Fig. 4: Thermoresponsive microneedles.

a Fabrication process for the thermoresponsive microneedles. DEX dexamethasone sodium phosphate, MN microneedles, HA hyaluronic acid, PCM phase-change material. Triadecanoic acid is selected as the PCM, exhibiting a phase transition temperature of ~ 42 °C. b Microscope image of the microneedles. Scale bar: 200 μm. SEM images of a single microneedle coated without (c) and with (d) tridecanoic acid. Scale bar: 100 μm. e EDS mapping images of the microneedles. f Optical image of the microneedles. g Force-displacement graph for evaluating the compressive strength of the microneedles. h Optical images of microneedles penetrating agarose gel. i H&E staining of tissue showing micro-sized holes (Inset: Picture of mouse back skin pierced by microneedles). Scale bar: 100 μm. j Melting process of tridecanoic acid layer coated on microneedles. k Thermal images showing temperature changes during heating