Magneto-mechanical effect of magnetic microhydrogel for improvement of magnetic neuro-stimulation

Superparamagnetic iron oxide (SPIO) nanoparticles play an important role in mediating precise and effective magnetic neurostimulation and can help overcome limitations related to penetration depth and spatial resolution. However, nanoparticles readily diffuse in vivo, decreasing the spatial resolution and activation efficiency. In this study, we employed a microfluidic means to fabricate injectable microhydrogels encapsulated with SPIO nanoparticles, which significantly improved the stability of nanoparticles, increased the magnetic properties, and reinforced the stimulation effectivity. The fabricated magnetic microhydrogels were highly uniform in size and sphericity, enabling minimally invasive injection into brain tissue. The long-term residency in the cortex up to 22 weeks and the safety of brain tissue were shown using a mouse model. In addition, we quantitatively determined the magneto-mechanical force yielded by only one magnetic microhydrogel using a video-based method. The force was found to be within 7–8 pN under 10 Hz magnetic stimulation by both theoretical simulation and experimental measurement. Lastly, electrophysiological measurement of brain slices showed that the magnetic microhydrogels offer significant advantages in terms of neural activation relative to dissociative SPIO nanoparticles. A universal strategy is thus offered for performing magnetic neuro-stimulation with an improved prospect for biomedical translation.