Functional fibers in fabric
Fabrics are among the most common and adaptable materials in history: Yarns can be made from plant fibers, animal wool, glass filaments, and synthetic polymers. And those yarns can be woven or knitted into resilient, lightweight, breathable, and foldable sheets. To those advantageous properties, mechanical engineer Rebecca Kramer-Bottiglio and her Yale University colleagues have added another—robotic.
For decades, engineers have developed flexible sensors, electrically conductive foils, nanotubes, and particles as building blocks for wearable devices (see Physics Today, May 2019, page 16 , and October 2008, page 18 ). But those earlier works focused on passive devices that are soft and flexible but don’t move on their own. The Yale researchers have now introduced a set of functional, active fibers that can be integrated into conventional fabrics and transform them into controllable robots and self-folding structures.
T. Buckner et al., Proc. Natl. Acad. Sci. USA, 2020, doi:10.1073/pnas.2006211117
The Yale group started with a fiber made of the shape-memory alloy nitinol. Electrically conductive, nitinol can be programmed to remember a particular shape of interest once it’s heated. Kramer-Bottiglio and coworkers flattened the alloy into a planar ribbon to prevent it from twisting and then integrated the ribbon into fabrics using couching, a common stitching technique. Antagonistic actuators allow the fabric to revert to its original shape. To give an alloy-embedded fabric some structure, the researchers used variable-stiffness (VS) fibers made from thermally responsive epoxy that softens and stiffens on opposite sides of a glass phase transition. Those VS fibers allow the fabric to make move-and-hold operations and sustain loads that would otherwise collapse on conventional fabric. Finally, to create a sensor, the group used a self-coagulating conductive ink they painted directly on the fabric. Composed essentially of bathtub caulk emulsified in a nanoparticle suspension, the ink bonds to individual fibers and makes them exquisitely sensitive to changes in strain.
To demonstrate the utility of such components, the researchers integrated them into conventional fabrics. For example, their tourniquet consists of a breathable fabric sleeve with rows of parallel ribbon actuators, VS fibers, and conductive-ink strain sensors. When the sleeve is severed along one of the ink traces, the damage is picked up as a broken circuit that prompts the fabric to squeeze at pressures up to 2 kPa (see the video ). That’s high enough to combat orthostatic hypotension, or high blood pressure. The researchers envision that the sleeve, powered by a battery, could find use in military and exploratory environments where medical aid is not immediately available.
Another prototype, shown here, is a shape-changing sheet. Initially flat, square, and napkin-sized, the sheet softens its VS frame when activated by temperature and then lifts itself up and stiffens into a rigid structure that supports up to 50 grams (see the video above). The structure can be softened again to change shape or return to its initial configuration, with each cycle taking about 90 s to complete. Carrying its own 11 V, 650 mA battery, the fabric can perform 25 shape changes on a single charge. The researchers imagine rolls of robotic fabric used to make self-reconfiguring machinery. (T. Buckner et al., Proc. Natl. Acad. Sci. USA, 2020, doi:10.1073/pnas.2006211117 .)
