Life-saving plastics

Tom Shelley reports on developments in plastics for the medical device market

There can be many reasons for turning from metals to plastics for medical products – easier processing, design freedom, colourability, lower weight and, of course, cost. But nothing is straightforward and new technologies have been and are being developed to give plastic products performance equal to or better than their metal equivalents. Medical products have their own particular demands, especially the need to be able to withstand sterilisation by various means. If used in a hospital environment, it may also be desirable to make them biocidal, by adding silver or copper, but if they are to be used internally in the human body, on the other hand, they will need to be biocompatible. It may also be desirable that they be transparent to NMR and X-Rays, which is hard to achieve with metals. Plastic products such as containers for the operating room are mainly based on high temperature-resistant amorphous and partially crystalline plastics such as polyetheretherketone (PEEK), polyphenylene sulphide (PPS), polyphenylsulfone (PPSU) and polyetherimide (PEI), according to Lehmann and Voss. If parts have to be slid relative to each other, it is possible to add lubricants such as polytetrafluoroethylene (PTFE) and 'Nano additives', according to Lehmann & Voss' European sales manager, Thomas Collet. Where required, however, electrical and thermal conductivity can be provided by carbon fibres and other nano additives. PEEK has for some years been used for implantable devices, especially by biomaterials company Invibio. A key reason for this is that the material can be engineered with fibre or filler reinforcement to have a modulus similar to that of cortical bone for spinal implants. Invibio's current top grade is 'PEEK Optima', which TWI's Dr Mehdi Tavakoli observes is characterised by high strength, extreme resistance to hydrolysis, and resistance to ionising radiation. "It can be repeatedly sterilised using steam, gamma rays and ethylene oxide without significant deterioration," he says. It can also be adhesively bonded to itself and titanium and nickel chrome alloys. An investigation led by Dr. Tavakoli found that grit blasting with fine alumina NK60 grit and treatment with a neodymium YAG laser were effective in enhancing the strength of PEEK to PEEK joints, but grit blasting was not effective on metal surfaces. The test pieces were lap joints, tested in shear and the strongest plastic to plastic joints after grit blasting came out at 6.2MPa using 'Loctite' 4061cyanoacrylate adhesive. A long-term implantable grade silicone adhesive achieved a shear strength of only 2.9MPa. The highest strength achieved with PEEK to 6-4 titanium joints was 5.9MPa with 4061 and 4.7MPa with Loctite M-31CL epoxy. For PEEK to Cobalt chrome, Allvac TJA-1537 from Titanium International, the strongest joints were 5.9MPa with 4061 and 4.8MPa with M-31 CL. Dr Tavakoli notes that, although the Loctite adhesive had the highest strength, it can only be used in short-term medical devices (i.e. less than 29 or 30 days), depending on whether ISO or FDA regulations are followed. For uses outside the human body, SABIC Innovative Plastics has announced what it calls an 'LNP Stat-Loy' technology that provides permanent anti-static properties for more efficient aerosol and powder dispensing inhalation devices. So far, the technology has been applied to three transparent resin systems: acrylonitrile butadiene styrene (ABS), polymethylmethacrylate (PMMA)/acrylic, and 'Xylex', a polycarbonate (PC)/polyester alloy. It can also be applied to opaque resins such as polyamide 6, polybutylene terephthalate (PBT), polyoxymethylene (POM) and polypropylene (PP). Other innovative uses of SABIC resins for medical products include the use of 'Cycoloy' PC/ABS resin for the housing components for the 'Breastlight' home use device for self examinations developed by Scottish company PWB Health. The latter chose Cycoloy because of its impact properties and high flowability, making it suitable for thin wall moulding. SABIC's 'Lexan' PC was chosen for the lens because of its clarity, impact strength and its ability to be ultrasonically welded to its housing.