Recently, Professor Zhao Hangbo and his team at the University of Southern California have created a new type of microneedle electrode array, which has the characteristics of high stretchability, customization and individual addressability. The paper was selected as the cover paper for Science Advances.
Figure | Cover page of the current issue (Source: Science Advances)
According to reports, this microneedle electrode array has 60%-90% stretchability, far exceeding the existing similar devices.
At the same time, its shape, length, detection area, impedance, and layout can all be customized in a low-cost, scalable way.
（来源：Science Advances）
In the study, the research group used laser etching technology, micromachining technology and transfer technology to propose a hybrid manufacturing scheme, through which this microneedle electrode array was fabricated.
For this hybrid manufacturing solution, it is a stretchable microneedle manufacturing process that is compatible with existing MEMS technologies.
Compared with the traditional microneedle array manufacturing process, this method is simpler and more efficient.
Selective etching of microneedles in three-dimensional space has long been a major challenge in microneedle manufacturing.
In this work, the above problems are ingeniously solved by the method of gel-assisted metal etching.
As a stretchable penetrating electrode array, it is expected that the microneedle electrode array will become a practical 3D biological interface platform.
In order to demonstrate the application prospects of this work, the team used microneedle electrode arrays for the measurement of intramuscular electromyography signals in sea slugs.
During this period, they inserted microneedle electrodes of different lengths into muscle groups at different positions and depths in the same microneedle array to measure the electromyography signals of different muscle groups during the movement of sea slugs.
Although this is only a one-time application in the muscles of mollusc nudibranchs, it is pre-assembled with this stretchable microneedle electrode array, which can be created into a bio-electronic interface, making it an effective tool for detecting the deep tissues of living organisms.
Especially for soft, deformable biological tissues, the three-dimensional bio-electronic interface created by this microneedle electrode array ensures a close fit between the electrodes and the living biological tissue.
This can lead to potential applications for electrophysiological sensing of brain-computer interfaces, electrochemical sensing of interstitial fluid of the skin, and electrical stimulation of nerves and muscles.
Figure | Zhao Hangbo (Source: Zhao Hangbo)
Marine molluscs inspire a top cover paper
According to reports, mollusks in the ocean are the "starting point" of this research.
The research group is very interested in how mollusks use their muscles to produce locomotion, such as how the tentacles of an octopus elongate and bend, and how a sea slug crawls and feeds.
Many mollusks have very special muscle structures. For example, the tentacles of an octopus contain longitudinal, transverse, and circumferential muscles, and the mouth and cheek muscles of a sea slug contain multiple layers of different muscle fibers.
For these multi-muscle groups distributed in three-dimensional space, it is important to study how they produce actions through synergy in the fields of biology, biomimicry, and robotics.
Measuring the EMG signals generated by these muscle groups can provide experimental data to answer these questions.
From these experimental data, it is possible to know which muscle groups contract first, which muscle groups contract later, and what kind of movement can be produced.
However, the team found that it was difficult to measure individual muscle groups from the small muscle tissues of these mollusks.
The reason for this is that these muscle groups are distributed in three-dimensional space on the scale of centimeters or even millimeters. At the same time, muscle groups are constantly undergoing drastic deformations.
Therefore, only tiny electrodes that can be addressed individually can be the ideal measuring tool.
They can measure local EMG signals by piercing muscle tissue to reach specific areas.
At the same time, the impact on the normal activities of mollusks can be minimized.
At present, microneedle electrode arrays have been used in many fields. For example, commercial microneedle neural probes have been used for brain-computer interfaces, and commercial subcutaneous microneedle patches have been used for electromyography sensing and electrochemical sensing.
（来源：Science Advances）
After a literature review, the team found that existing microneedle electrode arrays often have the following drawbacks:
First, the vast majority of microneedle electrode arrays are not stretchable, and a few microneedle electrode arrays are stretchable, but their performance is very limited, and they do not have the ability to address individually.
Second, it was previously difficult to create customized microneedle electrode arrays, such as having an array contain microneedles of different lengths, so that tissues at different depths could be measured.
Third, it is impossible to conveniently and accurately control the exposed area of the microneedle electrode array, so as to achieve local signal acquisition, and it is even more difficult to achieve in the face of microneedle electrode arrays of different sizes.
Fourth, the manufacturing process is cumbersome, which makes it difficult to scale.
The main reason for these shortcomings is that the process used to manufacture rigid microneedle electrode arrays is not compatible with stretchable flexible materials.
At the same time, for the three-dimensional structure of microneedles, it faces challenges in material integration and patterning.
Based on this, the team hopes to develop a new type of microneedle electrode array that can solve the above shortcomings.
（来源：Science Advances）
Create a three-dimensional bio-electronic interface
In the study, the research team started from the muscle structure and muscle function of mollusks, and clarified the requirements for measuring electromyography signals in three-dimensional muscle groups.
At the same time, they invented a new method for three-dimensional etching using hydrogels, which can selectively etch microneedles in a matter of seconds, so that the electrodes remain conductive only in the tip area.
This solution not only meets all the requirements for measuring 3D EMG 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. As a result, existing methods either do not allow for detection area control of different electrode sizes, or the manufacturing process is cumbersome and slow. ”
（来源：Science Advances）
After that, they worked with collaborators at the University of Illinois at Urbana-Champaign in the United States to test microneedle electrodes on sea slugs.
Together, they were able to measure the timing of electromyography signals (EMG) of multiple muscle groups in motion for the first time, explaining how muscle groups work together.
Through the above experimental cases, it is demonstrated that the microneedle electrode has the ability to act as a three-dimensional bio-electronic interface.
Although they successfully measured the signal from the isolated tissue of the sea slug, the research team often encountered various unexpected difficulties in the process of animal experiments.
For example, the fitting of microneedle electrodes to tissues, and the imaging positioning of microneedles in tissues are all problems they have encountered.
"Zhao Qinai, the first author of the paper and a doctoral student in the research group, spent a lot of effort on this seemingly simple animal experiment, and many times went to the University of Illinois to do experiments with collaborators." Zhao Hangbo said.
最终，相关论文以《用于肌内肌电图的高度可拉伸和可定制的微针电极阵列》（Highly stretchable and customizable microneedle electrode arrays for intramuscular electromyography）为题发在 Science Advances[1]。
Zhao Qinai is the first author, and Zhao Hangbo serves as the corresponding author.
Figure | Related papers (Source: Science Advances)
Currently, they are investigating 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 lightguide-assisted optical therapy, optogenetics, and targeted drug delivery.
Resources:
1.Zhao, Q., Gribkova, E., Shen, Y., Cui, J., Naughton, N., Liu, L., ... & Zhao, H. (2024). Highly stretchable and customizable microneedle electrode arrays for intramuscular electromyography. Science Advances, 10(18), eadn7202.
Operation/Typesetting: He Chenlong