Flexible Mechanical Sensors Fabricated with Graphene Oxide-Coated Commercial Silk

1. Introduction

Flexible electronic components have been steadily developed in various fields, including electrically conducting textiles [1,2,3,4], thermoelectric textiles [5,6,7], solar cells [8], secondary batteries [9], and gas sensors [10,11,12]. Among them, flexible mechanical sensors (strain and pressure sensors) have been of interest because of their utility for sensing human body functions for healthcare, and for use in the Internet of Things (IoT). In particular, textile-based mechanical sensors have the advantage of being lightweight, low-cost, and having superior flexibility [13,14,15,16]. Commercial textiles such as cotton, nylon, polyester, and silk have been used as flexible substrates for such sensors. However, other components were used to ensure high sensitivity and flexibility.

Cotton fabric has been decorated with Ag nanowires, Ag flowers, and Ag thread [17,18,19,20], and coatings of electrochemically exfoliated graphene film on cotton fabric [21], and coatings of Ag on cotton yarn using Ag paste followed by sintering [22] have been used to provide electrical conductivity. Likewise, electronic textiles for sensors fabricated with nylon and polyester have been achieved using conducting polymer [23], Au and MoS₂-coating [24], graphite-polyurethane (PU) coating on crochet knitted elastic [25], screen-printing of elastic conductive carbon ink on polyester fabric [26], wet-spinning of CNT/thermoplastic PU on silver-plated nylon electrodes [27], coating of CNT/carbon black-PU on the fabric with Ag-coated conductive nylon fiber [28], and spraying of multiwall CNT/TiO₂ conductive suspension on nylon textiles [29]. Mxene has also been introduced to textiles to improve sensitivity [30,31,32,33,34]. In the previous studies mentioned above, complex processes are inevitably required for the fabrication of the sensors.

In the case of silk, C. Wang et al. reported that the wearable strain sensors with carbonized silk fabric had a large gauge factor, but elastic polymer was used due to the thermal instability of carbonized silk [35]. A pressure sensor was fabricated using Ag nanowires coiled onto silk fiber-wrapped PU [36], and 3D graphene oxide (GO) and a silk fibroin mixture thermally reduced at 700 °C [37]. However, they also involved a complicated procedure and the sensitivity was very low (0.136 ~ 0.54 kPa^-1).

Here we report flexible pressure and strain sensors fabricated with reduced GO (rGO)-coated commercial silk (rGOS) fabrics and yarns. The GO was coated by a simple dipping method and then the GO-coated silk (GOS) was thermally reduced at 400 °C. The pressure and strain sensors were prepared by stacking small pieces of rGOS fabrics and twisting rGOS yarns, respectively. The pressure sensor showed superior sensitivity (2.58 ×10³ kPa^-1 at low pressure and 1.842 kPa^-1 at high pressure). The gauge factor (GF) of the strain sensor was relatively low (~ 0.307) but the strain sensor was able to detect human motion (bending) and had good cyclability even after 5000 cycles.

2. Materials and Methods

GO was synthesized with graphite powder (99.995% metals basis, Alfa Aesar) by the modified Hummers method [38]. GO was dissolved in DI water and sonicated for 1.0 h. The GO concentration was 1.0 mg/ml. GO was coated onto the commercial silk fabrics by simply dipping for 30 min and then the GOS fabric was dried at 40 °C in a fume hood. The process was repeated three times to make a uniform coating of GO [2,12]. The GOS fabric was thermally reduced. The rGOS fabrics for the pressure sensors were obtained by thermal treatment at 400 °C for 2.0 hr at a heating rate of 1.0 °C/min in N₂ atmosphere. The rGOS fabrics were cut by 1.0 cm × 1.0 cm and then three pressure sensors were made depending on the number of fabrics. The strain sensors were fabricated by twisting the GOS yarns with an electrically powered drill. After twisting the GOS yarns, the yarns were thermally treated in the same condition.

To observe the morphology of the samples, field emission scanning electron microscopy (FE-SEM, JSM-7800F, JEOL, Japan) was used. The structure of the rGOS was determined by X-ray diffraction (XRD, SmartLab, Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 1.5418 Å), X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe II, Ulvac-PHI 5000 Versa Probe, Phi(Ø), Chigasaki, Japan), and Raman spectroscopy (Raman-LTPL system, Witec alpha300, Witec, Ulm, Germany) using 532 nm laser. The size of GO was measured by atomic force microscopy (AFM, XE-NSOM, Park Systems, Korea).

3. Results and Discussion

3.1. Structural Investigation

As-synthesized GOs were dispersed in DI water and then they were dropped onto a SiO₂/Si wafer to measure the size of GO by AFM (Figure 1a). The distribution of the lateral size was from 50 nm to 1.95 μm (Figure 1b). The average size of the GOs was 349 nm. Figure 1c,d show optical images of the GOS and rGOS, respectively. After thermal treatment, the dark brown color of the GOS changed to black color (rGOS), and the size of the fabric was reduced. The shrinkage of the fabric was also observed in SEM images of the GOS (Figure 1e) and rGOS (Figure 1f).

The Raman spectroscopy of GOS and rGOS confirmed the existence of GO on silk (the inset of Figure 1e). D and G peaks were observed in the GO, GOS, and rGOS. The D band arises from defects (disorder) and the G band is related to C-C stretching of the sp² carbon [39]. The D peaks were at 1341, 1359, and 1330 cm^-1 and the G peaks were at 1584, 1587, and 1558 cm^-1 for the GO, GOS, and rGOS, respectively. The peak shifts in the GOS compared with GO are attributed to overlapping with the peaks from the silk. After thermal treatment (rGOS), the D and G peaks were red-shifted, from 1359 cm^-1 to 1330 cm^-1 for the D peak and from 1587 cm^-1 to 1558 cm^-1 for the G peak. This indicates the increase in translational symmetry of the GOS due to the thermal treatment. This behavior was also found in the I_D/I_G ratio. I_D/I_G ratios of the GO, GOS, and rGOS, which were 1.06, 1.06, and 1.01, respectively.

Figure 2a displays the normalized XRD patterns of the GO sheet, GOS, and rGOS. The peak at 11.44° ((001) plane) of the GO sheet indicates that the interlayer distance of the GO is 7.732 Å. The XRD patterns of the GOS were similar to those of pristine silk [40]. After heat treatment, the peak for the (002) lattice plane at 22.95° appeared in the rGOS. The relatively broad peak for the (002) plane originated from the overlapping of various amorphous structures. [41].

The decrease in oxygen functional groups due to thermal treatment was investigated using the XPS C1s peak (Figure 2b–d). A large amount of oxygen functional groups (COOH, C=O/O-C=O, C-O/C-O-C/C-OH) was clearly observed in the GO sheet (Figure 2b) and GOS (Figure 2c). In contrast, the functional groups significantly decreased in rGOS (Figure 2d). At the O1s peak, the amount of O-H, C-O, and C=O species in the GOS was similar to that of the GO sheet but C-O bonds decreased in the rGOC (Figure S1a–c). The nitrogen from the silk was found in the N1s peak of the GOS and rGOS (Figure S1d–f). Interestingly, pyridinic-N developed after thermal treatment, resulting from carbonized silk. The XRD, Raman spectroscopy, and XPS studies showed the reduction of GO and the carbonization of silk, which endow electrical conductivity to rGOS.

3.2. Pressure Sensors

Figure 3a is a side view of a single rGOS fabric. The average thickness is 349.6 μm. The flexible pressure sensors were made very simply with the rGOS fabrics. First, the fabric was cut into a square, whose length on a side was 1.0 cm. Next, the cut fabrics were stacked between Al plates (Figure 3f). We chose 3, 5, and 7 layers of fabric. The thickness of the 7-layer pressure sensor was about 4.52 mm (Figure 3b).

The sensitivity of the three sensors is depicted in Figure 3c-e. The sensitivity of the 3-layer pressure sensor was 3.32 kPa^-1 at low pressures (0.5 - 21 kPa), 0.662 kPa^-1 at 21 – 46 kPa, and 0.665 ×10^-3 kPa^-1 at high pressures (46 – 290 kPa) (Figure 3c). For the 5-layer pressure sensor, the sensitivity was 18.7 kPa^-1 at low pressures (0.4 – 10.9 kPa), 3.82 kPa^-1 at 10.9 – 91.3 kPa, and 2.57 ×10^-3 kPa^-1 at high pressures (91.3 – 402.1 kPa) (Figure 3d). Interestingly, an enormous increase in sensitivity was measured in the 7-layer pressure sensor. The sensitivities were 2.58 ×10³ kPa^-1 at low pressures (0.2 – 72.3 kPa) and 1.84 kPa^-1 at high pressure (72.3 – 421 kPa) (Figure 3e). To the best of our knowledge, the sensitivity at low pressure of the 7-layer sensor is the best among the textile-based sensors except for the pressure sensor fabricated with Ag nanowire (AgNW)-coated cotton fabric [17] (Table 1). The operation of the sensor is shown in Figure 3f. Without pressure the resistance (R) of the sensor was large because the rGOS layers barely contacted each other. The R significantly decreased as soon as pressure was applied, due to the electrical contact between the layers. The number of layer-dependent sensitivity was determined by the initial R of the sensors. The increase in the layer number increased the initial R. For example, the resistances of the 3-layer and 7-layer sensors were 0.42 MΩ and 958 MΩ, respectively.

With these results, we fabricated a two-by-two pressure module with the rGOS fabrics (0.5 × 0.5 cm²) and Au wires. The surface of the module was protected with pristine silk fabric (Figure 3g and 3h). The module directly measured the applied force, indicated by different colors with the Labview program (Figure S2). Figure 4 displays the measurement of the pressures of the four sensors in the module. The numbers in the red and white boxes indicate the applied forces. The response time of the sensors was 300 ms (Figure S3).

3.3. Motion Sensors

The strain sensors were fabricated with GOS yarns. First, the GO-coated yarn was twisted using an electrically powered drill (Figure 5a). Figure 5b shows the variation in force while twisting the yarn. Before being twisted, the force was unstable but the force was relatively stable after twisting began (yellow box). Then, the twisted GOS yarn was thermally treated using the same condition as that for fabricating the rGOS fabric. Evenly twisted yarn was found in the SEM image (Figure 5c).

Figure 6 depicts the strain-dependent R. As the twisted yarn was stretched from 4.40 cm (L0) to 7.15 cm (162.5%) (Figure 6a), the R changed from 2.295 MΩ (R0) to 1.855 MΩ (Figure 6b). The decrease in R can be explained by the increase in the electrically conducting paths because the rGOS fibers in the yarn contacted each other when the yarn was stretched. Although the R slightly increased (R = 2.308 MΩ, see the red triangle in Figure 6), R almost recovered when the strain sensor was released again. The increase in R was caused by breaking some parts of the fibers when the yarn was stretched to its maximum, as shown in Figure S4. The variation in R was 19.17% at 162.5% of stretching. The GF is defined by (ΔRL₀)/(R₀ΔL). The GF of the strain sensor was small (GF = 0.307) compared with previous studies [25,29,33,35] but it was larger than the cotton yarn with Ag-paste [22] and Ag-coated nylon [42]. Hence, we found that the twisted rGOC yarn could be used as a motion sensor.

Figure 7 shows the detection of finger motion. The twisted rGOS yarn was attached to a Latex glove with Ag foil to monitor the motion of the finger (Figure 7a). The current was approximately 240 nA when the finger was spread out. As the finger was bent about 90°, the current increased to ~ 280 nA. The change in the current was about 16.7% (Figure 7b). This indicates that the twisted rGOS yarns functioned as a motion sensor. To check the cyclability for bending, we made a hand using 3D printing with a motor (Figure 7c and Figure S5). The variation in current (the ratio, ΔI/I₀, I₀ is the current corresponding to stretching and ΔI = I – I₀) for bending and stretching was stable for 5000 cycles (Figure 7d). Figure 7e and 7f display variations in current near 2500 cycles and 5000 cycles, respectively. This indicates the good cyclability of the motion sensor even after 5000 cycles. The difference in current variation between the human finger (16.7%) and the artificial finger (~3.5%) was due to the bending degree. The human finger was bent 90° but the artificial hand could only be bent to a maximum of about 45°. The double peaks during bending were caused by two finger joints, as shown in Figure 7c (white circles).

4. Conclusions

In summary, we fabricated flexible pressure and motion sensors with GO and commercial silk fabric and yarns, respectively. The sensors were simply produced by dipping the silk into the GO solution followed by thermal treatment. The pressure sensors were achieved by stacking the rGOS fabrics. The sensitivity of the 7-layer pressure sensor was 2578 kPa^-1. As shown in Table 1, this value is very large compared to other flexible pressure sensors. The GF of the motion sensors obtained from the twisted rGOS yarns was 0.307. The motion sensors successfully detected the bending of a finger, and the response for bending and stretching was stable up to 5000 cycles. This study provides the possibility to synthesize ultrasensitive pressure and motion sensors without complicated procedures.