3D printed microneedles for transdermal drug delivery: A brief review of two decades

[Display omitted] •MNs are demonstrated as promising approaches for transdermal drug delivery (TDD).•Various conventional and modern techniques are applied for MN fabrication.•3D printing is a promising strategy for MNs fabrication.•3D printed MNs are applied for transdermal delivery of various drugs.•Inkjet printing has been widely applied for the development of MN patches. Microneedle (MN) technology shows excellent potential in controlled drug delivery, which has got rising attention from investigators and clinics. MNs can pierce through the stratum corneum layer of the skin into the epidermis, evading interaction with nerve fibers. MN patches have been fabricated using various types of materials and application processes. Recently, three-dimensional (3D) printing gives the prototyping and manufacturing methods the flexibility to produce the MN patches in a one-step manner with high levels of shape complexity and duplicability. This review aims to go through the last successes in 3D printed MN-based patches. In this regard, after the evaluation of various types of MNs and fabrication techniques, we will study different 3D printing approaches applied for MN patch fabrication. We further highlight the state of the art of the long-acting MNs and related progress with a specific look at what should come within the scope of upcoming researches.