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Cryogenic Device Warms Up to Lustran

Bayer MaterialScience AG : 30 January, 2001  (Company News)
Two Lustran
Gray Lustran ABS 248 resin is used for the probe's three- component handle while the snow white 348 grade is used for the disposable control unit that slips over the probe. Both applications require tight tolerances, which can reach as low as 0.0005 inches.

A new cryosurgical device from CryoGen, Inc. required formidable material requirements - tight design tolerances reaching as low as 0.0005 inches, radiation sterilization, excellent surface appearance and competitive pricing.

Bayer Corporation's Plastics Division met the challenge with two grades of its Lustran® acrylonitrile-butadiene-styrene resin. CryoGen's First Option1 Uterine Cryoblation Therapy1 system is used for intrauterine ablation (removal of tissue) to treat patients with abnormal uterine bleeding. It uses the Lustran 248 grade for the pistol-grip handle of the device's reusable cryoprobe and the Lustran 348 material for the disposable control unit, which slips over the cryoprobe.

'Eighty percent of the molders we talked to recommended focusing on ABS,' said Steve Kovaicheck, Senior Project Manager for CryoGen. 'We zeroed in on Bayer's materials because of a strong recommendation from our prototype designer. We also were very impressed with how quickly Bayer responded with literature and design and engineering assistance. That's important when you're a young, growing company that's moving fast,' he said.

Kovaicheck added that the materials also stood up to the tough challenges imposed by such a precise design. Both Lustran 248 and 348 resins offer dimensional stability, processability, strength, toughness, rigidity, heat resistance and chemical resistance. They also were price- competitive and readily available, two additional reasons underlying theirselection.

Unique Design Leaves Competition in the Cold

First Option, which is undergoing FDA review for the specific indication of endometrial ablation, is available for use now. The device is unique in its use of a closed refrigeration system that requires no replenishment and releases no coolant into the atmosphere. The tissue-freezing device also is lauded for its compact size and low operating pressure, which allow it to be used in an outpatient setting.

There are four main system components: the console, which contains the refrigerant compressors and other key system technology; the flexible transport line, which carries the coolant to the cryoprobe; the cryoprobe, which is inserted into the uterus; and the control unit, which is the disposable component that slips over the probe.

While the system can produce coolant that reaches minus 120 degrees Celsius, only the last inch of the probe tip reache s these temperatures. Everything else remains room temperature, making First Option convenient and safe for use in doctors' offices.

The probe's handle features a gray Lustran 248 ABS resin. This three-part handle measures 4.5 inches by 4 inches when assembled, with a diameter that ranges from 1.75 inches down to 1.5 inches in the grip area. The wall thickness ranges from 0.07 to 0.1 inches. An especially tricky design component is the 12 pinholes in the front section of the handle. These holes, which require a tolerance of 0.0005 inches, house 12 spring- loaded contacts. Any draft, and a contact may not be made, causing the device to not function.

'This Lustran material allows us to have low shrinkage, giving us the ability to handle these low tolerances,' said Kovaicheck. 'It also has a natural sheen, so we didn't have to go to any lengths to polish the molds to get the surface finish we wanted. And it gives us the rigidity we need with the tactile feel that's important for a medical device handle.'

Plastics Engineering & Development, Inc., of Carlsbad, Calif., injection molds the three probe handle components in a family mold. The components feature numerous inserts, including unique surface features and internal compartmentalizing. Cycle times average 30 seconds, and minimal secondary operations are required.

The triangular- shaped control unit uses snow white Lustran 348 resin for the locking tab unit, shell, nose cone and endcap. An uncommon feature is the use of O-rings in internal luer and fluid channels. Tight tolerances also abound, with many reaching 0.002 inches. The assembled unit measures 1.5 inches in diameter and 7.5 inches in length without the locking tabs and nylon composite catheter. Wall thickness is a consistent 0.1 inch.

Florida Custom Mold injection molds the four components in single molds, averaging cycle times of 35 seconds for each of three components and 60 seconds for the larger shell. The locking tab unit and nose are then bonded to the shell with adhesives, and the endcap is snap-fit to the nose.
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