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Cambridge University Science Magazine
The first 3D printer was built in the 1980s by Chuck Hall, and it has been suggested that the majority of first-world-country households would own their own 3D printer. Dropped and smashed your favourite mug? Not a problem, print another. Lost the remote control again? Hit start on the printer and you could have another one by morning. Recently, 3D printing has had a huge amount of attention — objects can be made cheaply and with very little waste. Its popularity is increasing from printing day-to-day household objects to industry to scientific discovery, providing an opportunity for engineers to collaborate with any other industry from construction to healthcare. Printing has developed considerably since the 1980s; in the early 2000s, Thomas Boland made the first bioprinter — which meant printing with live cells became a reality.

3D printing is the process of layering down material to build up a 3D object. The general process of 3D printing is to first make a model using computer-aided design (CAD) software, a software developed specifically to allow for the design of any 3D object, or to use measurements taken from imaging such as MRI scans. The model then needs to be transferred to a slicer software to convert the file into a language the printer can understand. Here, decisions such as how sturdy the structure needs to be, or how cheaply it can be manufactured, are made with the infill pattern and density. If the structure needs to be sturdy, a higher infill density is required but this leads to more material being laid down and therefore higher costs. Finally, the file is uploaded to the printer along with the appropriate ink, nozzles, and settings. Hit print and, depending on your materials and size of object, it will be generated in seconds or up to days later.

The variety of applications with 3D printing are vast — from aerospace, car manufacturing, scientific research, to construction — creating a more sustainable and cost-effective solution. The world’s first 3D-printed school has opened its doors to teach children in Malawi. The construction took just 18 hours and produced very little waste. The collaboration between 14Trees and Holcim has provided children an opportunity to have an education. With more schools being built by 3D printing by organisations such as Thinking Huts, more and more children will get the opportunities they deserve.

Moving on to healthcare, customisation is key to any effective treatment method. We are constantly learning that healthcare needs to be individualised to the patient’s needs for the best outcomes. 3D printing provides an easy way to customise implants that go into the human body. One area that requires this personalisation is in joint treatment. Researchers are working on treatments that can help avoid the inevitable and destructive total joint replacement, particularly for the younger age groups. One way of doing this is to insert a metal plate to relieve the weight and pressure on the damaged area of the joint. This procedure is known as a high tibial osteotomy.

Engineers at the University of Bath’s Centre for Therapeutic Innovation have been working in collaboration with 3D Metal Printing Ltd. to achieve a tailor-made plate that has fewer complications compared to the standard. They have 3D-printed medical-grade titanium-alloy plates designed from CT scans of patients, and have used computer modelling to determine the safety and risk of implant failure using the personalised plates compared to the standard. Although this study is currently in the computer-modelling stage, the plates have been approved for clinical trials.

The power of 3D printing is also shown through the creation of replica organs. These models can be made rapidly, cheaply, and in a lot of detail. Surgeons at Guy’s and St Thomas’ NHS Trust used a 3D-printed model of a cancerous prostate to not only pre-plan the procedure, but to also use as a guide during surgery. This particular procedure was successfully performed in 2016 using the minimally invasive robotic surgery. Despite the many advantages traditional robotic surgery has, the one drawback for surgeons is losing the ability to physically feel the tissue. Having a 3D-printed model replica directly in front of the surgeons gives them their sense of touch back and enables them to determine precisely where to cut. This was the first time a model was taken into surgery and used to ensure vital components of the tissue were not damaged, and complications were minimised. The model itself was made from MRI scans of the patient’s prostate, and 3D-printed in a lab at St Thomas’ Hospital. It took 12 hours to print the model, and cost as little as £150 – £200.

Replica organ models can save countless lives, but can we print real organs? This technique, known as bioprinting, allows cells to be printed into any desired shape. It seems simple enough — design the model or, better yet, take images of the patient’s organs and then print. When it comes to printing biological tissue, there is a lot more to consider than the model. Firstly, what is available to print the cells in?

The bioinks available in which cells can be printed are very limited, as bioprinters work by forcing ink out of a nozzle and onto a print bed for cross-linking. The bioinks need to be shear-thinning, meaning that its viscosity decreases as it is forced through the nozzle and still needs to be capable of holding its structure once it hits the print bed. This very important property minimises shear stress placed on cells during the printing process, and leads to a higher yield of live cells in the scaffold.

Secondly, there is a trade-off between cell survival and resolution of the final print. Increasing the resolution means using a small nozzle to print finer structures, but this places greater shear forces onto the cells, resulting in a higher proportion of cell death. A fine balance needs to be optimised to ensure cells remain alive during the print, but also that the desired model can be achieved. Material choice is limited, even more so by the need for biocompatible materials, porosity for nutrient transfer, and a cell-friendly cross-linking method to stabilise the scaffold. Hydrogels are the ideal materials to use for bioprinting; they have all the required properties, and mimic the extracellular matrix of the cells’ natural environment, leading to a higher cell survival rate.

Further complexities arise when thinking about the intricacies of organs. Multiple cell types, with their unique extracellular matrices, are often required for a single organ. The majority of tissues in the human body require vascularisation for oxygen and nutrient transfer; incorporating these into a print can be complicated. On top of this, getting a printed organ to perform the complex tasks that natural organs do naturally is a challenging feat.

Scientists are exploring ways in which 3D bioprinting organs may be possible in the future, but for now, it is still very much in the research stage. With huge cost implications, not only for the printer itself, but also for the bioinks, energy to run the printer, and equipment, time and skills required for the cells to produce a fully functioning organ prior to transplantation, it seems an impossible task. Even without the future of organ transplantation, 3D printing has revolutionised healthcare.

Sarah Lindsay is a post-doc in the Department of Surgery. Illustration by Rosanna Rann.