Layered: What the Future Might Hold for Medical Grade Additive Manufacturing Materials
Professional social networking websites like LinkedIn are becoming increasingly useful in technology-based industries like medical device manufacturing as one is able to connect with a specific group of interest and talk about innovative subjects with experts from all around the world.
One example is the Medical Additive Manufacturing and Rapid Prototyping group on LinkedIn created by Carsten Engel, a biomedical engineer from Sirris, the collective research centre of the Belgium technological industry. This group gives specific insight and reports about case studies for medical devices such as patient-specific implants and instruments.
One of the latest subjects was a question: “What materials are you expecting to be developed for the medical field and why?”. This question had quite an unexpected amount of reactions (more than 50 comments from different experts) around that topic.
Writing for Medical Plastics News, Carsten has summarised the responses from the group as follows.
Material properties and performances requirements of orthopedic implants continue to evolve. OEMs are looking for materials that are stronger, have a better wear resistance, and exhibit biocompatibility. Manufacturers serve patients who expect to remain active and want to resume daily activities sooner. The need for patient-specific designs and implants becomes not only necessary for cases of trauma or birth defects, but since every human and its anatomy are different, the implant has to be adapted to the patient’s anatomy as well and not the other way around. One additional trend would be to create multi-density parts built by Additive Manufacturing that offer solutions to medical imaging technology and anatomical models derived from real human scan data.
Current trends and latest material developments for implants show a tendency in all three big classes of materials: metals, ceramics and polymers.
Titanium (Ti6Al4V) is the material of choice for these applications. This is mainly because of its low density and extraordinarily strong biocompatibility, combined with excellent corrosion resistance and mechanical resistance for load bearing applications. Manufacturing patient-specific implants in the shape of the implant alone is not sufficient. To overcome stress-shielding related problems, cellular structures (“lattice structures”) are needed and have been studied more and more in the literature.
Indeed, the internal structure of the stiff implant causes a mismatch leading to the well known stress-shielding effect that leads to bone resorption around the implant and then loosening of the implant. One of the trends is to use tantalum or even beta-phase titanium alloys in order to reduce the gap between the elastic modulus of the metallic implants and the one of the bone.
Other ideas are to use Mg-alloys or even Fe-alloys which are able to degrade once implanted after a certain amount of time (up to two years). This idea is very promising, since it would mean that the metal is degrading progressively and being controlled in order to have one grow back, leaving only natural material in place.
Yet the use of Mg-alloys seems to be highly dangerous to additive manufacturing technologies because of explosion risks while using fine powder grain sizes. At Solid Concepts, developments are ongoing as to find a way to suspend heavy metals in cast urethane. This would allow the parts to be used for shielding purposes for x-ray devices. Other trends are to develop memory-shaped alloys, such as nickel-titanium alloys for additive manufacturing technologies.
On the other hand, ceramics such as hydroxyapatite (HAP) and tri-calcium phosphate (TCP) or a combination of both that are currently being developed and industrially used for spinal and cranio-maxillo-facial implants are biodegradable and bioactive. Since those can be produced through additive manufacturing technologies, they have a big advantage compared to ceramic foams.
Indeed, additive manufacturing allows a layer by layer fabrication, thereby making the creation of foams with interconnected pores possible. Those geometries are proven to be more efficient for the osteointegration and thus the acceptance of the implant.
Currently, more than 10,000 inter-vertebral cages in HAP are produced every year by Sirris. Compared to metals, ceramics remain very brittle however and cannot be used for load bearing applications.
Polymers are receiving more and more attention and seem to be easier to process or to developed with those technologies. A few examples are PAEK, PEKK and PEEK which can be processed on EOS-type machines, as well as more bioresorbable polymers such as PLLA. Bioresorbable materials do not posess the mechanical strength to enable them for being used for major load bearing applications such as spinal and cranio-maxillofacial applications (see image). PEEK and PEKK materials still remain a hot topic, but the high cost of entry continues to slow the growth of acceptance. Sintering PEEK as compared to CNC machining a PEEK product seems to make sense when as many as 50-60% PEEK shavings are wasted during machining.
EOS Victrex PEEK material for SLS has a 0% refresh rate. so even with a 50% full bed of parts (by volume)—which is unlikely—you would have worse that a 50% scrap rate (equivalent to CNC shavings). The only difference being that the wasted SLS powder will have cost considerably more to produce than the machining waste. On the P-800 PEEK machine, EOS systems have an adjustable build chamber, so we can effectively maximise the build efficiency. Because we can nest and stack multiple parts, total part volume to chamber volume is a more valuable consideration than part volume to billet volume.
The selective laser sintering (SLS) versus CNC decision often comes down to part orientation, geometry size and shape. Multiple parts per build can become competitive compared to CNC, where machining multiple parts out of a billet can be a challenging project. Simple machined parts will have a better surface finish and higher accuracy, whereas SLS PEEK process has the advantage with complex parts that are difficult, if not impossible to CNC. Several SLS projects we have done were initially 3+ components and by doing them in SLS PEEK the resulting part count is down to one. Obviously this does not work for every geometry, but for certain specific geometries it is a great match.
Among polymeric degradable and biocompatible materials, there are today a number of possibilities including polymers based on lactic acid, glycolic acid trimethylcarbonate, caprolactone, ethylene oxide/butylene terephthalate, and copolymers. Combinations are virtually infinite, but mechanical properties do not always match the bone, depending on the structure of the bone. For produced porous structures, the elastic modulus of the cellular solid that one would create is also very much dependent on total porosity, pore shapes and configuration in space. Yet, the challenge doesn't lie on the materials, but on the flexibility of the technological platform. So far, when considering biomedical applications, there are only a few systems available and they are rather closed. Developments should aim at an open system where one could plug and play different modules that are all governed by an open software.
Maybe the answer to all these requirements can’t just be found in one material but with multiple materials. One idea would be to use or develop a machine that would enable the manufacturing of functionally graded materials having thus multiple materials printed progressively or controlled for specific zones, where for example loads are more important or stiffness has to be taken into consideration. One should investigate the possibility to recycle the bulk material then as well as the resulting product.
When commenting on the current outlook for materials for additive manufacturing, Jim Woodcock, group editor at Rapid News, publisher of additive manufacturing and product development magazines TCT and Personalize, said: "Materials are often touted as one of the weak points in additive manufacturing—something that is particularly evident when it comes to polymers. The success of PA12 (either in pure form or as 'filled' variants) in selective laser sintering applications has lead to something of a material monopoly. The ease of processing of PA12 combined with excellent end-part properties (minimal water absorption, high impact resistance, good chemical resistance and fatigue resistance) has somewhat stifled new material development much to the detriment of more specialised applications."
He added: "While there are new additive manufacturing compatible polymers being developed all the time, most of them lack the full gamut of desirable properties that would lead to widespread deployment. The choice of metallic materials (either as pure metals or alloys) is much wider, however, and it is here that medical applications have historically been focussed. As Carsten points out, the closed nature of additive manufacturing machines (meaning that new material development and validation is unreasonably difficult) is one of the major hurdles standing in the way of wider adoption of additive manufacturing in medical applications across polymers, metals and ceramics." Medical grade materials for additive manufacturing will be an important focal point at TCT Live, an upcoming additive manufacturing, 3D printing and product development trade show and conference to be held at the NEC in Birmingham, UK, on September 25-26, 2013. TCT Live will be held in the same hall as Mediplas on the same days. Mediplas is a UK trade show for manufacturers of medical plastics.
For more information about PEKK SLS, see pages 36-36 of the January-February 2013 edition of Medical Plastics News.
Courtesey of the Linkedin disccusion group “Medical Additive Manufacturing and Rapid Prototyping”.
Special thanks to:
Carsten Engel (Sirris)—Belgium
Scott De Felice (Oxford Performance Materials)—USA
Igor Shishkovsky (Samara State Technical University)—Russia
Xibing Gong (University of Alabama)—USA
Lorenzo Moroni (University of Twente)—The Netherlands
Mattias Unosson (Exmet)—Sweden
Juan Garcia (Johns Hopkins University)—USA
Phil Reeves (Econolyst Ltd)—UK
Daniel Searle (Solid Concepts Inc)—USA
Carlos Garcia Pando (Fundacion Prodintec)—Spain
Sean Taffert (Cimetrix Solutions)—Canada
Matthew Schmidt (Jalex Medical)—USA
Andrew Snow (EOS)—USA
Shoufeng Yang (University of Southampton)—UK
To connect with any of these contributors please visit the Medical Additive Manufacturing and Rapid Prototyping LinkedIn group page.
Image courtesy of Xilloc Medical.