Owing to the abundance of copper versus other noble metals, as well as its excellent bulk electrical conductivity (5.96 × 105 S cm−1), copper-based nanomaterials may offer cost benefits for commercial applications of soft conductors and sensors. Up to now, copper nanowires (CuNWs) have been the most studied copper nanomaterial because of their facile synthesis and processing.[126] CuNWs possess excellent conductivity, good mechanical properties, and, upon passivation, sufficient resistance to oxidation.[127] CuNWs can be synthesized by several solution-phase methods including chemical reduction of Cu(I) and Cu(II) precursors, self-catalytic growth, and electrospinning.[128]
However, CuNWs tend to oxidize and it is difficult for them to be dispersed into a solvent.[129] The
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environmental stability of CuNWs could be improved by a core–shell structure design and passivation.[130] Addition of hydroxypropyl cellulose (HPC) to the solvent facilitates the preparation of homogeneous and well-dispersed CuNW suspensions.[131] Since, to our knowledge, there are no studies using Cu-nanomaterials-based e-textiles, we will only discuss e-skins in this section.
Zeng’s group reported highly stable, conducting Cu@Cu–NiNW elastomeric composites.[132] Ni-alloyed CuNWs were synthesized by heating a CuCl2 and Ni(acac)2 mixture (molar ratio = 2:1) in oleylamine at 180 °C for 4 h, followed by heating at 205 °C for another hour. SEM imaging demonstrated that the resulting Cu@Cu–Ni NWs were intertwined and thus they could provide a combination of good electrical contacts and mechanical flexibility. The TEM image showed that the entire NW had a smooth surface, and the interface between the Cu core (d ≈ 35 nm) and the 10 nm-thick Cu–Ni alloy shell was clear and abrupt. Uncoated CuNWs were also fabricated as a control. To fabricate CuNW-based and Cu@Cu–NiNW-based elastomeric composites, precured 1 mm-thick PDMS substrates were molded in a glass at room temperature. Next, the NWs were dispered in hexane, and the resulting suspensions were filtered to form a uniform film on a nitrocellulose membrane, which was transferred to the elastomeric PDMS film. A four-point probe test showed that the sheet resistances of the CuNWs/PDMS and Cu@Cu–NiNWs/PDMS films were almost identical (≈60 Ω sq−1) and they exhibited an optical transmittance of ≈80%. However, the changes in conductivity of the CuNWs/PDMS and Cu@Cu–Ni NWs/PDMS films upon stroage in ambient conditions were markedly different. The resistance of the CuNWs/PDMS platform increased by 10 times after 30 days of storage, while no obvious change in resistance was observed for the Cu@Cu–
Ni NWs/PDMS elastomer. Next, the conductivity of the Cu@Cu–Ni NWs/PDMS film was monitored under bending, stretching, and twisting tests. When the composite film was bent upward,
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downward, and vertically, the resistance (≈8 ) increased by about 4, 1.5, and 4 Ω, respectively.
Furthermore, when cyclic stretch–release tests were performed ( = 20%), the resistance increased by ≈3 times after 600 cycles. However, the authors pointed out that, during all mechanical stress experiments, the resistance of the Cu@Cu–Ni NWs/PDMS platform remained below 50 Ω. More importantly, charge transport remained very stable after bending/stretching even after 600 cycles.
To achieve high sensitivity to pressure, Liu’s group fabricated Ag-coated CuNWs on microstructured PDMS substrates.[47] The Cu–AgNWs were fabricated by a facile galvanic replacement reaction of Cu with Ag at room temperature to prevent typical oxidation of the pristine CuNWs. To this end, first a suspension of CuNWs (5 mg mL−1) was added into a AgNO3 solution and stirred for 30 min. Next, the dark-green Cu–Ag NWs were collected by centrifugation and stored in absolute EtOH for further processing. The SEM and TEM images demonstrated that the as-prepared CuNW surface was very smooth and the wires were ≈10–15 µm long and had a diameter of ≈100 nm (Figure 13a). After Ag deposition, the TEM image of the Cu–AgNWs demonstrated the formation of a core–shell structure having a ≈60 nm core and a rough coating with an average ≈20 nm shell (Figure 13b). The authors investigated the conductivity of several samples in ambient conditions, including those of the pristine CuNW films, Cu–AgNW films, and the CuNW/PDMS composites, to understand storage stability (Figure 13c). The data demonstrated that the Cu–AgNWs film is completely impervious to oxidation and it retains the original conductivity (not given) even after exposure for 30 h, which is much greater than those of both CuNWs and CuNWs/PDMS films after testing for the same period of time. To fabricate a flexible e-skin, a Cu–Ag NW suspension was drop-cast on a microstructred PVA film duplicated from a rose petal, serving as a mold. Then, a PDMS precursor mixture was drop-cast on top of the Cu–Ag NW/PVA film and cured at 60 °C for 24 h. After that, two microstructured Cu–
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AgNWs/PDMS were laminated with the microstructured surfaces facing each other, and copper wires were used as contact electrodes (Figure 13d). Cu–AgNWs/PDMS devices without a microstructured surface were also fabricated as control. The cross-sectional SEM image of the e-skin demonstrated that the microstructured patterns on two Cu–Ag NWs/PDMS conductors overlapped each other (Figure 13e). The resistance change of the e-skin with pressure from 0 to 20 kPa was tested (Figure 13f) and it was found negligible for the e-skin with the planar PVA surface. However, that of the e-skin with the microstructured PVA was found very sensitivite to presssure, with an Sp of
≈1.35 kPa−1 at low pressures (0–2.0 kPa) and ≈0.1 kPa−1 at higher pressures (2.0–5.0 kPa). The microstructured e-skin exhibited high operating stability with no Sp change after loading/unloading at a pressure of 2 kPa for more than 5000 cycles (Figure 13g). Also, an e-skin patch (0.5 cm × 2.0 cm) was attached to a person’s neck to monitor the voice vibrations (Figure 13h) and to the wrist to detect the blood pressure (Figure 13j). The relative resistance changes of the patch given in Figure 13i and Figure 13k, respectively, demonstrate a clear response to both stimuli, demonstrating that this e-skin may be a robust candidate for physiological diagnosis.
Based on percolation effect, Wong’s group reported stretchable CuNWs/SBS composites as strain sensors.[133] The as-prepared CuNWs exhibit a average diameter of 45 nm with a length more than 100 µm. An appropriate amount of CuNWs was mixed in an SBS/chloroform solution and then drop-cast on a mold or printed on paper to form stretchable CuNWs/SBS conducting features. SEM images show that the CuNWs are homogeneously dispersed in the SBS matrix. The authors found that the conductivity of the CuNWs/SBS composite increased from 217 S cm−1 to 1858 S cm−1 as the weight fraction of CuNWs was increased from 20 wt% to 50 wt%. For a 40 wt% CuNWs/SBS composite film, the ΔR/R0 increased negligibly with strain when the strain was less than 40%, then it increased
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significantly after a ≈80% strain reaching ≈1300 for a 160% strain. A test over 1000 cycles demonstrated that the resistance of the 40 wt% CuNWs/SBS film completely recovered when the stretching strain was maintained at ≤10%. Finally, the authors demonstrated implementation of this conductor for printed interconnects serving LED lighting. The LED remained functional when the printed interconnects were subjected to severe bending.
Aerogels are 3D network porous solid materials with cavities filled with a gas, typically air.[134] In recent years, conducting aerogels based on metal nanowires, carbon-based materials, organic polymers, etc. have begun to be utilized as pressure sensors because of their soft but robust structure, good electrical conductivity, and ultralow density.[134-135] Xu et al. fabricated a CuNW-based aerogel by freeze-drying CuNW hydrogels, which were synthesized and self-assembled into the sol flocs by reduction of CuSO4 with NH2NH2 and ethylenediamine (EDA) in a concentrated NaOH solution.[136] The as-produced CuNW aerogel exhibited very low densities (4.3–7.5 mg cm−3) as well as high porosity (>99.9%). The density was controlled by tuning the reaction parameters such as reactant concentration and the reaction temperature. Figure 14a,b display the highly porous network of this CuNW aerogel where the NWs spontaneously formed a 3D structure without the need of additional binding agents. SEM analysis indicated that these NWs exhibit a uniform size with an average diameter of ≈130 nm (Figure 14c,d). The aerogel monoliths were assembled into a pressure sensor by sandwiching them between a Ni tablet and an Al foil, then bonded with Cu wires by a silver conductive paste to form the circuit. The compressive and recovery stress–strain curve of the CuNW aerogel recorded two regions of deformation: the first, nearly linear, for ε < 40%, and a second densification region for ε reaching 60% (Figure 14e). Furthermore, sensors based on a CuNW aerogel with different densities displayed distinctly different pressure-sensing curves (Figure 14f).
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Thus, the sensor with the most porous structure exhibited the highest Sp of 0.7 kPa−1 while that based on the most dense one was considerably lower (0.02 kPa−1). In addition, the detection limit of the least dense structure was smaller (14 Pa) than that of the most dense aerogel sensor (43 Pa), demonstrating the importance of controlling the conductor bulk microstructure (Figure 14g).