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After the sleeve came a cage in which the silicone ball must bob up and down 40 million times a year without sticking, and Dow Coming's Chemist Silas A. Braley says confidently: "The Silastic ball cannot stick." The University of Oregon's Dr. Albert Starr has installed 18 such valves in six patients -three apiece, replacing the aortic, mitral and tricuspid valves.
But any ball valve is rougher on blood cells than nature's leaflet valves, so surgeons at the University of Wisconsin have developed butterfly valves of Teflon that come closer to the original in design. The demands on the plastic in such valves are tremendous: the leaflets must bend back and forth 40 million times a year. But so far, 39 patients have had them installed as replacements for aortic or mitral valves, and they are still working after as long as 20 months.
When the first entirely artificial heart is developed, it will probably be made of Silastic. This is the material that Houston's Surgeon Michael E. DeBakey used for the closest approximation to such an organ ever tried in a human patient. It was a substitute for the left ventricle, the heart's main pumping chamber, and it worked for 3½ days, until the patient died of other causes (TIME. Nov. 8, 1963).
Parts of major arteries, including the biggest of all, the aorta, are far easier to deal with. They are regularly replaced or bypassed with grafts of Dacron knit such as was used in surgery on the Duke of Windsor. Electrical pacemakers to regulate or replace the beat of a faltering heart have by now been implanted under the abdominal skin of 10,000 patients, with leads to their hearts; all these devices are encased in Silastic because of its inertness.
Filters & Glues. Artificial kidneys now in use are as big as a washing machine or an entire laundry, but medical engineers are making them ever smaller; they hope eventually to devise one that can actually be put inside the body. A hopeful lead comes from a plastic called polyvinyl pyrrolidone, now widely used as the setting agent in women's hair sprays. PVP membranes pass chemicals between the blood and the cleansing water of the artificial kidney about three times as fast as the cellophane membranes now used. PVP has another advantage for an implantable kidney: like Silastic, it seems thoroughly compatible with the blood, and has little tendency to provoke clotting.
As a replacement for bone in 700,000 operations a year, surgeons will now have available a regular supply of calf bone, specially treated to remove all dangerous protein. E. R. Squibb & Sons this week announced the first U.S. Government approval of a sterilized calf bone, vacuum-packed, which can be stored at room temperature.
Screw-In Cornea. Ironically, the earliest attempt to use a primitive plastic involved one of the most intricate organs in the body. It was an 1853 attempt to replace the cornea of the eye, and it failed. Then the technique of human corneal transplants was developed, and the urgency of finding a plastic seemed to diminish. But human transplants do not stay clear in all cases. An imaginative ophthalmic surgeon, Dr. William Stone Jr., working first in Boston, then in Los Angeles, has devised a corrective corneal implant of plastic.