Dipartimento di Chimica, UniVersita ` di Modena e Reggio Emilia, Via G. Campi 183, 41100 Modena, Italia ReceiVed: May 4, 2009; ReVised Manuscript ReceiVed: July 2, 2009 The effect of the replacement of CaO for MgO on the structural properties of the 45S5 Bioglass with composition 46.2SiO 2 · 24.3Na 2 O · (26.9 - x)CaO · 2.6P 2 O 5 · xMgO where x ) 0, 5, 10, 15, 20, and 26.9 mol has been studied by means of molecular dynamics simulations. The results confirmed the complexity of the local environment of Mg ions which are coordinated by 5 nonbridging oxygens of different TO 4 tetrahedra (T ) Si/P) leading to large rings in the structures. A rough correlation between the average dimension of the rings found in the structure and the computed Young’s modulus is obtained. The Young’s modulus decrease at low Mg-content reaching a minimum for the 46.2SiO 2 · 24.3Na 2 O · 16.9CaO · 2.6P 2 O 5 · 10MgO glass. At this composition, Mg is homogeneously distributed in the silica rich region together with Ca and Na ions but is almost totally absent from the Ca-Na-phosphate rich regions. The results suggest that the ideal glass composition for lowering the Young’s modulus preserving a specific bioactivity can be found below 10% of MgO content. Introduction Once implanted in the body, bioactive glasses react chemi- cally with body fluids 1,2 forming a layer of biologically active bonelike carbonate-containing hydroxyapatite on their surface, which triggers tissue reparation processes. The ability of the 45S5 Bioglass 3 (45%SiO 2 , 24.5%CaO, 24.5% Na 2 O, and 6%P 2 O 5 wt %) to induce new bone regeneration very shortly after being implanted explains its frequent use in clinical applications. However, the relatively poor mechanical properties of this material limit its uses to low-load bearing applications, such as otolaryngological, maxillofacial, dental, and periodontal implants. 4-7 Therefore, over the past few years, much research has been focused on the addition of doping atoms into the original 45S5 Bioglass to improve its elastic and other specific properties. 8-11 Among the dopants tested, magnesium, one of the most abundant cations in bone, 12,13 is of particular clinical interest since it has been shown to stimulate directly osteoblast proliferation. 14 Notwithstanding, the bioactivities of some Bio- verit glass-ceramics, containing a significant amount of MgO, have been clinically confirmed for years, 15 and contradictory explanations of the structural role of MgO on surface bioactivity of CaO-MgO-SiO 2 -P 2 O 5 sol-gel bioglasses have been provided; some in vitro results indicate that, in specific concentration ranges, MgO does not affect apatite formation, 16-18 but others suggest that it inhibits mineralization. 19 It is also now largely recognized that substitutions of CaO by MgO in the composition of silica glasses modify their chemical durability 20 and increase their mechanical properties: 21 the replacement of CaO for MgO in soda lime glasses leads to a significant increase of the fracture toughness and fracture surface energy with simultaneous decrease of the Young’s modulus. 21 It is also known that the partial substitution of CaO by MgO and Na 2 O by K 2 O is required to match the thermal expansion of bioactive glass coatings to that of Ti-based alloys. 22 Nevertheless, the structural role of Mg itself is still poorly understood due to the lack of unambiguous information on its local environment. In fact, depending on the glass composition and the experimental techniques employed, Mg was found to be present in four-, 23,24 five-, 25-28 and six-fold 29,30 coordination. Further investigation of key structural factors and their effect on elastic properties can be greatly improved with the atomistic resolution of modern computational techniques, such as classical MD simulations, provided that a reliable force-field is available. 31-37 A shell-model (SM) interatomic potential for phospho-silicate glasses including Na and Ca network modifiers, which provided a superior structural medium range properties description than the simple ionic rigid model, 38 has recently been developed by Tilocca et al. 37 Among the computational studies found in the literature, few works have been devoted to the simulation of the effect of Mg on the structure, elastic, and transport properties of glasses, except for windows soda-lime glasses. 39,40 In these previous works, carried out within the rigid body approximation, almost 90% of Mg was found to be 4-fold coordinated with the remaining amount being 5-fold coordinated; moreover, the substitution of CaO by MgO resulted in a reduction of the Young’s modulus and enhancement of Na diffusivity. 39 However, different conclusions were drawn from the results obtained in other computational works. 41-43 In this paper, MD simulations using the SM potential have been carried out in order to provide structure-property relationships on CaO substitution by MgO in the parent 45S5 Bioglass. To this purpose the SM potential has been “in- house” extended to include the Mg-O interaction. The compositions of the series of glasses studied is in the range 46.2SiO 2 · 24.3Na 2 O · (26.9 - x)CaO · 2.6P 2 O 5 · xMgO, where x ) 0, 5, 10, 15, 20, and 26.9 mol. Computational Methods Force-Field. Classical molecular dynamics simulations were carried out by means of the DL_POLY code 44 using the SM * Corresponding author. E-mail: menziani@unimore.it. Phone: +39 059 2055091. Fax: +39 059 373543. J. Phys. Chem. C 2009, 113, 15723–15730 15723 10.1021/jp904131t CCC: $40.75  2009 American Chemical Society Published on Web 08/07/2009