Trends in Device Design: Implications for Materials Selection

Medical devices are in a constant state of evolution, responding to trends within and outside the hospital environment to achieve better care at lower system costs. The trends toward miniaturization and portability are driving new requirements for housings and internal components. Cleanliness and sanitation continue to receive strong attention globally, requiring devices to withstand a range of chemicals and sterilization techniques.

As complex electronics and wireless features bring new capabilities to many devices, designers will need to consider the influence of external and internal radio-frequency or electromagnetic noise on the operation of their device as well as on nearby devices. Finally, continued expansion of regulatory agency guidances and regulations will affect the design of healthcare devices, which means designers will have to meet functional and aesthetic needs while complying with existing standards or planning for potential new ones.

A vital aspect of reengineering devices or designing for new applications is applying the latest materials technology to optimize functionality, usability, and aesthetics. The following five areas should be considered by designers when selecting materials:

Durability and Aesthetics

Devices designed for hospital use are often shared and, therefore, may need to be transported from one clinical area to another. Because they are subjected to repeated use and moved throughout a facility, often in unintended ways, some devices are inevitably dropped or damaged. For optimal portability and resilience, shared devices need to be durable, even when a primary goal is to make them as small and lightweight as possible.

Selection of the right materials helps make a device better able to endure the fast-paced, high-impact hospital environment. A material that addresses these needs can reduce maintenance costs, which means reduced equipment downtime. Hospitals therefore gain operational efficiency through availability of equipment and minimize administrative resources needed to manage repairs or replacements.

Standard industry tests such as notched Izod and instrumented impact energy tests help indicate the likely impact resistance of a material for a given design. These tests work by measuring triaxial mode failure and energy absorption. Because designs can vary, it is always advisable to conduct an impact study on the final iteration.

Three resin types are commonly used in housing components—typical unfilled acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), and PC/ABS blends. Their different notched Izod performances (ASTM D256; Izod impact, notched, at 23°C) can help guide material selection to achieve the performance needed. Specifically, the results show ABS at about 293 J/m, PC/ABS at about 550 J/m, and PC at about 694 J/m.

Polycarbonates typically can be injection, foam, or blow molded, and they are also offered in sheet, film, and profile extrusion formats. Different polymers offer different performance capabilities such as flow into and release from molding tools. Enhanced flow and release properties may enable more-challenging designs, ranging from complex geometries to low draft angles. Advanced designs can help to achieve simplicity of device functionality, cost savings from part reduction, or potentially easier molding. For example, although melt flows may be comparable, special copolymers may offer the advantage of practical flow (see Figure 1).

Engineered thermoplastics, including PC, PC blends, polyphenylene ether and polystyrene blends (PPE/PS), and specialty compounds, may offer resistance to impact, flame, and chemicals. Polycarbonates and blends are well known for offering impact and abuse resistance, excellent mechanical and electrical properties, and low- and high-temperature performance. These properties may be needed for a variety of climate conditions, including during transportation, where the influence of temperature on drop impact may result in different outcomes for device integrity.

New polymers with added capabilities, such as specialized copolymer PCs, offer greater ductility even at low temperatures, making them suitable for personal-use devices that may be exposed to seasonal temperature changes. A simple comparison of the ductile-brittle transition temperature of a standard PC versus a special copolymer PC demonstrates their differences. For example, standard PC is generally ductile to –10°C compared with potential ductility at or above –30°C for the special PC copolymer. Considering the increased use of small devices packed with electronics, heat management becomes an important consideration.

Enclosures for medical devices may require high-heat resistance, electrical properties such as dielectric strength, chemical resistance, or retention of properties in acidic and basic environments. In these situations, PPE/PS blends should be considered. When foam-injection molded, these lightweight materials provide structural integrity while supporting other performance objectives. Because these blends are lightweight, they can increase the portability of large devices such as anesthesia machines and patient monitors.

Additionally, the lower specific gravity of these blends may offer lower part weight than that of other resins. For example, standard PC has a specific gravity of 1.2 g/cm³ and PPE/PS blends have a specific gravity of 1.07 g/cm³. To achieve light weight in applications requiring flame resistance, consideration should be given to materials that meet UL 94V-0 or UL 94-5VB at thinner walls: for example, V-0 at 0.75 versus 1.5 mm, or 5VB at 1.5 mm versus 3.0 mm.

An important healthcare trend focuses on providing a reassuring and pleasant environment for patients. Enhanced aesthetics of medical devices can mitigate an institutional appearance that may contribute to an uninviting hospital environment. Colors and special effects, such as sparkle and frost, can improve the visual appeal of devices. Molded-in colors and effects are available directly from resin suppliers and can eliminate the cost of secondary operations, including painting.

Another facet of improved aesthetics is maintaining the appearance of the device over time. Indoor lighting may influence the color of a device by fading or shifting the original color. It is important to counteract the effects of ongoing exposure to lighting. Specially formulated engineering resins such as PC/polybutadiene-terephthalate (PBT) blends, PC/ABS blends, and acrylic-styrene-acrylonitrile (ASA) have been developed to lessen the effect of indoor lighting and, therefore, improve the long-term appearance of equipment.

Component Miniaturization

To reduce patient trauma and improve the safety and effectiveness of surgical procedures, some surgical instruments have been significantly repurposed or redesigned for use in minimally invasive surgery. Likewise, monitors and diagnostic equipment have been revitalized through miniaturization. Ensuring device robustness and functionality at smaller sizes goes well beyond resizing component parts to smaller dimensions.

If stress and load will be placed on smaller parts, their shape or interaction with other components may have to be redesigned. Changes may necessitate the use of different materials that can handle the forces and loads placed on those small parts where the force is distributed over a smaller area. Materials such as polyetherimide (PEI) resins are designed to provide high strength and stiffness for the management of higher loads on components such as gears. They are also designed to provide high compressive strength for use in surgical instruments with components such as handles that need to withstand axially directed pushing forces. An unfilled PEI has a compressive strength at yield (at 23°C) of approximately 130 MPa, compared with polysulfone, which has a compressive strength at yield of ~100 MPa and standard polycarbonate at ~70 MPa.

Stress-strain curves for a given plastic help designers understand physical property changes under different conditions. Figure 2 shows the stress-strain curves for standard PC versus standard PEI and demonstrates the superior stiffness of PEI. Stiffness is a key consideration when selecting materials for component parts such as gears and actuators that must sustain load or stress, especially as these components are miniaturized.

Tight tolerance is often another key property for miniaturized internal components. The functionality of the device may depend on the precise interaction of components. The selection of materials can be even more difficult given the variety of operating environments, which may include exposure to chemicals and high temperatures.

Over the years, many gears and components have been converted from metal to plastic, encouraging injection molding and eliminating machining or other secondary operations. This trend has continued with advances in the capabilities of specialty compounds. Such advances include greater lubricity, dimensional stability, strength, and stiffness. Glass and carbon-fiber fillers are two ways to enhance resin performance (see Table I).

Other properties, such as lubricity and dimensional stability, may also be important design requirements. Lubricity facilitates repeated use of gears and movable components by reducing wear or enhancing smooth, consistent part-on-part movement. When increased wear resistance, reduced coefficient of friction (COF), or reduced use of external or topical lubricants are called for, specialty compounds may be the appropriate materials to use.

Material selection is influenced by three factors: wear pairs (a combination of materials in contact with each other), wear conditions (e.g., wet, dry), and wear configurations (e.g., rotary, sliding, oscillatory, etc.). Table II shows the influence of different internal lubricants on a material's wear factor, and COF-dynamic, and COF-static performance.

Specialty plastics now offer device manufacturers the ability to customize performance. Miniature device parts can be injection molded rather than machined, and certain materials can eliminate secondary operations such as lubrication. Selecting the optimal material for a device can give manufacturers the flexibility to implement new design features and capabilities.

Cleaning and Sterilization Issues

Routine cleaning brings many devices into repeated contact with a wide variety of disinfection chemicals. Each exposure may contribute to deterioration of the plastics used in the devices. Therefore, it is critical to establish a product's desired durability in tandem with the expected cleaning regimen for the life of the device. The diversity of chemicals found in the healthcare environment can make the selection of materials challenging. Chemicals range from hydrocarbons to alcohols, acids, and alkalis. The chemical type, however, is only one factor to consider; others include the concentration and length of exposure, strain levels, and temperature. Although a wide range of resins are considered to be chemical resistant, it is important to pay close attention to specific chemical use considerations along with the functional requirements of the device when choosing among such materials as PBT, PPE/PS blends, PEI, and PC blends. Device testing under expected use conditions is recommended to provide input toward the final decision on material selection.

The need to sterilize devices using EtO, radiation, or high-heat steam autoclave (121–134°C) processes must also be considered. Depending on whether a device is disposable, limited-use, or durable, manufacturers and hospitals may use different approaches to sterilization. And depending on the sterilization technique, plastics can be affected in a variety of ways. Manufacturers and designers need to take this variability into account when selecting polymers for device construction.

For sterilizing disposables or reused devices and heat-sensitive medical instruments, manufacturers may choose EtO. EtO offers compatibility with many engineering thermoplastics, potentially facilitating the sterilization of devices made with multiple resins. For example, a device that includes certain nylons or polymers may not be able to withstand autoclave or gamma sterilization.

However, some devices are sensitive to residuals that can emerge from EtO sterilization, so it is important to understand how EtO interacts with a desired polymer. Also, after sterilization, devices are commonly aerated to allow for gas dissipation. This step adds to the cycle time before the device can be shipped. For example, EtO sterilization, including the aeration phase, may take several days or even weeks.

Another sterilization choice for devices is radiation. Both gamma and E-beam techniques allow many devices to be sterilized at one time. With gamma and E-beam, there is no concern about residuals and no degas step. Typical gamma sterilization takes only hours. Radiation methods can modestly influence the mechanical properties of plastics, although many engineering thermoplastics such as PC, PBT, and PEI generally maintain their mechanical properties.

Manufacturers should always ask their suppliers about a material's response to radiation. Color shifts of materials may occur and are typically most obvious with clear resins. Special formulations have been created to combat this effect, so when color is a key factor, the consideration of specialized gamma-compatible materials is advisable.

Many hospitals use steam autoclaving for repeat-use and durable devices. This sterilization method requires exposure to high-temperature steam. Because of the wide range of sterilization conditions over the life of the device—e.g., from one cycle to 2500 cycles, from 5 minutes to 30 minutes, and from 121° to 134°C—the influence of autoclaving on material performance varies widely.

Engineering thermoplastic resins offer performance capabilities to meet a range of autoclave sterilization conditions, from minimal to repeated high-temperature exposure. The combination of device performance requirements with sterilization exposure needs can influence material options. Some PCs may be exposed to only a few cycles, such as 121°C for 2–10 20-minute cycles, while certain copolymer PCs may undergo several hundred cycles. However, some PPE/PS blends and PEIs offer alternatives for higher heat and more cycles (e.g.,134°C for 1000–2500 5-minute cycles).

Electronic and Wireless Device Proliferation

The increased number of electronic and wireless devices calls for consideration of whether the device or its environment may be subject to radio-frequency or electromagnetic interference (RFI/EMI), or noise, that could hinder the functionality of the device or neighboring devices and equipment. This situation may result from the medical device itself, another medical device, or other electronics increasingly found in a hospital environment, including cell phones. Electrocardiograms and various health monitors are examples of devices that may be influenced by such noise.

EMI refers to electromagnetic waves that may distort communication signals being transmitted over the air. RFI generally refers to the disruption of signal reception by radio waves at the same frequency as the desired signal. Both conditions call for two considerations: protection from external interference and containment of internally generated interference. In the case of external interference, the goal is to protect the device's operation from disturbances generated by other equipment. However, when the device itself generates interference, the issue becomes how to avoid affecting other devices.

Primary methods for controlling EMI and RFI include optimal circuit design, good grounding, and electronic filtering of noise. In many cases, however, materials come into play. Some plastics are coated with conductive shielding materials. In addition, specialty compounds have been developed to facilitate shielding and eliminate the need for such coating or painting operations. It is possible to formulate materials to provide EMI shielding (up to 60 dB) and to offer surface resistivity from 10⁰ to 10² Ω/sq. Effectiveness can be influenced by the nature of the electromagnetic field, the operating frequency range, the conductivity of the shield materials, and the design of the part (i.e., wall thickness and presence of joints).

As with most devices, multiple functional requirements must be met concurrently. Fortunately, some specialty compounds may provide the Faraday cage effect of a housing, yet also offer flame-resistant properties, good dimensional stability, and good surface quality.

Environmental Regulations

Concerns about limited natural resources, waste, and polluted water and air have become global in scope. As a result, environmental regulations are being enacted across Europe and Asia as well as North America. These regulations are changing the design, manufacture, use, and disposal of products in many industries, particularly electrical and electronic (E/E) equipment. Moreover, these regulations may also affect device material selection.

Although the medical device industry is not always on the same implementation schedule for environmental legislation as products for other industries, manufacturers anticipate that existing regulations governing E/E products will ultimately be required of medical products as well. Therefore, many device manufacturers are bringing their products into compliance with such regulations, including EU Directive 2002/96/EC, “Waste Electrical & Electronic Equipment” (WEEE), and EU Directive 2002/95/EC, “Restrictions on the Use of Hazardous Substances” (RoHS). Manufacturers of electronic devices to be sold in Europe and Asia must understand how these directives may alter the range of acceptable materials for specific markets.

For E/E products, WEEE requires OEMs, component producers, and subassembly producers to take responsibility for their products at the end of the product's useful life. Manufacturers must assume collection, recovery, and treatment as well as accept financial responsibility for these activities. Plastics using brominated flame-retardants must be removed from the waste stream and treated selectively. To simplify end-of-life recovery and recycling, many manufacturers are consolidating materials and limiting choices to plastics that meet the performance requirements of the application without using flame-retardants. Suppliers offer a number of such materials across a wide range of resins.

Medical equipment that may be covered under WEEE includes radiotherapy equipment, pulmonary ventilators, laboratory equipment for in vitro diagnosis, and other appliances for detecting, preventing, monitoring, and treating illness. It is important to note that devices such as implanted and infected products are excluded from this directive. Infected products are those that have come into contact with blood or other biological contaminants.

RoHS applies to products placed into the EU market after July 1, 2006, and includes six families of restricted products: lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBBs), and some polybrominated diphenyl ethers (PBDEs). Medical devices are not currently within scope of the RoHS Directive; however, studies regarding the feasibility of including category 8 (medical equipment) and category 9 (monitoring and control instruments) in the directive are under way.

Fortunately, innovative plastics are currently available as alternatives to those that are subject to regulatory restrictions. For example, polyetherimide is inherently flame resistant, so it requires no flame-retardant additives to enhance performance. Alternative polymers with bromine-free flame-retardants include PCs, PC/ABS blends, PPE/PS blends, and a variety of specialty compounds. These alternatives can provide additional benefits such as cost-saving elimination of secondary operations. Through such innovations, plastics can help support environmental protection as well as the success of businesses that depend on them.

For example, structural panels made of thermoset fiber-reinforced plastic (FRP) composites may be replaced with thermoplastic alternatives. Multilayer coextruded sheet for thermoforming offers UV resistance with colors or other aesthetic designs. It may provide the economic advantages associated with automating a human-intensive process like hand-laid FRP or other composites. In addition, the use of thermoplastic sheets may eliminate the emissions and health hazards associated with the styrenes and acetones that are used to process composites and gel coats, as well as the cleanup afterward. Specific guidelines and timing for implementation of the WEEE and RoHS directives in the medical industry vary by equipment area and geographic location. However, to ensure compliance at the time regulations take effect, many manufacturers are already beginning to transition the materials used in their products to allow adequate time for selection, testing, and use of new resins.

Conclusion

As the trends toward miniaturization, portability, improved aesthetics, and environmental responsibility drive ongoing changes in medical devices, selecting the right materials has become more critical. To achieve optimal product usability, appearance, endurance, and performance, designers must carefully consider available materials.

Tools that facilitate the selection of materials are often available at suppliers' online service centers and polymer development centers. Field technical and application development support teams should also be able to assist manufacturers navigate the selection process. Given the range of influences on device design, designers may opt to involve suppliers early in the design process to identify the appropriate materials that support all of these considerations.

Clare Frissora is market director for healthcare at GE Plastics (Pittsfield, MA). She can be reached at [email protected].

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