## 2 Four-Dimensional (Bio-)printing  
4D printing is the latest advancement in the advanced manufacturing field [15]. It was initially defined as a procedure where the shape, properties, or functionality of a 3D printed structure can change according to time and/or stimuli (Figure 2) [26]. The foundational works were developed at the Massachusetts Institute of Technology (MIT), by considering time as the fourth dimension [27]. Given this, 4D (bio-)printing indicates that for the transition, over time, the 3D printed biocompatible materials may incorporate live cells by using bioinks [24]. Thereafter, sequential approaches have been applied to the concept of 4D (bio-)printing, according to the identified stimuli concurring to defined biomedical applications. The first approach to 4D printing closely followed the MIT concept, in which the material (e.g., smart biopolymer or responsive hydrogel), under the desired stimulus, folds in a predefined 3D configuration, and cells or tissues simply follow the substrate fold and form the desired shape. The second approach is the \"in vivo 4D bioprinting\" in which a 3D polymer is printed, acting as a medical device, initially implanted to accommodate tissue or organ growth after the postsurgical period. When the tissue or organ gets stronger, the scaffold degrades gradually, preferably in a controlled manner, and is absorbed by the body. In this approach, tissue growth can be viewed as a leading stimulus. However, considering 3D printed scaffolds under the 4D printing umbrella because of their biodegradation property (i.e., controlled biodegradation as a shape change over time), is utmost controversial [25]. Figure 2: Illustrative diagram representing addition of a predesigned stimulus to promote a desired change at the construct. [Original image from the authors]. [PERSON] et al. proposed a definition for 4D bioprinting, referring to groups of programmable self-assembling, self-folding, or self-accommodating technologies, which include three main defined or essential components: (i) man-made and not nature-made programmable design, (ii) two dimensional (2D) or 3D bioprinting process, and (iii) postprinting programmable evolution of bioprinted constructs that can be driven by cells or biomaterials and triggered by external signals [27]. The main advantageous characteristics of 4D bioprinting include additional degrees of freedom in terms of design, high precision cell placement, and the ability to produce tissues with adequate cellular density [15]. Many 3D printed scaffolds with programmable biodegradation fit within this definition, but are beyond the topic of this review. So far, a widely accepted definition of 4D printing is still missing, as from [PERSON] et al. : \"Whether we are really witnessing 4D printing, a process that should be defined as a programmed temporal shape change occurring during the 3D manufacturing itself, or not, is still to be clarified in the field\" [28]. In this scenario, many researchers are investigating different materials and approaches enabling controllable shape/property changes upon different stimuli and timeframes, which are suggested in this review under the name of 4D (bio-)printing. A category that naturally suits the 4D (bio-)printing concept is the printing of smart hydrogels [29]. Hydrogels are polymeric networks able to form water-swollen structures thanks to different types of forces between the polymer chains, leading to semi-crystalline or amorphous materials [14; 30; 31]. Smart hydrogels are able to respond to a desired stimulus (e.g., shear stress, electricity, ionic force, light, magnetic field, pH, temperature). The interest in developing smart hydrogels has been increasing within the last years, as they should be able to provide remarkable characteristics, such as shape memory, self-healing and controllable sol-gel transition.