In Diabetes Care for April 1988, there was a short letter that addresses a subject of interest to anyone who wears an insulin pump. The letter, "Medical Plastics and Insulin Pump Treatment" by Dr. Erich Sartorius, deals with issues related to the kind of plastic used in infusion set tubing. Dr. Sartorius points out several of the problems with polyvinyl chloride (PVC) tubing, and stresses the need for awareness of plastic and insulin compatibility.
Dr. Sartorius is not the only one concerned about PVC and its compatibility with insulin. A number of studies have shown that insulin pump tubing made with PVC has some significant drawbacks.
First, when insulin molecules pass through new PVC tubing, a significant quantity of them "stick" to areas on the surface of the plastic called binding sites. After a while, these binding sites all become occupied, and no more insulin sticks. What this means is that during the first few hours of infusion through a new set, as much as 70% of the insulin doesn't make it out the end of the tubing.
Insulin molecules are not the only component of insulin solution that interacts with the tubing. Special preservatives are added to insulin to maintain its sterility, and as much as 85% of these preservatives are absorbed into PVC tubing. Significant quantities of zinc, which is added to stabilize insulin molecules, is also lost to the tubing.
Carbon dioxide passes through PVC tubing relatively easily, and can change the pH of the insulin solution, making it more acidic. If it becomes too acidic, the insulin precipitates and can cause blockage of the infusion set. Materials that leach out of the PVC plastic can interact with insulin and damage the molecules, making them no longer able to function properly. If the infusion sets are sterilized by gamma radiation (an increasingly common method for sterilizing medical devices), this problem can be even greater.
To address the incompatibility problems between insulin and PVC plastic, a Medtronic Diabetes engineering team headed by April Konopka developed a new approach for infusion set tubing.
It has long been known that polyolefin plastics have fewer binding sites, are more resistant to carbon dioxide penetration, and leach far fewer materials into solution than PVC plastic. But polyolefin tubing by itself is not satisfactory, since it is brittle and can kink or break easily. By making a tubing with an inner layer of polyolefin plastic, a middle bonding layer, and an outer layer of PVC plastic for kink resistance and strength, April and her team were able to combine the best features of both plastics into tough insulin-compatible "Polyfin" tubing. They also decided that Polyfin tubing should not be sterilized by gamma radiation, but by another process that does not affect compatibility.
Medtronic Diabetes now uses Polyfin on our needle sets, and the non-needle Sof-set.
