Adhesive microneedles. (a) Optical images of arrow-shaped microneedles. Both scale bars are 250 μ
m. Reproduced with permission from Ref. [33
], copyright 2020, Elsevier B.V. (b) Serrated clamping microneedles: (i) schematic illustration of the clamping microneedles; (ii) digital photo of microneedles that bears 4 small magnet blocks adhering to porcine skin. Reproduced with permission from Ref. [19
], copyright 2019, Science China Press. (c) Optical image of the barbed microneedle array and the adhesion force tests. Reproduced with permission from Ref. [31
], copyright 2020, Wiley-VCH. (d) Schematic illustrations (i) and optical images (ii) of mechanical interlocking of swellable microneedles after penetration. The scale bar is 500 μ
m. Reproduced with permission from Ref. [35
], copyright 2013, Springer Nature. (e) Schematic illustrations of microneedles with polydopamine hydrogel supporting layer and suction-cup-like microstructures. Reproduced with permission from Ref. [36
], copyright 2020, American Association for the Advancement of Science.