Advances in Conductive Modification of Silk Fibroin for Smart Wearables

Abstract

Silk fibroin (SF)-based intelligent wearable systems represent a frontier research direction in artificial intelligence and precision medicine. Their core efficacy stems from the inherent advantages of silk fibroin, including excellent mechanical properties, interfacial compatibility, and tunable structure. This article systematically reviews conductive modification strategies for silk fibroin and its research progress in the smart wearable field. It elaborates on the molecular structural basis of silk fibroin for use in smart wearable devices, critically analyzes five conductive functionalization strategies, compares the advantages, disadvantages, and applicable domains of different modification approaches, and summarizes research achievements in areas such as bioelectrical signal sensing, energy conversion and harvesting, and flexible energy storage. Concurrently, an assessment was conducted focusing on the priority performance characteristics of the materials across diverse application scenarios. Specific emphasis was placed on addressing the long-term functional performance (temporal efficacy) and degradation stability of silk fibroin-based conductive materials exhibiting high biocompatibility in implantable settings. Additionally, the compatibility issues arising between externally applied coatings and the native substrate matrix during conductive modification processes were critically examined. The article also identifies challenges that silk fibroin-based smart wearable devices currently face and suggests potential future development directions, providing theoretical guidance and a technical framework for the functional integration and performance optimization of silk fibroin-based smart wearable devices.

1. Introduction

The prototype of smart wearable devices dates back to the 1960s, involving metal wires that integrated various physiological sensors attached to epidermal tissues, aiming to achieve continuous monitoring of the wearer’s vital signs for early warning of acute diseases and immediate medical intervention [1]. However, the inherent rigidity of metals limits their deformation capability. Consequently, early wearable devices severely restricted the wearer’s freedom of movement and struggled to form large-area conformal monitoring systems adapted to the skin, thus limiting their application scope [2].

Breakthroughs in materials science have led to the gradual iteration of smart wearable devices, enabling the widespread application of flexible and stretchable substrate materials [3]. Compared to traditional rigid metal systems, flexible substrates, with their excellent deformation capacity, low mass density, and high biomechanical compatibility, effectively overcome the technical limitations of early devices regarding epidermal conformal adhesion and motion parameter acquisition. Current research emphasis in this field has gradually shifted towards multi-dimensional human-centered design, emphasizing the optimization of comprehensive performance in physiological comfort and biocompatibility [4].

In recent years, advanced textile materials with optimized surface topologies have been widely used in constructing flexible substrates for smart wearable devices. As a long-established basic light industry, the textile sector has developed highly mature technical paradigms for raw fiber materials and manufacturing processes. Textile products exhibit outstanding physiological compatibility, aligning with the performance requirements of next-generation smart wearable devices [5].

Among various textile raw materials, silk stands out to researchers due to its exceptional comprehensive properties. Silk fibroin is a typical natural fibrous protein, possessing a series of superior characteristics such as excellent mechanical properties and interfacial adaptability, high surface luster, breathability, moisture absorption, and ease of processing and degradation, etc. [6,7]. Based on these attributes, silk fibroin has transcended the boundaries of traditional textile materials and found extensive application as a flexible substrate in smart wearable devices (Figure 1) [8]. Through structural design and engineering control, silk fibroin can be processed into multi-dimensional functional materials suitable for different application scenarios [9,10,11].

This paper will briefly describe the structure and properties of silk fibroin, review methods for its conductive modification, and provide application examples in smart wearable devices. Furthermore, the existing challenges pertaining to their operational stability and interface compatibility under varying conditions were analyzed. It summarizes the performance advantages and technical approaches of silk fibroin in this field and prospects its potential future development directions.

2. Structure and Properties of Silk Fibroin

Silk fibroin is a natural polymeric fiber obtained from silkworm cocoons (Bombyx mori) through an alkaline degumming process. Its molecular structure consists of two monomeric polypeptide chains with significantly heterogeneous molecular weights, connected by a single disulfide bond at the C-terminus [12]. The Light Chain (LC) has a molecular weight of approximately 26 kDa and exhibits a non-crystalline random coil conformation. The Heavy Chain (HC) has a molecular weight of about 390 kDa, featuring a typical alternating sequence structure of two distinct segment types (Figure 2). The hydrophobic segments constitute the core structural domain of the HC—a highly conserved sequence of hydrophobic repeat units. This includes high-density “GAGAGS” folding motifs, composed of glycine (Gly), alanine (Ala), and serine (Ser), which exhibit a strong propensity for self-assembly, interspersed with “GAAS” turn motifs arranged periodically. These segments form lamellar β-sheet nanocrystalline regions through non-covalent interactions. The hydrophilic segments are non-repetitive sequences connecting the core structural domains, rich in tyrosine (Tyr) and aspartic acid (Asp). They form a three-dimensional cross-linked system by spatially penetrating networks mediated by hydrogen bonding from side groups, linking the crystalline regions [13]. This unique microcrystalline network architecture is fundamentally determined by its molecular-level physicochemical properties: primarily arising from the ordered stacking of antiparallel β-sheet units within the hydrophobic repetitive domains forming rigid nanocrystalline phases, and the disordered amorphous phases formed by hydrogen-bond cross-linking within the hydrophilic non-repetitive domains [14].

The physicochemical properties of silk fibroin exhibit significant structure–property relationships [15], manifesting in multiple aspects:

(1)

Excellent biocompatibility and low immunogenicity: hydrophobic amino acid residues dominate the repetitive region of the heavy chain, exceeding 70% in total amino acid content. Their simple, stable, and electrically neutral side chains effectively inhibit non-specific protein adsorption and complement activation pathways, endowing silk fibroin with excellent biocompatibility and low immunogenicity.

(2)

Exceptional mechanical properties: The outstanding mechanical performance of silk fibroin is a direct result of its unique nanoscale structural organization. Highly ordered and densely packed β-sheet nanocrystalline regions act as rigid reinforcing phases, contributing high tensile strength and elastic modulus. Simultaneously, the hydrophilic non-repetitive sequence domains connecting these crystals and the disordered regions dominated by the light chain act as a flexible matrix phase, providing significant deformation capacity and toughness. This “rigid filler-flexible matrix” model, consisting of an amorphous network reinforced by nanocrystalline regions, is fundamental to its simultaneous combination of high strength and good flexibility [16].

(3)

Precisely controllable degradation: The in vitro and in vivo degradation rate of silk fibroin is one of its most critical parameters for applications, primarily controlled by the content, size distribution, spatial orientation of β-sheet nanocrystalline regions, and the density of the cross-linked network within the material. Physicochemical treatment strategies can systematically increase the material’s crystallinity and cross-linking density. This structural control can extend the degradation behavior from an initial timescale of days to weeks up to years. This characteristic significantly broadens the applicability of silk fibroin-based materials in fields requiring long-term functional maintenance.

(4)

Multi-scale processability: The inherent high processability of silk fibroin is closely related to its molecular assembly structure. The highly ordered arrangement of molecular dipole moments within the β-sheet layers endows the material with favorable solution rheological properties and solid-state plasticity. This allows silk fibroin solutions to be processed using various mature and highly controllable processing and molding technologies, enabling relatively easy transformation into precisely structured, morphologically diverse materials across scales—from micro/nano-scale microspheres, nanofibers, and porous scaffolds to macroscopic films, hydrogels, and bulk implants—tailored to customized requirements for diverse scenarios.

3. Conductive Modification of Silk Fibroin

Conductivity is a key functional characteristic for flexible wearable electronic devices. Given the inherent insulating nature of natural silk fibroin, functionalization strategies are required to construct conductive pathways to enable its adaptation to the core needs of smart wearable systems for electrical signal transmission and energy conversion [17].

3.1. Conductive Layer Deposition on Silk Fibroin Surfaces

Surface conductive layer deposition is a fundamental strategy for conductive modification of silk fibroin fibers. This approach involves constructing a nanoscale conductive functional layer onto the surface of natural silk fibers through liquid-phase dipping or spray deposition techniques, with the silk fiber serving as a mechanical support for directional electrical signal transmission [18]. Graphene oxide (GO) forms a stable interfacial bond with the amide groups of silk fibroin via a hydrogen bond network. Subsequent in situ chemical reduction converts it into a highly conductive reduced graphene oxide (RGO) composite system (Figure 3A) [19].

Feng’s team developed a highly conductive silk knitted composite scaffold based on a two-step electrostatic self-assembly process (Figure 3B) [20]. This scaffold forms an interpenetrating conductive layer on the silk fiber surface through RGO adsorption and in situ polymerization of aniline (ANI). The coated fibers exhibit minimal resistivity fluctuation under severe physical deformation (folding, twisting) and demonstrate good thermal stability. Compared to inorganic graphene, the conductive polymer polypyrrole (PPy) exhibits superior mechanical matching with silk fibroin. Vootla’s team employed in situ polymerization, achieving uniform coating without chemical modification via π–π stacking and van der Waals forces, preparing highly conductive polymer-coated silk fibroin fibers. After coating, the β-sheet conformation content of the silk fibers remained above 65%, while conductivity increased to the order of 10⁻² S/cm [21].

Compared to metal fibers, surface-coated conductive fibers possess superior bending resistance and chemical tolerance. Their conductivity is highly controllable, and they are easily integrable onto flexible substrates, thus demonstrating significant application value in flexible electronics. However, this structure also has notable limitations: the mechanical durability of the conductive layer is insufficient, prone to peeling or cracking under repeated friction, bending, or tensile stress, leading to conductivity decay. Significant differences in mechanical properties between the fiber core and the outer conductive layer can cause large internal stresses during deformation, resulting in fluctuating interface resistance and affecting system stability. Therefore, such fibers are suitable for applications requiring moderate absolute conductivity but high flexibility and chemical stability (e.g., static electromagnetic shielding, flexible sensing electrodes), but require careful evaluation in applications with high dynamic mechanical loads or demanding low-impedance stable connections.

3.2. Conductive-Core/Silk-Shell Fibers via Inverse Coating

Surface-modified conductive silk fibers suffer from the vulnerability of the epitaxial conductive interface. Since the silk fiber itself is non-conductive, damage to the outer conductive coating renders the modification ineffective. Thus, the inherent high stability and damage resistance of silk fibers are difficult to utilize in epitaxial systems. To address this, researchers have employed silk fibroin molecular self-assembly to form a continuous dense layer that inversely coats conductive materials, achieving core encapsulation to obtain more stable and durable conductive silk fibers [22].

Yadavalli’s team developed organic biodegradable conductive wires consisting of silk fibroin-coated polypyrrole (PPy) for flexible connectors in biosensors [23]. These wires achieved a conductivity of up to 716 S/cm. Photocrosslinked silk fibroin coating formed a core–sheath structure wire with controllable sheath thickness, maintaining axial conductivity while providing radial insulation. The material completely degraded within 28 days in a protease solution. Combining the high conductivity of conductive polymers with the biocompatibility and flexibility of silk fibroin, and capable of stable operation in a moist physiological environment, it provides an environmentally friendly solution for implantable transient electronic devices. Zhang’s team embedded carbon nanotube (CNT) fibers into a Bombyx mori silk matrix via electrospinning to form a core–sheath structure [24,25]. Coaxial spinning of SF solution and CNT suspension produced continuous conductive fibers with diameters of 50–200 nm. This material exhibits high conductivity, excellent mechanical strength, and fatigue resistance. The hydrophobic microdomains of the silk fibroin sheath impart wettability resistance. It also enables dynamic color change in the visible light region and wireless power transmission via voltage control, offering a solution for flexible electronic textiles that combines natural material properties with smart functions.

Inverse-coated conductive fibers possess excellent damage resistance. Selecting appropriate conductive core materials can provide good adaptability to different application scenarios. The cross-sectional area of the conductive core material is larger than that of an outer conductive coating, significantly improving intrinsic conductivity. However, the dense, continuous conductive core material typically has a higher bending modulus, imposing certain stiffness constraints and limiting its use in ultra-flexible devices. Heat generated when a current passes through the core material is also difficult to dissipate promptly, easily causing a mismatch in the thermal expansion coefficients of the core and sheath, leading to cyclic thermal fatigue cracking [26]. Therefore, compared to surface-conductive coated fibers, reverse-coated fibers are more suitable for application in the monitoring and conduction of low-amplitude, high-sensitivity physiological signals.

3.3. Electrochemically Deposited Silk Fibroin Conductive Films

Linear silk fibroin conductive fibers are typically integrated into textiles through co-spinning techniques. Textiles incorporating fibers in this linear form often lack the precision required for applications demanding large-area, conformal skin contact for reliable signal conduction and physiological monitoring. This limitation has consequently spurred the demand for higher-dimensional silk fibroin-based conductive materials. Silk fibroin electrochemical deposition technology represents a significant technical expansion from one-dimensional linear structures to two-dimensional planar functional film configurations for conductive fiber material preparation strategies [27]. This dimensional transition is evident in both the material system construction principles and surface functional engineering, showing significant homology with the previously described conductive layer deposition on silk fiber surfaces [28].

Dai’s research group developed reduced graphene oxide (rGO) composite silk fabrics, functionalizing plain, twill, and satin weaves using screen printing technology and liquid-phase dipping processes [29]. Process optimization experiments showed that five consecutive coatings combined with 120 min of hydrothermal reduction minimized surface resistivity, attributed to the formation of a three-dimensional interpenetrating conductive network by rGO nanosheets within fiber gaps. Cho-Hong Goh’s team constructed polypyrrole (PPy)-encapsulated silk fibroin conductive films [30]. Using a laccase-mediated in situ oxidative polymerization strategy, a continuous conductive layer was formed on a flexible SF substrate enzymatically grafted with carboxylated polyethylene glycol (cPEG). After 48 bending cycles, it maintained 92.7% of its initial conductivity.

Notably, the large-size continuous surface characteristics of two-dimensional film materials broaden the selection range of functionalization modifiers. Zhang developed a transparent conductive regenerated silk fibroin (RSF) composite film [31]. By chemically depositing a poly (hydroxymethyl-3,4-ethylenedioxythiophene) (PEDOT-OH) nano-conductive layer on the RSF film surface, the study employed a composite oxidant to optimize the aqueous polymerization process, forming a conductive layer with controllable thickness and strong adhesion. This endowed the material with both high conductivity and light transmittance. This interfacial engineering strategy effectively solved the mechanical compatibility challenge between the conductive layer and the biological substrate.

Due to the significant homology between the preparation technologies for electrochemically deposited silk fibroin films and surface-conductive silk fibers, their application limitations are similar. Compared to conductive fibers, the two-dimensional extension of silk fibroin films broadens their application scenarios. The flexible substrate’s synergistic effect and large specific surface area coverage of the conductive layer also enhance durability against mechanical impact. However, internal stress and interface resistance issues arising from organic–inorganic phase differences persist. Moreover, adding high-content conductive fillers can compromise the integrity of the silk matrix to some extent and accelerate its degradation rate in wet environments, posing another challenge for applications. In summary, silk fibroin conductive films are primarily suitable for short-term implantable biosensors, biodegradable neural electrodes, and conductive scaffolds for tissue engineering. Applications requiring high current-carrying capacity or long-term implantation still need to overcome material stability bottlenecks.

3.4. Silk Fibroin-Based Conductive Hydrogels

Regardless of whether the structure involves fibers or films, silk fibroin-based conductive materials inherently constitute heterogeneous composite materials where the conductive layer and the silk fibroin substrate constitute distinct phases. This macroscopic interphase interface inherently disrupts the electrical transport properties of the material. Critically, it also significantly elevates the material’s susceptibility to damage-induced failure mechanisms. Consequently, scaling silk fibroin-based conductive materials into three-dimensional constructs entails more than a straightforward spatial transformation; it necessitates fundamental adjustments to the distribution characteristics of the conductive constituent. The modification strategy for silk fibroin-based conductive hydrogels differs from the surface coating techniques used for traditional conductive fibers. Its core lies in uniformly dispersing conductive fillers within the gel network via in situ composite strategies (Figure 4A). This conductive phase construction mechanism, based on concentration control and the three-dimensional continuous phase distribution of conductive fillers within the silk fibroin matrix, not only endows the material with bulk conductivity but also significantly enhances the topological stability and mechanical compatibility of the conductive network [32,33].

Regarding functional system construction, Lv et al. developed a dynamic reversible cross-linking system [34]. By introducing conductive components like polypyrrole (PPy) or single-walled carbon nanotubes (SWCNTs) into silk fibroin hydrogels (Figure 4B), the resulting hydrogel possesses not only basic conductivity, flexibility, and environmental stability but also self-healing capability and high wet adhesion, supporting cell proliferation. As a strain sensor, it can sensitively monitor human motion and micro-expressions. The construction of ionic conductive hydrogels focuses on secondary structure regulation mechanisms. Shao et al. utilized ionic liquids to induce the transition from random coil to β-sheet conformation in silk fibroin aqueous solutions, constructing a double-network conductive hydrogel with uniform pore size distribution [35]. This hydrogel exhibits good conductivity alongside significantly improved mechanical properties and environmental stability.

Figure 4. (A) Schematic diagram for the fabrication of SIH gelatinous fibers. Reprinted with permission from Ref. [33]. Copyright 2024 Springer Nature. (B) Schematic representation of the network structure of the designed silk fibroin (SF)-based hydrogels. Reprinted with permission from Ref. [34]. Copyright 2019 American Chemical Society. (C) Schematic illustration of obtaining Au-silk hydrogel by a simple chemical process. Reprinted with permission from Ref. [36]. Copyright 2009 RSC Pub.

Figure 4. (A) Schematic diagram for the fabrication of SIH gelatinous fibers. Reprinted with permission from Ref. [33]. Copyright 2024 Springer Nature. (B) Schematic representation of the network structure of the designed silk fibroin (SF)-based hydrogels. Reprinted with permission from Ref. [34]. Copyright 2019 American Chemical Society. (C) Schematic illustration of obtaining Au-silk hydrogel by a simple chemical process. Reprinted with permission from Ref. [36]. Copyright 2009 RSC Pub.

Innovation in photo-responsive conductive hydrogels is exemplified by Ray et al.’s noble metal nanoparticle-doped system (Figure 4C) [36]. This hydrogel utilizes the photothermal effect of nanoparticles. By designing the match between the localized surface plasmon resonance (LSPR) wavelength and visible/near-infrared spectra, it achieves conductive regulation with a photothermal conversion efficiency exceeding 80%. Under sufficient light intensity, photothermal heating from the nanoparticles raises the internal temperature of the hydrogel, triggering a conformational change that causes a continuous decrease in material conductivity. This light-controlled resistance state switching has a very short response time, providing a novel mechanism for dynamic impedance matching in wearable devices.

The core advantages of silk fibroin-based conductive hydrogels lie in their tissue-like mechanical properties and dynamic interfacial adaptability. High fracture elongation gives them good tolerance to extreme deformation conditions. Their modulus is well-matched with biological soft tissues, making them suitable for body-adhesive or implantable materials. Furthermore, conductive hydrogels can achieve multiple stimulus responses (electrical/thermal/mechanical signal self-feedback) and self-healing, finding wide application in stretchable sensors and soft robotic actuators [37]. The inherent defects of silk fibroin-based conductive hydrogels include poor environmental stability. Solvent evaporation under dry conditions can cause long-term conductivity drift. Hysteresis effects due to ion redistribution under large strains are also significant, interfering with dynamic signal fidelity and leading to insufficient stability in high-frequency signal detection applications.

Owing to their distinct performance characteristics, electrodeposited silk fibroin films and conductive silk fibroin hydrogels (both serving as multidimensional silk fibroin-based conductive materials) exhibit differentiated suitability for specific application domains. Electrodeposited films demonstrate superior performance under high-frequency, large-amplitude strain conditions, rendering them highly effective for wearable motion detection devices. However, they exhibit inherent limitations concerning stability in hydrated environments and possess inferior deformability compared to conductive hydrogels. Consequently, their efficacy is substantially diminished in applications requiring conformal skin adhesion or direct implantation. In contrast, conductive hydrogels are markedly better suited to meet the requirements of physiological monitoring patches and implantable bio-signal transduction systems.

3.5. Silk Fibroin-Based Multi-Responsive Conductive Composites

The conductive modification of silk fibroin is not an isolated material fabrication strategy. Taking silk fibroin conductive hydrogels as an exemplar, it should be noted that they retain their function as an ideal matrix material capable of incorporating dynamic responsiveness—without compromising conductivity—through strategic modifications such as the introduction of dynamic covalent bonds or the doping of inorganic salts [38]. This approach facilitates the creation of composite materials. Critically, these composites synergistically retain the inherent high biocompatibility and bioactivity of the silk fibroin matrix while effectively integrating supplementary functionalities like stimuli-responsiveness, electrical conductivity, and self-healing properties, thereby substantially broadening their potential application scope [39].

Illustrating this concept, the Wahab research team fabricated an optically responsive, self-healing optoelectronic epidermal material synthesized from melanin nanoparticles and silk fibroin (Figure 5) [40]. Melanin nanoparticles were formed in situ via dopamine oxidative polymerization and subsequently co-incorporated with calcium chloride (CaCl₂) and glycerol into the silk fibroin hydrogel matrix. Subsequent drying yielded the final biomimetic electronic skin construct. This biomimetic e-skin demonstrated a conductivity of up to 6 mS/cm under 80% relative humidity. Upon photoirradiation, melanin nanoparticles absorbed light, generating heat, which induced localized expansion of the protein matrix. This photothermal expansion altered the interfacial resistance between electrode layers, resulting in a notable 40% enhancement in conductivity. Through this synergistic interplay between the functional additives, the biomimetic skin concurrently acquired three distinct, independent response modalities: pressure, humidity, and light stimulation. Furthermore, its electrical conductivity could be stabilized at enhanced levels through precise environmental modulation, significantly extending its functional applicability. It should be noted, however, that the requirement for incorporating and spatially organizing multiple distinct fillers substantially increased the fabrication complexity of these composite systems.

3.6. Piezoelectric Silk Fibroin Materials

Distinct from modification strategies predominantly reliant on the introduction of external conductive layers or fillers, unmodified silk fibroin can exhibit piezoelectric characteristics after stress treatment. The piezoelectricity of silk fibroin originates from the highly anisotropic nature of its molecular structure, particularly the non-centrosymmetric antiparallel β-sheet microcrystalline domain structure induced by mechanical stress fields. Alternately arranged Gly-Ala repeat units in this system form a local dipole matrix. Charge separation occurs through lattice displacement triggered by shear deformation, generating an intrinsic piezoelectric response.

Kaplan’s team revealed the correlation between the crystal structure of silk fibroin and its intrinsic piezoelectric coefficients using synchrotron radiation wide-angle X-ray scattering [41]. They clarified that the net polarization of the non-centrosymmetric monoclinic crystal system under shear force is the origin of piezoelectricity, confirming that the piezoelectric coefficient of silk fibroin is positively correlated with the orientation degree of β-sheet microcrystals, providing a theoretical basis for the design of silk fibroin bio-piezoelectric devices. Based on this, Zhang et al. developed a dry-spun piezoelectric silk fibroin fiber [42]. By regulating β-sheet content via post-treatment, the piezoelectric performance was enhanced, achieving a maximum output voltage of 27 V and a piezoelectric coefficient of 3.24 pm/V. Grooved surface structures were formed to enhance the stress response. The fiber can sensitively monitor finger joint bending movements and maintained 88% of its initial electrical output stability after cyclic loading tests (10⁴ cycles), demonstrating its practical value in wearable health monitoring and self-powered systems.

The piezoelectric properties of silk fibroin originate from the oriented arrangement of its internal β-sheet crystalline regions. Therefore, any environmental factor disrupting this orientation will weaken its piezoelectric response. Polymer materials also suffer from inherent defects such as low intrinsic piezoelectric constants and narrow characteristic frequency ranges. Consequently, silk fibroin piezoelectric materials are mostly suitable for short-term implantable scenarios like biodegradable in vivo mechanical sensors, tissue engineering dynamic stimulation devices, and low-power bio-energy harvesters. Significant application bottlenecks exist in high-precision active actuation or broadband vibration energy harvesting.

Due to its excellent solution processability and multi-level structural designability, silk fibroin can be derived into diverse modification routes. However, different modification strategies have advantages and disadvantages in terms of inherent physicochemical and electrical properties (

Table 1. Performance Comparison of Different Conductive Modification Methods for Silk Fibroin Materials.

Conductive Modification Method	Conductivity Range	Mechanical Stability	Biodegradation Performance	Fabrication Difficulty
Surface-Conductive Layer-Coated Silk Fibers	10−2–103 S/m	Low: Conductive layer prone to delamination	Reduced: Outer coating impedes core degradation	Low
Reverse-Coated Conductive Fibers (Core/Sheath)	102–104 S/m	High: Excellent impact/damage resistance	Moderate: Sheath structure fully degradable	High
Electro-Deposited Silk Fibroin Films	10−1–103 S/m	Moderate: Superior to fibers, but susceptible to delamination	Variable: Highly dependent on environmental conditions	Low
Silk Fibroin-Based Conductive Hydrogels (incl. Composites)	10−3–10−1 S/cm	Highest: Superior extensibility and self-healing capability	Excellent	High
Silk Fibroin Piezoelectric Materials	Negligible	Variable: Strongly influenced by internal microstructure	Excellent	High

). In summary, modification strategies utilizing conductive fillers externally attached to surfaces offer advantages in fabrication simplicity. However, the surface coating inherently compromises both the material’s operational stability and degradation performance. Conversely, approaches where conductive elements are embedded within the fibroin matrix—such as reverse-coated fibers and hydrogels (including responsive composites)—demonstrate superior stability and more predictable degradation profiles. This distinction further extends to performance under demanding conditions: stability in aqueous environments, oxidative conditions, and sustained mechanical stresses. Benefiting from the inherent structural integrity, density, and intrinsic oxidation resistance of the silk fibroin matrix, reverse-coated fibers and hydrogels effectively shield embedded conductive fillers, enabling stable functionality even in challenging environments. In contrast, surface modification strategies relying on conductive coatings remain vulnerable to a spectrum of failure modes, including coating swelling, oxidation-induced degradation, and mechanical delamination. Consequently, surface-coated modifications are predominantly suited for applications in short-term, nonconformal wearable electronics. The selection of conductive materials and modification methods requires consideration of the specific application scenario to maximize the material’s advantageous properties while circumventing its limitations, achieving device function optimization.

4. Applications of Conductive Silk Fibroin Materials in Smart Wearable Fields

Compared to traditional smart wearable materials, silk fibroin matrix materials possess multiple advantages, including excellent biocompatibility and safety, tunable mechanical properties matching epidermal tissue mechanics, wide temperature range adaptability and damage resistance, high breathability and moisture permeability, and significant controllable degradability, effectively reducing electronic waste pollution [43].

4.1. Motion Detection Devices

Deformation amplitudes induced by different sensing locations or physiological activities on the human body, and the range of compressive stresses they experience, exhibit significant differences. This demands that motion detection devices possess both the structural tolerance to withstand mechanical impacts caused by large deformations and shape stability, enabling rapid elastic recovery post-deformation [44,45].

Hu’s research group developed a reduced graphene oxide/silk fibroin (rGO/SF) composite hydrogel-based pressure sensor (Figure 6A) [46]. It achieved sensing characteristics combining high sensitivity with a wide pressure detection range (100 Pa–500 kPa), capable of simultaneously monitoring large-scale human motion behaviors and subtle pulse signals. By integrating a five-channel pressure sensing array into an intelligent glove combined with machine learning for gesture recognition, and achieving complete degradation in 0.1 M NaOH solution within 28 h, it provides an environmentally friendly solution for wearable medical diagnosis and human–computer interaction. Chen’s team functionalized silk fiber surfaces using in situ growth of nanosilver [47]. The functionalized silk fibers enable the simultaneous, precise detection of mechanical pressure and specific biomolecules in wearable sensing applications based on the synergistic effect of piezoresistive response characteristics and localized surface plasmon resonance.

4.2. Physiological Signal Detection Devices

Compared to motion signals, physiological signals have weak amplitudes and periodic generation characteristics. This imposes dual technical requirements on smart wearable sensing systems: high-sensitivity detection in low-threshold pressure ranges and long-term cyclic stability [52,53]. A polyurethane–silk fibroin eutectic film constructed based on an intermolecular template nucleation strategy, after vapor deposition conductive modification, exhibits high sensitivity in the sub-1500 Pa low-pressure range due to its bipolymer heterostructure [54]. A flexible wristband sensor based on this material system can achieve precise capture of dynamic meridian impedance responses and radial artery pulse waveforms at the human wrist.

Yapici’s team developed a biodegradable epidermal electronic device based on silk fibroin (Figure 6B) [48]. By modifying silk fibroin with Ca²⁺ ions to regulate material properties, an ultra-soft substrate was obtained, combining high self-adhesion, breathability, and rapid environmental degradation. Using inkjet printing to pattern serpentine silver nanoparticle circuits on the modified silk film, combined with a flexible wireless acquisition system, enables multi-modal physiological signal monitoring. The device achieves conformal contact with epidermal tissue for over 12 h without an interfacial adhesive layer. The soft silk fibroin substrate exhibits excellent mechanical matching with the epidermis and maintains functionality even after immersion in water, effectively ensuring the time domain integrity of continuous monitoring for bioelectrical signals like ECG and EMG.

4.3. Temperature Sensing Devices

The precise measurement of temperature changes essentially depends on the intrinsic correlation between the thermodynamic motion state of charge carriers in a conductive system and temperature. The core operating mechanism of smart wearable temperature sensors relies on charge transport modes dominated by ionic carrier migration. Therefore, current silk fibroin-based wearable temperature sensor devices generally adopt ionic conductive architectures.

Chen’s team constructed a high-sensitivity ionotronic skin based on the synergistic effect of silk fibroin and calcium ions (Figure 6C) [49]. Silk fibroin acts as the structural support and ion capture matrix, endowing the device with structural stability, mechanical flexibility, and interfacial adhesion properties. Calcium ions ensure highly stable ionic conductivity over a wide temperature range (−30 to 80 °C) by maintaining stable ion migration channels. Liu et al. blended carbon nanotubes with [EMIM] [N(Tf)₂] ionic liquid and coated them onto polyester yarn wrapped with silk, constructing a composite film with dual-mode temperature–pressure sensing functions [55]. The temperature sensor sensitivity reached 1.23%/°C and exhibited stable performance over 400 cycles. The pressure sensor used silk/polyurethane yarn modified with silver nanowires, achieving a capacitance retention rate of 90% after 5000 pressure cycles. The double-layer woven structure enables independent detection of temperature and pressure signals, maintaining over 85% sensing performance after washing tests, providing a new solution for wearable healthcare and human–computer interaction.

4.4. Wearable Energy Harvesting Devices

Harvesting mechanical energy and converting it into electricity based on the triboelectric effect induced by human motion on material surfaces is an important research direction for wearable devices. The core working mechanism of such energy harvesting systems relies on the dynamic contact electrification and electrostatic induction coupling process occurring at the interface between two materials with significantly different electron affinities under periodic mechanical force. This process involves repeated intense friction and deformation loads at the material interface. Consequently, it imposes extremely stringent mechanical durability requirements on the conductive functional media acting as charge collection layers and electrode layers [56,57,58].

Ling’s team employed core-spun yarn textile technology to prepare a ternary composite structure of silk fiber/polytetrafluoroethylene (PTFE) fiber/stainless steel fiber (Figure 6D) [50]. This design optimized the balance between mechanical and triboelectric properties. The resulting triboelectric textile exhibited excellent structural stability, with resistance fluctuation below 1% after 2.3 million mechanical bending cycles, providing a durable energy solution for wearable electronic devices. Ma’s team developed a regenerated silk fibroin–carbon nanotube (RSF/CNT) composite conductive film [28], possessing dual functionalities as a flexible capacitive pressure sensor and a wearable triboelectric nanogenerator (TENG), achieving device-level integration of mechanical energy harvesting and mechanical signal monitoring.

4.5. Bio-Integrated Electronic Devices

Aqueous silk fibroin solutions represent a novel pathway for application as bio-derived surfactants. Their unique amphiphilic balance enables them to spontaneously adsorb onto hydrophobic surfaces, forming uniform and dense conformal coatings [59]. Leveraging this property, spray-coating silk fibroin surfactant solutions or other surfactants [60] onto the surfaces of implantable electronic devices can significantly reduce the interfacial energy at the device–tissue interface, thereby enhancing biocompatibility and mitigating adverse immune reactions post-implantation. Kim et al. demonstrated this approach by spin-coating ultra-dilute silk fibroin surfactant solutions onto nanostructured device surfaces, such as transistors and photovoltaic cells [61]. Their results indicated a ~40% reduction in solid–liquid interfacial energy without compromising the devices’ inherent functionality. Furthermore, the silk fibroin coatings exhibited superior wetting behavior compared to commercial surfactants. Critically, these coatings possess excellent molecular compatibility, facilitating the co-deposition of proteins and nucleic acids on the modified surfaces. This intrinsic feature provides a fundamental platform for subsequent functionalization toward advanced biosensing applications.

4.6. Flexible Capacitor Devices

A special category of silk-based smart wearable materials utilizes carbonized silk fibroin film as a natural carbon source, co-prepared with silicon nanoparticles and graphene into sandwich composites (Figure 6E) [51]. Nitrogen-doped carbonized silk and graphene synergistically coat silicon nanoparticles to improve electrical connectivity and suppress pulverization. This composite material achieved a stable reversible specific capacity of 1104 mAh/g at a 0.2 C charge/discharge rate. Its mechanical compatibility and energy density characteristics demonstrate its potential for device integration in flexible self-powered wearable systems.

5. Conclusions and Outlook

This article systematically elaborates on the construction strategies for intelligent wearable systems based on regenerated silk fibroin (RSF) substrates, covering molecular structure design, multi-scale manufacturing processes, functional unit integration, and interfacial coupling mechanisms for flexible energy storage devices. It critically analyzes application progress in areas such as bioelectrical signal sensing, triboelectric/piezoelectric energy harvesting, and self-powered system integration. With the evolution of precise health monitoring demands towards real-time, multi-parameter, and implantable iterations, silk-based materials have become a research hotspot in the flexible electronics field due to their intrinsic biocompatibility, tunable degradation characteristics, and excellent mechanical properties.

As a macromolecular protein, silk fibroin degrades into natural amino acid monomers—its final degradation products—both in vitro and in vivo. These endogenous degradation products demonstrate negligible biotoxicity towards both human physiology and the natural environment. Consequently, the safety profile of silk fibroin-based smart wearable devices is predominantly governed by the degradation characteristics and byproducts of their incorporated conductive fillers. Analysis of the degradation performance of major conductive fillers (

Table 2. Comparative Degradation Performance of Major Conductive Fillers.

Conductive Filler	Degradation Products	Degradation Period	Biological Toxicity Analysis
Carbon Nanotubes	Polycyclic aromatic hydrocarbons (PAHs), CO2	6–12 months	Incompletely degraded residues may induce cellular damage and possess carcinogenic potential.
Graphene-based
Silver Nanomaterials	Metal ions (Ag+), metallic compounds	3–6 months	Silver ions exhibit mitochondrial toxicity and neurotoxicity.
Copper Nanomaterials
Polyaniline (PANI)	Aniline dimers, quinone derivatives	1–3 months	Degradation products are potentially carcinogenic.
Polypyrrole (PPy)	Nitrate ions, pyrrolidone derivatives	2–4 months	Degradation products may induce DNA damage in cells.

) reveals that when exposed to ambient environmental conditions, these materials generally undergo substantial degradation within several months to a year. While certain materials (notably carbon-based fillers) exhibit relatively extended degradation periods, the environmental impact of their degradation byproducts remains within manageable thresholds. However, the degradation scenario for implanted devices raises significant concerns. Pervasive issues include undesirably slow degradation kinetics and the presence of degradation byproducts carrying potential bio-toxic risks. This fundamental challenge is the primary rationale for the deliberate avoidance of additional conductive fillers in coatings designed for implantable smart electronics.

The scalable manufacturing of silk fibroin-based smart wearables constitutes another major research focus. Benefitting from silk fibroin’s excellent processability, the engineered production of multi-dimensional substrates can be achieved with relative simplicity and speed. Specific fabrication complexities, however, vary according to the conductive modification strategy employed: surface-conductive coated fibers and films can be directly integrated into existing textile manufacturing workflows, enabling simple preparation and high batch-to-batch reproducibility with consistent properties; conversely, the fabrication of reverse-coated and conductive hydrogel-based devices is inherently more complex, typically necessitating redesigned production lines. Nevertheless, high batch-to-batch consistency can still be attained; intrinsic piezoelectric materials, whose properties are critically dependent on the final internal microstructure, present considerable fabrication challenges and generally exhibit lower batch-to-batch consistency compared to other modification methods. Despite these variations, the large-scale manufacturing of silk fibroin-based smart wearables, in principle, poses no insurmountable technical barriers.

However, current research on silk fibroin-based smart wearable devices still needs to overcome the following technical challenges: Firstly, under frequent cyclic mechanical stresses (such as washing, friction, etc.), the conductive functional layers of silk-based smart textiles are prone to fracture failure due to repeated substrate deformation. There is an urgent need to construct stress dissipation micro-regions through interfacial engineering to enhance the long-term stability of conductive devices under dynamic deformation or high-humidity environments [62,63]. Secondly, the integrated assembly of multi-modal electronic components (e.g., sensors, energy modules) with silk fibroin substrates requires balancing functional superposition effects with conflicts arising from the intrinsic properties of the textile. Particular attention is needed to optimize conductive filler doping concentration, control interfacial thermal stress, and avoid deterioration in breathability and drapeability caused by excessive rigid components, ensuring comfort in the human skin microenvironment [64,65].

The summary of process routes and application domains for silk fibroin (SF)-based intelligent wearable systems is presented in Figure 7. Future research in the field of silk fibroin-based smart wearable devices should focus more on molecular and system design for dynamic adaptive materials, achieving multi-dimensional performance balance by constructing dual-responsive networks. At the molecular level, introducing photo/thermal-responsive dynamic covalent bonds (e.g., boronate esters, disulfide bonds) can build reversible bonding networks, endowing the substrate with the self-healing capability to reduce material failure probability under harsh conditions. At the mesoscopic level, developing heterogeneous interfacial nano-transition layers to regulate stress distribution and utilizing biomimetic wrinkled topological structures to suppress local strain can improve material stability and reduce performance conflicts between electronic components and the substrate. At the system level, introducing machine learning algorithms to optimize the spatial arrangement of functional modules can achieve functional matching and mechanical balance among energy harvesting, sensing, and energy storage units. Ultimately, through synergistic innovation in materials, structures, and processes, the technical barrier between device reliability and wearing comfort can be overcome.