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Study of Calcium Phosphate Formation Driven by the Dissolution of a 45S5 Bioactive Glass


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Authors

Lopez-Fontal, Elkin M 

Abstract

Although models have been proposed to explain the mechanisms of BG dissolution and subsequent CaP mineralisation, open questions remain. The processes in which phase transition occurs in aqueous solutions and their dynamics remain under-explored partly because traditional instruments/techniques do not allow for direct observations at the adequate time and length scales at which such phase transformations occur. For instance, given the crucial role of the silica gel in CaP formation during BG dissolution, uncertainty exists on how such silica gel forms on the BG surface. In the case of CaP formation driven by BG dissolution, questions can also be added, i.e., how CaP develops into an apatitic-like structure, how many transient phases there are, and, in general, the phenomena occurring in the solid-liquid interface during BG dissolution.

Several approaches were taken to study CaP mineralisation driven by bioglass dissolution, mainly examining the solid-liquid interface and the BG after-reaction surface. Electron microscopy techniques were used, including liquid phase transmission electron microscopy (LP-TEM), scanning electron microscopy, and FIB cross-sections. LP-TEM was expected to give a fresh look to the BG dissolution and its adhered phenomena by enabling direct observation. Therefore, one of the aims was to establish whether this in-situ imaging technique was suitable for studying this material system. However, the original BG's (Novamin™) particle size was too big to be directly used in the liquid cell (LC). Therefore, the first aim was to develop a method to obtain sub-micron BG particles. Ex-situ experiments were also performed, and results were compared with in-situ experiments. Other analysis techniques, such as XPS, PXRD, FTIR, ICP-EOS, ToF-SIMS, etc., were utilised to help indirectly understand the studied material system.

Ball-milled bioglass (BMBG) particles of suitable size were obtained and directly used in the liquid cell. Moreover, it was found that the grinding method utilised does not alter the BG particles’ chemical composition. Consequently, by judicially setting up LP-TEM experiments, novel observations of BG dissolution and CaP mineralisation processes occurred in their native liquid state. This study established the suitability of LP-TEM to study BG dissolution, determined its critical operational factors, the vast potential of in-situ imaging and how modern TEM microscopes could hugely benefit the investigation of material systems like bioglass. Furthermore, cross-sections of reacted BG blocks gave essential insights into the BG dissolution phenomena, particularly its strong dependency on experimental conditions, and tentative evidence has shown that soluble silica from BG dissolution may not reprecipitate/re-polymerise on BG particles’/BG blocks’ surface.

On the other hand, ex-situ experiments of BMBG dissolution were also performed, giving meaningful insights into this material system (BMBG). However, differences were found when comparing ex-situ experiments and in-situ experiment results. Additionally, under the experimental conditions used in this study, complementary analysis techniques determined that CaP, during BG dissolution, transitions from ACP to a calcium-deficient nanocrystalline apatitic structure with minimal contents of Si4+ and Na+ ions that may be molecularly part of CaP.

Some of the challenging topics of current and future BG research, i.e., controlled release of therapeutic ions or biomolecules, reliable BG coatings, and tunable mechanical properties, may be better tackled by improving the knowledge of BG dissolution. Moreover, for many years, the Hench model has been the core guidance of how bioglass dissolution and the subsequent CaP formation occur. However, this study shows tentative evidence that contributes to and somewhat differs from it.

Description

Date

2023-02-15

Advisors

Langford, Richard

Keywords

45S5 Bioactive glass, Ball-milled bioglass dissolution, Bioglass blocks dissolution, Bioglass cross-sectioned, Calcium phosphate, In-situ imaging, Liquid cell TEM, Liquid phase transmission electron microscopy

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Sponsorship
EPSRC (1803545)
The main funder of this project was the Engineering and Physical Science Research Council (EPSRC). Also, Glaxo Smith Kline (GSK) helped with sponsorship.

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