The overall goal of this manufacturing process is to develop a small caliber stent-graft by encapsulating a balloon expandable stent within two layers of electrospun polyurethane nanofibers. This research is helping us develop small caliber stent-grafts that actually heal very quickly, and can potentially open up new medical applications that have not been done before. Advantages of this technique are the stent-grafts are made from polymer nanofibers that mimic extracellular matrix.
The fabrication process is also straightforward, and provides favorable mechanical and cell adhesion properties. To begin first melt about 8 milliliters of bio-compatible food grade water soluble support material in a graduated cylinder at 155 degrees Celsius. Also, warm the 100 millimeter long, 3 millimeter wide, stainless steel mandrel to 155 degrees Celsius.
Next turn off the oven and dip the mandrel into the support material. Then hang the mandrel vertically in the oven and let the material solidify, which should occur around 140 degrees Celsius. Excess material should drip off the end.
Next set up the mandrel collector of the electrospinning system. This requires a laboratory mixer with an attached plastic rod that ends in a grip for the mandrel. Dissolve the support material off the tip of the mandrel so it can attach to the grip.
Next use rubber O rings on both sides of the mandrel to protect the electrospun material from the ground-wire, and electrically ground the mandrel with the U-shaped ground-wire. Next attach the exposed mandrel to a short grip at the far end for support. At the other end, attach the mandrel to the rotating rod from the laboratory mixer.
Secure both supports to the mandrel using screws, and electrically ground the mandrel. Now set up the liquid polyurethane extrusion system. First, load a five milliliter glass syringe that fits the syringe pump, with freshly made 15%polyurethane solution, and attach a blunt end stainless steel needle, which will function as a spinneret.
Taking into account the inner diameter of the syringe, program the pump to extrude the solution at zero point zero one milliliters per minute. To prevent electrical arcing insulate the syringe from the conductive parts of the syringe pumps using rubber sheets. Then position the syringe in the pump so the tip of the spinneret is 20 centimeters from the mandrel collector.
Next, connect the high voltage generator to the spinneret using an alligator clip. Before starting the electrospinning, turn off the fume hood exhaust. For operation, start the syringe pump and rotate the mandrel at about 50 RPM.
Then apply a voltage differential of 20 kilovolts across the spinneret and the collector mandrel. Polyurethane nanofibers will start depositing on the rotating mandrel and a thin layer will be visible within several minutes. Let the polyurethane nanofibers build up for two hours to make a uniform tube.
Once the nanofibers have been spun, turn the hood exhaust back on, and proceed to remove the mandrel from the plastic supports. Then carefully slide the balloon expandable stent onto the electrospun tube without damaging the tube. If necessary slightly expand the stent so that it slips on more easily.
Next crimp the stent at the desired location so it is tight to the tube and will not slide around. This will help prevent delamination of the inner and outer layers. Return the mandrel to the set up, and continue the electrospinning as before for three hours to fabricate an outer layer onto the stent-graft.
Next while still on the mandrel, circumferentially cut the excess polyurethane approximately one millimeter from each end of the stent using a scalpel. Next soak the mandrel and stent-graft in de-ionized water to dissolve the support material. Change the water as needed until the support material is completely dissolved.
Then gently remove the stent-graft from the mandrel and allow it to air dry. To test the stent-graft slide it onto a three millimeter trifold balloon, and crimp it onto the balloon using a handheld crimping tool. Under a microscope, inspect the crimped stent-graft for uniform crimping and any other signs of failure such as delamination or a puncture of the cover material which could occur with a deformed stent.
If it looks good, then expand the stent-graft to three millimeters by pressuring the trifold balloon with an inflation device and water. Once expanded use the microscope to inspect the stent-graft once again. Look for uniform expansion and any signs of failure.
Using the electrospinner, high quality polyurethane nanofibers can be produced. Electrospinning polyurethane nanofibers using this setup generates a uniform layer of polyurethane at 50 microns of nanofibers per hour. Crimping and expansion of the resulting small caliber stent-graft show that these devices are capable of being deployed using a standard trifold balloon without uneven expansion or signs of material failure.
Following this procedure other methods like animal model implantation can be performed to investigate the functionality, durability and performance characteristics of the device. While attempting this procedure, it's important to remember to work in a chemical fume-hood with the exhaust turned off during electrospinning and turned back on prior to removing the sample.
