Long-acting microneedles: a progress report of the state-of-the-art techniques

Microneedles (MNs) are a self-administrable and painless alternative to hypodermic needles for bolus drug delivery. Sustained drug release is the preferable drug delivery method because it reduces the chemical burden in patients and can deliver therapeutics that require long-term exposure. Here, we review recent advances in long-acting MNs, summarizing polymers used to fabricate long-acting MNs and mechanisms controlling drug release. We also introduce novel designs of separable needles to achieve implantable tips and bioinspired needles for enhanced skin adhesion, as well as strategies using the back layer as a drug reservoir.