Singapore: 3D Bioprinting with Magnesium Alloys to Create Bone Scaffolds

SEM micrographs of samples sintered at different temperatures in the regime of super solidus liquid phase sintering for 5 h, a) 535 °C, b) 550 °C, c) 565 °C, d) 580 °C, e) 595 °C, and f) 610 °C.

Strides in the medical field today via 3D printing have been staggering, and especially in bioprinting, with many different technologies and materials being created. Now, researchers in Singapore are exploring the use of alloys like magnesium in fabricating scaffolding, with their findings detailed in ‘Additive manufacturing of magnesium–zinc–zirconium (ZK) alloys via capillary-mediated binderless three-dimensional printing.’

Magnesium is an alloy that can be used in 3D printing and additive manufacturing, as a third-generation biomaterial useful in tissue engineering; however, as the researchers point out, there are myriad challenges. High affinity to oxygen and a low boiling temperature are issues, along with careful consideration that must be applied when disposing of magnesium powders due to the possibility of reactions with other chemicals.

High vapor pressure can be a major obstacle in using magnesium too, leading the researchers to explore AM processes with ambient temperature. This can allow for all the benefits of powder-bed inkjet 3D printing to be enjoyed, as it can be employed at ambient temperatures, no supports are required, and powder can be fully recycled. Here, the researchers have created a new 3D printing technique including a sintering process which transforms magnesium powder and green objects into functional parts that can be used in scaffolding, producing parts with mechanical properties as strong as human bone.

The research team customized their own ink-jet 3D printer for this study, working to overcome previous challenges with the use of magnesium. Maintaining oxygen percentages at the lowest levels possible was of ‘paramount importance’:

“Conserving oxygen in green objects in low level indicates the promise of formulated solvent for AM of Mg-based alloys,” stated the researchers.

3D printed green samples showed no change at all in composition after the sintering process, leaving the team to point out that this means it is a ‘compositionally zero-sum process.’ With temperature variations, both density and stability were affected. The researchers state that dimensional precision is another element of paramount importance and is influenced when deviations occur in printing. Swelling may cause substantial problems too, resulting in shape loss of printed objects, noted at an increased sintering temperature from to 595 °C and 610 °C. Swelling can also interfere with functionality of components.

Samples after 5 h sintering at different temperature in the range of 535 °C to 610 °C.

In continuing to examine other features, the researchers found that density increases with temperature. In studying the effects of holding time on physical and mechanical properties, they also found that strength may be low even though density has become high. Overall though, for overcoming the challenges required in creating scaffolds, mechanical integrity must be present, along with balanced stiffness and strength:

“Mechanical properties of scaffolds could significantly affect cells behavior and the osteointegration between host tissues and the scaffold; premature collapse of subchondral bonearound bone defects may happen if the scaffold provides more than enough mechanical support,” said the researchers. “Thus, stiffness and strength of scaffolds should be modulated to match with those of host tissues in order to avoid post-surgery stress shielding effects and promote tissue regeneration.”

Healthy scaffolds exhibit good pore percentage, size, and shape, offering osteointegration, nutrients transportation, tissue in-growth, and waste products removal. With all those quotients in order, bone tissue regeneration is possible.

“Mg based alloys classify as a third generation of biomaterials when it comes to clinical outcomes, and capillary-mediated binderless 3D printed Mg part after sintering can provide comparable properties with bone,” stated the researchers.

In their paper, the researchers explain more about the structure of human cortical bone, a hierarchical ‘organization’ of three sizes to include:

1. Haversian and Volkmann vascular canals having diameter in the range of 40 to 100 μm
2. Osteocyte lacunae with size ranging from 10 to 30 μm
3. Canaliculi having diameters on an order of a few tens of nanometers

Issues in porosity can be dealt with as larger pores are created in 3D to compensate for a required percentage, thus refining scaffold for better tissue engineering with bone.

“Increasing holding time from 5 h to 20, 40, and 60 h at optimum sintering temperature of 573 °C allowed steady improvements in microstructural, physical, and mechanical properties for each additional hold time while avoiding the undesirable dimensional loss. Interconnected open-porous structures with apparent porosity of 29%, average pore size of 15 μm, compressive strength of 174 MPa, and elastic modulus of 18 GPa were achieved,” concluded the scientists. “These values are well comparable with those seen for human cortical bone types.”

There is a huge momentum between 3D printing and the medical field today, and it just keeps growing as scientists and researchers continue to work toward the holy grail of fabricating human organs. Along with that, many different types of medical implants have been created and are now improving the quality of patients lives, from facial implants to those meant to facilitate knee replacements. Tissue engineering continues to be at the forefront of 3D printing also with the range of bioinks continuing to expand.

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Schematic illustrations of super-solidus liquid phase sintering process of 3D printed parts, a) total decomposition of interparticle bridges in a green sample after reaching 400 °C, b) nucleation of liquid phase along the grain boundaries and within the discrete islands throughout the grains at the temperature above the solidus, c) breaking MgO film for several particles with increasing temperature, leaking the liquid phase, forming liquid bridges among particles, and d) break down of MgO film, formation of liquid bridges between adjacent Mg particles, and growth of sinter necks diameter in the sample sintered at 573 °C for 40 h.