Microneedle (MN) technology is a minimally invasive technique using microscopic needles for the delivery of drugs through the stratum corneum (SC). Microneedles range in length from a few micrometers to as high as 2000μm. MNs are small in length to avoid nerve endings while touching the deeper skin layers and crossing the stratum corneum. MNs are preferred over invasive syringes due to their ease of manufacturing and their ability to provide painless drug delivery via the skin efficiently in comparison to other transdermal technologies. Likewise, as opposed to other traditional techniques, MNs do not require special skills and are very simple in design so that patients can self-administer. In addition, they are a single-use device, decreasing the risk of cross-contamination.

MNs can be divided on the basis of their differences in design: hollow, solid, coated, hydrogel-forming, and dissolving MNs. Solid MNs are known as the "stick and patch" approach; initially, micro holes are created and then the drug formulation is administered via a transdermal patch. The method followed by coated MNs is the "coat and patch" approach. In this case, before applying the MN to the skin, the drug formulation is coated on the microneedles. After penetration, the MN coating dissolves and the drug is absorbed into the skin. With hollow MNs, the drug is introduced into the hollow tip of the MN, which is then deposited directly into the upper dermis or epidermis of the skin upon insertion. This is known as the "poke and flow" technique. Dissolving MNs consist of biodegradable or dissolving polymers and act by a one-step application. Hydrogel-forming microneedles absorb large amounts of extra cellular fluid due to their polymeric nature that results in swelling and the formation of in-situ hydrogels. The main mechanism of drug delivery in hydrogel-forming and dissolving MNs is poke and release. In both types, there is no need for special measures to dispose of the needle, reducing the chances of MN reuse. Microneedle technology could be a promising technique to deliver drugs through skin ports with higher efficacy and fewer side effects.

The purpose of this Special Issue is to present advancements made in the field of microneedle-based technologies and strategies to deliver different therapeutics molecules across the stratum corneum. This Special Issue will be a valuable collection of articles related to microneedles-based drug delivery systems, with special emphasis on advancements in formulation techniques as well as the synthetization of novel materials. Secondly, this will also provide an insight for the international scientific community regarding transdermal delivery and will provide a platform for discussion of the importance of transdermal routes for the future delivery of macromolecules through skin.

Potential topics include but are not limited to the following:

• Transdermal drug delivery
• Targeted drug delivery
• Transdermal patches
• Microneedles
• Microneedle patches
• Iontophoretic drug delivery
• Drug permeability
• Drug permeation enhancers
• Skin as a safe route for drug delivery
• Lipophilicity and hydrophilicity