Table 1.

Different types of MNAs along with their advantages and disadvantages.

Type of MNAs	Properties	Advantageous	Disadvantageous	Ref (s)
Solid/coated	Contain no drugs, or drug can be absorbed on its surface, synthesized by metals	Increase the permeability of drugs	Needs two step application, broken needles result in irritation, the high cost of fabrication method, non-biodegradability of metals	[43]
Hollow	Can be loaded with drugs, fabricated by self-assembly and molding of soft materials	Higher stiffness, hydrophilic behavior and resistance can be achieved by using proper materials	Needs precise and expensive manufacturing technology, strong leakage or uncontrolled drug release, blocking the narrow channels	[44]
Porous	Large variety of pore sizes can be achieved for drug loading, porosity and pore size can be controlled during synthesis	High drug loading capability, functionalization with different moieties, simple fabrication methods	Low strength and penetration ability, pore blockage, drug release	[45]
Hydrogel/ swellable	Minimally-invasive devices used for controlled drug release	Significant biocompatibility, degradability, safe, simple, cheap, controlled drug release	Low mechanical strength, not suitable for very wet wounds	[46]
Merged-tip MN	Uses elasto-capillarity-driven self-assembly for construction a micro merged-tip system	Can be tunable, simultaneous drug loading and volume control, no need for micromolding, controllable cavity volumes and fracture approaches, flexibility and fabrication simplicity	–	[47]