FEATURE
Issue: 

Medical miracles

Gemma Church finds that additive manufacturing is being used to transform lives through advanced implants and guides for surgeons

Caption: Renishaw's Adept software is for healthcare professionals designing maxillofacial implants

Compared to other industries, the medical sector could be considered established in its use of additive techniques. Stephan Zeidler, business development manager for Concept Laser's medical division, said: ‘The medical sector is a mature industry that is driving additive manufacturing in other industries. The goal now is to increase the productivity and to make 3D printing machines more productive to compete with traditional manufacturing.’

The constraints of additive manufacturing have typically centred on productivity versus cost of investment. From a laser systems perspective, there are two methods to improve the throughput of additive techniques. The first is to increase both the number of lasers and their individual operating power within a system to produce a faster machine.

For example, to address productivity, Renishaw has implemented a quad laser system on its RenAM 500Q, where each of the four lasers can address any area of the plate. Ed Littlewood, marketing manager of medical products at Renishaw, explained: ‘Once we have optimised this for medical applications, it will certainly drive the move towards higher volume production using AM.’

It’s not just the number and power of the lasers that are changing; 3D printing systems need to integrate more components to meet medical regulations, as Lars Neumann, industry and application manager for medical AM at Trumpf, explained: ‘The medical device industry is subject to the highest quality requirements. Regulatory bodies require stringent process control and full documentation of the complete manufacturing process. As a machine manufacturer and laser specialist, we must design our 3D printing systems, such as our TruPrint, which addresses medical applications, accordingly.

‘For example, equipping our machines with sensors and cameras to monitor and document the production process is becoming an essential requirement. Our customers need to know as early as possible if an implant is not manufactured according to specification, and they need to prove to the regulatory bodies that they will detect any irregularities in the process,’ Neumann added.

The second method to boost productivity is to create a modular production line, which can be customised to meet the specific needs of the customer. For example, a post-processing machine may only take one hour to complete its work compared to a manufacturing machine that takes six hours to finish a job. So, by incorporating more manufacturing machines, it is possible to optimise the production line.

Zeidler said: ‘A modular production line also enables serial production. The dental industry is probably the best example for the mass production of patient-specific medical devices and has been using this technique for some time to create more than three million parts per year.’

Monitoring modular production lines presents challenges, as multiple machines have to be managed. Zeidler added: ‘We need to be open to redesigning our manufacturing processes and enable customers to use machines more intuitively. An intuitive user interface can help to overcome the constantly rising demand for highly-skilled operators.’

This prevailing skills gap is a pertinent point, as Littlewood explained: ‘Perhaps the biggest challenge is that additive manufacturing and powder-based fusion are relatively new. There’s no doubt that adoption of AM is strong, but we, as engineers, are yet to build up the massive knowledge base that subtractive manufacturing enjoys. This means that production and manufacturing engineers can have around 140 different parameters to get their heads around.’

Littlewood continued: ‘That’s why we optimise a customer’s machine to give certain build attributes that are required for the medical product they are building when we install it. This helps to reduce the learning curve and get them producing products sooner.’

This issue is further exacerbated by unfamiliar pre- and post-production processes, according to Littlewood, who added: ‘None of these are particularly complex or prohibitive, many are analogous to processes for subtractive manufacturing, but they are just not widely understood.’

Additive manufacturing is now reasonably well established in dentistry. Credit: Renishaw

Renishaw has developed several intuitive software tools that automate and streamline many activities. The first is Adept, which is a clinician-focused design package that can be used by healthcare professionals who have little or no CAD experience to design maxillofacial implants. Currently designs require complex design packages that can take hours of a skilled technician’s time. Adept can reduce this time to less than 15 minutes.

The second package is QuantAM and its dental-optimised version, QuantAM Dental. They enable quick and easy preparation of parts for building. The dental version goes a little further and automatically heals the CAD file, orientates the part, places ID tags, creates supports and nests with similar device types. ‘Along the entire journey we focus on ease of use, while maintaining flexibility and quality,’ Littlewood added.

Facial reconstruction

Additive manufacturing is regularly used to create personalised implants because of the flexibility of the production process. Littlewood explained: ‘It’s now as simple to produce a batch of custom implants as it is to serially produce a batch of one design. This means that custom implant production is 
now significantly easier and more cost effective.

‘Serial production of implants also gains from the ability to integrate complex lattice structures into a design. This can aid weight and material savings, and enable implants to mimic the stiffness of bone or to potentially improve osteo-integration,’ Littlewood added.

Additive manufacturing is used regularly during complex facial and cranial surgery to create both the custom implantable plates and the surgical guides to aid such procedures. In one example, surgical guides were printed for a patient suffering from a bone disorder in which scar-like tissue develops in place of normal bone, which severely disfigured the cranium and orbital bone surrounding the left eye.

A titanium spinal cage implant. Credit: 3D Systems

3D Systems printed a model using its ProX stereolithography technology that showed what the patient’s skull would look like once extraneous bone was removed. Stereolithography (SL) is the process by which a 3D printing machine, called a stereolithograph apparatus (SLA), converts liquid plastic into solid objects using a UV laser.

Two surgical guide templates were also created; one helped the surgeon position a temporary patient-specific titanium mesh to protect the patient’s eye socket region, and the other represented the bony overgrowth that needed to be removed from the patient’s skull.

Kevin McAlea, executive vice-president and chief operating officer of healthcare at 3D Systems, said: ‘Such templates are used to guide the surgeon, which takes all the guesswork out of the surgery – the surgeon can perform with confidence, spend less time carrying out the procedure and, therefore, the outcome is better for the patient.’

These templates helped the surgeon to make a reference mark on the bone to help ensure that the custom polyether ether ketone (PEEK) implant was positioned correctly. PEEK is a high-strength plastic used to create patient-specific cranial implants.

However, the use of additively manufactured plastic implants is usually reserved for complex and bespoke procedures, because of the slow processing times but extremely high quality and resolution of the parts. ‘Depending on the application and the technology, there is a trade-off between the productivity of the laser systems and quality of the parts,’ Zeidler added.

Material restrictions

While 3D printing works with some metallic powders, the conservative nature of the healthcare industry means it is yet to realise the widespread use of additively manufactured composites or more exotic materials. Neumann said: ‘Trumpf systems process metallic powder. Currently, these are mainly titanium and cobalt-chromium alloys in the medical domain thanks to their biocompatibility and suitability for additive manufacturing. Research institutions are investigating further materials, both metals and plastics, but a multitude of technical and regulatory requirements must be fulfilled.’

However, plastic additive techniques such as selective laser sintering (SLS) have been used for revolutionary medical applications. SLS uses a high-power CO₂ laser to fuse small particles of powdered material to create 3D parts. The laser selectively fuses powdered material by scanning X and Y cross-sections on the surface of a powder bed. The model is built one layer at a time from supplied 3D CAD data. SLS is capable of producing highly durable parts for real-world testing.

One of 3D Systems’ customers used a 3D body scan and SLS to make custom, lightweight parts for a bionic suit to help a patient confined to a wheelchair stand upright and walk again. McAlea said: ‘SLS is not quite as accurate as some additive techniques, but the durability is much better. Also, some plastics cannot be sterilised and therefore used in the body, so they are restricted to external use.’

A cranial plate produced by AM. Credit: Renishaw

However, additive manufacturing is used to create plastic spinal cage implants, as McAlea explained: ‘Spinal cages are small, have a lot of complexity and are hard to manufacture conventionally.’

German medical device manufacturer Emerging Implant Technologies (EIT) recently worked with 3D Systems to create a 3D-printed titanium fusion implant for a patient with a degenerative cervical spine condition. 3D Systems’ direct metal printing (DMP) technology was used, which is capable of building objects layer by layer in a variety of metals, using biocompatible titanium in this case.

The porous EIT cervical implant imitated the structure and characteristics of natural trabecular bone. Such porous structures are also used when additively manufacturing other implants, including hip cups, as McAlea explained: ‘We want to encourage bone growth to adhere the implant to the bone. We can do that in one print step, by creating a porous structure on top of a solid structure. Such integrated functionality is another reason why additive manufacturing makes sense for implants.’

The adaptability of additive to create a range of sizes at little additional cost is another benefit, as McAlea said: ‘When you get to low to medium volumes, 3D printing opens up other possibilities for the manufacturer, because they can choose to increase the number of sizes they pattern with no penalty. There’s no additional tooling or instrumentation required.’

The shift from prototyping to widespread manufacturing of additively manufactured parts is apparent, according to Neumann, who said: ‘For the past three years, we have seen metal additive manufacturing maturing from a technology predominantly used in prototyping to an almost normal manufacturing technology. This transition is changing the way our customers operate our systems and, in turn, how we design our TruPrint 3D printing systems. While, until recently, the ability to manufacture an implant in the first place was important, now factors such as production cost, efficiency and quality come into focus.’

Neumann concluded: ‘Additively manufactured implants will become cheaper and available to more and more patients, while more and more regulatory bodies create frameworks for additively manufactured implants. Over the long run, additively manufactured personalised implants will gain in momentum.’

Future metal AM

The Fraunhofer focus project, FutureAM, aims to accelerate the additive manufacturing of metal components to reduce manufacturing costs, which would overcome one of the barriers of widespread AM for medical components.

Although not directly related to the medical industry, the research platform was launched in November 2017 and brings together six institutes. Each will develop new digital process chains, scalable and robust AM processes, system technology and automation, as well as expand the range of affordable materials that can be processed. A virtual lab is also planned, in which demonstrator components will be created across the institutes and different disciplines.

Professor Johannes Henrich Schleifenbaum, coordinator of FutureAM and director of Additive Manufacturing and Functional Layers at Fraunhofer ILT, said: ‘We want to accelerate the actual AM process. All in all, apart from the process costs, we are also addressing post-processing, which is becoming increasingly important, and the automation of the entire process chain. For example, the previous, often still manual removal of the supports will be replaced by intelligent post-processing. To do this, we are working on newer methods, such as removing the supports in chemical baths.’

The FutureAM project will also address the challenge of making high-precision parts that often hampers large-scale manufacturing, as Schleifenbaum explained: ‘As a starting point, I would like to look at preheating, because with it the temperature gradients during the process are lower and fewer distortions and stresses occur in the component. This process can be supported by integrated simulation. In addition, according to our research, precision in the tenth of a millimetre range is generally sufficient. If not, then it is better to mill the components to the desired size.’

As a result, the impact of the FutureAM project means the medical world could be a lot closer to realising widespread additively manufactured personalised implants in three years’ time, when the project concludes. Watch this space.

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