Sustainable development of electroencephalography materials and technology

Electroencephalogram (EEG) is one of the most important bioelectrical signals related to brain activity and plays a crucial role in clinical medicine. Driven by continuously expanding applications, the development of EEG materials and technology has attracted considerable attention. However, systematic analysis of the sustainable development of EEG materials and technology is still lacking. This review discusses the sustainable development of EEG materials and technology. First, the developing course of EEG is introduced to reveal its significance, particularly in clinical medicine. Then, the sustainability of the EEG materials and technology is discussed from two main aspects: integrated systems and EEG electrodes. For integrated systems, sustainability has been focused on the developing trend toward mobile EEG systems and big‐data monitoring/analyzing of EEG signals. Sustainability is related to miniaturized, wireless, portable, and wearable systems that are integrated with big‐data modeling techniques. For EEG electrodes and materials, sustainability has been comprehensively analyzed from three perspectives: performance of different material/structural categories, sustainable materials for EEG electrodes, and sustainable manufacturing technologies. In addition, sustainable applications of EEG have been presented. Finally, the sustainable development of EEG materials and technology in recent decades is summarized, revealing future possible research directions as well as urgent challenges.


INTRODUCTION
Bioelectricity is the electrical activity generated by organisms during physiological activities.Recording and analyzing the phenotypes of bioelectricity play an important role in assisting in disease diagnosis.Bioelectrical signals from the brain regulate central and peripheral physiological functions and appear to be related to various kinds of diseases.However, monitoring the physiological functions of the brain was particularly difficult until the discovery of the electroencephalogram (EEG) technology.In 1875, EEG was first developed to record the electrical-current changes in animals' brains. 1,24][5] In 1950, spike activity from the surface of the cerebral cortex was successfully captured. 6In 1959, the visual cortex was first mapped by using a single-unit EEG recording method, 7 paving the way for clinical applications of EEG technology.This meaningful work won the Nobel Prize in Physiology or Medicine in 1981.During clinical applications, EEG records electrical signals through electrodes placed on the scalp, investigating brainwaves to detect different states of mental function.][15] EEG bioelectrical signals reflect synaptic activity, indicating a common denominator for the functional impacts of neurodegenerative processes.Qualitative EEG is routinely utilized to facilitate brain disease diagnosis. 16Driven by the continuous extension of clinical applications in neuroscience, cognitive science, and psychophysiological research, EEG technologies have been constantly evolving and advancing, as shown in Figure 1. 17 In the 1960s, the importance of spatial features in EEG analyses has been recognized, leading to significant breakthroughs in EEG technologies.In the 1990s, the development of BCIs promoted the development of EEG technologies with excellent spatial and temporal performance.EEG materials and technology have undergone rapid development over the past century.Nowadays, portable and wearable EEG systems are urgently required in continuously expanding applications in clinical and personalized medicine.
From the perspective of clinical medicine, EEG is a sensitive index for the state evaluation of brain function and is crucial to assist the diagnosis of neurological diseases and psychiatric disorders. 18Brain functions both in the physiological state and in the disease state could be evaluated by analyzing the recorded EEG bioelectrical signals, providing valuable information for assisting in the clinical diagnosis of diseases.Specifically, for the qualitative analysis and localization of paroxysmal brain dysfunction such as epilepsy (EP), EEG is the golden diagnostic technique that cannot be replaced by other methods. 19Particularly, it can provide useful information in EP diagnosis and treatment, including but not limited to the following six aspects: (1) determining whether the seizure is epileptic or psychogenic non-epileptic seizures; (2) determining the origin of abnormal brain electrical activity and the type of epileptic seizure; (3) assisting clinical diagnosis of EP syndrome; (4) searching for the causes of sudden cognitive decline in EP patients; (5) assessing the indications for surgery of EP patients; (6) estimating the risk of recurrence of EP after discontinuing antiepileptic drugs.Therefore, bioelectrical EEG signals are of great significance for assisting clinical decision-making and guiding drug treatment and EP surgery.][22] To access the sustainable future of EEG in clinical medicine, the development of EEG materials and technologies necessitates holistic thinking.Since first defined by the World Commission on Environment and Development, 23 sustainable development has been considered to balance the needs of the present generation with those of future generations. 24The broad concept of sustainability is related to environmental, economic, and social dimensions, 25,26 aiming to achieve a positive balance within all these domains.However, conventional life cycle assessments, 27 including environmental life cycle assessment, life cycle costing, and social life cycle assessment, are generally used for assessing products already on the market rather than technologies still under development.From the perspective of sustainable development for EEG materials and technologies, two main aspects in terms of F I G U R E 1 Evolution of human electroencephalogram (EEG) technology and its applications.

SUSTAINABLE DEVELOPMENT OF EEG SYSTEM
World reports on aging and health from the World Health Organization show the increasingly serious problems of global population aging. 28In this context, medical systems in many countries are overburdened with inadequate medical facilities and doctors/nurses. 29To reduce the burden on medical resources and improve public health, developing home-based health monitoring technologies, telemedicine, and personalized medicine become necessary.Therefore, the sustainable development of EEG systems in clinical medicine is toward miniaturization, portability, wearability, and wireless big-data modeling.A comprehensive EEG system generally comprises several important hardware components, including recording electrodes, signal amplifiers with filters, analog-to-digital (A/D) converters, recording modules, and analyzers. 30,31lectrodes detect weak bioelectrical signals over the head surface.Amplifiers amplify the weak analog signals and A/D converters convert the amplified bioelectrical signals into a digitalized range.The digitalized EEG signals are then stored, analyzed, and processed by the recording and analyzing modules.Here, the sustainable development of EEG systems is systematically elucidated from two perspectives: hardware developmental trends and big-data monitoring/analyzing techniques.

Developing trend toward mobile EEG systems
From a sustainability perspective, mobile EEG represents an important developmental trend.Mobile EEG is generally a miniaturized, wireless, portable, and wearable system.Miniaturization is beneficial to reduce energy consumption.Portability and wearability make mobile health monitoring outside hospitals possible, thereby relieving the burden on medical resources and carbon emissions from transportation.Wireless signal recording and transmission provide a zero-paper solution for longterm recording with reduced waste.EEG system is a hardware utilized to measure the small electrical potentials of the brain's neuronal activities by using recording electrodes on the scalp.Traditional EEG systems are bulky and stationary, which restricts the monitoring of subjects in laboratories and clinics.Over the last decade, significant research efforts have been devoted to the development of mobile EEG systems.Emerging state-of-the-art mobile EEG systems are usually defined as wireless, portable, and wearable EEG systems, allowing the long-term damagefree recording of brain electrical activities even when subjects leave the hospital and move around. 32,33onventionally, wired EEG systems with gel-based wetrecording electrodes have been the mainstay of EEG monitoring in clinics and laboratories.A schematic of a conventional wired EEG system with gel-based wetrecording electrodes is shown in Figure 2A.However, F I G U R E 2 Schematic illustration of electroencephalogram (EEG) system.(A) A schematic diagram of a conventional wired EEG system suffering from poor usability.(B) conventional wet silver/silver chloride electrode with indispensable conductive gel.(C) An example of a portable, wireless, and wearable system consisting of dry recording electrodes and integrated functional modules.Reproduced with permission. 35Copyright 2022, Royal Society of Chemistry.
several typical drawbacks exist that are not conducive to the sustainable development of EEG technologies for expanding applications.For example, traditional wired EEG systems have many wire connections, and the electrodes and cables at both ends are often disconnected, leading to poor usability.The large volume of the wired EEG system further deteriorates its accessibility.Furthermore, conventional EEG systems require complicated gel-based wet electrodes (Figure 2B) and scalp preparation processes performed by a trained specialist to ensure high-quality measured signals, which seriously limits the application fields and the sustainable development of EEG technology.
][36] EEG systems are evolving from bulky devices to portable, wireless, and wearable devices for long-time monitoring. 37igure 2C shows a schematic illustration of a novel wireless portable and wearable EEG system with dry recording electrodes.This type of hardware system generally consists of wearable recording electrodes and a functional modular box.The power supplying, signal processing, and wireless communicating functional modules are integrated together.Large-area multichannel dry electrodes have been highly desired in recent years because of their ability to obtain a comprehensive understanding of high-density EEG maps. 38,39Additionally, multichannel electrodes will relieve the motion artifact of electrode position shifts and compensate for individual electrode failure.For example, an EEG system with 68 electrodes has been used to completely cover across human scalp to record EEG signals. 38Data processing and communication units, which are essential for the visualization and further analysis of EEG signals, are highly desired for integration with large-area multichannel dry electrodes.However, it is chal-lenging to incorporate both flexible electrodes and rigid silicon-based IC chips into one system while maintaining non-degenerate functionality and acceptable reliability.Recently, hard-and-soft hybrid system integration schemes have been proposed to address this problem.
In recent years, several wireless multichannel systems have been developed to capture high-quality big-data EEG signals in a simple, convenient, and comfortable manner.It should be noted that monitoring EEG signals with traditional EEG systems suffers from lengthy preparation work, typically involving scalp preparation and electrode installation.This not only causes frequent scalp abrasion but also limits the application location and testing time, occupying a lot of medical resources.In addition, these inconveniences also limit the subject population, such as populations affected by dementia.In contrast, several currently available wireless, portable, and wearable EEG systems exhibit significant advantages, such as short preparation time, high-degree comfort without scalp damage, and long-term application.The removal of long-wiring connections is beneficial to the development of mobile devices with high-quality data recording.With advances in signal detection and quantitative analysis, wireless mobile systems are expected to be ideal candidates for the rapid clinical assessment of EEG electrical activities.Notably, replacing conventional wired EEG systems with gel-based wet-recording electrodes with a novel wireless, portable, and wearable EEG system with dry-recording electrodes is an unstoppable trend for the sustainable development of EEG technologies toward future continuously widening clinical applications and personalized medicine.
In particular, wireless, portable, and wearable EEG systems require all of the instruments to be integrated into a small wearable box that is self-powered by a miniature battery. 32Wireless data transmission is power intensive, so it is necessary to compress historical data in real time. 40,41 I G U R E 3 Electroencephalogram (EEG) measurement of brain waves with different frequencies.(A) Measured EEG signals over the scalp; Reproduced with permission. 17Copyright 2019, Elsevier.(B) Brain waves with different frequencies.Reproduced with permission. 30Copyright 2021, Wily-VCH.
Currently, despite addressing lots of technical challenges for the development of wireless, portable, and wearable EEG systems, [42][43][44] the cost and relatively limited battery lives still require improvement for future sustainable development.

Big-data Monitoring/Analysis of EEG Signals
Driven by future extended applications in home-based health monitoring, early warning of neurological diseases, telemedicine, and personalized medicine, the sustainability of EEG systems has also been considered in terms of big-data monitoring, analysis, and modeling.The incorporation of mobile EEG hardware with big-data computing and Internet of Things technology provides a promising solution for big-data monitoring/analysis of EEG signals toward precision diagnosis and health monitoring services. 45Specifically, big-data modeling with raw mass data in the EEG community promotes reuse at the end of life to minimize waste.
EEG signals are generally related to the post-synaptic potentials of the human brain and thus can be utilized to acid brain activity, cognition, and psychiatric diagnosis. 39,46EEG signals present brain electrical currents which are primarily consisting of Na + , K + , Ca ++ , and Cl − ion streams controlled by membrane potential. 30herefore, EEG signals over the scalp illustrated in Figure 3A  EEG recording systems are generally composed of signal amplifiers, analog filters, and A/D converters.Highquality signal amplifiers are utilized to amplify weak microvolt-level EEG signals.Peripheral electronic circuits are simultaneously implemented in amplifiers to reduce the unwanted environmental and systemic noise in the original EEG data.Analog filters are also applied to further remove specific noise components and enhance the signal-to-noise ratio (SNR) of the amplified EEG signals.
High-pass and band-reject (notch) filters can be used to optionally reject The low-frequency physiological noise, AC noise, and even unwanted distortion in the amplified EEG signals are optionally suppressed by using the high-pass filters, band-reject filters, and low-pass filter respectively.The A/D converters convert the amplified and filtered analog EEG signals into a digitalized range which could be conveniently analyzed. 47ne of the most important characteristics of EEG signals is the spatiotemporal variability.Multichannel electrode EEG measurement using the international 10-20 standard 37,48 is an internationally recognized method to monitor spatiotemporal EEG signals.In the 10-20 system, the electrodes were placed at certain points to measure the EEG signals with spatiotemporal variations.Driven by the widely used EEG technologies in various clinical applications, high-quality and high-temporal-spatialresolution EEG signals are highly desirable.Therefore, reliable measurement techniques to accurately acquire multichannel EEG signals in real-time are highly desired.Notably, replacing conventional gel-based low-density wet-recording electrodes with multichannel flexible dry electrodes is an unstoppable trend for high-quality and high-temporal-spatial-resolution capture of EEG signals.
Multiple channels with big data are developing trends in EEG monitoring.In general, recording as few as four EEG channels is clinically useful for EP and BCI-type applications. 49,50However, such four-channel systems are not commonly used in clinical diagnosis and treatment.Modern clinical diagnosis and treatment especially expect EEG systems with more than 256 channels.To avoid spatial aliasing, up to 600 channels have been developed. 51arallel to this channel-count trend, an increase in sampling resolution is also required.In contrast to traditional EEG systems with a dynamic range of 7 b, 52 most currently existing commercial EEG systems require 16 or more bits, which exceeds the 12 b recommendation. 37,42,53he continuously expanding EEG applications in the EEG community have led to big-data monitoring and analyzing challenges.Numerous large EEG datasets have accumulated rapidly.For example, more than 30,000 clinical EEGs from Temple University Hospital have been collected in an open databased named the TUH-EEG Corpus. 54Similarly, the MNI Open iEEG Atlas contained intracranial EEG data of 1772 channels from 106 subjects. 55Therefore, it is necessary to develop EEG tools and pipelines to handle such big-data EEG signals Up to now, MATLAB-based EEG tools (e.g.EEGLAB and FieldTrip 56,57 ) and MATLAB-based pipeline tools (e.g., PREP, 58 CTAP, 59 HAPPE, 60 and Automagic 61 ) have been developed successively.MATLAB-based EEG tools are suitable for batch process small-scale EEG data with offline personal computers.MATLAB-based pipeline tools have been utilized to preprocess large-scale raw EEG data and to assess the quality of the preprocessed EEG data offline.Recently, specific brain information computing platforms have been developed to handle rapidly accumulating EEG big data and support continuously expanding EEG applications. 62

SUSTAINABLE DEVELOPMENT OF EEG ELECTRODES AND MATERIALS
Electrodes are critical elementary components of EEG technology.The materials of EEG electrodes are significantly crucial to determining the EEG signal quality.EEG electrodes with various materials directly contact the scalp and capture the electrical signals generated during physiological activities.As the neural interface, EEG electrodes play crucial roles in recording neural activity.A key challenge in realizing mobile EEG systems is applying appropriate EEG electrodes connected to the scalp.The sustainable development of EEG electrodes and materials is analyzed from the following three aspects: performance of different material/structural categories, sustainable materials for EEG dry electrodes, and sustainable manufacturing technologies.

Performance of different material/structural categories
Based on different material and structural categories, EEG electrodes are subcategorized into three types: wet, semidry, and dry electrodes.It has been demonstrated that EEG electrodes are shifting from gel-based wet electrodes to dry electrodes.Schematic diagrams, real examples, and schematics of the wet, semi-dry, and dry electrodes are shown in Figure 4. Conventional gel-based wet electrodes generally exhibit excellent electrical stability, high SNR, and low contact impedance.However, sustainable development is still seriously hindered by complicated scalp preparation, scalp allergy, and long-term failure induced by the inevitable conductive gel.Considering the combination of clinical applicability and economic effects on sustainable development, semi-dry and dry EEG electrodes have been extensively investigated.Table 2 presents a comparison of the three different types of EEG electrodes (wet, semi-dry, and dry electrodes) used from a sustainability perspective.

Wet EEG electrodes
Wet EEG electrodes require conducting gel to achieve low electrode/skin contact impedance during EEG measuring. 64Traditional wet EEG electrodes are based on various metal materials.Ag/AgCl-based wet electrodes are the most commonly used wet EEG electrodes with excellent DC stability, good SNR, low noise level, and low resistance. 30,35,65Conductive gel is necessary to improve the interface contact between the skin and Ag/AgCl-based electrodes and minimize the movement of the electrode.
Wet electrodes are prepared as disposable surfaces, reusable discs, or saline-based electrodes.Disposable surface electrodes provide a simple and cheap method.However, directly applying disposable surface electrodes through the hair down to the scalp is difficult due to the large size of the adhesive pad.Importantly, disposable electrodes are against the sustainable development of EEG technology.
Wet EEG electrodes are commonly used in commercial EEG caps, where electrodes are embedded and positioned according to the international 10-20 system. 17,66During the use of an EEG cap, adhesive matter should be added between each electrode and the scalp to ensure good contact.Despite high-quality EEG signals, prolonged EEG monitoring in practical applications is still hindered by the complex preparation process, dry-out of the conducting gel, possible allergic problems, time cost of electrode replacement, and serious issues related to stability and biocompatibility.These drawbacks against the sustainable development of EEG technology and its applications have driven research into alternative electrodes.

Semi-dry EEG electrodes
Semi-dry EEG electrodes 63,67 aim to balance the advantages of wet EEG electrodes and the inconveniences induced by the amount of gel.Great efforts have been devoted to designing semi-dry electrodes that use aqueous  electrolytes instead of gels to lower contact impedance with a lifespan insufficient for practical EEG applications.Generally, the volume of electrolytes is approximately several hundred times smaller than that used for each wet EEG electrode.Due to the small amount of electrolyte typically in a few tens of microliters, semi-dry EEG electrodes exhibit advances in reducing hair dirt, avoiding short circuits, and maintaining low electrode/scalp impedance.However, recorded EEG signals are generally unstable because of uncontrolled or unexpected liquid release.Moreover, the saline solution is usually supplemented manually, leading to a complex EEG testing process.

Dry EEG electrodes
Dry EEG electrodes circumvent the requirement of gels and aqueous electrolytes, overcoming the drawbacks induced by adhesive materials.This is accomplished by using unique structures (e.g., small pins) or high-conductivity materials to reduce the interface impedance for current conduction.State-of-the-art dry EEG electrodes 30,35,67 are subcategorized into two types: invasive and noninvasive.Typically, an invasive dry electrode comprises an array of microstructures (e.g., microspikes, micropins, and microneedles) that could pierce the resistive stratum corneum (SC).Therefore, inva-sive dry EEG electrodes present advances in improving interface contact and relieving motion artifacts.These invasive electrodes are used in chronic experiments, such as neurofeedback, BCIs, and human-machine interfaces.However, invasive EEG electrodes could cause skin or tissue reactions including allergic reactions, rashes, and skin irritation.Seriously, fragile spikes often break off and cause infections or inflammations.9][70][71][72] The noninvasive dry EEG electrodes exhibit significant advantages in terms of user-friendliness and sustainable development.However, the dry electrode-skin contact impedances are relatively high.Additionally, they are sensitive to noise, electromagnetic interference, and movements.Despite partially alleviating the above issues, there is still a long way to go for high-quality recording of EEG signals by using noninvasive dry electrodes.

Sustainable materials for EEG electrodes
Like developing trends of other technologies, the development of EEG electrodes is continuously pursuing high performance.However, owing to its particularity of clinical applications, the sustainable development of EEG materials is to be simultaneously considered.Material development offers significant support for improving the main performance while considering sustainable development.For practical clinical applications, electrode materials play a crucial role in the performance of EEG systems and the sustainable development of EEG technologies.Highconductive materials with low electrode-skin interface impedance are required for electrodes to enable highquality EEG signals.This is a critical premise in exploring sustainability.From the perspective of sustainable development, resource richness, biocompatibility, comfortability, cost, and environmental impacts have been taken into account.The resource richness and biocompatibility depend directly on the materials themselves.Comfortability is generally determined by a combination of materials and structures.Flexibility is a typical factor affecting the comfort of EEG electrodes in wearable EEG systems.The economic and environmental impacts are required to be considered during EEG electrode fabrication.For example, high conductivity and low resource consumption determine the sustainable development of metallic material-based EEG electrodes.The sustainable development of polymer-based dry electrodes is motivated by comfortable, low-cost, and reusable applications.The emerging nanomaterials provide an opportunity for the sustainable development of dry electrodes fully considering high performance, comfort, biocompatibility, and environmental friendliness.Table 3 presents a comparison of EEG dry electrodes with different materials from a sustainability perspective.

High-conductivity metallic materials
Metallic materials exhibit no gap between the conduction band and the valence band, producing excellent electronic conductivity.High electronic conductivity is an essential prerequisite for EEG electrodes to enable low electrodeskin contact impedance and monitor high-quality signals.
Many metallic elements exist in the human body, indicating good biocompatibility.Considering conductivity, moisture stability, and reusability along with biocompatibility, noble metals (such as Ag, Au, and Pt) are preferred materials for metallic EEG electrodes.Ag-based materials have been widely used in EEG wet electrodes and are considered candidates for dry electrodes because of their outstanding physical properties. 30onventional wet Ag/AgCl electrodes could record highquality EEG signals with proper skin preparation and conductive gel usage. 35,65Regrettably, the unavoidable skin-preparation procedures lead to disadvantages in terms of time consumption, user-friendliness, and longtime monitoring feasibility. 71Ag-based materials have been considered for constructing dry EEG electrodes owing to their high electrical conductivity of 6.3E7 S/m.Ag/AgCl-based dry electrodes have been prepared by compacting the mixture of Ag and AgCl powders. 30owever, the high cost and scarcity of noble metal materials block their future sustainable practical applications.4][75][76][77][78][79][80][81] Fortunately, this roadmap provides an opportunity to enhance comfort when noble metals are coated on flexible substrates or structures.
Thin, lightweight, and conformal EEG electrodes with low electrode-skin impedance are highly desirable for long-term monitoring.Figure 5 presents comfortable dry electrodes with high-conductivity noble metallic coatings.Thin-film Au electrodes that are conformable to complex 3D biological surfaces have been developed to record highquality biopotentials, as shown in Figure 5A. 79Learning from this technical approach, Pt dry electrodes have also been reported, 30,80 as shown in Figure 5B.Pt electrodes with nano-porous surfaces exhibit an excellent EEG signal correction. 80To significantly reduce resource consumption, combining noble metal with other cheaper elements (e.g., Ti and Cu) is an effective strategy.For example, using Ag/Ti, Cu/Au/Pb, or BeCu/Au multilayer instead of noble metal coating offers a possible road to develop economical dry EEG electrodes with high-conductivity metallic materials.

Flexible polymer-based dry electrodes
Many polymer-based materials are relatively inexpensive, readily available, and biocompatible, exhibiting extensive applications in flexible bioelectronics.They are promising to be used to construct comfortable dry electrodes in flexible bioelectrical recording systems.However, apart from conductive poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), 82,83 most of the frequently used polymers have poor electrical conductivity.To improve the electrical performance while considering the sustainable development of polymer-based EEG electrodes, enhancing the conductivity of polymers by compositing with conducting films or materials is necessary to construct polymer-based dry electrodes.
Polydimethylsiloxane (PDMS) is preferred in various biomedical devices and systems due to its flexibility, biocompatibility, and resistance to chemical/thermal attack.PDMS-based microstructures can be fabricated conveniently by using molding technology at a low cost.Flexible F I G U R E 5 High-conductivity noble metallic electrodes related to dry electrodes.(A) Au dry thin film electrode.Reproduced with permission. 79Copyright 2018, Wily-VCH; (B) Pt dry electrode with nano-porous surface.Reproduced with permission. 30Copyright 2021, Wily-VCH.
PDMS-based dry electrodes with different pin structures could be fabricated by sputtering Au.The PDMS-based dry EEG electrodes have been used to record EEG signals without conductive gel and skin preparation. 77Similarly, flexible dry electrodes can be prepared by depositing a metal layer on other polymers such as parylene and polyimide (PI). 79,84PI is also extensively used as flexible substrates because of its excellent thermal stability, high chemical resistance, thin and flexible structures, and high-precision pattern with photo-lithography. 84Flexible PI-based dry electrodes with an Au layer have been reported, exhibiting feasibility to measure multichannel EEG signals.
To enhance mechanical properties and improve the electrode-skin contact, SU-8 could be a good choice for constructing polymer-based dry electrodes.SU-8 is superior to PDMS and PI in terms of hardness, facilitating the penetration of SU-8-based dry microneedle electrodes through the SC. 81In skin anatomy, skin is divided into epidermal, dermal, and subcutaneous layers.The epidermal layer contains SC and stratum germinativum (SG).The SC consists of electrical isolation of dead cells, exhibiting high electrical impedance.The SG consists of components of living cells, displaying electrical features.Therefore, dry electrodes are expected to be penetrated through the SC and stopped at the SG to reduce interface impedance while avoiding pain, bleeding, and infection.Therefore, SU-8-based microneedle-type electrodes have an opportunity to reduce electrode-skin contact impedance in the absence of tissue trauma.A SU-8-based dry microneedle electrode coated with metal film has been reported as shown in Figure 6, 81 demonstrating a sufficiently low skin-electrode contact impedance for EEG recording.

Promising nanomaterial-based dry electrodes
Nanomaterials offer a promising way to construct comfortable dry EEG electrodes with high performance and stability.From the perspective of sustainability, nanomaterialbased dry electrodes exhibit outstanding advantages of resource richness, biocompatibility, comfortableness, economic effect, and environmental friendliness.The combination of these advantages makes them ideal candidates for constructing dry electrodes for the future sustainable development of EEG materials and technology.
Due to the high electrical conductivity and large surface area, Ag nanowires (AgNWs) prospectively lead to higher performance than conventional Ag/AgCl wet electrodes.AgNW-based dry electrodes have been prepared by combining AgNWs with PDMS. 85,86Owing to the conducting, flexible, and stretchable features, AgNW-based dry electrodes are beneficial for conforming to skin surfaces, thereby reducing skin-electrode impedance and eliminating motion artifacts. 85In particular, fully embedded AgNWs into PDMS could keep long-time high-quality EEG measurements and be re-used for several months without signal degradation.Combined with the simple and costeffective fabrication, wearable AgNW-based dry electrode is a promising candidate for the sustainable development of EEG electrodes for long-term high-quality health monitoring.
Carbon nanomaterials, such as carbon nanotubes (CNTs) and graphene, also attract much attention due to their outstanding mechanical and physical properties. 30NTs were first applied to human EEG recordings in 2008, 30,87 where a brush-like structure was used to reduce electrode-skin contact impedance, as shown in Figure 7A.

F I G U R E 6
Flexible polymer-based dry electrodes.For example, SU-8 microneedle electrodes.Reproduced with permission. 81Copyright 2015, IOP.
CNT-based EEG dry electrodes offer a robust, stable, and low-impedance interface without penetrating the outer skin layer to achieve a kind of user-friendliness EEG measurement.Dispersing CNTs into viscous PDMS solution is a generally used method to construct electrically conductive CNT/PDMS composite, leading to a small, userfriendly, biocompatible, and wearable dry EEG electrode. 88he CNTs/PDMS-based EEG electrodes have been demonstrated to be comfortable during recording, exhibiting potential applications in u-health, BCI, and other biosignal recordings.Graphene is also a kind of commonly used material to construct wearable dry electrodes (as shown in Figure 7B) because of its low thickness, good mechanical properties, and biocompatibility. 89,90Low thickness could enable conformal contact with human skin, facilitating improved SNR. 89 Good mechanical property permits high stability and fidelity of signal quality even when compressing or stretching the graphene-based electrodes. 30The biocompatibility has been well demonstrated by the viability of the cells staying on the graphenebased electrodes for several days.Graphene can be prepared by various methods such as the chemical Hummer method, laser patterning, and electrochemical exfoliating, which indicates the feasibility of producing graphenebased wearable electrodes with low cost and large mass.Multichannel graphene-based wearable electrodes have great potential for future extensive applications which include but are not limited to clinical healthcare and BCI-related applications.

Sustainable manufacturing technologies
Manufacturing is a major concern in sustainable development.This is primarily because manufacturing activities consume significant quantities of natural resources and produce large amounts of waste.From the sustainable perspective, consuming fewer resources at a low cost while producing less waste toward net zero is preferred for an ideal manufacturing process.Several fabricating methods of dry EEG electrodes have been widely explored, including microelectromechanical system (MEMS)-based methods, screen printing, and 3D printing technology.The MEMS-based process is a conventional method to fabricate 3D spiked arrays consisting of metal, semiconductor, or polymer-based microneedles. 91,92This method can be used to produce large-area microstructure arrays with feature sizes as small as 10 µm.However, MEMS-based methods usually suffer from high costs and complex processes involving lithography equipment, which are not conducive to sustainable development.The screen-printing method can be used to introduce disposable, easy-to-use EEG electrodes at a low cost.Electrodes with conduction traces can be printed with silver ink directly onto a polyester film in a flatbed sheet silk screen printing unit. 93Despite being suitable for the preparation of nanomaterials-based films, this method is difficult to use to produce 3D structures.
As a sustainable manufacturing process, 94,95 3D printing (3DP) has attracted remarkable attention because of its potential to reduce waste, use less energy, lower costs, and create more sustainable products.The 3DP technology was developed in the 1980s.Since then, materials have been printed through layer-by-layer deposition using a computer-aided design (CAD) program.Despite the limits of the resolution in the range of around 20-40 µm, 3DP technology allows to fabrication of complex 3D electrodes with several kinds of materials in a short time.In recent years, 3DP has become widely available, affordable, and eco-friendly.Note that, 3D dry electrodes are beneficial for recording EEG signals in the presence of hair. 96The development of 3D hair-compatible electrodes would further improve user-friendliness.Various kinds of dry electrodes have been customized to adapt different hair types for high-quality and comfortable EEG measurements. 97From a practical perspective, it is highly desirable to incorporate personalization into the electrode fabrication.This finding is consistent with the sustainability.
It has been well demonstrated that 3DP allows personalized EEG electrodes to be fabricated on a nearreal-time basis. 75,76,96,98,99Owing to its versatility and cost-effectiveness, 3DP is considered a convenient and lowcost approach for fabricating dry electrodes.In addition to designing flexibility, the designed electrodes can also be easily printed with different materials, as numerous material options are commercially available.A wide number of electrode designs and configurations (e.g., finger, spider, spiny, and microneedles) are possible via 3DP.Various biocompatible polymers (e.g., PLA plastics 76,98 and acrylic-based resins 96 ) are selected in the 3DP process.Base electrodes fabricated by 3DP are nonconductive and not directly suitable for EEG measurements.With respect to the electrical contact performance, activation by depositing a conductive cover layer is required.If the base material is flexible enough to enable comfort, its conductivity under different amounts of compression/tension must be ensured constant. 100In addition, a full 3DP method has been developed to manufacture EEG electrodes entirely through 3D printing using a conductive filament. 101ver the last few decades, advances in 3DP techniques have facilitated the fabrication of accurate biological components and complicated 3D geometrics. 102As a wellknown sustainable manufacturing process owing to its reduced waste, reduced usage, recycling and repurposing of materials, and customization potential, 3DP is promising for the industrial revolution in the production of various EEG electrodes with the outstanding advantages of hair compatibility, biocompatibility, comfort, and personalized capabilities.

SUSTAINABLE EEG APPLICATIONS
EEG has been developed for nearly a century and motivates a rich and diverse spectrum of applications.Several typical applications exhibit significant sustainability based on the sustainable development of EEG technologies.For example, EEG is an essential method for diagnosing, classifying, and localizing seizure foci in patients with EP.Specifically, EEG has been the most popular method to directly measure neural activity in BCI systems.The sustainable development of EEG technologies is partially motivated by BCI-related applications and promotes the future development of sustainable BCI systems.

Epilepsy
Typically clinical applications of EEG include the diagnosis of convulsive and non-convulsive seizures, the assessment of encephalopathy, the identification of cerebral ischemia, and the monitoring and treatment of malignant intracranial hypertension. 103Epilepsy is a serious neurological disorder that affects approximately 70 million people of all ages worldwide. 104Despite the development of a variety of antiepileptic drugs over the past two decades, there are still 20%-40% of epileptics are medically intractable or drug-resistant.Epilepsy surgery is the gold standard therapeutic option when medications have failed. 105The goal of EP surgery is to rescue epileptics from seizures by completely eliminating the epileptogenic zone in their brain.The seizure onset zone (SOZ) is the area of the cortex that produces seizures and can be measured using EEG technology.
EEG is an essential method for diagnosing, classifying, and localizing seizures in epileptics.This application has evolved and extended with the sustainable development of EEG technology.Localization of the SOZ using EEG relies on its ability to detect potential changes occurring at the millimeter level of the cortex.However, the sustainable development of this application is seriously limited by its invasiveness and the restricted number of electrodes. 1068][109] Nowadays, scalp EEG is a dominating diagnostic tool for identifying SOZ.It is mainly utilized to facilitate judge 110 : (1) whether EP is occurring; (2) which types of EP it might be; (3) its specific location in the brain.After standard EEG evaluation for epileptic, interictal epileptic discharges (IEDs) are found in approximately 50% of patients with EP. 111 Long-term EEG monitoring could increase the chance of detecting IEDs.However, long-term inpatient EEG monitoring is not universally available due to the limitations of monetary and resource overheads. 112Ambulatory EEG exhibits obvious advantages over long-term inpatient monitoring, but several limitations in terms of portability, large amount of data, and time consumption still remain.
Recently, portable EEG used in closed-loop EP treatment systems has attracted significant interest.In addition, thanks to the development of analytical computing techniques, the combination of EEG with neuroimaging technology has enabled the extending clinical applications in the evaluation of patients with EP. 19,113,114 This EEGrelated presurgical evaluation is of great significance to the identification of SOZ and understanding EP occurrence.

Brain-Computer interfaces
The sustainable development of EEG technologies is partially motivated by BCI-related applications (as shown in Figure 1) and in turn, promotes the development of sustainable BCI systems in the future.In 1971, the concept of using EEG signals to control prosthetic arms was proposed, opening the research area of BCIs. 115,116Generally, BCI systems use recorded EEG signals to communicate between the brain and a computer, aiming to control the environment by using human intentions. 117BCI systems have been applied in two primary directions. 118The first is to explore a feedforward pathway used to control external devices by using recorded EEG signals.The other is a closed-loop BCI system for neural rehabilitation in which the feedback loop is used to regulate brain activity.EEG is the most popular method to directly measure neural activity in BCI systems.The development of the entire BCI field strongly depends on the technological level of the EEG system itself.The development of wireless, portable, and wearable EEG systems with dry recording electrodes offers good prospects for the future of BCIs in the realm of mobile EEGs. 119 specific protocol and paradigm must be selected to implement an EEG-based BCI system for a particular application.First, the subject performs a particular task to learn how to modulate their brain activity while EEG signals are recorded from the scalp.A neurodecoder of the paradigm is generated using the recorded EEG as training data.Afterward, the subject performs the task again and the neurodecoder is used for BCI control. 120uring the past decade, recorded EEG signals have been reported to control various devices and systems including wheelchairs, 121 communication aid systems, 122 and assistive rehabilitation devices. 1235][126][127][128] Particularly, finding the relationships between recorded EEG signals and BCI-based devices is still full of challenges.In recent years, exploring the relationships between EEG signals and upper limb movement has become a research hotspot. 129,130ustainable progress on BCI systems has been maintained through the sustainable development of EEG technologies.Despite the availability of commercial devices and established roadmaps, 131 critical limitations and challenges are longstanding related to EEG-BCI platforms.A wireless, portable, and wearable EEG system with dry recording electrodes could facilitate understanding the brain dynamics conveniently and promote the development of BCI systems.Recently, wearable and wireless EEG headsets have been reported. 132,1335][136][137] The sustainable development of EEG materials and technology could promote continuously expanding applications of BCIs.In addition, computational intelligence approaches will energize the sustainable development of EEG-based BCI studies in the near future. 138Looking ahead, establishing a sustainable developing path for responsible neuroengineering is essential. 139n light of combining performance with sustainability, there is no doubt that replacing conventional wired and bulky EEG systems with gel-based wet recording electrodes using a miniaturized, wireless, portable, and wearable mobile EEG system with dry recording electrodes is an unstoppable trend.Owing to the highly desirable highquality and high-temporal-spatial-resolution big-data EEG signals, the replacement of the conventional gel-based lowdensity wet recording electrodes with novel multichannel flexible dry recording electrodes is an overwhelming trend.Despite significant efforts and progress, several challenges in EEG technologies with respect to future sustainable development for extended applications in home-based health monitoring, early warning of neurological dis-eases, telemedicine, and personalized medicine should be addressed.

SUMMARY AND PERSPECTIVE
1. Typical limitation on power dissipation.Power dissipation is a crucial design parameter to be considered especially for a mobile EEG system.Battery size and capacity are the major determinants of the size of a mobile EEG system.Although the power constraints have been well summarized for wearable EEG systems, 140 these constraints have not been completely overcome.A front-end system has been demonstrated to produce a 25 µW power consumption per channel. 141A typical transmitter has been demonstrated to consume 50 nJ/b with approximately 120 µW for each transmitting channel.To realize 24-h continuous EEG recording, high efficiency is required for all fronts of a mobile EEG system.This presents a major challenge in realizing highly efficient mobile EEG systems.2. Typical limitation on reliability.Reliability is mainly related to the consistency of a measure across testing conditions.However, no metrics have been reported to assess the reliability of the EEG system.A recent study demonstrated that there was little variance (1%) compared to the contributions of "participants" or "systems," 142 suggesting that variability is induced by individual differences.Recently, test-retest reliability has been investigated and has been well demonstrated during seated and walking conditions under various external environments. 143,144Particularly, the reliability is lower for eyes-open resting states or cognitive paradigms owing to electrode misplacement. 144,1453. Lack of assessment criterion.The sustainable development and practical application of EEG technologies require the establishment of corresponding assessment criteria that include the following three aspects at least.The first is an assessment criterion for comprehensively evaluating the functions of EEG systems.The second is an assessment criterion for normalizing the practical operation of EEG technologies.The third is the assessment criterion for evaluating the sustainable development of EEG technologies.The sustainable development and practical application of EEG technologies are severely limited by the lack of assessment criteria.
It should be noted that together with the above challenges, several important directions also exist for the sustainable development of EEG technologies in the future.There is no doubt that material science is a key research direction not only at present but also in the future.On the one hand, the continuous exploration of novel materials will provide a stream of vitality for the sustainable development of EEG.On the other hand, investigating the optimization of micro-nano-structures based on the existing materials is another way to pursue the sustainable development of EEG.In addition to this, two possible future directions must be considered: power harvesting and multimodal recording.
Power-harvesting techniques harvest power from the ambient environment of the user, which is a promising technique for overcoming the power issues in the EEG system. 146Power-harvesting techniques are expected to harvest more than 100 µW, significantly relaxing the power constraints.In this context, a two-channel EEG system could be powered by body heat alone. 147However, the power source is usually nonconstant.Nevertheless, great efforts are required to establish the feasibility of such systems.
Multimodal EEG technologies will provide a more comprehensive map of brain functions.EEG is one of the most commonly used methods to assess the electrical activity of the brain in multimodal operations.Generally, the electrical signals associated with brain activities are passively recorded by the scalp electrodes of EEG systems.9][150][151][152] With multidisciplinary efforts, multimodal EEG technologies, such as EEG-fMRI and EEG-fNIRS, [150][151][152][153] could offer wide-reaching implications for the sustainable development of neuroscience, psychology, clinical neurology, BCI, neurorehabilitation, and personalized healthcare.

A C K N O W L E D G M E N T S
Ling Xiong and Nannan Li contributed equally to this study.This work was supported by the National Natural Science Foundation of China (grant number 62271458) and the Sichuan Province Central Government Guides Local Science and Technology Development Project (grant number 2023ZYD0015).

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

O R C I D
Nannan Li https://orcid.org/0000-0003-0964-9626 reveal changes in brain patterns by analyzing the recorded signals with the response to voluntary or involuntary mental commands.The electricity amplitude of EEG signals is generally in the range of 0.5-100 µV with four different frequency ranges, which are categorized as beta (>13 Hz), alfa (8-13 Hz), theta (4-8 Hz), and delta (0.5-4 Hz) regions, as shown in Figure 3B.Brain waves with Alfa frequencies are characteristic of EEG signals, especially in adults while blinking.Alfa waves with amplitudes of about 50 µV are easily obtained between the posterior, occipital, and central lobes.Beta waves are characteristic signals of rapid eye movement during sleep.Theta rhythms were observed in the semi-sleep state.Delta rhythms typically appear during sleep.EEG signals are extremely sensitive to successive state changes from stress, and alertness to resting, hypnosis, and sleep.

F I G U R E 4
Real examples and schematics of electrode-skin interfaces of wet, semi-dry, and dry electrodes.Real examples of (A) wet electrodes, (B) semi-dry electrodes, and (C) dry electrodes.Reproduced with permission.35Copyright 2022, Royal Society of Chemistry.(D-F) Schematic diagram of wet electrodes, semi-dry electrodes, and dry electrodes.Reproduced with permission.63Copyright 2017, Elsevier.

EEG technology Assessment aspects of sustainability Sustainability dimension
Sustainability assessment of electroencephalogram (EEG) materials and technologies.
TA B L E 1EEG systems• Developing trend toward mobile systems • Miniaturization, wireless, portability, wearability • Low energy; reduced carbon impacts • Reduced burden on medical resources• EEG signal: big-data monitoring/analysis • Big-data modeling • Zero-paper solutions • Customized telemedicine; public health For integrated systems, sustainability can be focused on the developing trend toward mobile EEG systems and bigdata monitoring/analyzing of EEG signals.Sustainability is related to miniaturized systems, reduced burden on medical resources, and big-data modeling, which are beneficial for customized telemedicine for human health.For EEG electrodes and materials, sustainability can be evaluated based on the following three aspects: performance of different material/structural categories, sustainable materials for EEG electrodes, and sustainable manufacturing technologies.In this review, we summarize the sustainable development of EEG materials and technology from five perspectives, as illustrated in Table1.Finally, we present sustainable applications of EEG and provide novel insights into the future development of EEG for clinical medicine.
Sustainable development of electroencephalogram (EEG) dry electrodes with different materials.
TA B L E 2 Comparison of different types of electroencephalogram (EEG) electrodes from a sustainability perspective.