Anisotropic Contraction in a Magnetically Hard but Mechanically Ultra-Soft Foam for Precise Drug Delivery

Hard magnetic soft materials (HMSMS) have been recently intensively explored in soft robots, owing to its native advantages such as untethered, rapid and reversible actuation, as well as large shape changes. However, in the existing studies, HMSMS is majorly applied by a magnetic field to produce bending, folding, and even twisting, that is, shape morphing, to realize multimodal locomotion. Meanwhile, such as contraction deformation, which may be employed in precise drug delivery, has been neglected in a long term. Here, an anisotropic contraction in a porous structure, hard magnetic foam (HMF), which is mechanically ultra-soft (with a shear modulus of 4.5 kPa) but magnetically hard (with a residual magnetic flux density of 12 mT), is reported. The investigation of HMF structures indicates that anisotropic contraction is dominated by the magnetodeformational effect and the magnetic body-force against support constraint. By adjusting the magnetic fields directions, the relationship between these two factors can be regulated to achieve a maximum contraction of 43% (synergy) or negligible deformation (antagonism). Finally, HMF is utilized to realize precision drug delivery since it has the advantages of no leakage during moving, precisely adjustable drug release rate (from 0.008 to 0.62 mL min⁻¹), and no residues after leaving.