A new method to fabricate soft electronics via particle engulfment printing

The electronics industry has been rapidly advancing over the past few decades, leading to the development of countless devices of different sizes and shapes, which are designed for a variety of applications.

These include stretchable electronics, which are sensors and other devices that can be used to create smart watches, fitness trackers, bio-medical sensors to monitor specific health conditions and soft robots.

Flexible electronics are typically made using soft polymers, elastic materials comprised of long molecular chains that can undergo significant deformation without breaking. To broaden their functions or boost their performance, these polymers are often combined with microscale or nanoscale particles that have specific optoelectronic or magnetic properties, also known as functional particles.

Most existing approaches to introducing functional particles in polymers work by dispersing the particles into liquid molecules that bond with other molecules to form a polymer before the material is solidified. Yet many of these strategies are only applicable to some polymer-particle combinations or are difficult to implement on a large scale.

Prof. John Ho’s research group at the National University of Singapore and Prof. Yong Lin Kong’s group at Rice University recently introduced a new method for fabricating soft electronics, which is outlined in a paper in Nature Electronics.

The first author of this paper is Dr. Rongzhou Lin, who was a post-doctoral student at the time and is now a faculty member at the South China University of Technology.

The new approach developed by the researchers leverages a soft matter physics phenomenon called ‘particle engulfment’ to embed particles into a soft polymer. Particle engulfment is a spontaneous process that occurs when the so-called elastocapillary length of a polymer’s matrix surpasses the characteristic length of particles.

“Elastocapillary length is the length scale at which the surface stress becomes important over bulk elastic stresses,” Prof.Yong Lin Kong, the co-corresponding author of the paper, told Tech Xplore.

“For example, when a particle with a size smaller than the elastocapillary length (e.g., a very soft substrate) is left on the surface, the surface stress of the substrate can dominate, so much so that it becomes energetically favorable for the particle to be ‘subsumed’ by the solid substrate itself.

“This phenomenon itself is not new and was already studied by others within the soft matter physics community, including as part of a beautiful work by Prof. Eric Dufresne at Cornell University. However, prior work studied particle engulfment with a few particles and it had so far not been employed to fabricate electronics.”

While they were trying to create an effective soft strain sensor, the researchers found that existing approaches to disperse CNT particles into polymer solutions did not reliably produce the conductive polymers needed for high-performance stretchable sensors. Yet accidentally, they discovered the advantages of leveraging particle engulfment to create these conductive polymers for stretchable electronics.

“Conventional methods for creating strain sensors using carbon nanotube (CNT) typically involve dispersing CNTs into a polymer precursor with the use of solvents,” Rongzhou Lin, first author of the paper, told Tech Xplore.

“However, we encountered challenges in producing conductive composites using this approach. Unexpectedly, we discovered that applying rubber CNTs onto cured silicone resulted in the easy formation of conductive composites.”

When they investigated this phenomenon further, the researchers found that nanomaterials can spontaneously embed themselves into the polymer matrix via the particle engulfment phenomenon. They then integrated this approach with a printing set-up, using a stencil mask to control the exposed areas, which was found to successfully incorporate a broad range of functional particles into soft polymers.

To demonstrate the potential of their proposed method, the team used it to create elastic electronics with multiple layers incorporating various materials. The devices they fabricated could be suitable for different real-world applications, as they were found to possess wireless sensing, communication and power transfer capabilities.

“To the best of our knowledge, this was the first successful use of particle engulfment to make soft electronics,” Lin said.

In addition to demonstrating the possibility of using particle engulfment to develop system-level electronics with tissue-like properties, this study also opens up new possibilities for soft matter physics research. Future studies could leverage the methods employed by the team to further study the physics underpinning particle engulfment.

“As this is the first experimental study on the engulfment of high concentrations of particles, our experimental data can potentially unravel new physical insights that have not been previously explored,” Kong added.

“Our intriguing observations open new questions in soft matter physics, such as multi-layer engulfment of particles that could inspire future applications.”

As the new strategy devised by the researchers is both scalable and reliable, it could soon also be employed by other teams, contributing to the further advancement of the electronics industry.

In the future, it could be used to fabricate a broader range of electronics, including electronic skins for robots and stretchable sensors for wearable devices.

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