STENT FABRICATION VIA TUBULAR CASTING PROCESSES
Tubular casting processes, such as dip-coating, may be used to form substrates from polymeric solutions which may be used to fabricate implantable devices such as stents. The polymeric substrates may have multiple layers which retain the inherent properties of their starting materials and which are sufficiently ductile to prevent brittle fracture. Parameters such as the number of times the mandrel is immersed, the duration of time of each immersion within the solution, as well as the delay time between each immersion or the drying or curing time between dips and withdrawal rates of the mandrel from the solution may each be controlled to result in the desired mechanical characteristics. Additional post-processing may also be utilized to further increase strength of the substrate or to alter its shape.
1 . A method of forming a polymeric substrate, comprising:
immersing a mandrel into at least a first polymeric solution such that at least a first layer of a biocompatible polymer substrate is formed upon the mandrel and has a first diameter defined by the mandrel;
curing the substrate;
subjecting the substrate to a first elevated temperature at or above a glass transition temperature of the substrate;
cooling the substrate in a controlled manner to a second temperature lower than the glass transition temperature such that the substrate transitions to a glass state and imparts a shape memory effect;
forming an expandable stent scaffold having the first diameter; and,
reducing the first diameter of the stent to a second smaller diameter, wherein the stent retains one or more mechanical properties of the polymer resin such that the stent exhibits ductility upon application of a load.
2 . The method of claim 1 wherein the first polymeric solution has an inherent viscosity of about 4.3 to about 8.4 dl/g.
3 . The method of claim 1 wherein prior to forming an expandable stent scaffold, further comprising:
controlling a number of immersions of the mandrel into the first polymeric solution;
controlling a duration of time of each immersion of the mandrel; and
controlling a delay time between each immersion of the mandrel.
4 . The method of claim 3 further comprising controlling a withdrawal rate of the mandrel from the first polymeric solution after each immersion.
5 . The method of claim 1 wherein the first polymeric solution comprises a polymer having a relatively high molecular weight.
6 . The method of claim 1 wherein the first polymeric solution is selected from the group consisting of polyethylene, polycarbonates, polyamides, polyesteramides, polyetheretherketone, polyacetals, polyketals, polyurethane, polyolefin, polyethylene terephthalate, polylactide, poly-L-lactide, poly-glycolide, poly(lactide-co-glycolide), polycaprolactone, caprolactones, polydioxanones, polyanhydrides, polyorthocarbonates, polyphosphazenes, chitin, chitosan, poly(amino acids), polyorthoesters, oligomers, homopolymers, methyl cerylate, methyl methacrylate, acrylic acid, methacrylic acid, acrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, glyceryl acrylate, glyceryl methacrylate, methacrylamide, ethacrylamide, styrene, vinyl chloride, binaly pyrrolidone, polyvinyl alcohol, polycoprolactam, polylauryl lactam, polyhexamethylene adipamide, polyhexamethylene dodecanediamide, trimethylene carbonate, poly(β-hydroxybutyrate), poly(g-ethyl glutamate), poly(DTH iminocarbonate), poly(bisphenol A iminocarbonate), polycyanoacrylate, polyphosphazene, methyl cerylate, methyl methacrylate, acryli acid, methacrylic acid, acrylamide, hydroxyethy acrylate, hydroxyethyl methacrylate, glyceryl scrylate, glyceryl methacrylate, methacrylamide, ethacrylamide, and copolymers, terpolymers and combinations and mixtures thereof.
7 . The method of claim 1 wherein immersing a mandrel comprises further immersing the mandrel into a second polymeric solution such that a second layer of polymer is formed upon the first layer.
8 . The method of claim 7 wherein the second polymeric solution comprises a drug or agent selected from the group consisting of antipoliferative, antineoplastic, antigenic, anti-inflammatory, antirestenotic, antilipid, antimitotics, metalloproteinase inhabitors, and anti-sclerosing agents.
9 . The method of claim 7 wherein the second polymeric solution comprises a BaSO 4 solution.
10 . The method of claim 1 further comprising applying a force within a lumen defined through the polymeric substrate such that the substrate expands from a first diameter to a second larger diameter.
11 . The method of claim 1 wherein the substrate has a length of 1 cm to 40 cm.
12 . The method of claim 1 further comprising controlling a crystallinity percentage of the substrate.
13 . The method of claim 1 wherein subjecting the substrate to a first elevated temperature comprises subjecting the substrate to the first elevated temperature above the glass transition temperature of the substrate while reducing the first diameter.
14 . The method of claim 13 wherein reducing comprises reducing the first diameter ranging from 3 mm to 9 mm to the second diameter ranging from 1.5 mm to 5 mm.
15 . The method of claim 1 wherein the expandable stent scaffold exhibits a 20% radial deformation when placed under a 0.1 N to 20 N load.
16 . The method of claim 1 wherein the expandable stent scaffold exhibits a percent reduction in diameter of between 5% to 70% without fracture formation when placed under a compressive load.
17 . The method of claim 1 wherein forming an expandable stent scaffold comprises processing the stent from the substrate.
18 . The method of claim 17 further comprising expanding a diameter of the stent via an expandable balloon by 5% to 70% without fracture formation.
19 . The method of claim 17 wherein the stent is adapted to curve up to 180° about a 1 cm curvature radius without fracture formation.
20 . The method of claim 17 wherein the stent is adapted to exhibit a percent reduction in axial length of between 10% to 30% without fracture formation when placed under an axial load.