## 3 Stimuli-Responsive Mechanisms: Avenues for 4D (Bio-)printing
### Temperature-Responsive Materials
Having more reliable biomaterial-assisted 3D in vitro models is very useful for drug development and therapy screening in many research areas, including liver cancer [76; 77]. The copolymer of PNIPAAm and N-tert-butylacrylamide (NtBAAm) have been used as delivery vehicles of an antimitotic agent to vascular smooth muscle cells [78]. [PERSON] et al. synthesized different thermally responsive dendritic core-shell polymers based on dendritic poly(ether-amide) (DPEA), modified with carboxyl end-capped linear poly(N-isopropylacrylamide) (PNIPAAm-COOH) or both PNIPAAm-COOH and carboxyl end-capped methoxy polyethylene glycol (PEG-COOH), in different ratios for drug delivery applications [79]. Results demonstrated that all lower critical solution temperatures were higher than that of traditional linear PNIPAAm, or its copolymers. In another attempt, colloidal suspensions of ceramic powder (Al\({}_{2}\)O\({}_{3}\)) dispersed by thermosensitive dispersant poly(acrylicacid)-poly(N-isopropylacrylamide) (PAA\(-\)PNIPAM) were designed for tailoring the rheology of suspensions, which is an indispensable matter for 3D printing applications [68]. For poly(N-vinylcaprolactam) (PNVC), toxicity tests did not reveal any evidence of cytotoxicity, confirming that for short exposure time at the aimed temperature, the polymers can be considered nontoxic (Figure 4c) [80]. Similarly, poly(L-lactic acid) (PLLA)-PEG block copolymer showed biodegradability, good biocompatibility, and amphiphilic characteristics, thus providing the adequate requirements for biomedical usage [81]. Finally, it was observed that the copolymer of polyethylene oxide (PEO) and polypropylene oxide (PPO), namely PEO-PPO-PEO displayed low levels of toxicity, even with a high percentage of PEO [82]. The ability of triethoxysilane-capped PEO-PPO-PEO triblock to perform as matrices for drug delivery was also investigated [83]. Findings showed that crosslinked gels released drugs, e.g., metronidazolezo, at a much slower rate than pluronic F127 (PEO99-PPO67-PEO99). Accordingly, [PERSON] et al. revealed a supreme potential of PLGA-b-PEG-b-PLGA thermogels to carry multidrugs for intraperitoneal (IP) chemotherapy in the cancer of peritoneal ovarian [84]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material(s)** & **Mechanism(s)** & **Application(s)** & **Ref.** \\ \hline Poly(N-alkyl substituted acrylamide) & Temperature & NA & [65] \\ \hline Poly(N-isopropylacrylamide) & Temperature & \begin{tabular}{c} Drug delivery, tissue \\ regeneration, yeast cell \\ encapsulation, artificial \\ muscle, programmable \\ actuators \\ \end{tabular} & [24, 63, 65, 67, 70, 73, 74, 85, 86, 87] \\ \hline Poly(N-vinylalkylamides) & Temperature & NA & [65, 79] \\ \hline Poly(N-vinylcaprolactam) & Temperature & \
\begin{tabular}{c} Stabilization of proteases \\ and controlled drug \\ delivery \\ \end{tabular} & [80, 88] \\ (PNVC) & &
\begin{tabular}{c} Potential vehicles for \\ delivery of an antimitotic \\ agent to vascular smooth \\ muscle cells \\ \end{tabular} & [78] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Overview of the experiments on 4D (bio-)printing and other manufacturing approaches using temperature as a predefined stimulus. Figure 4: Temperature-responsive materials: (**a**) The polymer bilayer is undeformed at elevated temperatures when the poly-(NIPAM–ABP) layer is shrunk. Reprinted with permission [70] © 2020 The Royal Society of Chemistry. (**b**) Thermally triggered response of hydrogels: (**i**) Morphology of a gradient porous hydrogel at 25 and 40 \({}^{\circ}\)C, (**ii**) Morphology and thermal responsive time of a gradient porous hydrogel strip after being put into 40 \({}^{\circ}\)C H\({}_{2}\)O. Reprinted with permission [71] © 2020 John Wiley and Sons. (**c**) Viability values for the thermosensitive polymers and monomers. Reprinted with permission [80] © 2020 Elsevier. These biomaterials have been used in several applications, including TE and drug release, and have been characterized according to their properties, biocompatibility in particular [24].