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Fiber‐filled composite materials offer a unique pathway to enable new functionalities for systems built via extrusion‐based additive manufacturing (or “3D printing”); however, challenges remain in controlling the fiber orientations that govern ultimate performance. In this work, a multi‐material, shape‐changing nozzle—constructed by means of PolyJet 3D printing—is presented that allows for the spatial distribution of short fibers embedded in polymer matrices to be modulated on demand throughout extrusion‐based deposition processes. Specifically, the nozzle comprises flexible bladders that can be inflated pneumatically to alter the geometry of the material extrusion channel from a straight to a converging–diverging configuration, and in turn, the directional orientation of fibers within printed filaments. Experimental results for printing carbon microfiber‐hydrogel composites reveal that increasing the nozzle actuation pressure from 0 to 100 kPa reduced the proportion of aligned fibers, and notably, prompted a transition from anisotropic to isotropic water‐induced swelling properties (i.e., the ratio of transverse to longitudinal swelling strain decreased from 1.73 ± 0.37 to 0.93 ± 0.39, respectively). In addition, dynamically varying the nozzle geometry during the extrusion of continuous composite filaments effects distinct swelling behaviors in adjacent regions, suggesting potential utility of the presented approach for emerging “4D printing” applications.

Mechanical metamaterials are architected manmade materials that allow for unique behaviors not observed in nature, making them promising candidates for a wide range of applications. Existing metamaterials lack tunability as their properties can only be changed to a limited extent after the fabrication. Herein, a new magneto‐mechanical metamaterial is presented that allows great tunability through a novel concept of deformation mode branching. The architecture of this new metamaterial employs an asymmetric joint design using hard‐magnetic soft active materials that permits two distinct actuation modes (bending and folding) under opposite‐direction magnetic fields. The subsequent application of mechanical compression leads to the deformation mode branching where the metamaterial architecture transforms into two distinctly different shapes, which exhibit very different deformations and enable great tunability in properties such as mechanical stiffness and acoustic bandgaps. Furthermore, this metamaterial design can be incorporated with magnetic shape memory polymers with global stiffness tunability, which also allows for the global shift of the acoustic behaviors. The combination of magnetic and mechanical actuations, as well as shape memory effects, impart wide tunable properties to a new paradigm of metamaterials.