In this section, we first examine the partially degradable systems predominantly used in current implementations, followed by a review of emerging strategies toward fully degradable conducting polymers. Furthermore, we discuss the applications of each system in electronics and the remaining challenges in the field. Initial strategies for partially degradable systems focused on blending conductive polymers with biodegradable matrices to impart mechanical softness and biological compatibility^[15] [Table 3]. Natural polymers such as chitosan (CS) and its derivatives have been widely used for this purpose due to their intrinsic biocompatibility and flexibility^[22,63-66]. However, the inherent incompatibility between hydrophobic conductive polymers and hydrophilic biodegradable matrices often leads to phase separation, weak interfacial adhesion, and disrupted charge-transport pathways^[67]. As a result, high loading fractions of conductive components are typically required to establish continuous conductive pathways, which suppresses degradation and compromises mechanical integrity^[68,69]. For example, polypyrrole (PPy) blended with biodegradable matrices such as poly(glycerol sebacate) (PGS)/collagen requires high conductive content to achieve measurable conductivity, highlighting the trade-off between electrical performance and degradability in physically blended systems^[54].

In situ polymerization within biodegradable templates provides a more effective route to improve interfacial compatibility and maintain conductive pathways^[16,18,70] [Figure 3A]. In this approach, conductive monomers such as aniline^[74], pyrrole^[19], and 3,4-ethylenedioxythiophene (EDOT)^[39] are polymerized directly within a polymer matrix, forming interpenetrating networks with improved dispersion and structural integrity^[75]. This strategy enables the simultaneous enhancement of mechanical and electrical properties.

Figure 3B demonstrates the synthesis of nanocellulose-templated polyaniline (PANI), which produces elastomeric composites with a mechanical strength of ~10 MPa, stretchability exceeding 500%, and conductivity in the range of ~10^-3 S·cm^-1. These results demonstrate that bio-templated architectures can mitigate phase separation while preserving conductive pathways, although precise control over the microstructure remains essential for reproducible performance^[71].

Block copolymerization offers a more controlled strategy by integrating conductive and biodegradable segments within a single macromolecular architecture^[76] [Figure 3C]. Synthetic biodegradable polymers such as polycaprolactone (PCL)^[21] and polylactic acid (PLA)^[77] are commonly employed to ensure structural uniformity and reduce side reactions. Figure 3D illustrates that block copolymers such as PPy-b-PCL significantly improve mechanical compliance while maintaining electrical conductivity on the order of 10^-2 S·cm^-1, with mechanical properties approaching those of soft biological tissues in the MPa range. These systems exhibit partial degradation, with mass loss reaching ~50% over several weeks under accelerated conditions^[72]. Nonetheless, phase segregation within polymer domains can still limit charge-transport efficiency and long-term stability, indicating that the balance between electronic connectivity and degradability remains unresolved^[78].

Graft copolymer approaches provide an alternative approach in which conductive side chains are attached to a biodegradable backbone, enabling greater molecular design versatility^[79] [Figure 3E]. For example, poly(3,4-ethylenedioxythiophene) (PEDOT) grafted with poly-D,L-lactic acid (PDLLA) retains electrical conductivity for several weeks during degradation, demonstrating improved stability relative to physically blended systems^[80]. However, increasing the fraction of biodegradable chains disrupts interchain π–π stacking, leading to a substantial reduction in conductivity, often by several orders of magnitude. Figure 3F shows the properties of gelatin-grafted poly(3-hexylthiophene-2,5-diyl) (Gel-g-P3HT), where P3HT segments are integrated onto a biodegradable gelatin backbone. In this architecture, structural ordering enables partial recovery of conductivity (~10^-7 S·cm^-1), although the overall electrical performance remains limited compared to pristine conductive polymers^[73].

Despite these advances, most current strategies yield only partially biodegradable systems, as the conductive polymer backbone often persists after degradation of the surrounding matrix. This limitation restricts their applicability in fully transient electronic systems and underscores the need for intrinsically degradable conjugated polymers that retain electrical functionality while undergoing complete breakdown^[81]. Future progress will require improved control over molecular design, microstructure, and phase behavior, together with strategies that balance conductivity, mechanical compliance, and degradation kinetics. Achieving this balance remains a central challenge in the development of biodegradable conductive polymers for transient soft electronics.

In contrast to partially degradable systems, fully degradable conductive polymers are designed to enable complete decomposition of the conductive network into environmentally or biologically benign products. Achieving this capability requires molecular design strategies that preserve charge transport while allowing controlled degradation under physiological or environmental conditions^[47].

Biologically mediated degradation represents one approach in which organisms facilitate the breakdown of conductive materials. Insects such as Zophobas morio (Z. morio) and mealworms have demonstrated the ability to metabolize hydrocarbon-based substrates, including materials containing stable C–C bonds^[82]. Moreover, because mealworms cannot ingest rigid polymers but readily consume soft materials, they may enable complete biodegradation of soft conductive polymers^[83,84]. Figure 4A illustrates that a composite based on poly(vinyl alcohol) (PVA) blended with melanin nanoparticles (MNPs) extracted from squid ink exhibits an electrical conductivity of 10^-1 S·cm^-1 and undergoes ingestion-driven degradation. Optimization of the MNP/PVA ratio enhances the feeding activity of Z. morio larvae by up to 5.2-fold^[85]. Furthermore, Fourier transform infrared (FT-IR) analysis of excreta confirms that degradation proceeds beyond physical fragmentation, reaching molecular-level transformation through biological digestion^[29]. Despite these advantages, reliance on biological pathways limits scalability and process control, while the underlying degradation mechanisms remain incompletely understood.

Enzymatic degradation within biological systems provides an alternative route toward complete bioresorption^{[13,22,24,26,86-90]}. This strategy employs short conjugated oligomers, typically consisting of 2 to 8 thiophene or aniline units, connected via degradable linkers to facilitate macrophage-mediated phagocytosis [Figure 4B]^[24,91-94]. For example, in vivo studies^[13] have shown that pyrrole–thiophene–pyrrole-based polymers implanted subcutaneously in rats undergo gradual degradation over 14, 21, and 29 days while eliciting minimal inflammatory responses comparable to those of Food and Drug Administration (FDA)-approved poly(lactic-co-glycolic acid) (PLGA). Figure 4C shows that aniline trimers incorporated into a biodegradable polyurethane matrix can maintain approximately 87% of their initial conductivity (10^-8-10^-5 S·cm^-1) over 150 h under humid conditions. Systems incorporating PCL as a soft segment achieve high elasticity, with over 97% instantaneous recovery at 10% strain^[30]. However, the limited fraction of conjugated segments in these materials restricts overall conductivity, thereby limiting their applicability in systems requiring high-performance charge transport.

Chemical strategies based on dynamic covalent bonds offer additional routes toward fully degradable conductive polymers. Hydrolyzable imine bonds preserve π-conjugation while enabling degradation through reversible bond cleavage^[95]. As shown in Figure 4D, diketopyrrolopyrrole (DPP)-based polymers incorporating imine linkages undergo complete degradation within 30 days under mildly acidic conditions (pH 4.6). Through this degradation process, potentially harmful residual species such as aluminum and p-phenylenediamine were found to remain well below commonly accepted safety limits, indicating minimal risk to human health and the environment^[31]. Degradation can also proceed under alkaline conditions, depending on polymer composition and environmental factors, as demonstrated in systems that undergo complete clearance in 0.5 M NaOH (pH 13.7)^[96]. However, the requirement for non-physiological pH conditions limits their applicability in biointerfaced environments.

Despite these advances, fully degradable conductive polymers remain constrained by the balance between electrical performance and degradability. Materials designed for complete degradation often rely on specific environmental triggers, whereas systems that operate under broader conditions tend to exhibit reduced conductivity. Further progress will require molecular designs that balance these competing requirements while aligning material properties with appropriate application environments in transient electronic systems.

Biodegradable conductive polymers enable a wide range of transient bioelectronic applications owing to their mechanical compliance, electronic functionality, and tunable degradation behavior^[97]. These materials have been explored in bioelectrical stimulation platforms^{[24,76,90,98]}, drug delivery systems^{[28,54,99-101]}, sensing devices^[67,73], and active electronic components^[31,102], where soft interfaces and controlled operational lifetimes are essential.

For bioelectrical stimulation, biodegradable conductive polymers are particularly attractive because relatively low conductivity levels are sufficient to elicit cellular responses^[103]. For example, electrospun PLGA/polydopamine (PDA)/CS membranes exhibit a conductivity of 2.85 × 10^-3 S·cm^-1, comparable to that of natural skin, and enhance fibroblast proliferation and collagen production under low-voltage stimulation^[98]. These results demonstrate that biodegradable conductive materials can support electrically assisted tissue regeneration while maintaining mechanical properties suitable for soft biointerfaces.

These materials also enable electrically responsive drug delivery systems. Early platforms primarily relied on passive drug release through matrix degradation or diffusion^[104]. For instance, drugs loaded into PGS/PPy composites are gradually released as the biodegradable matrix degrades^[54]. Extending this approach, electrically controlled release systems have also been demonstrated^[28]. Figure 5A illustrates a Dex (dextran)-aniline tetramer (AT)/hexamethylene diisocyanate (HDI) conductive hydrogel system that enables on-demand release via electrophoretic transport and redox-induced network contraction. Under electrical stimulation in phosphate-buffered saline (PBS) at 37 °C, dexamethasone release increases by approximately two- to three-fold at applied voltages of 1-3 V, demonstrating electrically controlled therapeutic delivery^[99].

Beyond stimulation and drug delivery, biodegradable conductive polymers also enable mechanically compliant sensing platforms. Their ability to preserve conductive pathways under deformation allows reliable signal transduction in soft and wearable environments^[105]. For example, Figure 5B demonstrates a biodegradable composite composed of cellulose nanofibers (CNF), PANI, and natural rubber (NR-8), which has been used to fabricate a strain sensor. The device maintains stable strain sensitivity over approximately 100 loading cycles (≈ 200 s), demonstrating operational durability^[71]. This performance originates from the CNF-derived framework, while the elastic NR matrix preserves conductive pathways during repeated deformation^[71]. This concept also extends to pressure sensing, where Gel-g-P3HT aerogel films exhibit pressure-dependent current responses under applied pressures of 5-20 kPa^[73]. These results demonstrate that biodegradable conductive polymers provide mechanically compliant sensing platforms suitable for wearable and biointegrated applications.

Biodegradable conductive polymers have also been extended to active electronic devices that require controlled charge transport and signal modulation^[106]. Early work by IrimiaVladu and colleagues demonstrated fully biodegradable organic transistors composed of degradable substrates, dielectrics, and semiconducting layers^[107]. However, these systems often relied on externally degradable components rather than intrinsically degradable semiconductors, prompting subsequent efforts to develop molecularly engineered biodegradable semiconducting materials. Figure 5C illustrates a degradable and stretchable semiconductor system based on a highly extensible urethane-based E-PCL elastomer matrix combined with the organic semiconductor poly(diketopyrrolopyrrole–p-phenyldiamine) [p(DPP-PPD)]. Optimized side-chain engineering of p(DPP-PPD) using branched alkyl substituents improves compatibility with the elastomer matrix and suppresses macroscopic aggregation^[102]. This design promotes nanoscale phase separation, forming nanoconfined semiconducting domains that suppress crystallization and delay crack initiation^[102]. As a result, the composite maintains crack-free film integrity under mechanical deformation. A film composed of 70% E-PCL and 30% p(DPP-PPD) retains semiconducting performance even at 100% strain, with a mobility of approximately 0.05 cm²·V^-1·s^-1[102]. These results highlight the potential of biodegradable semiconductors for stretchable and skin-inspired electronics^[102]. Figure 5D presents the integration of degradable semiconductors into transient logic circuits. Fully degradable ultrathin devices fabricated by integrating acid-hydrolyzable p(DPP-PPD) on cellulose substrates exhibit lightweight structures that can be supported by a human hair and show less than 5% variation in transfer characteristics under bending^[31]. Pseudo-CMOS inverters demonstrate a noise margin of 1.2 V with sharp switching behavior, while NAND and NOR circuits achieve near rail-to-rail voltage swings^[31]. These demonstrations illustrate that biodegradable semiconductors can support flexible logic circuits and enable more complex transient electronic functionalities^[31].

Collectively, these examples illustrate the broad potential of biodegradable conductive polymers across tissue engineering, drug delivery, sensing, and active electronics. However, their relatively low intrinsic conductivity remains a key limitation for applications requiring reliable charge transport in interconnects, electrodes, and circuit-level integration. In addition, achieving complete biodegradation under physiological conditions remains challenging. Addressing these challenges will require continued advances in molecular design and materials engineering to further enable high-performance, fully transient bioelectronic systems.

To precisely design the practical operational lifetime of composite devices, it is essential to quantitatively characterize the degradation kinetics of both the individual constituents and the resulting composites. First, biodegradable metals primarily used as conductive fillers undergo chemical dissolution following surface oxidation in aqueous environments. For example, Mo, Fe, and W thin films exhibit gradual electrical dissolution rates of approximately 1 nm·h^-1 (for Mo^[117] and Fe^[117]) and 3-5 nm·h^-1 (for W^[117]) in deionized (DI) water at room temperature, thereby maintaining relatively stable conductivity [Table 5]. In contrast, Zn^[117] and Mg^[117] thin films exhibit significantly higher corrosion rates of 50-90 and 200-400 nm·h^-1, respectively [Table 5], which makes them challenging to use as stable fillers because of their rapid disintegration. The degradation of these fillers is accelerated as water-soluble oxides or hydroxides formed on the surface diffuse into the surrounding biofluids^[125].

Polymer matrices also exhibit diverse degradation profiles depending on their chemical structures and components. Synthetic polyesters such as PBTPA (0.08-0.1 wt.% day^-1)^[118], PCL (0.03 wt.% day^-1)^[119], PLA (0.07-1 wt.% day^-1)^[120], PLGA (0.3-2 wt.% day^-1)^[121], and PBAT (0.38-0.54 wt.% day^-1)^[122] degrade primarily through the hydrolysis of ester bonds in their polymer backbones [Table 5]. Natural waxes, including beeswax (0.76 wt.% day^-1)^[123] and candelilla wax (0.05 wt.% day^-1)^[123], exhibit gradual degradation behavior due to their high water resistance. Meanwhile, the natural protein silk fibroin (0.17-0.34 wt.% day^-1)^[124] degrades through proteolysis, in which amide bonds are cleaved by specific enzymes [Table 5].

Unlike conventional pastes using stable hydrophobic polymers (e.g., epoxy, silicone elastomer)^[126,127] and chemically inert noble metal fillers (e.g., silver, gold)^[126,128], biodegradable conductive composites exhibit rapid initial functional degradation within physiological environments. Despite these individual degradation rates, the actual functional lifetime of these composite pastes is typically limited to approximately 2 to 19 days [Table 6]^{[11,12,32-35,108,111,112]}. Considering the typical filler size (0.5-100 μm) and paste thickness (50-1,000 μm), this operational duration is markedly shorter than the time required for complete dissolution of the components^{[11,12,32-35,108,111,112]}. The root cause of this premature functional failure lies in the extreme structural sensitivity of the percolation network in aqueous environments, which disrupts physical contact or the critical quantum tunneling distance between fillers, both of which are essential for maintaining electrical integrity^[111]. As a result, the functional stability of biodegradable composites is often governed more by early microstructural disruption of the filler percolation network than by the overall mass loss of the constituent materials. In other words, predicting the lifetime of the composite requires an approach that considers filler dispersion, interfacial stability, and water diffusion behavior rather than simply comparing the dissolution rate of metallic fillers or the mass loss rate of the polymer matrix. These characteristics highlight the importance of design strategies that preserve electrical connectivity throughout the intended operational lifetime before the onset of rapid transient degradation.

Figure 6A illustrates the microstructural evolution of Mo-based biodegradable conductive composites in a physiological-mimicking environment (PBS, 37 °C) as a function of the matrix type^[111]. The lower series, corresponding to the Mo/PLGA composite, shows a rapid decline in electrical performance over time. This behavior is mainly associated with the hydrophilic nature of the PLGA matrix, which promotes water uptake and swelling, thereby weakening the filler–matrix interface and leading to interfacial debonding^[111]. Previous studies report that PBS preferentially infiltrates the vulnerable interfaces between Mo particles and the PLGA matrix. The resulting expansion of localized voids physically disrupts the percolation pathways well before the metallic fillers undergo significant dissolution, leading to a sharp decrease in conductivity^[111]. By contrast, the upper series representing the Mo/PBTPA composite maintains stable conductivity for more than one week^[111]. PBTPA is a highly hydrophobic polymer that forms strong interfacial interactions with the native hydrophobic oxide layer (MoO₃) present on the Mo surface. This interfacial compatibility promotes uniform filler dispersion and suppresses void formation under aqueous exposure^[111]. As a result, the conductive network remains structurally stable because water infiltration along the filler–matrix interface is significantly reduced, allowing sustained electrical performance over extended periods^[111].

Since functional failure typically originates from the infiltration of physiological fluids^[125], matrix designs that limit water diffusion are important for preserving the conductive network. Figure 6B presents an optical analysis of water infiltration at the silicon–wax interface, showing no visible penetration into the wax layer for up to 3 days at 37 °C in PBS (pH 7.4)^[129]. This behavior arises from the high hydrophobicity and crystallinity of natural wax, which provide low moisture permeability and strong resistance to swelling. These moisture-barrier characteristics are associated with extended functional lifetimes^[129]. Specifically, the Mo/candelilla wax composite has been reported to maintain stable conductivity for up to 5 days at 37 °C in PBS, with complete electrical disconnection occurring only after approximately 19 days^[112]. Although irreversible water diffusion and eventual network collapse cannot be entirely avoided, such matrix design strategies can substantially prolong the operational lifetime of the conductive composite within the intended transient window.

Beyond matrix design, encapsulation strategies that introduce an additional protective layer provide an effective approach to improving functional reliability by delaying water diffusion and suppressing premature filler corrosion^[108]. Figure 6C shows a cross-sectional scanning electron microscopy (SEM) image of a W/PBAT composite fiber conductor encapsulated with a PBTPA coating^[108]. This structural protection enables the fiber to maintain strong chemical durability, exhibiting a conductivity change of less than 2% even after 20 washing cycles under various pH conditions^[108]. Similarly, Mo/PCL composite fibers coated with poly(propylene carbonate) (PPC) retain stable resistance for up to 10 days, whereas uncoated counterparts fail within 2 days^[11]. These observations highlight the role of encapsulation in extending the functional stability of conductive composites under practical operating conditions.

In addition to physical coatings, dispersants can enhance reliability by controlling the microstructure of the composite^[12,33,35]. Figure 6D schematically illustrates the fabrication process and functional stability of a W/beeswax conductive paste containing glycofurol (GF). When immersed in PBS at 37 °C, the GF-containing composite preserves its initial conductivity for approximately 13 days, even after two weeks of exposure^[35]. This behavior is associated with the role of GF in promoting uniform filler dispersion and modulating the surface tension of the paste, thereby stabilizing interfacial interactions within the percolation network^[35]. In contrast, additive-free control samples develop cracking that rapidly disrupts conductive pathways, leading to early electrical failure^[35].

In summary, the reliability of biodegradable conductive composites is achieved through the strategic selection of hydrophobic matrices, the integration of protective encapsulation layers, and interface optimization via dispersants. This multifaceted engineering approach provides a critical design foundation for ensuring that devices operate with stable reliability for a programmed duration, even under the harsh conditions of biological environments.

Biodegradable flexible conductive composite systems integrated into bio-interfaced electronics offer several advantages over conventional biodegradable conductive polymers, particularly complete biodegradability and reduced interfacial impedance resulting from their higher electrical conductivity^[12]. As discussed in Section “Material design strategies for the functional reliability of biodegradable conductive composite pastes”, combinations of matrices and conductive fillers provide the basis for maintaining functional reliability and programming device operational lifetimes. Owing to their fluidic processability and mechanical compliance, these composite pastes can be integrated into a wide range of biointerfaced electronic platforms, including transient interconnects^[35,111], sensors^[11,33,34], textile-integrated electronics^[108], and wireless implantable stimulators^[11,12].

Figure 7A illustrates the use of a W/beeswax conductive paste as an electrical interconnect in bioresorbable electronics, maintaining a stable electrical pathway during device operation and subsequently disappearing after the intended functional lifetime. This paste, characterized by a high conductivity exceeding 7 kS·m^-1, exhibits electrical properties comparable to those of conventional non-bioresorbable silver epoxy, thereby minimizing potential performance fluctuations or interfacial instabilities in bioresorbable electronics^[35]. In a separate study, similar performance characteristics were also observed in wireless stimulators employing W/candelilla wax paste as interconnects, which maintained stimulation voltages up to 30 V and exhibited excellent mechanical flexibility under deformation^[130]. In addition to functioning as compliant interconnects that buffer interfacial stress during mechanical deformation, these composite pastes can also serve as passive electronic components such as resistors^[112], capacitors^[112], and inductors^[33].

Composite pastes can also function as conformal sensors integrated with soft robotic systems^[131] or biological systems^[34]. Figure 7B presents a Mo/PBAT-based strain sensor attached to the surface of a fully compostable soft robotic finger^[131]. These conductive pastes are employed to ensure stable electrical interfaces while accommodating the high mechanical flexibility required for soft systems. Owing to its low bending stiffness, the flexible sensor closely conforms to the surface of a PGS-based soft robot and exhibits stable resistance changes at strains up to 80%^[131]. Stable electronic performance is maintained even under large mechanical deformation, after 2,000 cycles of repeated deformation, and following 4 months of storage^[131]. The non-toxic degradation by-products serve as nutrient sources, activating soil enzymes and supporting plant growth, allowing the system to return naturally to the ecosystem after use^[131]. Beyond strain sensing, similar composite systems have been used to realize temperature^[11,33,108] and physiological signal^[34] monitoring for bio-interfaced applications.

These materials can also be processed into one-dimensional fibers through drawing or spinning^[11,108], enabling seamless integration with conventional textile manufacturing. As illustrated in Figure 7C, a smart arm sleeve was fabricated by stitching PBTPA-coated W–PBAT composite fibers onto a biodegradable PLA fabric, enabling the integration of temperature sensors, electromyography (EMG) electrodes, and inductive coils for wireless power transfer^[108]. The EMG electrodes exhibit an interface impedance of 26.20 ± 8.71 kΩ at 1 kHz, enabling the detection of electromyographic signal changes during limb movement, even in the absence of conductive gels^[108]. This impedance profile corresponds to a signal-to-noise ratio (SNR) of 8.80 ± 0.49 dB, supporting the observation of neuromuscular coordination patterns^[108]. Within this context, physiological signals can be monitored in real time, including a 2 °C increase in skin temperature during running^[108]. The entire textile system decomposed naturally in soil within approximately 120 days^[108], highlighting the potential of these materials for environmentally sustainable wearable electronics.

Biodegradable composite conductors can also be utilized in fully implantable therapeutic systems^[12]. Figure 7D shows the implantation of a wireless radio-frequency receiver and a conductive nerve conduit (CNC) platform in a 10 mm sciatic nerve defect model, illustrating a system that eliminates infection risks associated with wired external connections. Specifically, the Mo/PCL conduit exhibits a high electrical conductivity of 7.4 S·cm^-1, which significantly exceeds the ranges typically reported for conventional conductive polymer or carbon-based materials (10^-6 to 1.57 × 10^-2 S·cm^-1)^[12]. This level of electrical performance provides a low-impedance environment, facilitating the consistent transmission of therapeutic signals across the nerve gap. Following daily monophasic pulse stimulation (100 µs pulse width, 20 Hz, three times per day), the stimulated group achieved a sciatic functional index (SFI) of -39.58 ± 2.36 after 12 weeks, demonstrating improved functional recovery compared with both the unstimulated group (-47.56 ± 5.48) and the autograft control (-43.6 ± 2.5)^[12]. These results suggest that the wireless stimulation platform, which combines high conductivity with mechanical flexibility, effectively supports axonal regeneration and functional recovery in damaged peripheral nerves.

In summary, biodegradable conductive composite systems combine high electrical performance with mechanical compliance compatible with biological tissues. Their versatility - from conformal wearable sensors to fully implantable wireless stimulators - offers a promising pathway toward bioelectronic systems that minimize long-term environmental impact. Continued advances in lifetime control, reliability engineering, and multifunctional integration will further expand the scope of sustainable biointegrated electronics.