Knitting has picked up a few new tricks.
Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences have used conventional knitting techniques to make fabrics that can change shape, sense movement and act as switches.
The team built specialized machine-knitted fabrics that can “snap” from one stable shape to another. Physicists call that multistability.
The research was led by recent Ph.D. graduate Kausalya Mahadevan, 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.
“I’ve always been excited about fabrics and textiles, and what we can engineer and build with them,” Mahadevan said.
“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.”
Instead of molding polymers and controlling residual stresses, the researchers used weft knitting, the industrial method commonly used to make hats and gloves, to create curved structures from yarn alone.
They chose highly elastic yarns and used a knitting method called plating, which puts different yarns on opposite faces of the textile. The combination produced dense, thick fabrics that naturally curl into three-dimensional forms.
“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.
The researchers arranged horizontal and vertical stripes in systematic combinations. That let them create textiles that could snap between different configurations and stay stable in each one, like a light switch staying on or off.
By studying the fabric’s geometry and material properties, the team identified the physical conditions that let knitted textiles become multistable. They also simulated the behavior by treating each textile as a continuous material instead of modeling every strand of yarn.
The team then added thin conductive yarns. That turned the knitted structures into soft, stretchable electric switches that change electrical state when the fabric snaps from one configuration to another.
In one demonstration, the researchers made a multistable knitted shell that switches an LED on and off as the textile moves between stable states.
They also made a wearable textile switch that can sit over a knee or elbow. The snapping movement created as the joint bends can be detected by an Arduino and used to count steps.
Another prototype was a reconfigurable lamp shade with three separate multistable switches. Each switch controlled a different light color as parts of the fabric were stretched and snapped between configurations.
The devices, along with other examples from the project, were featured in a recent Art Lab installation.
The researchers said the machines used in the work are similar to industrial knitting equipment already used in garment factories, suggesting the approach could potentially be scaled up relatively quickly.
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.
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