Human skin must flex and stretch to accommodate the body’s every move. Anything worn tight on the body must also be able to flex around muscles and joints, which helps explain why synthetic fabrics like spandex are popular in active wear. Wearable electronic devices that aim to track and measure the body’s movements must possess similar properties, yet integrating rigid electrical components on or within skin-mimicking matrix materials has proven to be challenging. Such components cannot stretch and dissipate forces like soft materials can, and this mismatch in flexibility concentrates stress at the junction between the hard and soft elements, frequently causing wearable devices to fail.
Now, a collaboration between the lab of Jennifer Lewis, Sc.D. at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering at Harvard University and J. Daniel Berrigan, Ph.D. and Michael Durstock, Ph.D. at the US Air Force Research Laboratory has created a new additive manufacturing technique for soft electronics, called hybrid 3D printing, that integrates soft, electrically conductive inks and matrix materials with rigid electronic components into a single, stretchable device.
“With this technique, we can print the electronic sensor directly onto the material, digitally pick-and-place electronic components, and print the conductive interconnects that complete the electronic circuitry required to ‘read’ the sensor’s data signal in one fell swoop,” says first author Alex Valentine, who was a Staff Engineer at the Wyss Institute when the study was completed and is currently a medical student at the Boston University School of Medicine. The study is published in Advanced Materials.
The stretchable conductive ink is made of thermoplastic polyurethane (TPU), a flexible plastic that is mixed with silver flakes. Both pure TPU and silver-TPU inks are printed to create the devices’ underlying soft substrate and conductive electrodes, respectively.
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