Micro‐Pore Composed Soft Pressure Sensor with Ether‐Surfactant via High‐Sensitivity in Wide Range for Robotic Machine Interface Application

Robotic machine interfaces with sensor devices in robotics applications can facilitate the proper handling of objects. The wide‐range responses of the sensors are important for achieving statement of recognition in tactile sensing systems. Thus, this study proposes a novel micro‐pore‐induced soft pressure sensor via wide‐range response by synthesizing functional composite materials with ether‐surfactant and conductive materials. The aim is to produce soft pressure sensors that are highly responsive and exhibit a wide sensitivity range for the detection of an applied pressure. The sensor comprises a multiwalled‐carbon nanotube, an ether‐based surfactant, and polymeric soft materials, which exhibit high‐speed response characteristics because of the microporous structure of the sensing layer. Moreover, a novel wearable robotic machine interface for facilitating object manipulation is achieved. Furthermore, the findings of this study in terms of the performance of the robotic‐tactile sensing sensor confirm its suitability for the machine interface of an artificial perception system. Furthermore, a real‐time tactile sensing system using the manufactured sensor in the form of wearable e‐skins is experimentally demonstrated.


DOI: 10.1002/adsr.202300045
perceptions and soft robotics interfaces. [1][2][3][4][5][6][7][8] Soft sensors with highly sensitive abilities are one of the important factors to be considered when aiming to enhance the human-like cutaneous sensation for the sensing device systems that imitate ecology and natural world. [9][10][11][12][13][14][15] Soft, high-sensitivity pressure sensors thus far have been fabricated with a relatively three-dimensional sponge structure. Upon the application of pressure to this sponge from the outside, the internal resistance component changes because of the deformation of the sensor housing. This is the principle of pressure sensing. The sensors that have been reported to date have realized a sponge structure via the insertion of a solute that dissolves in a solvent or pure water in the sensor housing followed by its removal in a post-process. [16][17][18] Zhao et al. reported a high-sensitivity and broad-range flexible pressure sensor using silicone rubber and silver nanoparticles. [19] Tang et al. reported a 3D-printed highly sensitive flexible pressure sensor. [20] A common feature of these methods is that their sensitivity was improved through the creation of a sensor housing with a pore structure.
However, in recent years, there have been reports of research on the construction of a pore structure in the sensing layer with a thin film. This has resulted from the aim of developing e-skin devices. In particular, high sensitivity has been achieved by forming pores in a thin-film sensor with a thickness of 1 mm or less through the combination of a conductive carbon material and a surfactant. Jung et al. reported a reverse-micelle-induced porous pressure sensor for a robot-machine interface. [21] Kumar et al. reported a highly sensitive chemo-resistant sensor based on a sulfonic surfactant. [22] These studies all involved the fabrication of thin-film pressure sensors and the introduction of pores. In addition, the sensing principle is based on the measurement of the resistance change. Furthermore, the most commonly used surfactants are low-molecular-weight materials with thiol groups, such as dodecylbenzenesulfonic acid. [23][24][25][26][27] This has been used to prevent the agglomeration of the carbon material and matrix polymer in the sensor and thus improve the film-forming properties of the sensor material ink on the substrate. However, it has been reported that a thiol group undergoes a cross-linking reaction with a matrix polymer containing a Si group typified by polydimethylsiloxane (PDMS), which results in an effect phenomenon. Thus, to realize a highly sensitive thin film-type soft pressure sensor, there is an urgent need for a new material system and device application that ensures the softness, high sensitivity, and thinness of the sensor housing.
This study fabricated a micropore using a reverse-micelleinduced soft pressure sensor to realize wide-range and highsensitivity responsive robotic e-skin applications. The fabrication of the high-performance soft sensors enabled the fabrication of soft and thin film device using several functional materials. The sensing performances of the soft sensor was used multi-walled carbon nanotubes (MWCNT), surfactants with ether group, and soft polymer materials to increase the micropore to the sensing layer. The surfactant used was a new ether-based nonionic material that prevented the agglomeration of each material constituting the ink. Furthermore, a specific property of the molecular chain length of the surfactant affecting the sensing properties was exploited. Consequently, the measurement of a low pressure and detection of a wide-range actuation was realized because of the softness improvement of the sensor. Thus, the proposed sensor was verified through its application to a robotic machine interface for object-tactile monitoring. Figure 1 displays schematic of fabrication procedure for manufacturing process. In this work, the fabrication of the ink and device was newly constructed by further brushing up the material system that can form the porous structure in our previous work. [28] Figure 1a shows the functional ink for sensing layer in our device. The ink for the sensor was prepared from 0.72 g of hexane as the main solvent (Tokyo Chemical Industry) and 400 mg of Polyoxyethylene-n-Stearyl Ether (PSE) surfactant (Fuji film-Wako) for ultrasonication by 15 min. In this study, the n values of the PSE were 2, 10, and 100. Thereafter, 1.30 g of PDMS as a silicon elastomer (Dow Chemical) and 71.5 mg of MWCNT (Aspect ratio: 100, Merck) were mixed. Following ultrasonication for 15 min, 700 mg of pure water was added, and the mixture was stirred for 30 min. Subsequently, 130 mg of stiffener for the PDMS was added to the ink and stirred for 5 min. Finally, vacuum evacuation was performed for 10 min. The viscosity of the ink was ≈1 Pa m, as measured using a viscometer (EKO Instruments). The optimization of the amounts of these carbon materials and basically sensor fabrication method was followed on our previous research. [28] It is known that the amount of water affects pore formation, and the added amount is an important factor in the formation of larger pores.

Device Fabrication
In this study, the sensor comprised electrodes and the sensing layer. Figure 1b,c displays a schematic image and photo of the fabricated sensor device. The electrode was prepared using PEDOT:PSS [poly(3,4-ethylenedioxythiophene)-poly(4styrenesulfonate)] (Clevios SV4 STAB, Heraeus) as a conductive polymer. The sensing layer was fabricated onto a 50-μ;m-thick poly(ethylene naphthalate) (PEN; Q65HA, DuPont) substrate. First, PEDOT:PSS was printed on the substrate using the stencilprinting method to form as the electrodes. [29] The interelectrode distance was 1 mm. The electrode was then annealed at 120°C for 30 min. In addition, we used this material for the electrodes because of its good wettability and high-conductivity. [30,31] Subsequently, the functional ink for the sensing layer was formed on the substrate using the stencil printing method. Figure 1d shows the chemical structure of the PSE surfactant. In this study, we selected three PSE lengths from n = 2 to n = 100. Figure 1e shows a schematic illustration of the behavior of the micro-micelles in sensing layer during the annealing procedure. PDMS existed around the micelles, and annealing caused water in the micelles to evaporate, thus creating pores. Further, MWCNT and PSE were chemically bonded, and current flowed in the sensor when each MWCNT was in physical contact. [32] Moreover, the pressure change in the sensor was obtained through measurements of the resistance value at that time. The fabrication method distributed evenly MWCNT in viscous media and formed reverse-micelles, which surrounded in water droplets. In addition, before annealing, surfactants existed in the matrix and solvents in the water micelle coating statement. After annealing, the water micelles evaporated, and micropores were generated in the sensing layer. The layers were annealed at 50 and 120°C for 2 and 1 h, respectively, after forming. Figure 2 shows the surface morphologies of the sensing lay-ers fabricated using the novel functional ink with different surfactant lengths (PSE) (n = 2, 10, and 100). Figure 2a shows several schematic images of this layer. There were no significant changes in the water content. Figure 2b shows the crosssectional images of the layers used to measure their thickness by a later microscope. Uniform films provided mechanical stability under applied pressure. [28] The measured thicknesses were all ≈500 μm. Figure 2c shows the surface scanning electron microscopy (SEM) images of the layers. The addition of surfactants of different lengths to the ink clearly produced a microporous structure in the sensing layer. The measured diameters were ≈30, 10, and 1 μm for the addition of surfactants with n = 2, 10, and 100, respectively. There was a correlation between pore size and layer flatness. These results indicate that the addition of a surfactants of different lengths distinctly affects the formation of micropores. In this study, the initial resistance of the sensor was ≈100 kΩ ( Figure S1, Supporting Information). The fabricated sensor had a housing with a Young's modulus of ≈150 MPa. This was calculated from the strain-stress curve ( Figure S2, Supporting Information). In addition, we showed the spectrum of FT-IR measurement of the functional ink used for the sensing layer in Figure S3 (Supporting Information). Distinct peaks were detected according to the functional groups of the mixed materials. Figure 3 shows the sensing performance of the soft pressure sensors as a function of applied pressure. Figure 3a shows photographs of the fabricated sensor upon the application of pressure. A compression tester was used to apply the vertical pressure. The application speed was 100 mm min −1 . The indenter on the compression test tip was composed of cured rubber. The change in resistance when pressure was applied is shown in Figure 3b. In this time, the sensor was constructed with or without PSE materials. The applied pressure was 100 kPa. A clear change in the resistance was observed from the initial resistance (100 kΩ) in response to the pressure application. Moreover, the resistance exhibited a constant value even when the pressure was maintained, and it can be concluded that the mechanical stability was high. The reaction speed in this time was ≈70 ms ( Figure S4, Supporting Information). This is a reasonable speed for a resistance change sensor for e-skin applications. [33][34][35] Figure 3c shows resistance changes as a function of applied pressure of sensors with and without PSE for sensing layer. In this time, the n value of PSE was 2. The applied pressure ranged from 5 to 150 kPa. Accordingly, a clear change in the resistance can be measured with each sensor for each pressure. Especially, it was found that the sensor containing PSE improved performances in both sensitivity and dynamic range. Figure 3d shows resistance changes as a function of applied pressure in using different PSE lengths. The pressure sensitivity was found to depend on the chain length. In particular, the shorter the chain length, the higher is the sensitivity. Long chains with n = 100 were sensitive to pressure; however, they were less sensitive than short chains. This may be attributed to that fact that long-chain surfactants aggregate during film formation, thus rendering the formation of pores difficult. [36,37] From these results, it can be concluded that the molecular chain length of PSE with n = 2 exhibited the highest sensitivity for fabricating a pressure sensor using the material system of this study. Figure 3e shows the mechanical stability under continuous application of pressure. In this time, the applied pressure was 100 kPa, and the applied speed was 100 mm min −1 . It can be concluded that the proposed sensor is a device with high mechanical stability because it exhibited stable behavior even for ≈1000 continuous applications. The outset graphs of Figure 3e show the changes in the resistance values extracted around 250 and 750 times. This can be attributed to the increase in the sensitivity and improvement in the softness of the sensor. In addition, the sensing layer has strong adhesion to the substrate. We showed the results of an adhesion strength test in Figure S5 (Supporting Information).

Sensor Application
The adaptability of the fabricated sensor to the robot e-skin was demonstrated. The application of soft sensors to tactile devices that mimic human skin sensations is important for constructing intelligent prosthetic hands and robotic skins. [38][39][40][41] We implemented the proposed sensor on a robotic hand and measured real-time tactile information when grasping and releasing an arbitrary object. Figure 4 shows a series of motions for gripping an object using a sensor mounted on the robot hand. In this study, we selected a rubber soccer ball as the grasped object. Figure 4a shows a tactile sensing system using our sensor for robotic gripping. Figure 4b shows the circuit configuration for measuring the change in the resistance value. A voltage-dividing circuit comprising the fabricated sensor and reference resistor was fabricated, and a signal was detected as a voltage change. A reference resistance of 100 kΩ, which was equivalent to the initial resistance of the sensor, was used. Figure 4c shows the results of the real-time monitoring of the actual signal when the robot hand gripped the object. The signal was displayed when grasping and releasing were repeated twice. A clear voltage signal was obtained for each grip. Furthermore, because the level of the voltage signal was the same between the first and second repetitions, it is evident that the electrical reliability of the device was high. Consequently, In addition, the sensing layer of our sensor has strong adhesion to the substrate. Thus, an issue attributed to adhesion do not occur when using the sensor mounted on the grip-per because of highly adhesiveness (see Figure S5, Supporting Information).
We could demonstrate the expandability of the proposed soft pressure sensor to the robot e-skin from the perspective of high sensitivity and reliability.

Conclusion
We fabricated a high-performance soft pressure sensor utilized soluble functional materials such MWCNT, PSE surfactants, and PDMS material. The sensor was capable of measuring and detecting low pressures with a wide-range response. The sensors achieved high performance in terms of good sensitivity, mechanical stability, and wide-range sensing ability. We performed grasping and tactile sensing in real-time using the developed sensor device. The system can be used for involving robotic e-skin via a tactile device. Moreover, we demonstrated potential of practical robotic handling system. These results indicated that the sensing abilities of the fabricated sensor is suitable for a robotic manipulation procedure. Thus these results facilitate construction of an intelligent sensing system for soft robot applications.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.