Making Sense of Plastics and Their Properties

Originally Published MDDI May 2004

Plastics



Plastics are quickly gaining popularity as the material of choice for medical device components. But with many options available, manufacturers need to know how to select the plastic that will work best for their devices.

Hedden Miller

There's no shortage of plastics to choose from for medical device and diagnostic components. But how do you find the one that performs best and at the lowest cost for an application and that also meets regulatory requirements?

The first step in meeting this challenge is to target the structural and functional criteria a material must satisfy during processing, component assembly, and product end use. Be aware that this may involve conflicting needs, such as high strength at low weight. In addition to the mechanical, physical, thermal, and other considerations that affect all plastic parts, the medical industry brings many unique issues to the fore, such as biocompatibility and repeated exposure to sterilization. 

Manufacturers can currently choose from either thermoset plastics or thermoplastics. Thermosets flow during molding and then cure or harden irreversibly. However, if they are overheated after curing, the polymer degrades rather than melts. Plastics in this category include phenolics, epoxies, alkyd polyester, urea-formaldehyde plastics, and natural and synthetic rubbers.

Thermoplastics, which soften when heated and harden when cooled, run the gamut from commodity to engineering plastics. Commodity resins include low- and high-density polyethylenes and polypropylene. 

Engineering resins include acetal, nylon, polycarbonate, polyphenylene sulfide, liquid-crystal polymer, cyclic olefin copolymer, and others. They generally cost more, carry greater loads, and withstand impact, high temperature, and chemical attack better than commodity plastics. Some engineering plastics, for instance, handle continuous temperatures of more than 500°F, 
resist most chemicals, and have strengths comparable to cast metal.

Many medical applications would be impossible without thermoplastics, which are the most prevalent class of plastics used in medical technology. The performance demands inherent in medical devices have turned designers increasingly toward engineered resins. 

A thermoplastic's properties depend on its chemistry, structure, chain length, and the bonds between chains. These plastics tend to be rigid and can have molecular structures that range from fully amorphous to those with organized chains that are mostly crystalline. Ordered structures add stiffness and strength as well as resistance to creep, heat, and chemicals.

Combining one or more polymers or using additives can alter a resin's properties. Most properties, such as strength, shrinkage, warpage, and lubricity, can be tailored by adding fillers and reinforcements. Reinforcing fibers, such as those made of glass, metal, or carbon, increase strength and other mechanical attributes. Particulate fillers like talc or ground calcium carbonate generally increase stiffness, while plasticizers enhance flexibility. Other common additives are antioxidants, stabilizers, and colorants. 

Engineering plastics are particularly useful because they can be molded or machined into almost any shape for use in medical devices and diagnostic equipment. Typical applications for some common engineering plastics include

•Acetal copolymer and thermoplastic polyester in inhalers and other intelligent dosing systems that require reliable parts in a confined space.
•Cyclic olefin copolymer in vials, prefilled syringes, needleless injectors, media containers, diagnostic devices, and pharmaceutical packaging, because of its transparency, clarity, shatter resistance, chemical resistance, moisture barrier, low water absorption, and thermoformability. 
•Liquid-crystal polymer in minimally invasive surgical instruments, given its stiffness in thin walls, high melt-flow properties, and superior sterilizability.
•Ultrahigh-molecular-weight polyethylene in knee and hip replacement parts, due to its low wear and lubricity.
•Polyphenylene sulfide for surgical instruments and other injection-molded parts, because of its dimensional stability, sterilizability, chemical resistance, low wear, and inherent flame retardancy.

Key Properties in Choosing a Plastic

In order to evaluate plastics, a manufacturer must understand the many properties that define them. Initial screening usually depends on data that are based on laboratory tests and published by suppliers and other sources. While this information helps in comparing different resins, it does not predict real-life performance. 

Once candidate resins are identified, they should be tested under the conditions they will be exposed to during processing, assembly, and end use. For use in a molded part, for instance, a polymer should have good flow, controllable shrinkage, and have the desired surface finish as it comes from the mold. It also should accommodate all anticipated assembly steps, such as solvent bonding, use of mechanical fasteners, ultrasonic welding, and snap-fits.

The material properties important to a product's end use are device-specific and typically involve a broad array of physical, mechanical, thermal, chemical, and electrical attributes. Polymers used in equipment that diagnoses or treats patients must be biocompatible. A summary of many common properties that come into play in medical devices follows.

Physical Properties of Plastics

Resins are described by many physical properties. Some of the most common ones are the following.

Density and Specific Gravity. Related measurements concerned with mass per volume. As a dimensionless number, specific gravity is a good way to compare materials.

Water Absorption. The percent increase in weight of a specimen before and after immersion. This property affects dimensional stability and some mechanical and electrical properties. 

Elasticity. A plastic's ability to return to its original size and shape after being deformed. 

Opacity and Luminous Transmittance. The ratio of transmitted light to incident light. 

Lubricity. Expresses a tendency not to be worn by friction and relates to a resin's coefficient of friction.

Mold Shrinkage. How much one dimension of a part changes as it cools and solidifies in a mold divided by the mold dimension. 

Mechanical Properties of Plastics

Mechanical properties are crucial in nearly all applications of plastics. Some of these properties include the following.

Tensile Strength. The maximum amount of tensile load per unit area a material can withstand.

Tensile Elongation. How much length increases in response to a tensile load as a percent of the original length. Elongation at break is the maximum elongation the plastic can undergo. 

Flexural Strength. How much bending a plastic can withstand before it ruptures. 

Creep. A plastic's deformation under load over time. Crystalline and glass-reinforced resins usually have low creep rates.

Impact Strength. Evaluates how well a part absorbs an impact without fracturing. Tests are done on samples with and without notches. Notched tests measure how easily a crack propagates through a material. 

Thermal Properties of Plastics

Exposure to elevated temperatures generally makes plastics more susceptible to mechanical stress and chemical attack, while exposure to low temperatures generally makes them less ductile. The thermal properties of plastics include the following.

Melting Point. The temperature at which a plastic becomes a liquid upon heating or a solid upon cooling. Crystalline resins have well-defined melting points, while amorphous ones soften and grow fluid over a range of temperatures.

Coefficient of Linear Thermal Expansion. Measures the change in one dimension compared to the original dimension per unit change in temperature.

Deflection Temperature under Load. The temperature at which a test bar under a given load deflects a set amount.

Thermal Conductivity. The rate heat energy travels along or through a plastic. 

Flammability. How well a material resists combustion. UL 94 is the most widely used flammability test. The limiting oxygen index measures how much oxygen is needed to sustain combustion. Plastics needing less than 21% oxygen burn in air. 

Electrical Properties of Plastics

As good insulators, plastics give medical equipment essential dielectric properties. Common electrical properties measured in plastics include the following.

Dielectric Strength. The highest voltage that can be applied to a plastic before it allows a current to pass. 

Dielectric Constant. The ratio of the capacitance of a capacitor made with the test substance to its capacitance when air or a vacuum is the dielectric. It addresses how well an insulator stores electrical energy so as to isolate electrical elements from each other and the ground.

Dissipation Factor. Measures the energy dissipated as heat during rapid polarization reversals, as with an alternating current. 

Arc Resistance. Measures how long in seconds it takes an electrical arc imposed on a plastic to create a conductive path. 

Comparative Tracking Index. Evaluates the voltage needed to create a conductive path between electrodes on a surface.
 
Volume Resistivity. Measures the reciprocal of conductivity when a dc potential is applied across a material.

Environmental and Chemical Stresses

If any solid, liquid, gas, or radiation will contact a plastic during processing, assembly, or end use, tests should be run to see whether it causes the polymer to craze, crack, discolor, lose properties, soften, or dissolve. A chemical, for instance, may reduce a polymer's molecular weight and alter short-term mechanical properties. It also may dissolve or be absorbed, reducing strength, stiffness, impact resistance, and other properties. Plastics are more subject to environmental stress when they are under load or when temperature increases.

While data from suppliers are helpful in understanding how the environment and chemicals affect a polymer, the real world can combine stresses to affect resins in unexpected ways. Testing of medical components should look at stresses to include those from cleaning agents and sterilization. Also, a plastic that contacts a drug directly should be tested to ensure one does not affect the other.

Meeting Regulatory Criteria

All materials used in medical devices must be screened for biocompatibility so they do not cause adverse local or systemic effects in people. These effects can occur through direct contact or through the release of impurities, extractables, or degradation products. Biocompatibility depends on a plastic's chemical characteristics and how a device will be used, which determines the nature, frequency, and duration of patients' exposure to a polymer.

Many tests used to evaluate biocompatibility are defined in the 12-part global standard known as ISO 10993, “Biological Evaluation of Medical Devices.” ISO 10993: Part 1 helps product developers select the tests needed for an application. Other parts of ISO 10993 detail the methods for the tests suggested in Part 1.

The battery of possible biocompatibility tests includes those for acute, subchronic, and chronic toxicity; irritation to skin, eyes, and mucosal surfaces; sensitization; hemocompatibility; genotoxicity; carcinogenicity; and reproductive effects. Tests that address specific organs or the immune or reproductive systems may also be needed if a device warrants it.

The USP Class VI standard, which is often used to determine biocompatibility in the United States, involves a series of in vivo tests that follow guidelines in FDA's blue book memorandum G95-1.

Testing for cytotoxicity is a rapid, sensitive, and inexpensive way to see whether a material has significant amounts of biologically harmful extractables. These tests use extractions drawn from a material to look for acute, adverse biological effects on mammalian cell cultures. They are defined in ISO 10993-5: “Tests for Cytotoxicity—In Vitro Methods.” 

USP Class VI comprises a set of nine tests, including acute systemic and intracutaneous toxicity. Each involves four extraction solutions and an implantation test. Acute systemic and intracutaneous toxicity tests involve solutions made by immersing a plastic in a saline solution, an alcohol-in-saline solution, glycol 400, and cottonseed oil. In acute systemic toxicity testing, extracts are injected into mice and are not considered toxic if the systemic reaction they cause is not significantly greater than that of a blank extractant. 

In intracutanous toxicity testing, the extracts are injected below the skin of rabbits and also must not produce a significantly greater tissue reaction than a blank extractant. In implantation testing, a material implanted in rabbits for 7 days must not produce a significantly greater macroscopic reaction than that of the USP negative control. 

In addition, regulatory requirements must be met by those who supply materials in the United States. An especially important requirement mandates that suppliers have FDA drug and device master files for their products. These confidential databases give FDA access to information on materials, facilities, processes, and health and safety studies, so the agency can validate device manufacturers' product approval submissions that include data from molders, converters, and other suppliers. These master files include information on where and how a material is made, polymer composition (including additives), sterilization data, and health and safety information.

As with any plastic component, other standards come into play according to how an assembled device will be used. These standards derive from a number of approval bodies, such as Underwriters Laboratories, and may involve specifications for heat resistance, flammability, and electrical and mechanical capabilities, among other properties. 

Supplier Support for Product Development and Manufacture

A supplier of engineering thermoplastics must offer far more than medical-grade polymers if it is to support the development of healthcare devices and diagnostic equipment. It should have solid experience in healthcare technology and be proficient with regulatory processes, so it can help its customers gain approvals in the United States and overseas.

It should also have USP Class VI and other biocompatibility data on hand for its products, so manufacturers can screen for plastics that will work in a particular application. Such information might include data based on cytotoxicity testing, hemolysis testing, and the USP physicochemical test. Suppliers should be willing to perform ancillary evaluations to substantiate material purity and even modify production to reduce extractables, if needed. 

A supplier should also have the expertise to work closely with the manufacturer, designer, molder, and others involved in creating a device in order to help minimize development costs and time to market. It should have the depth to support manufacturers with critical technical services, such as help in selecting the optimal resin, and such design services as computer-aided design, engineering, and finite-element analysis. It also should be able to support molding through mold-flow and other computer-based analyses, and to be a problem-solving resource as device production ramps up. 

Since medical devices usually have long development times and life cycles, a viable plastic supplier must be stable and able to make its materials available over the long term. It should be a global operation capable of supporting its materials anywhere in the world, if needed.

Conclusion

Engineering thermoplastics have become essential in healthcare technology because they perform well in stressful and demanding applications. They are often the material of choice in medical devices and diagnostic equipment because they work well across diverse environments and satisfy essential criteria for physical, mechanical, chemical, thermal, sterilization, and other properties.

When selecting a polymer, manufacturers need a solid understanding of the requirements that accompany their production end use. They also need to understand how to evaluate polymers to find candidates that will work best in a specific application. Often this involves close cooperation between a device manufacturer and material suppliers, who should have a wealth of information on their products, including biocompatibility data. In addition, the supplier of the chosen plastic should be global in scope, and have the capabilities needed to support devices at all stages from design to production. 

Copyright ©2004 Medical Device & Diagnostic Industry