# Tuning the Cell and Biological Tissue Environment through Magneto-Active Materials
## 4 Fundamentals of Smart Polymeric Materials: Focus on Magneto-Active Elastomers and Hydrogels
### Three-Dimensional Printing of Magneto-Active Polymers  
In recent years, additive manufacturing technologies, mainly 3D printing, have been posed as a good candidate for the fabrication of sophisticated structures, and allows for a low-cost, time saving and customizable result. However, 3D printed objects were always static [135]. A new concept has recently appeared, considering the design and fabrication through 3D printing technologies of structures that may undergo a structural change when subjected to an intended stimulus. This concept, known as 4D printing, was first introduced by professor [PERSON] as a new design of a complex spontaneous structure that changes with time as a response to a stimulus after the manufacturing process is completed [136]. Another important characteristic of 4D printed structures is the ability to revert to their original status [135]. Four-dimensional printing takes common features from 3D printing for the design and generation of the structure but controls a specific multifunctional response of the printed materials, thus programming the mechanical response to diverse stimuli. For example, some materials respond to temperature, light or a magnetic field through different mechanisms [137]. Magneto-active soft materials (MASM) are shape-programable active materials capable of controlled and intended shape transformation and object locomotion. These changes in structure or displacement of the materials are achieved in a remote manner through the application of a magnetic field, which offers numerous potential applications in the fields of soft robotics and biomedicine, amongst others [138]. There exist many approaches to additive manufacturing, such as fused deposition modelling (FDM) for thermoplastics, or stereolithography (SLA) for photocurable resins. However, one of the main techniques employed for the fabrication of MASMs is direct ink writing (DIW) [139]. This technique consists of the injection of a layer of material through a nozzle using either mechanical or pneumatic actuation. This material then undergoes polymerisation, allowing for the deposition of the next layer on top of the previous one. The stacking of these layers then leads to the final product. The other main technique used for printing of magnetic polymers is vat photo-polymerisation (VPP), which includes techniques such as SLA or digital light processing (DLP) [140]. These techniques consist in the polymerisation of a photo-crosslinkable resin. In this case, the differences between SLA  
Figure 2: Scheme of different manufacturing techniques for magneto-active hydrogels or ferrogels: (**A**) blending method; (**B**) in situ precipitation method; (**C**) grafting-onto method. and DLP lie on the area affected by the light source. In the case of SLA, a laser irradiates a path that will be crosslinked, while in DLP, the light source projects a whole image onto the vat, crosslinking the whole layer at once. In both cases, the fabrication is done layer by layer. These two techniques for the fabrication of MASMs are widely used for the ease of dispersion of the magnetic fillers in the polymeric matrix, which can be in a liquid state prior to printing in both cases [141]. This fluid nature of the printing material also allows for the orientation of the magnetic fillers prior to polymerisation, by application of a controlled magnetic field. Regarding MREs, some work has been developed in recent years concerning the printing of these materials. DIW, together with the application of controlled magnetic fields during the printing process, was employed by [PERSON] et al. [142] to obtain functional materials with a desired distribution of magnetic particles within the polymeric matrix. This process consisted in the application of a desired magnetic field during the printing process to achieve the free displacement of the magnetic filler particles within the printed matrix prior to its polymerisation.