Polytetrafluoroethylene

Polytetrafluoroethylene

  PTFE is thermosetting polymer very limited application in medicine, but its characteristic properties, which combine high strength and chemical resistance, are useful for some orthopedic and dental devices. It also has high modulus and tensile properties with negligible elongation. The polymer chains in this material are highly cross-linked and therefore have severely macromolecular mobility; this limits extension of the polymer chains under an applied load.

Biomaterials are used in many blood-contacting devices. These include artificial heart valves, synthetic vascular grafts, ventricular assist devices, drug releases, and a wide range of invasive treatment and diagnostic systems. An important issue in the design and selection of materials is the hemodynamic conditions in the vicinity of the device. For instance, mechanical heart valve implants are intended for long-term use. Consequently, the hinge points of each valve leaflet and the materials must have excellent wear and fatigue resistance in order to open and close 80 times per minute for many years after implantation. In addition, the open valve must minimize disturbances to blood flow as blood passes from the left ventricle of the heart, through the valve and into the ascending aorta of the arterial vascular system. To this end, the bileaflet valve disks of one type of implant are coated with pyrolytic carbon, which provides a relatively smooth, chemically inert surface.

Synthetic vascular graft materials are used to patch injured or diseased areas of arteries, for replacement of whole segments of larger arteries such as the aorta, and for use as sewing cuffs. Such materials need to be flexible to allow for the difficulties of implantation and to avoid irritating adjacent tissues; also, the internal diameter of the graft should remain constant under a wide range of flexing and bending conditions, and the modulus or compliance of the vessel should be similar to that of the natural vessel. A biomaterial used for blood vessel replacement will be in contact not only with blood but also with adjacent soft tissue. Experience with different materials has shown that tissue growth into the interstices of the biomaterials aids healing and integration of the material with host tissue after implantation. In order for the tissue, which consists mostly of collagen, to grow in the graft, the vascular graft must have an open structure with pores at least 10 micrometers in diameter. Fibroblasts synthesize the structural protein tropocollagen, which is needed in the development of new fibrous tissue as part of the healing response to a surgical wound.

Artificial heart valves and vascular grafts, while not ideal, have been used successful and have saved many thousands of lives. However, the risk of thrombosis has limited the success of existing cardiovascular devices and has restricted potential application of the biomaterials to other devices. Considerable advances have been made in the ability to manipulate molecular architecture at the surface of materials by using chemisorbed or physisorbed monolayer films. Such progress in surface modification, combined with the development of nanoscale probes that permit examination at the molecular and submolecular level, provide a strong basis for optimism in the development of specialty biomaterials with improved blood compatibility

Polyurethane

Seen today in everyday uses such as shoe soles, tires and foams, polyurethane holds an extremely import role in cardiac medicine today.  Polyurethane is a thermoset that is also a non-condensation step growth polymer^.Polyurethane has a very low molecular weight compared to many other polymers with a molecular weight average of only 47,000 g/mol.  The benefits of this material lie in the basics of it visible physical properties.  Polyurethane is often described to bridge the gap between rubber and plastic.  It holds one of the best load-bearing capacities of almost any materials around¹⁵.

Invented back in 1937 by Otto Baker, polyurethane was the result of a search for a material that has high strength and good environmental resistance.  For both reasons polyurethane today is one of the most important materials in use for ventricular assist devices.  Differing from artificial hearts, VAD   are for short-term assistance to cardiac circulation attached to one or both of the heart ventricles.Most commonly seen in the operating room during open-heart surgery, postoperatively, and in the cases of extreme cardiac trauma.  They consist of tubing attached to the heart valves leading to a pump that can be centrifugal, electrical, or pneumatic.

While Dr. D. Liotta of Baylor University developed the principles of this device in the 1950 two doctors, Pierce and Don achy in 1971, significantly refined the technology.  Rewriting the book on fluid mechanics (relating to blood flow) and taking advantage of polymers as a material.  Polyurethane (segmented) stabilized the VAD, making not only the contact barrier of the blood and machine the safest possible, but also using the compressive properties that it exhibits made it function more like the actual heart itself.  The once majority metal device was revealed in 1976 and approved for use by the FDA in 1980^.

There are two ways to produce polyurethane.  However, the most abundant source (95% of world production) is obtained through step growth polymerization of diisocyanates with dihydroxl compounds.  The result is a polymer that has a load bearing capacity comparable to cast steel.  Polyurethane is molded most often through injection molding.  Additionally as of recent years, reaction injection molding (RIM) has become one of the more popular ways to produce in industry.  For the most polyurethane used for VAD  are produced under careful supervision and not often RIM produced.  The largest debate over the use of these materials was potential for mechanical failure.  Past occurrences of failure have been attributed to poor processing and not the material itself¹⁷.

VAD  have made great strides in the past 20 years.  Where once limited to external unit only, today there are internally placed VAD  and as the technology improves, it may one day replace transplant surgery to cure cardiac conditions.  While there are many types of VAD , the only material that is a true alternative in some sense to the polyurethane is stainless steel.  Advancements in the use of ventricular assist devices can be seen in the decrease in number of deaths of patients awaiting transplant, even with a great increase of people on the waiting list.  Perhaps not a solution, the temporary alternative that exists in VAD can only be attributed the integration of polymers into their design.

Conclusion

 Indeed, biomaterials have already made a huge impact on medical practices. But, the opportunities that lie ahead of us are enormous.  Tissue engineering and related subjects have the potential to change paradigms for treating diseases that today cannot be treated effectively like certain forms of liver failure, paralysis, and certain disorders.   Clearly we are faced with big challenges. But, the message I try to get across to everyone mostly to young students is that the field holds a tremendous promise¹.