Stretchable conductive adhesives for connection of electronics in wearable devices based on metal-polymer conductors and carbon nanotubes

Abstract

Stretchable wearable devices with various functions are being continuously developed. However, the problem of how to connect rigid electronics to stretchable circuits has not been properly solved. In this study, we use stretchable conductive adhesives to realize stretchable connections between electronics and circuits, so that the overall reliability of stretchable wearable devices can be strengthened. The adhesives are based on metal-polymer conductors where polydimethylsiloxane is used as an elastomer matrix, while liquid metals form electrically conductive networks collectively with carbon nanotubes. The composites show superior adaptability to stretchable wearable devices due to excellent stretchability (maximum tensile strain: 40%) and stability of electrical conductivity in deformation ($\Delta$R/R < 16% at maximum tensile strain), which goes beyond existing conductive adhesives, and can satisfy the requirements of most applications. A stretchable light emitting diode screen is fabricated using the stretchable conductive adhesives and exhibits good performance under cyclic deformations.

Introduction

Stretchable wearable devices, as a new technology developed in the last two decades, provide a skin-contacted platform for body information monitoring [[1], [2], [3], [4], [5]], personalized medical treatment [6] and customized fabrication [1,7], so forth. Most research seems to focus on the development of conductors and various applications, leaving the basic problem of how to fit widely-used but rigid electronics into stretchable circuits unsolved. Conductors are designed to be deformable through serpentine structures or stretchable materials like silver and liquid metals [[1], [2], [3], [4], [5], [6]]. Nevertheless, three-dimensional electronics and two-dimensional conductors cannot form a strong combination or share enough contact area due to the poor compatibility between them. Connections of electronics in general wearable devices are easy to break under large deformations because of the concentrated stress and detriment the overall performance of these devices [8,9]. Some strategies were proposed to solve the problems of weak connections of electronics in wearable devices. For example, customized flexible printed circuit boards (FPCBs) were designed to fabricate complex circuits containing many electronics. However, the stretchability of FPCBs is rather limited and it usually takes a long time to redesign and manufacture FPCBs [2,3]. Besides, electrically conductive adhesives (ECAs) were also developed to replace highly toxic tin/lead-based solders and have been used in wearable devices [[10], [11], [12], [13]]. However, most reported ECAs exhibited weak tolerance to stretching and unstable electrical conductivity [10,11,13]. In addition, the high percolation thresholds of metal nanoparticle fillers used in ECAs leads to high cost of them. Therefore, a stretchable, stable, and low-cost ECA is urgently required to strengthen the internal connection of stretchable circuits.

Among various materials, liquid metals (LMs) and metal-polymer conductors (MPCs) with high stretchability and suitability on different kinds of substrates have been used as circuits, sensors, antennas and other electronic components in wearable devices [2,7,14,15]. By embedding metal particles (usually liquid metal droplets) into elastic polymers, MPCs can be easily fabricated and printed into any desired patterns [1,7]. More importantly, the properties of MPCs can be tuned by solely changing the mass ratio of liquid metals and polymers as they are in different phases (liquid versus solid). Besides, carbon nanotubes (CNTs) show unique advantages in fabricating wearable devices due to their excellent electrical conductivity, mechanical strength, structural flexibility, extremely high aspect ratio and so forth [16,17]. CNTs can be used as conductive inks/wires [[18], [19], [20], [21]], sensors, flexible electronics [[22], [23], [24], [25]], and so forth in wearable devices, as well as co-fillers to obtain considerable electrical conductivity with low content of metal particles [10]. A proper combination of MPCs and CNTs is thus expected to exhibit a better performance as stretchable conductive adhesives (SCAs).

In this work, a high-performance SCA connecting rigid electronics and deformable circuits was developed by employing PDMS elastomer-based MPCs and CNTs (Fig. 1a and b). The as-fabricated SCAs exhibited good stretchability, electrically conductive stability, and strong adhesion to polymer substrates. Besides, by transferring stress locally collected by electronics to larger areas, SCAs can also act as a stress buffer to avoid the breaking of circuits during deformation and thus can improve the overall performance of stretchable wearable devices. The SCAs exhibited synergistic advantages of mechanical stretchability of 140% of original length from PDMS elastomers and conductive networks with conductivity of ~10 S/m and a low resistance change of 16% from spherical liquid metal particles and linear CNTs. In addition, the tensile modulus of SCAs was ca. 2 MPa, which is similar to the elasticity of human skins [26]. We also fabricated a stretchable light emitting diode LED screen using the as-fabricated SCAs, which exhibited stable performance during multiple bending and stretching deformations. The excellent performance and low cost of raw materials make the as-fabricated SCAs promising potential for large-scale fabrication of stretchable wearable devices.