# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 4 Vat-Photopolymerization Based 4D Printing
### Digital Light Processing (DLP) of trSMPs  
The digital light processing (DLP) method employs a digital projector to flash a 2D image across the entire platform and cures an entire layer of material at once. As DLP technology is not restricted to the curing of a single spot at a time, it can significantly reduce printing times. Furthermore, through local variations in light intensity, DLP allows for inhomogeneous cross-linking within a layer, enabling the formation of gradients in mechanical and shape-memory properties [125]. DLP based 3D printing technologies cover a broad spectrum of printing resolutions from 0.6 \(\upmu\)m to 90 \(\upmu\)m by adjusting the magnification of the projection lens [69; 72]. Photocurable and electroactive shape-memory polymer nanocomposites (SMPCs) based on a thermosetting poly(ethylene glycol) diacrylate/poly(hydroxyethyl methacrylate) matrix containing multi-walled carbon nanotubes (MWCNT) were successfully prepared via DLP 3D printing [71]. The composition of the photocurable (meth)acrylate system was tuned to tailor the thermomechanical properties of the matrix, whereas the effects of CNTs on the photoreactivity and rheological properties of the formulations were investigated to assess the printability of CNT-containing formulations. When increasing the CNT content from 0.1 to 0.5 wt.%, a delay in the onset of photopolymerization was observed. This delay in curing time was attributed to the UV shielding effect of the MWCNTs, whose light absorption capability affected the efficiency of the photoinitiator. The 3D-printed demonstrators displayed high shape fixity and shape recovery ratios (\(R_{f}=100\%\), \(R_{r}>95\%\)) and an electrically triggered shape-memory effect realized via Joule heating. Furthermore, by changing the position of copper electrodes, sequential recovery of different segments of a printed zig-zag shaped demonstrator could be achieved (Figure 13a,b). [PERSON] et al. used the DLP printing method to produce multiple SMPs within a single construct using the same precursor [126]. They modulated the intensity of the light to enable the spatio-temporal tuning of material properties, including shape-memory transition temperature, rubbery modulus, and maximum elongation, within a single part. The photo-sensitive ink consisted of comonomers methyl acrylate (MA, 56.9 mole%) and isobornyl acrylate (40.3 mole%) with 1,6-hexanediol diacrylate (HDDA, 2.8 mole%) as the crosslinker and bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide as the photoinitiator. By digitally controlling the light pattern with different focus time periods, unusual shape shifting 3D nano-photonic and electronic devices combining softer and more rigid regions were constructed. [PERSON] et al. used high-resolution projection microstereolithography (P\(\mu\)SL) to print multi-material shape-memory structures [127]. P\(\mu\)SL is capable of printing high-resolution (to 0.6 \(\upmu\)m) complex 3D architectures covering multiple scales and with multiple materials over a relatively wide printing area (up to \(\sim\)90 mm \(\times\) 50 mm). To print multi-material SMP structures, an automatic material exchange mechanism was integrated into the P\(\mu\)SL manufacturing system. The photo-sensitive resin consisted of benzyl methacrylate (BMA) as linear chain extender and poly(ethylene glycol) dimethacrylate (PEGDMA), bisphenol A ethoxylate dimethacrylate(BPA), or di(ethylene glycol) dimethacrylate (DEGDMA) as crosslinker. Tailored glass transition temperatures and thermo-mechanical properties were achieved through variations in crosslinker type and concentration. Using this approach, a multi-material gripper was fabricated whose hinges consisted of an SMP and whose tips consisted of a soft material whose modulus could be adjusted based on the modulus of the target object (Figure 13c,d). [PERSON] et al. developed SMP/nanosilica composite resins for DLP 4D printing and evaluated the effects of nanosilica on the printing process and the shape-memory properties of the resultant parts [128].