this invention discloses a novel microneedle system , phase - transition microneedles , of which the microneedles formed of hydrophilic polymers are swelling but insoluble when absorbing water due to their internal cross - linked network through microcrystalline domains functioning as cross - linking junctions . the microneedles are sufficiently hard to penetrate the epidermis of the skin at dry state , but are converted to hydrogel state to release their loaded cargos by absorbing the body fluid in the dermis layer , and able to be withdrawn from the skin completely because of their insoluble network . moreover , formation of the insoluble polymeric network through microcrystalline domains of ptm is achieved by a mild freeze - thaw treatment , for which bio - active agents may be loaded safely in the microneedle tips by adding in the polymer solution prior to molding without denaturing .