Theoretical description of a responsive magneto-hydrogel transduction principle

A theoretical description and analysis of a novel responsive hydrogel transduction principle is given. It enables a very accurate measurement of the volume change of a stimulus-responsive hydrogel and might be used as an in-body sensor system. The hydrogel contains anchored magnetic nanoparticles and is deposited onto a substrate in which a GMR (giant magneto resistance) element is embedded. The sensor layout is such that a maximum signal is obtained when the hydrogel thickness is only 5 μm, enabling a fast sensor response. The GMR sensor is operated at 1 MHz to suppress noise. When the frequency dependence of the susceptibility is taken into account, the theory correctly describes the sensor response. To measure the response, a Fe3O4 nanoparticle dispersion having a susceptibility of 10 at 1 MHz has been used. The response curve of the sensor has been measured by varying the concentration of this dispersion. For a responsive hydrogel loaded with 1 vol% of these nanoparticles, an expansion of only 0.3‰ is theoretically still detectable. Experimental work with magneto-hydrogels to further validate this result will be performed in the near future.