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November 9, 1993
Polymer Solves Heart Problem

University Park, Pa. -- The same polymer that is used for tire inner tubes may be the solution to a problem that plagues the developers of electric artificial hearts, according to Penn State researchers.

"The Penn State Electric Total Artificial Heart is made up of two blood sacs on either side of a tiny electric motor," says Dr. Bernard Gordon III, associate professor of polymer science. "Unfortunately, the blood sacs allow small amounts of water to escape, filling the motor cavity."

Because the motor can not pump against the fluid that builds up, the liquid must be removed. Some artificial heart designers leave a tube open to the air so that the moisture can evaporate, but the designers at Penn State's Milton S. Hershey Medical Center want their heart to be completely implanted with no openings to the outside.

"Currently, in those Electric Total Artificial Heart models tested on calves, a small reservoir is placed just under the skin to collect the water," says Gordon. "A simple needle prick removes about 2 milliliters of fluid every two weeks." The Penn State designers would like to either lengthen the time between needle pricks or alleviate the need completely.

"Rather than look for another method of removing the fluid, we are looking for a material that is a better air and water barrier," says Gordon.

The blood sacs currently used and originally developed for Penn State's Heart-Assist Pump are made of medical polyurethane elastomers. These elastomers are composed of polyurethane and a polyether.

"What we have done is replaced the polyether with polyisobutylene segments, rubbery elastomeric compounds like polyether that has known properties as an air and water barrier," says Gordon.

This newly designed biomedical elastomer is not without problems. The current molecular weight is too low, which means the length of the chains formed by the carbon atoms in the substance is too short.

"We think we can fix the chain length and create a longer chained molecule," says Gordon. "The second, and more serious problem, is that this is a new biomedical elastomer and does not have Food and Drug Administration approval."

The walls of the blood sac in the Electric Total Artificial Heart come into contact with blood. Only substances approved by the FDA can be used in contact with blood or other biological tissues.

"The solution is to ensure that our polymer never touches the blood or any other tissue," says Gordon.

The blood sac is currently manufactured by dipping a mold into the Biomer_ and layering nine to 11 coats of polymer. Gordon and James Yeh, graduate student in polymer science, have manufactured films with one layer of the new elastomer sandwiched between three or four layers of the FDA-approved Biomer.

"The new biomedical elastomer, in only one layer between layers of the FDA-approved Biomer, provides at least 15 times the water barrier of the standard polymer," say Gordon.

With this amount of permeability, it would take six months for 2 milliliters of fluid to collect. The time between needle pricks to remove fluid could be even longer because the 15-times better permeability is the smallest amount the test equipment can accurately measure. The actual permeability may be far less. The researchers also note that this reduced permeability occurs with only one layer of short-chain polyisobutylene segmented polymer.

The researchers would eventually like to do away with the polyurethane/polyether Biomer and coat the Polyurethane/ polyisobutylene elastomer with fibernectin, a material that is FDA approved and has properties that work very well with blood.

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PENN STATE -- HERSHEY MEDICAL CENTER
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