# Tuning the Cell and Biological Tissue Environment through Magneto-Active Materials
## 4 Fundamentals of Smart Polymeric Materials: Focus on Magneto-Active Elastomers and Hydrogels
### Fabrication Techniques  
Ferrogels can be manufactured by different methodologies, among them:  
1. **Blending Method (Figure 2A)** MNPs and hydrogels are prepared separately in a sequential order. The prepared MNP sediment is then dispersed in an aqueous or oil phase (i.e., a ferrofluid) to avoid oxidisation and aggregation. Finally, the mixture of the ferrofluid and the hydrogel precursor solution are crosslinked, resulting in the encapsulation of the MNPs in the hydrogel. The main advantage offered by the blending method is the simplicity of the process. Furthermore, both components necessary for the fabrication are commercially available. Another advantage is the fact that both components are prepared separately, which offers the possibility to achieve a uniform particle size in hydrogels over a wide range from nanometres to micrometres, whether by adjusting the concentration of the reactants, the stirring speed, and the preparation cycle, or by purchasing the desired MNPs. However, it is challenging to achieve a uniform MNP distribution within the hydrogels, and MNPs may diffuse out of the magnetic hydrogels when immersed in a liquid solution [134].
2. **In Situ Precipitation Method (Figure 2B)** In the in situ precipitation method, the hydrogel networks act as a chemical reactor. An inorganic salt with iron ions reacts with precipitating agents to generate MNPs inside the hydrogel polymeric network. Firstly, hydrogels are fabricated via temperature change, radical polymerisation, or a photo-crosslinking reaction. Secondly, the hydrogels are placed into a concentrated aqueous solution containing Fe \({}^{2+}\) and Fe \({}^{3+}\), and the ferrous ions are taken at a molar ratio of 1:2 until swelling equilibrium is reached. Finally, the swollen hydrogels with absorbed Fe \({}^{2+}\) and Fe \({}^{3+}\) are immersed into an alkali solution for MNP precipitation. The in situ precipitation method has several advantages in the fabrication of magnetic hydrogels. Firstly, many inorganic particles can be introduced into the hydrogel network, ensuring that colloidal-sized particles can be well dispersed in the matrix. Secondly, the preparation process is simple and low-cost. However, the in situ precipitation method is only suitable for specific hydrogels with a stable network, otherwise the hydrogel network may be destroyed by alkali solution during the preparation process [134].
3. **Grafting-Onto Method (Figure 2C)** In the grafting-onto method, covalent bonds are formed between the hydrogel network and the MNPs. These covalent bonds are achieved by grafting several functional groups onto the surface of the MNPs, which work as nano-crosslinkers to form a covalent coupling with the monomers when polymerised. The main advantage offered by the grafting-onto method deals with the problem of particle diffusion into the media. Both previous methods consisted of a physical encapsulation of the particles. With this method, the direct covalent coupling between the hydrogel matrix and the magnetic particles leads to a more permanent particle encapsulation. However, the grafting of the functional groups onto the magnetic particles is a difficult task with a long preparation cycle and high cost, which makes this method a complex fabrication process [134].