Heart disease and circulatory disorders are responsible for more deaths in the world than any other common oilment. The most serious problems mise from arteriosclerosis and from the progressive narrowing of the cardiac arteries. A blockage of one of these arteries leads to “Heart attack”. For patients with an irreversibly damaged heart, two things can be done. First, a heart transplant may be possible, but it has limited scope due to obvious reasons. Secondly, the functions of the damaged heart may be taken over permanently or temporarily by an artificial pump. A lot of work has been done to design and testing of Artificial heart pumps. Unfortunately, most synthetic polymers accelerate the clotting of blood. This problem is so serious that animals on which the pumps are tested sometimes die within hours from the massive, gelatinous blood clots that form in the pumps. Avoidance of the clotting process is a complex problem because it depends mainly on the design of the pump and the materials used for its construction.
Heart Pump Designs.
Two types of heart pumps have been developed.
- Auxiliary blood pumps to bypass or supplement the action of a damaged heart until it can repair itself.
- Total Artificial heart pumps that can completely replace the living organ. Figure 4 shows a relatively simple “Artificial heart” device designed for implantation in the body.
Pulses of compressed air applied inside the rigid casing compress the silicone or polyurethane rubber inner tube which is connected to the aorta and this forces blood from the pump. Valves may be used to prevent backflow. The phase of the pumping cycle is synchronized with the pumping motion of the patient’s heart.
Figure 5 shows another design of a pump which uses hemispheres of titanium, polycarbonate. poly (tetrafluoroethylene) or poly (methyl methacrylate) containing a polyurethane diaphragm. Pulses of compressed air or carbon dioxide actuate the diaphragm and cause the pumping of the blood.
Synthetic polymers for heart pumps.
A wide variety of different polymers have been used for the fabrication of heart pumps. Some of the examples are silicone rubber, polyurethane rubber, Dacron polyester, Teflon, polycarbonate, poly (methyl-methacrylate), poly (vinyl chloride). Most of these materials cause blood clotting and destruction of red cells, although some are markedly better than others. Polyurethanes are among the most commonly used flexible biomaterials. They have excellent flexing strength. (The diaphragm in a heart pump would have to withstand about 90 million flexing motions without breaking over a 10 year period). However, they are chemically unstable during long term exposure to aqueous media.
A silicone rubber is also an ideal biomaterial. It is chemically inert, soft and flexible. However it can promote blood clotting if the blood is flowing slowly, and it can fail after continuous flexing. Another problem is the tendency of silicone rubber to absorb fats from the blood to swell and finally to weaken.
Researches show that fluoro-alkyl siloxane polymers or poly phosphazenes may prove to be more suitable for artificial heart applications. Also, the ability of a synthetic polymer to initiate blood clotting largely depends on the nature of the surface. (Smooth surfaces are better than rough). Highly water repellent polymers appear to be among the best materials for contact with blood. Since the inside lining of blood vessels is negatively charged, it has also been found that polymers with a surface charge is more effective than neutral polymers. If an anticoagulant like heparin is bonded to the surface or absorbed into the polymer, it may help in the problem of blood incompatibility.