This invention discloses a novel microneedle system, phase-transition microneedle patch, which overcomes all the limitations that existing microneedles encountered. The microneedle patch is formed of an integrated polymeric piece consisting of a microneedle array and a plate (called holding plate) on which the needles stand. The microneedles of the patch are hard and strong enough to penetrate epidermis at dry state but turn to be hydrogel state soft and permeable to hydrophilic agents when absorbing body fluid. The hydrogel state of the patch is a hydrophilic network held by physical or chemical cross-linking junctions. The pores of the network are opened up by body fluid for drugs and macromolecules to diffuse through. The polymeric materials used to form the microneedle patch have been used in the pharmaceutical field for years and have proven compatibility with the skin and with proteins. The drugs may be stored in the matrix of the microneedle array as well as the holding plate so that the requirement for high dose applications may be full filled. In addition, molding (casting) of this type of microneedle patch is simple, easy to achieve and needs no microfabrication systems and organic solvents. By a programmed molding (casting), the patch may be assembled in a layered structure with desired drug concentration in each layer, respectively. Due to this design, a programmed pulse or a zero order release of drugs may easily be achieved. In addition, delicate proteins loaded in the patch are kept in a dry and hydrophilic glassy state before being released, the most favored state for protein storage. Finally, during the swelling-based drug release, the microneedle patch increases their thickness gradually between the skin and the back cover (which holds the needles) lo create a sustained pressure to ensure good contact of the microneedles inside epidermis.
