The orientation and distribution of reinforcing particles in artificial composites are key to enable effective reinforcement of the material in mechanically loaded directions, but remain poor if compared to the distinctive architectures present in natural structural composites such as teeth, bone, and seashells. We show that micrometer-sized reinforcing particles coated with minimal concentrations of superparamagnetic nanoparticles (0.01 to 1 volume percent) can be controlled by using ultralow magnetic fields (1 to 10 milliteslas) to produce synthetic composites with tuned three-dimensional orientation and distribution of reinforcements. A variety of structures can be achieved with this simple method, leading to composites with tailored local reinforcement, wear resistance, and shape memory effects.

Shape change is a prevalent function apparent in a diverse set of natural structures, including seed dispersal units, climbing plants and carnivorous plants. Many of these natural materials change shape by using cellulose microfibrils at specific orientations to anisotropically restrict the swelling/shrinkage of their organic matrices upon external stimuli. This is in contrast to the material-specific mechanisms found in synthetic shape-memory systems. Here we propose a robust and universal method to replicate this unusual shape-changing mechanism of natural systems in artificial bioinspired composites. The technique is based upon the remote control of the orientation of reinforcing inorganic particles within the composite using a weak external magnetic field. Combining this reinforcement orientational control with swellable/ shrinkable polymer matrices enables the creation of composites whose shape change can be programmed into the material's microstructure rather than externally imposed. Such bioinspired approach can generate composites with unusual reversibility, twisting effects and site-specific programmable shape changes.

The assembly of complex structures out of simple colloidal building blocks is of practical interest for building materials with unique optical properties (for example photonic crystals and DNA biosensors) and is of fundamental importance in improving our understanding of self-assembly processes occurring on molecular to macroscopic length scales. Here we demonstrate a self-assembly principle that is capable of organizing a diverse set of colloidal particles into highly reproducible, rotationally symmetric arrangements. The structures are assembled using the magnetostatic interaction between effectively diamagnetic and paramagnetic particles within a magnetized ferrofluid. The resulting multipolar geometries resemble electrostatic charge configurations such as axial quadrupoles ('Saturn rings'), axial octupoles ('flowers'), linear quadrupoles (poles) and mixed multipole arrangements ('two tone'), which represent just a few examples of the type of structure that can be built using this technique.