Adding the value

Success in the medical sector can depend on the close relationship between designers and materials suppliers. Justin Cunningham finds out what you need to know and the new materials becoming available.

There are perhaps two things about the medical sector that engineering designers should be aware of. First is that because of its specialised nature, the sector has a lot of potential for adding value to a product or device.­­ The second is that there are increasing opportunities to use non-traditional materials.

While plastics and metals have been used for decades there is increasing push into, and pull from, the sector for more speciality compounds, carbon fibre composites, and even ceramics.

Medical engineering technology and material choices tend to be conservative by their very nature. The many standards and regulations in place put enormous constraints on design possibilities and can make the actual engineering of a product or device much more difficult as many traditional solutions, methodologies, and materials might not be valid or available.

Material selection is often particularly intimidating for the designer; what are the regulations that need to be adhered to? Does the material have the right level of biocompatibility? Plus, will it offer the performance and functionality it needs to and does it meet cost requirements? Equally, however, is the challenge to suppliers, which want to know just what designers expect from a material and what is it going to be used for.

Mark Edhouse, a principal engineer at Cambridge Consultants, says: "Polymer suppliers often want to know about the design, what the application is and how a material is going to be used.

"And, while many materials might be referred to as 'medical grade' this is not an all-encompassing term and can be rather misleading. Each material must be judged individually regarding its medical application, subsequent biocompatibility and regulatory requirements. It is a really broad topic and medical devices are highly regulated by several different bodies, depending where you are geographically, so there is quite a bit to think about."

There is a fair amount of risk when specifying non-medical grade materials for medical devices, as well as the possible expense of getting the material tested to the necessary standards. Typically, non-medical grade materials do not have the same supply chain guarantees in terms of process controls and availability. However, it may be a necessary step in developing a device with superior performance or attributes to get an improved unique selling point.

Material suppliers are, however, increasingly offering a wider portfolio of medical grade materials, which includes additional supply chain guarantees and biocompatibility information to help de-risk the material selection process.

"Typically pre-filled syringe bodies have been produced in glass up until now," says Edhouse. "Most pharmaceutical companies and manufacturers have steered away from using polymers because of the risks and expense associated with proving the stability and compatibility with the pharmaceutical compound."

Medical device manufacturers must ensure that the risk of hazardous substances leaching out of a product is minimised to an acceptable level to uphold patient and user safety as well as maintaining compliance with applicable regulatory bodies.

"They must also," says Edhouse, "ensure that the drug compound's effectiveness is not reduced by storage in the vessel. Given that glass is a fairly inert material, it has remained the favoured material. However, given the manufacturing tolerances and cost advantages of using polymers, there is some shift."

One of Europe's leading plastic manufacturers, Ensinger, has also had a long relationship with the medical sector and says it is continuing to grow. Ensinger develops and engineers stock shapes, compounds and profiles made of thermoplastics which are designed to comply with the stringent demands of medical technology. The increasing demands imposed on these materials such as elevated hygiene standards are addressed by Ensinger improving its range of medical grade materials and capabilities.

Hayley Powell, a technical application engineer at Ensinger UK, says: "Ensinger's medical grade thermoplastics have been specially tailored to perform well in terms of sterilisation. We invest heavily in regulations and certifications, as well as assuring seamless product traceability which adds value to Ensinger's services to the medical industry."

Ensinger's medical grade thermoplastics are suitable for use in applications including surgical implant trials, diagnostic equipment and surgical instruments.

Ensinger has recently launched the Tecatec product range of thermoplastic composites which offers excellent strength. Components made from this highly-filled carbon fibre composite material are characterised by an extraordinary degree of mechanical strength and high thermal dimensional stability.

Tecatec stock shape products are made of a thermoplastic matrix and woven carbon fibre bundles. This combination gives significantly higher tensile and flexural strength compared to fibre-reinforced, extruded materials. These lightweight composites also offer good chemical resistance and are permeable for X-rays, making them ideal for use in medical applications. Tecatec materials are physiologically harmless (with biocompatibility in accordance to ISO 10993-5) and are corrosion resistant.

Medical technology is the single most important field of application for Tecatec products. In orthopaedic applications, radiolucent, low-warpage targeting fixtures made of carbon fibre composites are used for positioning fixing pins. The high strength of the materials also offers benefits when used in the manufacture of spreaders or in components for the external fixture of bone fractures.The Tecatec material range is available in plate thicknesses of 3mm to 40mm, with larger dimensions available on request.

Morgan Technical Ceramics is a leading company in the design and manufacture of ceramic implants and complex ceramic assemblies for surgical tools, medical instrumentation, therapeutic and diagnostic equipment and is keen to let engineers know about the advantages of using ceramic materials in the medical sector.

Its new HIP Vitox ceramic material is mechanically and chemically stable and bio-inert and, it says, has a significantly lower wear rate than a metal-to-metal, or metal-to -polyethylene joint, offering much greater long term performance in a hip replacement.

The joints have a significantly longer predicted life of 20 years, which reduces repeat operations, saving healthcare costs, whilst offering improved quality of life for patients. The company's HIP Vitox ceramic-on-ceramic hip joints have demonstrated an exceptionally low wear rate of just 0.032mm3 per million cycles.

Another example of material replacement comes from thermoplastic elastomer (TPE) specialist Elasto, which has qualified a rubber alternative material called Mediprene. It has undertaken a number of projects to show where it can be used to replace traditional rubber. Mediprene is latex free, which reduces the risk of allergic reactions and representative grades have passed cytotoxicity tests (ISO 10993-5) and biocompatibility tests (USP Class VI).

It has been successfully used as a plunger seal of a single-use syringe. The TPE seal, which is mounted on the end of the plunger (see picture on page 14), needed to provide a leak-proof seal with the syringe barrel.

Niklas Ottosson, medical technical manager at Elasto, says: "The right TPE formulation is the key to a safe and successful medical product. That is why we have a collaborative approach, working with OEMs to develop custom materials that precisely match the properties they're looking for."

There are numerous examples of new materials becoming available all the time in the medical sector, offering designers and engineers new possibilities in terms of performance, processing and cost reduction. However, these need to be be treated with care and, although a material might be qualified for medical use, it is usual for designers to have to prove that the overall device is fit for purpose and will not cause any kind of diminishing effect on those that use it.

Medical device classification and material selection in the medical environment is not a cut and dry exercise. It is wholly dependent upon the markets the device and material will be used in, the associated regulatory body, the medical application, and potentially duration of use.

Need to know:
Medical devices generally fall within three main categories. 'Surface Devices' are items such as electrodes for monitoring, contact lenses and similar devices. Second is 'Externally Communicating Devices' such as laprascopes, blood administration devices, pacemakers, oxygenators and the like. Finally is, 'Implanted Devices' such as orthopedic pins or plates, heart valves, grafts and stents. In addition, there are three main timescales for biocompatible devices; 'Limited' is less than 24 hour exposure, 'Prolonged' is 24 hours to 30 days and 'Permanent' is 30 days and longer.

Design Pointers:
Reliability in medical devices and systems is vital, so it is perhaps unsurprising that the safety factor used is often two times greater than that of standard products.

For mechanisms such as springs, metals are still the preferred option as they are very good at capturing and storing energy. Similarly, plastic components offer the potential for more innovative geometry to be produced from a mould at a greatly reduce cost. Plastics are increasingly being used for medical devices as manufacturers and distributors now more commonly qualify and certify materials to guarantee performance and long-term stability.

Justin Cunningham

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