This invention discloses a novel microneedle system, phase-transition microneedles (PTM), 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to hydrogel state to release their loaded cargos by absorbing the body fluid in the dermis layer, and able to be withdrawn from the skin completely (without depositing needle tip materials in the skin) because of their insoluble network.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.