Research background of the field:
Conventional metal and semiconductor based strain sensors cannot fulfil these requirements because of their limited flexibility and sensitivity. To address the above issues and attain the required performance for an ideal flexible strain sensor, many researchers have investigated the use of various nanomaterials, such as nanowires, nanoparticles, carbon nanotubes and graphenes. Among them, graphene, graphene-metal nanowire hybrids, graphene foam and graphene on woven copper mesh have been intensively studied because of their superior flexibility, high conductivity and robust mechanical strength. However, the fabrication processes for these devices require either expensive chemical vapour deposition (CVD), harmful etching, complicated mixing processes, or a combination of these factors. Herein, we have presented a novel and facile approach for developing bendable and foldable strain sensors based on paper substrates patterned with reduced graphene oxide (rGO-paper) that allow full-range operation of wearable electronics. On the one hand, rGO-paper sensors maintain the simplicity of paper-based devices; on the other hand, these devices are inexpensive, scalable and highly sensitive to tiny deflections over a broad sensing range.
Our contributions and innovations:
I. Design and development of paper based reduced graphene oxide strain sensor
Our graphene oxide (GO) patterning methods use desktop digital craft cutters for the fabrication of the masking layer, which is followed by the drop casting of an aqueous GO solution onto the exposed surfaces to form the desired patterns. After this step, the dried GO patterns are reduced to rGO using photo-thermal energy by means of a commercial camera flash. To fabricate rGO-paper based sensors with silver patterns, we used a multi-layer masking method, consisting of alternating cutting and drop casting process for each material.
II. Sensitivity of rGO-paper strain sensor:
Gauge factor of our flexible rGO-paper sensors is about 66.6 ±5 within 6 % strain. Highly sensitive rGO-papers can detect a strain change as minute as 0.001 and the output signal is highly repeatable. The experimentally measured relative resistances with respect to incremental steps in the bending angles indicate that the limits of detection are as small as -0.2° and 1° for compressive and tensile bending, respectively. Benefiting from their high sensitivity, foldable rGO-paper sensors not only have a wide detection range, but can also reach an extremely small detection limit of 0.1° over the period of cycles.
III.Performance as wearable sensors:
rGO-paper allows freedom in the choice of creative and delicate designs. Although rGO-papers have limited stretchability, their high folding and bending sensitivity enables the design of paper-based wearable sensors for the real-time monitoring of large range human movements. Apart from their use as wearable electronics, the outstanding sensitivity of rGO-paper sensors enables their use as paper keyboards.
Novelty of rGO-paper strain sensors:
Inexpensive paper-based devices have generated considerable interest because these devices are ubiquitous, light in weight, portable, flexible, foldable and biodegradable. Whereas previous studies on paper-based devices have focused on labs-on-a-chip, supercapacitors, actuators, and transistors, no significant attempt has been made to develop wearable paper electronics before our work. We have demonstrated the use of rGO-paper sensors as wearable devices for detecting human body moments and controlling robotic hands. Furthermore, highly sensitive rGO-paper sensors were used to develop a paper-based keyboard.
Compared with conventional graphene/GO patterning approaches, our proposed method not only is simple, rapid and straightforward, but also allows freedom in the choice of creative and delicate designs. The key advantage of our rGO-paper sensor fabrication process is that it supports sensor production in remote locations with limited laboratory facilities.
The rGO-paper sensors possess excellent bending and folding sensitivity over a wide detection range from 0 to ± 180° with detection limits of 0.2° and 0.1° for the bending and folding angles, respectively. The rGO-paper sensors provide a stable change in conductance during initial bending/folding cycles and allow us to plan for a low-cost and use-and-through paper-based sensor, which open a new door of biodegradable electronics.