Table 1.

The summary of the years, advantages, disadvantages, and preparation methods of different types of MNs

Types of MNs	Years	Advantages	Disadvantages	Preparation methods	Refs
Solid MNs	1970s put forward 1990s rise	Excellent mechanical performance Strong puncture ability	Low security High toxicity Complex preparation process Serious matrix residue	Two-step etching method Free radical polymerization method Laser technology	[26, 36, 68–70]
Hollow MNs	1990s	High drug loading High drug delivery efficiency	Poor mechanical performance Complex preparation process	Solvent-casting method Microelectromechanical technology Laser technology	[38, 39, 41, 70, 71]
Dissolving MNs	At the beginning of the twenty-first century	Good biodegradability High security Low toxicity Wide range of use	Low drug loading	Solvent-casting method Three-step centrifugation method Droplet-born air-blowing method Two-layer centrifugation method	[72–79]
Hydrogel MNs	At the beginning of the twenty-first century	ISF extraction test Excellent swelling performance Adjustable mechanical strength Adjustable drug release rate	Narrow scope of application Complex matrix composition Insoluble matrix materials	Freeze–thaw cycle method Solvent-casting method	[51, 79–81]
Coated MNs	At the beginning of the twenty-first century	High drug delivery efficiency Excellent mechanical performance	Low drug loading Complex preparation process Low security	Wet etch process Dip coating method	[22, 59, 82–84]