Biodegradable polymeric microneedles were developed as a method for achieving sustained transdermal drug release. These microneedles have potential as a patient-friendly substitute for conventional sustained release methods. However, they have limitations related to the difficulty of achieving separation of the needles into the skin. We demonstrated that microneedle separation into the skin was mediated by hydrogel swelling in response to contact with body fluid after the needles were inserted into the skin. The hydrogel microparticles were synthesized by an emulsification method using poly-N-isopropylacrylamide (PNIPAAm). The microneedles were fabricated by micromolding poly-lactic-co-glycolic acid (PLGA) after filling the cavities of the mold with the hydrogel microparticles. The failure of microneedle tips caused by hydrogel swelling was studied in regard to contact with water, insertion of microneedles into porcine cadaver skin in vitro, stress-strain behavior, and insertion into the back skin of a hairless mouse in vivo. The drug delivery property of the hydrogel particles was investigated qualitatively by inserting polymer microneedles into porcine cadaver skin in vitro, and the sustained release property of PLGA microneedles containing hydrogel microparticles was studied quantitatively using the Franz cell model. The hydrogel particles absorbed water quickly, resulting in the cracking of the microneedles due to the difference in volume expansion between the needle matrix polymer and the hydrogel particles. The swollen particles caused the microneedles to totally breakdown, leaving the microneedle tips in the porcine cadaver skin in vitro and in the hairless mouse skin in vivo. Model drugs encapsulated in biodegradable polymer microneedles and hydrogel microparticles were successfully delivered by releasing microneedles into the skin.