The summer after my sophomore year in college I interned as a research assistant in the Stanford Hospital. I learned that mitral dilation is observed in a large majority of heart failure patients, but there was no good way at the time to study mitral dilation ex-vivo. The lab had already made an artificial heart that very accurately matched the conditions of the human heart, my task was to make a ring that could accurately model physiologic mitral dilation for use in the Left Heart Simulator they had created. I was published as second author for research performed using this ring in Medical Engineering & Physics ​in an article titled “A Novel 3D-Printed Preferential Posterior Mitral Annular Dilation Device Delineates Regurgitation Onset Threshold in an Ex Vivo Heart Simulator”
The most important design specification was that the ring dilate only the posterior leaflets of the valve with negligible change to the anterior leaflet. Other important specifications were: Strength of the blades, as they experienced significant stress throughout the cardiac cycle, a very compact size that fit into the simulator, and a seal between the ring and the simulator to ensure no leakage from one chamber to the next.
I worked on this project for several months. I began with dozens of ideas and sketches and went through many different prototypes until I had a product that finally worked. I used a Carbon M2 3D printer for the prototypes as well as the finished product. Most of the body of the device is made of Cyanate Ester; the base is made of Elastomeric Polyurethane to ensure a seal between the ring and the Left Heart Simulator.

Below and to the left can be seen the left heart simulator, this ring is placed in a chamber on the right. below and to the right is a video of the ring and mitral valve going through several cardiac cycles after the ring was dilated.
From left to right: 
one prototype involving a gear mechanism to dilate and contract the ring. 
schematic of left heart simulator
photo of ring connected to mitral valve and papillary muscles clearly visible
another early prototype made entirely out of PLA

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