A Highly Flexible, Stretchable and Ultra-thin

A Highly Flexible, Stretchable and Ultra-thin Piezoresistive Tactile Sensor Array using PAM/PEDOT:PSS Hydrogel Phi Tien Hoang1 , Hoa Phung1 , Canh Toan Nguyen1 , Tien Dat Nguyen1 , and Hyouk Ryeol Choi1 , Member, IEEE 1 School

of Mechanical Engineering, Sungkyunkwan University, Cheoncheon-dong, Jangan-gu, Suwon-si, Gyeonggi-do, South Korea (E-mail: [email protected], [email protected]) Abstract—This paper presents a highly flexible, stretchable and ultrathin piezoresistive tactile sensor array, which can be attached to a tactile display actuator array [1] without any influence on its motion. Its operational principal is based on the resistance changing of a tactile cell array prepared by blending a precise conductive polymer - Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) with Polyacrylamide (PAM) hydrogel. Through comparing the characteristics of the blended hydrogel mixed between PAM and PEDOT with different ratios 2.5:1; 3:1; 3.5:1 respectively, the 2.5:1 ratio is the appropriate sample selected as the sensor design. Then, the 12x8 tactile sensor array were fabricated by dropping the conductive hydrogel into the cavity of two electrodes cast on Sylgard 184 substrates (PDMS), which is flexible and thin - around 800µm thickness in total. Finally, a primary measuring circuit using Wheatstone Bridge with a low-pass filter, an amplifier, and a signal processing circuit are also built to demonstrate the sensor0 s performance successfully. Keywords—Piezoresistive tactile sensor, tactile display actuator, PEDOT:PSS, Polyacrylamide hydrogel.

1. I NTRODUCTION Recently, various tactile sensors have been reporting [2][4] and applying in many research fields, engineering and manufacturing. Along with the rapid development of soft polymer materials, tactile sensors have been more suitable for applications required better mechanical characteristics such as robust, flexible, stretchable, wearable, ultra-thin, arbitrarily shaped etc., allow them can perform the ”sense of touch” obligation for robot skin or tactile display actuator etc. Piezoresistive is one of the basic transductions for pressure or force sensor. Compared to the capacitive sensors, the piezoresistive sensors hold advantages in size and sensitivity. In addition, the low output impedance (kΩ) of the piezoresistive sensor allows it to be used with longer cables and a greater variety of signal conditioning devices without signal degradation [5]. There are many kinds of conductive materials that can be used as the piezoresistive materials such as carbon nanotube (CNT), graphite, carbon micro coils (CMC), silver nanowire (AgNW) etc., by incorporating them into elastomers. However, this method has some disadvantages such as poor resistance stability in case of limited in thickness, and unsatisfactory

Fig. 1: Ultra-thin 12x8 tactile sensor array prototype, about 800µm in thickness

performance in terms of flexibility, conductivity and sensitivity. In that situation, the conductive polymers become the good candidates because of its durable and reliable properties. Among these materials, PEDOT:PSS has gained an outstanding position for piezoresistive pressure or force sensors basing on its thermal stability, high conductivity [6]-[8]. However, it has a limitation in elasticity, cracking easily (strain ≤ 10%) and water-dispersed so that it could not blend with elastomers. To overcome its limitation, we use a hydrophilic polymer (PAM) as the blended polymer which can improve the elasticity and mechanical strain of the pristine PEDOT:PSS and the surface tension also. In this work, we present a highly flexible, stretchable and ultrathin 12x8 piezoresistive tactile sensor array as shown in Fig. 1. This paper is organized as follows: piezoresistive tactile sensor are introduced in section 2 including the operational principal and sensor design, blended conductive hydrogel PAM/PEDOT:PSS and fabrication process. In section 3, evaluation of the sensors is illustrated in experiments. Finally, the conclusion is given in section 4. 2. P IEZORESISTIVE TACTILE SENSOR 2.1. Operational principle and sensor design The basic principal of this sensor is based on the piezoresistive effect which is the changing in electrical resistivity of conductive materials when the mechanical strain or stress is applied. The resistance of the conductive materials are defined basing on Ohm0 s law:

Fig. 2: Schematic diagram of the proposed piezoresistive tactile sensor array: (a) The expected design; (b) Single cell exploded view; (c) Sensors zoomed-in top view; (d) Sensors exploded view.

Fig. 3: Dimensions of the proposed piezoresistive tactile sensor array. (a) Individual sensing cell, (b) Sensors top view, and its dimension.

L (1) A where ρ, L, A are the resistivity, length and the crosssectional area of the current flow of conductor, respectively. Equation (1) is also written in the derivative form:

Fig. 4: Resistance circuit model of the individual sensing cell.

dR dL dρ dA = + + (2) R L ρ A In addition, the piezoresistivity is also described by Gauge Factor (GF) which determines the amplification factor between the strain or stress and the resistance changing. In this paper, we introduce the tactile sensor based on normal stress, so:

the combination of the resistance of two electrodes (Re1 ,Re2 ), the resistance of the physical contacts between electrodes and PAM:PEDOT/PSS hydrogel (Rc1 ,Rc2 ) and the piezoresistor (R p ) of the piezoresistive material. The total resistance is given by equation (4) as:

R = ρ.

GF =

∆R R ∆L L

=

∆R εR

(3)

with σ = ε.E is normal stress. In this paper, a prototype of flexible, stretchable and ultrathin piezoresistive tactile sensor array has been proposed. As described in Fig. 2, it includes two Sylgard 184 (PDMS) top and bottom substrate films with 200µm thickness casted the 12x8 electrode patterns, which are assembled together with an isolating layer 400µm thickness sandwiched in the middle. The sensing materials (PAM:PEDOT/PSS hydrogel) will be dropped by micropipette to full-fill 12x8 pre-cut holes on isolating layer before half-drying and stack three layers together. Among stacking layers, Sylgard 527 (PDMS gel) was used to overcome the poor adhesive of PDMS substrates and prevent the completely drying process of the hydrogel cells after a period of time. Fig. 3 (a) and (b) illustrate the dimensions of the highly flexible, stretchable and ultra-thin piezoresistive tactile sensor array in details with (130mm x 90mm) in overall. There are eight rows and twelve columns with spaced 10mm among them. Each individual tactile cell has one 6mm diameter bottom electrode, one 7mm diameter piezoresistive sensing part, and one 5mm diameter top electrode respectively. Based on the sensor design above, each tactile cell can be modeled by a circuit described in Fig. 4. Its resistance is

R = Re1 + Re2 + Rc1 + Rc2 + R p

(4)

Therein, the electrode resistances were used by a highly conductive material about 10Ω with 130mm length and 30µm thickness. Besides, during the fabrication process, PAM:PEDOT/PSS hydrogel was dropped on the bottom electrodes surface directly. In addition, after half-drying PAM:PEDOT/PSS hydrogel, the top electrode was stacked above to minimize the contact resistances between it and hydrogel immediately . Therefore, the sensors resistance is equal to the conductive hydrogels resistance ideally: R ≈ Rp

(5)

2.2. The blended conductive hydrogel PAM/PEDOT:PSS The conductive hydrogel was made as follow: Firstly, the PAM particles was dissolved in water with 1:30 in ratio. Accelerating the process of dissolving, the mixture was heated at 60o C and stirred in 20 minutes to get the PAM hydrogel. Secondly, the PAM:PEDOT/PSS hydrogel was fabricated by blending the PAM hydrogel and the highly conductive polymer PEDOT/PSS (4-5% w.t in H2 O from Sigma-Aldrich) in accordance with the following proportions: ”2.5:1”, ”3:1” and ”3.5:1” to achieve different resistance values. Furthermore, 3% Ethylene Glycol (EG) in total weight was also dropped

Fig. 5: The performance of (a) the single cell sensor with the different blended ratio between PAM:PEDOT/PSS 2.5:1, 3:1 and 3.5:1: (b) Resistance-Normal force, (c) Normalized resistance change-Normal force relationship.

Fig. 6: 12x8 piezoresistive tactile sensor array fabrication process.

in the blended hydrogel to improve the PEDOT/PSS linking in the mixture. Finally, the mixture was dropped on the bottom electrode and dried at 60o C in 60 minutes. These finished sensor0 s samples have 10mm in diameter and 1mm in thickness. Fig. 5 illustrates the performance of single cell sensor with different ingredients ratio. Fig. 5 (b), (c) describe the resistance-normal force, and the normalized resistance change-normal force relationship, respectively. It is clear from the graphs that there was a sudden decrease in the sensor0 s resistance with applying force from 0 to 0.5N in case of 3:1 and 3.5:1 samples while it reduced slightly between 0.5 and 1N. In contrast, with 2.5:1 sample, sensor0 s resistance was reduced gradually when applying force increased from 0 to 1N. It is easy to see that the 2.5:1 proportion between PAM:PEDOT/PSS has the best performance comparing with two others.

2.3. Fabrication process As illustrated in Fig. 6, the fabrication process of the proposed sensor as follow: In the first, the PDMS and its curing agent are mixed with 10:1 ratio. Then, the 200µm (for the substrates) and 400µm (for the isolating layer) thickness PDMS films will be casted on the Teflon plate using casting rob and cured at 120o C in 20 minutes. Secondly, the electrode patterns made by mixing the conductive silicone and toluene will be deposited and casted on the bottom and the top of PDMS substrates using the acrylic masks. After curing these substrates, the isolating layer with the pre-cut holes using laser printer will be stacked upper the bottom substrate by brushing the PDMS gel. Next, the blended hydrogel (2.5:1 ratio between PAM and PEDOT/PSS) are dropped into the pre-cut holes using micropipette and dry at 60o C in 60 minutes. Finally, the sensors will be finished by stacking the top substrate using

Fig. 7: Schematic diagram of the experimental setup for measuring the sensor0 s response. Fig. 8: Sensor0 s response based on resistance changing with various normal force from 0.02N to 4.49N. PDMS gel. 3. E XPERIMENTAL SETUP AND RESULTS 3.1. Experimental setup After finished the sensor fabricating and wiring, we prepared the experimental setup to demonstrate the effectiveness of PAM:PEDOT/PSS hydrogel as a piezoresistive tactile sensor. The setup for this experiment is described in Fig. 7. The signal processing circuit includes an MCU (dsPIC30F5015) generates a signal to control the scanning block which is integrated by a transmitter and a receiver Multiplexers (MUX). The transmitting MUX includes one drain input pin which is connected to +5V power supply and 12 pins switching which can be controlled by MCU sequentially using 3-wired SPI communication. Similarly, the receiving MUX includes 8 pins switching and 1 drain output pin which will be connected to a Wheatstone Bridge (WB). Comparing with a simple voltage divider, WB is constructed by a pair of voltage dividers with some advantages such as smaller DC offset compared to the voltage swing, and the sensitivity can be boosted with a gain stage. Next, the differential signal generated from the Wheatstone Bridge will be amplified and sent to ADC pin (of MCU). A force gauge connected to a PC is also used to generate the specific normal force. Finally, MCU can communicat with PC using USB cable.

Fig. 9: The repeatability and linearity of taxels located at (2,2); (2,10) and (6,10) positions

3.2. Results Based on the setup above, the sensor0 s sensitivity and its hysteresis were evaluated in details using force gauge and LCR meter. On the one hand, the force gauge generates a various push-release ascending forces in magnitude from 0.02N to 4.49N. On the other hand, the LCR meter connected to an individual sensor0 s cell will send the resistance data to the PC. It is clear from the Fig. 8 that the sensor0 s resistance is quite stable with the remaining in the initial resistance around 1.6kΩ during the push-release of force gauge. Moreover, as is highlighted in the graph, the sensor0 s resistance change dramatically with a slightly changing in applying force. The graph demonstrates that the proposed sensor has a quite good sensitivity with 0.01N force resolution and the repeatability is

Fig. 10: Sensor0 s response under the specific F=0.05N.

also. In more details, Fig.9 illustrates the repeatability and linearity of various sensing taxels when we test in 3 independent

Fig. 11: Experiment results by single-touch and multi-touch on cells located at (a)-(6,10); (b)-(2,10); (c)-(2,2); (d) both (2,7) and (7,2) surroundings.

releasing process. This proves that the proposed tactile sensor has fast response and small hysteresis. 33.22

Gauge Factor

30

29.49

23

20.4

20

18.46 16.63 15.31 14.07

13.39

10 0.2

0.4

0.6

0.8

1.0

Force (N)

Fig. 12: Gauge Factor of the proposed tactile sensor array corresponding to the specific force.

cells located at (2,2), (2,10) and (6,10) postitions. To assess the sensor0 s hysteresis, the applying force will be pushed in 3 seconds and released in 3 seconds as illustrated in Fig. 10. When the force F=0.05N is applied, the sensors resistance decrease to approach the theoretical value after approximately 0.25ms. In contrast, it is about 0.35ms corresponding to the

Next, the 12x8 tactile sensor array was also built to demonstrate its tactile performance in a real application. The MCU can control two MUXs by sharing the same 3-wired SPI communication. Firstly, MCU will activate to control the transmitting MUX which its 8 output pins was connected to 8 rows of 12x8 tactile sensor array. After switching on the first row of the tactile sensor array, sequentially, 12 columns of it was also switched by receiving MUX which was controlled by MCU at the current. During this process, 12 resistance values were read and saved to MCU buffer as a 12 bytes scalar. Similarly, this scanning process was repeated for the remaining rows in a continuous circulation. Secondly, after finished one scanning cycle, MCU sent 8 of 12 bytes scalar to PC using USB communication. Finally, an user interface using Matlab has been written. This program read the 8 x 12 bytes scalar from MCU and display the results by 3D plotting. Fig. 11 (a) display the resistance data of cells which is located surrounding row 6, column 10 (6,10). By scanning method, when we touch on this area, its resistance was decreased cause an increasing of ADC output value without any influence to other areas. Similarly, Fig. 11 (b) and (c) illustrate the resistance data of cells located at (2,10) and (2,2) areas using Matlab, respectively. Finally, we also tested

the simultaneous response both of (2,7) and (7,2) diagonally areas, shown in Fig. 11 (d). As we can see, there are no ghost points occuring when the force applied to cells located on the diagonal. Finally, the gauge factor of the proposed tactile sensor array is measured as a function of applying force. As shown in Fig.12 that the gauge factor of the tactile sensor decreases when the applying force increases from 0 to 1N. To explain this behavior, the reason maybe that the normal stress creates the strain which reduce the bandgaps between conductive molecules. The result is that the conductivity of the blended hydrogel is increased. 4. C ONCLUSIONS In this work, a highly flexible, stretchable and ultra-thin 12x8 piezoresistive tactile sensor array has been proposed, which has an approximately 800µm in thickness, stretchability about 15%, and it can be bent and twisted. Its sensitivity, repeatability, and hysteresis were evaluated. Moreover, a signal processing circuit with a Wheatstone bridge was also built to demonstrate the sensors performance. According to Yoo-Yong et al [9], we have studied that the PAM hydrogel can be used as a blended polymer which can dissolve the water in the pristine PEDOT/PSS. The result is that blended PAM:PEDOT/PSS hydrogel has a smaller surface tension, better mechanical characteristics such as elasticity and stretchability. However, although PDMS gel has been used to prevent the gaps between layers during stacking process, the blended hydrogels still contact with the air. Consequently, the sensor’s performance start to decline after 2 months. In the next, we will focus on improving the mechanical characteristics of the blended polymer by optimizing the PAM hydrophilic polymer and process this hydrogels into organogels. ACKNOWLEDGEMENT This research was supported by the Commercializations Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science, ICT and Future Planning (No. 2016K000119). R EFERENCES [1] Phung, H., Nguyen, C. T., Nguyen, T. D., Lee, C., Kim, U., Lee, D., ... & Choi, H. R. (2015). Tactile display with rigid coupling based on soft actuator. Meccanica, 50(11), 2825-2837. [2] Hoshi, T., & Shinoda, H. (2006, May). Robot skin based on touch-areasensitive tactile element. In Robotics and Automation, 2006. ICRA 2006. Proceedings 2006 IEEE International Conference on (pp. 3463-3468). IEEE. [3] Elkmann, N., Fritzsche, M., & Schulenburg, E. (2011, February). Tactile sensing for safe physical human-robot interaction. In International conference on advances in computer-human interactions (pp. 212-217). [4] Silvera-Tawil, D., Rye, D., & Velonaki, M. (2015). Artificial skin and tactile sensing for socially interactive robots: A review. Robotics and Autonomous Systems, 63, 230-243. [5] Neumann, J. J., Greve, D. W., & Oppenheim, I. J. (2004, July). Comparison of piezoresistive and capacitive ultrasonic transducers. In Smart Structures and Materials (pp. 230-238). International Society for Optics and Photonics. [6] Latessa, G., Brunetti, F., Reale, A., Saggio, G., & Di Carlo, A. (2009). Piezoresistive behaviour of flexible PEDOT: PSS based sensors. Sensors and Actuators B: Chemical, 139(2), 304-309.

[7] Wang, J. C., Karmakar, R. S., Lu, Y. J., Huang, C. Y., & Wei, K. C. (2015). Characterization of piezoresistive PEDOT: PSS pressure sensors with inter-digitated and cross-point electrode structures. Sensors, 15(1), 818-831. [8] Xie, M., Aw, K. C., & Gao, W. (2015, November). Skin force sensor using piezoresistive PEDOT: PSS with arabitol on flexible PDMS. In SENSORS, 2015 IEEE (pp. 1-4). IEEE. [9] Lee, Y. Y., Kang, H. Y., Gwon, S. H., Choi, G. M., Lim, S. M., Sun, J. Y., & Joo, Y. C. (2016). A Strain-Insensitive Stretchable Electronic Conductor: PEDOT: PSS/Acrylamide Organogels. Advanced Materials, 28(8), 1636-1643.