Sustained-release microneedles for drug delivery: advances and prospects

Sustained-release microneedles (SR-MNs) are designed to establish a prolonged drug supply process within the dermis or local tissues after a single short-term application. This process can last from hours to months. In this way, it combines minimally invasive administration, reduced dosing frequency, and maintenance of local or systemic drug exposure. Unlike the broader category of controlled-release microneedles, the systems discussed in this review mainly rely on the intrinsic properties of materials and structural design to achieve prolonged drug output. They are not predominantly governed by externally triggered release. Over the past 15 years, the field has gradually evolved along three major technical routes: slowly degrading matrix microneedles, particle-loaded microneedles, and drug-loaded hydrogel microneedles. In addition, separable or implantable tips have emerged as an important structural strategy for constructing intradermal drug depots. Current studies show that different material systems and design routes correspond to distinct release mechanisms and application windows. These systems have also been extended to chronic disease management, vaccines, contraception, and anti-HIV therapy. Overall, the further translation of SR-MNs will depend on whether effective drug loading, preservation of drug activity, long-term safety, and scalable manufacturing can be optimized in parallel.