Figure 2.

A schematic diagram of microneedle (MN)-based drug delivery approaches with the cross section of the upper layer of the skin. The approaches are (a) solid MNs, (b) coated MNs, (c) hollow MNs, (d) dissolving MNs, and (e) hydrogel-forming MNs. The step-by-step process of each delivery approach is numbered from 1 to 3. Representative microscopic images of MN types and examples of deliverable payloads such as drugs and bio-macromolecules are also shown. Images was adopted with permission from [42,43]).