Carbon fibre replacing metals and polymers as material of choice in medical applications

If you dig a little deeper into composite material application, you’ll soon uncover a hugely diverse range of less obvious uses in equipment for patient treatment and diagnosis, sometimes even beneath the skin of the patients themselves.

Our typical image of a surgical instrument is the glint of polished stainless steel but in some branches of surgery the matt black finish of carbon fibre is becoming more common. In orthopedic surgery, the radiolucent properties of carbon fibre composites give these instruments almost complete transparency under a fluoroscope, the real-time X-ray imaging tool used in surgery.

Conventional metallic instruments must be repositioned or removed to allow clear imaging during surgery with composite replacements reducing surgery time and patient risk. Carbon composite instruments, typically epoxy or PEEK thermoplastic components using continuous or chopped carbon fibres, can be produced that are lighter in the surgeon’s hands, less prone to scratching highly polished metallic surfaces on implants and that fulfil all requirements of the high temperature autoclave sterilisation process.

The ability to repeatedly and comfortably position the patient with very high accuracy using lightweight equipment that doesn’t interfere with the treatment has driven the development of sophisticated restraint devices for radiotherapy applications. Carbon fibre prepreg table tops have been used in X-ray and imaging tables for many years as they are radiolucent and provide the clearest image results, but it is a more recent development to use carbon fibre to hold the patient perfectly still. Stiff and lightweight devices offer improved functionality as well as improving patient comfort and reducing the lifting load for nursing staff.

Unsized carbon fibres

Radiolucency, improved fatigue performance and the ability to design a semi-flexible fixation are the key benefits that have enabled carbon fibre reinforced thermoplastic components to establish a key position in highly loaded implants within the human body. Fixation plates for large bone fractures and spinal fusion, dental implants, anchors for sutures, fasteners and components for joint repair and replacement can all be produced with biocompatible composites.

Thermoplastic composites with chopped carbon reinforcement fibres have been approved for use in implants for some time and whilst these do offer increased mechanical performance over traditional reinforcements, the designers of the latest implant devices have been driving the development of continuous fibre solutions.

By working with Hexcel, the only manufacturer to supply continuous unsized fibre that can be efficiently and successfully processed into UD tapes, thermoplastic composite manufacturers are able to maximise interface bonding between a fibre and resin matrix. As the fibre is totally without sizing, it has allowed Hexcel customers to obtain Federal Drugs Approval for use within the body. The resulting composite components feature excellent strength coupled with a very high fatigue resistance.

In tests comparing components made in Ti-Al6-4V titanium and 316L stainless steel with a carbon fibre reinforced PEEK part, fatigue failure was seen at 40,000 cycles for titanium, 440,000 cycles for steel with the CF/PEEK component completing a massive 4 million cycles without failure.

This improved fatigue performance represents a huge patient benefit, extending the life of implants and avoiding costly and invasive revision surgery currently required for certain metal components such as replacement joints.

Crucially, the parts produced can also be designed to be less stiff than traditional metallic options, providing a much better match with the existing bone structure, encouraging load sharing and regeneration of the fractured bone. PEEK and PEKK resin matrices are common in these implant applications but an additional benefit of using unsized fibres is that the reinforcement remains compatible with any thermoplastic resin selected for manufacturing.

Lighter and faster

Hexcel has also supplied unsized fibres to converters who coat them with nickel and other metals, which enhances the conductivity of the fibre before it is processed into a braided or linear core for wires and cables. Nickel coated HexTow IM7 and AS4 carbon fibres from Hexcel’s Salt Lake City plant have been used in front line life saving devices such as automatic external defibrillators (AEDs).

Composite leads are flexible and lightweight, allowing the AED to be packaged into a smaller more portable unit. In addition, radiolucency means there is no need to remove the electrode leads from the patient before any X-Ray or other imaging, saving time and reducing patient risk.

When looking at the market’s future potential, it could be suggested that a key driver will be thermoplastic implants. An aging population, forecast to require a significant increase in fracture repair and joint replacement, also seems likely to only increase the amount of carbon fibre composites used. Of course, the development of new composite materials, components and the legislative approval by device manufacturers is expensive, but long term performance testing is confirming that carbon fibre containing in-body devices offer extended component lifespans and improved biocompatibility; offsetting this cost and ultimately proving carbon fibre to be the material of choice.

Engineering Materials

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Wonderful. I'm a former medical editor for CBS who has been waiting for needed improvements in body part replacements to last longer, weigh less, & function better than current options. This is great work, keep it up. Let's get Govt & medical approvals passed asap. Thank you engineers, scientists, industrial designers et al.

Comment Flavia von Vogrig, 08/10/2017

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