# Tuning the Cell and Biological Tissue Environment through Magneto-Active Materials
## 6 Magneto-Active Polymers in Biomedical Applications
### MAPs for Drug Delivery  
A biomedical field with great potential for MAPs is drug delivery. Conventional drug delivery lacks tissue specificity and one of its major drawbacks is to surpass natural physiological barriers [192]. The use of MAPs encapsulating the drug open the window to deliver specific therapeutic agents in situ to a specific target site. These MAPs consist of injectable nanoengineered hydrogels loaded with magnetic particles for localised, on demand delivery of therapeutics. To this end, an external magnetic field can be applied to direct the MAP carrying the drug to the desired region. Once there, the MAP allows for spatiotemporal modifications by applying an external magnetic field that tunes the drug release and concentration depending on metabolic changes with a minimal invasive surgery [189; 214]. These hydrogels must be shear-thinning and self-recoverable modulating the cross-linking degree [215]. Another advantage of these systems is that they prevent hitting the drug concentration peak which is metabolically undesirable. Instead, these MAPs allow for controlled drug delivery systems that are capable of keeping a constant drug concentration for longer, which is beneficial for the treatment of illnesses that follow a rhythmic pattern such as diabetes [189; 216; 217; 218]. Thus, these systems can be applied to release drugs for extended periods of time in illnesses that follow a rhythmic pattern, such as diabetes, depression, hypertension, arthritis or asthma [219; 220].  
In this field in particular, magnetic nanoparticles are extremely attractive for controlling drug delivery because their properties and behaviour can be easily modified by a magnetic field, which can be applied externally [221]. However, the main drawbacks of MNPs for drug delivery are the small amounts of drugs that can be linked to each NP, the possibility of drug deactivation once it is chemically bound to the NP, and the possibility of its immediate uncontrollable passive release (burst effect) [222]. These drawbacks can be addressed by introducing these magnetic particles into a hydrogel. [110] developed and characterised magnetic hydrogels by chemically cross-linking gelatine hydrogels and Fe\({}_{3}\)O\({}_{4}\) nanoparticles and loading with vitamin B12. By the application of an alternating magnetic field, both the swelling rate and the drug release rate were modified. While the magnetic field was turned on, the swelling rate decreased rapidly, even reaching \"deswealing\" rates, while the release rate was increased during the \"on\" times. The authors of [223] developed a novel hybrid magnetic hydrogel with CoFE2O4 nanoparticles as cross-linker agents for carboxymethylcellulose (CMC) polymer as a smart drug delivery system. Using MNPs as the crosslinking agent, the drawback of possible release of MNPs from the hydrogel to the external environment through diffusion was overcome. [224] developed B-cyclodextrintrim/cellulose hydrogel beads for drug delivery, which exhibited rapid swelling-deswelling properties under an external magnetic field (EMF) to remotely control drug release. The efficiency of the hydrogel in the stepwise drug release dose and rate can be controlled by switching on-off the EMF and by adjusting the MNPs concentration. An interesting approach to magnetic-based smart drug delivery is the development of hybrid thermoresponsive magnetic hydrogels. This approach combines both thermoresponsive hydrogels, such as poly(N-isopropylacrylamide) (PNIPAM) with magnetic nanoparticles. MNPs have been shown to generate heat under application of an alternating magnetic field (AMF). The heat produced by the relaxation mechanisms of magnetic nanoparticles influences the swelling and de-swelling behaviour of the thermoresponsive hydrogel, which allows for the controlled release of the encapsulated drug [225].