Conductive Plastics for Medical Applications

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

An MD&DI January 1999 Column

SPECIAL SECTION

Polymer materials compounded with a variety of additives provide design flexibility in protecting against static accumulation, ESD, and EMI/RFI.

The rapid growth of thermoplastics in medical markets is a testament to the suitability of these materials to meet the demands of today's healthcare industry. Thermoplastics can be compounded with a variety of common and specialty fillers, reinforcements, and modifiers to yield specific properties in a wide range of applications.

Among these additives are electrically conductive modifiers that, when compounded with thermoplastics, can provide protection against static accumulation, electrostatic discharge (ESD), and electromagnetic and radio-frequency interference (EMI/RFI). Although conductive thermoplastics are traditionally found in electronic, business-machine, computer, and industrial applications, the medical community is realizing enhanced performance and value in using these specialty materials for everything from tools to trays.

STATIC AND EMI/RFI

The effects caused by static and EMI/RFI are as familiar as sparks jumping from fingertip to doorknob, static cling in fabrics and films, and electronic noise in communications networks. Static accumulation and discharge and EMI/RFI can be either man-made or naturally occurring phenomena and may not necessarily pose a problem.

However, when present in, on, or near electronic circuitry, moving materials, or flammable environments, they create hazards that must be controlled or eliminated. ESD can damage or destroy sensitive electronic components, erase or alter magnetic media, and initiate explosions or fires in flammable environments. Accumulated static charge can halt mechanical processes by clogging the flow of materials. Static-attracted contaminants can affect the purity of pharmaceuticals.

A conductive thermoplastic compound from RTP Co. (Winona, MN) is used in the main housing, battery door, and end cap of this remote heart-monitoring device manufactured by GE Marquette Medical Systems (Milwaukee, WI). The transmitter gives patients freedom of movement while electronically linking them to a remote computerized output system.

Electromagnetic and radio-frequency waves radiate from computer circuits, radio transmitters (including cellular phones), fluorescent lamps, electric motors, lightning, and many other sources. They become undesirable when they interfere with the operation of electronic devices. Consequences can include corruption of data in information storage and retrieval systems, inaccuracy in diagnostic equipment, and interruption of medical devices such as pacemakers.MATERIAL SOLUTIONS FOR STATIC PROBLEMS

Static accumulation and electrostatic discharge are controlled or eliminated by adjusting electrical characteristics of at-risk materials or their immediate environment. Conductive thermoplastic compounds prevent static accumulation from reaching dangerous levels by reducing a material's electrical resistance. This allows static to dissipate slowly and continuously rather than accumulate and discharge rapidly—perhaps as a spark.

MATERIAL SOLUTIONS FOR EMI/RFI

Shielding of electronic circuitry controls electromagnetic or radio-frequency interference, thus ensuring operational integrity and electromagnetic compliance (EMC) with existing standards. Shielding preserves operational integrity by preventing electronic noise from penetrating to susceptible circuitry, and provides EMC by preventing emissions from escaping to adjacent susceptible equipment.

Figure 1. EMI/RFI is reflected off the source side of a shield or is rereflected off a second shield surface.

Conductive thermoplastic compounds provide this shielding by absorbing electromagnetic energy and converting it to electrical or thermal energy. These compounds also function by reflecting electromagnetic energy from the source side of the shield and also by rereflecting it from the second surface of the shield (Figure 1).

STRUCTURE OF CONDUCTIVE THERMOPLASTIC COMPOUNDS

A conductive thermoplastic compound is a resin that has been modified with electrically conductive additives, including carbon-based powder and fibers, metal powder and fibers, and metal-coated fibers of carbon or glass. Varying the percentage or type of conductive additive used in the compound permits one to control the degree of electrical resistivity (Figure 2).

Figure 2. Additive concentration effect on conductivity in a typical thermoplastic (nylon 6/6).

Recently, unique conductive additives such as metal oxide–coated substrates, intrinsically conductive polymers (ICPs), and inherently dissipative polymers (IDPs) have found commercial use in conductive thermoplastic compounds. Metal oxide–coated substrates were initially introduced as colorable substitutes for carbon black powder–filled plastics. When compounded into thermoplastics, these additives are able to provide a wide range of conductive properties and colors. ICPs are polymers with strong electrical conductivity. The newest type of additive, they are expected to play significant roles in conductive applications from static protection to EMI shielding. IDPs exhibit weaker electrical properties than ICPs; when compounded with other resins, they can impart antistatic properties to molded articles. IDP-containing compounds generally have lower ionic- and metallic-contaminant levels than conductive compounds containing traditional additives and are preferred for static-protective packaging of sensitive products.

SELECTION OF CONDUCTIVE ADDITIVES

Conductive thermoplastics are generally designed to meet physical performance criteria in addition to static or EMI/RFI control. Often, these materials must perform some structural function, meet flammability or temperature standards, or provide a wear- or chemical-resistant surface. In addition, conductive compounds may need to pass purity standards prior to acceptance in medical applications because of concerns with outgassing of volatile substances and contact with ionic or metallic contaminants.

The conductive additive for any application is chosen based on performance criteria of the molded article. If conductive performance is the only specification, almost any conductive additive can be used, and cost will ultimately control the selection. When some of these other criteria are included, the selection is determined by whether the cumulative effects of various additives are acceptable for the application. The specialty compounder should have qualified and experienced engineering personnel available to aid in the additive selection process.

MECHANICS OF CONDUCTIVITY

The mechanism of conductivity in plastics is similar to that of most other materials. Electrons travel from point to point when under stress, following the path of least resistance. Most plastic materials are insulative: that is, their resistance to electron passage is extremely high (generally >10¹⁵).

Conductive modifiers with low resistance can be melt blended with plastics—in a process called extrusion compounding—to alter the polymers' inherent resistance. At a threshold concentration unique to each conductive modifier and resin combination, the resistance through the plastic mass is lowered enough to allow electron movement. Speed of electron movement depends on modifier concentration—in other words, on the separation between the modifier particles. Increasing modifier content reduces interparticle separation distance, and, at a critical distance known as the percolation point, resistance decreases dramatically and electrons move rapidly.

Table I. Thermoplastics that can be used in conductive compounds.

THERMOPLASTICS IN COMMON CONDUCTIVE COMPOUNDS

Nearly every type of polymer can be compounded with conductive fillers (Table I). The following materials are some of the more common medical polymers that can be rendered electrically conductive.

Polyetheretherketone (PEEK). PEEK is sterilizable via autoclave, EtO gas, or high-energy radiation and offers good chemical resistance. Common uses include catheters, disposable surgical instruments, and sterilization trays.

Polyurethane. Available in a wide range of hardnesses, polyurethane is a high-clarity polymer that can be sterilized using dry heat, EtO, or radiation. Medical applications include tubing, catheters, shunts, connectors and fittings, pacemaker leads, tensioning ligatures, wound dressings, and transdermal drug-delivery patches.

Polycarbonate (and Polycarbonate Blends). Capable of being sterilized by all common methods, polycarbonate has especially good toughness and impact resistance. Equipment housings and reservoirs are among the most common medical components made from the material.

Polysulfone (PSO). Possessing excellent thermal stability and toughness, polysulfone is resistant to a variety of chemicals and can be supplied in transparent grades. The polymer can be sterilized using autoclave, EtO, or radiation. Applications include instrument handles and holders, microfiltration devices for immunoassays, reusable syringe injectors, respirators, nebulizers, prosthesis packaging, sterilizer trays, and dental tools.

Liquid-Crystal Polymer. High strength and stiffness are among the notable physical properties of liquid-crystal polymers. These materials can be sterilized by all common methods and are used in products such as dental tools, surgical instruments, and sterilizable trays.

FEATURES OF CONDUCTIVE THERMOPLASTICS

Conductive thermoplastics offer a number of advantages compared with other materials, such as metals, for ESD protection or EMI/RFI shielding (Figure 3). Finished parts are lighter in weight, easier to handle, and less costly to ship. Fabrication of finished parts is typically easier and less expensive, and all common thermoplastic processing methods can be employed. Conductive plastic parts are less subject to denting, chipping, and scratching and often demonstrate more-consistent electrical performance than painted metal parts.

Figure 3. Conductivity values of thermoplastic compounds fall between those of unmodified plastics and metals.

A common misperception is that conductive plastics are always colored black; this is not the case. In fact, most conductive thermoplastics can be made in a wide variety of colors. With a precolored conductive thermoplastic, the color is inherent in the material rather than added as part of a secondary operation.

In addition, specially developed additives offer both conductivity and matched substrate color when electrostatic painting is required for critical color matching of devices assembled from dissimilar materials. Matching the color of the conductive compound to the paint makes scratches, chips, and abrasions less noticeable and maintains a homogeneous surface appearance. These conductive compounds significantly improve paint transfer efficiency and eliminate the need for conductive primers, leading to dramatic reductions of volatile-organic-compound (VOC) emissions. Electrostatic painting also significantly reduces overspray, saving cleanup and disposal costs.

Neither is opacity the only option, as a number of conductive thermoplastic compounds retain transparency while exhibiting static-control properties. Particular static-control additives can match refractive indices of some thermoplastic polymers, rendering clear or translucent parts. Contact clarity—the ability to read objects through a directly contacting plastic material—is a desirable property that can be achieved in packaging applications, enabling bar code imprints or laser markings to be accurately detected and read by automatic equipment. Contents of packages can also be identified by color coding, without violating the package seal.

For environments in which ionic contamination and ESD can cause millions of dollars in damage to electronic components, pretested thermoplastics and conductive additives can be compounded to meet tight tolerances for a wide range of impurities. Such high-purity formulations adapt well to today's ultrasensitive electronics that often feature high device speeds, small geometries, and dense storage capacities.

TESTING FOR CONDUCTIVITY

Three major characteristics are used to evaluate the electrostatic properties of ESD compounds. These are resistivity, both volume and surface; electrical resistance; and static decay rate. EMI/RFI shielding materials are additionally evaluated by shielding-effectiveness testing.

The most common test method to determine the conductivity of plastics has been ASTM D 257, which measures both volume and surface resistivity. Since electrostatic charge is a surface phenomenon, surface resistivity tends to be the more meaningful of the two. Surface resistivity is the measured resistance between two electrodes forming opposite sides of a square, and is reported as ohm/square. Volume resistivity (also referred to as bulk resistivity) is measured resistance through the sample mass. It is an indicator of how well a conductive additive is dispersed, and is expressed as ohm-centimeter.

Electrical resistance is defined as opposition to the flow of electricity. The EOS/ESD Association Draft Standard 11.11 measures surface resistance as opposed to surface resistivity.

Static decay is measured with Federal Test Method 101, Method 4046. This test measures how quickly a charge is dissipated from a material under controlled conditions, which is one parameter of actual electrostatic performance.

Shielding effectiveness is evaluated under ASTM D 4935-89, in which coaxial transmission-line methodology analyzes planar specimens under far-field conditions over a frequency range from 30 MHz to 1.5 GHz. Shielding effectiveness is represented as the ratio of power received with and without a candidate material present and is expressed in decibels of attenuation.

APPLICATIONS

Suitable for a variety of applications, conductive thermoplastic compounds can satisfy the medical industry's need for miniaturized, high-strength parts. Most can withstand state-of-the-art sterilization procedures, including autoclave, and many are certified for purity and pretested to minimize ionic contamination. Medical applications under evaluation or currently using conductive thermoplastics include:

CONCLUSION

Conductive thermoplastics offer medical product designers unrivaled freedom in the control of ESD and EMI/RFI. These compounds do not generate high levels of static charge, can dissipate charges before dangerous levels accumulate, and can provide electrostatic and EMI/RFI shielding. Properly formulated, the materials can provide desired conductive characteristics while maintaining other required physical and mechanical properties. Already used in a varied range of applications—from strong, thin-walled sterilizable components to flame-retardant, precolored parts that can be electrostatically painted—conductive compounds are certain to become even more common as electronic devices proliferate and the technology evolves to meet new cost or performance imperatives.

BIBLIOGRAPHY

"Electromagnetic Shielding—A Material Perspective," Innovation 128 Tech Trends (Innovation 128, January 1996).

Huang, JC. "EMI Shielding Plastics: A Review." Advances in Polymer Technology 14, no. 2 (1995): 137–150.

Weber, ME. "The Processing and Properties of Electrically Conductive Fiber Composites." PhD diss., McGill University, 1995.

Larry Rupprecht is a senior product development engineer and manager of the Conductive Materials Group at RTP Company (Winona, MN). Connie Hawkinson is RTP's marketing communications manager.

Copyright ©1999 Medical Device & Diagnostic Industry