Applications of MXenes in human-like sensors and actuators

Human beings perceive the world through the senses of sight, hearing, smell, taste, touch, space, and balance. The first five senses are prerequisites for people to live. The sensing organs upload information to the nervous systems, including the brain, for interpreting the surrounding environment. Then, the brain sends commands to muscles reflexively to react to stimuli, including light, gas, chemicals, sound, and pressure. MXene, as an emerging two-dimensional material, has been intensively adopted in the applications of various sensors and actuators. In this review, we update the sensors to mimic five primary senses and actuators for stimulating muscles, which employ MXene-based film, membrane, and composite with other functional materials. First, a brief introduction is delivered for the structure, properties, and synthesis methods of MXenes. Then, we feed the readers the recent reports on the MXene-derived image sensors as artificial retinas, gas sensors, chemical biosensors, acoustic devices, and tactile sensors for electronic skin. Besides, the actuators of MXene-based composite are introduced. Eventually, future opportunities are given to MXene research based on the requirements of artificial intelligence and humanoid robot, which may induce prospects in accompanying healthcare and biomedical engineering applications.


Introduction
To perceive the surroundings, humans receive five types of senses, including sight, sound, taste, smell, and touch. These sensings ( Fig.  1) are correlated to the information perception by the eyes, ears, tongue, nose, and skin. The degradation or diseases of sensory organs may decrease the happiness of human beings. The development of artificial sensors, which mimic the five sensory organs, tends to bring back a clear perception of the outside world.
To understand the advantages of MXenes, we compare the differences between MXenes and other two-dimensional (2D) materials. Zhang et al. summarized the difference between MXene and other two-dimensional materials according to their optical properties [26]. The dominant features of typical two-dimensional materials are listed in Table 1. Compared with traditional bulk materials, two-dimensional materials [39,40] are more suitable for human-like sensing applications because of their large surface area [41], high carrier mobility [42,43], and flexibility [44,45].
In brief, their differences are depicted as follows. In ultrafast photonics, the low modulation depth of graphene due to atomiclayer thickness has limited its applications. Although topological insulators can achieve nonlinear broadband response, their manufacturing process is complicated, which leads to high costs [56]. When the thickness is fixed, two-dimensional transition metal chalcogenides have certain bandgaps, limiting the optical absorption in the mid-infrared region. Black phosphorus shows continuously tunable bandgaps by changing its thickness; however, phosphorene suffers from weak air stability. Other monoelemental 2D materials remain less intensively investigated, requiring more groups' participation. In contrast, MXene has the features of both adjustable bandgap and good stability, which makes it highly competitive in ultrafast photonics applications.
First of all, we introduce the progress of MXene synthesis. First is top-down such as etching of A layer from bulk MAX phases and delamination; another is bottom-up, including the direct chemical vapor deposition, as well as the sputtering reaction of Ti, Al, and C for epitaxial MAX film formation, followed by the etching of A layer for remaining MXene film over the supporting substrate. Four reprehensive approaches are listed in Fig. 2. The emerging trends of MXene synthesis include mild synthesis conditions such as alternative etchants other than HF.
Before discussing the device performances, we look at the structure of MXenes via the classification of MXenes. According to their morphologies, MXene can be divided into four categories, i.e., quantum dots, nanosheets, epitaxial film or large domains (by chemical vapor deposition), and the macroform.
We start with the top-down strategies for producing the quantum dots and nanosheets from the parent MAX phases. First, the MXene-based quantum dots have features in photoluminescence of different wavelengths according to the size of quantum dots. The quantum dots can be prepared by the microexplosion method [105] or high-frequency electromechanical vibration [106]. The quantum dots have demonstrated applications in light-emitting diodes [107], ultrafast photonics [26,100], cellular imaging [108], and catalytic cancer therapy. Second, the Ti 3 C 2 MXene nanosheets can be obtained by selected Al etching of Ti 3 AlC 2 MAX phase and delamination. These nanosheets (in dispersions) can be spun or drop-coated onto dielectric substrates for depositing patterned electrodes, which demonstrate the functions of sensors or transistors [109].
Then, we briefly introduce bottom-up synthesis strategies, including a thermal deposition from atomic or molecular precursors and the three-dimensional (3D) assembly from nanosheets. Third, the nanosheets can be assembled into macroform, including clay by powder hydration [110,111], freestanding membrane by vacuum filtration [21,112], or aerogel by freeze drying [102,113]. Fourth, the MXene films can be grown by bottom-up approaches, including the co-sputtering of Ti, Al, and C targets into MAX films and chemical vapor deposition of Mo 2 C [114]. The epitaxial film has potential in wafer-scale integrated electronic devices such as transistors, chemiresistors, and photodetectors. The macroform may find its applications in energy storage, such as batteries and supercapacitors, as well as energy conversion, including catalysis. Here, the MXene films become dominant in sensor applications. We come to the discussion of sensor applications of MXenes.
Here, we provide a timely review of the emerging MXene-based sensors for achieving the performances of artificial organs (Scheme 1), including light sensors, gas sensors, chemical biosensors, sound sensors, and tactile sensors. Besides, the MXenebased actuators are briefly introduced. We start with the sight sense, including photodetectors and image sensors.

Image sensor for vision sense
The principle of sight is depicted as follows. The eyes convert light to image and color signals for processing by the brain. The retina, a membrane composed of arrays of photoreceptor cells, converts the light beam into nerve pulses, which transform to the synaptic potential and transmit to the brain. Eventually, the brain reconstructs the visual image continuously.
MXene was employed as an electrode for a perovskite-based photodetector array for image sensing. Wang et al. designed an image sensor (Fig. 3) with MXene/perovskite/MXene structures by laser scribing technology. The MXene shows an energy band alignment with the perovskite-based photoactive layers, which facilitates charge transfer at the interface.
Cheng et al. report an MXene/Si-based image sensor array with a high resolution of 2 μm [140]. The Ti 3 C 2 T x /Si photodetector has a high-density integrated array of 1,024 pixels (Fig. 4), with a detectivity of 7.7 × 10 14 Jones and an ON/OFF current ratio (6.2 × 10 6 ).
With a transistor-based detector in each pixel, the MXene/Si image sensor [140] has been fabricated with conventional lithography protocols over a 4-inch wafer, which shows high compatibility with Si technology. The MXene stacking with other photosensitive materials, such as metal sulfides [141][142][143] or selenides [144][145][146], leads to the regulation of detection wavelength. For example, PdSe 2 [147] may extend to the nearinfrared for thermal imaging.
Another 32 × 32 pixel image sensor report has demonstrated a deer's shape [148] based on the heterostructures of MXene/RAN. Here, RAN polymer serves as photosensitive material and MXene as conducting electrodes. Such a photodetector shows transmittance of 70%, excellent mechanical stability, and performance retention under large bending angles.
Future opportunities remain in the coupling of MXene and photoactive materials. First, the metal halide perovskites [149], semiconductor nanowires [150], and metal chalcogenides [151,152] become emerging photosensitive materials for image sensors. In type I heterojunction, the bandgap engineering facilitates colorsensitive imaging, i.e., wavelength selective photodetection [153] via a bias voltage. Second, more active materials can be selected from conventional solar energy materials, including antimony selenides [154,155] and sulfides [156,157] and tandem structures [151], amorphous silicon [158], and four-element compound thin films [159][160][161]. Third, the charge transport layer may promote the sensitivity and responsivity of the photodetectors.
High-performance optoelectronic devices require significant absorption coefficients, long diffusion lengths, and adjustable bandgap width. High-quality photosensitive materials are among the research hotspots of next-generation optoelectronics. Early image sensors are developed based on light intensity. Future opportunities remain for recognizing color by speculating the wavelength of illuminated light.

Artificial eardrum for hearing sense
Human hearing perceives sounds via ear after receiving air vibrations with periodic pressure changes. The auditory system conducts human hearing, which converts the mechanical waves by the eardrum into neural pulse signals for brain collection. Hearing loss can be caused by heredity and congenital at birth, presbycusis in aging people, and acquired by living environments. Several solutions promote hearing ability, such as wearing a hearing aid device and restoring the perforated eardrum.
Firstly, hearing aid devices, which integrate the microphone, amplification circuit, and loudspeaker, have improved the living conditions of deaf people and others with hearing loss. The current technology of acoustic devices suffers from heavy rechargeable nickel-metal batteries, which are not comfortable to carry. The low energy consumption, lightweight, and integration with micro-batteries or micro-supercapacitors are core requirements for technology upgradation. In addition, the safety  [162] and the lifetime of batteries should be taken care of [163]. Therefore, low-dimensional nanomaterials are ideal alternative materials for acoustic membranes in microphones, amplification integrated circuits (if any), and loudspeakers. Secondly, lowdimensional nanomaterials have been implanted in the perforated eardrum of animal models for hearing recovery.
In this section, we discuss the progress in MXene-based acoustic devices, including fundamental components such as microphones and loudspeakers and system-level applications, e.g., artificial eardrums and artificial throats for voice recognition assisted by machine learning and data training.
We begin with the MXene-based microphone device. The sound provides a weak force of continuously regulated frequency that can be transduced to electric current by a microphone.
The vibration of air or water typically transmits the sound. Humans can hear the audio frequency ranges from 20 to 20,000 Hz. The acoustic wave induces the sound pressure (with a pascal unit), a frequency-dependent periodic change in local air pressure, which varies from one standard atmosphere. The microphone can measure the sound pressure.
The sound pressure level (SPL) depicts the relative pressure compared to a standard value in a logarithmic format, i.e., 20 log 10 (p/p 0 ) dB, where p is sound pressure and p 0 is 20 μPa as reference.
The typical acoustic membranes rely on isinglass, wood, mica, and polymers. They feature heavyweight and are not easily tailored. The ultimate goal for acoustic devices is to produce or detect a sound with high sensitivity, low detection limits, and a broad frequency spectrum [164]. Further improvement in acoustic performances remains challenging based on these conventional materials. Nanostructured materials may improve the performance of acoustic membranes. Acoustic sensors made of graphene [164] and carbon nanotubes [165] have been reported to detect human voices and recognize speech assisted with deep data learning.
Here, we discuss the artificial eardrum based on the MXene membrane. Ren et al. constructed an MXene-based piezoresistive pressure sensor that simulates artificial eardrums (Fig. 5) and combined it with machine learning to perform sound detection and recognition [166].
The pressure sensor [166] performs well, including a sensitivity of 62 kPa −1 and a detection limit (of 0.1 Pa). The MXene artificial eardrum ( Fig. 6) can precisely record audio waves comparable to commercial recorders. Based on the voice recognition algorithm, the MXene eardrum-based artificial auditory system can identify 280 voice signals with 96.4% accuracy. The concept of acoustic devices shows promising applications in wearable healthcare devices.
The artificial throat is introduced based on MXene. In the human throat, the larynx produces sound by periodic opening and switching off the vocal cords [167,168], which chop the continuous airflow to pulsed sound waves. The collisions of vocal cords produce harmonics, with a fundamental frequency of 120 Hz for men and 210 Hz for women. The artificial larynx or vocal cords become necessary for mimicking human voices, reparating voice loss, and restoring throat diseases.
Ding et al. prepared an MXene-based artificial throat that recognizes the pair (ɑː and ae) of a long and short vowel by sound detection sensor [169]. The MXene-based acoustic sensor achieves a perceived response to pressure and vibration (Fig. 7).
They used the convolutional neural network model [169] to recognize syllables in human pronunciation, which can identify long and short vowels. The study could broaden the applications of MXene acoustic sensors in sound generation for mimicking the larynx and vocal cords. The loudspeaker component based on MXene is still an ongoing investigation. The MXene has proven outstanding biocompatibility. But, the reparation of perforated eardrums by MXene membrane remains unexplored.

Gas sensors for olfaction sense
The olfaction sense is generated by the recognition of the gas molecules in the air. Here we discuss the MXene-based gas sensors.
We take examples of acetone, methanol, toluene, and humidity to demonstrate the MXene potentials in gas detection. Acetone, as a biomarker of diabetes [170,171], indicates insulin levels that promote glucose conversion. When blood sugar levels rise, the acetone concentration decreases in the exhaled gas and increases in insulin [172,173]. Therefore, an acetone gas sensor [174,175] provides an early diagnosis of diabetes and health monitoring. Most acetone sensors employ metal oxides for catalytic oxidation of acetone to change the conductance as a sensing mechanism. The MXene incorporation could elevate the sensor performances by a large specific area and excellent conductivity.
Wang et al. designed an acetone sensor based on chemiresistors of α-Fe 2 O 3 /MXene heterostructure [176]. The sensor can detect acetone content by the amount of electron transfer upon the acetone decomposition. The sensor has shown good selectivity of acetone detection, viz., the response to acetone has exceeded six times than that of other organic molecules such as toluene and alcohols [176].
Two mechanisms account for the sensitivity and selectivity of the sensor. First, the hydrogen bonds between MXene and acetone ( Fig. 8) guarantee the efficient chemisorption of acetone, which is favored compared to aromatic molecules without forming hydrogen bonds. Second, the bond dissociation energy of acetone (366 kJ·mol −1 ) is much lower than methanol and ethanol; therefore, the oxidation of acetone provides a large number of electrons for increasing the conductance of the chemiresistorsbased sensors.
The sensing mechanism is depicted as follows. The adsorption capacity of the sample surface was analyzed according to the density functional theory (DFT) calculation. The surface model of the heterostructure was optimized (Fig. 8) to α-Fe 2 O 3 /MXene. The gas sensitivity mechanism of the α-Fe 2 O 3 / MXene sensor to acetone was discussed.
Such a sensor, which operates at room temperature, exhibits specific selectivity for acetone, with a response of 16.6% to 5 ppm acetone and a response/recovery rate of 5/5 s. The acetone sensor has a large humidity tolerance ranging from 20% to 80%. Cycling tests show good performance retention after 28 days, which provides an essential reference for acetone sensing at room temperature.

Syllable recognition convolutional neural network
Train data Test data Methanol, which is converted from a waste of biomass, has been employed in fuel cells for consumer electronics and vehicles [177,178]. Methanol is toxic to the body's perception organs, blood, and nervous systems [179,180]. Often, the methanol sensors employ noble metals for catalytic oxidation of methanol to provide electron transfer, which induces resistance change as a sensing mechanism. Therefore, it is necessary to develop a quick and easy gas-sensing method to detect methanol [181][182][183].
Wang et al. fabricated a methanol sensor based on chemiresistors of In 2 O 3 /MXene heterostructure [184]. Here the In 2 O 3 serves as a sensitive material for providing electrons to chemisorbed oxygen molecules to form O 2 − . The highly reactive O 2 − oxidizes the methanol into carbon dioxide and releases electrons, which increases the conductance of the chemiresistors as a sensing mechanism. The MXene, as an electron-rich species, promotes the electron transfer to the depletion layer of In 2 O 3 .
When the sensor is initially exposed to air, the In 2 O 3 /MXene shows a decrease in carrier concentration due to electron depletion by forming O 2 − and an increase in resistance. When methanol is adsorbed to In 2 O 3 surface, O 2− can release electrons for methanal oxidation [185], which results in conductance changes and improves the sensitivity.
The sensor [184] exhibits a response rate of 29.6% for five ppm methanol and a response/recovery time of 6.5/3.5 s. This method of combining metal oxides/MXene provides methanol detection options.
Toluene is widely used in interior decoration as an adhesive solvent. Toluene is highly toxic to the respiratory system and can lead to nasopharyngeal cancer and bronchial disease [186]. Besides, toluene becomes an essential biomarker of lung cancer. Therefore, detecting toluene traces becomes significant for early lung cancer diagnosis [187,188].
Salama et al. designed MXene-based sensors for the selective detection of toluene [189]. They used ultrasound treatment to increase the specific surface area of Mo 2 CT x MXene materials, which eventually reflected in the improvement in device performances.
The MXene sensor exhibits remarkable selectivity for toluene sensing [189], which causes over three times larger responses than the other four gaseous molecules, including benzene, ethanol, methanol, and acetone. According to the ab initio calculations, the toluene shows the lowest adsorption energy at the MXene surface among the five types of organic molecules. First, the alcohol and ketone molecules are chemisorbed on MXene surfaces by hydrogen bonding [190]. Second, the interaction between the benzene ring and MXene is more robust than hydrogen bonds, reducing the charge carriers' concentration and increasing the MXene channel's resistance. Third, the methyl radical in toluene [191] enhances the activity of aromatic rings compared to benzene, which accounts for the best response in sensing toluene.
The toluene sensor based on MXene [189] has a limit of detection of 220 ppb, a linear detection range (35−170 ppm), and a sensitivity of 0.037 Ω·ppm −1 .
The Ti 3 C 2 T x MXene sensor can detect acetone, methanol, and ammonia at room temperature [192][193][194][195]. MXene has been reported in examining the concentration of volatile organic compounds (VOCs) in a nitrogen environment, which mimics air ambiance [196]. In these laboratory works, the impact of humidity on sensor performance has not been considered.
Sysoev et al. investigated the influence of humidity on the performances of gas sensors based on MXene chemiresistors in detecting organic and inorganic molecules [197]. The MXenebased humidity sensor has achieved a record limit of detection (10 ppm) for sensing H 2 O concentration (Fig. 9), which exceeds the performance of a commercial humidity sensor.
In the background of dry air, the sensor can be used to detect volatile organic gases, alcohols, acetone, and ammonia. But in wet air, only ammonia [197] causes the change in resistance, while other analytes do not change sensor resistance compared to the reference wet air. The strong hydrogen bonds between ammonia and MXene may account for the change in conductance. Indeed, other organic solvents, which do not form hydrogen bonds with MXene, cause no change in resistance in MXene channels. It shows the concerns of significant interference by the water [197], which makes the organic gases undetectable, i.e., no conductance changes in sensor resistance when incorporating organic gases in wet air. Therefore, the target analytes' dry treatment becomes necessary before the concentration examination.
Moreover, a multi-sensor array can be integrated with the same wafer with the Mo 2 CT x MXene as a sensing material, potentially detecting different vapors parallelly by multiplexer technology.

Chemical biosensors for gustation sense
The perception of food taste turns vital for human happiness when going to dinner. The tongue hosts the taste receptor cells for recognizing the taste of different chemicals. We discuss the chemical biosensors based on MXenes for detecting chemicals that produce the sense of taste. Samples for recognition are capsaicin, inosine monophosphate, and L-glutamate.
First, capsaicin, as the primary source of spiciness, has induced spicy Chinese dishes, which has a large consumer group. One evaluates the degree of spicy food by the content of capsaicin. Standardizing pre-cooked dishes requires precise control of capsaicin content in food. Hence, one can evaluate capsaicin concentration by various methods, including colorimetry [198], spectrophotometry [199], and liquid chromatography-mass spectrometry [200]. Among them, electrochemical sensors can reduce the cost of expensive equipment and save the detection time of complex operation protocols. Xu et al. designed an electrochemical sensor for capsaicin detection using MXene/poly(diallyldimethylammonium chloride) (PDDA)-carbon nanotubes/β-cyclodextrins [201]. The sensor utilizes MXene and carbon nanotubes to amplify the electrochemical current signal (Fig. 10) by enlarging the specific surface area of the composite. Indeed, the redox current gets extensively promoted by the MXene composite modified work electrode compared to bare glassy carbon. The β-cyclodextrins improve the degree of dispersion of the MXene-based suspension.
The MXene-based electrochemical sensor [201] was employed to detect capsaicin in three commercially available Chinese food, including pot-roast duck neck, pot-roast chicken claw, and potroast beef, which are heavily cooked with soy sources. The sensor achieves a linear detection range of 0.1−50 μmol·L −1 , with a limit of detection of 0.06 μmol·L −1 and a recovery rate of 84%−126%. This work has potential application in detecting food content, which mimics the function of gustation organs such as tongues.
Second, inosine monophosphate can be an essential indicator of meat quality. Liu et al. [202] adopt the MXene/enzyme-modified glassy carbon electrode in a biosensor for detecting inosine monophosphate. In the double-enzyme hydrolyzed inosine monophosphate process, the decomposition of H 2 O 2 leads to the transfer of charge, producing an electric current. Subsequently, the current change can determine the content of inosine monophosphate.
Such a biosensor, with a linear detection range of 0.04-17 g·L −1 and a detection limit of 2.73 ng·mL −1 , promotes an easy and quick detection of inosine monophosphate content. The MXene/enzymebased biosensor [202] has precisely detected the content of inosine monophosphate in four kinds of meat, including chicken and beef. Indeed, the amount of ingredient of inosine monophosphate was evaluated by such a biosensor, e.g., 1.88 mg·g −1 in chicken and  2.44 mg·g −1 in beef, which showed a low relative deviation (< 2%) compared to commercial liquid chromatography. Third, L-glutamate is one of the components involved in the human perception of food taste, and the content of free glutamate can determine the umami taste of food [203]. Monosodium glutamate, which is rich in L-glutamate, has been widely used in cooking Chinese dishes. The accurate determination of the glutamate content of foods can provide information on food safety and standardization of pre-cooked food [203] that the capital market favors.
Liu et al. developed an MXene electrochemical sensor based on Pt nanoparticle modifications for the selective detection of glutamate [203]. The sensor uses the decomposition of H 2 O 2 (produced during enzyme-catalyzed glutamate oxidation) to cause electron transfer to glassy carbon for electrochemical sensing (Fig.  11).
In evaluating glutamate concentration, the MXene-based biosensor shows a linear detection range of 10-110 μmol·L −1 , a sensitivity of 1.59 nA·μmol −1 , and a limit of detection of 0.45 μmol·L −1 . The sensor can be successfully used to detect monosodium glutamate as added to cooked foodstuff such as soy sauce, beef, and vegetable soups. The detection results of such an MXene-based biosensor are comparable to commercial liquid chromatography.
Besides, the field-effect transistors guarantee the recognition of miRNA [204]. Similarly, the photodegradation of organic dye molecules can be monitored by optical methods [205]. The transistor performances can be promoted by interface engineering [206] and Fermi-level depinning [207]. The transistor arrays guarantee industrial production compatibility [208].
MXene-based biosensors can detect more taste-related chemicals. The data fusion of several chemical sensors could lead to taste recognition.

Pressure sensors for touch sense
The tactile sense is produced by human skin through environmental pressure, temperature, and humidity. Electronic skin (e-skin) has become a crucial replacing material in prosthetic limbs, stretchable electronics, and biomedical monitoring.
Four work mechanisms exist in the pressure sensor for touch senses, including piezo-capacitive, piezoresistive, piezoelectric, and triboelectric modes. Here, we discuss the recent progress of MXene incorporation in these four types of pressure sensors. First, we start with the capacitive mechanism for pressure sensing.
Tung et al. [220] designed an electronic skin based on a piezocapacitive pressure sensor of MXene/polyacrylamide hydrogel heterostructures (Fig. 12(a)). In a parallel plate capacitor, the MXene/polypyrrole nanowires serve as both the top and bottom conductive plates. Meanwhile, the vinyl silica nanoparticlemodified polyacrylamide hydrogel and elastic tape play the role of dielectric (Fig. 12(b)).
In the elastomer performance, the abundant bonding between polymers and nanowires has reduced the energy dissipation of the hydrogel network [220]. Indeed, the composite exploits hydrogen bonds on polyacrylamide molecular chains and covalent bonding between polyacrylamide and vinyl-hybrid-silica nanoparticles, improving the sensor's toughness and responsiveness. Moreover, the formation of hydrogen bonds between bridging layers of polypyrrole nanowires and MXene can promote the sliding stability of MXene/hydrogel heterostructures. Therefore, the material has a skin-like strain-sensitive deformation and recovery behavior.
Furthermore, piezo-capacitive sensing is formed by pressing two heterogeneous structures together. When the external environment changes, the electric field of the electronic skin changes, and the charge transfer changes the capacitance, achieving sensing at a distance of 20 cm (Fig. 12), which is closer to the natural skin performance. The MXene sensor has significance in prosthetics or robots with a natural feeling.
When a significant strain applies (e.g., considerable gesture change when playing badminton and wearing e-skin sensors on forearms), the piezo-capacitive sensor does not operate accurately. Therefore, the piezoresistive mechanism applies to provide monitoring. Indeed, the MXene sensor [220] has a vast work range of up to 2,800% (breaking strain), a response of 90 ms, an elasticity of 240 ms, and reproducibility of greater than 5,000 cycles.
Eventually, an intelligent system is assembled to monitor finger approaching and pressuring (Fig. 13). Indeed, the MXene-based eskin is transmitted to a data analyzer circuit and connected to a smartphone app with a wireless Wi-Fi module for real-time human motion monitoring by attaching to the elbow joint. Now we turn to the piezoresistive sensor. Shen et al. designed a flexible piezoresistive sensor using MXene/polyacrylonitrile composite membranes [210]. The polyacrylonitrile and MXene nanosheets were blended thoroughly to form a composite (Fig. 14) 2e − Figure 11 MXene/Pt nanoparticles/enzyme composite modified glassy carbon as electrochemical sensors for detecting the concentration of L-glutamate. The amperometry was employed to collect the current at the work electrode from the electron transfer due to the oxidation of H 2 O 2 . L-glutamate oxidation (catalyzed by glutamate oxidase) generates H 2 O 2 , indicating L-glutamate concentration. Reproduced with permission from Ref. [203], © Elsevier Ltd. 2021. encapsulated and electrically connected by both MXene electrodes. When pressure is applied, the voids in the fiber network are compressed, allowing the MXene nanosheets to come into closer contact with each other. The close connection of MXene nanosheets increases the number of current paths, thereby significantly increasing the contact area of the composite fiber network. These structures minimize strain caused by bending.
Besides, they used the piezoresistive sensor to form a circuit with the light-emitting device (LED) (Fig. 15). When the sensor resistance changed, the voltage of the LED changed accordingly, thus characterizing the pressure received by the sensor with the brightness of the LED [210].
The sensor shows a sensitivity of 104 k·Pa −1 , with a response/recovery time of 30/20 ms and a limit of detection (1.5 Pa) to withstand 240 bending cycles. The sensitivity of this MXene electrode-based sensor is 20 times higher than similar sensors [221][222][223][224] using traditional nickel, copper, gold, or silver electrodes. The sensor can detect subtle movements of muscles, such as finger bending. The study allows MXene to combine with polymer fibers to design wearable devices.
The slight motion of humans can be quantified by MXene/polyimide aerogels-based piezoresistive sensors [225]. The gentle carotid artery has changed the resistance by 0.4% due to the small deformation (Fig. 16).
Indeed, the gestures of breath, swallowing, and pronunciation [225] were perturbed by the change in resistance, ΔR/R 0 , below 10%. But the bending of the finger and knee achieved changes in resistance of 20%-30%.
Next, we come to discuss the piezoelectric potential-based pressure sensor. Ko et al. used MXene/ferroelectric polymer-based composite to design the piezoelectric pressure sensor [226]. The sensor uses functional groups on the surface of MXene to form hydrogen bonds with polyvinylidene fluoride (PVDF) to bind them. The MXene has a charge accumulation effect in MXene/polyvinylidene fluoride. When subjected to external pressure, the enhanced polarization of the sensor interface induces electron transmission to balance the potential, which in turn enhances the piezoelectric potential output. In addition, the porous structure of the sensor produces a local stress concentration effect at the hole after being pressed (Fig. 17), which makes the sensor's sensitivity several times higher than that of the planar structure.
Subsequently, we update the report on triboelectric nanogenerator-based pressure sensors. Li et al. [227] proposed the MXene as electrodes for connecting the triboelectric layer to form a nanogenerator-based pressure sensor (Fig. 18). Polytetrafluoroethylene (PTFE) was employed as filtration paper to support MXene film formation during vacuum filtration.
Besides, the PTFE serves as a triboelectric layer in the MXene/PTFE/MXene-based triboelectric nanogenerator (TENG). The TENG works in contact-separation mode. The pressure sensor has shown significant sensitivity and an excellent retention ratio in a cycling test of 6,000 cycles.
The MXene-based TENG can operate in a single electrode mode driven by the falling and sliding of water droplets. Indeed, the device inside an infusion pipe works for remote drop counting (Fig. 19). Therefore, it may hold promise in biomedical and healthcare applications.
The MXene electrodes assisted TENG [227] have promoted the sensitivity (6.1 V·N −1 ) to subtle force, together with fast rising and The MXene/rGO aerogel-based pressure sensor [216] shows a linear range of 0-40 kPa and a response of 61.5 kPa −1 , which hold promises in monitoring human health statuses, such as heartbeats, pulse, and motions. The microbridges in human skin possess high  elastic moduli, which inspires the interlocked structure of MXene/microcapsule for pressure sensors [168]. Such an interlocked MXene composite has led to over 9-fold sensitivity compared to planar MXene-based pressure sensors. Moreover, the silk fibroin was employed as a template for crosslinking the MXene nanosheets into macroform [212]. Such a compositebased pressure sensor shows a low limit of detection (9.8 Pa) and high retention after 3,500 cycles. Indeed, the blends of MXene with polymers [211,213] become essential strategies for designing high-performance pressure-sensing materials. Biodegradable electronics reduce the quantity of solid waste in the environment [228,229], termed green materials, and green production techniques [230]. The bioresorbable [231][232][233], bioabsorbable devices [234,235], and transient electronics [236], may satisfy the in vivo-friendly requirements for implantable clinical applications [237]. Biodegradable devices consist of substrates and functional electronic components of different conductivity. For example, poly(vinylidene fluoride-cohexafluoropropylene) (P(VDF-HFP)) serves as conducting gate electrodes in top-gated transistors [238]. Often, one can choose biodegradable polymers, plant-derived biomass [239], natural wax [240], silk fibroin [241,242], and chitosan [243] as supporting substrates for hosting device fabrication.
In addition, self-healing [261] becomes a prerequisite feature for polymeric building blocks for wearable electronics. In the future, one needs to develop lab-to-fab potentials for the mass production of stretchable electronics in materials preparation [262].
The self-powering of electronic devices become emerging topics promoted by MXene-based energy storage. To date, the MXenebased new gadgets such as batteries [263][264][265][266] and supercapacitors [267] remain individual prototypes in the lab [268], viz., no commercial widgets are available in portable electronic devices such as smartphones and tablets. Indeed, the on-chip microsupercapacitors [269] facilitate the powering of miniaturized sensors but with a low technological readiness level [270,271] due to the limited efficient volume and package issues.
The technology readiness level of MXene-based composite and energy devices can be estimated as 1-2 [272]. It may take another ten years to achieve the technological maturity of MXene.
In the forthcoming decade, the community should tackle several technological issues. First, the shortening of charging time of batteries [273][274][275] and supercapacitors should exceed the conventional anode materials of graphite. Besides, the supercapacitors [276] are typically applied in the uphill climbing stage in an electric vehicle, and the batteries [277] are dominant power sources for motion on flat ground. When supercapacitors possess a high energy density comparable to a battery [278], supercapacitors may play a more important role in electric vehicles.
Besides, the wafer-scale production of micro-supercapacitor remains low in technological readiness level [279]. Similarly, clean electricity can be generated by the reduction of carbon dioxide [280,281], hydropower [282], and solar cells [283]. Second, the production cost should be reduced to less than the graphite. The MXenes have demonstrated successes in laboratory production, i.e., 50 g MAX transformed to MXene per batch in a 60 mL container [284]. The MXene nanosheets are as high quality as in a minimized synthesis protocol, i.e., 5 g per batch.
Third, incorporating MXene nanosheets into the yarns may push the textile electronics forward, including the core-sheath structure [285] of MXene-based capacitors. Here, the robust electrical contact inside a device and between the series and parallel connected device cascades remains challenging. Indeed, the stretching and strain can easily induce the deformation and breakage of the MXene-based composite electrodes, which causes electrical disconnect and device failure. Besides, the MXene plays the role of separation membrane [286] for the adsorption of the toxins in blood [287,288], which may regenerate the dialysate and eventually lead to a wearable kidney.
Fourth, integrating MXene-based displays with triboelectric nanogenerators may facilitate self-powered electroluminescence [219,289]. In addition, the electromagnetic shielding film [290] may be the first real-world product based on MXenes, which can suppress the interference of two adjacent microphones.
The sensors often require a battery for continuous operation. Wearable electronics [291], including implantable devices [292], demand lightweight and self-powering, i.e., without an external power supply. Indeed, the integration of pressure sensor with micro-supercapacitors [293,294] and photovoltaic devices [295,296] has shown energy storage of solar energy [297] for selfpowering of sensor operation [298]. Here, MXene can promote the performances of the solar cell as a charge transport layer [299] and suppress the dendrite formation in batteries [300] as an electrolyte additive [301]. The lithium-ion batteries [302] and zincion batteries [303] can connect with energy production units [304][305][306][307]. The nanogenerators themselves can serve as biosensors Here, a series of water droplets continue falling to and sliding away from the sensor. Reproduced with permission from Ref. [227], © Elsevier Ltd. 2021. [308]. Programmed macrostructures have enriched the choice of flexible sensors [309]. The data retention can be extended with the assistance of resistive random access memory [310] and neuromorphic computing [311]. Indeed, the interfaces could be built between data acquisition [312] (at electronic skins) and transmission to processing devices (for computing).

Actuators for artificial reflex
Simulating human perception and responding to stimuli is a fundamental challenge for bionic robots and neural prosthetics. Actuators for artificial reflex enable the perception of light, heat, and electricity and convert various signals into chemical or physical signals to complete the expected reflections.
Lee et al. proposed an MXene/cellulose bilayer heterostructure for a near-infrared irradiation-induced actuator [313]. Inspired by the leaf, the palisade mesophyll expands and shrinks upon water incorporation with a vein as robust support to retain structural stability. The polycarbonate filter supports the membrane formation by vacuum filtering the ink of MXene/cellulose blends (Fig. 20).
The sensor uses MXene nanosheets to convert light energy to thermal energy (simulating palisade mesophyll cells) to achieve thermal actuation. First, the flexibility of the sensor is achieved by constructing a biocompatible nanofiber skeleton (simulating the leaf vein skeleton). The MXene is mixed with cellulose (simulating the stratum corneum). An asymmetrical structure with polycarbonate (acting as the epidermis) achieves hygroscopic actuation. As the humidity increases, the cellulose absorbs water, which induces the shrinkage of the polymer architecture (Fig. 20).
The bending of an actuator occurs by the deformation of the polymer layer while MXene layer remains unchanged. With the thermal treatment, further deformation occurs by the interlayer spacing decreases.
First, the sensor uses MXene, and hydrophilic groups on the surface of the nanofibers can form hydrogen bonds with water. It responds to trace amounts of water of 44.4 μg·cm −2 by asymmetric structure and automatic bending at 10% relative humidity and improves the response speed through the porous structure of polycarbonate film. Second, the sensor can achieve bending folding under near-infrared wave illumination.
The realization of this function is mainly driven by the synergy of a large number of MXene-cellulose nanofibers. When irradiated with near-infrared light, a rapid heating up of 2.3 s of low power can be achieved. Third, according to the finite element model analysis, the volume mismatch between MXene-cellulose nanofibers and polycarbonate membranes in the near-infrared environment leads to the movement of the sensor, and as the light increases, the shrinkage increases. Fourth, the sensor can implement programmable behavior in a narrow rectangular area.
The MXene/cellulose could be tailored to two-column or threecolumn patterns (Fig. 21). By infrared irradiation, the MXenebased actuators experience periodical bending and recovery, which mimics the motion. Besides, the folding and unfolding of the box can be obtained. Moreover, the blooming and closure of the MXene-cellulose nanofibers to make bionic flowers by nearinfrared irradiation are realized. The drive process has an energy density of 0.74 W·kg −1 and a power density of 0.92 W·kg −1 .
Such MXene-based actuators lighten the display upon infrared irradiation, indicating good information encryption. The deformation upon infrared irradiation may lead to intelligent switches of night light. They also demonstrate the possibilities of application in the haptics. The human-like sensors and actuators have demonstrated excellent potential in future applications. First, artificial prosthetics [314,315] possesses huge markets, which is a system-level product that integrates tactile sensors (often termed electronic skins), temperature and humidity sensors [316], and interfaces [317]. The tactile sensor arrays [318] facilitate the imaging of palmprint [319].
And artificial muscles employ the concept of actuators. Second, the current service robotics remain machine-like and not ideal for human interaction, i.e., a shortage of human-like bending and stretching. Therefore, intelligent sensors and actuators will boost the actuate machine vision [320] and speed of sensing tiny objects and elevate actuation performances [321] with delicate displacement and small forces. Future service robotics may mimic human perception [322] and respond more vividly. Third, humanmachine interfaces arouse next-generation consumer electronics.
The metaverse-based electronic products and networking incorporate intelligent headsets, virtual reality [323], brain-machine interfacing [324], virtual community, flexible displays, three-dimensional vision, sixth-generation wireless systems [325], and the internet of things. Fourth, smart cities [326] may exploit intelligent sensors, innovative construction materials, smart switches based on actuators, the internet of everything, big data, and cloud computing. Human-like sensors and actuators have a bright future for evolving technological developments and growing huge markets.

Conclusion and future opportunities
This review provides the most recent advances in MXene-based sensors that demonstrate the fundamental functions of humanlike five senses and the actuators that serve as artificial muscles. First, the artificial retina based on MXene has been discussed for image pattern recognition. Second, the MXene-based gas sensors cover the detection of several representative gases, including methanol (biomass-derived fuel), volatile organic gases, including toluene (hazardous in vehicles and indoor decoration), and acetone as biomarkers of diabetes. Third, the taste sense has been achieved by detecting the chemicals in food that produces the gustation, such as L-glutamate (in monosodium glutamate), inosine monophosphate (in meat), and capsaicin (in spicy dishes). Then, the acoustic devices are discussed, including artificial eardrums, loudspeakers, microphones, and sound generators. In addition, the four types of pressure sensors are outlined based on MXene electrodes and sensitive materials. Eventually, the MXenebased actuators were conveyed in response to humidity, light irradiation, and magnetic field.
But the fundamental physics behind remains less known, i.e., carrier relation [343] and ultrafast dynamics [344], bandgap manipulation, and the conductivity-regulating mechanism. The investigation of the model MXene of Ti 3 C 2 T x , is relatively extensive [98,345], covering its conductivity and optical properties [346]. However, challenges remain for other types of MXenes [347] that have been less explored for both synthesis and properties. Hence, more efforts should investigate the semiconducting behaviors of MXenes by experimentally regulating their different types and amounts of terminational groups and theoretically predicting optical properties [348].
The synthesis of MXenes continues to evolute in both dry and wet conditions. First, substituting elements in MXenes leads to forming new compounds [374,375]. More efforts should be put into the controlled direct thermal deposition synthesis of MXene film over a wafer scale. Indeed, compound structures [376,377] analogous to MXene have been synthesized by chemical vapor deposition. Second, the solution processing of MXene [92] becomes promising for mass production of nanosheets in dispersions [378]. The MXene quantum dots [100] are efficient photoluminescent materials for near-infrared images [108]. Besides, the long-term stability of MXene-based aqueous solutions or dispersion could be improved by polymer hybridization [379] and surface engineering [93,378] without oxidation [95]. Indeed, printed electronics require the long durability of MXene-based inks [119] for reducing manufacturing costs and maintaining the uniformity of fabricated films. In addition, the MXene-based heterostructures [96,380,381] or composites [87,382] continue to expand the knowledge boundaries. The morphologies of hydrogel [125] and aerogel may elevate their mechanical and electrical performances.
Regarding the synthesis of MXene-based semiconductors, there is a lot of room to fill in between the ideal MXene materials and the currently available status. The MXene of different morphologies [96,98] will continue to evolve, aiming at the improved homogeneity of quantum dots, nanosheets, monolayer film, and thick membranes. Further developments are needed to optimize the stability of MXene nanosheets [383] and the homogeneity of nanosheet size and thickness. For the ultimate goal of MXenes, their terminational groups could be precisely manipulated, i.e., controllable fluorination [94], in the means of the density and the location of the MXene. The substitution of X element may induce novel phenomena [384]. The yield of MXene can be elevated during the etching and rinsing of nanosheets. More possibilities of heterostructures occur when coupling the MXene with different functional materials.
The MXene-based devices are mainly designed for individual functions, e.g., sensors [97,385], memories or data processors, and power units such as batteries and supercapacitors [386]. First, the individual device should break the theoretical limit for higher performances [387]. The integration of microfluidic chips and chemiresistor results in biosensors of good selectivity [120]. The Internet of Things era [388,389] requires high integration density of devices for lightweight, portable [390], and wearable electronics [391,392]. A few emerging devices and components have achieved dual functions by integrating two devices: temperature and humidity sensors, nanogenerators and supercapacitors for selfpowering, triboelectric nanogenerators for self-powered sensing, and humidity sensors and actuators. Indeed, the integration of supercapacitors [393] with logic devices leads to self-powering and self-charging [394]. More innovations may turn in for enriching the device design and fabrication.
The sensing-memory-computing in one remains challenging and requires more effort to push forwards. First, the memristor keeps boosting its performance as memory [395,396]. The insensor memory [397,398] and neuromorphic signal transmission have shown the integration of two functional devices [399]. Indeed, brain-like neuromorphic computing based on memristors is an emerging hot topic for elevating artificial data processing [400]. The MXene-based memristors and synapses [401] are still in their infancy. The system-level integration [402] requires the compromise of devices, hardware, algorithm, and interfaces.
MXene may eventually turn up in clinical applications after long-term mature surgery over animal models [418][419][420][421]. For example, the decoration of MXene by DOXjade groups [405] can employ the accumulation and localization of MXene nanosheets in tumor tissue, in which DOXjade liberates and serves for chemotherapy of tumors. However, the operation mechanism of MXene-based drug delivery and biomedicine has not yet been clarified. Efforts should be taken to achieve the in vivo release and biodegradation [422] after completing its role of therapy inside human bodies. Therefore, a more focused investigation on the reliability and robustness of MXene could promote its clinical applications.
Humanoid robotics remain the ultimate goal of artificial intelligence. Indeed, humanoids could be assembled by connecting various sensors and motors (actuators) for mimicking human behaviors such as speech, interactive dialogues, accompanying, and nursing. The wearable smart glasses and metaverse have driven the displays for virtual reality in healthcare and accompanying applications. Soft robotics based on MXene actuators [423] may provide efficient solutions for an artificial limb.
In sum, tremendous opportunities remain in MXene-related research, including materials synthesis, properties, device performances, and intelligent integrated systems composed of multiple interconnecting devices and their communications. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.