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Silver exhibits the highest bulk electrical conductivity (6.3 × 10⁵ S cm⁻¹) of all metals and it is quite resistant against oxidation, making it commonly used in electrical circuits and as metal contacts.^[97]

Silver-based nanomaterials, including 0D silver nanoparticles, 1D silver nanofibers (lower aspect ratio) and silver nanowires (higher aspect ratio), and 2D silver nanoflakes have been synthesized using different methodologies depending on precursor availability and specific applications.^[98] For instance, top-down fabrication techniques use photolithography and electron-beam lithography approaches to achieve nanostructures and fine patterns from the bulk metal. Bottom-up synthesis, starting from atomic and molecular precursors, also enables controllable nanostructures with low defects and a homogeneous chemical composition to be achieved, and include the polyol approach, the hydrothermal protocol, the electrochemical technique, and the UV-irradiation technique, to cite just a few.^[99] Among them, the polyol method remains the most common procedure due to the facile implementation into mass production, low cost, and simplicity.^[100] This method is based on the reduction of the metal precursor (e.g., AgNO3) by a polyol (e.g., ethylene glycol) in the presence of an appropriate capping agent (e.g., poly(vinylpyrrolidone)) at elevated temperatures, and enables the synthesis of all (0D, 1D, and 2D) Ag nanostructures by adjusting the synthesis parameters (e.g., temperature, ion type, concentration). Furthermore, thermal evporation and electroless deposition/plating (ELD) are also used for the production of AgNPs. ELD is a nongalvanic plating method that is commonly used for electroless plating of several metals, including silver, nickel, gold, and copper.^[101] In ELD, partial redox processes occur at the same electrode surface due to the potential difference exsisting between the equilibrium potentials for the oxidation and the reduction reactions and the potential at the electrode surface.^[102]

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31 4.1.1. Silver Nanomaterials for E-Skins

Owing to the large conductivity and good ambient stability, several inks based on silver nanomaterials are commercially available and are widely used in printed electronics and for transparent optoelectronics.^[103] However, stretchable and wearable electronics requires highly conductive electrical connenctions at large strains, ideally achieving a conductivity () > 1000 S cm⁻¹ at a strain (ε) of 100% and with >10⁵ cycle stability.^[104] Among the seminal contributions, Someya’s group reported the realization of printable elastic conducting lines for sensing applications comprising Ag nanoparticles (AgNPs) that were prepared in situ by blending micrometer-large Ag flakes, fluorine rubbers (DAI-EL^® G-801), and a surfactant, using a procedure based on previous studies.^[105] Specifically, as shown Figure 5a,b, the elastic conductor was realized by printing an ink comprising a poly(vinylidene fluoride-co-hexafluoropropylene) rubber, a polymeric fluorine surfactant (a nonionic chemical based on both hydrophilic ethylene oxide and hydrophobic perfluoroalkyl groups), and Ag flakes in methylisobutylketone (MIBK). After printing, these traces are dried at 80 °C for 1 h, followed by an additional heating at 120 °C for 1 h, which was found to be important for the formation of the AgNPs. The presence of AgNPs (2–10 nm) among micrometer-sized Ag flakes was confirmed by TEM imaging (Figure 5c). The formation of an optimal AgNP pattern was investigated by evaluating three different conditions, which include: i) the use of the surfactant, ii) without the surfactant, and iii) with the surfactant but the product being dried only at 80 °C for 1 h. The conductivity–strain characteristics of Figure 5d confirmed the contribution of the AgNPs to the conductivity. Thus, the stretchable conductors with the surfactant displayed high  values of

≈4900 S cm⁻¹ without strain and ≈700 S cm⁻¹ at a strain of 300%, which remain the highest values reported to date. On the contrary, the elastic conductor prepared without the surfactant showed 

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values of ≈3700 S cm⁻¹ and ≈300 S cm⁻¹ at strains of 0% and 300%, respectively. Interestingly, the elastic conductor with surfactant prepared at low temperature exhibits  values of only 5.5 × 10⁻⁵ S cm⁻¹ and 17 S cm⁻¹ at strains of 0% and 300%, respectively. The conductivity increase upon stretching was not rationalized. To demonstrate the utility of this stretchable device for interconnections, the authors fabricated a fully printed elastic sensor network on a stretchable elastomeric polyurethane (PU) substrate. Figure 5e displays the device structures of both pressure and temperature sensors on rigid islands fabricated with conventional photolithographic processes and electrically connected using an elastic conductor on a PU film. The pressure sensor consists of a Ni-particle–silicon composite, which can sense a weak force (1–5 N). The temperature sensor is a composite of graphite and an acrylic polymer, which can sense temperatures ranging from 28 to 36

°C. These devices remain functional even when stretched at a strain of 120%. More recently, Kim’s group achieved ultrahigh conductivity of 41 850 S cm⁻¹ by mixing ultralong (≈100 µm) gold-coated silver nanowires (Ag–Au) with SBS elstomer.^[106] Ultralong AgNWs were synthesized by the previously discussed polyol-mediated process.^[107] Then, Au layers were formed on the AgNWs through in situ reduction of HAuCl4 solution. After mixing Ag–Au NWs with SBS/toluene solution with appropriate ratio, the soltuion was cast into a mold and dried at 25–85 °C. As shown in Figure 5f, the Ag–Au/SBS composites, prepared at a 60:40 weight ratio of NW:SBS and dried at 25 °C, retained a conductivity of 10⁴ S cm⁻¹ when stretched to ≈150% and exhibited a high stretchability of 266%. Next, the Ag–

Au/SBS composites were patterned with serpentine features and assembled into a multifunctional e-skin consisting of recording electrodes to measure electrophysiological signals, bipolar stimulation contacts, and a heating element to deliver both electrical and thermal stimulation (Figure 5g). Figure 5h shows that the Ag–Au/SBS nanocomposite e-skin can record electromyography (EMG) signals and

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deliver electrical stimulation simultaneously. The stretchable heating element at the centre of the e-skin is used for Joule heating. The temperature–time plots shown in Figure 5i demosntrate that the temperature can reach 60 °C in 20 s by appling a 3 V bias. Also, the heating performance is stable under stretching up to 25%. The biocompatibility of the Ag–Au/SBS nanocomposite was confirmed by the high viability (≈100%) of heart myoblasts (H9C2), human skin fibroblasts (CCD-986sk), and mouse connective tissue (L929) cells exposed to it (Figure 5j).

One of the most attractive applications of e-skins would be motion monitoring, for instance detecting the degree of bending of a knee, extension of an arm, and hand motions. The goal is to achieve high gauge factor (GF) of these strain sensors while maintaining high stretchability. Wang’s group printed 0D AgNPs from a AgNP ink on polyurethane acrylate (PUA) elastomeric substrates in the form of planar serpentine structures, which can effectively mitigate the formation of localized strain areas and achieve high stretchability (Figure 5k,l).^[108] The radius (r) of the AgNP serpentines was changed from 200 to 1600 µm, while the width was maintained at ≈200 µm. Figure 5m reports the ∆R/R0 plotted as a function of ε for these AgNP features for different radii and for a straight line.

In agreement with expectations, the straight line can only be stretched to <2% while the serpentine-structured patterns are much more stretchable. Interestingly, ∆R/R0 exhibits a clear trend vs r, with a larger-radius-of-curvature structure leading to an enhanced stretchable structure. Specifically, the AgNP features with r = 200 µm on PUA could be stretched to ≈12% before becoming insulating, while the sample with r = 1600 µm exhibited a ∆R/R0 of only ≈13 when stretched to ≈25%

(elongation of PUA substrate). Importantly, the AgNP device with r = 200 µm possesses extremely large GFs of ≈6.6 × 10⁵ and 2 × 10⁷ at tensile strains of ≈10% and ≈12%, respectively. Finite element analysis (FEA) results for AgNP features, with r = 200 µm and 1600 µm, clearly demonstrated that

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larger r values can efficiently mitigate the strain sustained by the AgNP conducting features. Finally, the authors fabricated a demonstrator device with periodic segments of r = 400 µm serpentines to monitor the bending of human fingers. The electromechanical analysis of this device was carried out to calibrate the ∆R/R0 up to a tensile strain of 16%, which shows a GF of ≈700 at a 10% strain. As the finger was bent gradually, the resistance changed according to a stepwise function.

Spiders can sense minimal mechanical stress variations via crack-shaped slit organs located in the proximity of their leg joints.^[51a] Recent reports have shown that strain sensors based on nanoscale crack junctions exhibit much higher GFs than those achieved with other mechanisms.[42,51a,109]

However, most microcrack-assisted resistive strain sensors with high sensitivity generally feature the characteristic of a limited range (<5%) for strain detection. Jarkko et al. designed a highly stretchable, sensitive, and versatile strain device based on a cracking structure.^[43] For the fabrication, conductive fabrics (CFs) (MedTex P130 silver-plated nylon) were cut into two T/L-shapes and partially embeded into silicone rubber as shown in Figure 6a. Then the substrate was prestreched and the gap (5 mm × 1 mm) between two T/L-shaped CFs was filled with AgNPs (silver ink CI-1036) by screen printing. After thermal curing, the prestretched substrate was released to form wrinkles of the Ag-ink pattern. This wrinkled conducting structure was printed on precisely shaped stretchable CFs (called Ag-DS/CF), which exhibited a resistance of ≈170 Ω. Ag-DS/CF features, with different designs, were fabricated by tailing the T/L shapes to produce sensors that could be used to detect pressure changes, physical vibrations, and human motions. As shown in Figure 6b, high strectchability (up to 75%) was demonstrated with a fairly stable gauge factor (GF ≈ 1.5) when ε is between 7 and 58%; then, it increased significantly to ≈10⁴ for larger strains. For the ultrasensitive structure, the gauge factor remains at ≈130 when ε < 4%. Importantly, all the Ag-DS/CF devices

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exhibit excellent durability as demonstrated by 3000 repeated strain cycles and 5 cycles of machine washing. For the pressure sensor, the ∆R/R0 increased almost linearly in the three pressure regimes of 1–35 Pa, 35–200 Pa, and 0.200–10 kPa, and it showed a maximum pressure sensitivity of 0.82 kPa⁻¹, which then decreased to 0.075 kPa⁻¹ and 0.051 kPa⁻¹ as the pressure increased further (Figure 6c). The response/recovery times of Ag-DS/CF devices were in the range of 70–200 ms.

More recently, Zhang’s group demonstrated microcrack-assisted strain sensors using silver nanowires (AgNWs) and an elastomeric PDMS platform.^[54] These strain sensors exhibited a tremendous GF as high as 150 000 within a large stretchability of 60% strain. Figure 6d shows the typical fabrication process of these strain sensors. The key structural element is a patterned PDMS (P-PDMS) substrate (1 mm thick), which was fabricated using a piece of steel net (80 mesh) mold to provide a regular and uniform surface PDMS microstructure. Next, a AgNW (d = 35 nm, L = 25 µm) solution was spin-coated on the P-PDMS substrate and the modified substrate was prestretched (up to 100%) and dried at 56 °C for 10 min to enable the AgNWs to fill into the microcracks of the P-PDMS. This coating–prestretching–drying process was repeated three times before bonding electrical wires by a silver paste at the ends of the device. SEM images (Figures 6e,f) show that the microstructures of parallel concave lines and square-plate arrays in the P-PDMS substrate are regular and remarkably uniform. Notably, several microcracks were observed inside the parallel concave lines and square plates. However, percolating pathways of the AgNWs remain, resulting in a low electrical resistance (≈20 Ω). Optical images (Figure 6g,h) reveal that crack formation began when stress is applied, and the width of the cracks increases with stretching. Thus, as shown in Figure 6i, when the applied strain increased from 0% to 60%, the ∆R/R0 increased almost linearly from 0 to 90 000. The experimental results revealed that the GF of this strain sensor using a

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structure based on a flat PDMS substrate as a control was calculated to be ≈1100. In comparison, the GF of the strain sensor using P-PDMS approached as high as ≈150 000 within a 60% strain.

Furthermore, a fast response, low creep, and an ultrahigh stability (more than 30 000 cyclic loading tests) were achieved, which are appealing features of a strain sensor. Finally, the authors demonstrated the sensor use for monitoring finger/wrist bending and muscle movements (Figure 6j).

4.1.2. Silver Nanomaterials for E-Textiles

Song’s group reported Ag-based e-textile devices for sensing and processing the signals from the kinesis of a person.^[110] Their silver/polyester textile was fabricated by using a low-temperature in situ reduction of a silver precursor into AgNPs. Specifically, a piece of polyester textile with an interlock-stitch structure was first treated by air plasma to form a hydrophilic fiber surface, followed by dyeing with the Ag precursor. Next, the Ag precursor was reduced into AgNPs after annealing at 90 °C with formic acid, obtaining the fabric/AgNP conductive composite. Copper wires were then mounted (with the help of a silver paste) to the two ends of the fabric as external electrodes, resulting in an AgNP-based e-textile strain sensor. The R/R0–ɛ curve showed a monotonic resistance increase to the strain region between 6% and 50% and a monotonic decrease for strains less than 6%. A dynamic mechanical deformation (ɛ = 0–10%) test over 1000 cycles demonstrated the high stability of the composite fabric. Next, a multichannel and real-time monitoring of sensors at the heel, upper knee, and lower knee were mounted to a body to reveal the physical kinesis characteristics in detail. The sensors applied at the different joints exhibited different resistance

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responses during walking, which indicates the strain difference. Furthermore, the e-textile mounted on the upper knee of on the experimenter’s body exhibited different electronic signals (ΔR) for different body movements such as walking, squatting, jumping, relaxing, jogging, and extending.

An e-textile material can be completely conductive; thus, it can also function as the electrodes for electromyography (EMG), a diagnostic procedure wherein the electrical activity of muscle tissues is recorded.^[111] Jin et al reported a textile-permeable Ag ink that could be directly printed on a fine knitted fabric (FR3600: 76% nylon and 24% polyurethane).^[112] The conducting ink consisted of a fluoroelastomer (poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP)), 2-2(butoxyethoxy)ethyl acetate, and Ag flakes (average particle size = 2–3 µm) in a weight ratio of 1:2.45:4 (Figure 7a,b). After stencil printing (1 mm × 40 mm trace), the ink was absorbed from the surface to the interior of the textile fiber by a capillary process. The process was repeated for 5 passes; then the printed textile was dried at 90 °C for 2 h, followed by hot pressing at 160 °C for 30 s at 30 kPa. SEM images demonstrated that the fiber bundles retain the structure of a knitted textile and the stretchable conductor only filled into small gaps (<10 µm) between the fibers (Figure 7c,d).

Consequently, the initial sheet resistance of the printed traces was 0.06 Ω sq⁻¹, and it increased only by ≈70 after stretching it to 450% due to the good morphology of the textile-conducting composite bundles (Figure 7e). Next, an LED was connected to the printed electrode with a conductive epoxy.

As shown in Figure 7f, the LED remained functional while the system was stretched up to 400%.

Furthermore, a four-channel EMG monitoring system mounted on a skin-tight compression garment was fabricated to demonstrate the possible biology-related applications of this stretchable and printed textile circuit (Figure 7g). The data clearly demonstrated that this system can detect the electrical potential of the muscles after amplification/filtering of the signal collected from the

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surface electrode. The greater the voltage magnitude, the harder the muscle is tensing. The results showed that the noise levels of the texitle electrodes were only 0.07 mV and different activations of muscle (these electrodes were plated at different arm positions) during hand opening and closing motion were observed (Figure 7h). More recently, Chung’s group reported e-textile patches (12 cm × 12 cm) fabricated by printing a Ag-particle (2–3 µm in size)/flouroelastomer (DAI-EL^® G-801) composite ink on an electrospun porous PU textile (NANOSAN^®-Sorb).^[8b] The printed ink permeated the fabric affording serpentine patterns having a 60 μm-thick cladding layer since the Ag particles can pass through the pores of the PU textile, thus enabling both good mechanical and electrical properties. The e-textile featured a  of ≈3200 S cm⁻¹ which decreased only fractionally after 1000 cycles of 30% uniaxial stretching. Importantly, this platform can be used as the electrodes for EMG and electroencephalography (EEG) monitoring of muscle activity and brain waves (Figure 7i,j).

Importantly, the EEG signal from this sensor exhibited fewer movement artifacts than that commercially available.

Harvesting human-motion energy could be a promising way for powering integrated wearable electronics. For this purpose, Kwak et al. reported a fully stretchable TENG (S-TENG) based on differently knitted Ag/poly(tetrafluoroethylene) (PTFE) fabrics.^[113] To this end, fibers comprising either only PTFE yarns or mixed PTFE yarns (dtex 220, d = 300 µm) plus Ag yarns (280 denier) were knitted into plain-, double-, and rib-patterned fabrics with a gauge (needle/inch) of 12 using a commerical knitting machine, which resulted in three different insulating and conducting fibrotic structures. First, the authors investigated the response variation to stretching of each textile structure. The PTFE-only plain-, double-, and rib-knitted fabrics could return to their original structure after ≈10, 20, and 30% strains in the transverse direction, respectively. The S-TENG devices

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(10 cm × 10 cm size) were fabricated as a double-arc-shaped architecture consisting of two knitted Ag/PTFE fabrics as the top and bottom layers with an Ag-only fabric electrode in the middle. The output voltage (with a load of 40 MΩ) and current (with a load of 100 Ω) of the resulting S-TENGs having different knitted fabrics under the stretching conditions were measured. The results demonstrated that the plain- and double-knitted Ag/PTFE-based S-TENGs produced output voltages/current peaks of 2.00 V/0.09 μA and 12.25 V/0.53 μA, respectively. Interestingly, using the rib-knitted Ag/PTFE structure in the S-TENG led to a significantly higher increase of the output characterisitc (23.50 V/1.05 μA) compared with the other fabric architectures. The authors rationalized the data as the result of the greater contact surface area of the rib-fabric-structured S-TENG, which could enhance the total triboelectric-charge amount. Finally, to demonstrate the possible use of the S-TENG as a self-powered energy device, 15 commercial green LEDs were integrated with the S-TENG on a piece of sportswear. All the LEDs illuminated when the person wearing it was running, demonstrating the potential of human-motion energy production for powering wearable electronics.

Polyethylene with nanoscopic porosities (nanoPE) is transparent (≈96%) to mid-IR, human-body wavelengths but opaque to visible light owing to the unique pore size distribution (50 to 1000 nm).^[114] Cui’s group reported a nanophotonic structured textile with tailored infrared (IR) property for passive personal heating using nanoporous metallized polyethylene.^[102a] The nanoporous metallized PE textile consisted of a nanoporous Ag film on a 12 µm-thick nanoPE system (nano-Ag/PE). This fabric was then laminated on the outside of piece of cotton (called cotton/nano-Ag/PE) with the nanoPE side facing the ambient environment (Figure 8a). The porous Ag film was fabricated by electrodeless deposition (ELD) and the film nanoscale pores, with sizes of 50–300 nm (Figure 8b),

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were controlled by optimizing the plating time. The combination of nanoporous Ag film and the nanoPE provides a pathway for the transmission of water vapor, ensuring good breathability. The total IR reflectance of the nano-Ag/PE was measured using FTIR (Figure 8c) and a high IR reflectivity of 98.5% was exhibited, which is much greater than those of textiles such as cotton (8.4%), Omni-Heat metallic dots (31.9%) and the dense metal film of a Mylar blanket (91.2%). Owing to the high IR reflectivity of the nanoporous Ag film and the high IR transparency of the nanoPE (96.0%), the cotton/nano-Ag/PE textile exhibited a low IR emissivity (≈10%) on the outer surface, which was much lower than those of a Mylar blanket (60.6%), Omni-Heat (85.4%), and cotton (89.5%) (Figure 8d). Next, the authors demonstrated the radiative heating properties of the nano-Ag/PE textile by attaching it to a cotton shirt after being worn on a person and visualized heat transfer using an IR camera. The color comparison in Figure 8e,f demonstrated minimal IR emission when a person wears a garment made from the cotton/Ag/PE textile, while traditional textiles emit substantial heat to the environment.