Using a 3D printer, a shape memory polymer filament is extruded to form a branched tubular structure. The structure is patterned and shaped such that it can contract into a compact form once folded and then return to its formed shape when heated.
Shape-memory polymers can be programmed to change shape when certain conditions are met. 3D printers can produce these shapes by stacking layers after layers, allowing us to make any shape imaginable. In this video, we demonstrate how we can produce stents that can be shrunk to a compact form when being delivered, and recover to the most complex shape at the right moment.
By the use of this method, it is possible to design custom-tailored stents for each patient, because anything that can be designed can be exactly manufactured using the 3D printer. Set the diameter of the proximal main vessel to 25 millimeters. Then, set the diameters of the distal main vessel and the side branch equal to 22 millimeters.
Set the total length of the vessels equal to 140 millimeters. Next, set the length of the proximal main vessel to 65 millimeters, the distal main vessel to 75 millimeters, and the side branch to 65 millimeters. The computer model of the branched vessel is then printed by using a fused deposition modeling 3D printer and a polycarbonate filament.
Create a box shaped container that will house the 3D printed part. Set the container dimensions to 110 by 105 by 70 millimeters and use an acrylic plate. With a 3D printed branched vessel placed at the center of the box, gently pour the silicone inside the container to minimize bubble formation.
Dry the liquid silicone and harden it for 36 to 48 hours. Once hardened, remove the solidified silicone from the container, and cut it in half to remove the 3D printed part. Rejoin the divided silicone at the cut plane.
The resulting joined body is the blood vessel mock-up. Design the trunk of the bifurcated stent following wavy patterns similar to conventional stents, and design the bifurcated branches to be a cylinder. For the trunk, set the diameter to 22 millimeters and the length to 38 millimeters.
For the branch, set the diameter to 18 millimeters and the length to 34 millimeters. Finally, set the total length of the stent to 72 millimeters. Print the bifurcated stent in a fused deposition modeling 3D printer using a shape memory polymer filament.
The major composition of this filament is polyurethane. Use slicing software for model slicing and to control the settings of the 3D printer. Set the extruder temperature to 230 degrees Celsius and the temperature of the printer bed to room temperature.
Also, set the layer height to 0.1 millimeters to minimize the staircase effect. Then, set the printing speed to 3, 600 millimeters per minute and set the amount of interior fill percentage to 80 percent. Include the supporter formation during printing, which is needed because the interior of the structure is hollow.
Smooth out the surface as rough surfaces can damage the vessels by abrasion. Remove the supporters using cutters. The supporters are attached at the interior of the stent.
When removing the stents, exercise extreme caution to avoid tearing the stents. Rub the surface against sandpaper to remove the layer lines, striations, or blemishes on the printed surface. Repeated polishing may be needed where the supporters are removed by the cutters.
Prior to painting, clean, sand, and dry the stent surface. Paint the surface using a spray in a well-ventilated location and wear a personal mask. Protect from overspraying by applying thin layers of repeated paints.
Use black paints to enhance the contrast between the silicone vessel mockup and the stent. Place the bifurcated stents in warm water such that the temperature is above the glass transition temperature. When the stent becomes softened, push one half of the branch against the other half.
Nest one half within the other half. Perform the same nesting process to the other branch. Subsequently, the two halves of the cylinders are closed into one.
Now, fold the two branches into a single cylinder so that it can travel through the main vessel. Immerse the silicone vessel mockup inside the tank filled with warm water. Orient the mockup such that the main vessel is above, and the branches are below.
Now, insert the folded, bifurcated stent into the opening of the silicone vessel mockup. Orient the folded, bifurcated stent such that its branches are towards the opening. The folded, bifurcated stent will start to expand, and the lower branches will divide such that each branch will slide towards its mating pathway from the bifurcation core of the Y-shaped vessels.
The stent uses kiragami structure to allow the bifurcated stent to fold into a compact cylindrical tube, which is very suitable for sliding through the narrow pathways of blood vessels. The shape memory polymer allows the folded structure to return to its original shape when the temperature reaches the glass transition temperature. The original shape closely matches the branched vessels.
Make sure to set the temperature of the printer bed at room temperature. We experienced unwanted deformation of the structure when the printer bed's temperature was set higher. The polyurethane can be replaced by a polymer that has shape memory effect.
However, optimal temperature or memorising is different depending on the material used. The procedure produces bifurcated stents that can be inserted into a Y-shaped blood vessel with a single operation. Compared to multiple operations we use for conventional stents.
Our method can be used to treat coronary artery disease and blood vessel blocked by plaques. The procedure can also open passageway such as bile duct, bronchi, and ureters.
