The NUS team constructed a liquid-solid double-layer stretchable conductor that is not afraid of tens of thousands of stretches and 22 times tensile conductors, and is not prone to cracks and obvious electrical attenuation

Many people have always had an expectation that wearable bioelectronics or bioelectronics implanted in the body can be as soft and tough as the human body, and that it can be perfectly compatible with our physical bodies.

An important component of flexible stretchable bioelectronics is stretchable conductors. If a flexible stretchable bioelectronic system is likened to a "deformed building", bioelectronics is the "elastic brick" inside.

"Elastic bricks" can be combined with other materials to build many "functional units", such as sensors, transducers, communication modules, circuits and connecting lines, and finally build an entire "building".

At present, the preparation of "elastic bricks" is generally based on the composite of elastomers and various traditional solid-state conductive micro-nano materials such as copper, silver, and carbon.

These solid-state conductive materials are not inherently tensile and can withstand certain stresses or strains with the help of elastic polymers. However, after long-term or severe stress and strain, such as tens of thousands of stretches or more than 2 times stretching, the "elastic brick" will produce many microcracks, resulting in electrical or physical fractures, and eventually causing the paralysis or collapse of the "functional unit" or "entire building".

At the same time, the connection of these stretchable conductors to conventional solid-state microelectronics presents significant challenges. Conventional solder high-temperature soldering is not suitable for this kind of soft-hard connection, because the soft electronics are neither resistant to high temperatures nor compatible with solder.

Polymer-based conductive adhesives are compatible with soft electronics, but it is difficult to achieve fine micro-welding. For interfaces, it is also susceptible to similar cracks and electrical fractures under long-term or severe mechanical stimuli.

Later, people began to look for solutions in liquid metal. Liquid metal – This combination of names is due to the fact that it has both the electrical conductivity of a solid metal and the fluidity of a liquid. Therefore, it has the ability to deform itself.

For example, mercury mercury is a typical example of a liquid metal, and we use another alloy of gallium indium that is more biologically safe. If liquid metal materials are used to replace solid conductive materials to build "elastic bricks", although it is expected to solve the above problems, liquid metal has some characteristics that are difficult to control, that is, it has a large surface tension and a certain oxidation capacity.

Liquid metal is like a super active "little friend", and it is difficult to obediently let it stay in a certain place according to a certain posture. When a liquid metal block is broken into many small particles, a layer of oxide will be formed on the surface of the particles, which will be beneficial to manipulation and prevent further oxidation, but will cause the whole to be non-conductive.

Therefore, for the stability problems of long-term or intense mechanical stimulation and the problems of soft-hard connection, the current tensile conductors based on liquid metal have not been well solved. It is based on this background that Chen Shuwen, a postdoctoral fellow at the National University of Singapore, and his colleagues have created a liquid-solid bilayer stretchable conductor.

Figure | Chen Shuwen (Source: Chen Shuwen)

Construct liquid-solid double-layer stretchable conductors that are not afraid of tens of thousands of stretches and 22 times stretches

This liquid-solid bilayer stretchable conductor is based on liquid metal, liquid metal particles and elastic polymers. In preparation, it is not tiled layer by layer, but is formed automatically during the peeling process.

On the raw material of liquid-solid double-layer stretchable conductor, the research group uses liquid metal micro-nano particles and highly polar elastomer composite ink, which can be compatible with many substrates, so it is easy to manipulate, and after printing and forming, it is a layer of insulating composite.

When a substrate with a precursor is peeled off the adhesive, the stresses generated during the peeling process automatically drive the transformation of the single-layer insulator into a double-layer conductor. This results in a special structure: a liquid metal film on the upper layer and a solid composite on the bottom. Thereinto:

The liquid metal film on the upper layer facilitates the automatic soft-hard connection. In this case, the solid-state electrons are simply placed on top of the liquid-solid double-layer stretchable conductor, and after pressing, the solid-state electron pins automatically extend into the upper layer of liquid metal film to achieve an electrical connection.

The lower solid composite is conducive to stabilizing the connection between the liquid-solid double-layer stretchable conductor and the substrate. Therefore, even after tens of thousands of stretches and 22 times stretching, the liquid-solid double-layer stretchable conductor has no cracks and no significant electrical attenuation.

(Source: Data map)

What is particularly interesting is that when a crack occurs after a knife wound, the liquid metal particles in the lower layer will begin to rupture, thus releasing the liquid metal inside to automatically fill the gap, and then immediately restore electrical performance.

Therefore, the liquid-solid double-layer stretchable conductor can be as soft and elastic as the skin, which can not only stably withstand long-term or severe mechanical stimulation, but also can easily and quickly realize soft-hard electronic connection.

For this work, the reviewers believe that the level of this paper is in the top 15% of this type of paper compared with existing papers in the field.

One of the reviewers thought that the research group could obtain this peculiar liquid-solid bilayer stretchable conductor just by stripping. In terms of its electrical properties, it exhibits a very strong robustness to stress, which is attractive for stretchable electronics.

The second reviewer believes that this liquid-solid double-layer stretchable conductor has both self-welding ability and self-healing ability, and has almost constant electrical conductivity and resistance under large deformation.

On the whole, this liquid-solid double-layer stretchable conductor can be used to construct heaters, wireless systems, stretchable sensors, stretchable displays, stretchable heaters, and then build human-computer interaction systems based on stretchable biosensing.

One of the typical application scenarios for this human-computer interaction system is to monitor cardiac electrophysiology.

Atrial fibrillation is a common arrhythmia that usually occurs in the atrial part of the heart. When a person has atrial fibrillation, the atria beat no longer follows the normal orderly pattern, but beats in an irregular and rapid manner, which can cause the heart to not pump blood enough into the ventricles, increasing the risk of blood clots, strokes and other cardiovascular problems.

For people with severe chronic atrial fibrillation, especially if they don't respond to medication or are already intolerant, your doctor may recommend electrophysiology studies and ablation, in which electrophysiology studies are performed to identify abnormal heart tissue, and then cardiac ablation procedures are performed to destroy or isolate these tissues to help restore a normal heart rhythm.

Traditionally, the catheter technique has been used to locate abnormal sources of electrical activity to help restore a normal heart rhythm.

Due to their extremely low spatial resolution, these catheter electrodes are not able to map the electrophysiological state of the epicardium in a high-throughput manner, especially for the abnormal localization of multi-point epicardial tissue.

The use of liquid-solid double-layer stretchable conductors can be made into a heart-sized electrode array, which can be laid on or set on the heart for electrophysiological monitoring of the whole heart, and can be adaptively deformed with the beating of the heart, helping to achieve rapid, efficient and high-resolution localization of abnormal tissues.

"You can't just discover some scientific and engineering knowledge"

According to reports, when she first came to the team, she positioned flexible stretchable bioelectronics as her research direction according to the research group and her own research background.

Previously, the team had developed some fine fiber sensors by injecting liquid metal into the catheter.

Therefore, Chen Shuwen intends to try to use large-scale printing to prepare some bioelectronics. Before coming to Singapore, Chen Shuwen had been exposed to stretchable conductors based on solid-state micro-nano materials and was well aware of some of the problems.

Knowing the fluidity and deformability of liquid metal, she thought that liquid metal, as a very promising material, might be able to solve these problems.

Bugs occur when printing inks on elastic substrates at the beginning. Singapore's climate is very humid, and the elastomers precipitate out before the ink is applied to the substrate before the complete pattern is formed, or the dry pattern has not yet stretched or has many cracks when it is slightly pulled.

So, they tried to replace many elastomers, constantly adjusting the ink ratio, and finally configured an ink that was relatively compatible with water, and solved the problem of easy cracking.

However, seemingly flawless printed circuits do not conduct electricity. The research team first used tension to activate the ink, but the conductivity was still not high, and then activated it with pressure, and found that this contact would destroy the circuit pattern.

Later, they tried to mix a small amount of solid-state conductive nanowires to form a wider conductive network to improve the conductive effect, but the results were still very unsatisfactory.

Later, they tried to use non-contact ultrasonic stress instead of compressive or tensile stress, but again failed.

Later, after Chen Shuwen simply tore the sample off the viscous material, it turned out that the electrical conductivity was unusually high, and a glittering liquid metal film could be formed on it. Moreover, the original pattern of the circuit has not been destroyed, which surprised the entire research group.

Through mechanical analysis, they found that this peeling process can well integrate tension and compression. At the same time, there is no need to touch the sample, so the original pattern can be maintained to a great extent.

In addition, they also found that this liquid-solid double-layer structure can form a good soft and hard connection when it is directly pressed with solid-state electronics, and it can heal on its own even if it is injured by a knife edge.

Immediately afterwards, Chen Shuwen and his colleagues began to explore the use of this achievement in the fields of sensing, human-computer interaction, and wearables.

Finally, the related paper was published in Advanced under the title "Ultrahigh Strain‐Insensitive Integrated Hybrid Electronics Using Highly Stretchable Bilayer Liquid Metal Based Conductor". Materials[1]。

Chen Shuwen is the first author, and Professor Chwee Teck Lim, Fellow of the ASEAN Academy of Engineering and Technology, Fellow of the Singapore Academy of Engineering, Fellow of the Singapore Academy of Sciences, Fellow of the National Academy of Inventions, and Dean of the Department and Research Institute of Biomedical Engineering at the National University of Singapore serves as the corresponding author.

Figure | Related papers (Source: Advanced Materials)

"In fact, towards the end of all the work, Prof. Lin kept asking if there were any other applications? He is an engineering person, and under his leadership, our group has developed a lot of start-ups. Chen Shuwen said.

Therefore, Lin Shuide hopes that one day they can also industrialize this work, so he has been exploring the potential applications of this work, and hopes that Chen Shuwen and his colleagues will make products that can affect thousands of households, and finally serve biomedical and human life.

At the same time, Chen Shuwen was also deeply infected by this spirit, and she also realized that her work was not finished yet, and there was still a certain distance from her mentor's expectations.

That is, you can't just discover some scientific and engineering knowledge and verify it technically. It's about translating these sciences and technologies into really useful products.

Therefore, the next step is to develop a bioelectronic system for specific medical application scenarios, and after a large number of animal experiments and clinical experiments, it will finally be recognized by the end user, so as to fulfill the mentor's wish.

Therefore, although this paper has been published, the relevant work is still ongoing. "I believe that one day I will fulfill his wish, because this wish has already taken root in my heart. Chen Shuwen said.

Figure | From left to right: Lin Shuide and Chen Shuwen (source: data map)

"I hope that in the twilight of my life, I can recognize my life"

It is also reported that Chen Shuwen graduated from the Beijing Institute of Nano Energy and Systems with a master's degree, and his instructors include Academician Wang Zhonglin and Professor Cao Xia. He graduated from Huazhong University of Science and Technology with a Ph.D. degree from Professor Zhou Jun, winner of the National Science Foundation for Distinguished Young Scholars.

Currently, she is doing post-doctoral research at the National University of Singapore's Institute of Healthcare Innovation and Technology (iHealthtech) under the supervision of Prof. Lim Shui Tak above.

Chen Shuwen said that scientific research is a difficult road, and she is very fortunate to have met many excellent leaders along the way. Looking back, some of the teachings still echo in my ears:

For example, Wang Zhonglin often told her and her classmates that "scientific research is sometimes a lonely ride, but if you decide it, you must persevere"; Zhou Jun once told her and her classmates that "when doing scientific research, don't be a wave, if you want to do it, you must do the work like a lighthouse, and dare to gnaw hard bones"; Lin Shuide often told her and her colleagues that "we need to do some impactful works, not just papers" ”。

Chen Shuwen said that she often thinks that life is like a mayfly that passes away in a flash, but what kind of life she wants to live and what kind of scientific research she wants to do? She hopes that in her twilight years, she will be able to recognize her life and her career.

She continued: "Although the mentors have very different personalities, their influence on me is all-round, some teachers taught me to enjoy the loneliness and joy of scientific research, some teachers taught me to persist in my interests and explore the unknown, and some teachers taught me to pay attention to technology and industrial transformation. So far, I am still far from their expectations, and I hope that in the future scientific research, I can continue to collect wisdom with their teachings, and make more influential work that will benefit China and all mankind. ”

At the same time, she will continue to adhere to her original intention, do valuable and meaningful scientific research, and expand scientific cognition and technology accumulation step by step. Through innovation in the field of flexible bioelectronics, we will serve human health and intelligent life. ”

Resources:

1.Chen, S., Fan, S., Qi, J., Xiong, Z., Qiao, Z., Wu, Z., ... & Lim, C. T. (2023). Ultrahigh Strain‐Insensitive Integrated Hybrid Electronics Using Highly Stretchable Bilayer Liquid Metal Based Conductor. Advanced Materials, 35(5), 2208569.