## 3 Stimuli-Responsive Mechanisms: Avenues for 4D (Bio-)printing
### Magnetically Responsive Materials
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material(s)** & **Mechanism(s)** & **Application(s)** & **Ref.** \\ \hline Cobalt ferrite (CoFe\({}_{2}\)O\({}_{4}\)) & Magnetism & Controlled drug release & [29,108,115] \\ \hline Ferromagnetic cobalt nanoparticles & Magnetism & & [107] \\ \hline Iron platinum (FePt) & Magnetism & & [29,115] \\ \hline Iron (III) oxide (Fe\({}_{2}\)O\({}_{3}\)) & Magnetism & Drug delivery application & [29,115] \\ \hline Iron (II, III) oxide (Fe\({}_{3}\)O\({}_{4}\)) & Magnetism & & [29,115] \\ \hline CoPt & Magnetism & Drug delivery application & [115] \\ \hline (Fe\({}_{3}\)O\({}_{4}\)) with PNIPAM, PVA, Poly(acrylamide), alginate & Magnetism & Semiconductor, active scaffolds for on-demand drug and cell delivery & [105,106,109,110,112] \\ \hline Iron nanoparticles/[11-(2-bromo-2-methyl)-propionyloxy] undecyltrichlorosilane (Fenp-PS gel) & Magnetism & Cancer therapy, sensors, switches, various separations, membranes, and drug delivery systems & [113] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Overview of the experiments on 4D (bio-)printing and other manufacturing approaches using magnetism as a predefined stimulus. Exposure time, light intensity, and adequate photoinitiator should be examined and assessed to ensure that no damage will affect the cells [29]. Possible applications of photoresponsive polymers include reversible optical storage, polymer viscosity control, transduction and photomechanical performance, exchange of protein bioactivity, TE, and drug delivery [41; 133; 134]. For example, poly(1-pyrenylmethyl methacrylate) (PEO-PPyMA), poly(2-nitrobenzylmethyl methacrylate) PEO-b-PNBMA, and PEO-b-P(MA-DEACM) have been used as light-responsive polymers to produce micelles in anticancer therapy (Figure 7c) [135; 136; 137]. A list of materials using light as a 4D stimulus is summarized in Table 5. \begin{table}
\begin{tabular}{c c c c} \hline
**Material(s)** & **Mechanism(s)** & **Application(s)** & **Ref.** \\ \hline Polymers containing & Light (ultraviolet (UV) & Swimming soft robot & [119; 120; 121] \\ Azobenzene groups & light) & Swimming soft robot & [119; 120; 121] \\ \hline Polymers containing & & & [123] \\ Spiropyran groups & Light & & [124] \\ \hline Polymers containing & & & [125] \\ Nitrobenzyl groups & Light & & \\ \hline Poly (acrylic acid) (PAA) & Light & & [126] \\ containing Azobenzene groups & Light & & \\ \hline Poly (N-(2-hydroxopropyl) & & Polymeric micelles in & \\ methacrylamide (PHPMAm) & Light & anticancer therapy, scaffold & [127; 126] \\ containing Azobenzene groups & & for tissue engineering & \\ \hline PNIPAAm & Light & & [74] \\ \hline PEO-PPyMA & & & \\ (poly(1-pyrenylmethyl & Light & Micelles in & \\ methacrylate)) & & anticancer therapy & [137; 136] \\ \hline PEO-b-PNBMA & & & \\ (poly(2-nitrobenzylmethyl & Light (UV/NIR) & Micelles in & \\ methacrylate)) & Light (UV/NIR) & anticancer therapy & [138] \\ \hline PEO-b-P(MA-DEACM) & Light (UV/NIR) & Micelles in & \\  & & anticancer therapy & [136] \\ \hline \end{tabular}
\end{table} 
Table 5: Overview of the experiments on 4D (bio-)printing and other manufacturing approaches using light as a predefined stimulus.