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Polymer-Waveguide-Based Flexible Tactile Sensor Array for Dynamic Response Sungryul Yun, Suntak Park, Bongje Park, Youngsung Kim, Seung Koo Park, Saekwang Nam, and Ki-Uk Kyung*

Touch sensing in electronic devices is becoming more popular these days; for example, transparent touch-sensitive panels for portable devices such as smartphones and tablet computers are widely used, and are becoming more functional and versatile, allowing multipoint touch interaction as well as a single point touch.[1–3] Beyond contact detection, pressure-based interaction with a touchscreen leads to better usability, for example, on the touchscreen a keyboard that responds to contact forces improves key-clicking performance.[4] Recently, touch sensors have evolved that are flexible or even stretchable so that they can be compatible with flexible displays.[5–7] The combination of flexibility and a function that detects the magnitude of the contact force enables touch sensors to be used for various fields such as robots[8–10] and medical systems,[11,12] as well as conventional touchpads.[13] Nowadays, a flexible tactile sensor is considered a key component for an artificial skin, but its thin-film design remains a future challenge.[14–19] The features of tactile sensors are crucial to improve feasibility for practical applications, for example, the sensors need to be transparent if they are to be laid on a touchscreen device and thin and flexible if they are to be integrated within the curved surfaces of flexible displays or robot hands.[8,9,20] For detection of pressure distribution, the sensors should be able to recognize multipoint contacts.[5,14,15,17] Sometimes, components impervious to an electromagnetic field are required for compatibility with magnetic resonance imaging in neuromedical applications.[12,21,22] Various technical approaches to satisfy these practical requirements have been investigated in the last few years. Metal strain gauges with polymer substrates[14,18] and resistive,[5] capacitive,[6,23,24] or piezoelectric[25] tactile sensor arrays were reported for flexible force sensors with structural simplicity and implementability. In several tactile sensors, localized protrusion structures have been used to improve sensitivity in force distribution.[26,27] Nano-piezotronics based on a nanowire nanogenerator[28,29] and field-effect transistors with pressure-sensitive layers[30] were demonstrated for highly sensitive flexible force sensors. Another approach that used a layer of nanofibers Dr. S. Yun,[+] Dr. S. Park,[+] Dr. B. Park, Dr. Y. Kim, Dr. S. K. Park, S. Nam, Dr. K. U. Kyung Transparent Transducer and UX Creative Research Center Electronics and Telecommunications Research Institute Daejeon, Korea, 305–700 E-mail: [email protected] [+]These authors contributed equally to this work.

DOI: 10.1002/adma.201305850

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with a conductive coating showed the possibility of measuring shear and torsion as well as normal force.[31] However, since these approaches use electronic components for force sensing, there is a possibility of signal disturbance caused by physical stress such as bending, twisting, stretching, etc. An alternative approach, without the electronic components in sensing areas, is to use an optical-fiber-based sensor in which light-transmission efficiency is influenced by contact force.[32] The use of optical waveguides instead of optical fibers is advantageous for developing the force sensors in thin-film architecture.[33] Particularly, polymer waveguides coupled into an array structure on a glass wafer,[34,35] integrated into large-area organic photodiodes,[36] and with Fresnel-shaped collimating edges[37] enable a position-recognizable thin force sensor to be established on a touch panel for functional human interface devices. However, although polymer-waveguide-based sensors have been used to measure contact force and position simultaneously, positionestimation methodologies based on relations between signals of detectors aim at a single contact point rather than multiplepoint recognition. In addition, the robustness of sensors under bending conditions was not reported even though the sensors were composed of soft materials. Recently, an optical pressure sensor based on an elastomeric polydimethylsiloxane (PDMS) waveguide showed the possibility of the thin force sensors being transparent and stretchable.[19] However, polymerwaveguide-based sensors need to be improved with regards to dynamic response as the viscoelastic properties of the polymer materials may cause the sensor response to be delayed. For feasibility tests of such sensors, dynamic characteristics including hysteresis under high frequency input must be evaluated. Here, we report a polymer-waveguide-based transparent and flexible force sensor array which satisfies the principal requirements discussed above, such as thin-film architecture, localized force sensing, multiple-point recognition, and bending robustness, as well as fast response for a tactile sensor working on curvilinear surfaces. The force sensor array detects a contact force with a position at 27 points independently. The sensor array is thin (total thickness: < 150 µm), flexible, and highly transparent (transmittance: as high as 90%). The force sensor detects contact forces at single or multiple points with fast response (response delay: < 10 ms, evaluation frequency: as high as 16 Hz with 1 ms sampling), high reproducibility (Pearson correlation coefficient: as high as 0.994, hysteresis: as low as 6.7%), high sensitivity (as high as 16% N–1), and high bendability (10.8% sensitivity degradation at a bending radius of 1.5 mm) in response to dynamic input force (0–3 N) without any electronic components on sensing areas. The sensor is also

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Figure 1. a) An illustrated configuration and b) the working principle of a force sensor that detects input force applied to the touch layer.

capable of sensing pressure on curvilinear structures as well as on soft surfaces such as the human body without significant performance degradation. The force sensor has the potential to be used in a number of applications for measuring dynamic contact forces on various surfaces. The configuration of the force sensor is illustrated in Figure 1a. The force sensor consists of multiple waveguides, a light source, photodetectors, sensing areas, and a touch layer with refractive index (Ri)-tunable coating. Each waveguide is a combination of a core and two clad layers. The waveguide at the sensing area is designed with a bare core to allow the light passing through the core to be easily scattered when the touch layer is in contact with the core. Except for the sensing area, the clad layer fully covers the core to prevent the light from scattering in undesired areas. As described in Figure 1b, the sensor is designed so that a pressure contact between the touch layer and the core induces scattering of light passing through the core. The amount of light scattered is expected to change with contact force as well as Ri of the coating material on the touch layer. Our force sensor monitors output light intensity (Li) with respect to input Li from a source by using photodetectors, which are independently connected to each core. The fabrication process of the waveguide-based sensor is illustrated in Figure 2a. We used two synthesized photocurable fluorinated liquid prepolymers (prepolymers A and B) as functional materials to construct a polymer waveguide because of their different Ri, and other benefits such as flexibility, high optical transparency, thermal resistivity, and environmental stability.[38,39] The Ri of the prepolymers is adjustable by altering the fluorine contents of the prepolymers. Higher fluorine contents lead to lower Ris. For our waveguide-based sensor, the prepolymer A is designed to retain a Ri of 1.44 at a wavelength of 1031 nm, which is lower than that of the prepolymer B (Ri = 1.50) at the same wavelength. Prepolymer A and B coatings were assigned to clad and core, respectively. The materials were sequentially formed on a silicon wafer as a thin film by spin-casting, UV cross-linking, and thermal annealing. A thin prepolymer B coating (thickness: 11.7 µm) established on a prepolymer A layer (thickness: 10.5 µm) was turned into narrow rectangular cores with width of about 40 µm by using reactive-ion etching (RIE). Additional clad coating (thickness: 13.2 µm), which covered the top surface of the cores, was partially etched to construct a bare core and thin cylindrical spacers with diameter of 200 µm at each

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sensing area (Figure S1, Supporting Information). The relatively higher Ri of the core allows the propagation of light from the source into the sensing areas without significant loss. The use of solvent-free prepolymers facilitates the fabrication process of the waveguide structure. Finally, after peeling the waveguide layer off the silicon wafer, a touch layer with Ri-tunable coating of thickness of 5.5 µm was placed onto it (Figure S2). Figure 2b and c show a scanning electron microscope (SEM) image and an optical microscope image of the sensing area. The dense spacers and the core are clearly shown at the surface of the bottom clad. The spacers prevent electrostatic adhesion between the sensor and touch layers and maintain the position of the touch layer during repetitive force loadings. The waveguide layer has high optical transparency of 90% in the range of 550–1000 nm despite its multilayered configuration, which allows separation from the rigid substrate without mechanical damage (Figure 2d). The waveguide-based thin force sensor is capable of intimate contact on a forearm (Figure 2e) with high transparency and flexibility (Figure 2f) and the sensor is also mechanically robust to bending, twisting, or folding due to the soft nature of the materials used for both clad and core (Figure 2g). Figure 3a shows an illustration of the performance-testing system which is capable of measuring changes in Li at photodetectors with programmable input force. Figure 3(b–d) shows the averaged Li profiles of the force sensor being consistently loaded and released with sinusoidal input force ranging from 0–3 N at 16 Hz for 5 s. Since our sensor is designed for detection of finger tapping on a touchscreen, the range of the input force was decided by considering that the general human-finger keyclicking force does not exceed 3 N.[40] For the tests, thin films with different mechanical properties, which are made from three different polymers [cellulose acetate: CA; poly(tert-butyl acrylate): PTBA; polyethylene terephthalate: PET], were used for touch layers. Additional transparent coatings with three different Ri from 1.548–1.628 were formed on the bottom surface of each touch layer, which faces the bare core in the sensing area. When the touch layer is pressed toward the core, the Li of the force sensor exhibits a decreasing trend with increasing input force. We assumed that the sensor response might depend on the mechanical properties and Ri of touch-layer materials. In the case of the CA touch layer, the force sensor shows a tendency of consistent change in Li according to input forces with small deviations during repetitive force response

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Figure 2. a) Fabrication process of a waveguide-based sensor; b) SEM image and c) optical microscope image of a waveguide taken at the surface of the sensing area; d) photograph of the waveguide layer being peeled from an Si wafer after fabrication. Inset: optical transmittance profile of the sensor; a clean glass plate (UV transmittance: 91% at 550 nm) was used as a reference; e) photograph showing intimate contact of a thin-film sensor onto a forearm and f) its transparency; g) photograph and optical microscope image (inset) of the force sensor under mechanical-bending deformation.

tests at 16 Hz (Figure 3b). The Pearson correlation coefficient between the input force and the Li change was 0.994. When the input force ranged from 0 to 3 N, the maximum change in Li was as high as 49% as the Ri of the touch layer increases from 1.548 to 1.628. Even under an extended input force to 5 N, the sensor showed reliable performance (Figure S3, Supporting Information). Ri-dependent sensor responses were also observed by using the PTBA and the PET touch layers, which suggests that higher Ri materials can lead to a more sensitive 4476

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light-scattering response under the same contact conditions as for lower Ri materials. Especially, in the case of the PTBA touch layer, the sensor response is the most sensitive, but the statistical deviation of average Li values is the largest. The Pearson correlation coefficient between input force and Li decreases to 0.821 in this material. Figure 3e shows frequency-dependent sensitivities for the three different touch layers which retain a constant Ri of 1.628. All sensors exhibit consistent response to frequency change in

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the range of 1–16 Hz. The force sensor with the PTBA touch layer shows the highest sensitivity because the PTBA has the lowest Young’s modulus among the touch layers (Figure S4, Supporting Information), and therefore the PTBA touch layer can be more easily deformed and allows larger contact area with the core than other touch layers under the same input load. In each force sensor array with 27 sensing areas, the percentage change in light intensity responding to input force has a small deviation, below 5%, which means that the variation can be calibrated. To evaluate the performance of the sensor under bending, we prepared seven kinds of cylinders with radius of 15, 13, 10, 8, 5, 3, and 1.5 mm. For the bending test, the sensor was wound around three identical cylindrical rollers and then the dynamic force generator repetitively pressed and released the sensor

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at the top of the curved surface. Figure 3f shows the range of output intensity change with bending radius. Since a design of waveguide with a large difference in Ri for the sensor plays a role in preventing light leakage through bending, the output Li of our sensor is almost constant even when the bending radius decreases to 1.5 mm. Theoretically, transmission losses of our sensor for 360° bending with a radius of 1.5, 1, and 0.5 mm are 0.344%, 1.326%, and 2.032%, respectively (as simulated by using BeamPROP, RSoft). Although the output Li change decreases as the bending radius decreases, the performance of the sensor is not seriously damaged by bending. For example, the sensitivity of the sensor at a bending radius of 1.5 mm is 89.2% that of the unbent sensor. A video file, included in the Supporting Information, shows the sensor performance resistance to bending.

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Figure 4. Li profiles with time and hysteresis of the sensor response changing with the material of touch layer corresponding to a,b) PTBA, c,d) PET, and e,f) CA; g) durability tests of the force sensor using the CA touch layer during cyclic loadings of simultaneous input force in the range of 0–3 N at 1 Hz; h) the output performance of our force sensor placed onto the human arm. For all tests, the Ri of the functional coating on touch layer was fixed to 1.628.

Figure 4a–f shows the influence of touch layer on Li profile with time and on the hysteresis of the sensor responses. For the time-response tests, the Li profile of each force sensor was monitored throughout eight repetitive loadings of sinusoidal input force of 16 Hz. The hysteresis curves were obtained by replotting the time-response data. The average hysteresis values were determined from a ratio of hysteresis error to full scale output

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(FSO). The force sensor with the PTBA touch layer, which has the lowest Young’s modulus among the touch layers, showed the most sensitive response in the previous test. However, this sensor reveals a significantly high hysteresis value as large as 45.1 % and a response delay of 10 ms, which can be correlated with the viscoelasticity of the host polymer materials (Figure 4a and b).[41,42] For the PET touch layer, both hysteresis and

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Experimental Section Prepolymer A, 2,3,5,6,2′,3′,5′,6′-octafluoro-4,4´-bis-[2-{2-[1,1-difluoro-2(2,3,5,6-tetrafluoro-4-vinyl-phenoxy)-ethoxy]-1,1,2,2-tetrafluoro-ethoxy}2,2-difluoro-ethoxy]-biphenyl and prepolymer B, 2,3,5,6,2′,3′,5′,6′octafluoro-4,4′-bis-[2-(2-{2-[2-(2,3,5,6-tetrafluoro-4-vinyl-phenoxy)ethoxy]-ethoxy}-ethoxy)-ethoxy]-biphenyl were synthesized by following literature procedures.[38,39] After mixing each prepolymer with a photoinitiator (8:2 mixture of Irgacure 184 to Darocur TPO, Ciba Specialty Chemicals Inc.) and filtering through a 0.2 µm Teflon filter

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in sequence, each prepolymer containing the photoinitiator was spincast on a silicon wafer, photo-cross-linked by UV irradiation (power: 2 kW, wavelength: 365 nm) for 10 min and then thermally annealed on a hotplate for 20 min at 150 °C. To fabricate a waveguide structure with multiple sensing areas, the thin core and upper clad configured to array of cylindrical spacers at each sensing area were formed by using a PlasmaTherm 790 series RIE (power: 350 W, etching rate: ca. 0.3–0.4 µm min–1). The waveguide of the force sensor array was fiber pigtailed to THORLAB LPS-730-FC laser diodes (LD; power: 10 mW, wavelength: 830 nm) for a light source and RayCan 850 nm pin photodiodes for detectors. Finally, the sensor array was prepared by integrating the waveguide pattern into a touch layer with an Ri-tunable coating by using ChemOptics Exfine CO series. The sensor array was designed to be 27 pixels at 3×9 matrix similar to a QWERTY keypad (110 mm long, 60 mm wide). Each sensing area (5 mm wide, 5 mm long) was arranged with a spatial period of 9 mm. Details of the touch layer fabrication are described in the Supporting Information. The Ris of the prepolymers were measured by using a prism coupler with transverse electric-field (TE) polarized light at a wavelength of 1031 nm. The microscopic images of the sensing area were taken by using a Carl Zeiss AXIO Scope.A1 optical microscope and FEI SIRION 400 scanning electron microscope. The refractive indices of transmittance spectra of the force sensor were collected by using a Shimadzu UV-1700 spectrophotometer. Mechanical properties were obtained from their strain–stress curves, which were measured by using an Istron 4482. Test conditions were selected according to an ASTM standard D-882 (Tensile testing for thin plastic sheeting). For the performance testing, a sinusoidal vertical force input (force: ca. 0–3 N, frequency: ca. 1–16 Hz) was applied to the surface of the touch layer at the force sensor by using a PHANToM (SenAble Technologies) device which has a 1 ms sampling time. Light-intensity change with input force was monitored by using a data acquisition unit which was interconnected to the photodetector.

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response delay are remarkably reduced to 10.7 % and 2.5 ms, respectively, while sensitivity to input force is lower than for the PTBA touch layer (Figure 4c and d). On the other hand, the CA touch layer exhibits sensitive responses comparable to the PTBA touch layer and records the least hysteresis of 6.7% and the lowest response delay of 2.4 ms (Figure 4e and f). The results suggest that the soft nature of the touch layer contributes to realization of a more sensitive response to input force, but causes the response to be subject to significant hysteresis under repetition of the loading and unloading with high frequency. Figure 4g shows the Li profile of the force sensor with the CA touch layer during continuous cyclic loadings with a sinusoidal input force of 1 Hz in the range 0–3 N. For the first 600 cycles, the band of change in Li gradually moved downwards to 5 % and then stabilized. The range of change in Li is consistent over 3600 cycles with a negligible deviation of less than 1%. In the case of the PET touch layer, the response stabilized more rapidly, while the range of change in Li was smaller than for the CA touch layer. (Figure S5, Supporting Information). Figure 4h shows a demonstration scene of simultaneous force sensing at two different positions after overlaying the force sensor array onto the human arm. A video file, included in the Supporting Information, shows a single or multipoint contact force detection of the transparent flexible force sensor on the soft surface. We have developed a fast responding force sensor array that detects multiple points; it is is thin, highly transparent, and highly flexible and based on polymer waveguides that can be applied to curvilinear interfaces. The force sensor detects contact forces by monitoring light intensity transmitted through sensing areas that allow a touch layer to be in contact with the bare core of the waveguide patterns. The force sensor is capable of working without any electronic components on the sensing areas. The response characteristics including sensitivity, response time, and hysteresis depend on the mechanical properties of the touch layers. The use of the CA touch layer allows the force sensor to give a fast and sensitive response without significant hysteresis to the programmed sinusoidal input force (ca. 0–3 N) with frequency in the range of 1–16 Hz. The response is also highly durable over 3600 continuous loading and unloading cycles and resistant to bending. The thin-film architecture with the combination of the waveguide sensing mechanism and soft optical materials allows the demonstration of simultaneous and multiple force detection on a curvilinear surface such as the human arm. When the sensor is practically applied to electronic devices as a flexible input interface, we may additionally analyze environment effects such as temperature, humidity, etc. We also need to investigate the optimal spatial pattern for power consumption and materials for rapid response with low hysteresis.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author. The Supporting Information includes a video file showing single or multipoint touch force detection of the polymerwaveguide-based flexible tactile sensor array onto the human arm and a soft surface.

Acknowledgements The work reported here was supported by the Pioneer Program of Korea National Research Foundation (2013M3C1A3059557), the Creative Research Center Program of ETRI, and the TAXEL: Visuo-haptic Display (10035360) of MSIP/KEIT. Received: November 27, 2013 Revised: March 14, 2014 Published online: April 7, 2014

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Adv. Mater. 2014, 26, 4474–4480

Polymer-waveguide-based flexible tactile sensor array for dynamic response.

A polymer-waveguide-based transparent and flexible force sensor array is proposed, which satisfies the principal requirements for a tactile sensor wor...
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