# Tuning the Cell and Biological Tissue Environment through Magneto-Active Materials
## 4 Fundamentals of Smart Polymeric Materials: Focus on Magneto-Active Elastomers and Hydrogels
### Magnetorheological Elastomers (MREs)
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Stimulus** & **Materials** & **Applications** \\ \hline Temperature & PNIPAAm, PDEAAm, PVCL, PDMAEMA, PEG & Drug and gene delivery, tissue engineering (substrates and injectable gels). \\ Light & PEG and PMA hydrogels modified with light & Drug delivery (light can penetrate the skin); smart surfaces; nanostructures. \\  & resparse or coumarinly esters [107]. & Drug delivery, actuators in soft robotics, artificial muscles [109]. \\ Electrical field & Ionic polymer-metal composites: perfluorinated & Drug delivery, actuators in soft robotics, artificial muscles [109]. \\  & polymeric matrix laden with magnetic particles & Frug delivery, remote actuation, soft robotics, tissue engineering. \\ Magnetic field & (nano or micro, hard or soft) [110; 111; 112; 113; 114; 115]. & Frug delivery, remote actuation, soft robotics, tissue engineering. \\  & Polyelectrolyte polymers with weak acidic or basic & Drug delivery, gene carriers, biosensors. \\ pH & groups that accept or release protons in response to a & Drug delivery, gene carriers, biosensors. \\  & change in the environmental pH [116]. \\  & Polymers modified to respond to the presence of & Drug delivery, biosensors, tissue engineering and regenerative medicine. \\ Biomolecules & certain biological cues (signals, enzymes, ATP, etc.) & Drug delivery, biosensors, tissue engineering and regenerative medicine. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of multifunctional materials according to the nature of their stimulation and highlighting of applications. better mechanical properties and heat resistance. In particular, the use of natural rubber as matrices for MREs shows advantages, such as a good resistance to tearing at high temperatures and good dynamic performance. However, the use of natural rubber for MREs matrices present some disadvantages. The manufacturing of natural-rubber based matrices is much more complex than the fabrication using silicone-based rubbers, due to the raw material of the matrix being in solid state, which requires specialised equipment [122]. If the basic mechanical properties of MREs are given by the polymeric matrix, magnetic particles give these materials their field dependent properties. The shape, size and composition of these magnetic particles are factors that influence the behaviour of the resulting MRE. Regarding the shape of these magnetic particles, the most used particles are of spherical shape, since they are widely commercialised, while the size of the particles can vary. However, most CIPs reported in the literature have a size smaller than 10\(\upmu\)m. The reason why smaller sizes are preferred is that smaller particle sizes lead to a higher effective area of interfacial friction between the particle and the matrix [123]. Regarding the composition, it influences the way particles behave in the presence of and without an external magnetic field, where soft magnetic particles and hard magnetic particles can be distinguished. **Soft magnetic particles** are particles of low-coercivity ferromagnetic materials, or soft-magnetic materials, such as iron and iron oxides. These soft-magnetic materials can develop strong magnetisation along the applied magnetic field, but do not stay magnetised after the external magnetic field has been removed (low-coercivity). Therefore, soft magnetic particle-based MREs magnetorheological effect and magnetostriction effect are due to the attraction or repulsion forces of the induced dipole in the soft-magnetic particles by the magnetic field. Carbonyl iron powders (CIPs) are the most widely used magnetizable particles. CIPs use for MRE fabrication is widespread due to their properties: their high magnetic saturation and permeability allows for a high MR effect and magnetostriction, while their low coercivity means that they do not have high magnetic remnant properties, which allows for the mechanical properties of the material to return to normal once the external magnetic field is no longer present.