  We propose a new concept of magnetic focusing for targeting and accumulation
of functionalized superparamagnetic nanoparticles in living organs through
composite configurations of different permanent magnets. The proposed setups
fulfill two fundamental requirements for in vivo experiments: 1) reduced size
of the magnets to best focusing on small areas representing the targeted organs
of mice and rats and 2) maximization of the magnetic driving force acting on
the magnetic nanoparticles dispersed in blood. To this aim, several
configurations of permanent magnets organized with different degrees of
symmetry have been tested. The product B*grad(B) proportional to the magnetic
force has been experimentally measured, over a wide area (20x20 mm^2), at a
distance corresponding to the hypothetical distance of the mouse organ from the
magnets. A non-symmetric configuration of mixed shape permanent magnets
resulted in particularly promising to achieve the best performances for further
in vivo experiments.
