## 3 Stimuli-Responsive Mechanisms: Avenues for 4D (Bio-)printing
### pH-Responsive Materials  
Body pH can greatly vary as a consequence of both normal (e.g., in the gastrointestinal, vaginal) and diseased conditions (e.g., bone fracture, inflammation, tumors, some cardiovascular defects, infected wounds). The most remarkable example occurs in the gastrointestinal tract, where pH increases from 1 to 8, passing from the stomach to the intestine. Therefore, responsive biomaterials able to properly interact with such pH variations can be highly useful in drug delivery [139; 140; 141; 142; 143; 144; 145; 146; 147; 148; 149; 150]. Specifically, pH-responsive hydrogels react to pH changes in the microenvironment by volume expansion/contraction, namely via swelling/deswelling. This phenomenon is caused by the ionization of pendant groups, such as amine and carboxylic acid groups. Indeed, the presence of ions affects the osmotic pressure, which in anionic and cationic hydrogels causes swelling at pH \(>\) pKa and pH \(<\) pKa, respectively (Figure 8). Figure 7: Photresponsive materials: (**a**) Schematic representation of proteopolymers reconstituted with both BR and F0F1-ATP synthase. ATP synthase uses an electrochemical proton gradient generated by BR to synthesize ATP from ADP and inorganic phosphate. Reprinted with permission [138] © 2020 American Chemical Society. (**b**) 2D pattern with circular exposed spot: cells’ preferential adhesion, spreading, and proliferation are visible on the UV-exposed area after 4 d of culture. Reprinted with permission [130] © 2020 John Wiley and Sons. (**c**) Schematic illustration of light effect: (_i_) detachment of dye pendant groups resulting in the hydrophobic-to-hydrophilic switch and (_ii_) chemical structure of the pyrene-containing amphiphilic diblock copolymer and its photosolvolysis under UV light irradiation. Reprinted with permission [136] © 2020 American Chemical Society. Both synthetic and natural polymer hydrogels can show pH-responsiveness. Synthetic polymers include PAA [140; 143; 146], poly(methacrylic acid) (PMAA) [143], poly(acrylamide/maleic acid) [P(AAm/MA)] [150], poly(diethylaminoethyl methacrylate) (PDEAEMA) [143], poly(dimethylaminoethyl methacrylate) (PDMAEMA) [143; 149], PHEMA [142], some copolymers include poly(2-(diethylamino)ethyl methacrylate-co-2-(dimethylamino)ethyl methacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate) P(DEAEMA-co-DMAEMA)-b-PDMAEMA [151] and poly (DMAPMA-co-HEMA) [145]. Among natural macromolecules, some proteins (e.g., gelatin, albumin) [152] as well as polysaccharides (e.g., alginate, chitosan) [148; 153; 154] can give rise to pH-responsive hydrogels (Figure 2). Poly(N-isopropyl acrylamide-co-butyl methacrylate-co-acrylic acid) [155] and star-branched poly(\(\varepsilon\)-caprolactone)-b-poly(2-(dimethylamino)ethyl methacrylate) (HPs-star-PCL-b-PDMAEMA) [156] have been used as pH and temperature sensitive polymers for controlled drug delivery application. Several of these polymers have been used for 4D (bio-)printing by virtue of pH as a predefined stimulus to be exploited (Table 6). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material(s)** & **Mechanism(s)** & **Application(s)** & **Ref.** \\ \hline \multirow{6}{*}{Poly(acrylic acid) (PAA)} & & Peroral-controlled drug & \\  & & delivery, taste-masking of & \\  & & bitter drugs, and & \\  & & intravascular drug release & \\  & & during elevated blood pH in & \\  & & certain cardiovascular & \\  & & defects & \\ \hline Poly(acrylic acid) (PAA):PEO & pH & Drug delivery & [140] \\ \hline Based on gelatin (G) and acrylic acid & pH & Colon-specific oral drug & \\ (AAc) & pH & delivery & [146] \\ \hline Poly(methacrylic acid) (PMAA) & pH & & [143] \\ \hline Poly(acrylamide/maleic acid) (P(AAm/MA)) & pH & Drug delivery & [150] \\ \hline Poly(diethylaminoethyl methacrylate) (PDEAEMA) & pH & & [143] \\ \hline Poly(dimethylaminoethyl methacrylate) (PDMAEMA) & pH & Drug delivery, DNA transfer & [143; 149] \\ \hline \hline \end{tabular}
\end{table}