**4. Ionic liquids as carriers for magnetic fluids**

450 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

as semi-active (Jolly et al., 1999).

used in semi-active human prosthetic legs.

particle size does not increase (Tran et al., 2010).

The best known application for MRFs is their use as lubricants with field-dependent viscoelasticity in shock absorbers or dampers. It was in the early 1990s when *Lord Corporation* started commercializing the first MR dampers. Since then, further progress has followed, and commercial MR dampers have become available for large trucks or race and high-quality cars (Klingenberg, 2001). A typical monotube design of a MR damper used in vehicles was reported by Jolly et al. (Jolly et al., 1999). As shown in this previous work, this kind of MR damper has one reservoir for the MR fluid and an accumulator receptacle full with a compressed gas, which is used to accommodate the volume changes due to the entrance of the piston rod into the housing. When needed, an electric current in the coil generates a magnetic field in the required direction that makes the MRF change its rheological behaviour to a *solid-like* one; as a result, the vibrations provoked by external forces during driving are absorbed. Therefore, this kind of damping function can be labeled

The technological applications of MR dampers are not only restricted to vehicles. For example they are also used as shock/vibration absorbers in structures (i.e. seismic control of buildings or bridges). In this particular case, the stability against sedimentation plays a very important role, since the damper is expected to remain inactive most of its lifetime (Jolly et al., 1999; Klingenberg, 2001; Park et al., 2010). In addition, MR shock absorbers can also be

In the case of FFs, many biomedical applications have been described, which take advantage not only of their superparamagnetic behaviour, but also of their high surface-volume ratio, high reactivity, etc. Medical applications include cell labeling and targeting, separation and purification of cell populations, tissue repair, targeted drug delivery, magnetic resonance imaging or hyperthermia for cancer treatment (Durán et al., 2007; Tran et al., 2010). In all these applications biocompatibility and non-toxicity are of crucial importance. For this reason, iron oxides are preferred as the material for the dispersed phase and, in addition, they are often made biocompatible by means of surface coating by polymers (PEG, dextran, polyvinyl alcohol) or functional groups (thiols, amines, and carboxyls). All these additives prevent from particle aggregation too, which should be almost completely avoided so that

From a more engineering point of view, the control of both the position and the physicochemical properties of FFs by using magnets or solenoids makes them very interesting too. In fact, FFs have been used as lubricants, heat transfer agents or integrated in devices such as dynamic seals, dampers, magnetic inkjets or optic devices. As an example, companies like *Ferrotec* have successfully commercialised FF-based dynamic seals for 40 years. Dynamic seals are used when two different environments have to be isolated one from each other but energy has to be carried between them, by a shaft for example (see figure 6). Here, FFs can be used as filling materials, since they hermetically isolate both environments due to their fixed position if the hole for the shaft is made into a magnet. An important advantage of using a seal like this, apart from being almost completely hermetic, is a prolongation of the seal lifetime, since friction between its rotating and stationary components is almost negligible (i.e. the FF acts as a lubricant agent too) (Raj et al., 1990).

Traditionally, organic carriers (mineral oil, kerosene, etc.) have been used in the preparation of MFs, although in some cases (i.e. medical applications) water-based MFs are needed too. These conventional MFs have proved to be useful for many applications as pointed out above. However, the use of novel ionic liquids (ILs) as carriers may promote further improvements in the performance of MFs, especially in ultra-high vacuum applications, in which organic solvents would easily evaporate. But the preparation of IL-based MFs is not only interesting from an applied, technological approach, but also from the fundamental one. For example, their use as carriers allows studying phenomena involving MFs in environmental conditions that conventional carriers would not withstand. But, what are ILs and why do they appear as promising candidates in the preparation of MFs? These and other questions are discussed in next sections.
