Hybrid AM method prints flexible, durable wearable devices

4 A complete hybrid 3D-printed device flexes and conforms to the body’s shape. Credit: Alex Valentine, Lori K. Sanders, and Jennifer Lewis / Harvard University

A collaborative research team has discovered a new hybrid 3D printing technique that will help make flexible and durable wearable devices.

The 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, 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.

The technique 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 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.

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. 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, Ph.D., 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 those data, as well as 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 (through which ink is normally dispensed) to pick up electronic components and place them onto the substrate surface in a specific, programmable manner.

For the most part, surface-mounted electrical components tend to be hard and rigid. So the team was able to take 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 serve to anchor these rigid components and distribute stress throughout the entire matrix, allowing the fully assembled devices to be stretched up to 30 per cent while still maintaining function.

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, resulting in a wearable sleeve-like device that indicates how much the wearer’s arm is bending through successive lighting-up 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, whose 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.

The team believes that this is an important first step toward making customizable, wearable electronics that are lower-cost and mechanically robust.

“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. We believe that this is an important first step toward making customizable, wearable electronics that are lower-cost and mechanically robust,” 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.

Additional authors of the study include Travis Busbee, a graduate student in the Lewis lab and co-founder of Voxel8; Jordan Raney, Ph.D., former postdoc in the Lewis lab and current Assistant Professor in the School of Engineering and Applied Sciences at the University of Pennsylvania; Alex Chortos, Ph.D., and a postdoc in the Lewis lab; Arda Kotikian, a Graduate Research Fellow in the Lewis lab.

https://wyss.harvard.edu