Materials
Two 3D printed skulls created out of two different materials. The left hand side is nylon and resins, while the right hand side is polymers.
In August this year, surgeons at University Children’s Hospital in Krakow, Poland, saved the life of a baby girl who was born without a fifth of her skull shortly after she’d been given only four days to live. Using 3D-printed skulls produced by Polish hardware R&D company Sygnis, one made from nylon and resins, the other from polymers, they were able to assess the extent of bone loss and plan the operation.
These incredible achievements wouldn’t have been possible without material scientists and engineers working alongside doctors to find the best materials for 3D printing. Material is often chosen based on three key criteria – biocompatibility, sterilisability and bio-usability – whether it has physical properties similar to the area of the body being replaced, such as its flexibility and strength.
The right material
Plastics such as acrylonitrile butadiene styrene (ABS) or nylon are the materials of choice for 3D-printed prosthetics and medical models, because of their malleability and lightweight properties. These include polylactic acid (PLA), a thermoplastic polyester which is commonly used to create biodegradable limb sockets, screws and pins, but can also be used for bone scaffolds and drug delivery systems. Two popular thermoplastic polymers are Polyetheretherketone (PEEK) and Polyetherketoneketone (PEKK) and are commonly used for 3D-printed implants for bone and cartilage replacement because of their strength and biocompatibility.
Acrylic or polymethyl methacrylate (PMMA), otherwise known as ‘bone cement’ is used to fill in the gaps between implants and bones, as well as to make surgical tools. It makes good dentures and fillings too as it’s easy to dye to match the colour and size of a patient’s teeth. In the same way, bio- ceramics such as hydroxyapatite (HA) and tri- calcium phosphate (TCP) are used for dental implants like crowns because of their similarity to teeth and bone.
Although ‘skin friendly’ silicone is used for implants, 3D printing on this particular material is
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still in its early stages. Challenges include the time it takes to print and its high viscosity, which makes it difficult to print to the standard needed. However, engineers are busy developing and perfecting 3D printers for silicone to bring more medical devices created using the material to market.
Tough enough
One material that stands out for many surgeons and researchers using 3D technology for implants is titanium. Professor Mika Salmi is research director for the Aalto Digital Design Laboratory at Aalto University and has seen a huge increase in personalised treatment through 3D printing using titanium in recent years, especially in implants. He works in collaboration with surgeons at Finland’s Helsinki University Hospital on 3D printing personalised medical devices, for example, for patients with maxillofacial injuries caused by accidents, and tumours where bone has been damaged or needs to be removed.
“3D printing cannot yet make the perfect material and so you need to select something that is good enough, and quite often that is titanium,” he explains. “We obviously work very closely with doctors who use medical imaging and from the layer images, we can calculate what the patient’s bone would look like and use that to create our 3D model.” Polymers come in handy in Salmi’s lab too though, as his team create pre-operative models made of plastic so that surgeons can better plan operations. “A volumetric titanium net structure implant can then be 3D printed, which allows tissues to grow through and attach to other surrounding tissue,” he says. “Implants are attached to bone with titanium screws.” Titanium is also used to 3D print joint replacements and dental implants, while another type of metal, cobalt chrome, is widely used for replacement joints, like hips, shoulders and knees. As well as being used for surgical tools, stainless steel can also be used to print temporary implants.
The future is here
The world of 3D printing in medicine sometimes sounds like something straight out of science
Medical Device Developments /
www.nsmedicaldevices.com
Sygnis SA
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