Rethinking implants: high-performance plastics in medical technology

In an interview with K-Mag, Dr Philip Engel talks about the new carbon fibre-reinforced PEEK filament for 3D-printed long-term implants and what else could be possible in the field of smart biomaterials in the future.

Dr Engel, what is your product VESTAKEEP® all about?

Dr Philip Engel: Evonik markets polyether ether ketone (PEEK), a high-performance thermoplastic known for its mechanical properties, high temperature resistance and chemical resistance, under the brand name VESTAKEEP®.

Evonik is one of the leading technology providers in the field of high-performance polymers specifically for medical technology and offers various VESTAKEEP® PEEK grades, including so-called care grades for short-term body contact, solutions for dental products and for long-term implants. The properties of for medical applications include outstanding biocompatibility, very good biostability and resistance to all common sterilisation processes.

We were the first supplier worldwide to support filament 3D printing for the production of PEEK implants at a very early stage in close partnership with printer manufacturers and have continuously expanded our filament portfolio.

Our latest development is the carbon fibre-reinforced PEEK filament for medical 3D-printed long-term implants. The two filaments VESTAKEEP® iC4612 3DF and VESTAKEEP® iC4620 3DF contain 12% and 20% carbon fibres respectively. The two grades offer a choice of materials depending on the required strength and bending properties of 3D-printed implants such as bone plates and other reconstructive prostheses.

The filament has a diameter of 1.75 mm and can be processed with conventional PEEK printers using Fused Filament Fabrication (FFF).

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What advantages does your filament offer compared to other materials?

Engel: The use of carbon fibre increases the stiffness of the material by a factor of 3-4, which makes it suitable for applications that require high stiffness, such as bone plates, where unfilled PEEK is too elastic.

Another advantage is the ability to define the orientation of the carbon fibres during the 3D printing process.

In general, fibre-reinforced materials behave anisotropically, which means that their mechanical properties depend strongly on the orientation of the fibres and the fibre orientation in a component can vary.

For fibre-reinforced parts produced by injection moulding, for example, the fibre orientation and therefore the mechanical properties of the component can only be controlled to a limited extent. This is precisely the decisive advantage of processing a fibre-reinforced material from filament using FFF printing. With this processing technology, the alignment of the carbon fibres can be precisely controlled. The targeted reinforcement of certain areas of the component improves performance and increases flexibility for the user.

How was the balance between high strength, ductility and low weight achieved in the development of VESTAKEEP® filaments?

Engel: By adding fibres, the stiffness of a material is increased, but at the same time the ductility is reduced. In order to find the best compromise between these properties, we tested various fibre contents and evaluated them in collaboration with partners. As a result, we decided on two fibre contents in order to offer users the greatest possible flexibility.

The print tests showed that processing using 3D technology has particular advantages. This is because even with a lower fibre content, and therefore better ductility, a proportionally higher strength can be achieved through the targeted alignment of the fibres.

What specific properties does the 12% or 20% carbon fibre content bring to the PEEK filament and how does this influence its adaptability to the requirements of implants?

Engel: Carbon fibre-reinforced PEEK is particularly interesting when unfilled PEEK is too elastic for an application. This is often the case with bone plates. Another example is the replacement of larger bone parts that have to bear a high load, such as entire vertebral bodies. Metals such as titanium are still often used in medical technology, but they are too rigid and can lead to high stress on the bones. They also create strong X-ray artefacts. Carbon fibre-reinforced PEEK offers an alternative with greater rigidity and more physiological mechanical properties.

The various carbon fibre components and the processing of the filament using 3D printing provide customers with unprecedented flexibility and a wide range of applications. Other processing methods with carbon fibre PEEK are often unsuitable for patient-specific solutions, but by varying the printing strategy and the fibre content, a wide range of properties can be achieved. Carbon fibre reinforced PEEK can therefore be used in many different areas and offers a better alternative to the use of metals.

How do you see the future of smart biomaterials in medical technology and what further developments can we expect from Evonik in this area?

Engel: For us, it's always about understanding the requirements and unmet needs of our customers in order to further develop our material in a targeted manner. By "smart" we always mean the combination of different properties in a product, which in turn drives performance. Examples of this include our VESTAKEEP® Fusion, a PEEK material that combines the advantages of PEEK (biocompatibility, elasticity and radiopacity) with rapid bone integration.

Many other things are conceivable, such as the combination of our PEEK materials with other biomaterials from the Evonik portfolio, such as resorbable RESOMER® or our recombinant collagen VECOLLAN®. Many customised solutions can be developed here.