MATERIALS


3D


bio-printing uses bio-inks to fabricate scaffolds to support the growth of body tissues layer-by-layer


Researchers in China have developed in 2017 a new solvent-free, photo-curable poly- imide ink that hardens with exposure to light


that did not contain Laponite – which had ‘high initial (bursts) of protein release’ – the sustained release of the Laponite-based bio-ink could support long-term drug or protein delivery. As for the future, ‘our next


steps will focus on enhancing the cellular response within these printed constructs,’ according to Richard Oreffo, a researcher at the University of Southampton. ‘Either by subsequent preservation of the crosslinked state of the scaffolds or by usage of the attractive growth factor binding capabilities of the bio- ink with proteins that enhance cell responses.’


Polymer-based inks But it’s not just bio-inks for healthcare applications that are gaining increased recognition. There is also huge potential for polymer-based inks for industrial manufacturing purposes. Researchers at the Lanzhou Institute of Chemical Physics (LICP) at the Chinese Academy of Sciences, for example, have been working on a new solvent- free, photocurable polyimide (PI) ink that hardens with exposure to light.4 The project employs


two 3D printing techniques, stereolithography (SLA) and digital light processing technology (DLP), to create objects with good structural and mechanical properties. The use of polyimide is critical,


References 1 Armstrong, J.P.K, Burke M., Carter, B.M, Davis, S.A., & Perriman, A.W. 2016. Advanced Healthcare Materials, Volume 5, Issue 14, pp1724−1730.


2 Kim, K., Menard, F., Tian, Z., & Wang, Z. 2017. Biofabrication, Vol. 9, No.4. IOP Publishing.


3 Ahlfeld, T., Akkieni, A.R., Cidonio, G., Dawson, J.I., Duin, S., Gelinsky, M., Kilian, D., Lode, A., Oreffo, R.O.C., & Yang, S. 2017. Biofabrication, Volume 9, No. 3.


4 Guo, Y., Ji, Z., Yun, Z., Wang, X., & Feng, Z. 2017. Journal of Materials Chemistry A, 5, 16307-16314.


5 Dunand, D.C., Geisendorfer, N.R., Jakus, A.E., Shah, R.N., & Taylor, S.L. 2015. Advanced Functional Materials ,Volume 25, Issue 45. pp6985–6995.


explains lead researcher, Xiaolong Wang. ‘The majority of currently available resins for 3D printing are acrylic or epoxy, have poor mechanical strength, poor thermo- resistance and poor chemical stability,’ he says. ‘Therefore, the development of high-performance photosensitive resins, such as PI and polyetheretherketone (PEEK), for SLA and DLP, has become a vital focus to meet harsh application requirements.’ By designing the molecular


structure of polyimide, Wang and the team were able to prepare a polyimide oligomer with excellent solubility in acrylic reactive diluent monomer. 3D printing with the ink proved successful, enabling the team to produce complex structures that were tested for hardness, tensile strength and elongation – all of which are crucial mechanical properties for the use of an object in high- performance manufacturing. This


Our method creates great versatility, expanding the architectures and metals we’re able to print, which really opens the door for a lot of different applications


Ramille Shah McCormick School of Engineering


research, therefore, paves the way for the photo-curing polyimide ink that shows high promise for SLA/DLP 3D printing.


‘Design and synthesis of the polyimide molecular structure is probably the most novel and unique aspect of our work. The resulting 3D printing polyimide inks obtained mean that one can make polyimide material that can be directly used in devices,’ Wang enthuses. ‘This will significantly expand the practical applications of 3D printing in many fields, such as automotive, aerospace and electronics.’


Metal-based solutions


The 3D printing of metal structures opens up yet other opportunities, in end-use industries such as aerospace, automotive and rail. A team at Northwestern University, US, began in late 2015 the development of a process to create complex 3D-printed metallic structures5


with a new class of


inks using a range of metals, alloys, oxides and compounds, which can be synthesised into green-body structures – objects mainly constituted from weakly bound clay material – followed by thermochemical transformation into sintered metallic counterparts. The project made use of a biomedical polymer commonly used in clinical products, such as sutures. When used


as a binder, it makes green-bodies that are ‘very robust,’ despite the fact they are mainly made of powders. The process will even allow for the use of inks made from low- cost rust powder, which is lighter and more stable than other iron powders. The ink can be created in any volume required when mixed with solvents, powders and a biomedical elastomer called polylactic-co-glycolic acid (PLGA). The mixture starts out as a liquid ink made of metal or mixed metal powders, solvents and an elastomer binder. The team was able to use this liquid ink to rapidly print densely packed power structures through a simple syringe-extrusion process, in which ink was dispensed through a nozzle, at room temperature. The resultant 3D printed green-bodies are transformed into solid metallic counterparts through reduction and sintering in an H2


atmosphere


at elevated temperatures, creating large and robust objects that can be immediately handled. Ramille Shah, assistant


professor of materials science and engineering at the McCormick School of Engineering, explained: ‘This [research] is exciting because most advanced manufacturing methods used for metallic printing are limited as far as which metals and alloys can be printed and what types of architecture can be created. Our method creates great versatility, expanding the architectures and metals we’re able to print, which really opens the door for a lot of different applications.’ The ability to use cheap oxides, such as rust, which can be turned into metal through a reduction process, presents huge benefits for manufacturers. Possible applications for the ink include producing items such as batteries, solid-oxide fuel cells, medical implants or even mechanical parts for airplanes. Clearly, 3D printing inks have


huge untapped potential. They promise substantial environmental benefits, and cost savings through more efficient, lightweight product design, as well as the development of natural bio-inks and the possibilities for on-site construction. And while many of the 3D inks described here may yet be in their infancy, one thing is already clear: their future won’t simply be written on paper.


36 02 | 2018


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