Micro-cylindrical/fibric electronic devices: materials, fabrication, health and environmental monitoring

Conformal lithography by selective exposure

Photolithography is a viable strategy for fabricating ultra-high-resolution features, and adapting traditional planar photolithography to high-curvature micro-cylindrical surfaces is of significance and challenge. The key to micro-cylindrical surface photolithography lies in minimizing the patterning mapping errors on high-curvature surfaces^[9,20,72,73]. Various photolithography schemes have been developed for both mask-based and mask-less photolithography, enabling selective exposure and removal of photosensitive materials to meet the patterning requirements of high-curvature micro-cylindrical objects.

Mask-based photolithography utilizes high-precision masks and optimized exposure setups to reduce exposure mapping errors on micro-cylindrical surfaces, leveraging rotational exposure systems for effective lithography. Some studies focus on improving exposure equipment and strategies to adapt to micro-cylindrical objects. For example, Yang et al.^[40] used a cylindrical projection lithography system to pattern the surface of a 1 mm diameter capillary substrate, developing rotational exposure equipment to scan the mask projection along and around the substrate [Figure 2A]. Similarly, Toshiyuki et al. used rotary scanning projection lithography on a 2.5 mm diameter substrate with an edge-width slit mask to project high-performance imaging^[78], scanning a spiral pattern on the substrate [Figure 2B]. Other studies have taken advantage of the conformal properties of flexible masks and substrates. Doll et al. patterned an 8 mm diameter small cylindrical surface using a chromium-plated polymer flexible mask^[79], reducing exposure errors through the conformal characteristics of the elastic polymer with the substrate [Figure 2C]. Park et al. successfully fabricated high aspect ratio copper structures on a 20 mm diameter micro-cylindrical object using a flexible SU-8 lithographic mask, incorporating a planar micro-slit mask to prevent undesirable exposure from soft lithography^[80] [Figure 2D]. High-resolution and conformal mask design and fabrication were applied to minimize mapping errors.

Figure 2. Photolithography-based fabrication methods on micro-cylindrical surfaces. (A) Cylindrical direct projection lithography. Reproduced with permission^[40]. Copyright 2018, MDPI; (B) Variable-width slit mask-assisted lithography^[78]. Copyright 2015, SPST; (C) Chromium-coated polymer flexible mask-assisted lithography. Reproduced with permission^[79]. Copyright 2020, IOP Publishing Ltd; (D) Combination of flexible SU-8 photolithographic mask and planar slit mask lithography. Reproduced with permission^[80]. Copyright 2011, Elsevier; (E) Mask-less lithography using high-resolution light beams. Reproduced with permission^[19]. Copyright 2017, John Wiley and Sons; (F) Programmable improved UV lithography system^[81]. Copyright 2013, WORLD SCIENTIFIC PUBL CO PTE LTD. UV: Ultraviolet.

To further reduce the projection errors of masks on micro-cylindrical surfaces, researchers have developed mask-less lithography techniques, using high-resolution beams to directly pattern micro-structures with the aid of rotational exposure systems. Tamaki et al. performed mask-less lithography on the surface of a 360 μm diameter microprobe, utilizing high-resolution beam path planning for patterning^[19] [Figure 2E]. Yang et al. employed a custom-built rotating ultraviolet (UV) lithography system to plan path on the substrate^[81], achieving a patterned micro-cylindrical structure of 330 μm [Figure 2F]. Hwang et al. integrated various electronic components on a 150 × 150 μm square microfiber, using a mask-less aligner for patterning metal electrodes^[18]. These studies typically require both the beam and the motion platform to maintain high precision for minimizing mapping errors.

Laser processing for direct and controlled ablation

Laser processing technology utilizes high-energy laser beams to ablate complex patterns on the micro-cylindrical surface through precise path planning. This technique can be categorized based on its application to either the internal or external surfaces of cylindrical objects^[75-77].

For laser ablation processing on the outer surface, Haga et al. designed the structure of a hydraulically actuated bending catheter, using femtosecond laser processing to treat a super-elastic nitinol tube^[82] [Figure 3A]. These components feature multiple ring-arranged structures, with each ring having an outer diameter of 0.8 mm and an inner diameter of 0.75 mm. The innovation of this process lies in the use of a laser ablation system, which allows precise machining of the super-elastic alloy on micro-cylindrical surfaces while coating the thin-walled silicone rubber tube to enhance its functionality and durability. For processing inside the tube, Horiuchi et al. used laser scanning technology to create complex patterns within small-diameter tubes with 423 μm in diameter^[17] [Figure 3B]. They utilized a 408 nm wavelength semiconductor violet laser to project high-density multi-helix patterns onto the inner surface of the tube, by passing it through a pinhole and precision imaging optical system. This technique’s innovation lies in its capability to achieve submicron manufacturing precision within extremely small-diameter tubes, while combining photolithography and chemical etching to achieve the desired inner surface characteristics.

Figure 3. Laser processing fabrication technology on the inner and outer surfaces of micro-cylindrical objects. (A) Customizable path processing on the outer surface of a micro-cylindrical object via laser ablation. Reproduced with permission^[82]. Copyright 2011, John Wiley and Sons; (B) Laser scanning processing on the inner surface of a micro-cylindrical object. Reproduced with permission^[17]. Copyright 2014, IOP Publishing on behalf of the Japan Society of Applied Physics.

The common features of these two laser processing technologies include high-precision manufacturing, suitability for small-diameter tubes, the ability to apply complex patterns precisely, and meeting functional requirements for both the outer and inner surfaces through different processing methods.

Conformal printing for non-contact deposition

Inkjet printing is a non-contact additive manufacturing technique that, by planning the relative movement path of the nozzle and the substrate, enables the fabrication of patterned structures on high-curvature surfaces^{[12,83,85,86]}. Based on the principle of ink deposition, inkjet printing can be divided into extrusion printing^[87,88], aerosol jet printing^[13], EHD printing^[14], and electrospinning^[89]. Table 3 presents a comparison of different types of inkjet printing techniques.

For extrusion printing technology, it is the most commonly used inkjet printing method, utilizing substrate rotation equipment to achieve micro-cylindrical printing. Li et al. proposed the fabrication of one-dimensional stretchable fiber-shaped electronic devices through inkjet printing technology, with a minimum diameter of 500 μm^[15] [Figure 4A]. They developed precision rotary inkjet printing equipment and surface chemical modification processes, achieving wire structures with a line width of approximately 130 μm and customizable micro/nanofabrication. Zhang et al. also used direct ink writing technology to fabricate flexible capacitive strain sensors on the surface of fibers with diameters ranging from 0.5 to 1.5 mm, and demonstrated excellent sensing performance^[29] [Figure 4B].

Figure 4. Inkjet printing fabrication technology on micro-cylindrical surfaces. (A) Extrusion-based inkjet printing for customizable path fabrication. Reproduced with permission^[15]. Copyright 2023, Elsevier; (B) Spiral ink writing on micro-cylindrical surfaces. Reproduced with permission^[29]. Copyright 2023, Springer Nature; (C) (i) Schematic illustration of the LAJ printing process. (ii) Step-by-step fabrication workflow of a graphene sensor on a catheterized balloon. Reproduced with permission^[91]. Copyright 2024, Springer Nature; (D) Microscale printing based on electric field-driven jet. Reproduced with permission^[89]. Copyright 2022, Springer Nature; (E) Electric field-driven printing with a pre-set eccentricity strategy. Reproduced with permission^[14]. Copyright 2023, Elsevier. LAJ: Lathe-based aerosol jet.

To improve the quality of ink deposition, modifications have been made to both the ink materials and deposition methods. Aerosol jet printing involves spraying the material in the form of aerosol microdroplets, which can enhance the printing resolution. For example, by leveraging lathe-based aerosol jet (LAJ) printing in combination with a rotatable substrate platform, flexible multilayer CNT transistors and graphene-based sensors for temperature and inflation monitoring have been successfully fabricated on the surfaces of airbag catheters^[91] [Figure 4C]. In addition, Jordan et al. used aerosol jet printing to fabricate inductively coupled radio frequency coil markers on a polymer catheter with a diameter of approximately 2 mm, achieving a printed wire width of about 250 μm^[13].

EHD printing and electrospinning techniques involve applying a high-voltage electric field between the nozzle and the substrate. The high electric field force pulls the ink out of the nozzle, forming a submicron-scale jet, which improves ink deposition resolution. Zhang et al. proposed a microscale printing technique based on electric field-driven jetting, which only requires a single potential at the nozzle electrode to form the strong electric field, thereby eliminating the shape limitation of the substrate^[90]. Fang et al. used electrospinning to prepare ultrafine polyaniline fibers with diameters below 5 μm, achieving excellent performance^[89] [Figure 4D]. Peng et al. used electric field-driven printing to fabricate thin-walled tubular mesh structures on substrates with diameters ranging from 4 to 8 mm, proposing a pre-set eccentric strategy to mitigate the uneven electric field distribution caused by high-curvature substrates^[14] [Figure 4E].

The aforementioned techniques utilize substrate rotation equipment to adapt traditional printing methods to high-curvature micro-cylindrical surfaces, improving the positioning accuracy of the nozzle and the substrate to achieve higher-precision patterned structures.

Plating and coating for layer deposition

The coating/plating process is an important technique to prepare uniform and dense films around on the surface of micro-cylindrical substrates, aimed at altering surface properties or imparting new functionalities^[9,10,56,84]. The elongated characteristics of micro-cylindrical objects make them well-suited for coating and plating processes, which can be categorized into physical coating techniques and chemical plating techniques based on the principle of material deposition^{[11,18,92-94]}. Table 4 provides a comparison of different types of coating/plating techniques.

Physical coating techniques utilize mechanical, thermal, or other physical methods to apply functional materials uniformly to the substrate surface. For example, Ham et al. used a direct dip-coating method to uniformly coat organic ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] material on a 100 μm diameter Ag wire, fabricating substrate-free ferroelectric organic transistors that achieve one-dimensional artificial multi-synapse functions^[20] [Figure 5A]. Lee et al. proposed a suspended shear dip-coating method to attach deformable semi-solid liquid metal particles (LMP) onto fiber surfaces, significantly enhancing the coating’s durability, conductivity, stretchability, and biocompatibility^[95] [Figure 5B].

Figure 5. Fabrications of coating/plating technologies on micro-cylindrical surfaces. (A) Direct dip-coating method. Reproduced with permission^[20]. Copyright 2020, American Association for the Advancement of Science; (B) Suspension shear dip-coating method. Reproduced with permission^[95]. Copyright 2023, Springer Nature; (C) Electrochemical plating method. Reproduced with permission^[96]. Copyright 2023, John Wiley and Sons; (D) In-situ chemical reduction synthesis method. Reproduced with permission^[41]. Copyright 2022, Sage Publications; (E) Solution-redox method. Reproduced with permission^[97]. Copyright 2021, Royal Society of Chemistry; (F) Combination of direct dip-coating and electroplating processes. Reproduced with permission^[98]. Copyright 2021, American Chemical Society.

Compared to physical coating techniques, chemical plating involves chemical reactions during the process, such as electrochemical, redox, and in-situ chemical reduction mechanisms. Woo et al. incorporated hydrogel ionic diode systems onto fiber electrodes through electroplating and spin-coating, where the uniform coating ensured device stability^[96] [Figure 5C]. Huang et al. synthesized conductive composite fibers with AgNPs conductive sheaths via in-situ chemical reduction, embedding AgNPs within the core fibers to form a novel sensing mechanism, leading to the development of an AgNPs/double covered yarn (DCY) composite yarn strain sensor^[41] [Figure 5D]. Liao et al. synthesized textile-like V6O13 nanomaterials onto aligned CNT fibers using a solution-redox method to fabricate self-charging fiber electrodes, effectively mitigating active material detachment and cracking during deformation^[97] [Figure 5E].

Physical coating and chemical plating processes can also be combined. For example, Han et al. developed a multifunctional coaxial energy fiber with a multilayer structure, using a combination of direct dip-coating and electroplating to create different functional layers on the fiber surface^[98] [Figure 5F]. By integrating various film-forming techniques, it is possible to form uniform and dense multilayer functional structures on micro-cylindrical surfaces, suitable for applications in sensing, actuation, and other scenarios. Physical coating and chemical plating technologies each have their advantages but face challenges in achieving high precision and consistency. Physical coating is limited by coating speed and substrate shape, while chemical plating is affected by reaction kinetics and deposition conditions. Optimizing process parameters and developing novel materials is crucial for enhancing the performance of micro-cylindrical electronics fabrication.

Transferring flexible electronics from planar onto curved surfaces

Transfer from planar to high-curvature micro-cylindrical surfaces is based on preparing planar thin-film devices using microelectromechanical systems (MEMS) processes for flexible electronics, which are then wrapped onto the micro-cylindrical surfaces using molds, adhesives, and other means^{[21,35,45,99,100,103]}. Lee et al. used a conventional method to prepare planar flexible thin-film electronics, relying on the good ductility of the substrate^[44], nested with an implantable micro-cylindrical device of about 1 mm in diameter to achieve fit by conformal bending [Figure 6A]. Liu et al. first fabricated a 1024-channel Neuroscroll probe electronic structure using planar MEMS processes and then wrapped it around a micro-cylindrical surface using a micro-tungsten wire as a carrier^[32], achieving a theoretical diameter of the micro-cylindrical structure of 84 and 138 μm [Figure 6B]. Pothof et al. transferred a 64-channel circuit structure onto a SEEG probe with an 800 μm diameter using a specialized rolling mold, and employed a Cytop adhesive layer to enhance the conformal attachment of the flexible circuit^[104]. Fiath et al. also used the same process to fabricate probe electrodes^[49], but innovatively designed the transfer mold to reduce processing errors when wrapping flexible circuits onto micro-cylindrical surfaces [Figure 6C]. Schwaerzle et al. designed an optical stimulation electrode by covering a micro-cylindrical substrate with flexible circuits^[105], with mechanical assembly and adhesive connections between thin-walled multilayer structures [Figure 6D]. The above studies designed different processing molds to achieve conformal transfer to micro-cylindrical substrates, aiming to minimize wrapping attachment errors and enhance structural adhesion. Overall, transfer techniques demonstrate remarkable replication capabilities and offer an effective means for integrating multifunctional electronic devices onto micro-cylindrical substrates. However, future efforts should prioritize enhancing transfer accuracy and device consistency on high curvature surfaces to further advance this technology.

Figure 6. Transfer fabrication technology for micro-cylindrical electronics manufacturing. (A) Conformal bending of flexible thin film electronics. Reproduced with permission^[44]. Copyright 2024, Springer Nature; (B) Direct wrapping of flexible circuits on high-curvature micro-cylindrical surfaces. Reproduced with permission^[32]. Copyright 2024, Springer Nature; (C) Innovative wrapping mold design to reduce errors. Reproduced with permission^[49]. Copyright 2018, De Gruyter, Berlin/Boston; (D) Mechanical assembly and adhesive connection scheme for thin-walled multilayer structures. Reproduced with permission^[105]. Copyright 2015, Elsevier.

Nanoimprinting for rapid 3D fabrication

The nanoimprinting process can replicate micro-nanostructures from molds onto high-curvature micro-cylindrical surfaces, with imprint quality depending on the mold design and imprinting method^[101,102]. Mekaru et al. designed a sliding planar mold technique suitable for polyester fibers with a diameter of 180 μm by utilizing rolling and sliding movements^[106] [Figure 7A]. This technique addresses the fiber twisting issue by clamping the fiber between heated metal cylinders and dynamically rotating the setup to match the fiber’s rotation. During the process, the fiber rolls between two planar molds while being thermally imprinted with a 50 μm deep diffraction grating pattern, achieving a manufacturing precision of 50 μm.

Figure 7. Nanoimprint fabrication technology on micro-cylindrical surfaces. (A) Nanoimprinting based on sliding planar molds. Reproduced with permission^[106]. Copyright 2011, American Vacuum Society; (B) High-temperature-assisted thermal drawing imprinting technique. Reproduced with permission^[38]. Copyright 2020, Springer Nature; (C) High-speed rolling based on cylindrical molds with hybrid layered microstructures. Reproduced with permission^[107]. Copyright 2013, Elsevier.

The cylindrical mold technique includes two innovative approaches. Wang et al. used the draw-induced thermal drawing (DITD) method, in which a pair of rollers with the desired surface structures serve as a template^[38] [Figure 7B]. In this process, the softened material is stretched into fibers at a high drawing speed above the thermoplastic transition temperature, while the rollers imprint the fiber, achieving precise surface patterning. Ohtomo et al. employed a different method, using a cylindrical mold with hybrid layered microstructures to perform high-speed rolling on 240 μm diameter polymenthylmethacrylate (PMMA) plastic optical fibers, achieving continuous surface patterning^[107] [Figure 7C]. These techniques achieve nanoscale manufacturing precision and are suitable for the functional design requirements of micro-diameter optical fibers. However, they still encounter challenges related to mold design and material adaptability. With ongoing optimization of the nanoimprinting process, these technologies are anticipated to play a significant role in the advancement of optical fibers, sensors, and other functional devices in the future.

Fibric mechanical sensors

Fibric strain sensors are capable of swiftly detecting physical responses and converting mechanical deformations into electrical signals. These sensors exhibit fast response times, broad sensing ranges, excellent compliance, and can be seamlessly integrated into textiles, making them highly suitable for applications in human health monitoring and motion state detection. Fiber-based strain sensors can be categorized by their sensing mechanisms into resistive^[131], capacitive^[132], piezoelectric^[133], triboelectric^[134], and optical types^[135].

Resistive fibric strain sensors measure the magnitude of strain by detecting changes in resistance under mechanical stretching. These sensors are widely used due to their simple structure and ease of fabrication^[58,108]. Huang et al. developed a highly flexible and sensitive strain sensor based on a composite yarn^[41] for real-time effective recognition of sign languages. The performance of fiber sensors can be enhanced through material processing and unique microstructural designs^[136]. For example, optimizing the volume fraction of conductive nanofillers can improve sensitivity, while employing structural methods, such as wrinkled or helical configurations, can expand the strain sensing range^[120,137]. However, the viscoelastic behavior of polymer materials can lead to poor hysteresis properties in resistive fibric strain sensors, a challenge that must be addressed to enhance their stability and reliability.

Triboelectric fiber-based strain sensors operate on the principle of converting external deformations caused by the frictional interaction between two fiber-based materials with differing electron affinities into electrical signals^[138,139]. These sensors are widely employed in healthcare applications due to their simple structure, high accuracy, and unique self-powered characteristics^[140]. A typical triboelectric fiber-based strain sensor is designed with a helical structure^[141]. Upon stretching, the contact state between the two triboelectric layers changes, generating an electrical signal. Consequently, helical fiber strain sensors (HFSSs) are utilized to monitor human respiration [Figure 8A and B]. Additionally, triboelectric fiber-based strain sensors can be configured in a single-electrode mode [Figure 8C]. Conductive yarn is fabricated by twisting several polyester microfibers together with a stainless-steel microfiber, resulting in excellent sensitivity^[142]. This sensor can be easily integrated into gloves for gesture recognition [Figure 8D]. It is important to note that triboelectric fiber-based strain sensors often develop surface defects due to repeated friction during prolonged operation, which can negatively affect sensing performance. These surface defects can be mitigated or prevented through the application of advanced surface treatment technologies.

Figure 8. Principle, structure and application of fibric strain and pressure sensors. (A) Schematic diagram of a chest strap integrated with HFSSs; (B) Routine electrical signals from the HFSS-based chest strap during human respiration^[141]. Copyright 2022, American Chemical Society; (C) Illustration of the yarn-based stretchable sensing device; (D) Photographs of the wearable sign-to-speech interpretation system^[142]. Copyright 2020, Springer Nature; (E) Capacitive soft strain sensor mounted on textile across the knee; (F) Normalized decay time of the sensor output at different walking speeds^[143]. Copyright 2015, John Wiley and Sons; (G) Schematic illustration showing structural change of the pressure sensor under the applied pressure; (H) Photograph of a human hand wearing the smart glove with five pressure sensors on each finger when grabbing an apple; (I) Resistance response of the pressure sensor when grabbing and releasing an apple^[146]. Copyright 2020, American Chemical Society. Application of wearable devices for (J) detecting breathing and (K) monitoring pulses^[147]. Copyright 2019, American Chemical Society; (L) Schematic diagram of a fibric triboelectric sensor in combination with clothing. Reproduced with permission^[151]. Copyright 2020, American Association for the Advancement of Science. HFSSs: Helical fiber strain sensors.

Capacitive fiber-based strain sensors respond to strain by altering their capacitance. These sensors typically employ a core-shell structure^[143], which is effective for detecting elongation strain. These sensors can be integrated into textiles through sewing or weaving, enabling real-time monitoring of wearer movement during activities such as walking [Figure 8E and F]. Beyond the core-shell design, capacitive fiber-based strain sensors often utilize a double helical structure^[29]. For instance, Lee et al. reported a fiber strain-sensing system based on a helical structure consisting of two conductive fibers^[144], which offers stability, durability, and excellent sensitivity. Furthermore, the response of capacitive fiber-shaped strain sensors is particularly sensitive to changes in the dielectric constant of the surrounding environment. To address this limitation, it is essential to develop analytical models that account for various environmental conditions.

In addition to fibric strain sensors, fibric pressure sensors are also extensively utilized in motion monitoring, human-computer interaction, and e-skin due to their simple structure, low power consumption, and skin compatibility^[145]. Similar to fibric strain sensors, fibric pressure sensors can be categorized based on their sensing mechanisms into resistive, capacitive, electromagnetic, triboelectric, and optical pressure sensors.

Resistive fibric pressure sensors detect pressure by altering conductive paths under applied pressure, leading to a change in resistance. To date, various conductive materials, including CNTs, graphene, MXene, and metals, have been employed in the fabrication of fibric pressure sensors. Lan et al. developed conductive Au-MoS₂ composite-coated fibers exhibiting superior electrical conductivity, tensile strength, and stability^[146]. By stacking two Au-MoS₂ composite-coated fibers perpendicularly, a pressure sensor is formed at their cross-contact point [Figure 8G]. Integration of the pressure sensor into a fabric glove allows for multiple force mapping properties [Figure 8H and I]. The sensitivity of wearable sensors can be significantly enhanced through the use of advanced fiber materials and innovative microstructural designs. For example, fibric pressure sensors with high compressibility and sensitivity have been developed using hierarchical three-dimensional, porous reduced graphene oxide (rGO) fibers as key sensing elements^[147]. These sensors can effectively monitor human respiration and pulse [Figure 8J and K]. Additionally, highly sensitive fibric pressure sensors have been fabricated by fabricating stretchable MXene/CNT/polyurethane (PU) fibers via a wet spinning technique^[148].

Capacitive fibric pressure sensors are known for their high accuracy, wide detection range, and reliability. For effective performance, the fiber electrodes must maintain stable conductivity during stretching and bending^[149]. Additionally, microstructures can be incorporated to enhance sensing capabilities and reduce response time^[150]. Furthermore, one-dimensional conductive fibers, such as gel fibers, CNT fibers, graphene fibers, and polyaniline fibers, can be used as electrodes for capacitive pressure sensors, forming fiber electrode pairs. For example, fiber pressure sensors made with hydrogel exhibit high sensitivity, a wide operating range, and stable proximity sensing^[65].

When piezoelectric materials such as ZnO, BaTiO₃, and polyvinylidene fluoride (PVDF) are subjected to pressure, deformation of their crystal structure occurs, resulting in charge separation and the generation of voltage across the material. These piezoelectric materials are widely used in the development of piezoelectric fiber-based pressure sensors. For instance, integrating a fibric triboelectric sensor array into clothing enables simultaneous monitoring of arterial pulse and respiratory signals^[151], facilitating non-invasive and long-term health surveillance [Figure 8L]. Additionally, a piezoelectric PVDF nanofiber membrane (PVDF/ZnO NFM) was utilized as the pressure sensing layer^[152]. This textile was used for external pressure detection, human pulse monitoring, and tactile spatial mapping. Despite the high durability and flexibility of piezoelectric fibric pressure sensors, they are generally limited to dynamic measurements. However, by integrating a rectifier component, continuous measurements can be achieved.

Fibric thermal sensors

Body temperature monitoring is essential for assessing health conditions. However, traditional temperature sensors are unsuitable for wearable applications due to their rigidity and inability to provide continuous body temperature monitoring over extended periods^[153,154]. Fibric temperature sensors, by contrast, offer high sensitivity, rapid response, along with the flexibility and compliance to the skin^[5], making them widely applicable in e-skin^[155] and healthcare^[156]. Based on the temperature sensing mechanisms of the temperature-sensitive materials, fibric temperature sensors can be categorized into thermoresistive and thermoelectric types. The functional materials, structural designs, performance, and application areas of these two types of fibric temperature sensors will be discussed in detail in the following sections.

The principle of thermoresistive fibric temperature sensors is that resistance changes with temperature^[157]. The conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) shows strong potential for wearable temperature sensing due to its excellent flexibility, good electrical conductivity and high sensitivity^[153,158]. Furthermore, to mitigate the impact of additional stress, various specialized structural designs have been employed in the development of strain-insensitive fibric temperature sensors. One option is that a strain-insensitive fibric temperature sensor with periodic and uniform micro-wrinkles can be fabricated by applying pre-strain^[159]. Another feasible option is to sew PEDOT-thermoplastic polyurethane (TPU) composite fibers into normal textiles in an S-shape, which allows the sensor to measure skin temperature accurately during daily activities^[160] [Figure 9A]. However, as a polymer, PEDOT fibers are vulnerable to humidity, leading to inaccurate measurements, especially after perspiration. To address this issue, temperature-sensitive fibers have been developed by encapsulating PEDOT with a PU/graphene composite^[153] [Figure 9B]. Carbon-based materials are also widely employed in fibric temperature sensors due to their high electrical conductivity and thermal stability, such as graphene oxide (GO)-based fibers^[161], rGO/PU composite freestanding stretchable fibers^[162], and graphene-based fibers^[163]. Additionally, temperature-sensitive fabrics made from textile yarns coated with graphene-based inks offer both good washability and high flexibility^[163] [Figure 9C]. Fibric temperature sensors based on conductive polymers achieve high sensitivity, owing to their tunable semiconductor properties^[164,165]. When these sensors were integrated into textiles, they offered stable performance in detecting both body temperature changes and touch-induced temperature variations [Figure 9D].

Figure 9. Principle and structure and application of fibric temperature sensors and heaters. (A) Tester wearing a headband with sensors and sensor response during cycling^[160]. Copyright 2022, American Chemical Society; (B) Preparation of PU/graphene encapsulated PEDOT:PSS composite fiber with skin-core structure^[153]. Copyright 2023, American Chemical Society; (C) Garment knitted with ultra-flexible graphene textile sensors used for monitoring human physiological conditions^[163]. Copyright 2019, American Chemical Society; (D) Image of the fiber temperature sensor sewn onto the tip of a hand glove and temperature response of the fiber sensor to repetitive touch on a hot (45 °C) or cold (5 °C) object^[165]. Copyright 2023, Springer Nature; (E) Diagram of 1D SEF for wearable electrothermal applications; (F) Infrared thermal images of 1D-SEF at different applied voltages; (G) Optical and infrared thermal images of 1D-SEF embedded in a wearable kneepad with an applied voltage of 0.8 V^[15]. Copyright 2023, Elsevier; (H) Schematic of the integration process of WSPHS; (I) Photograph of WSPHS application at the chest position of an infant model; (J) Heating performance of stretchable heating fiber under various mechanical deformations^[170]. Copyright 2016, American Chemical Society; (K) DCFBs for all-weather personal thermal management textiles; (L) Temperature evolution and infrared thermal images of the fabric woven by DCFBs under stepwise voltage from 2 to 8 V^[171]. Copyright 2024, Elsevier. PU: Polyurethane; PEDOT:PSS: poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate); SEF: fiber-shaped electronics; WSPHS: wearable and smart personal heating system; DCFBs: dopamine-induced composite fiber bundles.

Thermoelectric fibric temperature sensors utilize the thermoelectric effect between two different conductors to measure temperature. When one end of the thermocouple is exposed to a temperature gradient, the potential difference between the conductors varies with the temperature^[108]. Wang et al. employed electrospinning technology to fabricate an elastic poly(styrene-block-butadiene-block-styrene) (SBS) nanofiber mat, which was subsequently drop-coated with a PEDOT:PSS solution to form a uniform shell layer. This process resulted in a temperature-sensitive layer with advantages including linearity, high sensitivity, and stability^[166].

Wearable electric heaters have broad applications across various fields, including healthcare^[167] and personal thermal management^[168]. Fibric electric heaters allows for more efficient integration of conductive materials, as well as their inherent flexibility^[169]. Moreover, they can be woven into a variety of flexible and breathable fabrics^[23]. Fibric electric heaters can be achieved by printing metallic materials directly onto the surface of elastic fibers to create. For instance, helical silver electrodes can be printed onto the surface of PU fibers and encapsulated with polydimethylsiloxane (PDMS)^[15] [Figure 9E and F]. The helical structure provides the fibers with excellent stability, softness, and stretchability, allowing them to be woven into items such as kneepads and gloves [Figure 9G]. Additionally, coating fibers or textiles with a layer of metal or metal oxide nanowires represents another viable approach for the development of fibric electric heaters. For instance, Cheng et al. designed a copper nanowire (CuNW) composite fiber with a unique hierarchical structure^[170] [Figure 9H]. This fiber not only exhibits excellent heating performance but also demonstrates remarkable tolerance to mechanical impacts, such as bending, twisting, and stretching. It has been applied as a heating layer for infant warming coats [Figure 9I and J]. Currently, the preparation of stretchable conductive fibers is complex and these fibers are prone to failure under significant deformation, which limits their application. To address this issue, Li et al. enhanced the wettability of PU multi-filaments using dopamine. They then coated the filaments with striated polypyrrole (PPy), resulting in composite fiber bundles with excellent stability and strain-insensitive properties. These composite fibers were integrated into smart textiles and demonstrated considerable potential for use in wearable thermal therapy devices^[171] [Figure 9K and L].

SPR-based optical fiber sensors

Optical fibers, as a typical fibric device, offer significant advantages in long-distance monitoring, miniaturization, and the flexible manipulation of light. Sensing based on the surface plasmon resonance (SPR) effect can be achieved by fabricating functional coatings on the surface of optical fibers^[110]. When the incident light and surface plasmon polaritons (SPP) satisfy the wavevector matching condition, a portion or most of the incident light energy couples into the SPP [Figure 10A]. When the refractive index of the surrounding environment changes, characteristics such as the SPR wavelength and the half-peak width detected by the spectrometer will also change in response [Figure 10B]. Nanomaterials such as AuNPs^[172], CNTs^[173], silicon oxide (SiO₂)^[174], and GO^[175] are commonly used in these coatings. Due to the extreme sensitivity of SPR effect to the refractive index of the surrounding environment, it is frequently utilized in biological and chemical detection applications.

Figure 10. Principle and application of fiber optic SPR sensors. (A) Principle diagram of the optical fiber sensor based on SPR and LSPR. Reproduced with permission^[110]. Copyright 2024, Elsevier; (B) Changes in the SPR spectra during the sensing process; (C) SPR optical fiber temperature sensing system for in situ monitoring of thermal effects in photoinduced catalysis; (D) Thermal sensitivity of HC mode, LC mode, and fiber core mode to varying ambient temperatures. Reproduced with permission^[177]. Copyright 2022, Springer Nature; (E) Cross-sectional of the fiber optic LSPR sensor based on AuNPs coating, along with a fundamental mode analysis of the sensor. Reproduced with permission^[179]. Copyright 2023, Elsevier; (F) Composition of a dual-channel SPR detection system designed for synchronized human IgG sensing and temperature compensation; (G) Wavelength shift of the dual-channel SPR sensor at varying human IgG concentrations. Reproduced with permission^[190]. Copyright 2019, Elsevier. SPR: Surface plasmon resonance; LSPR: localized surface plasmon resonance; HC: high-order cladding; LC: low-order cladding; AuNPs: gold nanoparticles; IgG: immunoglobulin G.

In general, the performance of fiber optic SPR sensors is heavily influenced by the material and thickness of the metal film. Gold is widely utilized for SPR sensing owing to its stable chemical properties and high dielectric constant^[176]. Li et al. proposed a sensing system based on gold-plated single-mode optical fibers with a tilted fiber Bragg grating (TFBG) etched in the fiber core^[177], as shown in Figure 10C. The spectral transmission of these devices can differentiate between rapid changes in localized temperature at the catalyst surface and rapid changes in ambient temperature. Thermal effects in interfacial photoinduced catalysis can be decoded with a temperature resolution of 0.1 °C and a time resolution of 0.1 s [Figure 10D]. Recently, Villatoro et al. employed the deposition of a gold film on one side of a TFBG to create a dual-resonance structure^[172]. It achieves high sensitivity in refractive index detection while also compensating for temperature fluctuations.

In addition to metal materials, a variety of new nanomaterials have been widely used, operating on a slightly different principle from SPR. This principle involves constructing localized surface plasmon resonance (LSPR) on the surface of nanoparticles^[178]. Ning et al. introduced a bent-structure J-shaped fiber optic probe, forming the basis of a LSPR biosensor for the detection of Helicobacter pylori (H. pylori)^[179]. This biosensor was achieved by coating the fiber surface with AuNPs and specific aptamers, as shown in Figure 10E. CNTs have been selected as sensing materials for fiber optic LSPR sensors due to their unique advantages, including high chemical stability and large surface area. An example is a fiber optic SPR sensor based on multi-walled CNTs (MWCNTs)-PPy matrix for dopamine detection^[180]. In addition, nanoparticles such as SiO₂^[181,182], ZnO^[183,184], TiO₂^[185-187], and MoS₂^[188] have been used in LSPR-based fiber-optic sensors for detecting gas concentration, humidity, specific ion concentration, and other parameters. LSPR generates a locally enhanced electromagnetic field, which makes LSPR particularly suitable for detecting localized environmental changes or specific markers.

To further improve the detection accuracy and expand the application range of fiber optic SPR sensors, multi-channel SPR sensors have been developed. These sensors compensate for non-specific binding and allow for the simultaneous determination of multiple analytes^[176,189]. Wang et al. proposed a dual-channel self-compensating fiber optic SPR sensor for the detection of human immunoglobulin G (IgG)^[190]. One sensing channel was coated with a bilayer of GO and gold to detect human IgG labeled with AuNPs. The other sensing channel was coated with a silver film only, serving as a reference channel [Figure 10F]. The sensor’s detection limit for human IgG was reduced to 15 ng/mL, which is superior to the detection limit of conventional SPR sensors [Figure 10G]. Siyu et al. designed a dual-channel SPR sensor based on a hollow-core fiber (HCF)^[191]. Coating the inner and outer surfaces of the HCF with silver and gold films, respectively, enables the simultaneous measurement of seawater salinity and temperature. Recently, Zheng et al. deposited AuNPs/β-cyclodextrin (β-CD) composites onto a gold-film-coated HCF and filled the inner channel with glycerol as a temperature-sensitive material^[192]. This setup effectively induced both the SPR effect and the multi-mode interference (MMI) effect, enabling simultaneous measurement of cholesterol concentration and temperature.

The application of optical fiber sensors in environmental monitoring has highlighted their significant advantages in high-sensitivity detection, particularly through SPR and LSPR technologies. However, current techniques face challenges, such as crosstalk among multiple physical signals and distributed sensing. Looking ahead, the ongoing development of new materials and advancements in multi-channel sensing technology are expected to expand the applications of optical fiber sensors in areas such as bio-detection and gas concentration monitoring, thereby facilitating the evolution of intelligent environmental monitoring systems.

POF sensor with surface functionalization

Traditional silicon-based optical fibers have been developed over several decades and are renowned for their high light conduction efficiency. However, their inherent brittleness constrains the range of potential application scenarios. In contrast, POFs, made from polymers with superior bending flexibility, can withstand larger strains, making them ideally suited for flexible sensors. Functional microstructures processed on the POF surface enable single or multi-mode sensing^[54,193], including strain^[194,195], temperature^[111], heart rate^[196], blood monitoring^[30,197], etc. Specific approaches include increasing the surface area to enhance inter-fiber friction^[198], creating arrayed micro-nano structures for specialized optical applications^[199,200], and even integrating electronic devices directly onto the POF surface^[42,201].

Nanoimprinting technology is well-suited for fabricating arrayed micro- and nanostructures on the surface of POFs or other functional fibers, offering high throughput and high resolution. Mekaru et al. conducted an extensive study using this process, preparing dot-array structures on the surface of 250-μm-diameter POFs with a length of 1.6 m using roller nanoimprinting technology^[202] [Figure 11A and B]. Additionally, smart fiber optic sensors with multiple sensing functions are expected to be realized through the structuring of arrays of diffraction gratings^{[106,199,203]} or MEMS structures^[200] on the POF surface. Nanoimprinting technology can achieve high-resolution manufacturing by combining with photolithography templates^[107], and the roll-to-roll process further improves production efficiency^{[22,199,204,205]}. However, it is necessary to ensure compatibility between the mold material and the fiber optic material during application to avoid compromising the basic function and structure of the optical fiber.

Figure 11. Application of plastic optical fibers with surface functional patterning. (A) Procedure of roller imprinting on the POF surface; (B) Photograph of intermittently patterned POF (top) and optical micrograph of 5 μm dotted pattern imprinted on POF (bottom).Reproduced with permission^[202]. Copyright 2010, Elsevier; (C) Setup scheme required for multi-sensing fluorescent POF sensors; (D) Multi-sensing device individually tested using nine complex solutions with different pH, glucose concentration, and MMP concentration. Reproduced with permission^[206]. Copyright 2023, John Wiley and Sons; (E) Schematic and optical image of a printed hybrid PbS QDs phototransistor structure. Reproduced with permission^[42]. Copyright 2023, John Wiley and Sons; (F) Photograph of multimodal human-computer interaction using an optoelectronic multimodal sensor; (G) Relative rates of change in intensity, capacitance, and resistance during the interaction with multiple stimuli existing simultaneously. Reproduced with permission^[201]. Copyright 2023, John Wiley and Sons. POF: Polymer optical fiber; MMP: matrix metalloproteinase; QDs: quantum dots.

In addition to nanoimprinting, chemical coating is a common method for preparing functional structures on POF surfaces. Unlike SPR-based fiber optic sensing, functional structures coated on POFs can be utilized for a variety of fluorescence-based sensing applications^[54]. Recently, Giovannini et al. developed a multi-sensing platform using three fluorescein isothiocyanate (FITC)-based POF sensors for the efficient quantification of pH, glucose, and matrix metalloproteinase (MMP) concentrations present in the wound exudate ^[206] [Figure 11C and D]. Moreover, inkjet printing technology is emerging as a promising method for preparing functional structures on POF surfaces and has been used to fabricate POF-based photodetector devices. For instance, Kara et al. printed colloidal PbS quantum dots (QDs) on a 1-mm-diameter POF surface wrapped with graphene, working as infrared phototransistors^[42] [Figure 11E]. The fabricated device detects photocurrents under laser excitation to monitor scattered light in a multi-mode POF without interrupting the optical path. In 2023, Wang et al. reported a flexible optoelectronic multimodal sensor capable of detecting and decoupling proximity, pressure, and temperature signals^[201][Figure 11F and G]. This was achieved by integrating an interdigital electrode on the POF surface to detect capacitance, light intensity, and resistance signals, respectively. This multifunctional and integrated self-decoupled multimodal POF sensor presents promising opportunities for human-machine-environment interaction applications. In brief, surface modification-based POF sensors exhibit significant potential for diverse applications in environmental signal monitoring, particularly due to their flexibility and versatility. In the future, these sensors are anticipated to play an increasingly vital role in fields such as medical monitoring, environmental protection, and smart home technologies.

Implantable SEEG and DBS electrodes

Implantable neuroelectrodes refer to electrophysiological devices that are implanted within a biological organism and connected to it for the purposes of recording and stimulation^[215]. They are particularly valuable in the diagnosis and treatment of neurological disorders, including epilepsy, migraines, and other related conditions^[115]. Implantable neuroelectrodes can be categorized based on their functions into two main types: Recording Electrodes: These include SEEG electrodes, which capture electroencephalogram signals through deep electrodes surgically implanted within brain tissue^[37]. Stimulation Electrodes: These encompass DBS electrodes, which, following the recording of nervous system signals, provide electrical stimulation aimed at treating neurological disorders^[33].

SEEG electrodes are surgically implanted into brain tissue to record electroencephalography (EEG) signals, which reflect collective transmembrane currents from multiple neurons, as well as action potentials from individual neurons or single units^[37]. The basic dimensions of these electrodes range from 0.8 to 1.23 mm. Fiath et al. employed a SEEG electrode fabricated based on the PI film^[49]. This micro-cylindrical SEEG electrode, with a diameter of 800 μm, was created by depositing a metal layer, such as platinum (Pt), on its surface and employing a thin-film convolution process [Figure 13A]. However, this SEEG electrode was limited to only 32 channels, which is relatively low. Pothof et al. employed time-domain multiplexing (TDM), reducing the number of leads by a 16:1 ratio while increasing the number of channels to 128^[216]. TDM allows external signals measured by multiple electrodes to be used as input signals and transmitted from the same output channel, greatly compressing the position occupied by the electrode leads^[216,217][Figure 13B and C]. Further, Steinmetz et al. proposed a neural pixel probe fabricated using complementary metal-oxide-semiconductor (CMOS) technology, which combines TDM with a very high integration of 1,280 channels in a single shank^[218]. In addition to the number of electrodes, the density of SEEG electrodes is also an important metric, and increasing the electrode density by decreasing the size of individual electrodes can improve the resolution of SEEG monitoring and even sense the electrical activity of a single neuron^[37]. In 2021, Gerbella et al. designed a neuroelectrode combining macroelectrodes and microelectrodes^[48], which saves the space occupied by the electrodes, improves the electrode density, and enhances the limit of the electrode’s perceptual resolution. Recently, Liu et al. proposed a neural probe with an extremely high number of channels (1024)^[32], very high stability (> 105 weeks) and reusability [Figure 13D and E]. Therefore, the current mainstream research direction of SEEG electrodes is to further improve the number and density of recording sites on a single-shank probe, the recording accuracy of a single recording site, and the stability and effectiveness of long-time recording of the probe by combining multiple fabrication techniques.

Figure 13. Structure and application of SEEG electrode and DBS electrode. (A) Basic structure of SEEG electrode. Reproduced with permission^[49]. Copyright 2018, Walter de Gruyter; (B and C) Multi-channel SEEG electrode lead integration. Reproduced with permission^[216,217]. Copyright 2015, Elsevier. Copyright 2017, Molecular Diversity Preservation International; (D) Whole-brain signal monitoring by four-handled 1024-channel SEEG electrode; (E) Multi-channel SEEG electrode prolonged whole-brain signal monitoring result sampling. Reproduced with permission^[32]. Copyright 2024, Springer Nature; (F) DBS electrodes consisting of PI cables with LEDs inserted inside the micro-cylindrical PI probes. Reproduced with permission^[105]. Copyright 2015, Elsevier; (G) Induced potential distribution within the respective depths at different times after stimulation. Reproduced with permission^[19]. Copyright 2017, John Wiley and Sons; (H) DBS electrode applied to stimulation experiments on mouse brain. Reproduced with permission^[95]. Copyright 2023, Springer Nature; (I) Signal feedback of mouse brain under different optogenetic stimulation. Reproduced with permission. Reproduced with permission^[220]. Copyright 2024, American Chemical Society. SEEG: Stereo electroencephalogram; DBS: deep brain stimulation; PI: polyimide; LEDs: light-emitting diodes.

DBS electrodes, as a type of implantable neuroelectrode, can both record pathological brain activity and provide adjustable stimulation for the treatment of neurological and psychiatric disorders^[33]. The current stimulation methods utilized in DBS electrodes predominantly comprise optogenetic and chemical stimulation techniques. In the realm of optogenetic stimulation, Schwaerzle et al. introduced a polymer-based neural probe that incorporates an integrated light-emitting diode (LED) chip^[105]. The electrode can deliver optical stimulation and conditioning directly to the targeted implantation site via the LED [Figure 13F]. Notably, the efficacy of optical stimulation is constrained by the inherent wavelength emission of the LED, necessitating precise modulation of parameters such as wavelength, intensity, and emission frequency to align with the therapeutic requirements at various treatment stages^[34,219]. In 2017, Tamaki et al. developed a neural probe equipped with eight recording sites and three light-emitting sites^[19]. This design successfully facilitated both optical stimulation and electrophysiological signal recording in rat brain experiments [Figure 13G]. As DBS electrode technology advances and exploration into novel stimulation methods continues, a broader array of techniques is emerging. In 2023, Lee et al. introduced a conductively stable and mechanically durable bilayer eutectic gallium indium (EGaIn) composite-coated stretchable fiber^[95], enabling both optical and electrical stimulation of neuronal cells of the mouse brain [Figure 13H]. Similarly, recently, Kim et al. proposed a multifunctional flexible thermally stretched fiber neural probe designed for bi-directional synapse probing within the brain^[220] [Figure 13I]. Collectively, these advancements in DBS technologies present new avenues for interrogating dysfunctional brain circuits and assessing the therapeutic potential of modulating their outputs.

To sum up, the current selection of implantable neuroelectrode types primarily depends on the specific requirements of the application. For long-term implantation, the stability of the electrode material and structure over time must be carefully assessed through extensive animal and clinical studies. Additionally, there is a growing trend towards the development of multifunctional neuroelectrodes that combine recording and stimulation capabilities. Future efforts should concentrate on reducing electrode size through reliable and high-precision manufacturing processes. Smaller electrodes result in smaller incisions and less post-operative pain for patients, promoting greater acceptance of surgical interventions.

Implantable cylindrical biosensors

Implantable biosensors are sensors that utilize materials sensitive to biological substances and physiological signals for the detection of biological organisms by implanting sensors into the human body^[24]. These biosensors play a crucial role in providing in vivo sensing capabilities, expanding the integration and application of sensors in medical devices. There are two main categories of biosensors based on the information they detect: implantable biosensors that focus on detecting biomarkers such as glucose sensors^[12,221], calcium sensors^[11], and neurotransmitter sensors^[222]; and implantable biosensors that focus on detecting both electrophysiological signals of the organism and biochemical signals, such as temperature sensors, pH sensors, and intracranial pressure sensors^[223-225].

Biosensors focusing on the detection of biomarkers have biosensitive materials as recognition elements, which mainly consist of appropriate physicochemical transducers and signal amplification devices. In 2018, Pu et al. proposed a cylindrical enzyme electrode glucose biosensor fabricated by inkjet printing^[12] [Figure 14A]. In vivo experiments in rats demonstrated that the sensor has the potential to be used for subcutaneous tissue implantation for continuous glucose monitoring. In 2020, Wang et al. suggested that functionalized MWCNTs twisted into helical fiber bundles could be utilized to detect changes in calcium ion and glucose concentrations in the blood, as well as hydrogen peroxide (H₂O₂) concentration and distribution^[11] [Figure 14B and C]. A continuous blood glucose monitoring system comprising a plastic optical fiber, a diboronic acid receptor-based chemical indicator, reinforcing wires, and thermocouples enables direct measurement of plasma glucose concentration within the patient’s blood vessels^[226]. Furthermore, the immobilization of glucose oxidase (GOx) on the surface of carbon fibers enabled the micro-cylindrical electrode to simultaneously monitor glucose and dopamine in vivo^[227] [Figure 14D-F]. The sensor demonstrated high sensitivity and selectivity, positioning it as a promising tool for reliable metabolic studies and neurotransmitter monitoring.

Figure 14. Applications of Typical Implantable Biosensors. (A) Structure of an implantable glucose/blood glucose sensor. Reproduced with permission^[12]. Copyright 2018, Lab on a Chip; (B) Detection of tumor growth utilizing H₂O₂ as a biomarker; (C) Sensitive response of an H₂O₂ sensor to ultra-low concentrations of H₂O₂. Reproduced with permission^[11]. Copyright 2019, Springer Nature; (D) Principle of identification and quantification of DA (pink) and H₂O₂ (green) on the micro-cylindrical biosensor surface; (E) Electrochemical modification of carbon fibers to immobilize oxidase on their surface; (F) Resultant graph of glucose and DA detected in a benchtop flow cell. Reproduced with permission^[227]. Copyright 2024, American Chemical Society; (G) Schematic depiction of esophageal and rectoanal pressure measurements using a porcine model. Reproduced with permission^[228]. Copyright 2022, Springer Nature; (H) Application of cardiac beat sensors in the detection of cardiovascular health; (I) Cardiac beat sensor for monitoring epicardial electrograms. Reproduced with permission^[229]. Copyright 2024, John Wiley and Sons. DA: dopamine.

Biosensors primarily detect electrophysiological and biochemical signals of biological. These sensors were used for monitoring intracranial pressure, PH level, temperature, and more. In 2016, Musolino et al. proposed an optical fiber temperature sensor based on rare-earth-doped tellurite glass, integrated at the tip of a conventional silica optical fiber^[224]. This innovative design enables long-term in vivo temperature measurements. Additionally, a piezoresistive pressure sensor catheter, constructed by injecting liquid metal into a silicone tube, was successfully employed to monitor gastrointestinal dynamic data in anesthetized pigs^[228] [Figure 14G]. Recently, Nam et al. proposed a stretchable microfiber composed of a phase-converted liquid metal core and a multifunctional nanocomposite shell^[229]. In rat muscle implantation experiments, the fibers successfully sensed muscle movements and converted them into electrical signals for further analysis [Figure 14H and I]. While current implantable biosensors have demonstrated a wide range of applications in sensing and monitoring, the long-term reliability of these biosensors and the biochemical effects of prolonged implantation in living organisms have been inadequately addressed in existing studies. Therefore, further research is essential to evaluate these aspects comprehensively.