Lignocellulose is the most abundant natural polymer on Earth, characterized by its renewable origin, intrinsic biodegradability, and broad availability across a wide range of lignocellulosic feedstocks^[51-53]. This natural material is mainly composed of three interconnected biological polymers: cellulose (40%-55%), hemicellulose (24%-40%), and lignin (18%-35%), along with minor extractives and inorganic components [Figure 1A]^[52,54-56]. Based on the unique crystal structure, chemical modification, and reconfiguration potential of lignocellulose, it provides excellent design flexibility for the construction of advanced flexible sensors. Various types of lignocellulosic materials [including CNF, CNC, bacterial cellulose (BC), xylan, and lignin] can be obtained through mechanical processing, enzymatic hydrolysis, and acid hydrolysis [Figure 1B]^[57,58].

Cellulose, the primary structural component, is a linear polysaccharide consisting of anhydro-D-glucose units connected via β-1,4-glycosidic bonds^[17,59]. Intrachain hydrogen bonds between hydroxyl groups on C3 and ring oxygen, as well as between C2 and C6 hydroxyls of adjacent units, confer linearity to the chains, while interchain hydrogen bonding and van der Waals forces drive the parallel stacking into hierarchical fibrillar structures. These cellulose chains assemble into microfibril bundles and ultimately macroscopic fibers, comprising highly ordered crystalline regions and disordered amorphous domains. CNC, normally isolated via sulfuric acid hydrolysis, exhibits a rod-like morphology with a cellulose I_β crystal structure (a = 0.778 nm, b = 0.820 nm, c = 1.038 nm, γ = 96.5°), and typically has diameters of 3-50 nm and lengths of 50-500 nm with high crystallinity (58%-88%)^[60]. CNF produced through mechanical nanofibrillation, often combined with chemical/enzymatic pretreatments such as 2,2,6,6-tetramethylpiperidinooxy (TEMPO)-mediated oxidation, possesses high aspect ratios exceeding 100 and contains both crystalline and partially amorphous regions^[61]. Finally, the abundant surface hydroxyl groups of cellulose provide versatile platforms for functionalization, grafting, and post-modification, enabling strong interfacial interactions within hydrogel matrices.

Hemicellulose, a heterogeneous polysaccharide composed of pyranose and furanose sugar units including D-xylose, D-mannose, D-glucose, D-galactosyl, L-arabinose, galacturonic acid, and glucuronic acid, is a short-chain polymer with a degree of polymerization of 80-200 that functions together with lignin as a filler and adhesive between cellulose microfibrils in the secondary cell wall [Figure 1A]^[24,62-64]. Among these sugar constituents, D-xylose constitutes the predominant backbone unit of xylan, which represents the most abundant hemicellulose type in hardwood and the second most abundant in softwood, thereby making xylan the archetypal model for hemicellulose-based functional materials. Its molecular chain adopts both two-fold flat-ribbon and three-fold helical screw conformations, enabling intimate non-covalent interactions with cellulose surfaces and lignin^[24,65]. Compared with the rigid crystalline structure of cellulose, hemicellulose is amorphous and highly branched, endowing it with structural flexibility and rich chemical modifiability^[24,65,66]. These structural features facilitate the introduction of functional groups or conductive components through chemical modification, thereby enabling the construction of sensing materials with responsive interfaces and the flexible regulation of detection signals^[67]. Its abundant free hydroxyl groups can act as dynamic cross-linking sites, which further enhance the cycling stability of the hydrogel sensor through hydrogen-bonding reorganization^[68,69].

Lignin, the most abundant natural aromatic polymer, is a complex three-dimensional amorphous network composed of phenylpropanoid units^[70-72]. These basic building blocks are p-coumaryl, coniferyl, and sinapyl alcohols, which give rise to p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units in the lignin macromolecular structure, respectively^[26,73-75]. The relative abundance of these three units varies with plant source: softwood lignin predominantly contains G units, hardwood lignin comprises both G and S units and minor H units, and herbaceous lignin possesses all three^[76-79]. These aromatic units are randomly interconnected through various ether and carbon–carbon linkages - primarily β-O-4, β-β, β-5, and 5-5 linkages - with the β-O-4 linkage being the most abundant^[80,81]. Extracted technical (or industrial) lignins retain abundant functional groups, including phenolic/aliphatic hydroxyl groups, carboxyl, and methoxyl groups^[82-85]. These functional moieties are heterogeneously distributed across the three-dimensional aromatic network, contributing to the structural complexity and polydispersity of lignin^[86,87]. Lignin can provide a rigid network structure for hydrogels through chemical crosslinking of the stable aromatic structure and the intrinsic hydroxyl groups. Moreover, its catechol structure can even form strong non-covalent bonds with the skin and be directly used for skin adhesion in hydrogel form^[88].

The sensing performance of lignocellulose-derived hydrogels is closely associated with their internal network architectures. Although lignocellulose components provide inherent structural advantages, the final mechanical properties and electrical responses are mainly determined by the interactions and organization within hydrogel networks. These network architectures, including hydrogen-bonded networks, covalently crosslinked networks, dynamic reversible networks, and conductive pathways, regulate the balance between mechanical strength, flexibility, toughness, conductivity, and sensing sensitivity^[89-91]. Generally, covalent networks provide enhanced structural stability, whereas dynamic reversible interactions contribute to energy dissipation and self-healing capability during repeated deformation. Therefore, rational regulation of network structures is essential for achieving high-performance lignocellulose-based hydrogel sensors^[90,91].

In lignocellulose-derived hydrogels, the hierarchical structures and abundant functional groups of cellulose, hemicellulose, and lignin provide versatile platforms for constructing multifunctional networks. CNFs and nanocrystals can reinforce hydrogel matrices through hydrogen bonding and physical entanglement, improving mechanical strength and structural stability^[17,92]. Hemicellulose contributes flexibility and dynamic interactions, while lignin enhances interfacial adhesion and network toughness through hydrogen bonding, π–π interactions, and other supramolecular interactions^[93,94]. Through the synergistic contribution of these components, lignocellulose-derived hydrogels can achieve tunable mechanical properties, improved structural stability, and enhanced sensing performance when integrated with appropriate functional components^[89].

Naturally, lignocellulose constitutes a uniquely suitable material platform for motion sensing, rather than merely a passive substrate that satisfies generic sensor requirements^[95]. Unlike synthetic petroleum-based polymers or inorganic nanomaterials, renewable lignocellulose possesses structural and chemical attributes that intrinsically align with the multifaceted demands of wearable motion sensing [Figure 1C]^[96,97]. The hierarchical architecture of lignocellulose - spanning multiple structural levels from molecular chains to nanofibrils, microfibrils, macrofibers, and tissue-scale pores - endows hydrogels with highly interconnected, multiscale porous networks [Figure 1D]^[98-100]. Such multiscale network structures not only enhance mechanical reinforcement but also influence ion transport, water retention, and deformation-induced signal conversion, directly affecting the sensitivity, response time, and long-term stability of wearable motion sensors^[91]. These networks facilitate efficient moisture wicking and gas permeation - functions critical to the reliable operation of skin-mounted motion sensors under dynamic physical activity^[101]. Without effective moisture management, sweat accumulation can lead to sensor signal drift, epidermal irritation, or premature device delamination^[6,102,103]. By contrast, lignocellulose-based hydrogels exhibit intrinsic breathability [Figure 1E], obviating the need for auxiliary perforation strategies and thereby ensuring sustained interfacial functionality during prolonged wear^[104-106].

The coexistence and synergy of rigid crystalline domains and flexible amorphous domains in lignocellulose microfibrils are governed by the plasticizing action of hemicellulose and the compressive resilience imparted by lignin, enabling precise, broad-spectrum mechanical tunability [Figure 1F]^[107,108]. Through controlled deconstruction or reconfiguration of these hierarchical structural components, hydrogels with tailored mechanical properties can be systematically engineered within a compositionally consistent lignocellulosic material system^[24,95,109]. In addition, the surface chemical diversity, sustainability, and structural anisotropy of lignocellulose are key advantages as a motion-detection platform. The abundant hydroxyl groups on cellulose and hemicellulose, together with the phenolic/aliphatic hydroxyl, and methoxyl functionalities of lignin, provide dense reactive anchors for covalent and non-covalent functionalization, enabling simultaneous integration of multiple sensing modalities - including conductive pathways (via grafting of aniline/pyrrole precursors), self-healing chemistries (via dynamic boronate ester or disulfide bonding), and bioadhesive interfaces (via catechol-mimicking lignin moieties) - without compromising the structural integrity of the gel network^[110-114]. PANI in its emeraldine salt form consists of alternating reduced (–C₆H₄–NH–C₆H₄–NH–) and oxidized (–C₆H₄–N⁺=C₆H₄=N⁺–) repeat units with protonated imine nitrogens that provide charge carriers (polarons/bipolarons) for electronic conduction^[36,38]. PPy consists of α,α′-coupled pyrrole rings with a conjugated π-electron backbone; upon oxidative doping, polarons delocalize along the polymer chain, conferring metallic conductivity^[36,38]. The aromatic backbones of PANI and PPy enable π–π stacking interactions with lignin aromatic units, enhancing interfacial adhesion and dispersion stability within hydrogel networks^[115,116]. Rather than treating environmental friendliness as an afterthought, lignocellulose offers intrinsic biodegradability, non-toxicity, and renewability as inseparable material attributes, eliminating electronic waste accumulation and reducing allergenic risks associated with synthetic polymer degradation products during long-term skin contact or disposable health monitoring applications^[117]. Furthermore, the natural alignment of cellulose microfibrils in plant cell walls, and the even more pronounced directional growth of BC nanofibrils, provide pre-organized anisotropic scaffolds that enable discrimination between different deformation modes^[57,118,119].

The multi-level structural hierarchy of lignocellulose - from molecular chains to nanofibrils, microfibrils, fibers, and tissue-level architectures - enables precise tuning of mechanical properties through specific assembly strategies. This structural versatility, combined with the inherent biocompatibility, biodegradability, and sustainability of lignocellulosic resources, establishes an unparalleled foundation for developing next-generation eco-friendly flexible sensors. Transforming lignocellulose-based hydrogel materials into functional flexible electronic devices requires key processes such as electrode integration, patterning, and interface encapsulation^[120-122]. Conductive electrodes can be constructed by introducing conductive fillers in situ such as CNTs^[39], graphene^[40], or MXene^[123,124] to form a self-supporting conductive network, or by compounding flexible electrodes such as carbon tapes and silver nanowire (AgNW) networks. Carbon-based conductive fillers, including CNTs and graphene, have different crystal structures. CNTs have a high aspect ratio (> 1,000) and a high specific surface area (> 1,000 m²·g^-1) and can form efficient permeation networks in hydrogel matrices at low content (< 1 wt%)^[39,125]. Graphene has a hexagonal honeycomb lattice, in which carbon atoms are sp² hybridized, and the π electron system is distributed in a delocalized manner, enabling ballistic electron transport^[40,126]. MXene (Ti₃C₂T) features a hexagonal closely packed layered structure. Each layer of Ti₃C₂ is composed of edge-shared Ti₆C octahedrons, and the surface terminals (T = -O, -OH, -F) can form hydrogen bonds with cellulose hydroxyl groups, promoting the electrostatic self-assembly of charged nanocellulose^[127,128]. The interlayer spacing (monolayer Ti₃C₂T approximately 1.0 nm) provides ion transport channels, thereby enhancing the ionic conductivity of the composite hydrogel^[33,129]. In terms of patterning, 3D printing technology enables rapid prototyping of sensors with complex structures, while screen printing and templating methods are suitable for large-area preparation of arrayed sensing units^[130-132]. To address the drawbacks of hydrogels, such as easy dehydration and swelling, elastomers like PDMS and TPU are commonly used for encapsulation, or organic hydrogels are prepared by introducing ethylene glycol/water binary solvents or deep eutectic solvents (DESs) to improve environmental tolerance^[133-135]. Additionally, by integrating sensing units with flexible circuits, Bluetooth modules, and energy-harvesting devices [e.g., triboelectric nanogenerator (TENG)], a self-powered and wirelessly transmissible intelligent sensing system can be constructed, thus realizing the leap from a single material to a multifunctional wearable platform^[136,137].

Compared with conventional synthetic and natural polymer-based hydrogels, lignocellulose-derived hydrogels exhibit a unique balance among mechanical robustness, flexibility, functional tunability, and sustainability^[89,138]. Synthetic hydrogels such as polyacrylamide (PAM) and PVA usually provide excellent stretchability and structural tunability but suffer from limited biodegradability and weak environmental adaptability^[139,140]. Natural polymer hydrogels, including alginate, gelatin, and chitosan, exhibit good biocompatibility but often require additional reinforcement or functional modification to achieve sufficient mechanical stability and sensing performance^[141,142]. In contrast, lignocellulose-derived hydrogels integrate renewable resources, hierarchical reinforcement, abundant functional groups, and multifunctional regulation, making them promising candidates for long-term wearable motion monitoring^[89,92,138]. A comparison of representative hydrogel systems based on their mechanical properties, electrical characteristics, and sustainability is summarized in Table 1.

The rational selection of lignocellulosic components is critical for designing hydrogel sensors with targeted sensing performance. Different lignocellulosic derivatives exhibit distinct structural characteristics and functional contributions. CNFs, owing to their high aspect ratio, excellent mechanical strength, and abundant surface hydroxyl groups, are commonly employed as reinforcing frameworks to enhance mechanical robustness and construct stable hydrogel networks^[35,36]. In comparison, CNCs possess highly crystalline domains and rigid rod-like structures, which are advantageous for improving stress transfer efficiency and regulating mechanical reinforcement^[58,59]. Lignin, with its aromatic structure and abundant phenolic hydroxyl groups, provides strong interfacial interactions, antioxidant capability, and environmental resistance, making it suitable for improving adhesion, durability, and multifunctional sensing performance^[26,71]. Hemicellulose, characterized by its amorphous structure and rich functional groups, contributes to flexibility, hydration regulation, and dynamic crosslinking^[24,60,65]. Therefore, the selection of lignocellulosic components should be determined according to specific application requirements: CNF/CNC are preferred for mechanically demanding sensing systems, lignin-containing components are beneficial for durable and adhesive sensors, whereas hemicellulose-rich structures are suitable for flexible and dynamic sensing platforms.

Natural intrinsic advantages establish lignocellulose not merely as a sustainable alternative to conventional sensor materials, but as a functionally superior platform whose hierarchical architecture and chemical diversity directly address the core challenges of motion monitoring. However, realizing this potential requires deliberate structural engineering strategies that either deconstruct the native lignocellulosic biomass into programmable nanoscale building blocks (top-down) or leverage biological/chemical self-assembly to construct optimized architectures from the molecular level (bottom-up). Figure 2 illustrates the advantages and preparation strategies of fabricating lignocellulose-derived gel sensors.

Piezoresistive sensing is the most extensively studied mechanism in lignocellulose-based hydrogel sensors^[17,18]. Its core principle lies in the structural changes of the conductive network within the gel caused by external mechanical stimuli, which in turn leads to corresponding alterations in resistance or conductivity [Figure 3A]^[175]. Lignocellulose plays a dual role as a structural framework and a functional regulator in this process.

According to percolation theory, when conductive fillers form a continuous conductive network in a polymer matrix, the resistivity of the material will undergo a sudden order-of-magnitude change^[176]. CNCs possess an extremely high aspect ratio and abundant surface hydroxyl groups, enabling the construction of an efficient percolation network within the hydrogel matrix^[177,178]. When the gel is stretched or compressed, the number of contact points and the contact area between the conductive fillers change, thereby regulating the electronic transmission path^[175,179]. CNCs, as functional nanofillers, can be incorporated into dynamic cross-linked hydrogel composites through surface modification, assisting in the construction of materials that possess high stretchability, self-healing ability, and sensing performance^[44,117]. Li et al. fabricated a self-healing piezoresistive sensor by in situ polymerization of a PVA-PAM double-network hydrogel with SA and tannic acid-modified CNCs (TA@CNCs)^[180]. Tannic acid (TA) is a natural polyphenol (C₇₆H₅₂O₄₆) rich in catechol and pyrogallol groups. It can coordinate polydentately with metal ions and form hydrogen bonds with cellulose hydroxyl groups^[13,49]. When TA is grafted onto the surface of CNC, TA forms a conformal coating, introducing catechol-mediated dynamic cross-linking sites while retaining the crystalline core structure of CNC^[13]. This dual dynamic cross-linking enables reversible dissociation and reassociation of conductive pathways under deformation, ensuring rapid recovery of the percolation network and thus restoring electrical conductivity upon self-healing. Similarly, Li et al. used CNCs modified with polydopamine (PDA) and gold nanoparticles (AuNPs) (CNCs@PDA-AuNPs) as functional nanofillers, which were embedded in a PVA hydrogel network^[49]. PDA, formed by oxidative polymerization of dopamine (3-hydroxytyramine, C₈H₁₁NO₂), exhibits a heterogeneous structure comprising uncyclized dopamine units, 5,6-dihydroxyindole (DHI), and indole-5,6-quinone moieties cross-linked via Michael addition and Schiff base reactions^[181]. The catechol groups of PDA were tightly bound to the PVA chains through hydrogen bonds, and AuNPs provided an electronically conductive path, thus constructing a stable 3D conductive network for human motion monitoring.

Lignin, as a natural aromatic polymer, can enhance the interfacial cohesion and dispersion stability of hydrogel networks through its abundant phenolic hydroxyl and sulfonic acid groups via multiple dynamic supramolecular interactions (hydrogen bonding, π-π stacking)^[115,116]. Zhu et al. prepared a thermoresponsive in situ adhesive bio-gel by co-embedding LS with MXene nanosheets into a gelatin/glycerol-water binary solvent matrix^[182]. The catechol and sulfonic acid groups on the LS surface form stable interfacial interactions with MXene, ensuring uniform dispersion of conductive nanosheets during deformation and reversible regulation of conductive pathways, thereby endowing the sensor with high sensitivity and a wide detection range. Moreover, as a natural polysaccharide biomass, hemicellulose contains abundant polar groups, such as hydroxyls, along its molecular chains, enabling the formation of multiple hydrogen bonds with conductive polymers or nanoparticles^[24]. Nano-polydopamine (nano-PDA) is uniformly dispersed and stably fills the pores through non-covalent interactions and electrostatic forces mediated by functional groups like amino and carboxyl groups in the hemicellulose network, forming continuous electronic conduction pathways. Meanwhile, ions within the hemicellulose matrix can migrate directionally through the three-dimensional porous network, contributing to ionic conductivity and offering unique advantages in flexible strain-sensing applications^[183].

These studies collectively demonstrate that rational surface engineering of CNCs with conductive nanomaterials and dynamic bonding motifs represents a versatile strategy for developing high-performance piezoresistive sensors with integrated self-healing and mechanical durability.

Ion mobility increases with rising temperature, decreasing resistance. The temperature sensor transduces thermal fluctuations into measurable ionic current variations within the hydrogel matrix, as illustrated in Figure 3E. The sensitivity is evaluated by the temperature coefficient of resistance (TCR), defined as

(2) $$ TCR=(R_T-R_0)/(R_0\times \Delta T) $$

where R_T is the resistance at the measured temperature, R₀ is the resistance at the initial temperature, and ΔT is the temperature change (unit: °C). The larger the absolute value of TCR, the more sensitive the sensor is to temperature fluctuations^[209-211].

In the temperature-sensing mechanism of hydrogels, lignocellulose enhances this effect by providing a porous network based on the above principle. Pang et al. developed a skin-inspired hydrogel based on a three-dimensional cellulose scaffold integrated with conductive fillers to form a composite conductive network^[212]. The resulting hydrogel displays a highly linear and reproducible resistance-temperature relationship, allowing temperature changes to be transduced into quantifiable electrical signals. Analogously, Zhang et al. developed a stretchable and environmentally tolerant hydrogel based on a CNF-stabilized liquid metal (LM)/MXene composite conductive network, where the synergistic effect of highly conductive LM droplets and Ti₃C₂T_x MXene nanosheets establishes efficient electron transport pathways^[213]; when temperature changes, the enhanced thermal excitation accelerates electron mobility while the microstructural evolution of the conductive network alters the resistance, enabling the hydrogel to convert temperature variations into quantifiable electrical signals with high sensitivity and excellent linearity. Beyond the conventional resistive sensing paradigm, an emerging iontronic approach leverages intrinsic temperature gradients rather than uniform temperature changes. Specifically, Cheng et al. engineered a cellulose hydrogel with regulated ion thermal migration channels, wherein the Soret effect drives selective ion diffusion under a thermal gradient to generate a thermoelectric voltage^[214]; by optimizing channel-ion interactions through counter-ion condensation and polyelectrolyte crosslinking, the hydrogel achieved an ultrahigh ionic thermopower of 22.09 mV·K^-1, establishing a self-powered temperature sensing mechanism that directly transduces heat differentials into electrical outputs without external power sources. The thermal response behavior of lignin is significantly different from its structural role in plant cell walls. Wu et al. utilized the abundant phenolic hydroxyl, hydroxyl, and sulfonic acid groups of LS to form a dual-network hydrogel scaffold through Ca²⁺ chelation with SA and PAA, and used this as a matrix to polymerize PANI in situ to form a semi-interpenetrating conductive network^[215]. Lignin provides a uniform and three-dimensional network support for PANI, while the P-type semiconductor properties of PANI endow the hydrogel with an inherent negative temperature coefficient effect. This ternary system can inhibit ice crystal formation and ensure high-temperature water retention properties.

Humidity and sweat sensors can measure changes in environmental humidity or analyze sweat composition (electrolytes, urea, lactate) in practical applications. The working principle is based on the hydrogel’s ability to absorb or release water molecules^[216-218]. When humidity increases, hydrophilic groups on lignocellulose bind to water molecules, causing the hydrogel to swell; this swelling alters either the ion transport pathways (resistance change) or the dielectric constant (capacitance change), generating a detectable electrical signal [Figure 3F]^[204].

The abundant hydroxyl functional groups on the surface of lignocellulose can form hydrogen bonds with water molecules, causing the hydrogel to swell upon absorbing water or shrink upon dehydration when environmental humidity changes. In low-humidity conditions, moisture in the hydrogel evaporates, increasing the distance between conductive materials and thus raising resistance; whereas in high-humidity environments, the hydrogel absorbs water and swells, making the conductive pathways denser and reducing resistance^[17,204,219]. Researchers reported a humidity sensor based on an acrylic/sugarcane bagasse cellulose porous hydrogel doped with graphene oxide (GO)^[40]. The sensor leverages the abundant hydroxyl groups in the cellulose framework to adsorb water molecules via hydrogen bonding, while the porous structure facilitates rapid moisture penetration. As humidity increases, more water is absorbed, enhancing the mobility of free ions within the hydrogel and reducing impedance; desorption reverses this effect. Furthermore, GO, with its surface rich in oxygen-containing functional groups, further improves sensitivity by providing additional binding sites for water molecules and promoting charge-carrier formation. Similarly, Bian et al. developed a transparent, intrinsically stretchable CNF-mediated conductive hydrogel by loading poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) onto a three-dimensional porous CNFs network^[220]. In this system, the CNF skeleton with abundant hydroxyl groups acts as a humidity-sensitive component: when environmental humidity increases, the hydroxyl groups capture water molecules through hydrogen bonding, causing the CNFs network to swell and enhancing the connectivity of the conductive PEDOT:PSS pathways, which reduces resistance; conversely, dehydration in low-humidity environments shrinks the network and increases resistance. This mechanism enables simultaneous strain and humidity sensing with high transparency (> 80%) and excellent stretchability (> 100% strain), demonstrating the versatility of cellulose-based materials in multimodal flexible sensors. By leveraging the abundant hydrophilic groups (–OH) on the surface of hydroxypropyl cellulose (HPC) molecules and the network structure formed within the hydrogel system, Song et al. developed a composite ionic conductive hydrogel incorporating HPC^[221]. This hydrogel retains the inherent self-adhesive stress-strain sensing capability of conductive hydrogels and achieves temperature-responsive transparency variation through the low critical solution temperature property of HPC, forming a multifunctional sensing system with zero crosstalk. Lignin has a higher hygroscopicity than cellulose, attributed to its amorphous structure and a large number of polar functional groups^[222]. LS acts as a “moisture capture center”, with its abundant sulfonic acid groups, phenolic hydroxyl groups, carboxyl groups, and other polar functional groups, which can quickly adsorb water molecules and dissociate to release free protons, forming a proton diffusion current under a humidity gradient^[175].

Meanwhile, the dynamic hydrogen bond-electrostatic cross-linked network composed of chitosan (CS) and PAA causes the ion transport channels of the hydrogel to undergo reversible changes during mechanical deformation, achieving sensitive responses in resistance, and these effects work together to endow the hydrogel with the dual sensing capabilities of humidity-electroconductivity and stress-resistance^[223].

Lignocellulose-derived hydrogel sensors can achieve diverse sensing functionalities through different signal transduction mechanisms, including piezoresistive, piezoelectric, triboelectric, capacitive, thermoelectric, and humidity/biofluid sensing pathways. Each mechanism exhibits distinct advantages and limitations in terms of sensitivity, response speed, environmental stability, and application adaptability. A comparison of the advantages, limitations, and representative applications of different sensing mechanisms is summarized in Table 2. Generally, piezoresistive and capacitive mechanisms are widely employed for pressure and strain sensing due to their simple structures, continuous signal output, and good capability for detecting static deformation. Piezoelectric and triboelectric mechanisms possess rapid response characteristics and self-powered sensing capabilities, making them suitable for dynamic motion monitoring and interactive wearable electronics^{[193,225,226]}. Thermoelectric and humidity/biofluid sensing strategies provide additional functions for environmental and physiological monitoring, although their response is relatively slower due to thermal diffusion, molecular adsorption, and ion transport^[200,227]. It should be noted that sensitivity among different sensing mechanisms cannot be directly compared using a single parameter because different transduction principles employ different evaluation criteria. For example, piezoresistive sensors are usually evaluated by GF or pressure sensitivity, while piezoelectric, triboelectric, capacitive, thermoelectric, and humidity sensors are characterized by parameters such as output voltage, charge density, capacitance variation, Seebeck coefficient, and resistance variation. Therefore, these parameters should be considered as representative performance indicators rather than absolute values for direct comparison.

Wearable electronic devices are the most representative application of lignocellulose-based hydrogel sensors in motion monitoring. Hydrogel sensors must serve as both soft substrates and stable signal-transduction interfaces on moving skin^[245,246]. The key value of these sensors lies in precisely capturing and digitally reconstructing human movements by converting macroscopic or microscopic deformations of the human body into quantifiable electrical signals. It also has excellent ductility, adhesion, air permeability, fatigue resistance, and environmental tolerance in different working environments^[34,138,228]. Cellulose and nanocellulose usually provide fibrillar reinforcement and hydrogen-bonded energy dissipation, whereas lignin contributes aromatic rigidity, phenolic antioxidation, ultraviolet shielding, and interfacial adhesion in the hydrogels^[247-250]. Therefore, lignocellulose-derived hydrogels are not simply renewable substitutes for synthetic elastomers but application-tailored wearable platforms for continuous and comfortable body-signal acquisition.

Macroscopic motion monitoring is the earliest and most mature example of wearable electronics, in which hydrogel sensors attached to fingers, wrists, elbows, knees, ankles, or the neck convert bending and stretching into resistance or ionic-conductivity changes^[251,252]. Lignocellulose-derived components play an active role in this sensing process rather than merely serving as passive fillers. By integrating lignin-containing cellulose nanofibers (LCNF) into PVA ionic conductive hydrogels, the trade-off between the mechanical properties and ionic conductivity of ionic conductive hydrogels has been effectively addressed. The ion-conductive hydrogel demonstrates outstanding response time (300 ms) and sensing stability (300 cycles at 80% strain)^[253]. Further, to balance viscosity and mechanical thermal sensitivity, CMC and LS are utilized to introduce LM into the gel network by dynamic cross-linking, enabling the long-term uniform diffusion of LM nanodroplets. The resulting hydrogel sensor features excellent adhesion strength [Figure 5A], sensitivity (GF = 3.8; the higher the GF value, the more sensitive it is), and a wide operating range (temperature 293-333 K). Meanwhile, it can detect movements at various human body joints (such as fingers, wrists, elbows, and neck) [Figure 5B-D]^[105]. However, this method did not address the defect of low-temperature freezing of hydrogels. To ensure the stable operation of gel sensors under low-temperature conditions, researchers introduced the DES into LCNF gels, endowing them with excellent electrical conductivity (2.29 S·m^-1) and anti-freezing properties (-80 °C)^[106]. Unfortunately, the hydrophilic swelling of the gel matrix can degrade performance when hydrogel sensors are used in underwater electronics. To address this defect, the catechol redox reaction between phosphorylated lignocellulose nanofibers and Ag⁺ is utilized to generate rapid self-gelation at room temperature, forming strong hydrogen-bond interactions and nano-reinforced effects^[103]. The hydrogel has excellent anti-swelling performance (equilibrium swelling ratio of 1.2% after 30 d), high electrical conductivity (2.12 S·m^-1), and strain-sensing ability (maximum GF = 3.98). Sensors based on this hydrogel can be conveniently attached to the limbs of the human body, achieving highly stable underwater communication and recognition of the swimming postures of marine organisms [Figure 5E and F]^[103]. For macroscopic motion monitoring, lignocellulose must balance hydrogel mechanical properties with electrical signal conduction to ensure sensor stability under large-scale motion. At the same time, under complex environmental conditions in real scenarios, functional design must demonstrate excellent environmental adaptability.

Beyond large joint movement, wearable electronics must also detect weak physiological and biomechanical signals such as throat vibration, facial expression, pulse waves, respiration, swallowing, and tremor-related microdeformation^[232,254]. This means gel sensors need extremely high sensitivity. Zeng et al. developed a lignin-modified MXene–Fe³⁺ dual-catalytic system to fabricate conductive hydrogels for wearable sensing under ambient conditions [Figure 5G]^[138]. The hydrogel exhibited a GF of 2.8, stable sensing performance over 200 cycles at 200% strain, and a broad working strain range of 0%-947%. Moreover, its integration into a smart-mask platform enabled the detection of breathing patterns and other subtle physiological motions [Figure 5H]^[138]. Another study on the introduction of MXene hydrogel sensors found that the hydroxyl, carboxyl, and catechol groups on the surface of TA-coated carboxylated CNCs could reduce the number of oxidizable sites on MXene that react with water, thereby inhibiting the oxidation of MXene within the hydrogel^[33,178]. In contrast, this wearable sensor features faster (161 ms) and more sensitive (measurement coefficient = 4.39) sensing performance, capable of accurately detecting subtle movements such as swallowing and pen strokes. Meanwhile, the dynamic crosslinking agent borax provides many dynamic sacrificial bonds, endowing the composite hydrogel with instantaneous self-healing performance and solving the problem that hydrogel sensors are sensitive to mechanical damage^[33]. In addition, by introducing biosynthesized selenium nanoparticles (BioSeNPs) into CNC and MXene hydrogel systems, the synergistic effect of the three has made the resulting hydrogels impressive in terms of tensile strain, elasticity and fatigue resistance. More importantly, the sensitivity of this hydrogel has been raised to 6.24, and it has excellent antibacterial properties^[169]. Compared with lignin and cellulose, the large number of hydroxyl groups in hemicellulose can endow hydrogels with stable mechanical properties and excellent adhesion characteristics. When hemicellulose is combined with MXene, the resulting hydrogel exhibits good self-adhesion (3.12 kPa on the skin), excellent stretchability (> 1,700%), and satisfactory electrical conductivity. This hydrogel serves as an adaptive sensor and can effectively capture subtle signals from various human movements^[255]. By contrast, another hydrogel prepared by combining modified esterified hemicellulose with MXene exhibits higher tensile properties (4,844%) and self-adhesion (12.3 kPa on pigskin)^[256]. The main drawback is that its detection acuity (GF = 2.36) is slightly lower than that of lignin and cellulose gels^[256]. In general, the dynamic cross-linked network constructed by the synergy of lignocellulose and conductive fillers endows the hydrogel sensor with remarkable sensitivity characteristics as a wearable electronic device. It can generate significant resistance changes under minor deformations and accurately identify minute strains such as pulse beats, swallowing movements, and breathing rhythms, providing a reliable technical means for health monitoring and early disease warning.

A more advanced direction in lignocellulose-based wearable hydrogel sensors is the transition from individual sensing materials toward system-level wearable platforms that integrate signal acquisition, power supply, energy harvesting, signal processing, and flexible circuit architectures^{[244,257,258]}. Self-powered sensing systems can achieve energy autonomy through energy conversion. Sun et al. developed a transparent, self-adhesive, and conductive lignin-based organohydrogels through an alkali lignin-Cu²⁺ (AL-Cu²⁺) self-catalytic system and assembled it into an extreme-environment-resistant TENG^[202]. It can achieve self-powered energy collection and movement monitoring of human joints (fingers, wrists)^[202]. In another representative study^[259], researchers prepared hydrogels with superior mechanical properties and electrical conductivity by using a self-catalytic system of sodium lignosulfonate and Fe³⁺. The assembled TENG also has an outstanding self-powered sensing capability^[259]. Significantly, the hierarchical architecture of electrostatic cross-linking, hydrogen-bonding interactions, and π-π stacking between LS and CMC can balance the mechanical properties, functional versatility, and sustainability of hydrogels. This integrated function enables the hydrogel to serve as a unified and all-in-one sensing platform, achieving the collection and conversion of biomechanical energy, ultra-wide range strain sensing, stable energy storage and output, and real-time self-powered monitoring of human physiological signals^[260]. Overall, wearable applications of lignocellulose-based hydrogel sensors are moving from single-point motion detection toward distributed, continuous, multimodal, and self-powered monitoring systems.

Healthcare-oriented applications impose stricter requirements on lignocellulose-based hydrogel sensors than general wearable electronics; the sensing interface must remain mechanically compliant, biologically safe, and signal-stable during long-term contact with skin or soft tissues, and achieve continuous, non-invasive, and real-time tracking of an individual’s vital signs and health status^{[172,261,262]}. Lignocellulose-derived hydrogels are particularly attractive for this purpose because their hydrated polymer networks, low modulus, biocompatible chemistry, and abundant functional groups can collectively support tissue-conformal adhesion, mechanical buffering, and functional biointerface design^[106,263]. Such tissue-like mechanical and interfacial properties help reduce modulus mismatch between the device and biological tissue, thereby improving signal fidelity during physiological monitoring^[264]. In addition, the renewable and biodegradable nature of lignocellulose-derived networks, particularly cellulose- and lignin-derived hydrogel systems, offers a promising pathway toward sustainable biomedical electronics and transient sensing systems with reduced environmental burden after use^[265,266].

Continuous physiological monitoring is one of the most crucial applications of lignocellulose-based hydrogel sensors in clinical use. Pulse, respiration, ventricular tremors, and even the slightest muscle contractions all generate weak and repetitive deformation signals. Long-term stable transmission of these signals requires close, low-irritation, and strong mechanical contact between the hydrogel and the skin or soft tissue^[267-270]. Lignocellulose-based hydrogels are renowned for their excellent biocompatibility and typically adhere closely to the skin surface to collect different signals, enabling continuous, non-invasive, and real-time tracking of an individual’s vital signs and health status^[26,116]. A self-powered sweat-sensing bioelectronic system was designed by leveraging the close hydrogen-bond interaction between sulfonated cellulose (SC) and PVA chains. Under dynamic mechanical stimulation, glucose, potassium ions, and sodium ions in sweat are directly converted into a stable voltage output. This system, which assembles a TENG, can achieve real-time, non-invasive, multi-channel monitoring during exercise, featuring wireless transmission and thresholding alarm functions that support exercise optimization, early warning of physiological imbalance, and rehabilitation guidance^[50]. Similarly, the ability of hydroelectric generators (HVEGs) to generate continuous electricity using a small amount of water vapor also makes them suitable for self-powered health monitoring systems. Compared with TENGs, the open-circuit voltage of HVEGs shows high sensitivity to both water volume and ion concentration, and can detect subtle physiological changes through interaction with body fluids such as sweat, saliva, or blood^[271,272]. Researchers obtained an efficient HVEG by introducing a CNT assembly into SC/PVA. This device can detect human breathing, coughing, and speaking activities by collecting energy from water vapor exhaled from the lungs, and also demonstrates outstanding sensitivity (> 40%) and a fast response time (~2 s), capable of monitoring various health conditions in real time^[272]. In addition to hydropower sensing, this device also features piezoelectric sensing capabilities, enabling precise real-time strain monitoring of subtle biomechanical inputs such as finger bending, wrist pulse, and neck movements. It has a response sensitivity exceeding 80% and a fast response time of 0.5 to 2 s. In addition, its piezoresistive behavior has been successfully applied in multiple real-world scenarios, including medical emergency alarm interfaces, smart doorbell systems, and public safety boundary crossing detection mechanisms [Figure 6A and B]^[273]. To enhance the sensitivity of piezoelectric sensing, the strong coordination selectivity of alginate is utilized to fix Al³⁺, which can significantly increase the ion diffusion difference under pressure^[274]. By adopting a controllable phase separation structure strategy, aluminum alginate is introduced into the cellulose network to improve the performance of hydrogel piezoelectric ion sensors. In contrast, the optimized hydrogel sensor features high sensitivity (14.13 mV·kPa^-1) and an ultra-fast response time (60 ms)^[274]. Compared with cellulose, the intermediate catechol groups generated by the dynamic catalytic system between lignin and metal ions (Al³⁺, Fe³⁺) provide hydrogels with excellent self-healing performance and repeatable adhesive performance^[275,276]. The hydrogel sensor manufactured by the LS-Fe³⁺ dynamic redox system features a wide sensing range (500%), fast response time (139 ms), and high sensitivity (GF = 8.98), enabling specific speech recognition and subtle body-movement detection for real-time elderly health and sleep management^[276]. The hydrogel obtained by LS-Al³⁺ also exhibits ideal mechanical properties, good self-recovery ability, and tensile sensitivity^[277,278]. Surprisingly, introducing Fe³⁺ into hemicellulose hydrogel can double its adhesion and sensitivity (the adhesion strength to pigskin increases from 12.3 to 23.85 kPa), and the sensitivity is high (the specification factor increases from 2.36 to 4.87)^[62,256]. Generally, continuous physiological monitoring requires hydrogel sensors that balance biocompatibility and functional universality, such as adhesion and signal stability, and provide personalized and operable self-powered bioelectronic interfaces for different health monitoring needs.

Motion-assisted diagnosis and rehabilitation represent another important biomedical direction for lignocellulose-based hydrogel sensors^[279]. Clinical rehabilitation often requires quantitative evaluation of joint range of motion, gait symmetry, muscle coordination, and repetitive training performance^[280]. Wearable hydrogel sensors mounted on limbs or joints provide an attractive platform for continuous and quantitative monitoring of biomechanical signals during therapeutic exercise, with relevance to stroke rehabilitation, orthopedic recovery, neurodegenerative disease management, sports injury, and postoperative motor-function assessment^[281-283]. A highly flexible and mechanically tunable all-wood hydrogel was developed^[284]. The neatly arranged cellulose nanochannels endowed it with remarkable anisotropic mechanical properties (longitudinal tensile strength of 36.5 MPa) and strain-sensitive electrical response. All-wood hydrogels can accurately distinguish various macroscopic or subtle human movements, including finger bending, pulse, and swallowing behaviors. In particular, when “Anqi” is called four times within 15 s, two pronunciation changes can be identified^[284].

Stroke patients often experience limb motor dysfunction, including impaired finger flexion/extension and decreased wrist/ankle joint mobility. Therefore, precise assessment methods are needed to develop personalized rehabilitation plans for patients^[285,286]. A dual-mode sensor (DM-BCESC) was designed by integrating the structural color interface of hydroxypropyl HPC with a black conductive polymer hydrogel (CPH) sensing component [Figure 6C]. It features high strain sensitivity (strain coefficient of 4.24) and excellent durability (over 4,400 cycles). In practice, the structural color shift of the HPC layer provides real-time visual interaction monitoring, enabling patients to self-adjust their movements. Meanwhile, the CPH layer can accurately quantify joint motion parameters, such as motion amplitude and frequency. This collaborative feedback mechanism not only enhances patients’ engagement through color changes but also provides digital assessment standards via electrical signals, demonstrating strong application potential for rehabilitation [Figure 6D and E]^[285]. In another study, researchers utilized the synergistic effects of PAA, dialdehyde carboxymethyl cellulose (OCMC), gelatin methacryloyl (GelMA), and lignosulfonate methacrylate (MLS) to achieve the best balance of adhesion, antibacterial activity, and conductivity in a multifunctional hydrogel through precise molecular-level regulation^[286]. The hydrogel’s wide range and high sensitivity (GF = 5.31) enable the sensor to reliably record human electromyography (EMG) and electrocardiogram (ECG) signals. One of the most promising applications lies in the management of neurodegenerative diseases. Machine-learning models can accurately identify the characteristic tremor signals associated with Parkinson’s disease [Figure 6F and G]^[286].

Wound microenvironments are mechanically dynamic and chemically heterogeneous, involving changes in pressure, exudate level, humidity, pH, inflammation, oxidative stress, and bacterial load. Therefore, hydrogel wound interfaces should not only adhere to irregular tissue surfaces, but also maintain moisture balance, resist infection, relieve oxidative damage, and monitor wound-related physiological parameters. However, existing designs and manufacturing methods for human skin interfaces still fail to meet the challenging clinical requirements of excellent adhesion. Therefore, researchers have proposed a printable hydrogel with adjustable adhesion force^[287]. In clinical cases, the hydrogel interface layer can maintain excellent adhesion and high-fidelity signal acquisition during flap/human skin measurement, while simultaneously exhibiting low adhesion after monitoring to avoid wound damage^[287]. Although the research focuses on characterizing tunable adhesion force on flaps/human skin, the responsive hydrogel interface can also achieve good adhesion force to various wet tissues, such as porcine intestines, stomachs, livers, and hearts^[287]. The self-adhesive property of hemicellulose hydrogels gives them a natural advantage in wound dressings. The multifunctional composite hydrogel composed of hemicellulose, PVA, and polydopamine-modified zinc oxide nanoparticles (PDA@ZnO NPs) has good cytocompatibility, blood compatibility, and the ability to promote fibroblast migration, showing great potential in wound-healing applications^[65]. Meanwhile, the shear adhesion strength of the nano-PDA-reinforced hemicellulose-based hydrogel to skin tissue reaches 7.52 kPa. The excellent adhesion performance extended the hydrogel’s service life in biomedical applications and enabled rapid loading of cationic drugs, serving as a drug patch to promote the transdermal introduction of electrically stimulated drug ions^[183]. These lignocellulose-derived hydrogels provide an important material basis for developing sustainable medical sensors that are both biocompatible and environmentally responsible^[288-290]. However, most of the lignocellulose-based hydrogel sensors are designed for epidermal or external biomedical monitoring, while truly implantable hydrogel sensing systems remain relatively few^[290]. Progress in this emerging direction will require a closer integration of biomass chemistry, hydrogel mechanics, bioelectronic design, and clinical translation^[291,292].

In recent years, with the deep integration of the Internet of Things, AI, and flexible electronics technology, HMI is undergoing a paradigm shift from the traditional rigid interface to flexible, adjustable mechanical performance and biocompatibility^[13,286,293]. Human intentions must be transformed into digital instructions through natural gestures, touches, pressures, and deformations. Hydrogel sensors can adhere to moving skin and provide continuous mechanical readings while avoiding the discomfort caused by hard input devices^[13]. Lignocellulose, with its sustainability, tunable mechanical properties, and biointerface compatibility, has become a promising material platform for building the next generation of HMI systems^{[291,292,294]}.

E-skins are a key component for achieving natural HMI, as the front-end interface that simulates the perception function of human skin^[1,295,296]. Inspired by natural skin, electronic skin engineering technology has attracted increasing attention, promoting the development of single-component multimodal sensors that can simultaneously respond to multiple external stimuli - temperature, pressure, and biochemical markers - and offer fast response and high spatial resolution^[12,244]. Lignocellulose-derived hydrogels offer unique advantages for constructing bionic tactile sensors because their mechanical properties resemble those of biological tissues (low modulus, high flexibility) and ultra-soft characteristics. Researchers have designed hydrogels based on the natural crystallization behavior of cellulose that can mimic human skin^[110]. By regulating the self-assembly of cellulose to form a porous, non-swelling supramolecular fiber framework, the mechanical strength and anti-swelling property of hydrogels can be significantly enhanced, enabling stable signal output in water environments^[110]. Introducing MXene into the cellulose gel skeleton to form a stable “brick and mortar” structure further enhances the piezoresistive performance of the pressure-sensitive layer, which can be used for accurately monitoring limb movement, pulse, and respiration. The flexible pressure sensor demonstrates high sensitivity (21.457 kPa^-1), a wide detection range (0.11-11.022 kPa), fast response/recovery time (41.84 ms/20.82 ms), as well as outstanding stability and cycle repeatability (up to 6,000 cycles). In addition, the pressure sensor array can also be used as an electronic skin to detect pressure distribution [Figure 7A]. With the help of the convolutional neural network (CNN) model [Figure 7B], it has achieved accurate recognition of 9 gestures (98.22%) [Figure 7C]^[297]. By contrast, the piezoresistive sensor array assembled with hydrogels prepared by combining LS and MXene can achieve uniform and precise signal acquisition. The sensor array on a badminton racket can achieve the spatio-temporal mapping of four different paddling postures, with a recognition accuracy rate of 93.75%^[182]. Gesture recognition is crucial for sign language interpretation, virtual and augmented reality control, remote operation of robots, and contactless command systems^[12,298]. Compared with rigid electronic devices, hydrogel sensors can better match the mechanical characteristics of joints and reduce discomfort during long-term use^[299]. The micro-nanostructured conductive network in lignocellulose hydrogels can maintain repeatable output under a wide range of multidirectional deformation.

Introducing an ionic liquid into the cellulose network skeleton can yield an ionic gel sensor with balanced mechanical and inductive properties. The spatial constraint effect of the cellulose skeleton regulates the growth and distribution of the ionic gel microphase structure, enabling the gel sensor to possess both excellent mechanical properties and electrical conductivity^[300]. Surprisingly, combining gelatin with cellulose can fabricate all-biomass hydrogels with strength and toughness comparable to cellulose/polymer hydrogels^[301]. Furthermore, a lignocellulose-based biomimetic skin hydrogel sensor features moisture resistance, self-repairing properties, and adhesion, and can directly detect the subtle stretches related to human speech and expressions. For instance, a study^[210] integrated LCNF and sorbitol to endow the hydrogel platform with outstanding self-healing capabilities and temperature resistance, ensuring stable sensing performance even when damaged or under extreme conditions. Meanwhile, this hydrogel also serves as a platform for speech recognition and hand-drawing, displaying repeatable and highly sensitive electrical signal changes^[210]. This route can support silent speech recognition, emotion-sensing interaction, and assistive communication technologies. Nevertheless, pure lignin can be modified to develop 3D-printed hydrogels through photopolymerization, which were incorporated with glycerol and lithium chloride to promote the formation of dynamic hydrogen bonds and correspondingly reduce the covalent cross-linking sites between monomers. This hydrogel exhibited excellent fatigue resistance (up to 10,000 cycles at 50% strain) and frost resistance (achieving effective sensing performance at -40 °C). As shown in Figure 7D, the 3D-printed hydrogel sensor array demonstrated a uniform stress response in controlling complex bionic hand movements^[34]. Overall, small-range tactile and pressure interaction interfaces, such as pressing, tapping, sliding, and grasping, are an important direction of HMI^[290]. Designing hierarchical conductive paths or ion channels can enable lignocellulose-derived hydrogels to achieve pressure sensitivity over a wide dynamic range, which supports the construction of intelligent flexible electronic skin integrating toughness, sensing, and feedback.

Intelligent control and information interaction interfaces are becoming central to next-generation HMI research. HMI is shifting from one-way instruction input to two-way coupling between human intentions and machine responses. Lignocellulose-based hydrogel sensors not only serve as passive signal acquisition units but also provide active control and information input functions. They thereby become a direct bridge connecting human intentions with machine execution. Researchers have designed a wearable one-handed keyboard with gesture-recognition capabilities, using a multifunctional hydrogel constructed from PAM, sodium CMC, and reduced GO^[302]. This sensor features outstanding sensitivity (GF = 8.18) and pressure-sensing capability. By integrating machine learning algorithms and a custom data acquisition system, the wearable keyboard developed has an accuracy rate of up to 98.13% in recognizing gesture signals^[302]. In contrast, another study developed a sensor that achieved a gesture signal classification accuracy rate of 99.8% under ultra-low temperature conditions (-76.6 °C)^[303]. By using a stepwise dual-salt adjustment strategy similar to bamboo basket weaving, the common trade-offs among sensing performance, mechanical strength, environmental tolerance, and biocompatibility in traditional design strategies have been effectively alleviated. Hydrogel sensors can still achieve stable Morse code communication and gesture-based communication in simulated high-salinity seawater^[303]. The complexity and uncertainty of EMG can reduce the accuracy of decoding movement intentions^[230,299]. As shown in Figure 7E, researchers have combined a multimodal sensing module with AI algorithms to achieve ultra-high-precision motion-intention decoding to drive robot operations ^[282]. This multimodal sensing module, composed of TA and PAM, can efficiently analyze motion intentions and form corresponding control commands. Compared with traditional multi-channel EMG signals, this sensor showed high sensitivity to weak pressure and can be used to monitor tiny force myography (FMG) signals^[260]. The complementarity of EMG and FMG signals significantly enhanced HMI efficiency. Finally, the multi-modal HMI system was used to control robotic gloves and achieve active rehabilitation training based on the human body’s movement intentions, demonstrating the system’s feasibility. Besides, the precise and repeatable control of the system over mechanical hands, smart cars and unmanned aerial vehicles was also demonstrated [Figure 7F]^[282].

TA is rich in phenolic hydroxyl groups and can form uniform metal-TA complexes with metals to achieve antioxidant effects. Significantly, introducing it into the MXene/TEMPO BC hydrogel can prevent oxidation of MXene. Moreover, this sensor can also wirelessly monitor human movement quickly (74 ms) and sensitively (GF = 15.65)^[123]. A major limitation of this approach is the relatively high cost of MXene. Surprisingly, researchers adopted TA as a “glue” to cross-link anisotropic CNFs decorated with PVA and PPy by imitating the anisotropic structure of human muscles, permanently fixing their hierarchical anisotropic structure through multiple hydrogen bonds^[281]. Thus, the hydrogel achieves a balance among excellent mechanical properties (tensile strength of 11.41 MPa and toughness of 12.44 MJ·m^-3), anisotropic conductivity, and biocompatibility. This assembled hydrogel sensor can stably monitor the multi-degree-of-freedom joint movements of the human body and facilitate the control of multi-axis virtual robot mechanical arms. In addition, the prepared hydrogel sensor has excellent biocompatibility and protein resistance as well as biological safety, as confirmed by its practical application as an implantable sensor^[281]. Interestingly, loading tannic acid onto hemicellulose (TA@HC) can endow the hydrogel with both self-healing property and repeatable self-adhesion. The resulting PAA-TA@HC-Al³⁺ ionic hydrogel has high tensile strength (1,060%), self-healing property (up to 87%), and high strain sensitivity (GF = 8.34). It can accurately monitor and distinguish between gross and micro-movements, as well as weak pulses and breaths^[304]. Further, replacing Al³⁺ with Fe³⁺ can significantly enhance the mechanical properties of hydrogels (with a strain of up to 5,600%)^[69]. In summary, lignocellulose-derived hydrogel sensors have been deeply integrated into multiple key scenarios of HMI, demonstrating irreplaceable material advantages from basic signal acquisition to multi-modal intelligent control, due to the high strength and mechanical robustness of lignocellulose^{[172,289,305,306]}. It is expected to become the crucial flexible interface connecting the physical human body with the digital world, driving the evolution of HMI technology towards a more natural, intelligent, and sustainable direction^[307]. Table 3 integrates key data on lignocellulose-based motion sensors in various motion-monitoring scenarios, including tensile strength, elongation at break, toughness, conductivity, response time, and GF.