Experimental setup on the beamline at the Diamond Light Source synchrotron facility. A photograph of the custom-built furnace used in the experiment and schematic of its internal chamber are shown in the insets.
Experimental setup on the beamline at the Diamond Light Source synchrotron facility. A photograph of the custom-built furnace used in the experiment and schematic of its internal chamber are shown in the insets.

Bioglass’s ability to stimulate tissue growth has made it the prime option for repair-guiding scaffolds, especially for bone defects. But fabricating scaffolds from traditional Bioglass is challenging because the high temperatures needed to form a 3D structure can cause crystallization, which is undesirable. Instead, altering the glass chemistry enables the use of sintering to fuse particles together into dense structures without crystallization. Now a team of researchers from the UK’s Diamond Light Source, Birmingham University, Imperial College London and University College London has watched this process as it happens.

“We used synchrotron X-ray tomography at the Diamond Light Source to image bioactive glass particles continuously as we heated them through a sintering cycle to form a 3D porous scaffold,” explains Amy Nommeots-Nomm of McGill University. “It is the first time the three stages of sintering of glass particles in a 3D object have been imaged and quantified.”

The process starts with a 3D printed structure of bioactive glass 13-93 particles, which include a mixture of silicon, calcium, magnesium, potassium and phosphorus oxides. During sintering, the particles melt and merge together to form a dense solid structure. In the first stage, larger angular particles become more rounded and smaller particles start to coalesce with each other, forming larger particles. Sintering is relatively slow during this part of the process and surface area and density change little.

“Our work showed that over 80% of all densification takes place during the second stage of sintering, which comes to an abrupt end once inter-particle pores become isolated,” says Nommeots-Nomm.

The whole of this vital stage, where the most important changes takes place, lasts just 16 minutes, the researchers found. There is a characteristic large fall in surface area and increase in density. In the final stage, the few remaining isolated pores become more spherical or disappear altogether. During the first two stages of sintering, local processes dominate, while global sintering takes over in the latter stage of the process.

“Our study unravels many important never-before-seen aspects of sintering of 3D objects, which we believe will impact research and development in viscous flow sintering, bioactive glass bone tissue scaffolds, and 3D printing of ceramic and glass powders,” she adds.

A better understanding of sintering rom the individual particle through to the macroscale could help bring patient-specific ‘bespoke’ scaffolds that take into account bone density, defect shape and location, as well as the patient’s age and sex a step closer.

“The vision in tissue engineering is to be able to scan a patient’s defect site, design them a scaffold tailored to their needs, print it, and then implant it in one short time-efficient process,” says Nommeots-Nomm.

Nommeots-Nomm et al., Materials Today Advances 2 (2019) 100011

Film showing the evolution of the whole scaffold through the sintering cycle: https://youtu.be/fa9LeXIMLrc

A more detailed view of one strut within the 3D scaffold: https://www.youtube.com/watch?v=KtbUHLmhO-I