nonsolvent for heparin such as methanol ethanol 1 propanol isopropanol t butanol acetone and the like 3 an aqueous solution of from 10 percent to 30 percent by weight heparin salt is continuously mixed into a stream of nonsolvent liquid and the microfibers are collected on a moving screen while the nonsolvent is recirculated and excess water is removed from the solvent stream the concentration of nonsolvent is maintained at 95 percent or higher see fig3 when a batch process is used with a liquid nonsolvent 2 above the concentration of the nonsolvent must be retained at greater than 90 percent by volume at all times during the process the remainder of less than 10 percent being the aqueous solution of the heparin salt and preferably the concentration of the liquid nonsolvent is retained at 95 percent or even better 99 percent or more by volume thus even at 90 percent of nonsolvent liquid the resulting microfibers are short and thin the concentration of the liquid nonsolvent is maintained at 95 percent by volume or greater in the continuous process in both cases the microfibers increase in length and hence in length to diameter ratio as the concentration of the water decreases the preferred liquid nonsolvent is isopropanol since it produces the best microfibers the microfibers obtained from the foregoing processes contain some water and where a liquid nonsolvent has been used normally some of it as well the combined nonsolvent.