Nano-fibrous microspheres and methods for making the same

1. A nano-fibrous microsphere, comprising:

a plurality of nano-fibers aggregated together in a spherical shape to form a shell of the microsphere, the nano-fibers formed of a star-shaped polymer including:

a multi-functional molecule having at least three branches; and

an oligomer or a polymer chain initiated from or coupled to the at least three branches of the multi-functional molecule;

a plurality of pores formed between at least some of the plurality of nano-fibers;

one hollow core formed in a center of the microsphere, the one hollow core being surrounded by the plurality of nano-fibers; and

a dispersing medium present in the one hollow core wherein the diameter of the microsphere ranges from about 10 μm to about 180 μm.

2. The microsphere as defined in claim 1 wherein each of the plurality of nano-fibers has a diameter ranging from about 50 nm to about 500 nm.

3. The microsphere as defined in claim 1 wherein the polymer chains are polymerized from or coupled to the at least three branches of the multi-functional molecule, and wherein the polymer chains are selected from the group consisting of poly(L-lactic acid), modified poly(L-lactic acid), polycaprolactone, polyamides, polyacrylonitrile, and gelatin.

4. The microsphere as defined in claim 1 wherein a molecular weight of the star-shaped polymer is equal to or greater than 10 kDa.

5. The microsphere as defined in claim 1 wherein the dispersing medium is glycerol.

6. A method for making the nano-fibrous microsphere as defined in claim 1 , comprising:

dissolving the star-shaped polymer in a solvent to form a solution;

adding a dispersing medium to the solution to form an emulsion;

thermally inducing phase separation of the emulsion to form nano-fibers;

extracting the dispersing medium; and

lyophilizing the nano-fibers to form the nano-fibrous microsphere.

7. The method as defined in claim 6 wherein the nano-fibrous microsphere is hollow, and wherein the method is performed in the absence of a template.

8. The method as defined in claim 6 wherein the solvent is tetrahydrofuran, and wherein the dispersing medium is glycerol.

9. The method as defined in claim 6 wherein thermally inducing phase separation is accomplished by quenching the emulsion in a sufficiently cold medium.

10. The method as defined in claim 6 wherein extracting the dispersing medium is accomplished via a solvent exchange with a non-solvent of the polymer.

11. The method as defined in claim 6 wherein the formation of the emulsion takes place in the absence of a surfactant.

12. The method as defined in claim 6 , further comprising:

adding a template to the solution;

wherein during the thermally induced phase separation, the nano-fibers form around the template such that the template acts as a mold for a hollow core; and

leaching the template, thereby exposing the hollow core.