Highly transparent tactile sensor based on a percolated carbon nanotube network

The demand for transparent and flexible electronic devices, which are an emerging technology for the next generation of sensors, continues to grow in both applications and development due to their potential to make a significant commercial impact in a wide variety of areas. Here, we demonstrate a highly transparent tactile sensor with 92% optical transparency in the visible range based on solution-processed 99% metallic CNTs attached on a polydimethylsiloxane (PDMS) film. We efficiently reconstructed the pressed, stimulated spatial location by increasing the injection current (Iinjection) during electrical resistance tomography (ERT) that computed the internal two-dimensional (2-D) resistivity distribution.The demand for transparent and flexible electronic devices, which are an emerging technology for the next generation of sensors, continues to grow in both applications and development due to their potential to make a significant commercial impact in a wide variety of areas. Here, we demonstrate a highly transparent tactile sensor with 92% optical transparency in the visible range based on solution-processed 99% metallic CNTs attached on a polydimethylsiloxane (PDMS) film. We efficiently reconstructed the pressed, stimulated spatial location by increasing the injection current (Iinjection) during electrical resistance tomography (ERT) that computed the internal two-dimensional (2-D) resistivity distribution.

with both satisfactory electrical characteristics and high optical transparency.In recent decades, there have been tremendous advances in an effort to obtain high-purity CNTs through the solution-based purification process (i.e., ultracentrifugation process), which allows CNTs to be rapidly and inexpensively separated according to their electronic type: semiconducting or metallic. 29,302][33] It has also been reported that high-purity metallic CNTs are more suitable for superior optical transparency because poorly conducting, strongly absorbing carbonaceous impurities, and semiconducting CNTs were eliminated through the densitygradient ultracentrifugation (DGU) process; 34,35 hence, high-purity metallic CNTs would be a good choice for use in the ERT-based tactile sensor with excellent optical transparency and high electrical performances for an aesthetic view of human applications.
The present work demonstrates a highly transparent ERT-based tactile sensor based on solutionprocessed 99% metallic CNTs adhered to polydimethylsiloxane (PDMS).We fabricated a highly conductive and transparent percolated network using metallic CNTs with high optical transparency of over 92% in the visible range.The percolated network of metallic CNTs was formed directly on a PDMS substrate using an easy, scalable, and low-cost spray-coating method.We investigated the mapping responses by the spatial resistivity changes, using ERT to evaluate the performance of the solution-processed metallic CNT-based tactile sensor.6][37] In particular, we focused on implementing efficient image reconstruction of the pressed position on our highly transparent tactile sensor by increasing the injection current (I injection ) to the outer electrodes when using ERT.We found that by increasing I injection , the resolution of the internal resistivity profile of the tactile sensor based on metallic CNT networks was improved when comparing the actual pressed area and the image reconstruction area. 27igure 1a shows the schematic diagram of the ERT measurement system.ERT reconstructs a two-dimensional (2-D) spatial resistivity distribution using the input-output current-voltage measurement method at the boundary of the conductive material.An adjacent ERT measurement requires a total of four electrodes, which must be set as follows: inject current into one electrode, another electrode to ground, and the other two electrodes to measure the voltage difference.The electrodes of the metallic CNT-PDMS film in this study were electrically interconnected with a copper alligator clip at the boundary.The electrical measurement was performed by a semiconductor parameter analyzer (Agilent 4156C) for applying a direct current (DC) source and measuring the output voltage difference and the switch mainframe (E5250A, Agilent) for sequential input-output current-voltage measurements.[9][10][11][12][13] An overview schematic of the ERT processes showing the spatial resistivity distribution is shown in Figure 1b.6][7][8] The inverse problem is a model that determines the spatial resistivity distribution of an unknown material based on numerous input-output experimental boundary electrode measurements.
For the transparent, conductive CNT-percolated film in an ERT-based tactile sensor, we produced the percolated network of the solution-processed 99% metallic CNTs on a thin PDMS substrate. 35igure 2a shows the spray-coating method for the 99% metallic CNT networks directly on the PDMS layer.The tactile sensor fabrication processes are briefly explained as follows.First, a flexible and transparent 1-mm-thick PDMS substrate was prepared by mixing PDMS prepolymer (Sylgard 184A, Dow Corning) and a curing agent in a ratio of 10:1 by weight.Next, the substrate was cleaned by oxygen plasma treatment and then functionalized with a poly-L-lysine solution (0.1% w/v in H 2 O; Sigma Aldrich) that acts as an effective adhesion layer for the deposition of the 99% metallic CNTs.The substrate was thoroughly rinsed with deionized (DI) water and dried with flowing nitrogen gas.Subsequently, the transparent metallic CNT network film was directly formed on the PDMS substrate by spray coating (150 spray coats) the pre-separated, metallic CNT solution (high metallic purity (99%) and a concentration of 0.01 mg/mL) on a hot plate at 100 • C, followed by thorough rinsing with isopropanol and DI water.Note that the spray-coating method is a facile and low-cost fabrication that can easily form the CNT percolated network on any large area substrate with high throughput.Moreover, the density of the CNT network film can be easily controlled to tune the performance of the sensor.As a final step, the substrate was annealed at 100 • C for an hour.
Figure 2b shows the image of the PDMS layer that was spray coated with 99% metallic CNTs with great flexibility and transparency.It is worthwhile to note that the percolated CNT network film formed by highly purified, solution-processed 99% metallic CNTs shows high optical transparency.Figure 2c shows the measured optical transparency of the CNT percolated film with greater than 92% in the visible range using a UV/Vis/NIR spectrophotometer (Cary 5000, Agilent), even though we used a large number of sprays for implementation of a highly conductive film (150 spray coats).Moreover, a reduction of only 5% in optical transparency was obtained compared to the bare PDMS.The transparency of the film was sufficient to render characters legible on background paper even in bright or dark environments.Until recently, the previously reported tactile sensors were based on carbon compound materials or a percolated network of CNTs and were generally opaque.However, our tactile sensors have a wider range of applications in terms of high transparency due to the advantages of highly purified, solution-processed metallic CNTs from the DGU process.In particular, electronic devices with high transparency, 35 including our tactile sensors, can be sufficiently utilized in human or smart display applications.Moreover, the scanning electron microscope (SEM) images of the percolated CNT network, as shown in Figure 2d, were found to be dense and uniform across the entire area, which is critical for reducing the variation of the sensing response and improving the sensing performance.The average CNT density obtained for 150 spray coats of the 99% metallic CNT solution was extracted as approximately 86 ± 3 tubes/µm 2 .
A total of 16 copper alligator clips were used to connect the electrodes to the metallic CNT network film; then, a total of 208 values of the voltage difference were collected through the adjacent ERT method.As shown in Figure 3a, a large PDMS frame was made to hold the copper alligator clips in place to minimize micro-shaking of the contact between the PDMS and electrodes.An adjacent ERT measurement requires a pair of voltage measurements (before and after stimulation).Therefore, an ERT mapping image can be reconstructed by acquiring 208 × 2 = 416 voltage measurements.Thus, in the adjacent method, the reconstructed image through ERT shows the rate of change of the relative resistivity value, not the rate of change of the absolute resistivity value.In addition, the reconstructed image is composed by the finite element method (6400 mesh elements in our work), and the change rate of the resistivity calculated approximately for each element is shown as a 2-D image.Note that the number of electrodes was arbitrarily set from 1 to 16 in order to determine the position where the pressure was applied.The performance of our squared-shaped ERT-based tactile sensor (30 mm × 30 mm) was tested using a 3.8-mm radius glass rod for applying constant pressure (62 kPa) at designated locations.The pressure was applied to the CNT networked film alternatively at positions between electrodes 14 and 15 and between electrodes 2 and 3.The resistance of the stimulated part of the metallic CNT-based tactile sensor was expected to increase by the morphological deformation of the percolated metallic CNT network; that is, in the stimulated part, the CNT network was loosened, resulting in a reduced current path and an increased spatial resistance, as schematically shown in Figure 3b.This is a well-known mechanism in network-based strain sensors composed of onedimensional materials such as CNTs. 35,38To verify the sensing performance of metallic CNT-based tactile sensor, the ERT measurements were performed under various I injection ranging from 20 to 80 µA to investigate the sensing response, as shown in Figure 3c.The best resolution spatial mapping image under the same applied pressure was reconstructed at the highest I injection of 80 µA because the highest I injection results in the largest potential difference.At this time, the estimated maximum resistivity change of the fabricated metallic CNT-based tactile sensor was extracted as 49.4 Ω•cm.In addition, the results showed that the error rate for the pressure response of our metallic CNTbased tactile sensor decreased by up to 6.1% for compared to the actual pressed area by the glass rod.However, unwanted images were also reconstructed near other electrodes due to measurement artifacts, such as micro-shaking of the contact between the CNT network during ERT measurements or the incompleteness of the one-dimensional segment model of the electrodes in the forward model. 39s I injection decreases, the pressure response of the reconstructed images begins to fade, and eventually the pressure response is hardly recognizable at the lowest I injection (20 µA).Our metallic CNT-based tactile sensor detects the accurately matched location with spatial stimulus because we use a spraycoating method that can uniformly and stably deposit the high-density metallic CNT film (150 sprays).From these results, we were able to obtain a high-resolution reconstructed image using ERT that can perform 2-D spatial resistivity change mapping by appropriately adjusting the I injection for the ERT measurements.Although we did not additionally measure the multiple pressure response induced by the simultaneous pressure of various shapes or the tactile response for the electronic skin, we believe that the distinguishable responses of stable and high-resolution sensors on the skin will enable human applications and smart displays.
In conclusion, we demonstrate a transparent tactile sensor based on solution-processed 99% metallic CNTs with high-density using a facile spray-coating method.In particular, tactile sensors based on high-purity metallic CNTs with reduced carbonaceous impurities that are poorly conducting and strongly absorbing through the DGU process, achieved a high optical transparency of over 92% in the visible range.In addition, the fabricated transparent tactile sensor was evaluated for resolution at a constant pressure using the ERT technique, which partially computes the internal resistivity change.Applying high I injection into the tactile sensor creates a high potential difference from constant pressure; thus, the reconstructed ERT image is clearly detected.In other words, acceptable sensing performance is controlled by the different I injection values of the tactile sensor.Therefore, we believe that our results will provide a significant turning point towards a transparent future for the sensor market.

FIG. 1 .
FIG. 1. (a)A schematic showing a measurement system for an adjacent ERT method for imaging a square-shaped CNT-PDMS film with 16 electrodes.(b) A schematic illustrating the ERT spatial resistivity reconstruction process.To solve for the spatial resistivity distribution, an iterative process utilizing a finite element model is used to solve the inverse problem until the solution converges.

FIG. 2 .
FIG. 2. (a) An image showing that a highly purified, pre-separated, 99% metallic CNT solution is spray coated on the PDMS layer during the sensor fabricating process.(b) An image of the fabricated metallic CNT-based tactile sensor with great flexibility and transparency.(c) Optical transparency of the bare PDMS film and the metallic CNT-PDMS film with 150 spray coats in the visible range.An image of the tactile sensor in bright or dark environments.(d) SEM images showing the 150 spray-coated metallic CNT networks on the PDMS.

FIG. 3 .
FIG. 3. (a) A real ERT measurement image of a tactile sensor with a copper alligator clip to connect to the metallic CNT networks on the PDMS.(b) Illustration of a CNT network formed on the PDMS before and after pressure is induced.(c) The reconstructed images that are shown through the ERT measurements according to the different I injection of the fabricated tactile sensor.
This work was supported by the National Research Foundation (NRF) of Korea under grants 2016R1A2B4011366 and 2016R1A5A1012966 and by the Future Semiconductor Device Technology Development Program (Grant 10067739) funded by MOTIE (Ministry of Trade, Industry & Energy) and KSRC (Korea Semiconductor Research Consortium).