Facile fabrication of Ag-doped graphene fiber with improved strength and conductivity for wearable sensor via the ion diffusion during fiber coagulation

Highlights

• We for the first time prepared Ag-doped reduced GO fiber (Ag-rGOF) via the introduction of Ag⁺ in the coagulation bath.

• The Ag⁺ in the bath could greatly facilitate fiber coagulation and improve the strength and conductivity of the fiber.

• The fibrous strain sensor based on Ag-rGOF showed high gauge factor, rapid response and outstanding durability.

Abstract

Graphene fibers with excellent mechanical and electrical properties are in great demand in the field of modern wearable microelectronics. However, presently the strength and electrical conductivity of graphene fibers are difficult to meet the requirements, limiting their application in wearable sensors. Herein, a facile approach is proposed for fabricating Ag-doped reduced graphene oxide fiber (Ag-rGOF) via introduction of Ag ion (Ag⁺) into fluid graphene oxide fiber (GOF) in the coagulation bath during wet-spinning followed by chemical reduction. The addition of Ag⁺ is beneficial to fiber coagulation and improves not only conductivity of the fiber, but also its mechanical properties. The tensile strength reaches up to 334 MPa and the conductivity to 1.14 × 10⁴ S/m, about 4 times as high as that of reduced graphene oxide fiber (rGOF). The fibrous strain sensor based on Ag-rGOF prepared with thermoplastic polyurethane as support shows high gauge factor of 177 at over 20% strain, rapid response time of 55 ms, and outstanding durability after 1000 stretch/release cycles. We also demonstrate that the fibrous sensors can be attached on skin to monitor sensitively facial expression and human motions.

Introduction

Nowadays, varieties of sensors have been adopted in smart detection devices used in industrial production, smart home, medical diagnosis and environmental protection [1], [2], [3], [4]. Particularly, the applications of wearable strain sensors have great potential in the areas such as electronic skin [5] and smart textiles [6]. Recently, many conducting nanomaterials [7], [8], [9] have been used in sensors, and particularly, as flexible nanosheet with outstanding electrical conductivity [10] and mechanical merits [11], graphene is extensively studied for being used as wearable sensors [12], [13], [14], [15], [16]. However, like most sensors, graphene-based sensors are generally film-shaped or foam-shaped because of their relative ease of fabrication [17], [18], [19], [20], [21], [22], [23]. Compared with films and foams, fibers with excellent mechanical and electrical properties are in great demand in the field of modern microelectronics, especially for the applications in wearable devices [24], [25]. Their better weaving performance is considered the main reason why the fiber-shaped sensors are more suitable to be used in wearable intelligent textiles. There are increasing sensors based on graphene fiber or the fiber containing graphene [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], but considering the performance of sensors, the former is expected to be more promising due to its special properties [36], [37].

In view of the practical applications, as stated above, good electrical conductivity and mechanical properties are demanded for a sensor, particularly wearable sensor. Indeed, graphene sheet itself has the excellent properties, but the performance of graphene fibers reported heretofore is far below those properties [38], [39]. The reduced graphene oxide (rGO) fibers show low mechanical properties and conductivity [40] because of harsh oxidation process for graphene oxide (GO) [41]. In addition, weak interaction between graphene sheets, poor orientation and voids inside the fiber are also the reasons for the ill mechanical properties of the fiber [42], [43]. Consequently, several attempts have been made to improve the performance of graphene fibers, and among these, introducing some components is one of the effective approaches [44], [45], [46], [47], [48], [49], [50]. As far as electrical conductivity is concerned, the addition of Ag has attracted much attentions. There are several methods of introducing Ag element into graphene fiber, such as direct addition to the spinning solution [51], immersion of the formed fiber into Ag ion (Ag⁺) solution [52], and electroplating Ag on the fiber surface. On the other hand, adding reinforcing components to spinning dope before fiber preparation is a common method to improve the mechanical properties, but there were few researchers who added reinforcing elements during fiber coagulating stage. Introducing Ag element into the fluid fiber in the coagulating stage is easy to operate, which can avoid nonuniform dispersion of spinning dope. What’s more important, it can facilitate fiber coagulation through formation of salt according to our previous work [53]. It is expected to simultaneously improve the conductivity and mechanical properties of the fiber due to the compacting fiber structure [54].

Because of the conductive mechanism of quantum transmission, the sensors based on Ag nanoparticle usually have high sensitivity [55]. In addition, the recoverability and carrier mobility of graphene enable the strain sensors with high reproducibility [56]. Thus sensors based on Ag and graphene have attracted increasing interest [57], [58], [59], [60], [61], [62], [63], but most of them are non-fiber based sensors [57], [58], [59], [60], [61]. There are only limited researches on the temperature sensors or pressure sensors composed of the fibers with Ag and graphene as conductive filler or coating [62], [63]. However, there has been no report on Ag-doped graphene fiber based strain sensor so far. Detecting human motions is the primary and important function of wearable sensors, and recently, limited work has been done for the wearable sensors containing fiber in which the dominant ingredient is graphene [26], [30].

Hence, in this work, we reported strain sensors composed of Ag-doped reduced graphene oxide fiber (Ag-rGOF) embedded in thermoplastic polyurethane (TPU) matrix. A facile approach was presented for fabricating Ag-rGOF with improved both mechanical properties and conductivity via introduction of Ag ion (Ag⁺) into fluid GO fiber (GOF) in the coagulation bath during wet-spinning followed by chemical reduction. The sensors based on Ag-rGOF could well monitor human motions such as muscle movement, finger bending, blinking, smile and swallow with high sensitivity, rapid response and outstanding durability.