Table 2. Comparison of vat photopolymerization and extrusion printing techniques for 3D printing hydrogel actuators.

	Technique	Ink requirements	Resolution	Advantages	Disadvantages
Vat photopolymerization		• Photocurable resin124	• Lateral: 5–50 µm, dependent on laser spot size120,125	• High resolution – enables smaller feature sizes	• Anisotropy may occur within layers due to curing gradient120
		• Viscosity: <4 Pa s120	• Vertical: dependent on light penetration depth125	• Smooth surface finish	• Difficult to print multiple inks120,124
			• Fabrication rate: ∼105 mm3 h−1 128		• Additives may increase viscosity
					• Large amounts of precursor solution needed (∼500 mL) and most remains unused129
		• Photocurable resin124	• Lateral: 5–50 µm, dependent on projector pixel size120,125,130	• Faster printing speed than SLA120,125	• Lower resolution compared to SLA
		• Viscosity: <4 Pa s120	• Vertical: dependent on light penetration depth125	• Typically less affected by oxygen inhibition than SLA125	• Difficult to print multiple inks120,124
			• Fabrication Rate: ∼106 mm3 h−1 128		• Additives may increase viscosity
					• Large amounts of precursor solution needed (∼500 mL) and most remains unused129
Extrusion 3D printing		• Viscoelastic	• Lateral: 1 – 150 µm, dependent on nozzle diameter and printing speed93,120,124	• Material versatility124	• Slower printing speeds and lower resolution compared to SLA and DLP
		• Shear thinning122	• Vertical: dependent on ink yield stress and capillary forces131	• Easy to co-print multiple inks	• Bubbles trapped between layers122
		• Viscosity: 0.1–1000 Pa s96	• Fabrication Rate: ∼103–104 mm3 h−1 132	• Anisotropic alignment of fillers	• Difficult to print overhangs122
		• Storage modulus (G′): 104–105 Pa122	• Time ∝ (print speed) × (nozzle diameter)2	• Requires less ink than SLA and DLP129	• Unpolymerized structures may collapse under the weight of subsequent layers124
		• Yield point (G′ = G″): 500–2500 Pa122			• Nozzle clogging due to viscous inks or fillers133
		• Shape fidelity: dependent on tan δ (G′ > 1)93