Magnetically Remanent Hydrogels with Colloidal Crosslinkers

Hydrogels are widely used in biomedical applications such as drug delivery and tissue engineering. In this research, the feasibility of a hydrogel with embedded magnetic nanoparticles, also called a ferrogel, for biosensor applications was tested. A pH sensitive poly(acrylic acid) hydrogel was used where the magnetic nanoparticles serve to translate pH-induced volume changes of the ferrogel into magnetic field changes. Swelling and shrinking cycles of a ferrogel could cause leakage of the nanoparticles, due to enhanced diffusion as a result of uptake and expulsion of large amounts of water during such cycles. Parting magnetic nanoparticles cause loss of functionality of the ferrogel, therefore, the surface of the nanoparticles was functionalized to incorporate the nanoparticles in the hydrogel network, so that the particles function as a crosslinker and are fixed in the hydrogel network. To obtain a magnetically remanent ferrogel that only needs a single magnetization treatment prior to the use of the ferrogel, we incorporated magnetic nanoparticles with slow thermal relaxation of the magnetic dipole moment inside the nanoparticle, the so-called Néel relaxation. Fixation of nanoparticles in the hydrogel network and slow relaxation of the magnetic dipole moment enable magnetic remanence of the ferrogel when magnetized with an external magnet. To predict the magnetic field of a ferrogel disk, theory has been developed. The magnetic field dependence on ferrogel content and geometry is calculated and confirmed by experiments: the measured magnetic field increases as a function of nanoparticle concentration and increases for thicker ferrogels. However, the ratio of radius to thickness of the ferrogel should be taken into acount, as this becomes more important at small distances between the ferrogel and the sensor. For optimal results, one should decrease the size of the ferrogel disk and optimize the distance between the sensor and the ferrogel disk. Thin ferrogel layers were swollen and shrunken to equilibrium in three consecutive cycles showing reversibility and reproducibility. Theory to calculate the ferrogel thickness as a function of time and pH has been presented, and simulations are in good agreement with experimental data for ferrogel swelling. On the other hand, for shrinking of a ferrogel, more processes appeared to be involved than only simple diffusion. Swelling and shrinking experiments for magnetic field measurements showed interesting results: it had been expected that for dilution of the nanoparticles as a result of swelling, the magnetic field would decrease, whereas for a shrinking ferrogel, the magnetic field would increase, as the nanoparticle concentration increases again. However, swelling of a ferrogel initially resulted in an overall increase of the magnetic field with a maximumat 30% swelling, and a shrinking ferrogel mainly showed decrease of the magnetic field. These observations were discussed on the basis of a swelling-induced enhancement effect, where the orientation of nanoparticle dipole structures changes during stretching of the polymer network as a result of swelling and causes an increasing magnetic field. In conclusion, the current ferrogel is sensitive to pH, but interpretation of changes of the magnetic field in terms of pH changes of the surroundings is not straightforward.