Materials
Tesla was a human calculator. His ability to work out complex maths and physics equations helped him achieve early career success in Europe. In the digital age, this approach has been taken to the extreme by machine learning, with computing power capable of processing such equations in a matter of seconds at times. Across swathes of heavy industry, pharmaceuticals, software design and even in arts and culture, AI has brought experimental efficiency and the slow eradication of systematic trial and error in the lab. The super- expanding field of biosynthetic materials and 3D- printable bio-inks is no different. In fact, without machine learning, a new 3D-printing technique that is helping amass a vast library of materials for use in all industries.
The work of aerospace and mechanical engineer professor Yanliang Zhang and his team at the University of Notre Dame, Indiana, the US, has been a magnet for global attention in this field. His career is built around research into thermoelectric materials, but he has accrued knowledge about bio-inks and their flexibility for 3D printing. His most recent research contributions show how high-throughput combinatorial printing (HTCP) can aid in the quest for materials used in multidisciplinary contexts. Although Zhang has an aerospace engineering background, the tentacles of his groups’ work cut across many sectors. Zhang has been working towards an autonomous system for designing mixes of multiple aerosolised nanomaterial inks drawn through a single printing nozzle. The “art”, he says, is in controlling the ink blend ratios rapidly during the printing process. “Before, we were looking at one or maybe two decades to discover a material with a valuable application, be it in engineering, medicine or other sectors,” Zhang adds. “With medical applications, we hope to reduce that timeline of discovery to less than a year, even down to a few months, if we can create a functioning autonomous and self-driving process for materials discovery and device manufacturing.” Using HTCP, Zhang and his team can control both the printed materials’ 3D architecture and ink compositions. His group can now produce materials with “gradiated compositions” and properties at microscale spatial resolution. In medicine, the potential application for these materials is vast for use as protective coatings around joints and tendons and many as yet unexplored areas of orthopaedics.
The new alchemy
The momentum for this revolution in both engineering and chemistry stems from the urgency to develop inks and machines to handle the tolerances needed to create bespoke materials.
Medical Device Developments /
www.nsmedicaldevices.com
The physical chemistry involved is fascinating too, with a smorgasbord of chemical complexes available. HTCP allows direct fabrication of thin films of metals, nitrides, carbides, chalcogenides and halides – materials that outwardly would seem to be incompatible. The method of screening combinatorial materials is opening doors to structures thought impossible only a few years ago. For example, Zhang’s team report that the aerosol jet printhead has nozzles engineered with variable sizes that can deliver spatial resolution as low as around 20μm in the x–y plane and a deposition thickness as low as approximately 100nm.
“We printed a polymeric scaffold using material laced with Sporosarcina pasteurii, a bacterium that in nature starts the process of mineralisation to calcium carbonate deposition. After four days, the bacteria triggered the mineralisation process in the scaffold, and we were handling a fi nal product with a mineral content of over 90%.”
Esther Amstad
In an optimised range of ink flow rates, Zhang and co-workers found that this monotonic trend can be applied to a variety of nanomaterial inks, including silver nanowires (AgNW), graphene, thermoelectric compounds like bismuth telluride and even polystyrene. Before HTCP, the trial- and-error system required extensive processing time. It not only caused difficulty with high- throughput fabrication, but could lead to messy side reactions, in some cases via a mismatch of starting materials related to their surface charge, pH values or ionic strength. Zhang uses nanoparticles of MXene – a 2D inorganic compound with a similar structure to graphene – and antimony telluride (Sb2Te3), as an example of such a mismatch. “MXene and Sb2Te3 nanoparticles exhibit opposite surface charges in a certain pH range, [which] leads to the formation of larger aggregates with poor colloidal stability.” But HTCP, Zhang adds, enables the rapid fabrication of combinatorial “samples”, minimising these unwanted side effects and resulting in a stable material with advantages conveyed by both compounds.
Sustainable and resilient In Europe, a laboratory in Switzerland is also pushing the technological envelope. Esther
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