Is 3D printing the solution to design hurdles in wearable tech?
A new technique combines precision printing of stretchable conductive inks with pick-and-place manufacturing of electronic components to make flexible, wearable sensors.
Wearable electronic devices that aim to track and measure the body's movements need to be able to flex and move with the body, yet integrating rigid electrical components on or within skin-mimicking matrix materials has proven to be challenging. This mismatch in flexibility concentrates stress at the junction between the hard and soft elements, frequently causing wearable devices to fail.
One team of researchers believe the answer could be hybrid 3D printing, which integrates soft, electrically conductive inks and matrix materials with rigid electronic components into a single, stretchable device.
Jennifer Lewis at the Wyss Institute for Biologically Inspired Engineering at Harvard University has teamed up with J. Daniel Berrigan and Michael Durstock, at the US Air Force Research Laboratory to study the approach.
“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,” said first author Alex Valentine, who was a staff engineer at the Wyss Institute when the study was completed. The study is published in Advanced Materials.
"The silver flakes in the conductive ink align themselves on top of one another like overlapping leaves on a forest floor"
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.
“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,” said Valentine. “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.
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.
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 serve to 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 LEDs attached to a flat TPU sheet created using this method was able to be repeatedly bent into a cylindrical shape without 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 the 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.