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Materials


got three different materials there straight away, that all have to work together and all have to be made almost at the same time. Then the blood vessels need to be open to be filled with blood and to hold it at the right pressure. The first one was probably one of the hardest things I’ve ever made, in terms of its technical difficulty.”


The models produced by the Nottingham Trent Advanced Textile Research Group are able to match the visual and tactile qualities of real diseased livers.


speeds into the hushed, excited tone that one might take when watching an actual procedure. “As he started cutting away and revealing the tumour, he realised that it was invading one of the blood vessels, which was obviously filled with blood, and when he started to try and remove it, it started to seep, so immediately he said, ‘Right, we need to clamp that’,” Arm recalls. “He was able to see something he wouldn’t have otherwise, and then respond accordingly, and then to plan his surgery slightly differently to what he originally intended. Even if a surgeon doesn’t complete the [simulated] procedure, they know and understand what’s actually happening with the liver, so when they go into theatre, they’re much more prepared to deal with any problem scenarios that might arise.”


Live and learn


The Nottingham and Minnesota teams use 3D printing very differently, which is testament to Arm and McAlpine’s shared belief that the technology is secondary to the way it is employed. Whereas McAlpine uses a custom 3D printer that can extrude a range of silicone thermosets inks from different syringes using air pressure, Arm describes the Nottingham team’s approach as a “post-print processing technique” for achieving specific material goals. By making a mould of an organ 3D printed in a more traditional thermoplastic and casting it in other materials, the Nottingham research group have sidestepped the difficulties of adapting commercially available products to print tissue-mimicking materials. “It’s very much about manipulating the prints using these post-print processing techniques of casting and coating, and extrusion and moulding,” says Arm. “Materials are applied in different ways depending on what we want to achieve. With these livers, we also recreate the vasculature and the tumour, and embed it all in the right place. If you think about it, you’ve


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By contrast, the Minnesota team directly prints specific mixes of different types of greases, silicones and colourings – which isn’t much easier, as McAlpine explains. Quite apart from mimicking the properties of biological tissue, these inks need to cure fast enough to solidify into the desired structures and slowly enough not to clog the print nozzle. The strategic use of chemical agents inside the silicone makes that possible, ensuring that enough polymer chains link together at room temperature to solidify the ink into the desired shape – although, as with the Nottingham liver, a separate supporting material is needed in the meantime. As such, McAlpine’s aortic root models are made with four specifically tuned and simultaneously printed inks: three of which mimic the aorta and one that holds them in place as they cure. In Arm’s terms, printing and processing these organ models is a way of translating between the generic descriptions in clinical literature, the visual language of radiologists and the tactile understanding of surgeons. Both research groups start by aligning their materials with the results of published tests on the mechanical properties of tissue and blood vessels. From there, they employ scan data and radiological insights to recreate patient-specific attributes, like calcification geometries on aortic valves, or fibrosis in the liver, which stiffens the tissue and makes it appear lighter in scans. McAlpine and Arm both started their work on 3D printing tissue mimicking materials because of requests from medical practitioners and throughout the development process, each relied on the expertise of radiologists and surgeons, alongside polymer experts and mechanical engineers. For his part, Arm prefers to call the Nottingham livers multidisciplinary products over 3D printed ones. Most importantly, though, they reached their final iterations by learning from their mistakes. “The first one took us probably two months to recreate,” says Arm. “The next one took about two weeks, and the third one took two days. We learned from just making and failing, and then making again – learning from the mistakes and recreating things with different steps and different materials. I think we’re all our own best teachers – anybody who has any sort of wisdom about themselves has learned from their mistakes, and not only is that teaching yourself, but it’s teaching yourself in the way that you know how to learn. It leads to such individuality and such growth.” Surgeons are too important not to be able to do it too. ●


Medical Device Developments / www.nsmedicaldevices.com


Nottingham Trent University


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