## 3 Stimuli-Responsive Mechanisms: Avenues for 4D (Bio-)printing
### Magnetically Responsive Materials
4D (bio-)printed materials using magnetism as a stimulus are reported in Table 4.  
Figure 6: (**a**) A cylinder of a nanoporous ferrogel reduced its height \(\sim\)5% when subjected to a vertical magnetic-field gradient of \(\sim\)38 A/m2. (**b**) The corresponding macroporous ferrogel deformed \(\sim\)70% under the same magnetic field. (**c**) SEM images of a freeze-dried macroporous ferrogel in the undeformed and deformed states. (Scale bar: 500 μm). Reprinted with permission [112] © 2020 National Academy of Sciences.3.5. Photoresponsive Materials. Photosensitive polymers are macromolecules that alter their structural properties upon irradiation with light at a given wavelength. These changes occur when light radiates specific functional groups along the polymer chain [41]. The strong light attenuation in biological tissues can present a challenge to this strategy, causing nonuniform illumination and transformation. On the other hand, near-infrared (NIR) light, representing a biosafe wavelength range, can be used to improve light penetration in bioprinted photosensitive tissue constructs [24]. Photocatalyzed reactions have shown a strong correlation to the light irradiance (i.e., radiant flux by surface area), thus becoming attractive tools for hydrogel manipulation [116]. The light can be applied instantaneously directly on the surface of the polymer or in distant locations using optical fibers, and under specific conditions with high precision, making photosensitive polymers quite advantageous for various applications. The wavelength of the laser must be tuned, near the infrared spectrum, in the so-called biologically \"friendly\" window, because it is less harmful than UV and penetrates deeper into tissues than light in the visible spectrum. In this case, the light is absorbed as little as possible by the cells and tissues and as much as possible by the polymers, thus enabling controlled therapy and diagnostics [117,118]. Most photoresponsive polymers contain light sensitive chromophores, such as azobenzene groups, spiropyran groups, or nitrobenzyl groups. A variety of photovoltaic polymers containing azobenzene [119, 120, 121, 122], spiropyran [122,123], nitrobenzyl [122,124] have been reported, such as poly(acrylic acid) (PAA) [125], poly(N-(2-hydroxyethyl)) methacrylamide (PHPMAm) [126,127], and PNIPAAm [74]. Azobenzene is one of the most well-studied photochromic molecules because of its reversible photoisomerization depending on the wavelength of light. Photosensitive polymers contain photoactive groups that may undergo reversible structural changes when they come into contact with UV-Vis light. These functional fragments alter size and shape, or form ionic species after irradiation (Figure 7a) [65,128,129]. Recent efforts have demonstrated the possibility of controlling 2D and 3D hydrogel fabrication along a time and space scale (Figure 7b) [130]. According to recent studies with PAA and human liver cancer cells, they demonstrated to have a desired cytotoxic activity [131]. The polymer PHPMAm is hydrophilic, biocompatible, and is not immunogenic [132]. Tests performed on different cell types with PNIPAAm revealed that the cell sensitivity varies according to cell type, being advisable to always perform toxicity tests; the purity of the polymer also influences the results [74]. An important feature of light sensitive polymers is that irradiation as a stimulus is a relatively simple and noninvasive mechanism to induce a responsive behavior [41,89,133]. Cell-containing polymers can be readily crosslinked with exposure to UV light. During the curing process, photoinitiators generate free radicals to initiate the polymerization process.