Harvard scientists turn knitting into shape-shifting smart fabric

Knitting is no longer limited to making familiar items such as sweaters, hats, and blankets. Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have shown that conventional knitting techniques can be used to create functional fabrics capable of changing shape, sensing movement, and operating as switches. The approach could help advance a new generation of programmable textiles. The team developed specialized, machine-knitted fabrics that can "snap" from one stable shape to another. Physicists describe this ability to maintain multiple stable configurations as multistability. The research was led by recent Ph.D. graduate Kausalya Mahadevan, who is now a postdoctoral associate in the lab of Katia Bertoldi, the William and Ami Kuan Danoff Professor of Applied Mechanics. The findings were published in Advanced Functional Materials. Bringing Physics Into Knitted Textiles "I've always been excited about fabrics and textiles, and what we can engineer and build with them," said Mahadevan, who began working in Bertoldi's lab as an undergraduate. "Our ideas around multistability in textiles arose from being inspired by textile artists and how they approach structures, combined with how [Bertoldi's] lab has traditionally thought about nonlinear mechanics in solids. We tried to approach thinking about textiles in that context." Materials designed to curve and then hold that form are often produced by molding polymers and carefully controlling residual stresses inside them. In this work, however, the researchers demonstrated that weft knitting, the same industrial method commonly used to produce hats and gloves, can create complex curved structures using yarn alone. The team selected highly elastic yarns and used a knitting method called plating, which places different yarns on opposite faces of the textile. This combination produced dense, thick fabrics that naturally curl into three-dimensional forms. The effect relies on the same basic behavior that can cause the bottom edge of a cut T-shirt to curl upward. "The yarn selection and machine parameter choices allowed us to basically select a fabric that is going to be as snappy as we can possibly get," Mahadevan said. Fabrics That Snap Between Stable Shapes The researchers then arranged horizontal and vertical stripes in systematic combinations. This allowed them to create textiles that could snap between different configurations and remain stable in each one, somewhat like a light switch that stays either on or off. By studying how the fabric's geometry and material properties influenced this snap-through behavior, the team determined the physical conditions that allow knitted textiles to become multistable. They also successfully simulated the behavior by treating each textile as a continuous material rather than attempting to model every individual strand of yarn. The researchers next added thin conductive yarns to demonstrate how the technology could be used in practical devices. These conductive fibers transformed the knitted structures into soft, stretchable electric switches that change electrical state when the fabric snaps from one configuration to another. From Step Counters to Color Changing Lights In one demonstration, the team created a multistable knitted shell that switches an LED on and off as the textile moves between its stable states. They also produced a wearable textile switch that can be placed over a knee or elbow. The snapping movement generated as the joint bends can be detected by an Arduino and used to count steps. Another prototype was a reconfigurable lamp shade containing three separate multistable switches. Each switch controlled a different color of light as portions of the fabric were stretched and snapped between configurations. These devices, along with other examples from the project, were featured in a recent Art Lab installation. The researchers made a reconfigurable lamp shade with multistable switches that correspond to different colors of light. A Potential Path to Scalable Smart Textiles Importantly, the machines used by Mahadevan and her colleagues are similar to the industrial knitting equipment already found in garment factories. That compatibility suggests devices based on the technique could potentially be scaled up relatively quickly. From a scientific perspective, the work also brings textiles closer to the field of nonlinear mechanical metamaterials. These engineered structures are designed to bend, buckle and snap in controlled ways that provide useful functions. Mahadevan sees additional opportunities for using multistability to create fabrics that combine softness, seamless construction and functionality. In the future, the researchers envision textiles that could quietly track body movement, deliver tactile feedback, or change their physical shape when needed. The research was supported by NSF grant DMR-2011754 and ARO MURI program W911NF-22-1-0219. Equipment was supported by ONR DURIP Award N00014-19-1-2220.