Carbon Nanotube-Based Self-Powered Sensors for Autonomous Environmental and Biomedical Monitoring

Abstract

Self-powered sensor technologies are receiving increasing attention owing to their ability to operate independently without the need for external batteries or power supplies. This autonomy enables continuous and real-time monitoring in various applications. Carbon nanotubes (CNTs) are particularly promising as electrode materials and energy-harvesting components, owing to their excellent electrical conductivity, mechanical robustness, and tunable surface properties. This review provides a concise overview and critical perspectives on recent progress in CNT-based self-powered sensors, focusing on their structural designs, operating mechanisms, and application areas. The sensors are classified according to their practical application environments, including environmental, wearable, and implantable applications, rather than by their energy-harvesting mechanisms or detection targets. Furthermore, current critical challenges, such as durability, scalable fabrication, and in vivo validation, which must be solved to achieve fully autonomous CNT-based sensors for healthcare and environmental monitoring, are discussed. This review underscores the pivotal role of CNT-based self-powered sensors in driving next-generation autonomous monitoring technologies and offers insights for the implementation of such sensors in practical biomedical and environmental applications.

1. Introduction

The increasing demand for continuous and real-time monitoring in healthcare, environmental sensing, and wearable devices has intensified the need for autonomous low-maintenance sensors. However, conventional battery-powered sensors are often limited by their dependence on replaceable batteries or wired supplies, the need for frequent maintenance (e.g., battery replacement and calibration), and system-level integration issues, which hinder scalability and long-term usability. Self-powered sensors can harvest ambient energy through piezoelectric, triboelectric, thermoelectric, photoelectric, and emerging electrochemical mechanisms. These include faradaic systems such as redox-type biofuel cells and non-faradaic systems where electric double-layer modulation drives current generation, offering a promising solution to current limitations [1,2]. Among various materials, carbon nanotubes (CNTs) are particularly attractive owing to their high electrical conductivity, mechanical flexibility, high aspect ratio, and tunable surface chemistry. Beyond these general features, CNTs offer unique practical advantages over graphene and MXenes. First, continuous CNT fibers and yarns can be directly spun via floating-catalyst CVD, enabling scalable production of textile-grade conductors, whereas graphene and MXene fibers largely rely on wet-spinning of colloidal inks, often requiring binders or post-treatments [3,4,5]. Second, CNT networks exhibit superior long-term stability under humid and physiological conditions, while MXenes such as Ti₃C₂T_x are chemically prone to oxidation and graphene fibers require additional functionalization for stability [6,7,8]. These unique characteristics enable CNTs to act not only as conductive materials but also as active sensing elements and energy harvesters, making them highly adaptable to diverse operational demands. Rather than following the traditional classification by working principles, this review organizes recent progress according to application domains (environmental, wearable, and implantable systems), because each context imposes distinct requirements on device design, stability, flexibility, and biocompatibility (Figure 1). By focusing on application-driven scenarios, it reveals how CNT-based platforms are advancing the field of autonomous sensing for biomedical and environmental monitoring.

2. Multifunctional Roles of CNTs in Self-Powered Sensors

The first TEM observations of hollow graphitic filaments were reported in 1952 [9]. Subsequently, the formal identification of multi-walled carbon nanotubes by Iijima in 1991 [10] marked a milestone that drew significant scientific attention to CNTs. The catalyst-assisted synthesis of single-walled nanotubes in 1993 [11,12], together with subsequent advances in laser ablation, HiPco, and CVD (including water-assisted ‘supergrowth’) [13,14,15], transformed CNTs from nanoscale curiosities into scalable, device-ready materials. Subsequent progress in alignment, sorting (metallic/semiconducting), and processing into large-area films and direct-spun fibers/yarns shifted the field from single-tube physics to robust, application-level engineering across electronics, energy, and biointerfaces [16,17,18,19]. Against this backdrop, the role of CNTs in self-powered sensing is particularly compelling. Their structural and electronic attributes, including large interfacial area and tunable Seebeck/work functions for charge generation, low-loss percolation networks for charge transport, and fiber/textile form factors that enable mechanical compliance and scalable integration, collectively match the core requirements of self-powered sensors [1,20].

In this context, CNTs emerge as multifunctional platforms that simultaneously serve as energy harvesters, active sensing elements, and performance enhancers. As harvesters, CNTs can convert ambient energy into electrical signals through piezoelectric, triboelectric, and thermoelectric effects by leveraging their high conductivity, large surface area for enhanced charge generation, and a significant Seebeck coefficient for thermal energy conversion [21,22,23]. Especially, this energy harvesting capability is intrinsically related to their function as active sensing materials, as the electrical signal generated simultaneously serves as the quantitative sensing output. This paradigm enables an ideal self-powered system in which the sensor itself serves as the power source, thereby eliminating the need for external batteries. Furthermore, the excellent mechanical strength and flexibility of CNTs ensures the durability of these devices. Moreover, their versatile processability into various forms, ranging from films to fibers, makes them highly adaptable as electrodes for devices with diverse geometries. This potential is most profoundly realized when they are fabricated into fibers and yarns for the creation of textile-integrated wearable sensors that can continuously monitor human motion and physiological signals by harvesting biomechanical energy, seamlessly embedding smart technology into everyday clothing. Therefore, CNTs are a promising material for next-generation electronics, uniquely combining energy harvesting and active sensing to unlock truly independent and functional wearable systems.

3. Environmental Sensors

In environmental-sensing platforms, CNTs are often integrated into energy-harvesting modules such as electrochemical systems, triboelectric nanogenerators (TENGs), piezoelectric elements, photoelectric systems, and thermoelectric systems. These integrations enable autonomous signal generation and transduction without external power sources, as illustrated in Figure 2. Owing to their ability to function simultaneously as sensing interfaces and active electrodes, CNTs enable the development of compact multifunctional devices capable of detecting diverse environmental stimuli, including volatile organic compounds, wind, pressure, humidity, and chemical pollutants. We categorize CNT-based environmental sensors based on their integrated energy-harvesting mechanisms, providing insight into how each approach captures ambient energy while leveraging the intrinsic properties of CNTs to enhance sensitivity, durability, and performance.

3.1. Electrochemical Sensor

Electrochemical self-powered sensors utilize redox reactions or electric double-layer phenomena to harvest energy from the surrounding environment while detecting target analytes. Owing to their electrical conductivity, large specific surface area, and excellent electrochemical stability, CNTs serve dual roles as both active electrodes and sensing elements. These characteristics make CNTs highly suitable for integration into diverse electrochemical systems designed for autonomous environmental monitoring.

Majdecka et al. utilized BFCs to environmental sensing by developing a hybrid platform capable of simultaneously detecting catechols and oxygen. This system involved multi-walled CNT (MWCNT)-modified carbon paper electrodes to improve electron transfer efficiency and enzyme immobilization. It generated a power density of 2.5 mW/cm² and operated autonomously without requiring external power, highlighting the potential of CNT-based BFCs for real-time environmental monitoring [24].

Electric double-layer capacitance (EDLC) at CNT–electrolyte interfaces has been explored to convert mechanical energy into electrical signals in self-powered environmental sensing systems [25]. Sim et al. developed a mechano-electrochemical generator using vertically aligned CNTs embedded in silicone rubber. This device harvested energy through deformation-induced changes in EDLC as seawater intermittently wetted the CNT surface. Its superhydrophobic structure eliminated the need for external encapsulation. The generator achieved a power density of 6 W/m², detected ultralow-frequency ocean waves (<0.01 Hz), and operated stably in the battery-free mode for over 90 days; thus, it is well suited for extreme and long-term marine monitoring (Figure 3A) [26]. CNT-based electrochemical systems have also exhibited strong potential for ion and humidity sensing and improved ionic conductivity and signal transduction in composite materials [27]. Eryanto and Tsengdeveloped a self-powered paper-based humidity sensor composed of a magnesium chloride (MgCl₂)/CNT composite. The device generated a maximum voltage of 1.07 V and a power output of 1.984 µW across a wide relative humidity (RH) range of 11–97%, without external power. The sensor exhibited rapid response (~10 s) and high reproducibility; thus, it is suitable for real-time humidity monitoring. Furthermore, the device is capable of powering small electronic components such as light-emitting diodes (LEDs), demonstrating its potential for humidity-driven energy harvesting in low-power applications [28].

Collectively, the aforementioned studies highlight the versatility and effectiveness of CNT-based electrochemical self-powered sensors in environmental monitoring, where energy harvesting and analyte detection are integrated into a single functional platform. These systems—including enzymatic BFCs, electric double-layer capacitors, and redox-based devices—enable the detection of various environmental parameters such as pressure, humidity, gases, and ions without reliance on external power sources. CNTs play a pivotal role by facilitating electron transfer, increasing the effective surface area for electrochemical reactions, and enhancing the mechanical and operational stability of sensing interfaces. However, despite these advantages, several challenges persist. Enzymatic sensors, in particular, suffer from poor long-term stability due to enzyme degradation and susceptibility to environmental factors such as fluctuations in the temperature, pH, and ionic strength. In addition, the low power output of certain self-powered systems may limit their application in energy-intensive scenarios. Overcoming these limitations will require advancements in material engineering, enzyme stabilization techniques, and the development of hybrid energy-harvesting strategies to unlock the full potential of CNT-based self-powered sensors for practical and scalable environmental applications.

3.2. Triboelectric Sensor

TENGs have garnered attention for use in energy harvesting for self-powered sensing, particularly in environments subjected to mechanical stimuli such as vibrations, airflow, and motion. These devices operate through contact electrification and electrostatic induction, effectively converting mechanical deformations into electrical signals. Owing to their electrical conductivity, mechanical resilience, and surface tunability, CNTs are widely employed in triboelectric sensors to enhance charge collection efficiency, flexibility, and sensitivity. In environmental sensing, CNT-integrated TENGs have proven effective for real-time monitoring of gaseous pollutants, pressure variations, and mechanical motion without reliance on external power sources. The structural versatility of CNT-based TENGs enables the fabrication of fibers, films, and microstructured devices adaptable to various environmental interfaces. Moreover, CNTs can serve dual roles as both active triboelectric materials and sensing elements, enabling the design of compact integrated platforms capable of simultaneous energy harvesting and stimulus detection. These characteristics make CNT-based TENGs promising candidates for sustainable maintenance-free environmental-monitoring applications [34,35,36].

Vafaiee et al. developed CNT–PDMS foams as self-powered humidity sensors based on triboelectric nanogenerators driven by finger tapping. The incorporation of CNTs enhanced the hydrophilicity and porosity of PDMS, enabling broader humidity detection and a higher response compared to pure PDMS. As ambient humidity increased from 30% to 80% RH, adsorption of water molecules reduced the surface charge density, resulting in a gradual decrease in output current from 2.9 µA to 1.6 µA, with a linear response up to 108%. This work demonstrates how CNT compositing not only improves humidity sensitivity and linearity but also ensures lightweight, flexible, and fully self-powered operation (Figure 3B(i)) [29]. Wang et al. developed a self-powered ammonia (NH₃) sensor using a spherical TENG that harvested mechanical energy from vibrations during transportation. The harvested energy powered a gas sensor made of polydopamine-functionalized CNTs coated with polyaniline, enhancing hydrophilicity, electron transfer, and material dispersion. The battery-free system displayed high sensitivity to NH₃; thus, it is suitable for cold-chain logistics and freshness monitoring (Figure 3B(ii)) [30]. In health applications, Wang et al. also introduced a respiration-driven TENG integrated with MXene/NH₂-MWCNTs for gas sensing and respiratory diagnostics. The device harvested energy from human breath, generating voltages of up to 136 V through contact-separation cycles. It also functioned as a formaldehyde sensor with high selectivity (detection limit of 10 ppb) and could distinguish pathological respiratory patterns using machine learning [37]. Xia et al. developed a dual-mode pressure sensor by integrating a TENG with a reduced-graphene-oxide/CNT piezoresistive sensor. Operating in the contact-separation mode, the TENG generated an output voltage of 250 V, which powered the piezoresistive element for pressure sensing. This device exhibited high sensitivity (26.4 kPa⁻¹) and could detect biological signals such as pulses and speech, indicating potential for use in wearable electronics and real-time health monitoring [38]. For biological applications, Lan et al. created an on-plant self-powered sensing platform that harvested biomechanical energy from plants. CNT-coated silk fibroin and polytetrafluoroethylene (PTFE) layers were utilized to directly apply breathable TENGs to plant leaves to power humidity and temperature sensors. This battery-free platform supports continuous environmental monitoring while preserving plant physiology, showing potential for precision agriculture and sustainable ecosystem sensing (Figure 3B(iii)) [31].

Thus, CNT-based triboelectric sensors provide a versatile approach to autonomous environmental and wearable monitoring by harvesting mechanical energy from motion, vibration, airflow, and respiration. CNTs enhance charge transfer, structural flexibility, and mechanical durability, significantly boosting triboelectric performance. However, challenges such as limited energy output at high frequencies, sensitivity to humidity and temperature, and the long-term stability of triboelectric layers remain. Continued advances in nanomaterials and device architectures are addressing these issues, reinforcing the potential of CNT-based triboelectric sensors in sustainable, self-powered technologies.

3.3. Photoelectric Sensor

Photoelectric self-powered sensors convert light energy into electrical signals for sensing applications without external power. These devices capture solar or ambient light via photoelectrochemical reactions or photovoltaic effects. Owing to their electrical conductivity, large surface area, and efficient charge transport, CNTs are critical for improving the light absorption, charge separation, and energy conversion efficiency of photoactive materials.

In the field of gas sensing, Liu et al. developed a self-powered hydrogen sulfide (H₂S) sensor using a single-walled CNT (SWCNT)–n-type silicon heterojunction. This system harvested energy from a built-in electric field resulting from the Fermi level difference between p-type SWCNTs and n-type Si, producing an OCV of 181 mV and detecting H₂S concentrations of as low as 100 ppb. This solar-powered platform operates passively, offering a compact and real-time solution for toxic gas detection [39]. Similarly, Guo et al. created a self-powered nitrogen dioxide (NO₂) sensor by integrating SWCNTs with a SWCNT/Si heterojunction solar cell. The sensor exhibited a 230% increase in sensitivity and a 330% faster response than devices driven by external power and wirelessly transmitted data via Bluetooth; thus, it is highly suitable for monitoring the quality of mobile and distributed air (Figure 3C) [32]. Xie et al. introduced a self-powered photoelectrochemical biosensor for detecting methyl parathion, a pesticide, using a MoS₂–TiO₂ heterostructure as the photoanode. This system facilitated efficient photogenerated electron transfer and battery-free operation, achieving a low detection limit of 3.2 nM [40,41]. In summary, CNT-based photoelectric sensors offer a transformative battery-free approach to environmental and health monitoring. By utilizing light energy, they exhibit high sensitivity and fast response. CNTs improve light absorption and charge transport, supporting diverse applications such as gas and pesticide detection and biological-marker monitoring. However, dependency on light, material degradation, and limited power output remain key challenges. Addressing these limitations through material innovations and system-level design is crucial for real-world deployment of CNT-based photoelectric sensors.

3.4. Piezoelectric Sensor

Piezoelectric sensors convert mechanical stress or deformation into electrical signals through the piezoelectric effect, wherein an applied force aligns internal dipoles within a material, resulting in a voltage difference. These sensors are commonly employed for pressure, vibration, and motion sensing, as they directly transduce mechanical energy into electrical energy without requiring external power sources. The integration of CNTs enhances sensor performance by improving charge transport, mechanical robustness, and sensitivity.

Bhaduri et al. [33] developed a self-powered fluoride (F⁻) ion detection system using a piezoelectric nanogenerator (PENG) composed of a polymer matrix embedded with CNTs, barium titanate (BaTiO₃), and poly (vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE). Under mechanical stress, the system generated electrical energy, with the piezoelectric output increasing considerably from 1.2 to 5.2 V owing to the incorporation of CNTs. This sensor achieved a detection limit of 2.9 μM for fluoride ions and operated entirely without an external power source. The study highlights the potential of CNT-enhanced PENGs for battery-free environmental monitoring applications (Figure 3D). Thus, piezoelectric CNT-based sensors offer a reliable and self-sustaining approach to environmental monitoring by converting mechanical energy into electrical signals. The incorporation of CNTs enhances performance by promoting efficient charge transfer and improving mechanical stability and sensitivity. Despite these advantages, several challenges remain, including limited power output at high frequencies and the need for material optimization to ensure long-term durability under harsh or fluctuating environmental conditions.

3.5. Thermoelectric Sensor

Thermoelectric self-powered sensors convert temperature gradients into electrical energy via the Seebeck effect, where a voltage is generated across a conductive material because of a temperature difference between two regions. These sensors operate independently of external power by harnessing ambient temperature fluctuations, including body heat, environmental temperature differentials, or waste heat from industrial systems. Incorporating CNTs improves thermoelectric performance by enhancing energy conversion efficiency, promoting charge transport, and providing mechanical flexibility [42].

Wan et al. created a self-powered thermoelectric composite by embedding self-folded CNT networks within an epoxy matrix, harvesting thermal energy via the Seebeck effect. The spontaneous folding of CNTs formed localized thermoelectric junctions, resulting in a Seebeck coefficient of approximately 36 µV/K. The system generated measurable voltage outputs under small temperature gradients (<1 °C), sufficient to power low-energy sensing modules. Additionally, the composite exhibited high mechanical integrity and thermal conductivity (~9.5 W/m·K); thus, it is well suited for aerospace, automotive, and structural health-monitoring applications [43]. Overall, CNT-based thermoelectric sensors offer a sustainable battery-free solution for temperature monitoring by converting ambient thermal fluctuations into usable electrical energy. However, challenges such as low power output under small temperature gradients and limited long-term stability in fluctuating environments remain. Further material and device optimization is necessary to improve performance and reliability in real-world applications.

Table 1. Environmental self-powered sensors covering working principles, active materials, performance and applications.

Working Principle	Sensor Type	Active Materials	Total Power	Limit of Detection	Application	References
Electrochemical	Pressure	CNT sheet	OCV: 2.4 mV Power Density: 1.28 mW/m2	10 Pa	Ocean wave monitoring	[25]
	Pressure	VACNT forest + Silicone rubber composite	OCV: 350 mV Power Density: 6 W/m2	0.01 Hz	Ocean wave monitoring	[26]
	Humidity	MgCl2	OCV: 1.07 V Power: 1.984 μW	N/A	Humidity monitoring	[28]
Triboelectric	Bio	CuO + Chitosan + PPy	N/A	0.75 μM	Pesticide monitoring	[34]
	Pressure	PVA	OCV: 26.5 V Power Density: 4.57 W/m2	0.2 kPa	Human motion monitoring	[35]
	Pressure	PVDF	OCV: 0.127 V/Pa	5 kPa	Pressure monitoring	[36]
	Humidity	PDMS	OCV: 35 V	N/A	Humidity monitoring	[29]
	Gas	PDA-PANI	OCV: 30 V	38 ppb	Meat spoilage detection	[30]
	Gas	MXene (Ti3C2Tx)	OCV: 136 V Power: 27 μW	10 ppb	Respiratory monitoring	[37]
	Pressure	rGO + PI + Copper	OCV: 250 V Power Density: 700 mW/cm2	5 kPa	Pressure monitoring	[38]
	Pressure	PVDF-HFP	Power Density: 330.6 μW/cm2	N/A	Plant health monitoring	[31]
Photoelectric	Gas	n-type silicon	OCV: 181 mV	100 ppb	Gas monitoring	[39]
	Gas	n-type silicon	OCV: 0.5 V	1 ppm	Gas monitoring	[32]
	Chemical	CuO/ZnO nanostructure	Current Density: 0.18 mA/cm2	8 μM	H2O2 monitoring	[40]
	Chemical	B-TiO2 photoanode + Cu2O/3DNG photocathode	OCV: 0.664 V Power Density: 10.77 mW/cm2	0.33 pg/ml	On-site SMZ monitoring	[41]
Piezoelectric	Chemical	Mn-doped BaTiO3	OCV: 43.6 V	1.18 μM	Fluoride detection in water	[33]
Thermoelectric	Physical	honeycomb structures	OCV: 21 mV	N/A	Healthcare	[43]

summarizes representative CNT-based self-powered environmental sensors, comparing their working principles, sensor types, active materials, power outputs, detection limits, and application fields. This comparative overview highlights how different device architectures achieve sensitivity and energy harvesting efficiency in environmental monitoring.

4. Wearable Sensors

Wearable electronics enable continuous physiological monitoring and real-time feedback, revolutionizing both healthcare and human–machine interfaces. A critical challenge in ensuring the long-term operation of wearable systems is achieving energy autonomy without external power sources. Self-powered sensors that harvest energy from the human body or the surrounding environment provide an effective solution. Owing to their exceptional electrical conductivity, flexibility, and biocompatibility, CNTs are particularly well suited for integration into wearable self-powered systems. CNTs serve various roles, including active sensing elements, electrodes, or charge-transfer mediators, depending on the energy transduction mechanism such as electrochemical, piezoelectric, triboelectric, or thermoelectric processes, as illustrated in Figure 2.

4.1. Electrochemical Sensor

Electrochemical self-powered sensors incorporating CNTs are promising for wearable applications requiring continuous biosignal monitoring and simultaneous energy harvesting. Their high electrical conductivity, large surface area, and mechanical resilience enable CNTs to function as both sensing electrodes and active energy conversion components in electrolyte-based transducers.

A foundational study conducted by Kim et al. [44] showed that coiled CNT yarns immersed in electrolytes can generate electricity through capacitance modulation induced by mechanical deformation (Figure 4A(i)). This reversible, non-faradaic mechanism allowed for stable and rapid energy harvesting. Building on this, Moon et al. developed muscle-mimetic sensors by embedding CNT yarns within elastomeric matrices and utilizing electrochemical swelling and ion flux to detect strain under dynamic, body-like deformations (Figure 4A(ii)) [45]. Sim et al. advanced this concept by engineering coiled CNT yarns with optimized geometry and surface chemistry to detect physiological motion. These sensors operated via electrochemical impedance modulation in response to ionic perturbations, delivering high sensitivity to skin deformation while maintaining mechanical durability and conformability (Figure 4A(iii)) [46].

To further expand functionality, Gwac et al. designed a multi-ion potentiometric sensing platform using poly (3,4-ethylenedioxythiophene) (PEDOT)-coated CNT yarns integrated with ion-selective membranes. These fiber-based sensors could detect K⁺, Na⁺, and H⁺ ions and maintained stable Nernstian responses under mechanical strain, bending, and knotting. When integrated into a wearable glove, the system effectively measured ion concentrations in real samples such as seawater and fruit juice, demonstrating real-world utility in hydration and food safety monitoring (Figure 4A(iv)) [47]. Similarly, Tai’s groups developed two distinct self-powered strain sensor systems based on different electrochemical principles [48,49]. One approach utilizes an ion gradient-driven mechanism within a flexible yarn structure composed of CNTs and lithium chloride (LiCl) [48]. In this design, a humidity gradient along the yarn, created by the hygroscopic properties of LiCl, induces directional ion movement and generates stable voltage. The sensor monitors strain through changes in current, which are dominated by the strain-induced resistance variations in the CNT network [48]. A second approach employs a galvanic cell-type electrochemical reaction using copper and aluminum as electrodes separated by a LiCl-CNT electrolyte-infused elastic yarn [49]. This configuration functions as a primary battery, generating power through the redox reactions between the metals, while strain is detected via the modulation of the yarn’s internal resistance, which alters the output current. Both designs successfully achieve self-powered operation and can detect static and dynamic strain, demonstrating their potential for applications such as continuous respiratory monitoring. A unifying principle across CNT-based electrochemical sensors is the reliance on CNT–electrolyte interactions. Signal generation stems from modulation of the electrochemical double layer or ion-selective potentials. The coiled CNT yarn architectures and their derivatives have expanded the design possibilities for electrochemical wearables by coupling mechanical deformation with ion–electron interactions. This synergy enables highly sensitive, flexible, and multifunctional sensing, laying the foundation for next-generation self-powered e-textiles capable of autonomous biosignal acquisition and transmission.

Figure 4. Wearable applications of CNT-based self-powered sensor. (A) Images for electrochemical based sensors. (i) The coiled CNT yarns, called “Twistrons,” electrochemically harvest mechanical energy by converting tensile or torsional motion into electricity without an external bias voltage. (Reproduced with permission from Kim et al. [44], Science; published by the American Association for the Advancement of Science, 2017.) (ii) A self-powered inertial sensor based on a coiled CNT yarn detects vibrations, impacts, and body motions by harvesting the mechanical energy of an internal mass. (Reproduced with permission from Moon et al. [45]. IEEE Transactions on Industrial Electronics; published by IEEE, 2021.) (iii) A soft, highly elastic hygroelectric fiber sensor generates electricity from ambient humidity changes, using asymmetrically oxidized CNT buckles to enable wearable respiration monitoring. (Reproduced with permission from Sim et al. [46]. Chemical Engineering Journal; published by Elsevier, 2024.) (iv) A stretchable, fiber-based potentiometric sensor made of multi-plied and coiled carbon nanotube yarns functions as a wearable electronic tongue by selectively detecting various ions like K⁺, Na⁺, and H⁺. (Reproduced with permission from Gwac et al. [47], Advanced Materials Technologies; published by John Wiley & Sons, 2024.) (B) Images for piezoelectric based sensor. (i) A robust, self-powered piezoelectric sensor is fabricated from electrospun nanofibers with CNT electrodes and encapsulated with a superhydrophobic coating to ensure stable motion monitoring in harsh environments. (Reproduced with permission from Su et al. [50], Nano Energy; published by Elsevier, 2023.) (ii) A knittable, self-powered piezoionic sensor is developed from a single, gel-electrolyte-coated CNT yarn that generates voltage along its length when stretched, enabling precise gesture recognition. (Reproduced with permission from Li et al. [51], ACS Applied Electronic Materials; published by the American Chemical Society, 2021.) (iii) Inspired by human skin, a self-powered piezoionic sensor with hierarchically porous graphene and CNT composite electrodes detects strain and stress for wearable applications like facial expression recognition. (A) Schematic image for mechanism of facial expression recognition. Sensing signals with (B) happy, (C) angry, (D) sad, and (E) surprise expression. (Reproduced with permission from Yu et al. [52], Small; published by John Wiley & Sons, 2025.) (C) Images for triboelectric based sensor. (i) A shape-adaptive and self-healing triboelectric nanogenerator is created using a viscoelastic putty as both the electrification layer and the matrix for a CNT composite electrode, enabling soft solid–solid contact electrification. (Reproduced with permission from Chen et al. [53], ACS Nano; published by the American Chemical Society, 2019.) (ii) A stretchable, self-healing, and environmentally stable triboelectric electronic skin was developed using a CNT-doped ionic liquid elastomer as the electrode and an electrospun PVDF/PU nanofiber membrane as the triboelectric layer. Response of voltage signals for (A) walking, (B) running, and (C) jumping. (D) schematic image of the principle for pressure sensing. (E) 3 × 3 pixel fabric sensor. (F,G) Output voltage and luminosity of each pixel for 3 × 3 pixel fabric sensor. (Reproduced with permission from Cheng et al. [54], Nano Energy; published by Elsevier, 2022.) (iii) A stretchable and self-powered mechanoluminescent triboelectric fiber, made from a CNT fiber core and a luminescent composite shell, provides simultaneous electro-optical signals for wearable and amphibious sensing. (Reproduced from Wu et al. [55], Advanced Science; published by Wiley, 2024.) (iv) A flexible, fiber-based triboelectric nanogenerator was fabricated by electrospinning a silk fibroin substrate and electrospraying a silk/CNT composite layer for wearable power supply applications. (Reproduced with permission from Su et al. [56], ACS Applied Nano Materials; published by the American Chemical Society, 2020.) (D) Images for thermoelectric based sensor. (i) A flexible and durable thermoelectric composite fabric was fabricated by simultaneously electrospraying CNTs onto electrospun poly(lactic acid) (PLA) nanofibers for wearable energy harvesting and self-powered sensing. (Reproduced from Liu et al. [57], Composites Science and Technology; published by Elsevier, 2024.) (ii) A wearable thermoelectric generator is fabricated with vertically aligned p-type (PEDOT:PSS) and n-type (SWCNT) thermoelements in a flexible PDMS frame for efficient body heat harvesting from the wrist. (Reproduced with permission from Hasan et al. [58], International Journal of Energy Research; published by John Wiley & Sons, 2022.) (iii) A highly stretchable, durable, and breathable thermoelectric fabric is prepared by spraying a CNT/polyvinyl pyrrolidone composite onto an electrospun polyurethane skeleton for body energy harvesting and self-powered sensing. (A,B) Schematic images of the flexible wearable TEG. (C) Dimensions and surface area for each thermoelements. (D) Temperature gradient as a function by the length of PEDOT:PSS and SWCNT films. (E) Simulation results of the temperature gradient across by the length of the thermoelements. (F) Temperature distribution for thermoelements by arc length. (Reproduced from He et al. [59], Carbon Energy; published by Wiley, 2022).

Figure 4. Wearable applications of CNT-based self-powered sensor. (A) Images for electrochemical based sensors. (i) The coiled CNT yarns, called “Twistrons,” electrochemically harvest mechanical energy by converting tensile or torsional motion into electricity without an external bias voltage. (Reproduced with permission from Kim et al. [44], Science; published by the American Association for the Advancement of Science, 2017.) (ii) A self-powered inertial sensor based on a coiled CNT yarn detects vibrations, impacts, and body motions by harvesting the mechanical energy of an internal mass. (Reproduced with permission from Moon et al. [45]. IEEE Transactions on Industrial Electronics; published by IEEE, 2021.) (iii) A soft, highly elastic hygroelectric fiber sensor generates electricity from ambient humidity changes, using asymmetrically oxidized CNT buckles to enable wearable respiration monitoring. (Reproduced with permission from Sim et al. [46]. Chemical Engineering Journal; published by Elsevier, 2024.) (iv) A stretchable, fiber-based potentiometric sensor made of multi-plied and coiled carbon nanotube yarns functions as a wearable electronic tongue by selectively detecting various ions like K⁺, Na⁺, and H⁺. (Reproduced with permission from Gwac et al. [47], Advanced Materials Technologies; published by John Wiley & Sons, 2024.) (B) Images for piezoelectric based sensor. (i) A robust, self-powered piezoelectric sensor is fabricated from electrospun nanofibers with CNT electrodes and encapsulated with a superhydrophobic coating to ensure stable motion monitoring in harsh environments. (Reproduced with permission from Su et al. [50], Nano Energy; published by Elsevier, 2023.) (ii) A knittable, self-powered piezoionic sensor is developed from a single, gel-electrolyte-coated CNT yarn that generates voltage along its length when stretched, enabling precise gesture recognition. (Reproduced with permission from Li et al. [51], ACS Applied Electronic Materials; published by the American Chemical Society, 2021.) (iii) Inspired by human skin, a self-powered piezoionic sensor with hierarchically porous graphene and CNT composite electrodes detects strain and stress for wearable applications like facial expression recognition. (A) Schematic image for mechanism of facial expression recognition. Sensing signals with (B) happy, (C) angry, (D) sad, and (E) surprise expression. (Reproduced with permission from Yu et al. [52], Small; published by John Wiley & Sons, 2025.) (C) Images for triboelectric based sensor. (i) A shape-adaptive and self-healing triboelectric nanogenerator is created using a viscoelastic putty as both the electrification layer and the matrix for a CNT composite electrode, enabling soft solid–solid contact electrification. (Reproduced with permission from Chen et al. [53], ACS Nano; published by the American Chemical Society, 2019.) (ii) A stretchable, self-healing, and environmentally stable triboelectric electronic skin was developed using a CNT-doped ionic liquid elastomer as the electrode and an electrospun PVDF/PU nanofiber membrane as the triboelectric layer. Response of voltage signals for (A) walking, (B) running, and (C) jumping. (D) schematic image of the principle for pressure sensing. (E) 3 × 3 pixel fabric sensor. (F,G) Output voltage and luminosity of each pixel for 3 × 3 pixel fabric sensor. (Reproduced with permission from Cheng et al. [54], Nano Energy; published by Elsevier, 2022.) (iii) A stretchable and self-powered mechanoluminescent triboelectric fiber, made from a CNT fiber core and a luminescent composite shell, provides simultaneous electro-optical signals for wearable and amphibious sensing. (Reproduced from Wu et al. [55], Advanced Science; published by Wiley, 2024.) (iv) A flexible, fiber-based triboelectric nanogenerator was fabricated by electrospinning a silk fibroin substrate and electrospraying a silk/CNT composite layer for wearable power supply applications. (Reproduced with permission from Su et al. [56], ACS Applied Nano Materials; published by the American Chemical Society, 2020.) (D) Images for thermoelectric based sensor. (i) A flexible and durable thermoelectric composite fabric was fabricated by simultaneously electrospraying CNTs onto electrospun poly(lactic acid) (PLA) nanofibers for wearable energy harvesting and self-powered sensing. (Reproduced from Liu et al. [57], Composites Science and Technology; published by Elsevier, 2024.) (ii) A wearable thermoelectric generator is fabricated with vertically aligned p-type (PEDOT:PSS) and n-type (SWCNT) thermoelements in a flexible PDMS frame for efficient body heat harvesting from the wrist. (Reproduced with permission from Hasan et al. [58], International Journal of Energy Research; published by John Wiley & Sons, 2022.) (iii) A highly stretchable, durable, and breathable thermoelectric fabric is prepared by spraying a CNT/polyvinyl pyrrolidone composite onto an electrospun polyurethane skeleton for body energy harvesting and self-powered sensing. (A,B) Schematic images of the flexible wearable TEG. (C) Dimensions and surface area for each thermoelements. (D) Temperature gradient as a function by the length of PEDOT:PSS and SWCNT films. (E) Simulation results of the temperature gradient across by the length of the thermoelements. (F) Temperature distribution for thermoelements by arc length. (Reproduced from He et al. [59], Carbon Energy; published by Wiley, 2022).

4.2. Piezoelectric Sensor

Piezoelectric self-powered sensors convert mechanical stimuli—such as bending, stretching, or pressure—into electrical signals via intrinsic dipole reorientation within piezoelectric materials. Their compatibility with body motions makes them ideal for wearable applications. Incorporating CNTs enhances these systems by improving electrical conductivity, mechanical resilience, and piezoelectric efficiency—particularly by promoting β-phase crystallinity in polyvinylidene fluoride (PVDF)-based polymers and facilitating efficient charge transport. For example, electrospun CNT/PVDF-hexafluoropropylene (HFP) nanofibers achieved β-phase concentration of up to 93% and retained performance after 150 laundering cycles, demonstrating strong potential for integration into durable wearable textiles [60]. Su et al. further enhanced environmental resilience by developing a superhydrophobic PENG using initiated chemical vapor deposition) to coat P(VDF-TrFE) membranes with CNT-based electrodes (Figure 4B(i)). This device exhibited a water contact angle of >150° and maintained sensitivity during body motion sensing, highlighting its applicability in real-world wearables [50]. Biswajit et al. integrated both piezoelectric and pyroelectric sensing into a PVDF–MWCNT-based nanogenerator, achieving an output of 35 V and a power density of 34 μW/cm² under mechanical stress. Featuring wireless data transmission, the device supports IoT-enabled health-monitoring systems [61]. Li et al. incorporated barium titanate nanoparticles into PVDF, enhancing piezoelectric sensitivity to 11.6 V/bar and enabling stable operation over 12,000 cycles—suitable for gait and joint monitoring [62].

Beyond film-based sensors, fiber and yarn architectures improve wearability and facilitate seamless textile integration. Li et al. developed a gel-coated CNT yarn sensor generating up to 15 mV under 80% strain via asymmetric ion squeezing, enabling precise gesture recognition in smart gloves (Figure 4B(ii)) [51]. CNT/MXene hybrids and CNT–ionogel composites have also demonstrated high sensitivity, durability, and adaptability for monitoring physiological signals such as pulse, facial expressions, and posture (Figure 4B(iii)) [52,63].

4.3. Triboelectric Sensor

TENGs convert mechanical contact and separation between dissimilar materials into electricity through contact electrification and electrostatic induction. With high voltage output, responsiveness to low-force stimuli, and broad material compatibility, TENGs are especially suited for self-powered wearable systems.

CNTs are widely incorporated into TENG composites—often embedded in polydimethylsiloxane (PDMS), polyurethane (PU), or PVDF matrices—to enhance charge trapping and conductivity. For example, a CNT/PDMS-based TENG exhibited 91% transparency and a power density of 8 W/m², enabling LED activation from body motion [64]. A shape-adaptive CNT–putty TENG delivered 140 V output and self-healing capability via viscoelastic recovery, outperforming conventional PTFE-based systems (Figure 4C(i)) [53]. To improve environmental durability, Wang et al. created a droplet-driven underwater self-healing TENG using PVDF-HFP and eco-friendly plasticizer ATBC. This device retained 85% of its performance after 12 h of healing and delivered 20 V output across 5000 cycles [65]. Cheng et al. enhanced stretchability and mechanical resilience with a CNT-doped ionic elastomer/PVDF composite, achieving 2.44 W/m² power density and 93.7% stress recovery, enabling applications such as ECG sensing and tactile mapping (Figure 4C(ii)) [54]. Fiber-integrated TENGs also offer better compatibility with textiles. For example, a stretchable triboelectric fiber made from PVDF-TrFE and CNT coatings generated 13–24 mV under strain and operated stably across 10,000 cycles when woven into fabric [66]. Multifunctional designs, such as mechanoluminescent TENG fibers with CNT cores and ZnS:Cu shells, enabled optical and electrical sensing in environments ranging from underwater to non-contact scenarios (Figure 4C(iii)) [55]. A tri-layer CNT–silk fiber TENG also achieved power densities up to 317.4 μW/cm², featuring scalable fabrication and high breathability for daily wearable use (Figure 4C(iv)) [56].

4.4. Thermoelectric Sensor

Thermoelectric generators (TEGs) harvest electricity from temperature differences through the Seebeck effect, offering continuous and passive power for low-power wearable electronics. The temperature gradient between human skin (32–37 °C) and ambient air enables sustained energy generation for long-term biosignal monitoring.

CNTs, with their high electrical conductivity, flexibility, and tunable doping, are ideal for use in flexible TEGs. They can be fabricated as films, yarns, or fabrics, and doped to function as either p-type or n-type materials. To boost performance, CNT-based TEGs are often combined with conductive polymers or additives to enhance the Seebeck coefficient and power factor while maintaining mechanical resilience. Cui et al. developed a PEDOT/MWCNT@PLF fabric with 320% stretchability, 121 S/m conductivity, and a 13.8 μV/K Seebeck coefficient. Liu et al. developed CNT/PLA composites via electrospray and electrospinning, achieving water resistance, mechanical durability, and 37.3 nW/cm² power density under a 17 K temperature gradient (Figure 4D(i)) [57]. The fabric supported both strain and temperature sensing with sub-second response times and seamless integration into garments [67]. Zhang et al. advanced this further by incorporating Ti₃C₂Cl into PEDOT:polystyrene sulfonate (PSS)/SWCNT composites, achieving a power factor of 79.23 μW/m·K² with stable performance under repeated bending [68]. Structural innovations have further advanced TEG multifunctionality. Gao et al. used porous PDMS foam coated with PEDOT:PSS/CNTs for dual-mode pressure and temperature sensing [69]. Hasan et al. engineered a vertical TEG with optimized p/n thermoelements using doping and finite element analysis, harvesting 1.75 mV and 10.17 nW/cm² from an 11.24 °C skin-temperature gradient while maintaining flexibility under a 52° bend (Figure 4D(ii)) [58]. Alternative polymers such as polyvinylpyrrolidone (PVP)/PU and polylactic acid (PLA) offer improved CNT dispersion and compliance. He et al. fabricated an electrospun CNT/PVP/PU fabric with 250% stretchability and a 51 μV/K Seebeck coefficient, enabling temperature and motion sensing on the wrist (Figure 4D(iii)) [59].

Table 2. Wearable self-powered sensors highlighting device configurations, functional materials and monitoring applications.

Working Principle	Sensor Type	Active Materials	Total Power	Limit of Detection	Application	References
Electrochemical	Fiber	CNT	OCV: 50 mV	30 mM	Electronic tongue	[47]
	Fiber/Strain	CNT	OCV: 1.46 mV	1.9 m/s2	Inertial sensor	[45]
	Fiber/Humidity	CNT	2.5 mW/m2	RH 90%	Respiration monitoring	[46]
	Fiber/Strain	CNT	250 W/kg	30%	Strain monitoring	[44]
Piezoelectric	Film	PVDF/MWCNT	34 μW/cm2	7.5 V/kPa	Healthcare monitoring	[61]
	Film	PVDF-HFP/MWCNT	0.62 V	15 N	Electric skin	[60]
	Film	PVT/PFE/CNT	65 V	RH 90%	Human behavior monitoring	[50]
	Film	PVDF/BTO/CNT	-	5 bar	Human behavior monitoring	[62]
	Strain	CNT	0.47 mW/g	30%	Human behavior monitoring	[51]
	Film/strain	MXene/CNT	9.56 μW/cm2	100%	Human behavior monitoring	[52]
	Film	Graphene/CNT	0.62 μA	7 mm	Human behavior monitoring	[63]
Triboelectric	Film/strain	PDMS/CNT	10 mW	35%	Glove sensor	[64]
	Film	PDMS/CNT	410 mW/m2	2 Hz	Human behavior monitoring	[53]
	Film/strain	PVDF/PU/CNT	2.444 mW/m2	100%	Healthcare monitoring	[54]
	Fiber/Strain	PVDF/CNT	24 mV	50%	Human behavior monitoring	[66]
	Fiber/strain	ZnS:Cu/CNT	28 V	200%	Human behavior monitoring	[55]
	Film	Silk/CNT	317.4 μW/cm2	N/A	Human behavior monitoring	[56]
Thermoelectric	Textile	PEDOT/CNT	227.1 nW/mK2	13.8 μV/K	Human behavior monitoring	[67]
	Film	PEDOT:PSS/CSWCNT	79.23 μW/mK2	21.69 μV/K	Human behavior monitoring	[68]
	Foam	PEDOT:PSS/PDA/CNT	N/A	40.5 mV/K	Electric skin	[69]
	Film	PEDOT:PSS/SWCNT	0.15 μW/cm2°C	11.24 °C	Wrist band	[58]
	Film/strain	PVP/CNT	586 pW	51 μV/K	Human behavior monitoring	[59]
	Fabric	PLA/CNT	37.3 nW/cm2	62.9 μV/K	Human behavior monitoring	[57]

provides a comparison of CNT-based self-powered wearable sensors, listing their working principles, sensing modes, active materials, power characteristics, detection limits, and demonstrated applications. This table illustrates the design strategies that enable flexibility, stability, and performance in wearable scenarios.

5. Implantable Sensors

Owing to their high electrical conductivity and intrinsic biocompatibility, CNTs enable miniaturized real-time sensing because functional biomolecules, such as enzymes and other biochemicals, are incorporated into sensor architectures. They also convert mechanical strain into electrochemical signals, supporting advanced healthcare monitoring applications.

5.1. Electrochemical–Redox-Type Biofuel Cell

Enzymatic biofuel cells (EBFCs) represent a promising approach to self-powered implantable systems by converting endogenous biofuels such as glucose and lactate into electricity under biocompatible conditions. In a typical EBFC, glucose oxidase oxidizes glucose at the bioanode, generating electrons that flow through an external circuit, while oxygen is reduced at the biocathode. The resultant electrochemical reaction yields non-toxic byproducts, making EBFCs suitable for long-term biomedical implants [70,71].

Building on CNTs, Zhu et al. and Zhao et al. developed self-powered electrochemical aptasensors for myoglobin detection using CNT–Au nanoparticle hybrid electrodes and glucose-driven EBFCs, with the OCV as the key performance indicator. Zhu et al. reported a sensor with a detection limit of 0.011 ng/mL and a voltage sensitivity of 23.0 mV per log-unit increase in myoglobin concentration (0.1–10⁴ ng/mL). Zhao et al. reported a 0.23 ng/mL detection limit, 19 μW/cm² peak power density, and high specificity across the 5–5000 ng/mL range. Both sensors exhibited excellent biocompatibility and stable signal output, highlighting their suitability for implantable real-time cardiac biomarker monitoring [72,73]. Park et al. developed a dual-functional CNT yarn-based sensor that operated both as a reactive oxygen species (ROS) biosensor and a supercapacitor powered by a glucose-fueled EBFC. This sensor showed high H₂O₂ sensitivity (49.02 μA μM⁻¹ cm⁻²), retained 96.45% capacitance after 10,000 cycles in biological fluids, and maintained a stable OCV of 0.42 V. Its robust selectivity and operational stability in physiological environments make it promising for implantable ROS monitoring, such as in smart stent applications (Figure 5A) [74].

To validate in vivo applicability, De la Paz et al. evaluated a CNT-based glucose biosensor powered by a glucose-driven BFC in a porcine model. The capsule integrates CNT-coated Ni-foam electrodes modified with GOx/TTF-TCNQ at the anode and BOD/ABTS²⁻ at the cathode, together with a custom low-power microchip and an mHBC antenna compactly packaged in a 2.6 × 0.9 cm shell. This design enabled real-time monitoring of intestinal glucose across the 3–90 mM range, with a detection limit of 4.656 mM and stable OCV of 0.57 V—eliminating the need for external power. By directly converting glucose oxidation into electrical signals and transmitting them wirelessly, the sensor exhibited minimal interference from dopamine and ascorbic acid and maintained signal drift of <7% over 3 h in artificial intestinal fluid, showcasing its feasibility as a battery-free fully implantable glucose monitor (Figure 5B) (

Table 3. Implantable self-powered sensors presenting electrochemical and mechano-electrochemical systems for biomedical monitoring.

Working Principle	Sensor Type	Active Materials	Total Power	Limit of Detection	Application	References
Electrochemical (Biofuel cell)	Aptamer-based Mb sensor (Bio)	CNT–AuNP, GOx, aptamer	OCV drop (sensitivity 23.0 mV/decade)	0.011 ng/mL Mb	Implantable myoglobin detection	[72]
	Aptamer-based Mb sensor (Bio)	Carboxylated CNT-AuNP electrospun nanofiber	OCV drop, Power ~19 μW/cm2	0.23 ng/mL Mb	Implantable myoglobin sensing	[73]
	ROS sensor + supercapacitor (Bio)	CNT yarn, Cytochrome c	OCV 0.42 V, supercapacitor retained 96.45%	49.02 μA μM−1 cm−2 (sensitivity)	ROS monitoring, smart stent	[74]
	Glucose biosensor (Bio)	CNT-coated Ni foam, GOx	OCV 0.57 V	LOD 4.656 mM, range 3–90 mM	In vivo glucose monitoring (porcine model)	[75]
Mechano-electrochemical	Glucose-responsive actuator	MWNT yarn, HA-cholesterol-boronic acid nanogel	N/A	5–100 mM glucose	Implantable glucose sensing via actuator motion	[78]
	Strain/volume sensor (Twistron)	Coiled CNT yarn	OCV ~37 mV at 30% strain	10–30% strain, ~3–12 mL volume	Gastric motility monitoring	[79]
	Strain/pressure sensor	Microbuckled CNT yarn + elastomer	OCV ~20 mV, SCC ~1.5 μA	~60 kPa, 40–60 mL volume	Bladder and cardiac motion sensing	[76]
	Organ motion sensor	MWNT-coated elastomeric SCBF	OCV ~15 mV, 121 A/kg, 16 W/kg	0.12 mV/% strain, 0.78 μA/kg/% strain	Implantable organ pressure and volume sensing	[77]

) [75]. In biofuel cell based implantable sensors, CNTs serve as conductive scaffolds that lower internal resistance, accelerate electron transfer, and provide stable sites for enzyme immobilization, thereby improving power output and sensing reliability.

5.2. Mechanoelectrical

Mechano-electrochemical energy harvesting leverages mechanical stimuli (e.g., organ motion) to generate electricity or convert chemical energy into mechanical movement. This is achieved by strain-induced changes in capacitance or redox activity at the electrode–electrolyte interface, enabling power generation without batteries or fuels [1,44].

Using CNTs, Lee et al. introduced a self-powered torsional yarn actuator that converted glucose signals into mechanical motion without external energy sources. The system comprises a MWCNT yarn embedded with a glucose-responsive nanogel containing hyaluronic acid, cholesterol, and 3-aminophenylboronic acid. Glucose concentrations of 5–100 mM triggered boronate ester formation, causing ~230% volumetric expansion and 10–40°/mm torsional motion. The actuator displayed fast response (~20 min) and recovery (~30 min) and high efficiency (97.2%) in phosphate-buffered saline (PBS), highlighting its potential in implantable or wearable glucose-responsive devices [78]. Jang et al. developed a mechano-electrochemical sensor based on coiled CNT yarns to monitor gastric motility by transducing mechanical strain into electrical signals. The sensor detected gastric wall strain (10–30%), contraction frequency (0.04–0.1 Hz), and volume fluctuations (~3–12 mL) in media such as PBS and serum. In an artificial stomach model, the sensor generated an OCV of approximately 37 mV at 30% strain and 0.06 Hz—consistent with physiological gastric activity. Its biocompatibility was confirmed using C2C12 cell culture, and the coiled design enhanced contact with dynamic tissues, demonstrating its suitability for real-time gastrointestinal monitoring [79]. Sim et al. developed a mechano-electrochemical fiber sensor comprising microbuckled CNT yarns encapsulated in stretchable elastomeric sheaths for implantation in soft organs. The buckled architecture modulated the electrochemical double-layer capacitance in response to strain, enabling reversible, non-Faradaic charge storage while maintaining softness (2 MPa modulus) and high elasticity (up to 100% strain) comparable to biological tissue. This device generated OCVs of ~20 mV and short-circuit currents of ~1.5 μA through EDLC and Faradaic processes under 50% strain. With high sensitivity (~100 A/kg/MPa), it detected pressures as low as 60 kPa and successfully tracked bladder dynamics and 1 Hz cardiac motion in porcine models (Figure 5C) [76].

Building on this, Sim et al. developed a coiled–buckled hybrid fiber sensor that achieved a current density of 121 A/kg and power density of 16 W/kg. The sensor exhibited exceptional mechanical properties—1050% stretchability, 0.2 MPa modulus, and compatibility with suturing—enabling stable, self-powered monitoring of both dynamic and static organ motions. Its synergistic architecture, combining coil opening with buckle unfolding, enhanced capacitance modulation under strain and was soft enough to be directly wrapped around a porcine bladder without impairing function. In this model, the sensor consistently produced voltage outputs (~0.4 mV/mL) under strain and showed high sensitivity (0.12 mV/% strain and 0.78 μA/kg/% strain in the 1.3C-SCBF configuration), displaying its potential for continuous, minimally invasive soft-organ monitoring (Figure 5D) [77]. In mechanoelectrical implantable sensors, CNT yarns act as flexible conductors that maintain charge transport under deformation and efficiently convert strain or pressure into electrical signals. Table 3 outlines the performance of CNT-based self-powered implantable sensors, including their operating mechanisms, sensor configurations, active components, total power generation, detection limits, and biomedical applications. The summarized data offer a concise reference for evaluating how CNTs contribute to biocompatibility, reliability, and functionality in implantable platforms.

6. Conclusions and Perspectives

This review has summarized CNT-based self-powered sensors developed across electrochemical, triboelectric, piezoelectric, thermoelectric, and photoelectric mechanisms, with applications spanning environmental monitoring, wearable systems, and implantable devices. Owing to their high conductivity, mechanical flexibility, and surface tunability, CNTs can serve simultaneously as active sensing elements and energy harvesters. Furthermore, scalable fabrication strategies, including floating-catalyst direct spinning and textile integration, have accelerated the transition of CNT-based technologies from laboratory demonstrations to practical device platforms [1,3,5]. Nevertheless, several challenges remain. First, long-term operational stability is still limited, particularly in enzymatic and bio-interfacing devices, where enzyme deactivation and signal drift are recurrent issues [75,80,81]. Second, the power output of many CNT-based harvesters remains insufficient to continuously operate wireless modules or integrated processors [1]. Third, scalable and reproducible fabrication remains a significant challenge. Although direct spinning and printing techniques are promising, issues such as batch-to-batch variability and difficulties in integrating CNTs into complex textile architectures remain notable obstacles [3,5].

At the materials level, optimizing CNT alignment and junction structures and developing multifunctional composites may enhance both sensitivity and power output. In addition, surface and chemical modifications (e.g., controlled oxidation, antifouling coatings, and dopants) will be critical for sustaining stability under physiological and other harsh environmental conditions [1]. On the fabrication side, reproducible direct spinning, printing, and soft encapsulation must ensure washability in wearables and chronic operation in implantables [1,3,5,74,75]. Equally important is establishing reliability benchmarks for output stability, cycling endurance, and in vivo validation [75]. At the system level, CNT harvesters should be integrated with low-power circuits, wireless modules, and hybrid energy management. Application-oriented goals include ≥90-day durability in environmental systems, washability/stretchability in wearables, and chronic wireless operation in implantables [1,74,75,76,78,79]. Looking ahead, advances in printable electronics, hybrid energy harvesting, and wireless management will be pivotal. Recent demonstrations in CNT-based energy harvesting and implantable/wearable platforms highlight feasible routes to overcome current limitations [1,74,75,76,78,79]. With interdisciplinary collaboration, CNT-based self-powered sensors are positioned to enable the next generation of intelligent, battery-free monitoring technologies.