## 4 4D (Bio-)printing: Cutting Edge Applications and Future Perspectives  
3D printing has become a growing industry. New materials, printing methods, software, and machines are constantly being developed and improved, making 4D printing a more realistic and affordable option. The main difference between 3D and 4D printing is the possibility to trigger a planned time-scale modification of specific feature (e.g., shape, size) in the printed constructs, thus opening the opportunity of programmed response to changing environments and needs [180]. The concept of 4D printing is relatively young, becoming very fashionable for numerous engineering applications, including structural, aerospace, and biomedical, namely, in those applications where the microenvironment changes play a fundamental role in the material function [181; 182; 183; 184; 185; 186]. 4D (bio-)printing, despite a full consensus on the exact definition, has the potential to physically replicate the path of developmental biology and bring organ printing a step closer to reality, thus combining life sciences with engineering in a dynamic fashion. This technology has emerged as a useful tool for biomedical applications because of its innate advantages, including (i) complex 3D structures based on materials that respond to environmental stimuli and can reshape or alter their function, and (ii) postmaturation of cell populations printed on scaffolds with programmable architectures enable the generation of tissue constructions with functionalities similar to native tissues [24]. Recent advances in understanding the structure of blood vessels and the evolution of 4D (bio-)printing can improve vessel constructions and various other constructs that mimic native tissue. In the case of blood vessels, these are the ones that maintain cellular function through the supply of nutrients and oxygen and waste removal. 4D printed constructs for blood vessels are manufactured as layer-by-layer solids, where the cells are mixed in the hydrogels and the cylindrical structures obtained resemble the blood vessels [187]. These cellularized hydrogels are subsequently activated by maturation factors, and the cells rapidly mature, resulting in vascularization. Polymers that are sensitive to different stimuli also have applicability in regenerative medicine. They can be classified into polymers with smart surfaces and polymers subjected to sol-gel transition for injectable implants. The former can be used as scaffolds with excellent ability to control the properties of surfaces, enabling the adsorption or desorption of molecules and cells. Depending on the application, polymers that respond to stimuli may enable the production of smart scaffolds for TE via 3D printing also aided by other technologies [65]. Interesting examples of regenerative applications include 4D biofabrication of fibrous artificial nerve graft [188], and skeletal muscle microtissues [189]. However, the printability of these polymers must be assessed. It is fundamental to precisely control deposition of hydrogels during printing. Therefore, the printing parameters, such as air pressure, feed rate, and printing distance need to be mastered [190]. Printability of hydrogels can be assessed by depositing one dimensional (1D), 2D, and up to 3D structures in order to understand the real versus designed outline accuracy. Piezoelectric materials have been successfully 3D printed with methods that also integrate poling treatments [191]. 3D printed PVDF structures have indeed