Cohering particles via weak electromagnetic waves for highly conductive polymer composites

Domain: physics
Published: 2025-11-25
Editorial outcome: major_revision

Abstract
Highly electrically conductive polymer composites are fundamental materials for today’s electronic engineering. However, these composites are very expensive since heavy loading of costly conductive particles (fillers) is required to guarantee high electrical conductivity. This work successfully reports that weak electromagnetic waves (EMWs) could cohere the microparticles to optimize conductive networks for highly conductive polymer composites. Such an EMW strategy arises from the earliest EMW detector (coherer) invented by Marconi and Popov in 1895 via the breakdown of surface oxides and cohering of the micro-conductors by EMWs. The flexible composite (metal microfiber-polymer) shows a unique insulator-conductor transition with drastic resistance change (50 MΩ to 1 kΩ) under weak EMWs (power: 10-6 mW). The EMW strategy is extremely rapid (0.34 µs) and highly stable (104 cycles). Such EMW effect universally describes the interactions between EM fields and composite materials that are suitable for general conductors (e.g., carbon- and metal-fillers) and most polymers.

Introduction
Electrically conductive polymer composites are the fundamental materials in today’s electronic engineering and are broadly applied as soft conductors, plastic electronic components (e.g., antistatic shielding cover), and electronic packaging materials due to their combined advantages of electrical conductivity, polymer flexibility, and interfacial bonding capability1–3. The current market for conductive polymer composites achieves as high as $4.4 billion per year in the electronic industry4,5. Although broadly utilized, the highly conductive composites are usually quite expensive since very high loading of costly fillers (usually beyond 75wt% for noble metal particles) is required to achieve sufficient conductive networks to guarantee good electrical performance3. Lots of investigations have been performed to lower the loading values to reduce the cost, including using specific fillers (silver nanowires, silver flakes, carbon fibers, and graphene) and unique blending strategies (solvent or bicontinuous phase assisted)6–9. However, these techniques or specific fillers are not prevalent in the industry due to the difficult synthesis, complicated steps, and strict requirements for the processing. So far, using a large quantity of costly conductive fillers in polymers is the most favored industrial method in electronics for high electrical conductivity in composites.

Electromagnetic waves (EMWs) have been the undoubted foundation for human society for more than 140 years since Hertz’s discovery in the 1880s10,11. The study of the interactions between EMWs and conductors is an important aspect of electromagnetics, yielding fruitful results for today’s technologies12. For example, radio technology uses EMWs and conductors to transmit signals, serving as the foundation of modern communication disciplines and making significant contribution to human society13,14. In the early history of radio technology, both Marconi and Popov utilized the interaction between metal powders and EMWs to receive EMWs as the earliest radio signal detector (coherer) in 189515. Such interaction is called the Branly effect, in which the high electrical resistive metal powders could be conveniently turned into low resistive materials by cohering the metal particles upon weak EMWs16,17. Although the coherer-type radio detector was quickly replaced by other advanced detectors due to the stability issue, the phenomenon between metals and EMWs strongly reminds us of a promising and convenient strategy to obtain highly conductive composites by low-energy EMWs.

In this work, we revisit the oldest EMW detector (coherer) from over a century ago and report a universal and convenient strategy that can cohere the conductive microfillers in polymers for highly electrically conductive composites through weak EMWs. Such an EMW effect arises from the Branly effect that can break down the thin oxides and then produce the joule heat from the induced voltage by EMWs to cohere the microfillers in polymers. Excitingly, applying weak EMW (power: 10−6 mW) on the composites remotely for a very short time (<1 µs) is sufficient to optimize the conductive network in the composites, leading to a significant change in material performance. The flexible metal microfiber polymer composite can have a unique insulator-conductor transition with a drastic resistance change in 4 orders of magnitude from 50 MΩ to 1 kΩ through emitting EMWs at a long distance (5 m). Systematic investigations in this work find that the EMW effect is heavily influenced by the amplitude and the rising/falling edge time of EMWs. Remarkably, the resistance transition is completely reversible (recovery ratio: ~100%) upon external micro-force (29 mN) and can withstand more than 104 cycles of such transition. It is worth noting that the EMW effect is a general strategy that universally describes the interactions between dynamic EM fields and conductors without strict requirements for metals and polymers. Flexible resistive memory devices with similar behaviors to magnetoresistive materials can be successfully and conveniently fabricated based on EMW effects. More importantly, different from the phenomena observed by Marconi and Popov, this work creatively finds that cohering of microfillers via EMWs could be permanently locked in the polymer composites if further treated by joule heating via transient voltage, which provides the electronic industry with a convenient strategy for highly conductive composites.

Results
The composite in this work is prepared by simple shear mixing of metal micro-conductors (metal microfiber) and polymers (Figs. S1, S2 and Table S1). Styrene-Isoprene-Styrene (SIS) rubber (Mw: 30 kDa) is used as the polymer matrix since it shows high extensibility, while the metal microfibers in the work are steel fibers (SAE 316L, diameter: 12 μm and length: 500–800 μm, shown in Fig. S3) that are utilized as micro-fillers in polymer composites. In this work, unless specified, the EM materials refer to steel microfiber-SIS composite using SIS as a polymer matrix with 15 wt% above steel fibers (Fig. S4). Figure 1a, b show that emitting EMWs for a short time on a single metal microfiber will induce a high voltage between fibers. The induced voltage is also monitored (approximately 20 V, Figs. 1c and S5) when emitting the EMWs by an electrical fast burst (EFB) generator (Fig. S6) with a rising edge at 8 ns. Such induced voltage could produce a strong electric field strength (2.2 × 107 V/m based on the simulation of COMSOL Multiphysics, Fig. S7) among the fibers, exceeding the dielectric breakdown threshold of copper oxide18. The power-frequency spectrum of the EMWs is also recorded by the real-time spectrum analyzer (Fig. S8), which shows that the weak EMWs (power: 10−6 mW, triggered by EFB or piezoelectric ignitor with a total energy below 75 μJ) are sufficient to cohere the microfiber. Figure 1d exhibits that when the metal microfibers form composite materials with polymers, the microscopic breakdown of the surface oxide of microfibers in the polymer matrix before and after the EMW treatment will optimize the conductive network of the materials. Importantly, the EMW effect can be a general strategy that universally describes the microscopic interactions between EMWs and conductors. Figures S9–S13 and Table S1 demonstrate that other common conductors (such as copper particles, copper fibers, and carbon) and general polymers such as polycarbonate (PC), styrene-ethylene-butylene-styrene (SEBS), thermoplastic polyurethane (TPU), low-density polyethylene (LDPE), ethylene-vinyl acetate (EVA), and polylactic acid (PLA) show similar EMW effects.

The mechanism of the EMW effect for composites can be demonstrated by an old phenomenon reported over one century ago called the Branly effect. The Branly effect was used to construct the earliest radio signal detector (coherer) by Marconi and Popov via cohering metal powders upon weak EMWs. As shown in Fig. 2a1–a4, and 3D microscopic images (Fig. 2a3), after EMW (τ = 4 ns, Fig. S14) treatment, tiny dents are observed on the points where the steel beads (SAE 316L, Φ: 10 mm) contact. It indicates that EMWs can generate sufficiently high voltage between the steel beads, which will break down the surface oxide and cohere the metals through the induced voltage. To clearly observe such a cohering effect, a 2 kV transient voltage (500 µs, Fig. S15) is applied to the ends of the string with four contact beads. Due to the surface oxides of the beads, the string is electrically insulating. As shown in Fig. 2b, visible electric spark is generated among the four beads under the transient voltage, and the resistance is reduced to 3.8 Ω immediately. The four beads are cohered together at their joints (Fig. 2c) due to the local joule heating without dropping off upon lifting (inset in Fig. 2c). Building on this, copper fibers are positioned 1 cm away from the EMW source and treated for 20 s. As shown in Fig. S16, after EMW treatment, the surface at the contact point of the metal fibers is disrupted, leading to cohered fibers. These experimental results directly demonstrate the breakdown of the surface oxide layer on the microfibers. To further verify the transient heat for the composite sample under induced low voltage, a transient voltage (80 V, 412 μs) is applied across the EM material, and the resulting thermal effects are observed by an infrared camera (Fig. S17). The surface oxide breakdown and joule heating offer the efficient EMW effect for conductive microfillers in polymer composites. In addition, the previous report19 and information from prototype experiments in this work (Fig. S18) illustrate the Branly effect for pure carbon and metal fillers. EMW induces the insulator-to-conductor transition primarily through localized oxide layer breakdown and removal at the metal. As shown in the scanning electron microscopy (SEM) mapping (Fig. S19), after EMW treatment, the reduction of the O element (from 50.01 to 10.32%) in the energy-dispersive X-ray spectroscopy (EDS) image of the treated oxidized copper surface directly demonstrates the removal of the oxide layer. In addition, samples with copper particles whose oxide layer is removed (Fig. S20) exhibited no such EMW effects. The surface oxide breakdown and joule heating offer the efficient EMW effect for conductive microfillers in polymer composites.

Figure 2d provides an intuitive view of the conductive network optimization upon measuring the electrical conductivity of the materials. Interestingly, similar to radio technology, EMW effects can be applied remotely (Fig. 2d). When EMWs are emitted at different distances from the sample (Movies S1, S2 and Figs. S21, S23), EM materials will show a unique insulator-conductor transition immediately. When EMWs are emitted at a distance of 5 m from the EM material, the composite exhibit a dramatic reduction in electrical resistance from >50 MΩ to 1 kΩ, and the lighting of the light-emitting diode (LED) is observed (Figs. 2d and S21–S23). EM materials are placed at various distances (0–6.6 m) from the EMW source to further investigate the influence of the EMW source-to-material distance on the EMW effect. As shown in Fig. S24, the effect attenuates exponentially with increasing distance from the EMW source. High-temporal-resolution measurements obtained through oscilloscopic analysis reveal that the EMW-induced conductivity transition of the EM material occurred with remarkable rapidity, demonstrating a characteristic response time of 0.34 µs (Figs. 2e and S25). Under EMW exposure, the resistance transition times for the steel microfiber-EVA composite and copper microfiber-EVA composite are 0.35 μs and 0.25 μs, respectively (Fig. S26). This demonstrates that varying the type of conductive filler and polymer matrix enables the modulation of transition time, thereby facilitating faster or slower resistance transitions. It is noteworthy that the LED remains on after the electric transient is present for only a very short time, and the EMW disappears. As shown in Fig. S27, the low resistance state of the EM material will not change with time extension even after the EMW source is turned off.

The amplitude and rising edge of the EMWs significantly affect the EMW effect, which is evaluated by experiments shown in Fig. 2f–h. Since the EMWs are dynamic electromagnetic fields, we utilize a signal generator connected with two copper plates to provide the dynamic parallel electric fields (Fig. 2f). The induced voltage across the EM material, measured at various points within the central region between the plates, is consistent (Fig. S28). This uniformity confirms the homogeneity of the applied electric field. Such changing electric fields will offer electromagnetic fields as EMWs to activate the EMW effects. The resistance of the EM material is monitored to evaluate the degree of particle cohering based on the current and voltage in the circuit. In addition, to avoid resistance measurement errors caused by induced currents (0.9 μA, Fig. S29) and to ensure the accuracy of the experimental results, the resistance measurements are carried out after switching off the electromagnetic field. As shown in the inset of Fig. 2g, electric pulses from the signal generator provide different transient EM fields with varied amplitude (E) and rising time (τ). With a certain rising edge (τ) of the amplitude for EM fields at 500 ns, the R/R0 ratio is reduced when increasing the amplitude of EMWs from 100 V/cm to 750 V/cm. In this work, for R/R0, R is the real-time resistance, and R0 is the initial resistance of the EM material. Besides, with the same amplitude, the fast-rising edge (τ) provides a stronger EMW effect. For τ at 5 ns, a small amplitude (5 V/cm) can generate a significant resistance decrease (Fig. 2g, inset). While for large τ at 150 ns, a much higher amplitude (>400 V/cm) is required to activate the significant conductive network optimization. It should be noted that static electric fields will not be able to cohere the microfillers in polymers. As shown in Fig. S30, the resistance of the EM material does not change when applying static electric fields up to 750 V/cm for a sufficient time (>600 s). Actually, the faster-changing electric fields can induce a higher electric voltage in the conductors to break down the surface oxides among microfibers. According to Faraday’s law ε = dΦ/dt = A(dB/dt) (1), higher-frequency sinusoidal voltages exhibit a greater E, thereby generating stronger induced voltages. As shown in Fig. S31, when sinusoidal voltages of varying frequencies (50 and 500 kHz) but identical amplitudes are applied to the copper plates, higher-frequency electric fields induced higher-amplitude voltages in the EM material, thereby enhancing the efficacy of the EMW effect. The electric fields with low E/τ values (e.g., strong static electric fields with the greatest τ value) cannot excite the EMW effects due to the low induced voltage in the composites.

Similar to the earliest EMW detector in radio technology, such cohering of particles is only temporarily fixed, and the sample will recover to the high resistance state upon slight vibration. A reverse resistance change can be conveniently achieved when utilizing external ultra-small strain (<0.14%, 29 mN) to disturb the connected microfibers in the EM material, which can be further verified by the resistance recovery of the materials. Movie S3 and Fig. S32 show that slight vibrating of the sample treated by EMWs can retrieve the high resistance (recovery ratio: >99.99%) of the EM material. Dynamic mechanical analysis (DMA) is used to investigate the relationship between the network recovery and the external force (Inset in Fig. 3a, initial spacing between the two fixtures is 1 cm) upon measuring the resistance change. Figure 3a shows that when the tensile strain is above 0.14%, the EM material (length: 15 mm, width: 10 mm) can have the reversible conductor-insulator transition. It should be noted that during the stretching process corresponding to the resistance recovery, no cracks are generated in the polymer, and the vibration disrupted the fragile micro-welded points between fibers through a physico-mechanical process, thereby altering their contact state. The reverse conductive network transition under external micro-force is also very fast, which can be achieved in 20 μs (Fig. 3b, the external force on the EM material is offered by a commercially available micro-vibrator controlled by square signals in Fig. S34). As a result, with the micro-vibrator (Figs. S33 and S34), the EM materials can be successfully applied as an electronic component. The electronic design is illustrated in Fig. 3b (inset). The EM material membrane can be fixed on the surface of the micro-vibrator. As shown in Fig. 3c, upon EMWs and a vibrator, the reversible insulator-conductor transition can be achieved. Considering the above enthralling characteristics, the EMW effect provides fantastic control over the conductive network, which can make substantial contributions to modern electronics. For example, similar to magnetoresistive materials20,21, a kind of colossal electromagnetic resistive material with an astonishing 99.99% resistance change (4 orders of magnitude change in resistance from above 50 MΩ to 1 kΩ) can be conveniently created (steel microfiber-SIS composite, Fig. 3d). This resistance variation remains remarkably stable beyond 104 on/off switching cycles, showing no significant change in baseline or peak-to-peak resistance values (Figs. 3e and S35). Moreover, the resistance switching state (on/off) of the EM material is flexibly tunable. By adjusting the frequency and amplitude of the control signal, the material’s resistance can maintain stable switching from above >5 MΩ to 1 kΩ over 104 cycles with a 3 s period during cycling (Figs. S34 and S36). Therefore, this work gives a fresh perspective on the field of future flexible resistive memory electronics. Figure 3f shows the typical electronic design for the EM materials to work as a resistive memory device with the capability of a rapid switch with multiple states. The EMWs for the sample work like the third electrode in the device, which can control the multiple resistive states remotely. Figure 3f (middle) and Fig. 3c have already indicated that EM materials can show rapid and stable on/off (0 or 1 states) features in electronics upon EMWs with high E/τ values. Interestingly, different from most previously reported resistive memory materials22–24, EMW effects can provide multiple resistive states that can be conveniently and remotely programmed by the E/τ values of EMWs (Fig. 3g, six different states, steel microfiber-SIS composite). These steps remain nearly constant over multiple cycles (Fig. S37), showing no signs of hysteresis or write fatigue. The different resistive states can be read through the current-voltage curves (Fig. S38, steel microfiber-SIS composite) in the circuit of the electronics. Figures 3h and S39 show one prototype experiment in which two EM materials are connected with the LEDs in series. Using EMWs, four states of “10”, “01”, “11”, and “00” can be easily and quickly produced and recorded. These prototype results strongly indicate that the EMW effect can be utilized to fabricate a type of resistive memory device. It should be noted that for samples or circuits asking for high spatial resolution or under electromagnetic shielding/interference, special efforts should be applied to realize the EMW effects. Currently, most magnetoresistive and resistive memory materials are based on rigid inorganic materials. This work provides such soft electronics based on flexible polymer composites that can be remotely controlled via EMWs.

More importantly, the optimized conductive network via the EMW effect can be permanently locked in the polymer composites if further treated by quick joule heating via transient voltage. As shown in Fig. 4a, steel fibers can be strongly cohered at the contact nodes by a transient voltage if the surface oxide has been broken down by the EMW effect. Specifically, as shown in Fig. 4b, a capacitor with a voltage of 100 V and a capacity of 3 μF is utilized to provide a transient voltage in a short time (165 μs) on the sample that has been treated with EMWs. The sample can be permanently locked in a low-resistive state without recovery. The micrographs (Figs. 4c and S40) clearly show the local cohering of steel microfibers encapsulated by polymer within the EM material. With the combined strategy of EMW and transient voltage treatment, the EM material shows permanently stable resistance, which can withstand the 1000 cyclic vibration tests as shown in Fig. 4d. To further investigate the stability of the cohered micro-filler network, long-term aging, temperature cycling, and strain testing are conducted. The integrally treated materials exhibit no detectable change in electrical resistance after 8 weeks (Fig. S41). Furthermore, the treated materials maintain a stable low-resistance state (<10 Ω, Fig. S42) across a temperature range of 25–120 °C. Fig. S43 additionally demonstrates that the processed composite retains its low-resistance state (below 1 kΩ) under 24.5% tensile strain. After the combined strategy treatment, the cohered micro-filler network exhibits unchanged mechanical and chemical stability following prolonged aging, thermal cycling, and mechanical stress application. In addition, it should be noted that the transient voltage via the capacitor is very important to guarantee the fast and high transient power for the local cohering. If using direct voltage (>30 V), it will burn the EM material (Fig. S44) due to the too long time (>1 s) of heating. While applying a transient low voltage (30 V), it will not generate high enough transient power for micro-cohering to lock the resistance permanently (EM material cannot withstand vibration testing, Figs. S45 and S46). It is well-known that the high loading value of expensive conductive fillers greatly increases the cost for the industry of composite materials. This combined strategy will show a heavy impact on the current electronic industry by offering a feasible and convenient way to lower the threshold value of the conductive composites. As shown in Fig. 4f, for steel microfibers as fillers, the conductive percolation threshold value after the combined treatment is almost one-third of that without the treatment (15 wt% vs 45 wt%), indicating a significant decrease in the demand for the conductive fillers. In addition, the impacts of filler size, distribution, and conductivity on the EMW effects are also investigated in detail, as shown in Figs. S47–S49. In addition, based on previous reports that carbon materials also exhibit the Branly effect19, combined with the experimental information of this work, collectively show that the composite treatment approach has broad applicability across different conductive filler systems, including carbon (carbon nanotube and graphene) and metal (copper powders) fillers. The electrical conductivity is significantly enhanced (2–4 magnitude) for all the composites with these fillers (Figs. 4g and S49–S51), suggesting a universal strategy for the current electronic industry. However, the composite with the graphene and graphene oxide hybrid is significantly greater than that observed in composites with the pristine graphene alone (Fig. S20). This again demonstrates that the surface oxide layers on carbon materials also play a crucial role in the EMW effects. Especially, costly silver particles are the most commonly used noble metal fillers for the electronic industry25–28. This work utilizes the simple shear mixing of silver particles (silver diameters: 1–1.5 μm) with resin to prepare the representative example for conductive polymer composites in electronics. With low silver usage (56 wt%), the electrical conductivity of the silver-resin composite after the combined treatment can achieve as high as 1.3 × 106 S/m (Figs. 4h, S52 and Table S2), surpassing most of the currently used silver-based composites in industry and academic research as electrical conductive paste, soft conductors, and electronic packaging materials8,29–43. In addition, the combined EMW strategy treatment can transition oxidation-prone silver-copper paste (65 wt%) from a non-conductive to a conductive state (8.0 × 103 S/m, Fig. S53). Such a strategy will undoubtedly alleviate the heavy cost for the current industry and meet the strict demands for most electronic applications.

Methods
Materials: Steel fibers (SAE 316L, Φ: 12 and 22 μm, length: 10 cm) and Copper fibers (T1, Φ: 22 μm) were purchased from Huitong New Materials Co., Ltd. Carbon nanotubes (CNT, Φ: 5–12 nm, length: 3–10 μm, 90–95%) were purchased from Wuxi Yifeng Technology Co., Ltd. Graphene oxide (GO, Φ: 20–40 μm, thickness: 1–10 nm, O: 40–45 wt%) was purchased from Nakate New Materials Co., Ltd. The few-layer graphene (Φ: 7–12 μm, 11–15 μm, diameter-thickness ratio: 9500, 98%) was purchased from Shenzhen Suiheng Graphene Technology Co. Steel beads (SAE 316L, Φ: 10 mm) were purchased from Glisen Hardware Co., Ltd. Silver powders (1 μm, 99.99%) were purchased from Exceed Technology (Suzhou) Co., Ltd. Conductive silver paste (50 wt%) was purchased from Suzhou Xinwei High-tech Material Co., Ltd. The silver-coated copper particles (Φ: 13 μm, 5 wt% silver content) were purchased from Jingrui Alloy Products Co., Ltd. Low-melting-point glass powder was purchased from Lingzhuo County Huachuan Mineral Products Processing Co., Ltd. The phenoxy resin (AR) was purchased from Shanghai Ron Chemical Technology Co., Ltd. Benzoyl Peroxide (BPO, AR) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. 2-(2-Butoxyethoxy)ethyl acetate (DBAC, 98%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. SIS (YH-1105, 30 kDa) was purchased from Yoshida Chemical Co., Ltd. SEBS (YH-502) was purchased from Baling Petrochemical Co., Ltd. The Polydimethylsiloxane (PDMS, Sylgard 184) was supplied by Dow Inc. PC (C-110, 30–100 kDa) was purchased from CHIMEI Co., Ltd. TPU (BTE-75A) was purchased from Risheng Chemical Co., Ltd. LDPE (1965P5) was purchased from Saudi Basic Industries Co. Ltd. EVA (40 W) was purchased from DuPont Co., Ltd., USA. The PLA (NatureWorks Ingeo 3260HP) was purchased from Dongguan Qisu & Plastic Materials Co., Ltd. Copper electrode (length: 5 m, width: 10 mm, height: 0.06 mm) was purchased from Shanghai Weitesi Electronic Technologies Co., Ltd. Piezoelectric ceramics (piezo ignitor, rising edge: 10–20 ns, amplitude: 10–50 kV). The micro-vibrators (DC 3–5 V) were purchased from the Jinhua Tuglisi Technology Co., Ltd.

Instruments: Metal micro-conductors (metal microfibers) and polymer were mixed by an open mill (STD-2KL) and a melt-compounder (AP2113, AUPLEX) to obtain the metal microfiber-polymer composite. EMWs were generated by the EFB generator (EFT61004TA, PR9240343897) and the piezoelectric ignitor. The power of the emitted EMWs at a distance of 5 cm from the emitting center was 10−6 mW, which was monitored by a real-time spectrum analyzer (SIGLENT SVA1032X) with an electric sensor (Fig. S8). Dynamic electromagnetic fields were generated by waveform generators (SIGLENT SDG1022X) and power amplifiers (ATA-2021B). The electrical resistance was measured by means of multimeters (VICTOR VC890C, Keysight 34461A, and Keithley DMM6500) and a precision four-point probe resistivity measurement system (HPS2663). The internal structure of the metal microfiber-polymer composite was observed by a digital microscope (Leica DVM6). The rising edge of the EMWs was measured by an oscillograph (UTD2152S-E). The response and recovery time for the metal microfiber-polymer composite upon EMWs and external micro-force was measured by an oscillograph (UTD2152S-E). The relationship between the network recovery and the external force upon measuring the resistance change was investigated by DMA (Discovery HR-20). The metal microfiber-polymer composites were stretched by an electro-mechanical stretching machine (E42.503, MTS Systems (China) Co., Ltd.). Infrared photos were taken by an infrared thermal imaging thermometer (FLUKE TiS 20). The SEM images and EDS mapping were captured using an emission scanning electron microscope (FEI Inspect F50) equipped with EDS capabilities.

Preparation of metal-polymer composite: As shown in Fig. S1, the preparation of steel fiber-SIS composites with a 15 wt% mass fraction was taken as an example. SIS (4.00 g) and steel fibers (0.71 g) were shear-mixed in an open mill (STD-2KL) at 90 °C at 2 rad/s for 5 min to uniformly distribute the percolation network. The composite film with a thickness of 1 mm was then obtained by hot pressing at 120 °C for 2 min under 20 tonnes of pressure using a melt-compounder (AP2113, AUPLEX). To unite the standard of samples, the composite film was cut into identical rectangular samples (length: 30 mm ± 1 mm, width: 10 mm ± 1 mm, height: 1 mm). Copper electrodes (length: 13 cm, width: 10 mm) were attached to the ends of the samples for test standby as shown in Fig. S2. Copper electrodes were placed on the two ends of the rectangular sample 1 cm apart in the center. The copper electrodes were thermally bonded into rectangular samples at 120 °C for 2 min to ensure firm adhesion. Similarly, other samples with different fillers were prepared using the method described above.

Evaluation for the percolation network transition based on resistance change upon electromagnetic fields with different rising edges and amplitude: The main components in the experiment were steel microfiber—SIS composites, waveform generators, power amplifiers, two copper plates, and a multimeter. The waveform generator was connected to an amplifier. The signal from the waveform generator was modulated as a transient wave with a high level of 4 V, a low level of 0 V, a period of 1 s, and a duty cycle of 0.02%. The amplified signal was connected with wires to two parallel copper plates. The composite was connected to a multimeter and placed between parallel copper plates to format test circuit. Then, the rising edge and the signal from the waveform generator were controlled, and the amplification of the power amplifier was used to make the different maximum values of the electric field strength between parallel plates in the working circuit. The electric field between two copper plates was controlled from 0–750 V/cm. The falling edge of the signal was much larger (above 20 μs) than the rising edge (τ) to ensure a single fast pulse in one period. The modulation of the composite’s percolation network by different electric field strengths was evaluated by reading the change in resistance of the multimeter.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this paper, the authors present a novel method for fabricating highly conductive composites by utilizing weak EMW to cohere the conductive microfillers in polymers. The rapidity and stability of the EMW strategy were quantitatively investigated. Meanwhile, flexible resistive memory devices with similar behaviors of magnetoresistive materials could be successfully and conveniently fabricated based on EMW effects. Furthermore, the authors demonstrated that microfibers can be permanently locked in the polymer composites under transient voltage. This EMW effect is applicable to common conductors and most polymers, offering a highly convenient strategy for developing high-conductivity polymer composites in electronic engineering. However, I don’t think this manuscript meets the publication criteria of Nature Communications because of the lack of physical significance. To enrich the impact of this work, I have some questions:\n1. The steel bead experiment is used to illustrate the principle of the EMW effect, as show in Figure 2a-c. I suggest that the authors provide direct experimental results in the microscopic system. Please provide more experimental details, such as the frequency of EMW source on both microfiber composites and steel bead.\n2. Does the distance between the EMW emission position and the sample location affect the conductive properties of the composite material? The study discusses scenarios with distances of 10 cm and 5 m, both of which can rapidly reduce the material's resistance to 1 kΩ. Does this imply that 5 m is not the maximum effective control distance?\n3. Is the microscopic image in the inset of Figure 2d (upper right corner) taken after EMW irradiation? Please label it accordingly, and consider adding a microscopic image taken before EMW irradiation, as this would create a more pronounced contrast between the pre- and post-irradiation states.\n4. P8，Please verify the statement: “As shown in the inset of Fig. 2g, electric pulses from the signal generator provide different transient EM fields with varied amplitude (E) and rising time (τ). With the certain rising edge (τ) of the amplitude for EM fields at 500 ns, the R/R0 ratio is increased when increasing the amplitude of EMWs from 100 V/cm to 750 V/cm.” The content of the inset in Fig. 2g does not align with this description. Additionally, the R/R0 ratio is observed to decrease (not increase) when the amplitude of EMWs is raised from 100 V/cm to 750 V/cm. This discrepancy requires correction.\n5. The description exhibits excessive repetition. For instance, the term \"(steel microfiber-SIS composite)\" is used repeatedly throughout the manuscript. It is advisable to provide a declaration at the beginning of the article and implement simplification in the main text.", "reviewer_2": "Thank you so much for giving me a chance to review the manuscript entitled “Cohering Particles via Weak Electromagnetic Waves for Highly Conductive Polymer Composites”, submitted to Nat Comm. The authors present an intriguing and technically sound strategy for enhancing electrical conductivity in polymer composites via weak electromagnetic wave (EMW) stimulation - a concept inspired by the classical Branly effect.\nThe manuscript is well-structured and offers a compelling combination of historical context and modern application. I find the demonstration of EMW-induced coherence of conductive fillers along with its reversible and lockable conductivity transitions to be a significant and novel contribution, particularly in the context of flexible electronics and low-cost resistive memory design.\nThat said, I have raised several comments regarding the interpretation of the chemical and physical mechanisms and the clarity of data presentation in the figures. I believe addressing these points will further strengthen the quality and impact of the manuscript.\nI appreciate the opportunity to contribute to the peer review process and would be happy to provide further input if needed.\nPlease see the comments below:\n1. The induced voltage during EMW exposure is stated to arise from electric fields with high E/τ values, but what are the actual voltages generated across individual fiber junctions in the composites? Are these voltages directly measured or inferred? What modeling or simulation supports this?\n2. The authors claim that Joule heating from transient voltages induced by EMWs is responsible for oxide breakdown and fiber cohering. However, is there any thermal imaging or time-resolved IR thermography to directly verify this localized heating? Could this be separated from general dielectric heating of the polymer?\n3. While the breakdown of surface oxides is a central claim, are there any XPS, TEM, or EDS analyses performed on the fiber surfaces before and after EMW treatment to confirm oxide layer removal or alteration?\n4. The setup employs parallel plates or localized EFB emitters. How uniform is the field within the composite? Could non-uniformity cause uneven cohering, creating hotspots or weak regions in the percolation network? Are cross-sectional conductivity maps or tomographic images available?\n5. The response time of 0.34 µs is very fast. How was this time constant extracted—was it limited by the RC time of the measurement circuit or the intrinsic material switching? Can faster switching be achieved by optimizing filler distribution or polymer viscosity?\n6. The resistance recovery upon strain is attributed to the mechanical separation of cohered fibers. Has this been confirmed by in-situ SEM or micro-CT under strain? Are cracks formed in the matrix or is the disconnection occurring only at the fiber-fiber contact?\n7. The paper shows >10,000 switching cycles, but is there any degradation in peak-to-peak resistance or gradual baseline drift? Has the endurance been tested under different strain amplitudes or frequencies? This is critical for realistic flexible electronics applications.\n8. The memory behavior with multiple states (Fig. 3g) shows discrete resistance steps. Is this scaling linear, logarithmic, or stochastic with respect to EMW amplitude or energy? Does this show any hysteresis or write fatigue over multiple cycles?\n9. The work briefly shows various fillers (CNT, graphene, Cu particles), but how does filler aspect ratio, conductivity, and dispersion affect the EMW-induced transition? Are there statistical analyses comparing percolation thresholds and resistance change magnitudes across these systems?\n10. The paper states that 10⁻⁶ mW EMW power is sufficient to trigger transitions. But what is the total energy input per activation event (considering pulse width, capacitor discharge, and delivery losses)? How does this compare to thermal sintering, photonic curing, or microwave treatments in terms of J/cm³?\n11. How do the authors differentiate between the effects of EMW-induced electrical breakdown and electrostatic discharge or capacitive coupling artifacts?\n12. To what extent has the electric field strength induced across microfibers been quantitatively verified or modeled (e.g., electric potential gradient, capacitive coupling, or field enhancement at fiber tips)?\n13. Can the mechanical or chemical stability of the “cohered” microfiller networks be validated over time, temperature cycling, or mechanical strain, especially in the “permanent” state?\n14. What are the potential limitations (e.g., shielding issues, spatial resolution, EM interference) of using EMWs as a remote and selective tool for device fabrication or activation in complex circuits?", "reviewer_3": "Summary Comments: The authors studied a novel approach to enhance electrical conductivity in polymer composites by utilizing weak electromagnetic waves (EMWs) to cohere conductive microfillers. The study demonstrates a significant conductivity improvement (4 orders of magnitude resistance reduction) with reduced filler loading requirements. The EMW effect is based on the historical Branly effect discovered over a century ago, showing remarkable innovation in applying classical electromagnetic phenomena to modern materials engineering. This creative revival of century-old technology for modern materials applications is highly commendable and represents the type of interdisciplinary innovation that would be suitable for Nature Communications after addressing the concerns below. A stretchable LED device was also fabricated to demonstrate the practical applicability of the materials. However, several critical issues need to be addressed before publication.\nComment #1: \"While the resistance reduction from 50 MΩ to 1 kΩ is impressive, the final resistance value of 1 kΩ remains too high for most practical electrode applications. Typical conductive electrodes or inks require resistance values in the range of 1-100 Ω/sq. The authors should address how this technology can achieve lower resistance values suitable for real-world applications or identify specific applications where 1 kΩ resistance is acceptable.\"\nComment #2: \"The mechanism of surface oxide breakdown and joule heating induced by EMWs is not sufficiently explained. How exactly do weak EMWs (10⁻⁶ mW) generate enough energy to break down oxide layers? What is the theoretical basis for the E/τ ratio dependency, and why is this parameter critical for the EMW effect?\"\nComment #3: \"A fundamental concern exists regarding the experimental sequence and mechanism feasibility. The authors prepare the composite by mixing SIS and steel fibers at 90°C followed by hot pressing at 120°C to create a solidified 1mm thick film, and then apply EMW treatment to this already-solidified matrix. This raises critical questions: How can conductive particles move and reorganize within a solidified polymer matrix? If the SIS matrix has high elastic modulus and elasticity, wouldn't any particle rearrangement be limited and quickly revert to the original state due to polymer chain elasticity? The authors need to provide a more detailed explanation of how significant particle movement and network reorganization can occur in a solid polymer matrix, and why this rearrangement doesn't immediately reverse due to polymer elasticity.\"\nComment #4: \"The authors claim universal applicability across different conductors and polymers, but the evidence is limited. Only a few material combinations are tested, and the mechanism may vary significantly between different types of conductors (metals vs. carbon materials). More comprehensive studies across diverse material systems are needed to support the universality claim.\"\nComment #5: \"Statistical analysis and reproducibility data are lacking throughout the study. Most experimental results appear to be based on single samples without error bars or statistical significance testing. What is the sample-to-sample variation in EMW response? How reproducible are the resistance changes across different batches?\"\nMinor Issues:\nComment #6: Several grammatical errors and typos throughout the manuscript need correction (e.g., \"exhibite\" should be \"exhibited\" on page 6).\nComment #7: Terminology inconsistency: \"EMW effects\" and \"EM-PNR effects\" are used interchangeably without clear definition of the latter term."}, "author_response": "RESPONSE TO REVIEWE RS' COMMENTS\nReviewer #1:\n1. This EMW effect is applicable to common conductors and most polymers, offering\na highly convenient strategy for developing high-conductivity polymer composites in\nelectronic engineering. However, I don’t think this manuscript meets the publication\ncriteria of Nature Communications because of the lack of physical significance.\nResponse: Thanks for the reviewer’s very professional comment. We sincerely\nexpress our gratitude to the reviewers for their highly professional comments and\nsuggestions on our manuscript. We acknowledge that the initial version provided\ninsufficient depth and clarity in elucidating the physical phenomena and their\nunderlying mechanisms. Based on the constructive feedback from reviewers 1 to 3,\ncomprehensive revisions have been undertaken. This includes redesigning and\nsupplementing multiple experiments to advance the understanding of EMW effects on\ncomposite materials. We have further elaborated on the microscopic physical\nmechanisms and supplemented supporting experimental evidence, yielding five key\nconclusions:\n(1) EMW induces the insulator-to-conductor transition primarily through localized\noxide layer breakdown and removal at metal microfiber interfaces within the EM\nmaterial.\n(2) The transient heat is generated under EMW exposure, which promotes intimate\ncontact and localized welding between conductive particles.\n(3) The micro-welding induced by EMW at the contact points of conductive fillers,\nthereby modifying the conductive network and significantly enhancing the electrical\nconductivity of the material.\n(4) The EMW effect is negatively correlated with the distance of the emission source.\n(5) A high E/τ of EMWs induces EM materials to generate a higher induced voltage,\nachieving a stronger EMW effect.\nThese enhancements substantially strengthen the evidentiary support for the\nmanuscript's central claims, resulting in significantly enhanced clarity regarding the\nphysicochemical mechanisms governing EMW-composite interactions.\nAdditional experiments and physical mechanisms during this revision\nMicroscopic\nExperiment Conclusion\nmechanisms of physics\nDetailed discussion about the mechanism of the\nBranly effect. EMW induces the\nRemoval of the oxide SEM and DES images insulator-to-conductor transition primarily\nlayer of oxide layer removal through localized oxide layer breakdown and\nremoval at metal microfiber interfaces within the\nEM material.\nThe mechanism for transient Joule heating is\nAn infrared camera is further investigated. The transient heat is\nThe generation of Joule\nused to observe the generated under EMW exposure, which promotes\nheat\ngeneration of Joule heat intimate contact and localized welding between\nconductive particles.\nThe deep understanding of the micro-welding\nSoldering at the contact\ninduced by EMW at the contact points of\nThe formation of points of tin (Sn)\nconductive fillers, thereby modifying the\ncontact point welding particles cured in\nconductive network and significantly enhancing\nEcoflex\nthe electrical conductivity of the material.\nEMWs are emitted at\nThe influence of The detailed distance dependence for the EMW\ndifferent distances from\nEMWs' distance on the effect: as the distance from the emission source\nthe sample, and the\nEMW effect increases, the effect attenuates exponentially.\nresistance is recorded\nMeasure the induced The theoretical relationship from Faraday's law\nThe relationship\nvoltage and resistance with the high E/τ (ratio of electric field intensity\nbetween the rising edge\nof the EM material by to rise time) to determine the high induced\nof EMWs and the\nchanging the frequency voltage intensity, thereby resulting in a stronger\ninduced voltage\nof the electric field EMW effect.\nThe work revitalizes the century-old Branly effect (1890s) in a contemporary\nmaterials science context, demonstrating its utility for optimizing conductive\nnetworks in polymer composites. This bridges historical physics with cutting-edge\napplications.\nWe have undertaken a comprehensive revision of the manuscript. This includes\nsubstantial revisions and additions to the introduction, discussion sections within the\nmain text, as well as the supporting information. These revisions deepen the\nexplanation and elaboration of the physical phenomena, significantly enhancing the\npaper's physical significance.\n2. The steel bead experiment is used to illustrate the principle of the EMW effect, as\nshow in Figure 2a-c. I suggest that the authors provide direct experimental results in\nthe microscopic system. Please provide more experimental details, such as the\nfrequency of EMW source on both microfiber composites and steel bead.\nResponse: We appreciate the reviewer's expert comment. To address the reviewer’s\nconcern, a copper foil, prepared through prolonged storage in air and moisture to have\nsufficient oxidation, was selected for the EMW experiment and SEM mapping\nstudy. By employing an EMW source positioned 5 mm from the foil to generate an\nelectric field, as illustrated, the surface oxides were eradicated (fig. S18A -\nB). Analysis of the EDS image (fig. S18) reveals that Cu and O elements are\nuniformly distributed on the untreated copper foil surface. Conversely, the treated,\nexposed copper foil exhibits a significantly denser concentration of Cu elements and a\nmarked reduction of O elements at its center. EDS analysis was thus employed to\nconfirm the elimination of the oxide layer.\nTo further investigate interfacial phenomena under electromagnetic stimulation, tin\nmicroparticles (Sn, Φ: 835 μm) were selected for their low melting point (232℃),\nenabling clear visualization of coalescence. When embedded in cured Ecoflex and\nexposed to EMWs (1 cm source distance), two Sn particles exhibited localized\ncoalescence at their contact junction (fig. SA). This confirms that conductive fillers\nwithin polymer matrices can undergo field-induced restructuring through localized\ncoalescence.\nAdditionally, we have provided further experimental details regarding the effect of\nEMWs on steel spheres in the supplementary information. To characterize the EMW\ninteraction, the waveform across two steel spheres was measured using an\noscilloscope probe attached to either side, as illustrated in fig. B. The measured\nwaveform (fig. S14) exhibits a rise time of approximately 4 ns.\nThe direct experimental results from microscopic systems have been incorporated into\nthe manuscript and are highlighted in yellow.\nFig. S18. Microstructural characterization of oxide layer removal from copper foil\nsurface under EMW treatment. The copper foil was preserved at 4°C and 60% RH for 6\nmonths until a uniform surface oxide occurred. An EMW source was positioned 5 mm from\nthe copper foil to generate an electric field. Surface morphology of the untreated region (A)\nand treated region (B) was characterized by SEM. (A , A ) show EDS elemental mapping of\n1 2\nregion (A), revealing homogeneous distribution of Cu and O elements. (B , B ) show EDS\n1 2\nelemental mapping of region (B), with the oxygen-depleted zone delineated by white dashed\ncircles. In the treated region (B , B ), the central area shows a significant reduction in O and a\n1 2\npronounced increase in Cu. The oxygen atomic percentage in region (C) is 50.01%, whereas\nin region (D) it is 10.32%. These results demonstrate that the electric field effectively\nremoved the oxide from the copper foil surface, indicating EMW treatment capability for\nremoving metal surface oxide layers. Scale bar: 50 µm.\nFig. A. Observation of coalescence at the interface between two Sn particles cured\nwithin Ecoflex via microscopy. Liquid components A and B of Ecoflex-0035 were\nuniformly mixed at a 1:1 ratio and poured into a Petri dish. Before complete curing, two Sn\nparticles were embedded within the Ecoflex mixture. The sample was then cured at room\ntemperature for 5 min. EMWs were irradiated at a distance of 2 cm from the Sn particles.\nMicroscopic imaging captured changes at the particle contact point before and after EMW\ntreatment. Compared to the initial state (A), pronounced coalescence between the two Sn\nparticles is evident following EMW treatment, as shown in (B). The right panel displays a\nmagnified view of the region indicated in the left panel. Scale bars: 200 μm and 500 μm.\nFig. B. Two contact steel beads are placed on a flat surface, and EMWs are generated\nusing the EFB generator. EMWs (< 1 µs) were emitted by the piezoelectric system (EFB\nand copper) from the two contact steel beads at 5 cm, and the photographs showed that a\ndistinctive insulator-conductor transition was immediately displayed in the two steel beads.\n(A) The initial non-conductive state of two contacting steel balls. (B) After the EMWs were\nemitted, the two contacting steel beads became conductive.\nFig. S14. The induced voltage at both ends of two contacting steel balls when EMWs are\nemitted. Transient EMWs were emitted at a distance of 5 cm from the steel beads. The design\nof the experimental setup is shown in fig. SC. Connect the oscilloscope to the copper wires on\nboth sides of the two steel beads and record the voltage at both ends. The measured waveform\nexhibits a rise time of approximately 4 ns.\n3. Does the distance between the EMW emission position and the sample location\naffect the conductive properties of the composite material? The study discusses\nscenarios with distances of 10 cm and 5 m, both of which can rapidly reduce the\nmaterial's resistance to 1 kΩ. Does this imply that 5 m is not the maximum effective\ncontrol distance?\nResponse: Thank the reviewer for the constructive comment. In response to\nreviewers' comments, we systematically investigated the influence of EMW\nsource-to-sample distance on the electrical conductivity of composites. All\nexperiments employed Cu microfiber-SIS composites (Φ: 22 μm, 30 wt%) positioned\nat varying distances from the EMW source. As depicted in fig. S23, the electrical\nresistance of the treated sample exhibits a gradual increase with rising distance from\nthe EMW source within 5 m. Beyond this threshold distance, further source distance\naugmentation results in a pronounced escalation of resistance. Consequently,\nincreased source distance directly correlates with enhanced electrical resistance and\ndiminished conductivity. This demonstrates that EMW source proximity critically\ngoverns the electrical properties of treated composites. In this work, optimizing EM\nmaterial design or implementing more powerful antenna systems may enable\noperational distances exceeding 5 m.\nAll additional data discussions have been highlighted in yellow in the main text and\nthe supporting information for clarity.\nFig. S23. The influence of the distance between the EM source and the sample on the\nresistance of the composite material. The Cu microfiber-SIS composites (Φ: 22 μm, 30 wt%)\nwith an initial resistance greater than 20 MΩ were positioned at different distances from the\nEMW source, respectively. The inset shows the zoomed-in area of the distance from 1 – 5 m\n(the unit for the y-axis is kΩ). Following the EMW emission, sample resistance was recorded\nusing a multimeter. Within 5 m, the electrical resistance of the EMW-treated sample increases\ngradually with increasing distance from the EMW source. Beyond this range, a substantial\nincrease in resistance is observed as the source-sample distance extends further. This\ndemonstrates an inverse relationship between the distance of the EMW source from the\ncomposite material and the composite's electrical conductivity.\n4. Is the microscopic image in the inset of Figure 2d (upper right corner) taken after\nEMW irradiation? Please label it accordingly, and consider adding a microscopic\nimage taken before EMW irradiation, as this would create a more pronounced contrast\nbetween the pre- and post-irradiation states.\nResponse: Thanks for your kind comment. The micrograph inset in Fig. 2d (upper\nright) illustrates the intrinsic microstructure of the EM material, confirming its\ncomposition of styrene-isoprene-styrene (SIS) polymer matrix and steel microfibers.\nThis inset describes the composite architecture before EMW treatment. Fig. 2d\nprimarily demonstrates long-distance EMW effect transmission. At the 5 m\nsource-sample distance, after EMW treatment, oxide breakdown occurs at steel\nmicrofiber contact points. Consequently, the conductive network reorganized without\nfiber displacement, with micro-welding occurring exclusively at contact points.\n5. P8, Please verify the statement: \"As shown in the inset of Fig. 2g, electric pulses\nfrom the signal generator provide different transient EM fields with varied amplitude\n(E) and rising time (τ). With the certain rising edge (τ) of the amplitude for EM fields\nat 500 ns, the R/R ratio is increased when increasing the amplitude of EMWs from\n0\ndescription. Additionally, the R/R ratio is observed to decrease (not increase) when\n0\nthe amplitude of EMWs is raised from 100 V/cm to 750 V/cm. This discrepancy\nrequires correction.\nResponse: We express our sincere appreciation to the reviewers for their meticulous\nassessment of our manuscript. Upon re-examination, we identified a typo in the\nmanuscript's description of Fig. 2g. The relevant sentence has been revised to:\n\" As shown in the inset of Fig. 2g, electric pulses from the signal generator provide\ndifferent transient EM fields with varied amplitude (E) and rising time (τ). With the\ncertain rising edge (τ) of the amplitude for EM fields at 500 ns, the R/R ratio is\n0\nreduced when increasing the amplitude of EMWs from 100 V/cm to 750 V/cm.\"\nThese corrections appear in yellow highlighting throughout the manuscript.\n6. The description exhibits excessive repetition. For instance, the term \"(steel\nmicrofiber-SIS composite)\" is used repeatedly throughout the manuscript. It is\nadvisable to provide a declaration at the beginning of the article and implement\nsimplification in the main text.\nResponse: Thanks for your reasonable suggestion. To simplify the description, we\nprovide a statement, \"In this work, unless specified, the EM materials refer to steel\nmicrofiber-SIS composite using SIS as a polymer matrix with 15 wt% of steel fibers.\"\nin the first paragraph of the Results and Discussion section in the main text.\nMeanwhile, we deleted the repeatedly used term \"steel microfiber-SIS composite\" in\nthe manuscript and modified it to \"EM material\".\nAll the corresponding descriptions in the main text have been changed and\nhighlighted in yellow.\nReviewer #2:\n1. The induced voltage during EMW exposure is stated to arise from electric fields\nwith high E/τ values, but what are the actual voltages generated across individual\nfiber junctions in the composites? Are these voltages directly measured or inferred?\nWhat modeling or simulation supports this?\nResponse: We appreciate the reviewers' constructive comments. The voltage can be\ndirectly measured by an oscilloscope to record the induced voltage value by EMWs.\nTo quantitatively measure the induced voltage intensity across the sample (1 cm × 1\ncm), it was divided into a uniform 6 × 6 grid. A probe from an oscilloscope was\nconnected to the sample's copper wire at one end, and positioned at the center of each\ngrid cell at the other end to record the peak voltage during EMWs emission. As shown\nin fig. S4, the induced voltages across different regions were approximately 20 V, with\nno significant variation observed. The content regarding the induced voltage in the\nsample has been added to the main text and supporting information, with the additions\nhighlighted in yellow.\nFig. S4. Distribution of induced voltage on steel fiber-SIS composite during EMWs.\n(A)The 1 cm × 1 cm sample of the steel microfiber-SIS composite was divided into a uniform\n6×6 grid. An oscilloscope probe was connected to the copper wire of the sample at one end,\nwhile the other end was positioned at the center of each grid cell to record the peak voltage\nacross different regions of the sample during EMWs emission. (B) The induced voltage\nmeasured approximately 20 V across all regions, with no significant variation observed.\n2. The authors claim that Joule heating from transient voltages induced by EMWs is\nresponsible for oxide breakdown and fiber cohering. However, is there any thermal\nimaging or time-resolved IR thermography to directly verify this localized heating?\nCould this be separated from general dielectric heating of the polymer?\nResponse: We sincerely appreciate the reviewer's insightful questions regarding the\nlocal Joule heating mechanism. Regarding whether the Joule heat generated by the\ntransient voltage induced by EMW is the direct cause of oxide breakdown and fiber\nadhesion, we fully understand the reviewers' concern that they expect to see direct\nthermal imaging evidence.\nThe most critical challenge lies in the extremely high required temporal and spatial\nresolution of IR camera. The response time of the conductivity change we observed\nwas only 0.34 μs (Fig. 2e). However, the frame rates of the currently most advanced\ncommercial high-speed infrared thermal imagers (such as FLIR X8500, Child window:\n29,134 Hz, 64 × 4 pixels) are typically in the range of tens of thousands of hertz, and\nthe single-frame acquisition time is in the order of 10 microseconds, which is far from\nsufficient to capture the sub-microsecond transient heating process. In addition, for\nthe local Joule heating, the size of the heated region on the microfibers (diameter: 12 -\n22 μm) is usually in the nanoscale (as shown in Fig. 2a, the nanoscale breakdown\nindentations). However, these most advanced IR thermal imagers usually have a\nresolution of over 10 μm. Furthermore, there are also significant obstacles in terms of\nspatial resolution. The polymer matrix (such as SIS rubber) has a strong absorption\nand scattering effect on infrared signals, significantly attenuating the thermal radiation\nsignals from deep layers or tiny contact points. As a result, the weak signals of the\ntarget hotspots are overwhelmed by background noise and the signals of the matrix.\nWe have tried different IR thermal imagers in our university. It is quite challenging to\nobserve the temperature difference within such a small region.\nWe acknowledge that, under the current technological conditions, there are significant\nchallenges in directly, in situ, and with high spatial and temporal resolution observing\nthe transient Joule heating process at the micro-nano level contact points within\ncomposite materials. In previous studies (Journal de Physique III, 1997, 7, 303-310),\nby directly visualizing (using infrared cameras) the conduction path when a single\nlayer of aluminum beads was subjected to high voltage, the phenomenon of\nconduction transition could be observed. Therefore, to simulate the Joule heating\nphenomenon by the transient induced voltage of EMW, we utilized a capacitor (3 μF)\nto apply a pulsed voltage (fig. SD , 80 V, 412 μs; fig. SD , 200V, 425 μs) directly at\n1 2\nboth ends of the composite material to enhance the phenomenon and facilitate\nobservation. As shown in fig. S16, as the voltage at both ends of the EM material\nincreases, the internal heat generated also significantly increases. It has been\nconfirmed that Joule heat is generated under the transient pulse voltage.\nThe conventional dielectric heating of polymers generates heat through molecular\nfriction, resulting in a uniform temperature rise throughout the material. The typical\ntimescale is on the order of seconds (Macromol. Rapid Commun. 2019, 40, 1800669;\nACS Appl. Mater. Interfaces 2022, 14, 50215−50224). In this work, the surface oxide\nlayer on the conductive filler is induced to break down by a high-rate electromagnetic\nfield, causing an increase in temperature at the contact points. The typical timescale is\non the order of nanoseconds.\nThe relevant content will be incorporated into the paper as supporting information and\nwill be highlighted in yellow.\nFig. S16. A transient voltage was applied across the EM material sample, inducing heat\ngeneration within it. Apply a transient voltage across the ends of the material using a\ncapacitor (3 μF). Different transient voltages (fig. SD , 80 V, 412 μs; fig. SD , 200V, 425 μs)\n1 2\nwere applied across the sample, and the resultant heat generation was monitored via an\ninfrared camera. As shown in (A-C), the infrared images revealed a progressive color shift\nfrom blue to red at the sample location with increasing applied voltage. Graphs (D ) and (D )\n1 2\ndisplay the voltage waveforms applied across the sample in (B) and (C), respectively,\nshowing a discharge duration of (D ) 412 μs and (D ) 425 μs. This indicates that higher\n1 2\nvoltages generate more heat within the sample. Scale bar: 5 mm.\n3. While the breakdown of surface oxides is a central claim, are there any XPS, TEM,\nor EDS analyses performed on the fiber surfaces before and after EMW treatment to\nconfirm oxide layer removal or alteration?\nResponse: We acknowledge the reviewers for their meticulous feedback. To address\nthe reviewer’s concern, a copper foil, prepared through prolonged storage in air, under\nmoisture to have sufficient oxidation, was selected for the EMW experiment and SEM\nmapping study. By employing an EMW source positioned 5 mm from the foil to\ngenerate an electric field, as illustrated, the surface oxides were completely\neradicated. Analysis of the EDS image X reveals that Cu and O elements are\nuniformly distributed on the untreated copper foil surface. Conversely, the treated,\nexposed copper foil exhibits a significantly denser concentration of Cu elements and a\nmarked reduction of O elements at its center. EDS analysis was thus employed to\nconfirm the elimination of the oxide layer.\nRevisions about oxide layer modifications have been incorporated into the main text\nand supporting documents, with all additions highlighted in yellow for visual\nidentification.\nFig. S18. Microstructural characterization of oxide layer removal from copper foil\nsurface under EMW treatment. The copper foil was preserved at 4°C and 60% RH for 6\nmonths until a uniform surface oxide occurred. An EMW source was positioned 5 mm from\nthe copper foil to generate an electric field. Surface morphology of the untreated region (A)\nand treated region (B) was characterized by SEM. (A , A ) show EDS elemental mapping of\n1 2\nregion (A), revealing homogeneous distribution of Cu and O elements. (B , B ) show EDS\n1 2\nelemental mapping of region (B), with the oxygen-depleted zone delineated by white dashed\ncircles. In the treated region (B , B ), the central area shows a significant reduction in O and a\n1 2\npronounced increase in Cu. The oxygen atomic percentage in region (C) is 50.01%, whereas\nin region (D) it is 10.32%. These results demonstrate that the electric field effectively\nremoved the oxide from the copper foil surface, indicating EMW treatment capability for\nremoving metal surface oxide layers. Scale bar: 50 µm.\n4. The setup employs parallel plates or localized EFB emitters. How uniform is the\nfield within the composite? Could non-uniformity cause uneven cohering, creating\nhotspots or weak regions in the percolation network? Are cross-sectional conductivity\nmaps or tomographic images available?\nResponse: We thank the reviewers for their careful reading and insightful comments.\nThe parallel plates used in this study generated a uniform electric field. As illustrated\nin fig. S26, a sinusoidal voltage was applied across the parallel plates. The sample\nwas positioned at various locations within the interplate region, and the peak induced\nvoltage signal across the sample was recorded using an oscilloscope. Fig. S27\ndemonstrates that an identical voltage signal was generated by the sample at all\npositions, confirming electric field uniformity within the central region of the parallel\nplates.\nEM material (3 cm × 3 cm × 1 mm) resistivity was measured using a precision\nfour-point probe resistivity measurement system. As shown in fig. S46A, the probe\nwas positioned at the center of each subgrid square for measurement. Fig. S46B\nshows that the electrical conductivity values measured at the center of each subgrid\nsquare were closely matched. Consequently, at the macroscopic level, the conductive\nnetwork of the material exhibits no hot spots or weak zones.\nActually, the homogeneity of the composite materials is more critical to controlling\nthe homogeneity of the conductivity distribution under EMW treatment. In fig. C, we\nutilized a microscope to observe the distribution of the contact area on the sample\nsurface. The white dashed circles denote areas with higher contact point density, while\norange-yellow dashed circles indicate regions with lower contact point density. Within\nthe composite material, the distribution of steel fiber contact points is not perfectly\nuniform. Consequently, EMW effects are more likely to occur in regions of higher\ncontact point density and less likely to occur in regions of lower density. Therefore, at\nthe microscopic level, the electrical conductivity of the material is not entirely\nuniform, as its homogeneity is determined by the conductive filler uniformity\nresulting from the contact point distribution.\nThe relevant experimental results have been added to the main text and supporting\ninformation, with the additions highlighted in yellow.\nFig. S27. The sample generates identical induced voltages at different positions within\nthe central region of the parallel copper plates. The waveform generator was connected to\nan amplifier. The signal from the waveform generator was modulated as a sine wave, with a\nhigh level of 3 V, a low level of -3 V, and a frequency of 500 kHz. The amplified signal was\nconnected with wires to two parallel copper plates (8 cm × 10 cm). (A) The sinusoidal voltage\n(the amplified signal was connected with wires to two parallel copper plates) was applied\nacross the parallel plates, and the sample was positioned at distinct locations (1, 2, 3, ...,35)\nwithin the plates. (B) The voltage across the sample was recorded using an oscilloscope. A\nplot was constructed using the peak induced voltage values at each position, revealing\nvirtually identical voltage magnitudes (4.4 V - 4.56 V) across all measurement locations. This\nthus demonstrates that the field generated by the parallel plates is uniform.\nFig. S46. Measurement of electrical conductivity in EM materials via the precision\nfour-point probe method. To measure the local electrical conductivity of the steel fiber-SIS\ncomposite material, the larger EM material sample (3 cm × 3 cm × 1 mm) was subdivided\ninto a 3 × 3 grid array. (A) The probe of the precision four-point probe resistivity\nmeasurement system was positioned at the center of each subgrid square for measurement. (B)\nThe electrical conductivity values measured at the center of each subgrid square were closely\nmatched, with values ranging from 0.7266 to 0.9385 S/m. Macroscopically, this indicates the\nabsence of hot spots or weak zones within the conductive network structure of the material.\nScale bar: 5 mm.\nFig. C. Distribution of steel microfibers within the composite material. The composite\nconsists of steel fibers (316L, Φ: 12 μm) and SIS. White dashed circles denote areas with\nhigher contact point density, while orange-yellow dashed circles indicate regions with lower\ncontact point density. Due to the random formation of the percolating network by the steel\nfibers, contact points of the conductive filler exhibit an imperfectly homogeneous distribution\nwithin the material. Therefore, at the microscopic level, the electrical conductivity of the\nmaterial is not entirely uniform, and the uniformity of the material determines the uniformity\nof its electrical conductivity. Scale bar: 100 μm.\n5. The response time of 0.34 µs is very fast. How was this time constant\nextracted—was it limited by the RC time of the measurement circuit or the intrinsic\nmaterial switching? Can faster switching be achieved by optimizing filler distribution\nor polymer viscosity?\nResponse: We appreciate the reviewers' professional comments. To address the\nreviewer’s concern, we measured the switching times for different conductive fillers\nwithin a polymer matrix using the same methodology to see whether we could\nimprove the resistance switching time. The EM material was connected in series with\na fixed resistor across a DC power supply (4.5 V), and an oscilloscope captured the\nvoltage transition across the fixed resistor (5 MΩ) upon EMW emission. As shown\nin fig. S24, the resistance switching time was 0.35 μs for the steel fiber-EVA composite\nand 0.25 μs for the Cu fiber-EVA composite. Switching to a different conductive filler\ncould achieve faster transition time. Measurements within the same circuit revealed\ndistinct switching times for different conductive fillers, indicating that the measured\ntime is influenced by the intrinsic switching characteristics of the material (types of\nmetal and polymer matrix).\nThe content regarding resistance switching time has now been incorporated into the\nmain text and supporting literature, with all additions highlighted in yellow for clarity.\nFig. S24. The resistance of the EM material was monitored in real-time before and after\nthe emission of the EMW. The measurement configuration is shown in Fig. S18. The EM\nmaterial was connected in series with a fixed resistor across a DC power supply (4.5 V), and\nan oscilloscope captured the voltage transition across the fixed resistor (5 MΩ) upon EMW\nemission. The resistance transient of the (A) steel microfiber-EVA composite and (B) Cu\nmicrofiber-EVA composite during EMW emission. An inset on the dashed curve provides a\nmagnified view showing the change in time of (A) 0.35 µs and (B) 0.25 µs. This demonstrates\nthat varying the type of conductive filler and polymer matrix enables the modulation of\ntransition time, thereby facilitating faster or slower resistance transitions.\n6. The resistance recovery upon strain is attributed to the mechanical separation of\ncohered fibers. Has this been confirmed by in-situ SEM or micro-CT under strain?\nAre cracks formed in the matrix or is the disconnection occurring only at the\nfiber-fiber contact?\nResponse: We thank the reviewers for their insightful comments. The suggestions\nprovided are highly professional and effectively highlight areas where additional\ndetail regarding the resistance recovery process would be beneficial. However, SEM\nand micro-CT facilities in Nanjing and surrounding cities lack the capability for the\nsimultaneous application of force during sample testing (in situ force application).\nGiven that the deformation of the sample occurs on the microscale, it is readily\nobservable using microscopy.\nConsequently, to further elucidate the details of the material's recovery to an\ninsulating state following deformation, we have supplemented the study with in situ\nmicrostructural analysis. The 15 wt% steel microfiber-SIS composite, pretreated with\nEMW, was examined microscopically after the application of 25% tensile strain. As\nshown in fig. D(B), under 25% strain, obvious increases in the angle between\ncontacting fibers, aligned along the loading axis, are evident, indicating pronounced\ndeformation. Crucially, the polymer matrix maintained its structural integrity\nthroughout this transition, with no observable cracking. The composite's electrical\nresistance recovered to a non-conductive state.\nThese findings demonstrate the mechanism by which the strain-induced mechanical\nseparation of interconnected fibers leads to an increase in the composite's electrical\nconductivity. The relevant content has been incorporated throughout the manuscript\nand supporting information, with all additions highlighted in yellow.\nFig. D. After EMW treatment, the sample's resistance returned to non-conductivity due\nto the structural change of the fiber contact points after strain. Following EMW\ntreatment, the steel microfiber-SIS composite (15 wt%, fiber diameter: 12 μm) was examined\nmicroscopically while concurrently monitoring electrical resistance via a multimeter.\nMicroscope image (A) displays the pre-strain microstructure, corresponding to the\ncomposite's highly conductive state. In situ, tensile deformation was then applied to 25%\nstrain. Microscope image (B) shows structural reorganization at the adhered fiber connections,\nwhile the polymer matrix remains crack-free. The white dashed lines indicate the trajectories\nof the two contacting fibers, revealing an obvious increase in the contact angle following\ndeformation. The resistance of the composite material becomes a non-conductive state (R >\nstructural change caused by the strain at the contact points of the conductive filler disrupts the\npermeation path. Scale bar: 50 μm.\n7. The paper shows >10,000 switching cycles, but is there any degradation in\npeak-to-peak resistance or gradual baseline drift? Has the endurance been tested under\ndifferent strain amplitudes or frequencies? This is critical for realistic flexible\nelectronics applications.\nResponse: We thank the reviewer for their expert and insightful comments. We have\nzoomed in on the 2,000–10,000 cycle segment of Fig. 3e (10,400 total cycles) to\nenhance the clarity of the baseline, supplemented with additional details and data\nanalysis. As evidenced by Fig. 3e and fig. S34, the composite material exhibits\nreversible resistance switching between high- and low-resistance states across\nmultiple cycles, with no observable degradation in peak-to-peak resistance or baseline.\nThe composite demonstrates robust cycling stability in repeated switching operations.\nAdditionally, we have supplemented durability testing under varying strain amplitudes\nand frequencies across 10,000 cycles, while maintaining identical conditions for all\nother experimental parameters. As depicted in the control signal diagram for the\nmicro-vibrator (fig. S33B), the operational period was extended to 3 s while reducing\nthe voltage amplitude of the micro-vibrator. Concurrently, the control period for the\nEMW signal was prolonged to 3 s. The resistance switching characteristics of the EM\nmaterial—modulated by EMWs and the micro-vibrator—consistently maintained a\ntransition range of ~1 kΩ to > 5 MΩ over 10,000 cycles (fig. S35), demonstrating its\npotential for flexible electronics applications.\nData about the cycling stability and broad applicability of the EM material have been\nincorporated into the main text and supporting information, with additions highlighted\nin yellow.\nFig. S34. The baseline of over 10,000 on/off cycles (Fig. 3e) for the EM material under\nEMWs and a micro-vibrator. The parallel plate and micro-vibrator were independently\ncontrolled via two distinct electrical signals. A variable electric field, applied by the parallel\nplate, induced a decrease in the resistance of the EM material. Its control signal is modulated\ninto a pulse wave with a high level of 4 V, a low level of 0 V, a period of 1 s, and a duty cycle\nof 0.02%. Concurrently, the mechanical force provided by the micro-vibrator restored the\nmaterial's resistance to its high-resistance state. Its signal is modulated into a trapezoidal\nwaveform with a high level of 5 V, a low level of 0 V, a period of 1 s, a duty cycle of 10%,\nand a phase of 144° (fig. S33). Fig. S34 presents a magnified segment of the 10,000 cycles\nfrom Fig. 3e. The absence of baseline drift demonstrates the exceptional stability and\nreusability of this EM material for flexible electronics applications.\nFig. S33. The square wave for controlling the micro-vibrator (fig. S32). The\nmicro-vibrator is connected to the CH1 channel of a waveform generator. A continuous\nsquare wave with an adjustable duty cycle is applied to the micro-vibrator. The micro-vibrator\nis active during high-level periods and inactive during low-level periods. This square wave is\nused to exert cyclic control over both the micro-vibrator and the metal microfiber-polymer\ncomposite material. (A) The signal output from the CH1 channel is a square wave with a high\nlevel of 5 V, a low level of 0 V, a period of 1 s, a duty cycle of 10%, and a phase of 144°. As\nshown in Fig. 3e, this signal controls the EM material for 10,400 on/off measurement cycles.\n(B) The signal output from the CH1 channel is a square wave with a high level of 3.5 V,\na low level of 0 V, a period of 3 s, a duty cycle of 10%, and a phase of 144°. As shown in fig.\nS35, this signal controls the EM material for 10,100 on/off measurement cycles.\nFig. S35. The 10,100 cycles of the on/off measurement of the EM material under EMWs\nand the micro-vibrator. The parallel plate and the micro-vibrator are controlled\nindependently via two distinct electrical signals. The variable electric field applied by the\nparallel plate induces a reduction in the electrical resistance of the EM material. Its control\nsignal is modulated into a pulse wave with a high level of 4 V, a low level of 0 V, a period of\n3 s, and a duty cycle of 0.02%. Simultaneously, the mechanical force provided by the\nmicro-vibrator restores the material's resistance to a high-resistance state. Its signal is\nmodulated into a trapezoidal waveform with a high level of 3.5 V, a low level of 0 V, a period of\n3 s, a duty cycle of 10%, and a phase of 144° (fig. S33B). A total of 10,100 cycles with a\nperiod of 3 s and lighter vibration amplitudes demonstrate the versatility of the EM material\nfor flexible electronics applications.\n8. The memory behavior with multiple states (Fig. 3g) shows discrete resistance steps.\nIs this scaling linear, logarithmic, or stochastic with respect to EMW amplitude or\nenergy? Does this show any hysteresis or write fatigue over multiple cycles?\nResponse: We thank the reviewers for their helpful comments. To analyze the\nrelationship between the resistance steps and the EMW amplitude, fig. S36A has been\nadded, plotting R/R on the ordinate against the amplitudes corresponding to the six\n0\nsteps in Fig. 3g on the abscissa. As shown in this figure, the resistance steps (R/R )\n0\nexhibit a logarithmic decrease with increasing EMW amplitude. These steps remain\nnearly constant over multiple cycles (fig. S36B), showing no signs of hysteresis or\nwrite fatigue. This stability is related to the extremely rapid and sensitive transition\nspeed within the conductive network, coupled with the easy reversibility of the\nresistance changes.\nAdditional information regarding the multi-state resistance steps has been added to\nthe main text and supporting information, with the additions highlighted in yellow.\nFig. S36. The material exhibits cycling of memory behavior featuring multiple resistance\nstates, induced by the EMW effect. (A) The resistance change (R/R ) – amplitude (E) curve\n0\nfor the EM material under different amplitudes (0 - 600 V/cm) of dynamic fields. (B) The\nresistance (R/R ) is controlled by the amplitude of the EMWs (each stage corresponding to a\n0\ncertain amplitude, rising time: 150 ns). Five cycles of the measurements were performed.\n9. The work briefly shows various fillers (CNT, graphene, Cu particles), but how does\nfiller aspect ratio, conductivity, and dispersion affect the EMW-induced transition?\nAre there statistical analyses comparing percolation thresholds and resistance change\nmagnitudes across these systems?\nResponse: We thank the reviewer for their insightful comments.\nTo determine the influence of filler aspect ratio on the EMW-induced transition, the\npercolation threshold diagrams of 12 µm steel fiber-SIS composites and 22 µm steel\nfiber-SIS composites were compared in fig. S47A (reproduced from Fig. 4f, Main\nText) and fig. S47B. The 12 µm steel fiber-SIS composite exhibited a lower\npercolation threshold (15 wt% vs. 25 wt%) than its 22 µm counterpart, attributable to\nits higher aspect ratio. However, the magnitude of |ΔR/R | remained comparable,\n0\nindicating that the enhancement in conductivity induced by EMW is not governed by\nthe aspect ratio of the conductive filler.\nTo evaluate the effect of filler conductivity, the threshold diagrams of 22 µm steel\nfiber-SIS composites and 22 µm copper fiber-SIS composites were compared in fig.\nS47B and fig. S47C. The copper fiber-SIS composite showed a higher threshold than\nthe steel fiber composite (30 wt% vs. 25 wt%), despite copper’s superior intrinsic\nconductivity (5.96×10⁷ S/m vs. 1.40×10⁶ S/m). This demonstrates that filler\nconductivity exerts a negligible influence on both the percolation threshold and\n|ΔR/R |.\nhomogeneity of the composite materials is more critical to control the homogeneity of\nthe conductivity distribution under EMW treatment (fig. S46 and fig.C). As shown in\nfig. C, when the filler distribution is less uniform, the overall conductivity of the EM\nmaterial is more non-uniform. As indicated in fig. S47, due to the higher degree of\nconductive filler aggregation, the resistance after EMW treatment is smaller (89 Ω).\nWhereas, when the filler distribution is more uniform, the overall conductivity of the\nEM material is more uniform; the resistance after EMW treatment is larger (862 Ω).\nTherefore, the dispersion of conductive fillers influences the EMW effect from two\naspects: the uniformity of the material resistance and the magnitude of the resistance\nafter EMW treatment.\nAll the data and discussions regarding the influencing factors of all relevant EMW\neffects are included in the main text and supplementary information sections. New\ncontent is highlighted in yellow font.\nFig. S47. The change in resistance of the Cu fibers-SIS composites and steel fibers-SIS\ncomposites with different mass percentages of the steel microfibers, with or without the\npresence of EMW. Composite materials were prepared by blending steel fibers (Φ: (A) 12\nμm [reproduced from Fig. 4f, Main Text] and (B) 22 μm) and (C) Cu fibers (Φ: 22 μm) of\ndifferent mass fractions with SIS, respectively. The resistance of the composites was recorded\nwith different mass fractions of the steel fibers with 12/22 μm diameters in the presence or\nabsence of EMW. With EMW, the conductivity threshold of the composites was 15/25 wt%\nof the steel fibers' mass fraction. Without EMW, the conductive threshold of the composite\nwas 45/50 wt% of the steel fibers' mass fraction. There was a difference in the conductivity\nthreshold between the two cases. The resistance of the composites was recorded with different\nmass fractions of the Cu fibers in the presence or absence of EMW. With EMW, the\nconductivity threshold of the composites was 30 wt% of the Cu fiber mass fraction.\nWithout EMW, the conductive threshold of the composite was 60 wt% of the Cu fiber mass\nfraction. There was a difference in the conductivity threshold between the two cases. The steel\nfibers with a diameter of 12 μm have a large length-to-diameter ratio and a relatively small\nseepage threshold. The EMW treatment showed no significant difference in the\nlength-to-diameter ratio of conductive fillers and the resistance variation amplitude of fillers\nwith different electrical conductivities.\nFig. S45. Uniformly dispersed and non-uniformly dispersed samples (steel\nmicrofiber-SIS composites) exhibited EMW effects. Reducing the shear-mixing time from\n5 min to 3 min resulted in non-uniform dispersion of the fibers within the polymer matrix,\nincreasing the aggregation of metal fibers. Both the uniformly dispersed sample and the\nnon-uniformly dispersed sample were placed at the same location, and their resistance\nchanges were recorded using a Keysight instrument. EMW (source: EFB generator or\npiezoelectric igniter; power: 10⁻⁵ - 10⁻⁷ mW; rise time: 5 - 8 ns) was emitted near both\nsamples (distance: 5 cm) to initiate the EMW effect. Microscopy images show the internal\nstructure of the uniformly dispersed sample and the non-uniformly dispersed sample. Scale\nbar: 500 µm.\n10. The paper states that 10⁻⁶ mW EMW power is sufficient to trigger transitions. But\nwhat is the total energy input per activation event (considering pulse width, capacitor\ndischarge, and delivery losses)? How does this compare to thermal sintering, photonic\ncuring, or microwave treatments in terms of J/cm³?\nResponse: Thanks for the reviewer’s very insightful comment. The power of the\nEMW emitted by the piezoelectric igniter or an electrical fast burst (EFB) generator,\nas measured with a spectrum analyzer, was 10⁻⁷ mW (-66 dBm, fig. S5). To address\nthe reviewer's concern, we examined the internal circuit of the EFB in detail. The\nelectrostatic generator was set to an output voltage of 1 kV, and its discharge circuit\nemployed a capacitor of 150 pF. The total energy delivered per discharge event is\ncalculated using the formula for the energy stored in a capacitor:𝐸 = 1 𝐶𝑉2.\n2 2\nIf all the energy is applied to the sample, considering the volume of the sample\n(𝑉 = 10 𝑚𝑚×10 𝑚𝑚×1 𝑚𝑚 = 0.1 𝑐𝑚3), the corresponding energy density is\n𝐸 = 7.5×10−5𝐽 = 7.5×10−4𝐽 𝑐𝑚⁻³ . The EMW-assisted cohering strategy\n𝑉 0.1 𝑐𝑚3\ndemonstrates exceptional energy efficiency, operating at approximately 7.5 × 10⁻4\nJ/cm³, which is 3–5 orders of magnitude lower than conventional methods: thermal\nsintering typically requires 30–100 J/cm³ due to prolonged high-temperature\nprocessing; photonic curing consumes 0.5–5 J/cm³ despite millisecond pulses; and\nmicrowave treatments expend 1– 10 J/cm³ owing to bulk heating inefficiencies. This\nultra-low energy density arises from the special mechanism: weak EMWs (10⁻⁶ mW)\nselectively break oxide barriers at particle contacts, inducing micro-welding that\nreconfigure percolation pathways. The traditional methods usually require overall\nheating for processing, thus demanding higher energy density.\nThe relevant energy calculations have now been incorporated into the main text and\nsupporting information, and are highlighted for clarity.\n11. How do you differentiate between the effects of EMW-induced electrical\nbreakdown and electrostatic discharge or capacitive coupling artifacts?\nResponse: We appreciate the reviewers' professional comments. In this work, we have\nalso considered and addressed the potential influence of artifacts arising from\nelectrostatic discharge (ESD) or capacitive coupling.\nTo avoid the effects of ESD, the sample was grounded during EMW emission. The\nresults showed that the EMW effects persisted. Crucially, the EMW effects were\nobserved both when the sample was grounded and when it was not, effectively ruling\nout ESD as a contributing factor.\nTo avoid artifacts from capacitive coupling, the experimental design of the\nparallel-plate setup (Figs. 2g - 2h) incorporated a specific protocol: resistance\nmeasurements were conducted only after the dynamic electromagnetic field was\nswitched off. This approach specifically prevents measurement errors caused by\ncurrents induced through capacitive coupling. As demonstrated in fig. S25, the\nmaterial maintains its low-resistance state even after the EMW source is deactivated.\nThis observation provides additional evidence that the results are not artifacts of\ncapacitive coupling.\nRelevant discussions have been added to the main text and highlighted in yellow.\n12. To what extent has the electric field strength induced across microfibers been\nquantitatively verified or modeled (e.g., electric potential gradient, capacitive\ncoupling, or field enhancement at fiber tips)?\nResponse: We thank the reviewer for the insightful comments regarding electric field\nstrength. Simulations were performed in COMSOL Multiphysics to analyze the\nelectric field distribution across copper fibers under the induced voltage. The model\nassumed two parallel copper fibers (diameter: 12 μm) with negligible bulk resistance\n(σ ≈ 10⁷ S/m), where the interfacial oxide layer (0.1-1 μm thickness) dominated the\nimpedance distribution. An induced voltage of 20 V was applied across the fiber pair.\nAs shown in fig. S6A, the voltage drop primarily occurred within the oxide region.\nFig. S6B demonstrates that with a 1-μm-thick oxide layer, the inter-fiber field strength\nreached 2.2×107 V/m, exceeding the dielectric breakdown threshold of copper oxide\n(2 - 5×107 V/m, Appl. Phys. Rev. 2022, 9, 01131). Consequently, EMW exposure can\ninduce sufficient voltage between adjacent metal fibers to generate extreme electric\nfields that rupture the metallic oxide layer, thereby altering electrical conductivity.\nFig. S6. The distribution of the electric field on metal fibers under induced voltage,\nsimulated using COMSOL Multiphysics. The model assumed two parallel Cu fibers\n(diameter: 12 μm) with negligible bulk resistance (σ ≈ 10⁷ S/m), where the interfacial oxide\nlayer (0.1-1 μm thickness) dominated the impedance distribution. An induced voltage of 20 V\nwas applied across the fiber pair. (A) The voltage drop primarily occurred within the oxide\nregion, demonstrating that with a 1-μm-thick oxide layer, the inter-fiber field (B) strength\nreached 2.2×107 V/m, exceeding the dielectric breakdown threshold of copper oxide.\nConsequently, EMW exposure can induce sufficient voltage between adjacent metal fibers to\ngenerate extreme electric fields that rupture the metallic oxide layer, thereby altering\nelectrical conductivity.\n13. Can the mechanical or chemical stability of the “cohered” microfiller networks be\nvalidated over time, temperature cycling, or mechanical strain, especially in the\n“permanent” state?\nResponse: We thank the reviewers for their valuable suggestions. To demonstrate the\nstability of the composite after treatment, we evaluated its performance across three\nparameters: thermal cycling, mechanical stress/strain, and time.\nIn our supporting information (fig. S39) showed that after 8 weeks, no resistance\nchange was observed. To further prove the stability over temperature and strain from\nthe reviewer, we performed additional experiments during the revision. As shown in\nfig. S40, the treated composite maintains a stable low-resistance state (< 10 Ω)\nthroughout the temperature range of 25°C to 120°C. Fig. S41 demonstrates that under\na tensile strain of 24.5%, the treated composite retains a low-resistance state below 1\nkΩ.\nThese experimental validations of conductivity stability have been incorporated into\nthe revised manuscript and supporting information, with additions highlighted in\nyellow.\nFig. S39. Temporal stability of the composite material treated with the combined\napproach under low-resistance state. EM materials (steel fiber-SIS composites) were\ntreated using a combined strategy of EMWs and transient electrical current. The resistance\nacross the composite was measured with a multimeter over a period of 60 days, exhibiting no\nsignificant change. This demonstrates that the conductive network cohered by the fused\nmicrofillers remains stable after prolonged aging following treatment with the combined\nstrategy.\nFig. S40. The stability of electrical conductivity of EM materials with different\ntreatments to temperatures. EM materials (steel fiber-SIS composites) with an initial\nresistance >1 MΩ were divided into two groups: a group subjected to EMW treatment to\nachieve a reversible low-resistance state, and a group receiving a combined strategy of EMW\nand transient voltage treatment to achieve a stable low-resistance state. As the temperature\nincreased, the control group consistently exhibited resistance exceeding 1 MΩ. (A) The\nEMW-treated composites maintained a low-resistance state (< 0.01 MΩ) within the 25°C to\n80°C range. However, upon further temperature increase to 90-100°C, their resistance rapidly\nincreased to values greater than 1 MΩ, and remained above 1 MΩ within the 100°C to 120°C\nrange. (B) In contrast, the comprehensively treated composites retained a low-resistance state\n(<10 Ω) throughout the entire 25°C to 120°C range, with resistance showing only a slight\nincrease as the temperature rose. Furthermore, samples from the EMW-treated group\nexhibited no significant change in resistance during cooling from 120°C to 25°C. Notably,\nwhen subjected to a subsequent EMW treatment at 25°C, a significant reduction in resistance\nwas observed. This indicates that EMW treatment remains effective in restoring the\nlow-resistance state after the material undergoes thermal cycling.\nFig. S41. Stress-strain stability of the low-resistance state in composites treated with the\ncombined approach. The steel fiber-SIS composite was treated using the combined strategy\nof EMWs and transient electrical current. Samples stabilized in the low-resistance state were\nmounted on a tensile testing machine for stress-strain testing, while\nsimultaneously connecting both ends of the samples to a Keysight instrument to record\nresistance changes during stretching. Following the combined treatment, the composite\nmaintained a low resistance state (below 1 kΩ) under a tensile strain of 24.5%.\n14. What are the potential limitations (e.g., shielding issues, spatial resolution, EM\ninterference) of using EMWs as a remote and selective tool for device fabrication or\nactivation in complex circuits?\nResponse: We thank the reviewer for raising these critical limitations. When\nimplementing EMW technology as a remote selective tool in complex circuits, key\nconstraints include:\n(1) Electromagnetic Shielding: Metallic enclosures or PCB copper layers (> 35 μm)\nattenuate EM fields, acting as de facto Faraday cages. Mitigation: Localized\nRF-transparent substrates (e.g., alumina ceramics) may reduce attenuation but require\nthermal/mechanical validation.\n(2) Spatial resolution: Since the EMWs generated by the EMW source are spatially\ndiffuse, it does not have spatial selectivity when acting on EM materials.\n(3) Electromagnetic Interference (EMI): Fast-rise pulses (5–8 ns) exhibit significant\ncoupling to adjacent traces, risking digital switching errors and analog SNR\ndegradation. Mitigation: Localized shielding (Mu-metal enclosures) reduced crosstalk\nin tests but increased form factor complexity.\nThese limitations and mitigation analyses have been incorporated into the manuscript,\nwith revisions highlighted in yellow.\nReviewer #3 (Remarks to the Author):\n1: \"While the resistance reduction from 50 MΩ to 1 kΩ is impressive, the final\nresistance value of 1 kΩ remains too high for most practical electrode applications.\nTypical conductive electrodes or inks require resistance values in the range of 1-100\nΩ/sq. The authors should address how this technology can achieve lower resistance\nvalues suitable for real-world applications or identify specific applications where 1\nkΩ resistance is acceptable.\"\nResponse: We appreciate the reviewer's comments. In this work, steel fiber-SIS\ncomposites were selected as representative examples to demonstrate the potential\napplication value of the EMW strategy in flexible electronics through a simple\nstructure, while simultaneously proving its broad applicability. As stated in the main\ntext, for composites containing 15 wt% steel fibers, the electrical resistance decreased\nfrom 50 MΩ to 1 kΩ after EMW treatment. The final resistance further decreases with\nincreasing filler loading; for instance, when the mass fraction is increased to 25 wt%,\nthe final resistance can reach below 100 Ω.\nFurthermore, to enhance suitability for practical applications, alternative fillers can be\nemployed. We have supplemented the study with experiments documenting the\nresistance changes in conductive silver copper paste subjected to EMW treatment.\nDue to the susceptibility of copper particles to oxidation, the initial paste exhibits\nextremely low conductivity. Following EMW treatment, its final conductivity\nincreases to 8.4 × 10³ S/m, corresponding to a sheet resistance of 2 Ω/sq (fig. S51).\nWe have supplemented both the main text and the supporting information with\nadditional content. All modifications are highlighted in yellow.\nFig. S51. Calculation of the electrical conductivity of 65 wt% silver-copper particle-resin\ncomposites after combined treatment. (A) The initial electrical resistance of the conductive\nsilver-copper paste was extremely high (> 20 MΩ). (B) Following EMW combined treatment,\nthe resistance of the conductive silver-copper paste decreased to 48.9 Ω. The length (C),\nwidth (D), and average thickness (E -E ) of the conductive silver-copper paste specimen were\n1 2\n7.6 cm, 3.1 mm, and 0.06 mm, respectively. The electrical conductivity (σ) of the silver and\nCu particle-resin composites was calculated using the formula for volume conductivity: σ =\n1/ρ = L / (R × A), where L is the length, R is the resistance, and A is the cross-sectional area.\n𝟏 𝟕.𝟔∗𝟏𝟎−𝟐 𝐦\nThus: 𝛔= = ≈ 𝟖𝟑𝟓𝟓.𝟖𝟕 𝐒/𝐦.\n𝛒 𝟒𝟖.𝟗 𝛀∗𝟑.𝟏∗𝟏𝟎−𝟑 𝐦∗𝟎.𝟎𝟔∗𝟏𝟎−𝟑 𝐦\n2: \"The mechanism of surface oxide breakdown and joule heating induced by EMWs\nis not sufficiently explained. How exactly do weak EMWs (10⁻⁶ mW) generate\nenough energy to break down oxide layers? What is the theoretical basis for the E/τ\nratio dependency, and why is this parameter critical for the EMW effect?\"\nResponse: We would like to express our gratitude to the reviewer for the comments.\nThe core mechanism by which weak EMWs (10⁻⁶ mW) disrupt oxide layers lies in\nthe spatiotemporal concentration of energy at nanoscale contact points: Nanoscale\nprotrusions on metal microfiber surfaces (with curvature radii ≈ 50 nm) compress the\nactual contact area to 10⁻¹²–10⁻¹⁰ m². This enables geometric field enhancement (β ≈\n10³–10⁴) to locally amplify weak external fields (e.g., 20 V/cm) beyond 2×107 V/m\n(Chem. Commun. 2025, 61, 9343-9358; Appl. Phys. Lett. 2006. 88. 104101).\nConcurrently, the nanosecond-scale transient nature of EMWs (rise time τ ≈ 5 ns)\ngenerates an induced voltage gradient (dV/dt ∝ E/τ) that produces instantaneous\npower densities up to 10¹⁰ W/m³ at contact gaps. This value far exceeds the dielectric\nbreakdown threshold of the oxide layer. The resulting micro-arcs trigger localized\nJoule heating within thermally confined zones (< 100 nm), where suppressed thermal\ndiffusion permits temperatures to surge transiently high, thereby achieving molten\nwelding at metallic contact points.\nThe ratio E/τ (electric field strength to perturbation time window) is critically\nimportant for the EMW effect, as demonstrated both theoretically and experimentally.\nClassical Electromagnetism Perspective:\nFaraday's law states: 𝜀 = −\n𝑑𝛷𝐵\n≈ A\n𝑑𝐵\n𝑑𝑡 𝑑𝑡\n𝑑𝐵\nWhere A is the conductor's effective area and is the magnetic field change rate,\n𝑑𝑡\n𝑑𝐸\nproportional to the electric field change rate (since B = E/c).\n𝑑𝑡\n𝑑𝐸 △𝐸 𝐸\nFor steep-rising EM pulses: ≈ ≈\n𝑑𝑡 △𝑡 τ\n𝐸\nSubstituting into Faraday's law yields 𝜀 ∝ .\nτ\nExperimental validation using parallel plates under sinusoidal voltage confirms that\nhigher frequencies (fig. S30B, 500 kHz) generate sufficient 𝜀 to induce the EMW\neffect, whereas low-frequency AC (fig. S30A, 50 Hz) or DC fields fail (fig.\n𝐸\nS29)—directly evidencing the critical role of .\nτ\nThe critical importance of E/τ for the EMW effect has been added to the main text\nand supporting information, with additions highlighted in yellow for emphasis.\nFig. S30. EMW effect versus induced voltage in samples under dynamic electric fields.\nSimilar to the setup of Fig. 2f, which provided a dynamic parallel electric field, the signal\ngenerator was connected to two copper plates with a spacing of 2 mm. In addition, the sample\nwas placed between the two copper plates, and the voltage across the sample was recorded\nwith an oscilloscope. Two sinusoidal voltages of (A) 90 V, 50 Hz and (B) 90 V, 500 kHz\nwere applied at the ends of the two copper plates, and the induced voltages at the ends of the\nsample were recorded. (C) A dynamic parallel electric field was provided with the same\ndevice as above, and the sample ends were connected to a multimeter to record the resistance.\nAccording to Faraday's law, the induced electromotive force (EMF) is proportional to the\n𝑬\nratio of electric field strength to the characteristic time constant (𝜺 ∝ ). Experimental results\n𝛕\ndemonstrate that high-frequency excitation (τ = 0.5 μs) generates significantly stronger\ninduced voltages. This enhanced voltage effectively triggers the EMW effect, inducing a\ntransformation in the percolation network.\nFig. S29. The resistance change (R/R ) - amplitude (E) curve for the metal\n0\nmicrofiber-polymer composite under different amplitudes (0 - 750 V/cm) of the\nspecimen with applied DC field. A signal generator connected to two copper plates was used\nto provide a parallel electric field. The sample (steel microfiber-SIS composite) was placed\nbetween two copper plates, and a multimeter was connected at both ends to record the\nresistance. The applied electric field was maintained at different amplitudes (0 - 750 V/cm)\nfor sufficient time (> 600 s), respectively, and the resistance of the samples did not change\nsignificantly under the DC field, indicating that the changing rate and the amplitude of the\nelectric field were both important for EMW effects. The static electric fields would not be\nable to induce the conductive network change.\n3: \"A fundamental concern exists regarding the experimental sequence and\nmechanism feasibility. The authors prepare the composite by mixing SIS and steel\nfibers at 90°C followed by hot pressing at 120°C to create a solidified 1mm thick film,\nand then apply EMW treatment to this already-solidified matrix. This raises critical\nquestions: How can conductive particles move and reorganize within a solidified\npolymer matrix? If the SIS matrix has high elastic modulus and elasticity, wouldn't\nany particle rearrangement be limited and quickly revert to the original state due to\npolymer chain elasticity? The authors need to provide a more detailed explanation of\nhow significant particle movement and network reorganization can occur in a solid\npolymer matrix, and why this rearrangement doesn't immediately reverse due to\npolymer elasticity.\"\nResponse: We appreciate the constructive feedback. The theoretical foundation of this\nwork lies in the Branly effect, wherein conductive metal particles undergo cohering\nunder EMW exposure, substantially enhancing electrical conductivity. Branly\ndemonstrated this phenomenon using particles immobilized in wax or resin, proving\nthat particle displacement is unnecessary. Instead, conductive network reorganization\noccurs exclusively through optimization of existing contact points without long-range\nmigration. EMWs induce localized oxide layer breakdown at filler contact interfaces,\nfollowed by micro-weld formation via Joule heating-induced fusion, which resists\npolymer elastic rebound.\nTo experimentally validate this mechanism, Sn microparticles (diameter: 835 μm)\nwere selected owing to their low melting point, facilitating clear observation of\ncohering. As depicted in fig. B, two Sn microparticles embedded in cured Ecoflex\nwere subjected to EMWs (1 cm distance). The resulting cohering at their contact\njunction confirms that conductive fillers within solidified polymer matrices can\nachieve localized coalescence under EMW treatment without macroscopic\ndisplacement, thereby enabling conductive network restructuring.\nThe relevant content has been revised accordingly in the main text and supporting\ninformation, with changes highlighted in yellow.\nFig. B. Observation of coalescence at the interface between two Sn particles cured within\nEcoflex via microscopy. Liquid components A and B of Ecoflex-0035 were uniformly mixed\nat a 1:1 ratio and poured into a Petri dish. Before complete curing, two Sn particles were\nembedded within the Ecoflex mixture. The sample was then cured at room temperature for 5\nmin. EMWs were irradiated at a distance of 2 cm from the Sn particles. Microscopic imaging\ncaptured changes at the particle contact point before and after EMW treatment. Compared to\nthe initial state (A), pronounced coalescence between the two Sn particles is evident\nfollowing EMW treatment, as shown in (B). The right panel displays a magnified view of the\nregion indicated in the left panel. Scale bars: 200 μm and 500 μm.\n4: \"The authors claim universal applicability across different conductors and polymers,\nbut the evidence is limited. Only a few material combinations are tested, and the\nmechanism may vary significantly between different types of conductors (metals vs.\ncarbon materials). More comprehensive studies across diverse material systems are\nneeded to support the universality claim.\"\nResponse: Thanks for the reviewer’s comments. To address the reviewer’s concerns,\nwe supplemented the universality tests of Cu fibers across various polymer matrices.\nAs shown in Fig. S13, the composites incorporating Cu fibers with different polymers\n(PC, TPU, LDPE, EVA, PLA) all exhibited a significant reduction in electrical\nresistance following EMW treatment. This demonstrates that Cu fiber-polymer\ncomposites universally exhibit the EMW effect.\nMechanism studies were conducted focusing on metallic materials and carbon-based\nmaterials. The surface oxide layer on metallic conductive fillers is crucial for the\nEMW effect. Previous studies (Phys. Rev. 1900, 11, 22 - 39) reported that noble\nmetals, which are resistant to oxidation, do not exhibit the EMW effect. The Cu\nparticles (diameter: 75 μm) were immersed in 3 mol/L hydrochloric acid solution and\nstirred for 5 min to remove surface oxides, followed by drying in a vacuum oven at\n25°C for 10 h. We provided direct evidence for the removal of the metal oxide layer\nunder EMW exposure (fig. S18). Furthermore, as demonstrated in fig. S19, removal\nof the surface oxide layer from copper particles resulted in no change in particle\nresistance before and after EMW treatment. The above results indicate that the Branly\neffects (oxide breakdown) are critical for EMW treatment of the composites.\nRegarding carbon materials, we also performed additional experiments to support our\nconclusions. As shown in fig. S19, graphene exhibited a slight decrease in resistance\nafter EMW treatment. In contrast, a mixture of graphene (Φ: 7 - 12 μm,\ndiameter-thickness ratio: 9500, 98%) and graphene oxide (GO, Φ: 20 - 40 μm,\nthickness: 1 - 10 nm, O: 40 - 45 wt%) demonstrated a significantly larger reduction in\nresistance following EMW treatment. These results confirm that similar to metal\nfillers, the surface oxidation of carbon materials is also critical to show the\ninsulator-conductor transition under the EMW effect.\n)\nΩ107\n(\ne\nc\nn\na105\nt\ns\ni\ns\ne\nR103\nPolymer types\nFig. S13. Universality of the EMW Effect of Copper Fibers in Different Polymer\nMatrices. Composites were prepared by mixing 40 wt% Cu fibers with 60 wt% of various\npolymers (PC, TPU, LDPE, EVA, and PLA) under heat (160°C, 125°C, 100°C, 85°C, and\n130°C) and shear using a two-roll mill. The mixtures were subsequently hot-pressed and cut\ninto thin film specimens (3 cm × 1 cm × 1 mm). EMWs were emitted at a distance of 5 cm\nfrom the EMW materials, and the electrical resistance of the different composites was\nrecorded using a multimeter. After EMW treatment, a significant reduction in electrical\nresistance was observed across all types of composites. This suggests that the EM effect\nexhibits universality in conductor-polymer composites.\nTable S1.\nComposition of metal micro-conductors and polymer composites\nMass fraction\nMass fraction of micro-conduc Dimensions for the\nPolymer of microfillers\npolymer (%) tors microfillers\n(%)\nsteel\nSIS 85% 15% Φ: 12 μm and 22 μm\nmicrofibers\nSIS 70% Cu microfibers 30% Φ: 22 μm\nSIS 40% Cu particles 60% Φ: 75 μm\nΦ: 5 - 12 nm, length: 3 -\nSIS 93% CNT 7%\n10 μm, 90% - 95%\nΦ: 7 - 12 μm, 11 - 15 μm,\nSIS 80% graphene 20% diameter-thickness ratio:\n9500, 98%\nPLA 60% Cu microfibers 40% Φ: 22 μm\nEVA 60% Cu microfibers 40% Φ: 22 μm\nLDPE 60% Cu microfibers 40% Φ: 22 μm\nTPU 60% Cu microfibers 40% Φ: 22 μm\nPC 60% Cu microfibers 40% Φ: 22 μm\nsteel\nSEBS 85% 15% Φ: 12 μm\nmicrofibers\nsteel\nPC 70% 30% Φ: 12 μm\nmicrofibers\nResin 44% silver particles 56% Φ: 1 - 2 μm\nFig. S19. Oxidized conductive fillers exhibit significant EMW effects. Copper particles\n(diameter: 75 μm) were immersed in a 3 mol/L HCl solution and stirred for 5 min to remove\nsurface oxides, followed by drying in a vacuum oven at 25°C for 10 h. Three prepared\nmaterials—graphene, a mixture of graphene and graphene oxide, and oxide-removed copper\nparticles—were positioned flat between two parallel copper tapes spaced 1 cm apart.\nEMWs were emitted at a distance of 10 cm (EFB generator or piezoelectric ignitor, power: 10-5 - 10-7\nmW, rising edge: 4 - 8 ns) from the sample. (A -A ) Graphene exhibited a slight decrease in\n1 2\nresistance after EMW treatment. (B - ) In contrast, a mixture of graphene (Φ: 7 - 12 μm,\n1 2\ndiameter-thickness ratio: 9500, 98%) and graphene oxide (GO, Φ: 20 - 40 μm, thickness: 1 -\n10 nm, O: 40 - 45 wt%) demonstrated a significantly larger reduction in resistance following\nEMW treatment. Removal of the surface oxide layer from copper particles resulted in no\nchange in particle resistance before (C ) and after (C ) EMW treatment. The presence of\n1 2\nsurface oxidation layers on carbon materials and metal fillers plays a critical role in enabling\nthe insulator-conductor transition under EMW exposure.\n5: \"Statistical analysis and reproducibility data are lacking throughout the study. Most\nexperimental results appear to be based on single samples without error bars or\nstatistical significance testing. What is the sample-to-sample variation in EMW\nresponse? How reproducible are the resistance changes across different batches?\"\nResponse: We thank the reviewer for their comments. In Figs. 2g, 2h, and 4g of the\nmain text, the data were based on three samples and included error bars. To address\nthe reviewer's concerns, we have supplemented the threshold plot depicting\nelectromagnetic response variations across different samples (Fig. 4f) and added error\nbars based on three samples in figs. S47-S49. The electromagnetic response variations\nbetween different samples exhibit similarity, and the resistance changes demonstrate\nhigh reproducibility across different batches. Furthermore, as requested by the Editor\nto ensure data reproducibility, we have provided the original data in both the main text\nand the supporting information. The relevant modifications have been incorporated\ninto the main text and supporting information and are highlighted in yellow.\nMinor Issues:\n6: Several grammatical errors and typos throughout the manuscript need correction\n(e.g., \"exhibite\" should be \"exhibited\" on page 6).\nResponse: Thank you for your careful reading of the manuscript and your comments\nabout grammar. We have carefully examined the full text and corrected the\ngrammatical errors (including singular and plural usage and phrase usage) throughout\nthe text. All the corrections are highlighted in yellow.\n7: Terminology inconsistency: \"EMW effects\" and \"EM-PNR effects\" are used\ninterchangeably without clear definition of the latter term\nResponse: Thank you for your useful comment. The full name of EM-PNR is\nelectromagnetic-percolation network regulation. In order to simplify the expression\nand unify the entire text, we choose to express the entire text using the EMW effect.\nThe relevant positions in the main text have been modified and highlighted in yellow."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have carefully read the revised manuscript and the response from the authors. Thank you for the detailed response with additional experiments to my previous comments. Unfortunately, I still have several questions and concerns that have not been resolved in the revised manuscript.\n1. The authors studied the Branly effect by electromagnetic wave on two steel beads to explain the fundamental mechanism in polymer with metal microfibers. As shown in Fig. 2c and d, they have different shape, scale, and materials. Can the authors directly show the breakdown of surface oxide on two metal microfibers?\n2. The authors show a resistive memory device with the fabricated polymer composites. They claimed that the resistance can be recovered rapidly in 20 μs under small vibration. Can the metal microfibers surround by polymer contact with air and generate surface oxide in 20 μs?\n3. In Fig.4, C1 and C2 are different sample or in different positions. Does the cohering of microfibers only exist on the surface of polymer composites?", "reviewer_2": "The authors have carefully revised the comments from the reviewers. The current version of this manuscript can be suitable for publication on Nature Communications.", "reviewer_3": "The authors have substantially improved the manuscript and addressed most major concerns effectively. The demonstration of 2 Ω/sq sheet resistance using silver-copper paste and the expanded mechanistic understanding significantly enhance the work's scientific merit and practical applicability.\nTechnical Concern:\nThe thickness measurement using vernier calipers (0.06 mm) for conductivity calculation in Fig. S51 introduces significant uncertainty. I recommend using alpha-step profilometry or similar precise measurement techniques for accurate electrical characterization of thin films.\nRecommendation:\nThe manuscript quality has been considerably enhanced through comprehensive responses with comprehensive responses with comprehensive responses with new experimental data. Despite the minor measurement concern, I recommend acceptance of this manuscript for publication. This innovative application of the Branly effect to modern polymer composites represents valuable interdisciplinary research suitable for high-impact publication."}, "author_response": "Reviewer #1:\n1. The authors studied the Branly effect by electromagnetic wave on two steel beads\nto explain the fundamental mechanism in polymer with metal microfibers. As shown\nin Fig. 2c and d, they have different shape, scale, and materials. Can the authors\ndirectly show the breakdown of surface oxide on two metal microfibers?\nResponse: Thank the reviewer for raising this insightful question for highlighting the\ndirect observation of oxide breakdown in microfibers. In direct response to the\nreviewer's comment, we have supplemented our findings with scanning electron\nmicroscopy (SEM) images. The oxidized copper fibers were fixed on the surface of\nthe SEM substrate and were naturally contacted with each other. Copper fibers were\npositioned 1 cm away from the EMW source and treated for 20 s. The cohered fibers\nare clearly observed after EMW treatment as shown in Fig. S16B. These experimental\nresults directly demonstrate the cohering of the microfibers.\nThe relevant experimental data confirming the cohering of the metal fibers have been\nadded to the main text and the supporting information.\nSupplementary Fig. S16. Cohering is observed by SEM at the interface among the\ncopper fibers. The copper fibers (Φ: 22 μm) were oxidized on a hot stage at 250°C for 20\nmin. (A) In the initial state, the copper fibers were fixed on the surface of the SEM substrate\nand were naturally contacted with each other. Scale bar: 100 µm. Subsequently, the copper\nfibers were positioned 1 cm away from the electric spark generated by an EMW source. The\nsource was then activated and maintained for a sustained exposure time of 20 s. (B) SEM\nimages indicated that a distinct cohering phenomenon occurred at the points of contact\nbetween the copper fibers. The dash circles show the fibers cohered. These surface fibers\nhave slightly displacement during the process of the EMW treatment and SEM\ncharacterization. Scale bar: 100 µm.\n2. The authors show a resistive memory device with the fabricated polymer\ncomposites. They claimed that the resistance can be recovered rapidly in 20 μs under\nsmall vibration. Can the metal microfibers surround by polymer contact with air and\ngenerate surface oxide in 20 μs?\nResponse: We thank the reviewer for raising this insightful question. The\ninstantaneous recovery of the high-resistance state following vibration is not\ndependent on the rapid re-oxidation of the oxide but is instead a physico-mechanical\nprocess. The \"micro-welding\" points formed by the action of EMW in this case are\nfragile. Minor vibrations can mechanically disrupt these fragile conductive bridges,\nseparating the contact points and thereby instantaneously (in the microsecond scale)\ninterrupting the conductive pathway and restoring the high-resistance state.\nPost-vibration, the fibers undergo slight displacement within the polymer matrix,\nmaking it highly improbable for them to return precisely to their original contact\nconfiguration. This displacement allows the composite material to stably maintain its\nhigh-resistance state.\nThe discussion regarding the cause of the high-resistance state recovery has been\nadded to the main text and supporting literature, and is highlighted in yellow.\n3. In Fig.4, C1 and C2 are different sample or in different positions. Does the cohering\nof microfibers only exist on the surface of polymer composites?\nResponse: Thank the reviewer for the constructive comment. For Fig.4c1 - 4c2, we\nused the same sample for the microscopic characterization. However, since the metal\nfibers are randomly distributed within the polymer matrix without the control over the\ncontact distance and the degree of surface oxidation for the fibers, it is quite\nchallenging to know in advance which fibers will be cohered. So, tracking the cohered\npositions will be very difficult. As a result, we used different positions in the same\nsample to take the microscopic images.\nIt should be noted that Fig. 4c - 4c used a regular optical microscope to take images.\n1 2\nSince our composites are mixture of transparent polymers (SIS) and micro-sized\nfibers, light can penetrate into the composite surface layers. So, the metal microfibers\nobserved in Fig. 4c and 4c are not directly exposed on the outermost surface but are\n1 2\nrather encapsulated by the surrounding matrix material. This conclusion can also be\nsupported by the Fig. S11B and Fig. S12B in our supporting information. The\nside-view iamge shows little metal fiber on surface but top-view image shows a lot\ndue to the light penetration through the polymer surface. To further verify this\nconclusion, SEM observations were further employed. As shown in Fig. S40A, for the\nsame sample, most of the metal fibers are encapsulated by the polymer matrix, which\nis consistent with the optical microscopic images in Fig. 4c, S11B, and S12B. Thus,\nthe cohering of the fiber network we observed is within the polymer rather than being\na surface phenomenon.\nSupplementary Fig. S40. SEM was employed to characterize the morphology of the steel\nmicrofiber-SIS composite. The detailed procedure for the steel microfiber-SIS composite\nfabrication can be found in Fig. S2. (A) The material surface appears smooth and continuous,\nwith the steel fibers (Φ: 12 μm) being almost completely encapsulated within the SIS matrix,\nleaving no exposed protrusions. (B) The sample was broken into halves for cross-section\ncharacterization. In side-view, steel fiber protrusions are clearly observed.\nReviewer #3:\n1. The thickness measurement using vernier calipers (0.06 mm) for conductivity\ncalculation in Fig. S51 introduces significant uncertainty. I recommend using\nalpha-step profilometry or similar precise measurement techniques for accurate\nelectrical characterization of thin films.\nResponse: We sincerely thank the reviewer for this insightful comment. In direct\nresponse to this comment, we have re-measured the thickness of the conductive\nsilver-copper paste film using optical microscopy with the function of the\nprofilometry (as detailed in the revised Fig. S53). This technique provides a higher\nresolution and accuracy compared to mechanical calipers. The newly obtained and\nmore precise average thickness value is 63 μm. We have updated all subsequent\nconductivity calculations in the manuscript (particularly in Fig. S53 and the related\ndiscussion) using this revised value. The updated results confirm that our central\nconclusions regarding the material's conductivity remain valid.\nThe relevant data regarding the thickness measurement of the conductive\nsilver-copper paste, as obtained by optical microscopy, have been included in both the\nmain text and the supporting information, with additions highlighted in yellow.\nSupplementary Fig. S53. Calculation of the electrical conductivity of 65 wt%\nsilver-copper particle-resin composites after combined treatment. (A) The initial electrical\nresistance of the conductive silver-copper paste was extremely high (> 20 MΩ). (B)\nFollowing EMW combined treatment, the resistance of the conductive silver-copper paste\ndecreased to 48.9 Ω. The length (C), width (D), and average thickness (E -E ) of the\n1 2\nconductive silver-copper paste specimen were 7.6 cm, 3.1 mm, and 63 μm, respectively. The\nelectrical conductivity (σ) of the silver and Cu particle-resin composites was calculated using\nthe formula for volume conductivity: σ = 1/ρ = L / (R × A), where L is the length, R is the\n𝟏 𝟕.𝟔∗𝟏𝟎−𝟐 𝐦\nresistance, and A is the cross-sectional area. Thus: 𝛔 = = ≈\n𝛒 𝟒𝟖.𝟗 𝛀∗𝟑.𝟏∗𝟏𝟎−𝟑 𝐦∗𝟎.𝟎𝟔𝟑∗𝟏𝟎−𝟑 𝐦\n𝟕𝟗𝟓𝟕.𝟗𝟕 𝐒/𝐦."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have addressed all my concerns. In my view the paper is now much improved, the results are robust, reliable and original, the presentation is clear. The paper is now suitable for publication.", "reviewer_3": "The authors have carefully revised the comments from the reviewers. The current version of this manuscript can be suitable for publication on Nature Communications."}, "author_response": ""}]

Decision letter