Liquid metal networks as an ideal platform for stretchable electronics
(Nanowerk Spotlight) The burgeoning fields of e-textiles, soft robotics, and stretchable electronics – that can be bent, folded, crumpled and stretched – are major research areas towards next-generation wearable and implantable devices.
Unlike today's electronics based on rigid silicon technologies, stretchable devices can conform to almost any kind of surface shapes and provide unique functionalities which are unreachable with simple extension of conventional technologies.
A key component of stretchable devices are the conductors and several strategies have been used to fabricate them. They generally fall into two categories: Structures that stretch (such as textiles or buckled substrates) and materials that stretch (such as polymers).
However, most conductors like silver or copper are rigid and won’t stretch past 1-2% before they break. Other conductors that are stretchable, such as polymer composites or conductive polymers, decrease in conductivity as they are elongated.
A novel and very promising design strategy for stretchable electronics is based on liquid metals (read more about liquid metals in our previous Nanowerk Spotlight: "Using liquid metals in nanotechnology"); specifically the use of the oxide shell of liquid gallium alloys to fabricate polymerized liquid metal networks (Poly-LMNs).
The novel attribute of these Poly-LMNs is that they increase in conductivity as they are elongated, resulting in the measured resistance across the conductor remaining nearly unchanged as they are stretched to 700% their original length. The reason for this is that, when stretched, particles within the Poly-LMNs rupture and release their liquid metal payload, resulting in a rapid 108-fold increase in the network’s conductivity.
"The new scientific core of our findings is based on the concept of using the naturally forming oxide skin on the surface of gallium alloy particles as anchor points to attach molecular monomer units that can be chemically linked together to form a polymer network that incorporates the oxide itself," Dr. Christopher E. Tabor, who led this work, explains to Nanowerk. "By doing this, mechanical strain is transferred from the substrate through the entire Poly-LMN, allowing the initial film to transition between individual particles that are not connected to a stretched state whereby each particle 'rips' open and the liquid cores connect to their neighbors. This forms a tortuous conductive path that autonomously responds to strain by changing the volume of the conductive metal path."
He points out that, without this oxide or the ligands that chemically tie the oxide together, the particles would not form the same network and would remain individual isolated particles, providing no conductive path through the particle film.
Formation and activation of polymerized liquid metal networks. Depictions of liquid metal particles at different stages of network formation and activation. Reactive core–shell liquid metal particle with inset showing the outer oxide functionalized with an acrylate-containing phosphonic acid ligand (outer left). Polymerized particles in a highly cross-linked network with inset showing acrylate-based polymer connections (inner left). Strain releases liquid metal from ruptured particles to form a contiguous conductive network (inner right). Relaxed liquid metal network structures are compressed from the release of strain (outer right). (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
Poly-LMNs retain their network of interconnectivity when they returned to a zero-strain state and thus remain electrically conductive. The implication of this effect is that a circuit can now be designed with a stretchable Poly-LMN wire that won’t change its resistance when stretched. This should eliminate excessive compensation electronics that would otherwise have to 1) measure the change in resistance as the device is strained and then 2) change the electronic inputs such as an applied voltage in real-time in order to accommodate that change.
During their tests, the researchers also found that Poly-LMN resistances were stable over 10 000 cycles of 0%–100% strain.
"Liquid metals have been used for several years to achieve highly stretchable electronics, such as embedded liquid wires from Professor Michael Dickey at NC State and composite materials such as the Liquid Metal Embedded Elastomers from Prof. Carmel Majidi’s group at Carnegie Mellon University," notes Tabor. "However, the existence of the liquid metal as a fluid requires an encasing material to keep it in place. We wanted to utilize the unique mechanical properties of the surface oxide that forms spontaneously on the surface of the liquid metal to design new systems that have unique properties, such as our Poly-LMNs."
The use of liquid metals as a stretchable conductor fundamentally changes the paradigm of what commonly fails first in a stretchable circuit: In a traditional stretchable circuit the solid conductor will mechanically fail; whereas the substrate will mechanically fail first when using a liquid metal instead.
The researchers are currently working on understanding how encapsulation effects the properties of the Poly-LMN and the fundamental science of the interfacial properties between the Poly-LMN and the substrate. With regard to devices, they are targeting applications in wearable electronics in the short term, such as looking to try to integrate this material as a consistent resistive heating element in textiles covering joints or other areas of the body where the fabric stretches to a significant degree.
"For the research field of flexible electronics as a whole, integration and connectivity remain challenging," Tabor concludes. "The difficulty of moving from rigid traditional high-performance electronics based on silicon to wearable textiles that undergo numerous cycles of strain is inherently difficult. We are exploring how the Poly-LMN and other liquid metal systems may help connect two materials with very different moduli."