Probing the fate of interstitial water in bulk bioactive glass by ab initio simulations

Abstract

As also observed for conventional silicate glasses, water can be incorporated in the bulk interstitial regions of a bioactive glass (BG) matrix during the glass preparation and/or upon exposure to an aqueous environment. However, in the case of BGs, very little is known about the effect of hydration on the bulk structure, and then on key properties of these materials, such as biodegradation and bioactivity, that depend on the bulk structure itself. Here we employ a combination of atomistic simulation techniques to explore the nature and effects of water-BG interactions in the bulk of a bioactive glass. The fate of water inserted in the bulk interstitial region of 45S5 bioglass has been studied by ab initio geometry relaxations and Molecular Dynamics (AIMD) simulations. We probed the interaction of a water molecule with silica rings and cages of different size, as well as the stability of potentially relevant configurations involving manually dissociated water and opened rings. The local stability of selected configurations was further assessed by subjecting them to AIMD runs, in order to overcome possible kinetic barriers for water diffusion and dissociation. Small rings do not appear as favourable absorption sites in the bulk of a bioactive glass as they are for bioinert glasses. Moreover, water dissociation through rupture of Si-O bonds of silica rings formed in the bulk was thermodynamically unfavourable. However, a hightemperature AIMD run led to a dissociated state involving no broken Si-O bonds and a free hydroxyl: because re-optimization of this state produced the most favourable hydration energy identified in this study, dissociative absorption through this mechanism appears a likely outcome of the water-45S5 interaction at low water content. We discuss the structural and dynamical basis for the stability of this and other water-glass adducts identified, and the potential consequences of these interactions for the behaviour of the glass in a biological context.

Electrospun nerve guide scaffold of poly(ε‐caprolactone)/collagen/nanobioglass: an in vitro study in peripheral nerve tissue engineering

“…Consequently, a lot of water molecules are absorbed by these hydroxyl groups through hydrogen bonds formation (Fig. ) . Overall, collagen and NBG decrease contact angle of conduit.…”

Bioactive Sol–Gel Glasses at the Atomic Scale: The Complementary Use of Advanced Probe and Computer Modeling Methods

“…However, as just noted, a significant fraction of the hydroxyl oxygen atoms bond directly to the (sodium and) calcium, and not to silicon. In this case, the effect on the glass structure is to increase the silicate network connectivity above that which might be expected, as also found in more complicated hydrated yttrium aluminosilicate glasses . Clustering of modifier cations is also potentially important because it increases bioactivity, and these distributions of modifier cations are readily addressed by MD simulations .…”

“…These “free”–OH come from the dissociation of added H 2 O molecules. Likewise, a quantum mechanical study of water in Bioglass also contained free ‒OH, which arose from the dissociation of water molecules. A similar observation was made by Tilocca on a 45S5 glass in which a quantity of Na were replaced by protons.…”

“…The glass surface is first enriched with sodium, which interacts with water outside the glass, allowing water to enter the glass itself, which begins the dissolution . For these glasses, Ca–water interactions occur after Na has been leached into the solution; for the Na–free (CaO) 0.3 (SiO 2 ) 0.7 composition, the Ca–water interactions will presumably occur first, although more slowly as Ca is less mobile through the glass structure than Na . Ca also binds to hydroxyl groups created by water dissociation, stabilizing this process .…”

“…One might speculate that the migration of the Ca cation in such cases (e.g., surface gel layers) is a cooperative process involving the hydrating ‒OH species as well as the Ca cation itself. Among the most reactive sites on the glass surface are nonbridging oxygen atoms (often associated with modifier cations), these can promote water dissociation and the formation of silanol Si–O–H groups on the surface of the glass, which is one of the early steps in the bioactive glass dissolution process. In addition, a large number of the free hydroxyl groups introduced into the glass structure are bound to the modifier cations, as also observed in (CaO) 0.3 (SiO 2 ) 0.7 .…”