Flexible Conductive Composites with Programmed Electrical Anisotropy Using Acoustophoresis

Developing mechanically flexible composite materials with high electrical conductivity is currently hindered by the need to use high loading of conductive filler, which severely limits flexibility. Here, acoustic focusing is used to control arrangement of conductive particles in photopolymer matrices to create composites with both tunable conductivity and flexibility. Acoustophoresis patterns filler particles into highly efficient percolated networks which utilize up to 97% of the particles in the composite, whereas the inefficient stochastic networks of conventional dispersed-fiber composites utilize <5%. These patterned materials have conductivity an order of magnitude higher than conventional composites made with the same ink, reaching 48% the conductivity of bulk silver within the assembled silver-particle networks (at 2.6 vol% loading). They also have low particle loading so that they are flexible, withstanding >500 bending cycles without losses in conductivity and changing conductivity only 5% within cycles on average. In contrast, conventional unpatterned composites with the same conductivity require such high loading that they are prohibitively brittle. Finally, modulating the applied acoustic field controls the anisotropy of the conductive networks and produces materials which are either 2D conductive, 1D conductive, or insulating, using the same nozzle and ink, paving the way for versatile multifunctional 3D printing.