Medical Plastics Help Meet Changing Requirements

By: L.W. Johnson, Global Healthcare Marketing Director, PolyOne Corp., Avon Lake, Ohio, USA

Regulatory compliance, although challenging and sometimes frustrating, is also creating new opportunities. Designers can use this body of constraints as a means to drive positive change within their products. With the increase in regulations governing plastics and additives from flame retardants through to plasticisers,^1,2 opportunities can be found by anticipating new regulatory requirements and selecting compliant materials that speed the approval process. Sometimes there may not be an actual regulation in place, but consumer demand alone will force changes in material selection. One example is the current anxiety over bisphenol-A (BPA). Although there are no national laws restricting the use of BPA in medical or consumer applications, consumers want BPA-free baby and child products and the market is responding.

 

Anticipating future regulations is also having an effect on device design. Trying to anticipate future changes in the Restriction of Hazardous Substances (RoHS) Directive,¹ which may ban the use of lead in medical devices, manufacturers and designers whose products generate radiation are seeking alternative materials that can provide the equivalent radiation shielding to lead.

 

Among material suppliers, consolidation of operations and rationalisation of product lines are taking place to streamline production and improve efficiency. However, as grades of polymer products are discontinued, manufacturers need to find equivalent materials, ensure proper substitution exists and requalify their products with the appropriate regulatory bodies.

 

Changing demographics are also driving new opportunities for plastics. The growing ranks of ageing populations around the globe are having a significant impact on product design. Senior citizens are living longer and requiring more varied healthcare devices. They also want comfort and ergonomic design. Materials such as soft-touch thermoplastic elastomers (TPEs) and technologies such as overmoulding can help meet those requirements.

 

Another effect of this demographic shift is increased demand for better healthcare services. As well as improved hospital care, older patients want increased home health care, which is leading to requirements for consumer friendly medical equipment. Plastics that can provide thin wall moulding capability, high performance and pleasing appearance for home use will help designers succeed in this market.

 

Finally, scientific breakthroughs in medical therapies continue to create the need for new materials. Noninvasive or less-invasive surgical techniques, for example, require miniaturised devices and specialised equipment. Plastics can offer the design freedom to create ultra-small devices with complex functionality. Endoscopic surgical instruments with soft touch, overmoulded handles, syringe components, gaskets and seals are just a few examples.

 

Research suggests that minimally invasive medical devices tend to be recession resistant and are expected to grow by 8–12% in the near term.³ Demand is also rising for radiopaque plastics used to make cardiac and urological catheters.

 

Antimicrobial polymers that resist bacteria, fungus and algae are valuable in the fight against nosocomial infections. As the death rate for hospital-acquired infections rises to between 80000 and 100000 cases per year in the United States, hospitals are seeking to control the spread of infection and reduce their legal liability.^4–6 This trend is driving estimated growth of 15% per annum globally of formulated product for these materials. Application areas include bed rails, infusion sets, equipment housings, virtually any surface that the patient, doctor or nurse may touch.

 

Specialised engineering thermoplastics, developed specifically for healthcare applications, can offer a set of powerful tools for optimising products, increasing speed to market and improving product function. For example, a multitude of options are available for flexible applications such as traditional vinyl, nonphthalate vinyl, thermoplastic polyurethanes, TPEs, copolyesters and ethylene vinyl acetate. To help designers decide which materials are best for specific applications, the following section describes some healthcare environments.

 

Patients first step onto a weighing scale that is covered with a mat. These mats can be formulated to meet colour and durability requirements, in part through the choice of a cross-linked thermoplastic vulcanisate (TPV) or an elastomer based on styrenic block copolymers. Gel-soft TPE materials are available that can be formulated with antimicrobial additives to protect surfaces from contamination.

 

Examination tables can consist of multiple polymer components. For example, housings that cover motors can be moulded from a flame retardant compound designed with vibration dampening characteristics to reduce noise. There are a variety of traditional and halogen-free alternative flame retardants depending on the resin system chosen.

A housing for a light stand may be overmoulded with a soft touch TPE. TPEs offer a good balance of softness and bondability. Recent developments allow TPEs to bond to a variety of substrate plastics ranging from nonpolar olefins to more polar materials such as polyester or nylon. The materials can be formulated to meet the desired softness rating and provide good wet and dry grip performance.

 

Some applications require United States Pharmacopeia (USP) Class VI rating. One example is an oxygen mask and tubing, where high performance vinyl or TPE compounds provide economical and highly functional offerings. They are able to achieve required softness levels combined with resilience, high elongation and ease of manufacture.

 

A caster wheel represents traditional metal-to-plastic conversion where the primary decision driver is system economics. For the wheel hub, the plastic compound can be made from a variety of resins and filler systems. Moving from talc fillers to glass fibres to carbon fibres and even long glass and long carbon fibres provides increasing strength and practical impact performance. TPEs and particularly thermoplastic vulcanisates can provide the good compression set performance required for the wheels. These materials can be formulated with carbon black or other fillers and resins to supply electrostatic dissipation protection, thereby tailoring the surface resistivity of the filled polymer system.

 

To create a material that is antistatic requires the surface resistivity of the polymer to be reduced from 10¹² or 10¹³ ohm/sq by a few orders of magnitude to 10¹⁰ to 10¹². If these materials need to dissipate static charge, the surface resistivity should be reduced by another couple of orders of magnitude to 105 to 1012. The most stringent requirements of conductivity and electromagnetic interference (EMI) shielding must be down as low as 10–1000 ohm/sq.

 

In a typical X-ray room (Figure 1), comfort and reliability also apply, but material modifications bring specific benefits to equipment performance. Electronic equipment gains functionality when the right polymeric material is selected. Plastics with high flow and high temperature resistance such as liquid crystal polymers and polyphenylene sulphide that are traditionally used to mould connectors and other electrical devices have enabled connectors to get smaller and more intricate. In addition, advancements in filler technology, including metal fillers and metal-coated fibres allow compounding of electrically conductive materials that provide EMI or EMI shielding performance. This example refers to X-ray electronics, but these types of shielding connectors are used in an array of medical electronics, particularly when the device frequency increases or device size decreases. Computed tomography scanners and other X-ray generating equipment usually include components machined from lead or lead alloys that collimate, or guide, the X-ray photons. In addition, X-ray rooms are often lined with lead and the patient is draped in lead to minimise exposure to radiation in nondiagnostic parts of the body.

 

Although the RoHS and Waste Electrical and Electronic Equipment Directives^1,7 currently exempt healthcare devices, tighter restrictions on the use of lead are expected in the future. Advances in polymer systems can provide an alternative. By using tungsten fillers, specific gravity of 11 can be achieved in a rigid nylon compound, reaching the same density as lead alloys. In comparison testing for shielding efficiency, these materials show comparable performance to lead across a variety of radiation sources, including 100 and 125 kV X-rays. In testing for shielding efficiency, high specific gravity compounds showed equivalent performance to lead in shielding 35 keV gamma radiation from iodine-125 isotopes, 71 keV gamma radiation from thallium-201 isotopes, and 152 keV gamma radiation from technecium-99m isotopes. Table I shows a range of material considerations for products used in an X-ray room. These products are marked on Figure 1.

 

In the operating theatre (Figure 2), respirator bulbs can be moulded from vinyl, which still makes up a significant portion of medical plastics used today. Vinyl products display excellent clarity and chemical resistance, are easily processed and can be formulated in a range of colours and durometers. They are sterilisable by steam, gamma radiation and ethylene oxide (EtO) and

provide an economical option. For this reason, they are used in many fluid container applications, from intravenous and dialysis fluids to blood storage bags. In these bags, the low oxygen permeability and good clarity makes vinyl ideal. Medical vinyl compounds are also used in a broad range of tubing such as wound and chest drainage tubes, catheters and endotracheal tubing.

 

In the operating room, formulated polymer systems can be used in reusable versions of formerly disposable items. There are now materials that have the temperature and mechanical performance properties required for multiple uses and sterilisations. Materials such as polyphenylsulphone and polyetheretherketone (PEEK) can withstand more than a thousand steam sterilisation cycles, making them useful in surgical and dental instruments or in sterilisation trays. Not only are these materials resilient in steam sterilisation, but their excellent chemical resistance extends to many common hospital disinfectants to give longer life for multiple use applications. Table II lists a range of material considerations for products required in an operating theatre. These products are marked on Figure 2.