Stretchable and self-healing conductors for soft electronics

by Jim Dietz, DoSupply

The demand for soft and conformable electronics is rapidly building. The emerging applications for these next-generation electronic devices, including on-body, on-skin, and biomedical implants, generate high demand. Modern-day electronics require electrical conductors because they are the fundamental building block for stretchable electronic devices and systems. This article will present an overview of conductive, stretchable, and self-healing materials. Specifically, it will explore the nanomaterial MXene and its relevant applications. 

Applications for soft electronics

Stretchable electronic devices have received increasing attention from researchers globally. They can be applied in many innovative fields, such as epidermal electronic devices, biomedical engineering, healthcare monitoring, soft robotics, electronic skins, and human-machine interfaces (HMI). Stretchable electronics’ development has achieved remarkable progress based on nanomaterials’ tremendous growth and new nanofabrication technologies during the past few decades. Stretchable electronics are devices that can add to traditional, rigid, silicon-based electronic devices for interfacing with human skin or curved, deformable interfaces.

Broadly, stretchable electronics consist of integrated circuits on pliable and stretchable material composed of elastomeric fabric. Compared with traditional printed circuit boards, stretchable electronic circuits can mechanically bend, twist, compress, stretch, and self-heal if damaged from being stretched using soft substrate materials to adapt to the attached surface’s contour.

Researchers have found that stretchable and adaptable electrical conductors are “the critical building blocks in potential electronic applications, such as artificial electronic skins, smart sensors, energy harvesters, transistor arrays, light-emitting diode (LED) displays, health monitoring, touch panel, and energy storage devices”. Stretchable conductors must withstand high mechanical strains greater than 50% and high electrical conductivity based on these new generation devices’ performance requirements.

It is essential to understand nanoscale mechanics, material properties, and structure-property relationships, with micro-fabrication and material processing techniques, to visualize the various forms of stretchable conductors. Everyday usage of the device, including stretching, twisting, impact, and temperature fluctuations, can lead to damage. This damage is generally invisible to the naked eye. Still, it will reduce the performance level and serviceable lifespan of soft electronics. The design of these devices must consider self-healing since, visibly, the damage is undetectable. The repair must be automatic, accurate, and able to restore the electronic component to full functionality.

Intrinsically conductive materials for soft electronics

MXenes are newly emerging two-dimensional (2D) nanomaterials. MXenes are transition metal carbides, nitrides, and carbonitrides with the general formula Mn+1XnTx. M is a D Block transition metal (see periodic table below), and X is either carbon, nitrogen, or both. Tx is a functional group of elements that make up the substrate surface fabric.

MXenes is a recent discovery (2011) that has received immense attention. Industries representing electrochemical energy storage, electromagnetic interference (EMI) shielding, antennas, transparent conductors, sensors, membranes, catalysis, and medicine are all interested. MXenes has many advantages, including outstanding metallic conductivity, low density, large specific surface area, tunable surface chemistry, and solution processability.

Present applications of MXenes

The discovery of single-layer graphene’s unique physical properties and two-dimensional (2D) materials has been widely researched. This interest led to a replacement wave of research on known 2D materials, like metal dichalcogenides and boron nitride, and, therefore, to discover many new 2D materials. Many of these materials have not yet been used in any practical application; they are primary sources of ongoing research. However, other newly identified materials, like MXenes, have jumped into the limelight due to their attractive properties, which have led to practical applications in creating flexible and contour forming components for soft electronics use.

The explored applications and properties of MXenes to date are shown in the graphic above. Currently, over 70% of all MXene research has focused on the first discovered MXene, Ti3C2Tx. The exploration of this MXene is so profuse that, for many, the name MXene has become synonymous with Ti₃C₂Tx. So they use ‘MXene’ without specifying the composition. This lack of clarity can create confusion for those not familiar with nanomaterials, as there are many different formulations to be considered. At least 100 stoichiometric MXene compositions and a plethora of reliable solutions provide a unique combination of material goods. Varying the ratios of the M or X elements will accomplish that purpose. The large, still underexplored family of MXenes and their unique combination of properties could very well lead to various novel applications. The possibilities of the latest compositions provide good evidence that we are still in the early stages of MXenes research. It is possible that many exciting discoveries are yet to come.

An environmental benefit of MXene

MXene is a promising new catalyst to induce an electrochemical nitrogen reduction reaction (NRR) that produces NH₃. This new process is vital as an eco-friendly replacement for the traditional Haber-Bosch process for making agricultural fertilizer and various other chemicals. The NRR synthesis makes NH₃ using much less electricity that can be sourced from renewable sources rather than fossil fuel or nuclear plants. Haber-Bosch uses a hydrogen evolution reaction (HER) that delivers much more hydrogen than it does NH₃ at a much higher production cost. Thus, the electrochemical NRR process provides a sustainable and distributed alternative to the Haber-Bosch process.

The NRR process is indicative of how easily and flexibly the MXene nanomaterial can be manipulated to realize properties for applications like high amounts of surface area, tunable electronic components, and the electronic conductivity necessary for the production of soft electronics.

About the author

Jim Dietz is a content writer for Do Supply Inc. Jim is an experienced systems engineer and technical writer with over 18 years of experience. He holds a bachelor’s degree in Electrical Engineering from the Pennsylvania State University (1977) and a Master’s Degree in Electrical Engineering from the Naval Postgraduate School in Monterey CA (1985).