Four-dimensional imaging and quantification of viscous flow sintering within a 3D printed bioactive glass scaffold using synchrotron X-ray tomography

Research output: Contribution to journalArticle

Authors

  • A. Nommeots-nomm
  • C. Ligorio
  • A.j. Bodey
  • J.r. Jones
  • P.d. Lee

Colleges, School and Institutes

Abstract

Bioglass was the first material to form a stable chemical bond with human tissue. Since its discovery, a key goal was to produce 3D porous scaffolds which can host and guide tissue repair, in particular, regeneration of long bone defects resulting from trauma or disease. Producing 3D scaffolds from bioactive glasses is challenging due to crystallisation events that occur while the glass particles densify at high temperatures. Bioactive glasses such as the 13-93 composition can be sintered by viscous flow sintering at temperatures above the glass transition onset (Tg) and below the crystallisation temperature (Tc). There is however, very little literature on viscous flow of bioactive glasses and none which focuses on the viscous flow sintering of glass scaffolds in 4D (3D + time). Here, high resolution synchrotron-sourced X-ray computed tomography (sCT) was used to capture and quantify viscous flow sintering of an additively manufactured bioactive glass scaffold in 4D. In-situ sCT allowed the simultaneous quantification of individual particle (local) structural changes and the scaffold’s (global) dimensional changes during the sintering cycle. Densification, calculated as change in surface area, occurred in three distinct stages, confirming classical sintering theory. Importantly, our observations show for the first time that the local and global contributions to densification are significantly different at each of these stages: local sintering dominates stages 1 and 2, then global sintering is more prevalent in stage 3. During stage 1, small particles coalesced to larger particles, due to higher driving force for viscous flow at lower temperature, while large angular particles became less faceted (angular regions had local small radius of curvature). A transition in rate of sintering was then observed in which significant viscous flow occurred, resulting in large reduction in surface area, total strut volume and interparticle porosity as the majority of the printed particles coalesced to become continuous struts (stage 2). Transition from stage 2 to 3 was distinctly obvious when interparticle pores became isolated and closed while sintering rate significantly reduced. During stage 3, at the local scale, isolated pores either became more spherical or reduced in size and disappeared depending on their initial morphology. Globally, during stage 3, sintering of the scaffolds continued at the struct level, where strut diameter increased in size while inter-strut porosity reduced, suggesting overall shrinkage of the scaffold with flow of material via the strut contacts.
This study provides novel insights into viscous flow in a complex non-idealised construct, where, locally, particles are not spherical and are of a range of sizes, leading to a random distribution of interparticle porosity, while globally a pre-designed porosity between the struts exists to allow the construct to support tissue growth. This is the first time that the three stages of densification have been captured at the local and global scales simultaneously. The insights provided here should accelerate the development of 3D bioactive glass scaffolds.

Details

Original languageEnglish
Article number100011
Number of pages9
JournalMaterials Today Advances
Volume2
Publication statusPublished - 1 Jun 2019

Keywords

  • Glass, Sintering, In situ x-ray tomography, Bioactive, Scaffold, Viscous flow