Matrix-Independent Highly Conductive Composites for Electrodes and

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Matrix-Independent Highly Conductive Composites for Electrodes and Interconnects in Stretchable Electronics Wei Guo, Peng Zheng, Xin Huang, Haoyue Zhuo, Yingjie Wu, Zhouping Yin, Zhuo Li, and Hao Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21836 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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ACS Applied Materials & Interfaces

Matrix-Independent Highly Conductive Composites for Electrodes and Interconnects in Stretchable Electronics Wei Guo†,#, Peng Zheng‡,#, Xin Huang†, Haoyue Zhuo‡, Yingjie Wu‡, Zhouping Yin†, Zhuo Li*,‡ and Hao Wu*,† † Flexible

Electronics Research Center, School of Mechanical Science and Engineering,

Huazhong University of Science and Technology, Wuhan, Hubei, 430074, P. R. China. ‡ Department

of Materials Science, Fudan University, Shanghai, 200433, China.

* E-mail: [email protected] (L. Zhuo); [email protected] (H. Wu); KEYWORDS: silver flakes, iodization, silver nanoparticles, conductive composites, on-skin electronics, electrophysiological monitoring, human-machine interface.

ABSTRACT: Electrically conductive composites (ECCs) hold great promise in stretchable electronics due to their printability, facile preparation, elasticity, and possibility for large area fabrication. A high conductivity at steady state and during mechanical deformation is a critical property for ECCs, and extensive efforts have been made to improve the conductivity. However,

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most of those approaches are exclusively functional to a specific polymer matrix, restricting their capability to meet other requirements such as the mechanical, adhesive and thermomechanical properties. Here we report a generic approach to prepare ECCs with conductivity close to that of bulk metals and maintain their conductivity during stretching. This approach iodizes the surfactants on the commercial silver flakes, and subsequent photo exposure converts these silver iodide nanoparticles to silver nanoparticles. The ECCs based on silver nanoparticles-covered silver flakes exhibit high conductivity because of the removal of insulating surfactants as well as the enhanced contact between flakes. The treatment of silver flakes is independent of the polymer matrix and provides the flexibility in matrix selection. In the development of stretchable interconnects, ECCs can be prepared with the same polymer as the substrate to ensure strong adhesion between interconnects and the substrate. For the fabrication of on-skin electrodes, a polymer matrix of low modulus can be selected to enhance conformal contact with the skin for reduced impedance.

INTROUDUCTION Stretchable electronics show great potential in various applications such as health monitoring,1-5 prosthetics,6-8 and robotics.9,

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As vital parts of stretchable electronics systems,

electrodes and interconnects are required to exhibit several features at the same time,11 including high conductivity, capability to maintain the conductivity during mechanical deformation, elasticity, and strong adhesion/mechanically matching with the substrate. While it can be challenging to accommodate all those requirements simultaneously, even more properties of electrodes/interconnects are also expected in some application scenarios. For instance, in


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epidermal electronics or on-skin electronics, where stretchable electronic devices have great promise for health monitoring1-5 and human-machine interface (HMI),12-15 it is desirable that electrodes can achieve and maintain conformal contact with human skin through fine tuning of modulus to reduce the electrode-skin impedance.1, 2, 12, 14, 16 Generally there are two categories of strategies for the fabrication of stretchable interconnect materials. The first one is to adopt metal lines (including liquid metal lines) with deliberately designed microstructures, including serpentine,17, 18 nanomesh19, 20 and accordion motifs .21 The stretchability is accommodated via the releasing of these microstructures and the adhesion with the substrate is achieved by Si-O-metal bonding after hydrophilic treatment of substrate (mostly polydimethyl siloxane (PDMS)) at selective areas.22 The other category is to fill polymer composites with conductive fillers, such as carbon nanotubes,23-27 graphene,28 metal nanowires,2937

metal nanoparticles,38 and metal flakes.39-43 Compared with the first strategy, the polymer

composite materials have the advantages of facile preparation, printability, intrinsic elasticity, and therefore can enable cost-effective fabrication of large area stretchable electronics. However, their electrical conductivity is usually inferior to pure metals. Extensive studies have been performed to enhance the conductivity of stretchable conductors at steady and stretching states. Kim et al. reported self-organization of gold nanoparticles in polyurethane, giving rise to high conductivity during stretching.38 Matsuhisa et al. utilized phase separation of silver flakes in a fluorine rubber with a fluorine surfactant to obtain high conductivity.40 Matsuhisa et al. and our previous work demonstrated in-situ formation of silver nanoparticles in silver flake filled ECCs that can promote the conductivity in the presence of a fluorine surfactant and a reducing agent, respectively.42, 44 Despite the effectiveness of these methods, they are usually exclusively applicable to specific polymer matrices. When a polymer matrix has to be selected to mechanically match the low


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modulus substrate, or improve the adhesion with the human skin, the above mentioned methods may not work. Therefore, a generic method that can enhance the conductivity of stretchable composites in any polymer matrices is highly desirable in the construction of stretchable electronic systems. Here we report the development of highly conductive silver micro-flakes filled electrically conductive composites (ECCs) independent of polymer matrices. Commercial silver flakes are iodized with KI and subsequently exposed to sunlight to convert the surface lubricant and silver oxide to silver nanoparticles. These nanoparticles-covered flakes can be added to any polymer matrix and obtain much higher conductivity at steady state and during deformation in comparison to pure flakes filled composites. Through this approach, for the fabrication of stretchable interconnects, ECCs can be prepared by mixing the treated flakes with polymer matrix of the same materials as the substrate to form a strong chemical bonding between the ECCs and the substrate. Our PDMS-ECCs has a resistivity of 9.43×10-5 Ω·cm at 75 wt.% filler loading and keeps the resistivity increase within three times at 119% elongation. The PDMS-ECCs interconnects on the PDMS substrate demonstrate great interface reliability. We also extend the utility of this approach for electrophysiological (EP) monitoring and implementation of HMI tasks through the development of on-skin electrodes. The adoption of a much softer silicone matrix for the fabrication of the electrodes leads to more conformal contact with human skin and improved performance in the capture of EP signals.

RESULTS AND DISCUSSION Matrix-independent treatment of silver flakes


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Silver flakes are excellent conductive fillers for ECCs due to the commercial availability and their capability to maintain the electrical conductivity by parallel alignment during stretching.44, 45 However, commercial silver flakes are generally covered with a layer of lubricant in order to prevent cold welding during processing. This lubricant layer, usually in the form of silver salts of long chain fatty acid,46, 47 hinders the electrical conduction between neighboring silver flakes in ECCs. Previous efforts to improve the conductivity include using polymer matrices with reducing groups to chemically reduce the lubricant and oxides to silver nanoparticles,42, 44 using thermally resistant polymer matrices to decompose the surface lubricants during high temperature curing,48 and using polymer matrices that can induce filler aggregation by phase separation.40 Despite of the effectiveness of these methods, all of them can only be applied to a specific type of polymer matrix. Different from previous studies, the present method has silver flakes treated before mixing with the polymer matrix and is completely independent of the properties of the polymer matrix. Commercial silver flakes are firstly iodized to convert the surface lubricant and silver oxide to silver iodide, which subsequently gets decomposed and forms the surface silver nanoparticles upon exposure to sunlight. These silver nanoparticles covered silver flakes have less energy barrier for electrical conduction because of the removal of surface lubricants. Moreover, the nanoparticles may sinter with neighboring silver flakes and further enhance the conductivity. Figure 1a and Figure S1 show the X-ray photoelectron spectroscopy (XPS) survey scan and high resolution spectra of untreated and KI treated silver flakes, respectively. After KI treatment, the intensity of carbon and oxygen signals from the surface lubricant decreases, indicating the partial replacement of the surface lubricant. Moreover, after KI treatment, iodine trace is clearly seen in the survey spectrum (Figure 1a) even after the flakes are intensively washed with ethanol and water. In the high resolution spectra (Figure S1 (c)), the peaks at 619.3 eV and 630.7 eV


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represent I 3d5/2 and I 3d3/2, respectively, which corresponds well with the literature value of 619.4 eV for AgI.49 This substitution reaction is further confirmed by Raman spectra and thermogravimetric analysis (TGA) results. The presence of silver carboxylate based lubricant is evident by the stretching of COO− (1393 cm−1), the C–COO− group (927 cm−1), the C-C backbones (1092 and 1149 cm−1), and the scissor (1431cm−1) of methylene group in Raman spectra (Figure 1b).50 These peaks gradually reduce while the KI concentration increases; the peaks almost disappear when 1.99 mg KI per gram silver flakes is applied. All mentioned above indicate that the lubricant layer has been mostly removed. In the TGA results (Figure 1c), the untreated silver flakes have an obvious degradation at around 200°C, owing to the decomposition of the lubricant layer.48 At temperatures higher than 200°C, the gradual weight loss may be from the decomposition of silver oxide.50 It is interesting to find that the weight loss attributed to the silver carboxylate lubricant decreases with increasing the KI concentration, indicating more lubricant is removed by increased amount of KI. Correspondingly, a new degradation appears at around 680°C probably due to AgI,51 and this weight loss increases with increasing the amount of KI. According to our calculation, the decreased amount of silver carboxylate lubricant after KI treatment matches well with the increased amount of AgI formation (Table S1), indicating that silver carboxylic lubricant is converted to silver iodide after KI treatment and this conversion is more complete at higher KI concentration. The morphologies of pristine, and KI treated silver flakes are shown in Figure 1(d-e). Silver flakes have an average diameter and thickness around 2.75 μm, 125 nm respectively (Figure S2 b, c). After treatment with KI, a number of AgI nanoparticles appear with a diameter in the range of tens of nanometers (Figure 1e). Due to the unique photosensitivity of AgI, we expose KI treated silver flakes to sunlight and expect the AgI nanoparticles to be decomposed into silver nanoparticles and iodine. As shown in


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Figure S3, after photolysis, the weight loss due to AgI decreases with the increase of the light exposure time in the TGA curves, indicating that AgI nanoparticles are converted to Ag nanoparticles after light exposure. AgI nanoparticles converting to Ag nanoparticles can also be seen in transmission electron microscopy (TEM) / Energy Dispersive Spectroscopy (EDS) (Figure S4) where the weight percentage of Ag in the nanoparticles greatly increased after sunlight exposure. The morphology of silver flakes remains the same with plenty of nanoparticles (Figure 1f and Figure S2 (c)). The generated elemental iodine can be removed through repeatedly washing with ethanol and deionized water. The generated silver nanoparticles can sinter during ECC curing to provide effective metallic bond in ECCs. As shown in the SEM images (Figure 1f), most of the silver nanoparticles are located on the flake surface and are part of the flakes, which are beneficial in forming metallic contact with neighboring flakes especially after sintering. This is in contrast with previous reports of in-situ generated nanoparticles where most of those particles did not contribute directly to the conductivity of ECCs since they were scattered in the polymer matrix without contact with each other or flakes.42 Indeed, for ECCs consisting of 75wt.% silver flakes in PDMS matrix (Dow Corning Sylgard 184), the resistivity is 7.04×105 Ω⋅cm if pristine silver flakes are used. After the flakes are treated with KI (1.33 mg KI per gram silver flakes), the electrical resistivity reaches 1.11×10-3 Ω⋅cm because AgI has a higher conductivity and a shorted chain length than silver carboxylate lubricants. The resistivity further decreases to 6.08×10-4 Ω⋅cm after the AgI nanoparticles are exposed to sunlight for 10 minutes and are partially converted to silver nanoparticles.


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Figure 1. a) XPS survey scan of the untreated and KI-treated silver flakes (1g silver flakes treated with 1.33mg KI). b) Raman spectra of untreated, and KI treated silver flakes with different KI


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concentrations. c) TGA of pristine and KI treated silver flakes with different KI concentrations. Morphologies of d) pristine silver flakes, e) KI treated silver flakes (1g silver flakes treated with 1.33mg KI) and f) KI treated silver flakes with 60 min sunlight exposure. Stretchable ECCs based on treated silver flakes Stretchable ECCs can be synthesized simply by mixing the treated silver flakes with a polymer matrix. Figure 2a demonstrates the universal applicability of the present method in different stretchable matrices, including 3 silicone matrices (Ecoflex 00-30, Sylgard 184, Silbione LSR 4305), isobutylene-isoprene rubber (IIR), styrene butadiene rubber (SBR), nitrile rubber (NBR), and butadiene rubber (BR). In all cases, especially silicone matrices, a dramatic decrease in electrical resistivity is observed after using treated silver flakes. To be noticed that, the resistivity changes are different for different polymer matrices adopted. It is attributed to several factors, including different interactions between Ag flake surface and polymer matrices and different curing kinetics of polymer matrices. The resistivity can be further reduced by optimizing the KI treatment and curing conditions. Nevertheless, this result demonstrates the possibility to obtain similar high performance in other polymer systems. The versatility of the present method brings the freedom in selection of the polymer matrix in fabrication of different components in stretchable circuits. Here we demonstrate the interconnects and EP electrodes as two examples. In preparing the interconnects in stretchable electronics, we select one of the most common polymers used as stretchable substrate, PDMS (Sylgard 184), as the matrix for the ECC. It is expected that the ECCs can bond chemically with the PDMS matrix to eliminate reliability issues after various deformation. We firstly optimize the resistivity of PDMS-ECCs from three aspects, namely the concentration of KI (Figure 2b), the sunlight exposure time (Figure 2c) to find the


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optimum conversion amount of the surfactant on the Ag flake surface and the curing temperature (Figure 2d) to promote the sintering of Ag nanoparticles before they are fully cured. Higher concentration of KI is able to convert more surface lubricant layer to silver nanoparticles, but it will render the mixing of flakes and polymer matrix more challenging. According to the lubricant amount estimated from the TGA result, about 4.15 mg KI is needed to completely remove the lubricant in 1g silver flakes. When the KI amount increases from 0 to 1.33 mg, the electrical resistivity shows a dramatic decrease while further increasing the concentration of KI leads to increased resistivity probably due to the worse dispersion of flakes. Longer sunlight exposure time allows more sufficient conversion to silver nanoparticles and lower resistivity. A low resistivity of 9.43×10-5 Ω⋅cm is obtained with 60 minutes sunlight exposure. Although even longer exposure time may lead to a lower resistivity, we use 60 minutes exposure time to achieve the balance with efficiency. In terms of curing temperature optimization, two processes occur simultaneously during the curing of ECCs, namely, the coalescence of Ag flakes and the solidification of PDMS matrix. While high temperature can promote the coalescence of Ag flakes, the time for the curing of PDMS is shortened significantly. When the PDMS matrix is cured, the Ag flakes cannot coalescence due to the lack of mobility46. Therefore, the electrical resistivity decreases when the curing temperature increases from 140 °C to 160 °C, as the higher temperature promotes the sintering of silver nanoparticles. However, when the temperature reaches 180 °C, the curing of PDMS is too fast to allow sufficient sintering of silver nanoparticles before matrix solidification. Consequently, 160 °C is selected as the optimized curing temperature. The resistivity changes of PDMS-ECCs during stretching for strain up to 119% and under cyclic tests with 30% and 80% strain are investigated. The resistivity of PDMS-ECCs can be maintained below 3 times of original values at stretching of 119% strain. Figure 2e presents the


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excellent conductivity and stretchability of ECCs through comparisons with previously reported stretchable conductors.27, 29, 31-37, 42, 43, 52 It is worth noting that the stretchability of PDMS-ECCs is limited by the elongation limit of PDMS (~140% according to product data sheet), i.e., the stretchability of the PDMS-ECCs can be enhanced if polymer matrices of larger breakage strain are adopted. Moreover, in contrast to previously reported stretchable composite conductors whose resistivity increases a few orders of magnitude after only hundreds of loading cycles with strain

Matrix-Independent Highly Conductive Composites for Electrodes and

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