Micro- and nano-sized particles, agglomerates and fibers are generated from high molecular weight water-soluble biopolymers applying supercritical fluid technology. A method of producing micro- or nanoparticles from an aqueous solution of a high molecular weight biopolymer includes the step of spraying the aqueous solution together with a mixture of a compressible gas and a water-soluble co-solvent/antisolvent into a pressurized chamber. The method may be adapted to impregnate the micro- or nanoparticles with a bioactive material. A method for microencapsulating a bioactive material with a biopolymer is also provided.