Hybrid 3D printing with thermoplastic urethane creates low-cost, mechanically robust wearables 

A frequent cause of failure in wearable electronic devices is the mismatch between rigid electrical components and soft, flexible materials that conform to the body’s movements. This disparity in flexibility concentrates stress at the junction between the hard and soft elements. Now, researchers have created a new additive manufacturing technique for soft electronics, called hybrid 3D printing, that integrates soft, electrically conductive inks and thermoplastic urethane (TPU) with rigid electronic components into a single, stretchable device. It is described by the researchers as an important first step toward making customizable, wearable electronics that are lower in cost than current devices while being mechanically robust.

hybrid 3D printing, Wyss Institute
A device composed of 12 light-emitting diodes (LEDs) attached to a flat thermoplastic urethane sheet produced by hybrid 3D printing was repeatedly bent into a cylindrical shape without causing a reduction in the intensity of the LEDs’ light or mechanical failure of the device. Image courtesy Alex Valentine, Lori K. Sanders and Jennifer Lewis at Harvard University.

The breakthrough is the result of a collaboration between the lab of Jennifer Lewis, Sc.D., at the Wyss Institute for Biologically Inspired Engineering at Harvard University and the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and J. Daniel Berrigan, PhD, and Michael Durstock, PhD, at the US Air Force Research Laboratory. A press release on the Wyss Institute website, which is reprinted below, describes the research in some detail. A paper also has been published in the Advanced Materials journal.

The stretchable conductive ink is made of TPU mixed with silver flakes. Both pure TPU and silver-TPU inks are printed to create, respectively, the devices’ underlying soft substrate and conductive electrodes, explains the press release. “Because both the substrate and the electrodes contain TPU, when they are co-printed layer-by-layer they strongly adhere to one another prior to drying,” says 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. “After the solvent evaporates, both of the inks solidify, forming an integrated system that is both flexible and stretchable.”

The printing process causes the silver flakes in the conductive ink to align themselves along the printing direction, so their flat, plate-like sides layer on top of one another, like overlapping leaves on a forest floor. This structural alignment improves their ability to conduct electricity along the printed electrodes. “Because the ink and substrate are 3D printed, we have complete control over where the conductive features are patterned, and can design circuits to create soft electronic devices of nearly every size and shape,” says Will Boley, PhD, a postdoctoral researcher in the Lewis lab at SEAS and co-author of the paper.

Soft sensors composed of conductive materials that exhibit changes in their electrical conductivity when stretched, which is how they detect movement, are coupled with a programmable microcontroller chip to process the data and a readout device that communicates the data in a form humans can understand. To achieve this, the researchers combined the printed soft sensors with a digital pick-and-place process that applies a modest vacuum through an empty printing nozzle to pick up electronic components and place them onto the substrate surface in a specific, programmable manner.

Because these surface-mounted electrical components are by nature hard and rigid, the team took advantage of TPU’s adhesive properties by applying a dot of TPU ink beneath each component prior to attaching it to the underlying soft TPU substrate. Once dried, the TPU dots anchor these rigid components and distribute stress throughout the entire matrix, allowing the fully assembled devices to be stretched up to 30% while still maintaining function. A device composed of 12 light-emitting diodes (LEDs) attached to a flat TPU sheet created using this method was able to be repeatedly bent into a cylindrical shape without causing a reduction in the intensity of the LEDs’ light or mechanical failure of the device.

As a simple proof-of-concept, the team created two soft electronic devices to demonstrate the full capabilities of this additive manufacturing technique. A strain sensor was fabricated by printing TPU and silver-TPU-ink electrodes onto a textile base and applying a microcontroller chip and readout LEDs via the pick-and-place method. The resulting wearable sleeve-like device indicates how much the wearer’s arm is bending through successive light ups of the LEDs. The second device, a pressure sensor in the shape of a person’s left footprint, was created by printing alternating layers of conductive silver-TPU electrodes and insulating TPU to form electrical capacitors on a soft TPU substrate. The deformation patterns are processed by a manual electrical readout system to make a visual heat map image of the foot when a person steps on the sensor.

While the team is continuing to optimize both their materials and methods, hybrid 3D printing is broadly applicable to manufacturing myriad electronic devices. “We have both broadened the palette of printable electronic materials and expanded our programmable, multi-material printing platform to enable digital pick-and-place of electronic components,” says Lewis, who is the corresponding author of the paper, a Core Faculty member at the Wyss Institute, and the Hansjörg Wyss Professor of Biologically Inspired Engineering at SEAS.

“This new method is a great example of the type of cross-disciplinary collaborative work that distinguishes the Wyss Institute from many other research labs,” says Wyss Founding Director Don Ingber, M.D., Ph.D., who is also the Judah Folkman Professor of Vascular Biology at Harvard Medical School and the Vascular Biology Program at Boston Children’s Hospital, as well as a Professor of Bioengineering at Harvard SEAS. “By combining the physical precision of 3D printing with the digital programmability of electronic components, we are literally building the future.”

This research was supported by the Air Force Research Laboratory Materials and Manufacturing Directorate and UES, the Vannevar Bush Faculty Fellowship Program under the Office of Naval Research, a generous donation from the GETTYLAB, and the Wyss Institute at Harvard University.

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