Table 2.

Summary of emerging 4D printed hydrogels for drug delivery applications.

Materials	Printing Parameters	Crosslinking	Printed Shape Transformation	Drug & Loading Method	External Stimulus	Drug Release Profile	Ref.
Pluronic diacrylate macromer, and alginate	DIW: Nozzle diameter—400 µm; nozzle temperature—25 °C; print speed—15 mm/s; pressure—0.04 kPa; bed temperature—60 °C	UV curing	Square mesh to folded mesh	Methotrexate—Co-mixing	Ion (Calcium chloride)	Fast release up to 6 h followed by steady release up to 12 h	[148]
Gelatin methacryloyl	DIW: Nozzle diameter—210 µm; nozzle temperature—26 °C; print speed—20 mm/s	UV curing	Sheet to tubular	Heparin—Co-mixing	Solvent (Water)	Fast release up to 8 h followed by steady release up to 28 h	[149]
Poly(N-isopropylacrylamide)	DIW: Nozzle diameter—340 µm	UV curing	Expanding core–shell capsules	Brilliant blue and lemon yellow—Injection	Temperature (22 °C)	Fast release up to 15 h followed by medium release up to 48 h	[150]
Poly(vinyl alcohol)	FDM: Nozzle diameter—400 µm; nozzle temperature—180 °C; print speed—23 mm/s	-	I to U, U to I, and helix to extended conformation	Caffeine—Co-mixing	Solvent (Water)	Fast release up to 2 h followed by steady release up to 6 h	[151]
Methacrylamide Chitosan	2PP: Sub-micron resolution	-	Expandable microswimmers	Doxorubicin—Immersion	Light (UV 365 nm)	Fast release up to 1 min (light ON) followed by steady release up to 6 min (light OFF)	[152]
Gelatin methacryloyl	2PP: Sub-micron resolution	-	Expandable microswimmers	Fluorescein isothiocyanate—Immersion	Enzyme (Metallo-proteinase 2)	Fast release up to 2 h followed by medium release up to 48 h	[153]
Poly(ethylene glycol) diacrylate	µSLA: Layer thicknesss—50 µm	UV curing	Straight to backward-facing curved barbs in microneedle array	Rhodamine B—Immersion	Ion (Phosphate buffered saline)	Fast release in 1 min followed by medium release up to 2.5 h, and slow releases up to 3 h	[154]
4-hydroxybutyl acrylate, and urethanepolyethylene glycol-polypropylene glycol	DLP: Layer thicknesss—100 µm	UV curing	Stretchable nerve guide tubing	Doxorubicin, and ovalbumin—Immersion	Magnetic field (1 MHz)	Fast release in 4 h (doxorubicin) or 30 min (ovalbumin) followed by medium release up to 50 h, and steady releases up to 75 h	[155]