Recently, Professor Zhao Hangbo and his team from the University of Southern California have created a new type of microneedle electrode array that is highly stretchable, customizable, and individually addressable. The related paper was selected as the cover article for Science Advances.

It is reported that this microneedle electrode array has a stretchability of 60%-90%, far exceeding similar existing devices.

At the same time, its shape, length, detection area, impedance, and layout can all be customized through low-cost, scalable methods.

In the research, the team used laser etching technology, micro-fabrication technology, and transfer technology to propose a hybrid manufacturing solution, which was used to create this microneedle electrode array.

For this hybrid manufacturing solution, it is a stretchable microneedle manufacturing process that is compatible with existing micro-electromechanical systems (MEMS) technology.Compared to traditional microneedle array manufacturing processes, this method is simpler and more efficient.

Always, selective etching of microneedles in three-dimensional space has been a significant challenge in microneedle manufacturing.

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This work cleverly solves the above difficulties through the method of gel-assisted metal etching.

As a stretchable penetrating electrode array, it is expected that this microneedle electrode array will become a practical three-dimensional biointerface platform.

To demonstrate the application prospects of this achievement, the team used the microneedle electrode array for measuring electromyographic signals within the muscles of the sea hare.During this period, they inserted microneedle electrodes of different lengths from the same microneedle array into different positions and depths within muscle groups, thereby measuring the electromyographic signals of different muscle groups during the movement of the sea hare.

Although this is only an application in the muscles of the mollusk sea hare, it signifies that this stretchable microneedle electrode array can be crafted into a bio-electronic interface, thus becoming an effective tool for detecting deep tissue activity in living organisms.

Especially for soft, deformable biological tissues, the three-dimensional bio-electronic interface created by this microneedle electrode array can ensure a tight fit between the electrodes and the moving biological tissues.

This could potentially lead to applications in electrophysiological sensing for brain-computer interfaces, electrochemical sensing for interstitial fluid in the skin, and electrical stimulation of nerves and muscles.Inspired by Marine Mollusks: A Cover Paper in a Top Journal

According to the introduction, marine mollusks are the "starting point" of this study.

The research team is very interested in how mollusks use their muscles to generate motion, such as how the tentacles of an octopus can stretch and bend, and how sea slugs crawl and feed.

Many mollusks have very special muscle structures. For example, the tentacles of an octopus contain longitudinal, transverse, and circular muscles, and the buccal muscles of the sea slug's oral cavity contain multiple layers of different muscle fibers.

Studying how these multiple muscle groups distributed in three-dimensional space work together to produce movement is of great significance to the fields of biology, bionics, and robotics.Measuring the electromyographic signals generated by these muscle groups can provide experimental data to answer the aforementioned questions.

With these experimental data, it is possible to determine which muscle groups contract first, which muscle groups contract later, and what kind of movement can be generated.

However, the team found it difficult to conduct measurements on individual muscle groups in the small muscle tissues of these mollusks.

The reason is that these muscle groups are distributed in a three-dimensional space on a scale of centimeters or even millimeters. At the same time, the muscle groups are constantly undergoing significant deformation.

Therefore, only microelectrodes that can be individually addressed can become the ideal measuring tool.They can reach specific areas by piercing muscle tissue, thereby measuring local electromyographic signals.

At the same time, the impact on the normal activities of soft-bodied animals can also be minimized.

Currently, microneedle electrode arrays have been applied in many fields. For example, commercialized microneedle neural probes have been used for brain-computer interfaces, and there are also commercialized subcutaneous microneedle patches that have been used for electromyographic sensing and electrochemical sensing.

After a literature review, the team found that existing microneedle electrode arrays often have the following shortcomings:

Firstly, the vast majority of microneedle electrode arrays are not stretchable, a few microneedle electrode arrays have stretchability, but the performance is very limited, and they do not have the ability to address individually.Secondly, it was previously difficult to create customizable microneedle electrode arrays, such as an array that includes microneedles of different lengths to measure tissues at different depths.

Thirdly, it is not convenient or precise to control the exposed area of the microneedle electrode array, thereby achieving local signal collection, and this is even more challenging when facing microneedle electrode arrays of different sizes.

Fourthly, the manufacturing process is cumbersome, making it difficult to scale up.

The above shortcomings exist mainly because the manufacturing process used for rigid microneedle electrode arrays is not compatible with stretchable and flexible materials.

At the same time, for the three-dimensional structure of the microneedles, they face challenges in material integration and patterning.Based on this, the team hopes to develop a new type of microneedle electrode array that can address the aforementioned shortcomings.

Creating a 3D Bio-Electronic Interface

In the study, the research group started from the muscle structure and function of mollusks, clarifying the requirements for measuring electromyographic signals within three-dimensional muscle groups.

At the same time, they invented a new method of using hydrogel for three-dimensional etching, which can achieve selective etching of microneedles in a few seconds, thereby keeping the electrodes conductive only in the tip area.This plan not only meets all the requirements for measuring three-dimensional myoelectric signals but also has the advantages of low cost, high stretchability, and scalability.

Zhao Hangbo said: "This is very necessary for the realization of local sensing. Therefore, existing methods either cannot achieve the control of the detection area for electrodes of different sizes, or the manufacturing process is very cumbersome and slow."

After that, they, together with collaborators from the University of Illinois at Urbana-Champaign, tested microneedle electrodes on sea slugs.

Through cooperation, they measured the myoelectric signal timing spectrum of multiple muscle groups in sea slugs during movement for the first time, and explained the principle of muscle group cooperation.

The above experimental cases demonstrate the ability of the microneedle electrodes to act as a three-dimensional bio-electron interface.Despite successfully measuring signals from the isolated tissues of sea slugs, the research team often encountered various unexpected difficulties during the animal experiments.

For example, the adhesion of microneedle electrodes to the tissue and the imaging and positioning of microneedles within the tissue were all challenges they had faced.

"The first author of the paper and the doctoral student of the research group, Zhao Qinai, has devoted a lot of effort to this seemingly simple animal experiment, and many times he personally went to the University of Illinois to conduct experiments with collaborators," said Zhao Hangbo.

Ultimately, the related paper was published in Science Advances with the title "Highly stretchable and customizable microneedle electrode arrays for intramuscular electromyography."

Zhao Qinai is the first author, and Zhao Hangbo serves as the corresponding author.Currently, they are researching the application of microneedle electrode arrays in the medical field.

At the same time, they are also exploring how to combine microneedle electrode arrays with sensors, optics, optoelectronics, and microfluidics for use in light-guided optical therapy, optogenetics, and targeted drug delivery.