# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 3 Extrusion Based 4D Printing
### Direct Ink Writing
#### 3.2.2 DIW of Liquid Pre-Polymers
The application of 20 V and 150 mA induces a change from the programmed shape to the printed shape after 180 s. Thermal images were taken with an IR camera [119] (CC BY license). The same group reported a multi-material 3D printing approach involving the use of dynamic photomask-assisted DIW together with a two-stage curing hybrid ink [29]. In particular, a nanocomposite ink containing a photocurable acrylate prepolymer, a thermally curable epoxy resin and fumed silica were utilized. Dynamic photomasks were generated by a nearby projector during the printing process to enable different degrees of cure in different locations, which was further modulated by irradiation time. This enabled the generation of local variations in the extent of cure in each layer being printed, resulting in a graded structure (Figure 11c-i). The capability to print materials with dramatically different properties was then utilized to give structures with controllable buckling or fracture behavior or sequential shape memory characteristics. The multi-step compression of a truss structure is shown in Figure 11c-ii. Such printed multi-materials structures were then combined with triboelectric nanogenerators (TENGs) to fabricate multi-level TENG devices. Highlighting the merits of sequential deformation, the multilevel TENG generated multilevel electric signals in response to different levels of mechanical force, providing opportunities for both energy harvesting and sensing. Recently, the high-resolution DIW printing of filler-free epoxy-based SMPs with high transition temperatures and mechanical strength was reported [121]. In the first step, a cyanate ester and an epoxy resin were reacted to yield a prepolymer with a high viscosity. In the second step, 3D printing and photocuring of the prepolymer with acrylate- or vinyl-functional additives resulted in a hard gel, which was thermally cured in the third step, resulting an IPN structure. DIW through a combination of hybrid pre-polymer formation and a two-step curing process resulted a complex mesh structure with high transition temperature (~170 \({}^{\circ}\)C) and tensile strength (~70 MPa) along with excellent shape memory (\(R_{r}>86\%\), \(R_{f}>95\%\)). Furthermore, the applicability of this 3D printed SMP was demonstrated via the formation of a spiral spring with high modulus and a deformable sealing ring for easy installation (Figure 11d). Figure 11: (**a**) Schematic demonstration of printing of epoxy based SMP IPNs assisted by UV and thermal curing. (**b**) A lattice structure with a single layered wall and its SEM image. Reproduced with permission from reference [120], copyright 2018 Royal Society of Chemistry, London, UK. (**c-i**) Sequence of three dynamic photomasks and the total effective dose pattern by combining all the light dose for each photomask during photomask assisted DIW printing of graded lattice structures (**c-ii**) results of compression test at different stages of deformation for the truss structure. Reproduced with permission from reference [29], copyright 2019 John Wiley & Sons. (**d**) A high temperature shape changing of process of a spiral and a sealing ring 3D printed structure. Reproduced with permission from reference [121], copyright 2019 Elsevier.