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Article Cite This: ACS Omega 2018, 3, 1110−1116
Customizable, Flexible Pressure, and Temperature Step Sensors with Human Skinlike Color Seonggi Kim,†,‡ Sunjong Oh,† Youngdo Jung,† Hyungpil Moon,*,‡ and Hyuneui Lim*,† †
Department of Nature-Inspired Nanoconvergence Systems, Korea Institute of Machinery and Materials, 156 Gajeongbuk-Ro, Yuseong-Gu, Daejeon, 34103, South Korea ‡ School of Mechanical Engineering, Sungkyunkwan University, 2066 Seobu-Ro, Jangan-Gu, Suwon, 16419, South Korea S Supporting Information *
ABSTRACT: Durability and multifunctionality are very important factors for human skinlike tactile sensors for measuring physical stimuli if they provide reasonable pressure measurement range and sensitivity. Here, we propose a step tactile sensor with a simple processing unit, showing high repeatability and mechanical stability without drifting caused by thermal and geometrical noise. The proposed sensor, similar to a switch mechanism, detects the applied pressure discretely and has a wide pressure range of 2 kPa to 1.2 MPa according to its geometry. The developed tactile sensor can be designed and fabricated in various morphologies to detect a wide range of tactile stimuli, which help in customizing the sensor as per user demand for practical applications such as a prosthesis arm or hand. It is also easy to extend the sensor size to cover a large area owing to the simple fabrication process by using a 3D printer. Furthermore, with the addition of a flexible exterior layer of leuco dyes and the polydimethylsiloxane mixture, the color of a step tactile sensor not only resembles that of human skin color but also changes its color depending on the temperature changes as human skin does. Thus, the function of a pressure and temperature indicator in a flexible step sensor finds practical applications in various fields, including but not limited to prosthetic applications for the customized and comfortable usage. between a structured conductor and an electrode.31−33 Resistive tactile sensors require less electronics and are easy to fabricate and integrate; however, they suffer from hysteresis and drifting due to thermal noise. Capacitive tactile sensors mostly detect the variation in thickness of the dielectric elastomer sandwiched between the electrodes.34−36 They exhibit high spatial resolution and have a large dynamic range; however, they are susceptible to noise in mesh configurations because of crosstalk noise and field interaction. Magnetic tactile sensors are highly sensitive and free from hysteresis but suffer from crosstalk noise and electromagnetic interference. Optical tactile sensors exhibit high repeatability and spatial resolution; moreover, they are unaffected by electromagnetic interference. However, they are not suitable for skin-type sensors because of their bulky size and rigidness.37−39 Therefore, many studies are still underway for developing skinlike tactile sensors for practical applications. This paper proposes a customizable skinlike step sensor that detects not only applied pressure discretely but also temperature visually. The step sensor, based on the polydimethylsiloxane (PDMS) substrate, is a contact resistive-type sensor, and its
1. INTRODUCTION Human skin is a multifunctional sensory organ having the largest area in our bodies. Human skin senses physical stimuli such as pressure, temperature, and humidity from mechanoreceptors.1−3 Humans can control body motion by reacting to external physical stimuli via tactile sensing and feedback. Many research studies have been conducted to develop skinlike tactile sensors such as electronic skin, which can detect not only a touch but also a variety of stimuli for the application in robotics,4−6 healthcare,7−11 and entertainment.12,13 Skinlike tactile sensors should have the features of flexibility, electrical and mechanical stability, and easy shape deformation for practical applications because of complex geometries and harsh movements. In the past decade, numerous efforts have been undertaken to develop skinlike sensors with versatile morphology based on flexible and stretchable materials.14−21 Various strategies, such as the use of a very thin substrate22,23 or a soft material24,25 and a combination of soft and hard materials,26−28 have been adopted to achieve flexibility or stretchability. Typical transducers for flexible tactile sensors mostly rely on capacitive, resistive, magnetic, and optic mechanisms, each having its own advantages and disadvantages. Resistive tactile sensors are usually classified as piezoresistive type, which uses the piezoresistivity of intrinsic materials,29,30 and contact resistive type, which uses the variation in resistance © 2018 American Chemical Society
Received: November 28, 2017 Accepted: January 16, 2018 Published: January 26, 2018 1110
DOI: 10.1021/acsomega.7b01868 ACS Omega 2018, 3, 1110−1116
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and bottom layers, the PDMS substrate is treated with oxygen plasma before spray-coating. The spray-coated bottom and top electrode layers are annealed to improve the electrical conductivity of the coated AgNW on a 200 °C hotplate for 20 min. The 40 μm thick intermediate layer with the hole area in the range of 0.16−8.41 mm2 is printed with the PDMS solution by a 3D printer (PROTEX Co.) on the bottom electrode layer, as shown in Figure 1c. The liquid bridge phenomenon might arise in the patterning due to the low viscosity of PDMS when the 3D printer creates a void region for the hole pattern. To avoid this problem, the temperature of the hotplate and manipulation direction of the injector should be appropriately selected during PDMS printing, as shown in Figure S1, Supporting Information; hence, the bottom electrode layer is placed on a 110 °C hotplate during intermediate layer printing. The cover, that is, the inverse bumper layer is developed from a mixture of leuco dyes of red and yellow, white silicone pigment, and the PDMS solution for exhibiting human skin color. Depending on the ratio of the leuco dye and white pigment in the PDMS solution, the cover of the step sensor can represent various colors. The mixture is coated by using a spin coater at 350 rpm, and then, an inverse bump is patterned on the spin-coated PDMS mixture of 140 μm thickness using a 3D printer, as shown in Figure 1d. Finally, all the layers are carefully aligned on the hotplate heated at 80 °C, as shown in Figure 1e. Figure 1f shows the fabricated 4 × 10 multiunit step sensor. Single cells and multiunits of step sensors are characterized by measuring the variation in their electrical resistance. Pressure is applied to each single cell of a step sensor through the bump with a contact area of 8.41 mm2 by using a custom-made pressure measurement system at a speed of 0.5 mm/min, and then, the resistance value is transferred to a processing computer through RS232 serial communications. Each electrical line of the 4 × 10 multiunit step sensor is connected to digital I/O ports of the processing board (Arduino Uno, Arduino Co.), and the data are transferred to a processing computer through USB serial communications. The collected data are analyzed by MATLAB software and then represented graphically using the MATLAB GUI function. A single cell of a step sensor consists of the following four layersbottom electrode layer, intermediate layer with various areas of the hole pattern, top electrode layer, and inverse bumper layeras shown in Figure 2a. In each single unit, the bottom electrode is intersected with the top electrode, the intermediate layer with various holes is placed between the bottom and top electrodes, as shown in Figure 2b, and the applied pressure is concentrated on the sensing cell by the inverse bumper layer over the top electrode. Under pressure, the step sensor, similar to a switch mechanism, detects an on/off current signal through an interconnection between the top and bottom electrodes. In the initial state, the resistance of the sensing cell is almost infinity because the bottom electrode stays away from the top electrode. The electrodes are connected directly by deformation of the top electrode layer and the resistance of the single cell decreases to a few kΩ under applied pressure. Then, the signal processing system detects the pressure by the flow of current to the digital I/O port in the processing unit. The deformation of the top electrode layer is determined from its Young’s modulus, inverse bumper area (w2), distance between two electrodes (s), hole area (d2), and thickness of the top electrode layer (t). Therefore, the response pressure of the step tactile sensor is designed by tuning several factors. Here,
operation principle is based on the on/off mechanism similar to that of a switch. When pressure is applied, the top and bottom electrodes are connected directly through a hole in the intermediate layer, and then, the current flow is detected from the digital I/O port on the processing unit. This sensor exhibits different pressure responses depending on the ratio of the hole area and inverse bumper area, and such a simple pressure-sensing mechanism covers a wide pressure range from a few kPa to several MPa. Step sensors with an appropriate combination of single cells can measure various pressure ranges discretely. A simple fabrication process using a 3D printer allows outstanding expandability to multiunits and serves to customize the pressure range and spatial resolution according to the required specification, for example, prosthesis parts such as hand, forearm, and foot. Although step sensors cannot measure the applied pressure precisely, they have a simple processing unit, high repeatability, and mechanical stability without drifting caused by thermal and geometrical noise. Furthermore, the inverse bumper layer, composed of PDMS and thermochromic materials, not only detects temperature visually but also provides the sensor a natural color resembling the human skin.
2. RESULTS AND DISCUSSION Figure 1 describes the fabrication process of a step tactile sensor. It consists of four flexible layers: bottom electrode layer,
Figure 1. Fabrication process of a step tactile sensor. (a) Spin-coating of PDMS solution on an aluminum plate at 350 rpm for 60 s. (b) AgNW spray-coating on the 140 μm thick spin-coated PDMS film for the top and bottom electrode layers. (c) Printing an intermediate layer with various hole areas using a 3D printer with PDMS solution on the bottom electrode layer (in red box: real photograph of the printing process). (d) Bump pattern printing for the inverse bumper layer on the spin-coated thin film, which is a mixture of leuco dye and PDMS, by using a 3D printer. (e) Assembly of all layers. (f) Real photograph of a 4 × 10 multiunit step sensor with human skin color. The area of the sensor is 76 × 40 mm2.
intermediate layer, top electrode layer, and inverse bumper layer. The bottom and top electrode substrates are made of PDMS (Sylgard 184, Dow Corning Co.), comprising a base prepolymer and a crosslinking agent (10:1 weight ratio), by spin-coating at 350 rpm, as shown in Figure 1a. Electric signal lines, shaped as stripes, are developed by AgNW spray-coating using a 3D-printed shadow mask on a 140 μm thick PDMS substrate, as shown in Figure 1b. To improve the adhesion strength between the AgNW and PDMS substrate of the top 1111
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is designed by modulating the hole area. In the proposed step tactile sensor, four single cells with different hole areas compose a single unit, which can detect four different pressures. This means that the step sensor measures the applied pressure discretely, such as a threshold indicator, because this sensor exhibits a noncontinuous signal. Furthermore, the step sensor exhibits outstanding extendability because of its simple structure and sensing mechanism. Figure 2c shows a 4 × 10 multiunit sensor, which encompasses four steps. A resistive pressure sensor typically employs the following two mechanisms: intrinsic materials’ piezoresistivity and contact resistance between the resistor and the electrode. A resistive pressure sensor using piezoresistivity not only exhibits high sensitivity but also suffers from large hysteresis and a large thermal effect. To overcome these drawbacks, a contact resistive-type sensor with a pyramid or a microdome-patterned piezoresistor has been extensively studied.40 The speed of recovery to the initial state of the microstructure is faster than that of the solid structure because of the void region created between the micropatterns under decreasing pressure. Therefore, this sensor can reduce hysteresis because of its intrinsic viscoelastic characteristics. In the case of a step sensor, when pressure is applied, the top electrode is suspended on the hole pattern of the intermediate layer, and the suspended part of the top electrode is deformed into a cone shape before making contact with the bottom electrode. Eventually, the step sensor
Figure 2. Schematic of a step tactile sensor (a) cross-section of a single cell, which consists of four layers, where w is the side of the square of the inverse bumper, t is the thickness of the top PDMS layer, s is the thickness of the intermediate layer, and d is the side of the square of the hole in the intermediate layer. (b) Schematic of a single unit of a step sensor with four hole areas in the intermediate layer. (c) Schematic of the multiunit step sensor with 7 mm spatial resolution between each unit.
PDMS, with a Young’s modulus of 833 kPa, is used in the formation of each layer for flexibility, and the thickness of the intermediate layer, that is, the distance between two electrodes (s), is fixed at 40 μm. The main factor of the response pressure
Figure 3. Performance of a single-cell step sensor with t = 140 μm, s = 40 μm, w = 4 mm2, and d = 4.62 mm2. (a) Hysteresis curve of a single-cell step sensor under loading and unloading. (b) Response times of the step sensor under loading (upper) and unloading (lower). (c) Voltage variation of the step sensor without drifting under repeated pressure for 1000 times. (d) Enlarged graph of voltage variation at first 15 s (upper) and last 15 s (lower). 1112
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Figure 4. Pressure-sensing performance of a step tactile sensor. (a) Pressure response of the step sensor with different thicknesses of the top electrode layer (inset: enlarged pressure response of the top electrode layer of 140 μm thickness under low pressure). (b) Real-time measurement of response of a single-unit step sensor with four hole areas under pressure (inset: real photograph of printed single unit including four hole areas on the bottom electrode layer). (c) Variation in pressure response of a single cell sensor having 4.62 mm2 hole area under different temperatures. (d) Variation in pressure response of a single cell of 4.62 mm2 hole area attached on surfaces of various curvature radii, especially for the application of a prosthetic arm.
4a describes the pressure response of a single cell with three thicknesses of the top electrode layer according to increasing area ratio (w2/d2) of the inverse bumper and hole areas. The area of the inverse bumper is fixed to 4 mm2, the hole area of the intermediate layer is changed gradually from 0.16 to 8.41 mm2, and finally, the ratio is varied from 25.0 to 0.48. Therefore, the larger the hole area, the lower the pressure response. Also, the effects of the top electrode layer of 140, 170, and 190 μm thicknesses on pressure are shown in Figure 4a. A single cell of 140 μm thickness detects the lowest pressure of 2 kPa at a hole area of 8.41 mm2. The pressure response of a single-cell step sensor is measured up to 1.23 MPa at a top electrode layer of 190 μm thickness, and the response is linear with an increasing ratio (w2/d2). These results show the potential of a step tactile sensor for customized function in sensitivity, pressure range, and spatial resolution just by tuning the hole area and layer thickness. Furthermore, the 3D printing process helps to control the size and spatial resolution of the step sensor. Figure 4b shows the real-time voltage response of a single unit for each single cell. A single unit of the step sensor is composed of four hole areas of 4.62, 2.56, 1.96, and 1.32 mm2. Each single cell responds at different pressures, as expected. On the basis of the suggested tactile sensing mechanism, a single unit of a step sensor can detect several pressure ranges and design various step sensors according to the practical application demand. For instance, on average, the human foot feels a pressure of up to 200 kPa.43 The step sensor for a prosthetic foot can be composed of single cells with a ratio of less than 5.0 for detecting pressure under 200 kPa when the top layer has a thickness of 170 μm, while the step sensor with a
has a fractional hysteresis of within 4.76% because the geometry of the cone shape contributes to the low, similar to the case of a pyramid structure, as shown in Figure 3a. Figure 3b shows that the response times of the step sensor, whose resistance reaches a few kΩ from hundreds of MΩ, are 120 and 90 ms under loading and unloading, respectively. To investigate the reliability of the step sensor, a repeated pressure of 0.5 Hz is applied to the single cell while monitoring the variation in resistance in real time. The step sensor shows a uniform response under pressure applied repeatedly for 1000 times, as shown in Figure 3c; the extracted result at several intervals of the step sensor is shown in Figure 3d. These results demonstrate the robust and durable sensing performance of a step tactile sensor. However, there is a limit to show the good response at high frequency because of its visco-elastic characteristics. To apply a pressure sensor for feedback on practical applications, the performance of the pressure sensor in terms of sensitivity, pressure range, and spatial resolution should be customized according to the requirements of usages because overload might arise because of excessive pressure feedback.41 For instance, a prosthetic hand requires high spatial resolution and high sensitivity, whereas the industrial robot arm requires low sensitivity and a wide range. A step tactile sensor that acts as a pressure indicator can provide adequate choice of the pressure range, sensitivity, and spatial resolution as per user demand. Deformation of the top electrode layer under applied pressure is determined by the ratio (w/d) and thickness of this layer. The relationship of pressure response and the ratio of the length (w/d) have been analyzed in a previous study.42 Figure 1113
DOI: 10.1021/acsomega.7b01868 ACS Omega 2018, 3, 1110−1116
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ACS Omega top electrode layer of 140 μm thickness can use larger ratios of up to 25.0. Figure 4c shows the variation in response pressure under different temperatures. The temperature controlled by the thermoelectric element is transferred to a single-cell step sensor, and the pressure response is measured. A typical resistive-type sensor is affected by temperature because the mobility of electrons is determined by temperature. However, step sensors show only 3% error in pressure response under the temperature variation in the range of 10−90 °C because they respond only when the top electrode contacts the bottom electrode. The robustness in flexibility of the step sensor is also investigated with bending in various curvatures. Figure 4d shows the robustness of the step sensor under surfaces of various curvature radii. The step sensor is placed on surfaces of various curvature radii, which are selected by measuring the curvature radius for several body parts and the pressure at the point of a sharp drop in resistance. The pressure response has a scarce error of within 4%. The multiunit step tactile sensor is fabricated by the distribution of a single unit with four hole areas and skinlike color in the inverse bumper layer. The fabricated multiunit sensor has 40 single units, 7 mm spatial resolution, and pressure range of 6.37−17.15 kPa. Therefore, a single unit of the sensor can detect four pressure ranges, and the cover of the sensor with leuco dye indicates the variation in temperature. The shadow effect arises in the step sensor of a multiarrayed matrix form by the scanning method, and it is known that an unexpected sensing cell is detected when many sensing cells are pressed simultaneously. A typical mechanism for avoiding the shadow effect is inserting a diode in each sensing cell.44 We design a step sensor with electric signal lines of resistance as high as several kΩ because an unexpected cell operated by the shadow effect has a higher resistance than the original cell due to the long path. Then, the shadow effect is removed completely, as shown in Figure S2, Supporting Information, because the current over 0.1 A is only detected on the digital I/ O port used in the microcontroller unit. Figure 5a,b shows the performance of the multiunit step sensor under varying pressure and temperature. The white, yellow, and red boxes indicate the applied pressures of 6.37 kPa, 11.64 kPa, and over 17.15 kPa, respectively. As shown in Video S1, the multiunit step sensor detects a weight of 100 g, and the color of the sensor skin changes from human skin color to white color when hot air at around 100 °C is blown, and once the hot air is removed, the original color is recovered. The pressure feedback system is demonstrated simply without any process unit, using a single-cell step sensor, 3 V battery, and vibrator, as shown in Figure 5c. The single-cell step sensor is attached on the fingertip, and each element is connected by a gold wire of 50 μm thickness. The single-cell step sensor responds by hitting the ground, and then, the vibrator is vitalized as shown in Figure 5d and Video S2. This shows a customized, flexible pressure, and temperature step sensor with a natural appearing color and applied pressure feedback system with a simple element.
Figure 5. Pressure and temperature step sensor with skinlike color. (a) Performance of the multiunit step sensor with human skin color under pressure. (b) Change in skin color and pressure sensing when hot air and pressure are applied simultaneously (Video S1). (c) Demonstration of the pressure feedback system with step sensor, 3 V battery, and vibrator. (d) Pressure feedback on the physical stimuli at the fingertip in a vibration form (Video S2).
unit with four different single cells detects four different pressure levels. The step sensor shows customizable function in sensitivity, pressure range, and spatial resolution just by modulating the hole area and layer thickness. Moreover, the simple sensing mechanism provides mechanical stability without hysteresis and thermal effect (range from 10 to 90 °C). The sensor skin with natural appearance is made of a mixture of PDMS and thermochromic dyes, and the skin color changes over 40 °C with a human response under hot physical stimuli. The proposed step tactile sensor with outstanding extendability, fabricated by an easy process using a 3D printer, shows the potential for robotic and prosthesis applications with various shapes, sensing ranges, spatial resolutions, and sensitivities.
4. EXPERIMENTAL SECTION 4.1. Details of AgNW Electrodes. An AgNW (AgNW ink, NANOPYXIS Co.) of 35−45 nm diameter and 15−25 μm length is diluted in 0.3 wt % isopropyl alcohol. The AgNW is spray-coated using a shadow mask created by a 3D printer (Object pro, Stratasys Ltd.) on the oxygen plasma-treated PDMS substrate. The spray nozzle is placed 15 cm apart from the plate, and the spraying speed is about 25 mm/s. The AgNW is spray-coated three times for electrical stability; the resistance of the single electrode line, whose line width is 2 mm, is about 20 Ω/mm, and the transparency of the spray-coated AgNW is about 84.95%, as shown in Figure S3, Supporting Information. 4.2. Details of a Temperature Indicator with a Thermochromic Cover Layer. Thermochromism is a property of substances to change color with variation in temperature. It exists in two forms: liquid crystal type and leuco dye type. The leuco dye easily disperses in silicone rubber such as a PDMS prepolymer, and the thermochromic dye offers more intuitive color than liquid crystal-type thermochromic dye. In the case of step sensors, two types of leuco dyes with
3. CONCLUSIONS In summary, we developed a customizable, flexible pressure, and temperature step sensor with a natural appearing color. The developed step sensor has 4 × 10 single units of a wide sensing range of 2 kPa to 1.23 MPa pressure. A single cell of the step sensor responds at different pressures according to the geometric condition of the sensor structure, and then, a single 1114
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different temperature responses are selected, and the initial color can be customized according to the ratio of each dye depending on user specification. As shown in Figure S4, Supporting Information, the color of the mixture of thermochromic and PDMS materials changes with temperature, from the intrinsic characteristic of the leuco dye, which is a colorless form, to scarlet and white at 40 and 70 °C, respectively.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01868. The hole patterns of the intermediate layer created using a 3D printer, the shadow effect phenomenon on the multiarrayed matrix formed by the scanning method, scanning electron microscopy images of spray-coated AgNW on PDMS, photographs and UV−visible spectra of AgNW spray-coated glass, and color change of the thin film made by the mixture of thermochromic dye and PDMS with temperature (PDF) The performance of the multiunit step sensor under varying pressure and temperature (AVI) Demonstration of the pressure feedback (AVI)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Fax: 82-31-290-7507 (H.M.). *E-mail:
[email protected]. Fax: 82-42-868-7106 (H.L.). ORCID
Hyuneui Lim: 0000-0003-2692-647X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Convergence Technology Development Program for Bionic Arm through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (no. 2014M3C1B2048177).
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REFERENCES
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DOI: 10.1021/acsomega.7b01868 ACS Omega 2018, 3, 1110−1116
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DOI: 10.1021/acsomega.7b01868 ACS Omega 2018, 3, 1110−1116
