the present invention provides an apparatus and methods of producing particles that include a polymer , a wax and / or lipid and , optionally , a biologically active substance . in accordance with the methods of the invention , a load stock including a polymer , a wax and / or a lipid that is a solid at standard temperature and pressure and , optionally , a biologically active substance is provided . the load stock is contacted with a supercritical fluid to form a melt . the melt is contacted with a polar solvent under suitable conditions to form an emulsion . the emulsion is expanded across a pressure drop to form solid particles that include the load stock . the methods and apparatus facilitate the production of very small particles that have a narrow particle size distribution .