Recently, Dr. Hongyan Gao and his team at the University of Massachusetts in the United States have built a grid bioelectronic system.
It is a sensor embedded with multifunctional graphene nanoelectronics, which can track the excitation-contraction process of the cardiac microtissues thanks to the integration of a single-layer graphene transistor.
图 | 高洪岩（来源：高洪岩）
In this device, graphene acts as a transistor, that is, using the field effect and piezoresistive effect of graphene, it can simultaneously detect the action potential and mechanical signals of the heart's microtissues.
（来源：Nature Communications）
In this way, the research group not only integrates multi-functional sensing in the same device, but also avoids the challenges of signal synchronization, device addressing, and integration heterogeneity in traditional strategies.
The flexible mesh network provides tissue-level flexibility and cell-level characteristic dimensions to achieve close contact with the heart tissue.
This allows the device to simultaneously monitor the electrical and mechanical signals inside the cardiac microtissue, which are the two most important reference signals for the human heart to function.
Through a complex excitation-contraction process, these two signals can be coupled together. Therefore, the action potential signal and mechanical contraction signal monitored by the device at the same location can reflect the state of the cardiac microtissue during development.
It is known that mature heart tissue can exhibit greater amplitude of action potential and mechanical vibration, and these indicators can be reflected by the change of conductance of graphene.
In addition, the research group also uses drug treatment methods to suppress the signal, so the process of signal change in the device, such as amplitude change and frequency change, can intuitively reflect the impact of related drugs on cardiac microtissue.
Taken together, this sensing platform allows the entire organization to reliably track complex excitation-contraction couplings throughout the organization, and is able to continue throughout the organization's development.
At the same time, it can also comprehensively assess the maturity of heart tissue, distinguish the effects of drugs, and perform disease modeling. Compared with single-modal sensor technology, rich datasets can be created through this perception platform.
It is expected to achieve more accurate quantitative results for the function, development, and pathophysiology of cardiac tissue, thus providing an important tool for improving cardiac tissue engineering.
It is understood that since the device can be wrapped inside the cardiac microtissue, environmental factors such as temperature and oxygen content that will not affect the cardiac microtissue will not affect the output signal.
At the same time, this work overcomes the limitations of optical imaging sensing and unimodal electronic sensing, and provides a comprehensive evaluation of microtissue developmental dynamics by combining bioelectrical sensing and biomechanical sensing.
This allows the results to be seamlessly integrated into microtissues, resulting in a minimally invasive and stable tracking of tissue development, which in turn provides a promising tool for studying tissue engineering.
Subsequently, if the test conditions are optimized, the device will also be used for drug screening in 3D cardiac tissue.
For cardiac microtissues, it can also provide a microenvironment that is more similar to that of the human heart, and is promising for the monitoring of living cardiac tissues, thereby improving the efficiency of drug screening.
It is also reported that mesh electrons and graphene transistor electrons have been widely used for the monitoring of living nerve signals. The mesh electronics constructed this time provide a tool for intuitively analyzing the side effects of drugs on the heart.
（来源：Nature Communications）
Moreover, due to its high monitoring sensitivity and good chemical stability, graphene can be a more suitable material than silicon nanowires.
Overall, the use of graphene can bring the following advantages:
First, graphene is a two-dimensional material, so a homogeneous thin film can be obtained by chemical vapor deposition, which provides the basis for large-scale integration and device homogeneity.
Second, graphene is a material with both field and piezoresistive effects, which can theoretically monitor changes in voltage and strain at the same time, thus integrating both functions on the same device.
Third, graphene is also a carbon material with excellent chemical stability and can achieve long-term physiological monitoring.
At the same time, graphene can also be replaced with other 2D semiconductor materials, provided that the material has both piezoresistive and field effects, as well as physiological environmental stability and the ability to synthesize on a large scale.
The road to monitoring the microtissues of the heart is long and obstructive
As mentioned earlier, this result can be used for the detection of cardiac signals. Heart disease is one of the leading causes of morbidity and mortality in humans. The lack of animal models makes in vitro cardiac tissue an important alternative.
Among them, cardiac tissue constructed from human stem cell-derived cardiomyocytes is the most commonly used in vitro model. They can retain patient information that can be used to study genetic diseases and personalize drug screening.
Compared to planar tissue culture, 3D cardiac microtissues are the preferred tissue models because they can better reproduce cell phenotypes, microenvironments, and cell-cell interactions, presenting experimental results close to those of living organs.
Electrical and mechanical signals are two of the most intuitive parameters for monitoring the development of cardiac microtissues, which can be used to evaluate the effects of drugs and diseases.
In the case of electrical and mechanical activity in the microtissues of the heart, they are intrinsically interconnected through excitation-contraction coupling. Therefore, the simultaneous measurement of the relevant dynamics is extremely important for the study of cardiac microtissue.
For example, in patients with chronic myocardial infarction, cardiomyocyte dysfunction is associated with impaired excitation-contraction coupling. At the same time, many arrhythmias are also due to a weakened excitation-contraction coupling.
Therefore, spatial mapping by tracking electrical or mechanical reactions alone has some limitations in identifying impaired excitation-contraction coupling links.
In development-related studies, tissue maturity or the degree of aging are often better characterized by the associated excitation-contraction coupling dynamics, rather than simply by information on electrical or mechanical activity.
Previously, optical means were used to track electrical and mechanical movements, which in turn were used in three-dimensional microstructures.
However, due to the opacity of the tissue, only superficial physiological activity can be obtained.
The same problem exists with electronic sensors integrated on a flat or flexible substrate.
Although the development of flexible mesh electronics provides an opportunity to embed addressable sensors in deep tissues, the flexible ribbon feature in particular allows the mechanical properties of mesh devices to be close to those of tissues, which can reduce invasiveness and extend the stability of the interface.
However, existing platforms can only detect a single modality of electrical or mechanical activity, which is indeed sufficient to detect tissues such as nerves, but not enough to characterize the mechanical and electrical responses associated with the cardiac system.
It is understood that due to the existence of cardiotoxic side effects, about one-third of the drugs cannot achieve clinical application.
Previously, the team has demonstrated that 3D silicon nanowire transistors constructed by microfabrication technology can be fabricated into bifunctional electronic devices that can simultaneously measure planar monolayer cardiomyocytes with the help of field and piezoresistive effects [1].
However, the system is not suitable for cardiac microtissues because it is difficult to assemble 3D silicon nanowires on independent substrates in a bottom-up approach.
（来源：Nature Communications）
A clever combination of cell crimping and electronics
The idea of electromechanical monitoring of cardiac microtissues originated from Gao Hongyan's previous project of silicon nanowire transistors.
In his previous project, he found that monolayers of cells often detach from the substrate and spontaneously curl up to fold into three-dimensional microtissues.
This is undoubtedly a disadvantage for monolayer cell signal monitoring, as the separation of the device from the cell can directly lead to signal interruption.
After communicating with his mentor, Gao Hongyan came up with the following idea: to build an electronic device that can be directly embedded in the heart tissue as the cells curl and fold in order to monitor mechanical and electrical signals.
Following this line of thinking, as well as the experimental feasibility provided by the previous silicon nanowire project, Gao Hongyan began to design the device.
As a more traditional transistor, graphene has been widely used to monitor cellular action potentials. However, the monitoring of graphene's mechanical activity of cells is still in a blank area of scientific research.
Here's why: Graphene is a monolayer of carbon atoms with a diameter of about 0.3nm. Therefore, special attention needs to be paid to the breakage of graphene during transfer and the contact with metal electrodes during device processing.
In addition, because the graphene transistor is built on the surface of another polymer with a thickness of only 400nm, the processing process is more delicate.
At the same time, how to realize the integration of flexible mesh devices and cardiac microtissues is also a challenge faced by this study. In addition, ultra-thin mesh devices can bring a lot of uncertainty to the operation process, resulting in low yield of early devices.
Later, by optimizing the cell culture process and the integration method, the fixed mesh device was fused with cardiomyocytes on the substrate, and the experimental efficiency was finally greatly improved.
（来源：Nature Communications）
最终，相关论文以《具有融合多功能性的石墨烯集成网状电子器件，用于跟踪心脏微组织中的多模态兴奋收缩动力学》（Graphene-integrated mesh electronics with converged multifunctionality for tracking multimodal excitation-contraction dynamics in cardiac microtissues）为题发在 Nature Communications[1]。
Hongyan Gao is the first author, and Professor Jun Yao of the University of Massachusetts serves as the corresponding author.
图 | 相关论文(来源:Nature Communications)
Overall, this study demonstrates that the interface between reticular electronics and three-dimensional biological tissues can provide rich biological information.
In addition, this study uses in vitro experiments, so can the conclusions obtained from in vitro experiments truly reflect the behavior of tissues in living organisms?
In this regard, the team said that the experiment uses microtissues constructed by cardiomyocytes differentiated by human embryonic stem cells, which can reflect the development process of myocardial tissue to a certain extent.
However, due to the complex multi-level structure and function of the living heart, there is still a certain distance from truly reflecting the behavior of the living body.
Therefore, the team will further explore the application of this work in the analysis of three-dimensional biological tissue physiological signals.
Resources:
1.Gao, H. Y. et al. Bioinspired two-in-one nanotransistor sensor for the simultaneous measurements of electrical and mechanical cellular responses. Sci. Adv. 8, eabn2485 (2022).
2.Gao, H., Wang, Z., Yang, F., Wang, X., Wang, S., Zhang, Q., ... & Yao, J. (2024). Graphene-integrated mesh electronics with converged multifunctionality for tracking multimodal excitation-contraction dynamics in cardiac microtissues. Nature Communications, 15(1), 2321.
Operation/Typesetting: He Chenlong