Patent ID: 8545742

Claim:
A method of forming a stent comprising: heating and mixing a polymer comprising poly(L-lactide) (PLLA) in an extruder to form a polymer melt; forming a tube from the polymer melt, the forming of the tube comprising conveying the polymer melt through the extruder into a die assembly to form a film, and after the film exits the die assembly, cooling and drawing the film to form the polymer tube, wherein the tube is cooled to below a glass transition temperature (Tg) of the polymer; wherein the die assembly comprises an inlet for the polymer melt and a conical section that fits into a structure having a conical bore, wherein the polymer melt flows through a spiral channel downstream of the inlet which is formed by spiral grooves in the conical section and a surface of the conical bore, the spiral channel cross-section gradually tapering as the polymer melt moves downstream, wherein the spiral channel merges with an annular passage way between surfaces of the conical section and the conical bore to allow the polymer melt to flow through the spiral channel downstream into the annular passage way, wherein a clearance between the surfaces of the conical section and the conical bore increases as the spiral channel tapers and merges with the annular passage way and when the tapering spiral channel merges with the annular passage way, some of the polymer melt spills over an edge of the spiral grooves into the annular passage way which changes the direction of polymer melt flow, wherein the spiral channel and annular passage way provide for a balanced flow through the die assembly, wherein the polymer melt flows from the annular passage way to an outlet of the die assembly, and wherein the crystallinity of the polymer in the formed tube after cooling is not more than 5%; wherein spiral flow provided by the spiral channel and the balanced flow allow for the crystallinity of not more than 5%, processing the polymer tube at a temperature below the Tm of the polymer to increase the radial strength of the polymer tube and increase the crystallinity of the polymer tube from not more than 5% to a crystallinity in the range of about 20% to about 50%, wherein the processing is at a temperature between 60 and 100° C. to provide crystallinity with small crystallite domains which contribute to both the strength and fracture toughness of the polymer of the polymer tube; and forming a stent from the processed polymer tube, wherein the crystallinity of the polymer tube prior to processing of not more than 5% allows for the increase in crystallinity of the tube to be formed in one step under processing conditions that enhance facture toughness.