Properly designed seals protect against dirt, dust, fluids, and EMI.
DALE M. ASHBY
Material Technology and R&D Manager
Parker Hannifin O-Ring Div.
Designing elastomeric seals for pressurized-fluid systems in military and aerospace vehicles presents interesting challenges. In many cases, medical applications pose the same challenges, but with some unusual twists. In medical devices, seal failures can pump too large a drug dose into a patient, or make an electric wheelchair spin out of control due to cell-phone interference, or spark adverse and deadly reaction in an artificial heart. Issues like these are real threats that biomedical engineers deal with daily.
A material problem
Many sealing applications considered to be "medical" never come in direct contact with human tissue. For these, engineers can rely on standard principles of seal design and material selection. But for applications in which seals touch human tissue or bodily fluids, or contact drugs and other medical fluids, biocompatibility of the seal compound must also be taken into account.
Another consideration is the level of impurities in the seal material. Impurities could react with fluids or tissues themselves. The typical concern is that impurities will leach out of the seal over time. The leachate may be carcinogenic or toxic. Therefore, engineers specify elastomers with few or no impurities.
Medical-seal materials are usually required to be certified as suitable for use. These requirements are summarized in Part 21 CFR 177.2600 and related sections. The Food and Drug Administration lists the rubber ingredients generally recognized as safe. Making rubber seals from FDA-approved ingredients is usually sufficient for overall FDA approval. In critical applications, however, seals must be made from an even "cleaner" list of ingredients. The U.S. Pharmacopeia (USP) Class VI outlines requirements for system toxicity and intracutaneous toxicity for these "cleaner" compounds. These USP Class VI compounds must be made of ingredients with clear histories of biocompatibility and meet tighter requirements for leachates. Typical USP Class VI elastomer compounds include platinum-cured silicones and certain TPEs. Platinum-cured silicones are favored because platinum is a relatively clean curative. In addition, ASTM standard F604 describes silicone elastomers for medical seals.
It is not only the seal material and design that biomedical engineers must consider, but the processing and packaging as well. Federal regulations (21 CFR Part 820) outline manufacturing requirements for medical devices and components. It defines the Good Manufacturing Practice Regulations (GMP) that must be used in making elastomeric seals for medical devices. In some instances, it mandates that manufacturing be done in rooms with positive pressure to keep out contaminates. In other cases, medical seals must be fabricated in "rated" clean rooms.
After manufacturing the rubber seal, it must be handled with care. Usually seals are washed in deionized water before packaging for shipment. Packaging for medical seals is also different than that used for typical rubber parts. Double bagging is commonly used as a safety factor against contamination. And labeling to provide complete traceability of raw materials is critical to ensure parts are not mixed at assembly.
Design and fabrication
Although there are many complex molded seals in the medical industry, there is still a large demand for simple O-rings and flat gaskets. These sealing solutions are readily available and cost effective, and can be manufactured in many compounds and virtually any size.
Another cost effective and flexible way to manufacture medical-grade seals is by extruding rubber to form solid or hollow cross-sectional profiles. The rubber is sliced into gaskets as thin as 0.018 in. or spliced into longer lengths and formed into rings or four-corner "picture-frame" gaskets.
For spliced gaskets, a hollow cross section lowers the amount of force it takes to compress the seal. Low closure forces are important if a device will be made from thin plastic parts.
Smaller devices typically use extruded-andcut seals (washer shaped). Cut gaskets are ideal for static face seals, and the flexibility of the extrusion process lets operators form complicated geometries. Large sterilizer units, for example, incorporate hollow cross-sectional spliced gaskets on their doors. The hollow seal lets the doors close easily while still maintaining an effective seal.
Shielding against EMI
Complying with global electromagnetic compatibility (EMC) regulations is a major challenge in developing new medical devices. Faster-running systems in smaller, lighter designs two trends in medical design are more likely to have EMI problems. And clinics are increasingly filled with signals from other medical electronics, computers, cell phones, and telecommunication devices. Consequently, EMC standards are becoming more comprehensive.
The European Union's Medical Device Directive and related standards (EN 60601-1-2) are evolving into world references for EMC in the medical industry. They set levels for both emission leakage and EMI immunity. Medical devices that meet European standards are typically assumed by the FDA to comply with U.S. standards. Techniques for EMI shielding of medical devices are similar to those in other industries. Effective board design and regulating chip speeds minimize noisy circuit-board components. But some form of shielding is often still needed to comply with standards. For example, conductive seals and other EMI-shielding seals are available in a wide array of sizes.
In medical equipment, shielding at the outer enclosure is the most common design approach. Whether metal or metallized plastic, the outer housing of a medical device will have openings for cables, switches, monitors, keyboards, and more. In addition, housing-panel seams present pathways for unwanted signals to leave or enter.
EMI gaskets are typically used at these seams for shielding protection. A conductive elastomer gasket provides current continuity between conductive panels. The gasket's inherently conformable nature also provides a level of environmental sealing. More often, a separate environmental seal is used outboard of the EMI gasket to guard against clinical fluids such as saline and bleach.
Another option is a coextruded elastomer gasket. These feature a conductive side for shielding combined with a nonconductive environmental seal in a single, continuous strip. Other EMI gaskets include strips of metal fingers or wire mesh, or soft foam rubber covered with conductive fabric. Metal fingerstock and knitted wire mesh are both highly conductive and popular choices for shielding medical cabinetry. Conductive fabric-over-foam gaskets are ideal when only low closure forces are available between enclosure parts. They are made of low-durometer urethane cores and may offer secondary dust protection. The fabric and foam gaskets can also be shaped and used at connector sites, backplanes, and other holes through the enclosure.
All of these EMI seals come in continuous lengths and cut-to-length pieces. They're usually attached to panels via integral adhesive strips or tangs that clip onto cabinet edges.
Monitors and displays on medical equipment provide another pathway for spurious signals. EMI shielded windows, conductively mounted to an enclosure, can eliminate most EMI problems. These windows contain a fine-wire mesh, and recent developments have dramatically improved their optical clarity. They can also be treated with glare-resistant coatings for better image quality.
At the printed-circuit-board level, stamped metal cans or metallized plastic covers can shield individual components. The plastic covers carry a thin layer of conductive elastomer or paint for shielding properties. These cans and covers mount to metal traces on the board to complete the conductive path. Integral conductive gaskets on the can edges provide a more flexible EMI seal than soldering an attractive feature for handheld medical devices in plastic housings.
In the future, expect more stringent regulations on the sealing industry. Currently, European standards already exceed those in the U.S. The bulk of new regulation. will focus on developing cleaner materials with fewer extractable and lower toxicity levels. Emphasis may also be placed on manufacturing conditions. Regulators are considering forcing seal production into clean-room environments.
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Artificial heart valves
Blood transfusion equipment
|In some medical devices, materials come in direct contact with bodily fluids, tissues or drugs and solutions that will come in contact with the human body.|