## 3 Stimuli-Responsive Mechanisms: Avenues for 4D (Bio-)printing
### Other Stimuli-Responsive Materials
Different piezoelectric nanoparticle-polymer composite materials with the ability of conversion compressive/tensile stresses to an electric charge, or vice versa, have also been used in 3D printing to the creation of a wide variety of smart structures [171; 172; 173; 174; 175; 176]. The one-step 3D printing of piezoelectric materials opens up a new demographic of sensor designs, and also 3D piezoelectric scaffolds could be used as electroactive scaffolds in TE, providing a biomimetic environment for cell culture. By chemically modifying the surface of the BaTiO\({}_{3}\) nanoparticles with linker molecules that crosslink with the polymer matrix under light exposure, [PERSON] et al. increased the mechanical-to-electrical conversion process of nanocomposites, which can be used for different applications such as implantable medical devices [176]. [PERSON] et al. investigated the effect of addition of barium titanate (BaTiO\({}_{3}\)) nanoparticles in nucleating piezoelectric \(\beta\)-polymorph in 3D printable PVDF and fabrication of the layer-by-layer and self-supporting piezoelectric structures on a micro- to millimeter scale [171]. Thermo-mechanically responsive materials, such as glassy shape memory polymers (GSMPs) have also been used for some specific applications such as active origami [177; 178]. [PERSON] et al. [178] created a printed active composite (PAC) consisting of GSMPs fibers with tailored thermomechanical behavior in an elastomeric matrix to be used as hinges to connect inactive plates of stiff plastic to enable a self-folding box. The multiple film layers have been printed to create the 3D laminate. PAC displays the shape memory effect in the range of T\({}_{\text{L}}\) = 15 \({}^{\circ}\)C to T\({}_{\text{H}}\) = 60 \({}^{\circ}\)C. Thus during a temperature cycle between T\({}_{\text{L}}\) and T\({}_{\text{H}}\), the matrix with a glass transition temperature of Tg = 5 \({}^{\circ}\)C is always rubbery, while the fibers undergo a glass temperature (Tg) that lets them be used as the switchable segment in the composite. Chemically responsive hydrogels show swelling or degradation in response to a defined target molecule. The primary example is glucose-responsive hydrogels that exploit the competitive reaction and best affinity for glucose of concanavalin A [158]. Glucose-responsive polymers have received considerable attention because of their application. One of the applications is the huge potential in the self-regulated modes of insulin delivery where a smart delivery system is used, in which insulin delivery is automatically triggered by a rise in blood glucose levels [41; 65]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material(s)** & **Mechanism(s)** & **Application(s)** & **Ref.** \\ \hline Glucose-responsive hydrogels & Chemistry & Diabetes monitoring & [41; 65; 158] \\ \hline Glucose oxidase & Chemistry & Drug delivery & [159] \\ \hline Glucose oxidase & Chemistry & Controlled release of insulin & [160] \\ \hline Cysteine-containing hydrogel & Mechanical force & Injectable biomaterial & [157] \\ \hline Barium titanate (BaTiO3, BTO) & & & \\ nanoparticles & & Implantable medical & \\ into a polyethylene glycol diacrylate & & devices & [176] \\ (PEGDA) matrix & & & \\ \hline BaTiO\({}_{3}\) (BTO) filler in a poly(vinylidene) & Mechanical force & & \\ fluoride (PVDF) & & (piezoelectric) & Pressure sensor & [179] \\ \hline Ultraviolet-curable resins (acrylate ester & & & \\ for Resin 1, and methacrylated & & & \\ monomers/oligomers for Resin 2) & & & N/A & [172] \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Overview of the experiments on 4D (bio-)printing and other manufacturing approaches using chemical and mechanical-based forces as a predefined stimuli.