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From Stephen Dundon on January 22, 2017

Synthetic Heart Valves
Synthetic heart valves are devices used to replace damaged or diseased valves in the human heart. They are used as an alternative to biological valves, bovine or porcine valves in the case of a xenograft or harvested human heart valves in the case of an allograft. Synthetic heart valves are mechanical in nature but some research in tissue engineering is heading towards a more blurred distinction between mechanical and biological devices.

History
According to Richard DeWall, a cardio thoracic surgeon at the Cox Heart Institute, the first artificial heart valve was a ball and ring valve implanted and designed by Dr. Charles Hufnagel in 1952. This valve could only be placed in the descending aorta and not the heart itself so it only alleviated symptoms instead of replacing the heart valve outright.

The next major improvement in synthetic heart valves was the tilting disc design from the late 1960’s. The tilting disc was superior to the ball valve because it could replace any valve in the heart.

Designs for a bi-leaflet valve were worked on through the 1960's and 70's. The bi-leaflet valve further improved over the tilting disc by allowing blood to pass through the center of the valve causing less turbulence and improved fatigue properties.

Other designs have been experimented with throughout the years including the single leaflet, caged ball, and tri-leaflet valves as well as countless variations of these designs. However, the bi-leaflet valve is currently the most commonly implanted.

Indications for use
According to Drexel University's medical school synthetic heart valves are typically used when medication and corrective surgery have already failed. Synthetic heart valves are selected over biological (bovine or porcine) replacement valves when the patient is younger and able to tolerate anticoagulant use. However, children and young adults are not candidates for synthetic heart valves due to the valves inability to resize as the patient grows. This is partially due to the longer viability of synthetic heart valves. While biological valves often begin to fail in a little as ten year a synthetic heart valve will likely function for more than double that time. Avoiding the need for a second surgery makes synthetic heart valves the better option if the patient can tolerate the anti coagulants. Synthetic valves are rarely used in patients over the age of 65.

Materials
The outer ring of the modern synthetic valve is made from titanium to provide structural strength and because it is bio-compatible. Pyrolitic carbon is used for the inner ring, the valve leaflets, and any other parts that will be contacting blood. Pyrolitic carbon has the strength and resistance to wear and fatigue to withstand the repetitive loading from the heart while also having the necessary thromboresistant properties to allow it to contact blood. The suture ring where the valve is attached to the heart is often made from Dacron or Teflon because they are relatively inert and their porosity allows for the ingrowth of tissues after implantation.

Problems with use
The primary problem with implanting synthetic hear valves is the common problem for any implant that contacts blood, thrombogenisis. Any substance contacting blood besides the endothelial lining of blood vessels encourages the coagulation of blood and leads to blood clots which can cause death in the patient. While substances like pyrolitic carbon have proven to be less thrombogenic and anticoagulant drugs can lessen the problem there is currently no method of making a synthetic substance completely non-thrombogenic.

Current research
While there is some research in advancing valve designs and replacing larger portions of the heart with mechanical devices most research in synthetic valves revolves around improved materials. The discovery of pyrolitic carbon's thromboresistant property was revolutionary to the field of implants and so researches are now looking for a new material thatwould in fact be completely non-thrombogenic. Lakshmi Prasad Dasi, a professor of Biomedical Engineering and Surgery at the Dorothy Davis Heart and Lung Research Institute at Ohio State University, is leading a research team to develop a hyaluronan based plastic that would be non-thrombogenic and thus be able to be used in patients who could not tolerate the use of anticoagulants.