Our group is exploring the use of magnetic fields as a tool for actuation in cardiac function. In the cover image of the April 15 issue of Biophysical Journal, we visualized what happens to magnetic nanoparticles after they have been applied to cardiomyocytes and are subjected to a low-strength uniform static magnetic field along the plane of the cells.

The image was obtained by using an inverted fluorescence microscope at 60´ magnification and shows the engineered tissue of human stem cell–derived cardiomyocytes (actin cytoskeleton in green, nuclei in blue, and channelrhodopsin in yellow) and the magnetic nanoparticles (orange and red) with which they were treated. Before capturing the image, we used a Halbach array to apply a magnetic field of ~45 mT to the cells, and we saw that the magnetic nanoparticles that attached to the cells formed clusters, which instantaneously aligned to the direction of the imposed field. From this observation, we think that the structure of the magnetic nanoparticle clusters and the forces exerted by the field on the cells through these clusters might have contributed to the observed effects on the cardiac electromechanical waves that we found in our report. Our group is actively working to uncover the precise mechanisms.

The ability to use a simple combination of low-strength magnetic fields and magnetic nanoparticles to modulate cardiomyocyte behavior is an exciting new discovery. With further development of the idea, this approach could be used to improve tissue-engineering efforts in the heart or to steer waves to treat cardiac arrhythmias.

 —Maria R. Pozo, Yuli W. Heinson, Christianne J. Chua, and Emilia Entcheva