The skin is an amazing organ with a high level of flexibility that’s difficult to reproduce synthetically. Fabrics like Spandex can mimic the stretchability of skin, but when it comes to wearable electronics, that flexibility gets tricky. Devices that are worn on the skin need to be able to move and stretch with the skin, but that’s difficult to do and still integrate the kinds of functional electronics required for such devices. Generally, electrical components are rigid and hard, and the difference in flexibility between them and more elastic substrates causes stress that makes many wearable electronics fail.
Jennifer Lewis, Sc.D. of the Wyss Institute for Biologically Inspired Engineering at Harvard University and the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) is an expert in advanced materials and 3D printing, having worked on everything from ceramic foam material to bioprinted kidney subunits. She and her lab partnered with J. Daniel Berrigan, Ph.D. and Michael Durstock, Ph.D. at the US Air Force Research Laboratory to develop a new 3D printing technique for soft electronics, called hybrid 3D printing. The technique combines soft, electrically conductive inks and matrix materials with rigid electronic components into one stretchable device.
“Our team has demonstrated a new approach for creating low-cost wearable devices,” Lewis told 3DPrint.com.
“We fabricated soft sensors using our hybrid 3D printing method, which allows stretchable conductive inks and soft matrices to be printed and integrated with surface mount electrical components, such as LEDs, resistors, and microprocessors, that are digitally ‘pick-and-placed.’ Our soft sensors can be printed directly onto textiles to enable monitoring of human motion during sporting activities or rehab.”
The conductive, stretchable 3D printing material is made from TPU mixed with silver flakes. Pure TPU and silver TPU are both 3D printed to create, respectively, the device’s flexible substrate and conductive electrodes.
“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,” explains first author Alex Valentine. “After the solvent evaporates, both of the inks solidify, forming an integrated system that is both flexible and stretchable.” Valentine, a Staff Engineer at the Wyss Institute when the study was completed, is now a medical student at the Boston University School of Medicine.
When the silver TPU is 3D printed, the silver flakes align themselves along the printing direction so that they overlap each other like plates, improving their ability to conduct electricity along the 3D 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,” said Will Boley, a postdoctoral researcher in Lewis’ lab at SEAS.
If you’ve ever wondered how wearable electronics obtain the data they provide about the human body, it’s a multifaceted process. Soft, conductive sensors exhibit changes in their electrical conductivity when stretched, which is how they detect movement. A programmable microcontroller chip then processes that data and communicates it via a readout device. To get this process to work for their device, the researchers combined the 3D printed soft sensors with a digital pick-and-place process that applies a vacuum through an empty printing nozzle to pick up electronic components and place them onto the substrate surface in a specific, programmable manner.
The electrical components included parts like LEDs, resistors and micro-chips, which are hard and rigid, so to impart more flexibility, the researchers applied a dot of TPU printing material beneath each component before attaching it to the stretchy TPU substrate. Once they dried, the TPU dots anchored the components and helped to distribute stress throughout the entire matrix, allowing the devices to be stretched up to 30% while still maintaining their function. For example, a device made of 12 LEDs attached to a flat TPU sheet could be repeatedly bent into a cylindrical shape without mechanical failure or reduction in the light given off by the LEDs.
Two soft electronic devices were created in order to demonstrate the capabilities of the 3D printing technique. A strain sensor was created by 3D printing TPU and silver TPU electrodes onto a textile base and applying a microcontroller chip and readout LEDs via the pick-and-place method. This resulted in a sleeve-like device that indicates how much the wearer’s arm is bending through successive lighting up of the LEDs. They also created a pressure sensor in the shape of a person’s left foot. Alternating layers of conductive silver TPU and insulating TPU were printed in order to form electrical capacitors on a soft TPU substrate, and a manual electrical readout system processes the substrate’s deformation patterns to form a heat map of the foot.
“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,” said Lewis. “We believe that this is an important first step toward making customizable, wearable electronics that are lower-cost and mechanically robust.”
“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,” added 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.”
The research is published in a paper entitled “Hybrid 3D Printing of Soft Electronics”, available here. Authors include Alexander D. Valentine, Travis A. Busbee, John William Boley, Jordan R. Raney, Alex Chortos, Arda Kotikian, John Daniel Berrigan, Michael F. Durstock, and Jennifer A. Lewis. The work received support from 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. Discuss in the Hybrid 3D Printing forum at 3DPB.com.[Images: Alex Valentine, Lori K. Sanders, and Jennifer A. Lewis / Wyss Institute at Harvard University]