Originally Published October 2000
Polymer Improves Performance of Laparoscopic Device
Medical-grade plastic molds around metal insert to maintain seal.
As surgical procedures become more sophisticated, so must the instruments that support them. One such instrument is the ElectroScope, a suction irrigation electrode developed by Encision Inc. (Boulder, CO; formerly ElectroScope Inc.) that enables surgeons to conduct suction, irrigation, and electrosurgery at the surgery site.
The company used Questra 2730 polymer from Dow Plastics (Midland, MI) for the body of the ElectroScope. Questra 2730 is a semicrystalline polymer that is 30% glass filled, impact modified, and manufactured to meet medical specifications.
One of the major hurdles that the manufacturer had to overcome was insulation failure. Insulation failure in a laparoscopic device occurs when the insulation breaks down along the instrument's shaft (active electrode) and the electric current burns human tissue in contact with it. The insulation on reusable electrified instruments wears down with use, and the most damage occurs during repeated disinfection, cleaning, and sterilization.
Bob Mitchiner, director of engineering for Encision, explains, "Several plastics, such as polysulfone, were tried, but we found that these would allow water to migrate between the metal and plastic interface and form a conductive path between the high- and low-potential areas and cause the instrument to fail."
The body of the ElectroScope is made of Questra 2730 polymer from Dow Plastics.
Tom Wessel, a development leader at Dow, explains, "The challenge was to find a plastic that would mold around the metal insert, achieve part dimensions, and maintain a seal between the metal and plastic through initial autoclave sterilization after manufacturing and after subsequent sterilization cycles during its use."
Dow's Questra 2730 provided several advantages. It could be molded in an existing mold and it meets the critical part dimensions for assembly and use, provides the chemical resistance necessary for cleaning, and is capable of withstanding multiple autoclave sterilization cycles.
"The molding optimization provided by the Dow polymer enables the formation of a good seal, and therefore the electrical connection can be made and maintained over the life of the device," says Wessel.
Failure Rate Reduced
The failure rate with the polysulfone parts was at least 25% after 25 autoclave and cleaning cycles. "It was apparent after the first run with Questra 2730 polymer that the resistance to water migration was significantly better than with polysulfone," says Mitchiner. "Fifteen parts were molded in the first trial run and all survived 25 autoclave and cleaning cycles. Successive mold trials proved the superiority of the Questra 2730 polymer in this application over polysulfone."
Molding trials were run on small lots of parts, which were tested to verify that fluid migration had been stopped. "We found that the Dow polymer encapsulated the internal parts more effectively and stopped the migration of fluid caused by multiple autoclave and cleaning cycles," says Mitchiner.
The application is injection molded in two stepsa primary mold to encapsulate the active parts and an overmold to accommodate various applications. Shot sizes are less than one ounce on both operations and the cycle time is approximately 30 seconds.
"Questra 2730 has been a cost-effective choice for the Electro-Scope," says Mitchiner. "When we made the conversion to it, the only material change we made was to convert from polyfluoraniline insulated wire to polyetheretherketone (PEEK) insulated wire. We also made some mechanical changes to improve the ability of the material to encapsulate the conductors."
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