Table 1.

Different smart materials that can be used in 4D printing.

S No	Smart material	Description	References
1	Shape memory alloys	•This material gets deformed when cooled and gets to its pre-deformed shape when heated •It is lightweight which is used efficiently to manufacture hermetic joints in metal tubing •Most common shape memory alloys are nickel-titanium and copper-aluminium alloys	Li et al., 201621; Yuan et al., 201822
2	Electroactive polymers	•This polymer changes their shape and size when stimulated by an electric field •It is used for sensor and actuators •Used for the development of artificial muscle and microfluidic devices	Ning et al., 201823; Palza et al., 201924
3	Shape memory polymers	•Shape memory polymers are used in 4D printing due to its low cost, low density, ease of use and its chemical modification ability •It has great potential to return to its original shape from the deforming stage with the change in temperature •The shape of the material can be changed by triggering off the magnetic or electric field	Díaz Lantada, 201725; Govindarajan and Shandas, 201726
4	Magnetic shape memory alloys	•These materials produce deformation and force with the response of magnetic field •These alloys include Manganese, Nickel and Gallium •These materials operate between −40 and 60 °C and can change its electric resistivity and magnetic permeability during deformation	Haldar and Lagoudas, 201427; Bruno et al., 201728
5	Smart inorganic polymers	•These materials are fully inorganic polymers having stimuli-responsive like rheology, conductivity, bioactivity, sensing etc •These materials have smart properties and useful for biosensing applications •Have diverse applications for tissue regeneration, tissue repair and drug delivery	Yan et al., 201629; Tomczykowa et al., 201930
6	Electro-responsive polymers	•This polymer can change its volume and shape with the response to various electrical stimuli •Materials are useful to convert the electrical energy into mechanical work •These materials are used to manufacture soft artificial muscle using 4D printing technology	Rahimi et al., 201431; Verbrugghe et al., 201532
7	Temperature responsive polymers	•These polymers show a discontinuous change in their physical properties concerning temperature •These are stimuli-responsive materials which can change their properties with environmental conditions •Major applications are drug delivery, liquid chromatography and bioseparation	Karimi et al., 201633; Nagase et al., 201934
8	Photoresponsive polymer	•It undergoes the physical and chemical transformation which induces changes like polarity, conformation, hydrophilicity and optical properties •It is unique stimuli-responsive materials which can change its properties using light irradiation •Show good processability during printing and suitable for the biomedical research	Marturano et al., 201635; Yang et al., 201736
9	Smart hydrogels	•These are stimuli-responsive hydrogel having the ability of external and internal stimuli changes •This hydrogel material changes the shape with exposure to temperature changes •4D printing uses smart hydrogel to create the living structure of human tissue and organs	Du et al., 201537; Ribeiro et al., 201838