Controlling Waterglass Gelation for Binding Composite Bone Scaffolds
Creators: Matinfar, Marzieh
Publisher: name: University of Alberta Library
Year: 2024

<p>Bioactive glass bone scaffolds are promising for bone regeneration due to their ability to bond with bone, but current processing techniques face challenges, particularly regarding heat treatment and formability. To address these issues, our lab explored a new approach by creating formable composite scaffolds using 45S5 bioactive glass powder mixed with a sodium silicate binder (waterglass). However, achieving clinically practical setting times remained a challenge.<br>
This dissertation addresses this issue by optimizing the sol-gel transition of sodium silicate solutions (waterglass) with acid initiators to create formable bioactive glass composites. Since the setting of the composite depends on waterglass gelation, the focus was on controlling this gelation process. Given the complex nature of waterglass and the existing knowledge gap compared to well-understood bioactive glass, the sol-gel transition of waterglass was the primary focus. The main goal was to reduce the gelation time of waterglass to a practical range, as well as to understand the relationship between processing conditions, molecular structure, and mechanical properties of the resulting gels. The experimental section was divided into three parts, each addressing a specific aspect related to the hypotheses: (1) gelation kinetics, (2) molecular structure, and (3) mechanical properties and microstructure.<br>
In the first part of the research, by varying pH (from 2-11), waterglass concentration (from 15-50 wt.%), and acid initiator types (boric and phosphoric acids), the gelation time was successfully reduced from 10 days to ~10 minutes. The pH and waterglass concentration were found to significantly impact gelation kinetics and the final gel microstructure, while the type of acid was less significant. The gel point identified through a quantitative method (UV-VIS spectrophotometry) indicated the onset of gelation (~8 minutes), while the qualitative method (tube inversion) indicated the point where the gel network became rigid.<br>
In the second part, molecular structure analysis using Raman spectroscopy revealed that basic gels contained higher-order silicate rings, with ~60-72% Q3 units and ~10-17% Q2 units, whereas acidic gels had ~70-80% lower-order silicate rings, predominantly Q2 (~62-80%) and Q0 (~20-38%). Moreover, real-time Raman spectroscopy revealed that gelation proceeds by agglomeration of silicate particle through physical interactions. <br>
In the third part of the study, mechanical testing under compression showed that all hydrogels initially had similar strengths (7.3-9.9 MPa). Aging significantly increased their strength, with basic gels reaching 16.4-38.0 MPa and acidic gels reaching 21.2-53.7 MPa one hour after gelation. Engineering stress overestimated true stress by 21-66%. Real-time monitoring during compression, along with fractography, revealed radial, circumferential, and splitting cracks. Basic gels, with larger structural units, formed looser, mesh-like structures with large pores (3-5 µm), resulting in higher ductility and shrinking pores over time. In contrast, acidic gels, with smaller units, formed dense, brittle gels with small pores and strong internal connections.<br>
This research provides a detailed understanding of the sol-gel transition in sodium silicate solutions, highlighting the influence of processing condition on gelation kinetics, molecular structure, and mechanical properties. These insights facilitate the design of optimized bioactive glass scaffolds, improving their clinical applicability for bone regeneration by enabling tailored scaffold properties.</p>