## 4 4D (Bio-)printing: Cutting Edge Applications and Future Perspectives
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material(s)** & **Mechanism(s)** & **Application(s)** & **Ref.** \\ \hline BaTiO\({}_{3}\) powder and photocurable & Mechanical force & Energy focusing and & [173] \\ resin SI500 & (piezoelectric) & ultrasonic sensing & [173] \\ \hline BaTiO\({}_{3}\) nanoparticles in nucleating & & & \\ piezoelectric \(\beta\)-polymorph in 3D & Mechanical force & & \\ printable polyvinylidene fluoride & (piezoelectric) & Integrable sensors & [171] \\ (PVDF) & & & \\ \hline Multiwall carbon nanotubes & Mechanical force & Sensor & [174] \\ (MWCNTs)/PVDF/BaTiO\({}_{3}\) nanoparticles & (piezoelectric) & & \\ \hline Shape memory polymer & Thermomechanical & Active origami & [177] \\ \hline Glassy shape memory polymer & Thermomechanical & Active origami & [178] \\ \hline \hline \end{tabular}
\end{table} 
Table 7: _Cont._boosted the emerging field of piezoelectronics, which has remarkable applications in the biomedical field to produce devices and interfaces with transduction capability [192]. [PERSON] et al. reported on a photocurable piezoelectric ink suitable for additive manufacturing [193]. In the bioprinting research, additional factors must be investigated, as the optimal printing parameters for manufacturing a 3D architecture could not be suitable for cell survival, thus pointing out that the rheological properties of the bioink are fundamental [194]. Hydrogel composition together with printing parameters strongly affect printability in terms of desired shape resolution, as demonstrated using pure alginate or alginate with gelatin and methyl-cellulose [195]. Moreover, the solvent used for hydrogel formation, the resultant viscosity, the applied pressure and temperature, as well as the nozzle size and geometrical factors of the printing head all influence cell viability, due to the shear stress experienced by the cells upon extrusion, which represents a key factor [196]. Shear-thinning properties are crucial for keeping bioink shape fidelity and cell viability. Preliminary studies have demonstrated the bioprintability by different 3D printing methods of thermosensitive p(NIPAAm-AA) hydrogel, proposed as a vascularized skin substitute [197]. In similar fashion, a number of smart materials are reported to be 3D printed every day, thus suggesting intriguing opportunities for 4D (bio-)printing [198; 15]. As such, a great effort still regards the exact spatial and temporal control of the property change in smart material printing, which includes novel studies via computational modeling [199]. Controlled drug delivery has a high potential to improve drug efficacy, as well as personalized and local drug administration. Having a controlled delivery reduces the patient's pharmacokinetic variability inherent to oral and intravenous applications. One of the options is the use of magnetic nanoparticles capable of incorporating a reasonably high weight fraction of the drugs, of forming a stable suspension in biocompatible, biodegradable aqueous media, and of not being rapidly eliminated in the bloodstream. The main advantage of using these nanoparticles relies on the exploitation of magnetic stimuli to achieve zonal confinement and administration of drugs to be remotely directed [200]. Finally, 4D (bio-)printing allows the spatial distribution of different materials to be controlled, which can be self-fold or self-unfold, thus obtaining the encapsulation and release of drugs or cells in a programmable way [24]. For example, small droplets of oil that encapsulate aqueous droplets called \"multisomes\" can be printed in water [201]; these droplets adhere to each other forming bilayers. After being stimulated with temperature, the components in the droplets can be released [24]. Other macroscale materials with self-morphing [202], and self-folding properties have shown promise in tissue regeneration [203; 204; 205].