Plastics for the medical industry

Plastics products in the medical device industry have become a successful story because of the intensive and fruitful co-operation of polymer scientists, physicians with diverse medical background, and experts for fine mechanics and micro system technology. Often, the enormously time-consuming and expensive development of new drugs receives the most attention from the general public.

High-tech plastic products become more and more a performance and quality of life determining factor for the medical care and treatment of the population. Only a few people know or are aware of the fact that Bayer supplies medical device manufacturers with a number of thermoplastic materials and films, rubber, silicone rubber, and polyurethanes. The wide range of engineering plastics is used to solve problems whenever the property profile of a standard plastic resin does not comply with the requirements of a customer's medical product.The use of synthetic plastics in the human body, nowadays summarised as biomaterials, is quite a new field. Precious metals such as gold or silver were already used in dentistry by the Romans, Chinese, and Aztec.

Basic research efforts focus on a better understanding and measuring of biocompatibility and are the key to the success of plastic resins in medical device industry. We are learning about the fundamentals of the interactions of device surfaces with tissue, blood, and other body fluids. Through these contributions and product orientated development in device industry biocompatible materials with less foreign body reactions and less complications become available. Furthermore, the modification and development of new resins is in the focus of industrial research to comply with the changing customers´ requirements: New technologies and diagnostic and therapeutical techniques require new materials with improved product profiles including e.g. heat resistance, chemical resistance and sterilisability. How Bayer as one of the major suppliers of engineering resins for the medical device market accounts for these demands, will be shown through the following examples.

Market Plastic resins are increasingly being used and are replacing more expensive and heavier traditional materials such as glass or metal. Because of the still increasing cost saving efforts in healthcare systems new technologies and innovative drug delivery applications are looking to be successful, as they support this trend. World demand for plastics rose from about six million tons in 1959 to about 160 million tons in 1999. The Business Communication Company reports that about 850,000 tons were used worldwide for medical devices (1). The standard resins like polyvinylchloride (PVC), polyethylene (PE), polypropylene (PP) and polystyrene (PS) represent more than 80% of the demand. The remaining 187 thousand tons are divided between polycarbonate (PC), thermoplastic elastomers (TPE), styrenics (ABS, SAN) and other so-called engineering resins.

The main markets are currently the traditional markets: North America, Europe, and Japan. During the last five years, the average annual growth rate in the medical segment was 6 to 9%. Innovative technologies like drug delivery systems have been significantly above this range.

New polymer developments and applications Bayer offers a wide range of engineering thermoplastics as so-called construction materials, that are used to solve problems whenever the property profile of standard resins does not comply with the requirements of a medical device application.

Apart from the already established resins, development work in Bayer's research labs has led to enhanced and new tailor-made resins for the medical device industry in recent years.

Polycarbonate Polycarbonate is characterised by a wide range of outstanding properties. The amorphous bisphenol-A-polycarbonate was invented at Bayer in the middle of the 1950s and has been manufactured and brought into the market since the early 1960s. It has been used in medical devices at the same time. Polycarbonate combines inherent clarity, heat resistance, and rigidity with toughness, fracture resistance, and high dimensional stability within a wide temperature range and is therefore used in many applications where it competes with traditional materials such as glass or stainless steel. The outstanding light transmission of the transparent grades made PC the ideal material for clinical and diagnostic devices requiring visual monitoring of tissue, blood or other body fluids. The good processability, low shrinkage and low water absorption of PC enables the safe manufacturing of thin-walled, lightweight parts. The heat deflection temperature of up to 135°C permits sterilisation with steam at 121°C.

Other suitable sterilisation methods are gassing with ethylene oxide (EtO). Special grades with better colour stability are available for gamma radiation. Numerous Makrolon® grades meet the requirements of the United States Food and Drug Administration (FDA). These grades have passed the tests according to the United States Pharmacopoeia (USP) XXIII class VI and according to ISO 10993-1 up to 30 days contact with blood.

The above mentioned combination of properties has early led to polycarbonate´s almost complete dominance as a material for artificial kidney housings. Furthermore PC housings are advantageous as they enable the safe and economical assembling of housing and hollow fibre or flat membranes by the means of polyurethane potting materials.

During open heart surgery, e.g. coronary artery by-pass operation or heart valve replacement the patient's heart was brought to standstill and the pump and the oxygenator of the heart lung machine take the function of the heart respectively of the lung. For more than 20 years polycarbonate has been used for manufacturing oxygenators, blood reservoirs and blood filters for the extra-corporeal blood circulation. The glass-like transparency is required to monitor blood flow and condition during the course of the operation. The fracture resistance of PC guarantees the safety of the devices. During many surgical interventions the patient's blood is collected, filtered and returned to the patient, to avoid transfusion of conserved blood. A variety of devices for blood treatment are equipped with filters and drums from polycarbonate, serving to purify and separate blood components for reuse. Those filter drums rotate with high speed, that causes high forces. The material used needs to show the adequate mechanical strength, to prevent the drum from breaking and leaking of the filtrate.

Surgical instruments from polycarbonate for minimally invasive operations have shown excellent behaviour in use. Toughness and stiffness make PC an suitable alternative to stainless steel or glass for trocar tubes. In principle, these are long pipes serving the physician as a lead for the insertion of surgical instruments into the patient's abdominal cavity. During the positioning procedure trocar tubes have to be unbreakable and resistant to bending and deformation. A further important requirement is clarity in order to monitor the instruments in the trocar tube. All these requirements are met by polycarbonate. Further examples for use of engineering thermoplastics are dialysis membranes, housings for separators, stop-cocks and manifolds, and petri dishes.

High temperature polycarbonate Even for higher service temperatures Apec®, the Bayer high temperature polycarbonate, is designed. In addition to the typical characteristics of Makrolon®higher heat deflection temperature is a key feature of this tailor-made polycarbonate grades. Consequently, Apec® can be repeatedly hot steam sterilised at 134°C and even at 143°C. Galenic formulations of poorly water soluble pharmaceuticals are increasingly based on emulsions using phospholipids like Intralipid and enabling the intravenous administration of fluids and drugs. Plastics used for infusion technology need to meet more stringent requirements in the view of chemical resistance and strength of the connection to other components like connectors to blood lines.

Makrolon® Rx 1805 was developed with a special stabilisation for medical applications especially for infusion technology, and is one way of contributing towards the safe, optimum care of patients receiving lipid-containing intravenous drugs.

The enhanced chemical resistance of parts from Makrolon® Rx 1805 also requires a stress free fabrication. As a consequence in comparison to middle and low viscous standard polycarbonate grades these lipid resistant grades require higher processing temperatures.

As the table shows, at 5% strain Makrolon Rx 1805 is more resistant towards 20 weight-% Intralipid solution that a standard grade. Products from Makrolon Rx 1805 can be sterilised by the two commonly used methods: Hot steam at 121C or ethylene oxide gas. Superior colour stability and resistance to yellowing following gamma sterilisation is an additional key feature of Makrolon Rx 1805.

For safe connections of parts from Makrolon Rx 1805 with blood lines from plasticised PVC the commonly used adhesives or solvents can be applied. Product examples are the needle free injections system SAFESITE™ from B. Braun Medical Inc., three-way stopcocks, Luerfittings, connectors and other components A new, closed centrifugation system from Dendreon Corp. separates the hematopoietic progenitor cells (CD 34 cells) from the white blood cells within a period of 30 minutes, with a centrifugal acceleration of 850 g.

The three-part centrifuge system comprises a cup-shaped base, a cap and a disk-shaped support – all made of Makrolon® Rx 2530. The cap is glued on the base. The slight purple tint of Makrolon® Rx 2530 reverts to clear after gamma sterilisation. No yellowing occurs.

Many patients are afraid of injections because they hate the pain of the needle. The new needle-free injection system Injex™ from Rösch Medizintechnik AG, Berlin, is a very good example for the replacement of glass through plastic. The system can be used for many different liquid medicaments that have to be administered subcutaneously. Liquid medicaments are sucked up in an one-way ampoule, that is made of Makrolon® Rx 2530. The actual injector is made of metal, which shoots the medicine with high pressure through the disinfected patient's skin about 6 to 9 mm into the fatty tissue. The ampoule needs to be break and impact-resistant and has to withstand injection pressures of up to 300 bar.

Inhalation devices Besides plastic products, that directly come in contact with human body, high-quality device components and functional devices have contributed to the development of new innovative treatments by the application of plastic. For the easier administration and regularly taking of medicines e.g. for the treatment of respiratory diseases, intelligent designed plastic applications have contributed. Powder dosing systems, so-called inhalers, were developed as alternatives to gas propulsion systems. The new inhalation technology enables the patients to administer a precisely measured dose of an asthma drug to themselves. The DISKUS™ Powder Inhaler from GlaxoSmithKline, London, has been designed with a particular eye on safety and ease of operation.

The mechanics, inside of a housing from impact-resistant Lustran ABS, authorised for food contact, is made up of different injection-moulded plastic parts, ensuring an accurate fit. The pressure lever of the mechanics initiating the metering operation is made of Makrolon.

Polycarbonate films High-quality films with the trade name Makrofol® - (PC films) and Bayfol® (PC/Polybutylene terephthalate (PBT) films) have found a wide range of applications. Makrofol® films provide outstanding optical, mechanical, thermal, and electrical properties The films are available in various thicknesses and are highly suitable for thermoforming, embossing and printing and also support new processing technologies such as IMD (In-Mould-Decoration).

Thermoplastic polyurethanes Texin and Desmopan are the thermoplastic polyurethanes (TPU) from Bayer. The range of grades is based on various raw material groups, such as polyesters, polyethers, and blends, and is available in formulations with hardnesses ranging from 75 Shore A to more than 70 Shore D. The common properties of all grades are high impact strength and flexibility, good resistance to abrasion, tearing and tear propagation, and high resilience. Thermoplastic polyurethane elastomers are processed by injection moulding or extrusion, in the same way as thermoplastics. Special Texin grades are available for blood contact applications, such as thin-walled tubings and catheters, incisions films and tube connectors.

Sterilisation can be performed using ethylene oxide gas, gamma radiation or hot air, but not with superheated steam. Add-ons in the form of prescription reading lenses can make sunglasses and safety spectacles into reading spectacles. These highly transparent flexible prescription lenses are crescent-shaped. They are produced by injection moulding using UV-stabilised, aliphatic polyether TPU, Texin DP 7-3006. The lenses are available in a width of up to 1.25 inches with between 1.25 and 3.00 diotres. The application of the lenses is easy: First the spectacles are cleaned with water, and the lens placed on the inside of the spectacle while they are still wet. The lens is then pressed down briefly and dabbed dry.

Development trends Device manufacturers always use plastics for medical applications when cost, performance or manufacturing advantages can be realised. Therefore plastics more and more compete with traditional materials like glass or metal. Lightweight, good processability and design versatility are key properties for the use of engineering resins in medical applications. These materials contribute to the development of new devices, being the prerequisite for new surgical, diagnostic and therapeutic procedures. The healthcare systems of all industrialised countries are challenged by the increase of the average age of their population and the dramatic explosion of the expenditures for the health insurances. To manage this development stringent cost saving measures are running in all countries. To guarantee the quality of life for the increasing number of older people the trend to mobile diagnostic and therapeutic systems as well as to self medication systems will increase.

New developments are directed towards property enhancements such as barrier properties, scratch resistance, weatherability and chemical resistance of plastic. Procedures for surface finishing such as "hardcoating" processes are under development. Three coating systems have primarily become established: acrylate laquers, siloxane laquers and polyurethane-based laquers. They meet the requirements for scratch resistance, weather resistance and chemical resistance. These developments are supported by high volume applications like automotive engineering and can be modified or adapted – if necessary – for solutions in the medical device industry.

With the tendency to have smaller surface textures, miniaturisation of medical device components and parts, and more functionality, more economical manufacturing in future polycarbonates with modified chemical structures will become of greater importance. Copolycarbonates with tailor-made characteristics, particularly regarding lower water absorption, better rheological and optical properties could be the key for further successful developments in the field of medical technology.