Micromachined polymer actuators as tactors for tactile ... - IEEE Xplore

Proceedings of the 2003 IEEE Intemational Conference on Robotics,Intelligent Systems and Signal Processing Changsha, China - October 2003

Micromachined Polymer Actuators as Tactors for Tactile Display Tony K. H.

TO^,^, Jennifer W. L. ZhoulS2,Hoyin Chan132,W e n J. Li’,2,and Yunhui Liu2

’Centre f o r Micro and Nan0 Systems, The Chinese University of Hong Kong ’Department of Automation and Computer-Aided Engineering, The Chinese University of Hong Kong Hong Kong SAR

Abstract MEMS fabricated smart polymer actuators such as Nafion and PANI are proposed as tactors in the development of tactors-

on-chip for applications in virtual reality and tele-operations. Two different polymer materials, i.e., Nafion and PANI, were investigated as potential materials to construct the tactors. Thus far, we have successfully demonstrated that these polymer materials can in fact be used to fabricate MEMSscale actuators with actuation voltage of less than 1OV. Nafion actuators ( ~ 1 0 0pm, 1=1200 pm, G0.4pm) were actuated with input power of -50mW and showed dynamic , response up to 9Hz. PANI actuators ( ~ 5 0 p m 1=200pm, t-lpm) were shown to actuate in 0.5M HCl solution using 2.5V input voltage. The fabricationprocess for these polymer actuators, their experimental results, and a proposed tactorson-chip design is presented in this paper.

1. Introduction A desirable element in the application of virtual reality and telerobotic operations is the ability for a human subject to “touch” or “feel” the virtual or remote objects. Efforts are underway worldwide to develop tactile display systems that will transduce “feelings” from robotic hands to the human operator. However, several factors impede the display systems under development from gaining universal acceptance: bulkiness, small tactile force output, large required driving voltage, slow system response, system complexity, and non-integrability with integrated control circuits. We propose to use micromachiningMEMS technologies to process new breeds of artificialmuscular materials, possibly include ion-metal-polymer composites (IMPC) or Polyaniline (PANI), to build an tactor array, integrate it with IC controller units, and produce a novel tactile-display system. Our aim is to create a low-cost, mass-producible, tactile-display actuator chip integrable with ‘IC control circuits. Our ultimate goal is to demonstrate a tactile-actuator chip that transduces shape information from a dynamic virtual environment or from sensors of a teleoperated This project is fimded by the Hong Kong Research Grants Council (CUHK4206/00E). +

0-7803-7925-X/03/$17.00 02003 IEEE

dexterous five-finger robotic hand, and allow a human user to “feel” the texture and shape of the virtual or real remote objects. This chip will have applications in virtual presence, remote palpation, handling of surgical instrument, surgical robot controller, tele-manipulation, and sensory substitution aids for the blind. Micromachining/MEMS technologies will be employed , to build the device because they will allow us to build distributed, dense, and highly precise tactors in a small surface, and will allow these tactors to be individually addressed and integrable with IC controller circuits. We explore IMPC and PANI as materials for meso- and micro-scale actuators because they offer some very promising characteristics when compared with other smart and MEMS thin film materials: 1) relatively large output force given a small input voltage, 2) give much greater displacement over shape memory alloys or MEMS actuators, 3) a few orders of magnitude better in frequency response than SMA, 4) can be actuated with CMOS compatible input voltage of 2 to 5V, and 5) micromachinable and integrable with IC components. Several researchers have developed tactiledisplay arrays, but none had used integrated micromachining techniques or smart polymer materials (i.e., IMPC or PANI) as a base material to create their tactors. Taylor et al. (1996) [l] developed a 5x5 electrorheological fluid-based tactile array for virtual environments. The compliant surface of the tactile array offers a variable and controllable resistance to finger motion as finger is pressed onto and moved across the array. Each tactor in this array was 1.lcmxl.lcm separated by 2mm space and needed 1.5 to 2.4x103V ’ driving voltage, which is much larger in size and consumes much more energy than the presently proposed Nafion actuator array. Howe [2] and Kontarinis [3] (1995) have contributed much work in tactile shape sensing and display system for teleoperated manipulation and described a 6x4 array of pins actuated by SMA linkages. The system requires springs, shafts, levers, and wires to actuate the pins, and hence, is much more complex to fabricate than the proposed system. In addition, since SMA wires were used, time response of the system is limited to a few seconds, and the system is non-integrable with IC controllers. Hasser and

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mechanisms for medical and miniature device applications (e.g., see [12]). Experimental results have been reported in the past decade from researchers using ICPFs (e.g., based on NafionTM polymer film) as an actuator material. However, most of these ICPF actuators are in macro scale (e.g., see Shahinpoor and Ogura’s work). Typically, Nafion actuators can lift about 40 times their own weight and requires about 2 to 5V for actuation. The deflection amplitude .is dependent on the actuator structural geometry and applied voltage (e.g., a 5mmx20mmx200pm cantilever Nafion beam can be deflected by 10” at the tip with 2.5V at 0.5Hz [SI). Nafion actuators operate best in a humid environment but can be used as self-contained encapsulated actuators to operate in dry environments as well (Shahinpoor and Bar-Cohen reported that Saran@wrapped actuators can .>perate for up to 4 months). Our group has developed a laser-based fabrication method to “cut” millimeter (meso) scale Nafion actuators and performed extensive experimental analyses on these ICFP actuators in the past few years (e.g., see [13] and [14]). However, to our surprise, we were the first group that reported such small scale Nafion actuators at the time of publication of [13], as most researchers were interested in using Nafion based ICPFs to produce actuators for artificial muscle applications. Through our research, we have found that if commercial Nafion films were used (200um thick), then the deflection of ICPF actuators would be limited by the mechanical stiffness of Nafion. Hence, we have initiated research efforts in using MEMS related processes such as spin-coating and microphotolithography to reduce the thickness of polymer films and fabricate MEMS-scale micro actuators. This paper will present our recent successful results in fabricating MEMS-scale Nafion and PANI actuators. We believe that these MEMS-scale actuators will eventually allow us to produce tactors-on-chip that will allow tele-operators to “touch” and “feel” virtual or remote objects.

Wiesenberger (1993) [4] reported an 6x6 tactor array actuated with SMA wires in a flexure design. Although high array density and small package requirements were met for mounting in surgical instruments, but force level were limited to 0.2N per element. The system was also harnpered by slow time response and non-IC integrable. Cohn, Tam, and Fearing (1992) [5] built a 5x5 array with pneumatically-actuated pins. Since the pins were controlled by pulse-width modulated solenoid valves, mounting on surgical instruments is difficult due to the large number of valves and tubes connected to its elements. Most of other research on tactile shape display has focused on sensory substitutioc aids for the blind. The OPTACON uses vibrating pins to represent the intensity pattern as the device is manually scanned across a printed page [6]. Our current project is only concemed with static tactile displays. To the best of our knowledge, no one has developed a tactile-display that contsins 5x6 tactors in a l.jcmxl.5cm surface that is integrable with CMOS chip controllers, and runs at CMOS compatible voltages. We are currently developing MEMS based techniques to process new polymeric materials and produce arrays of large force micro-actuators driven at CMOS compatible voltages This is a novel approach to create a tactile display system because no one had used integrated MEMS fabrication technique to process Nafion or PANI as a base material to create their tactors, and combining the two technologies has become viable only recently owing to the recent advances in micromachining and IMPC research. Up to now, to our knowledge, all conventional MEMS actuators’ are hindered by small force output (typically micro- to milliNewtons) and small deflection due to the available thin films and actuation principles, and hence, can not be used for tactile-display applications. Shahinpoor [SI and Oguro et al. [9] have contributed and published a significant amount of work in using IPMC materials for robotic applications this decade. Shahinpoor et al. [IO] are currently developing robotic actuators for space applications with JF’L and Langley of NASA. IMPC materials promise to bring efficient miniature actuators that are light, compact and consume low power to telerobotic and space systems [ 111. Oguro’s collaborators are using ICPFS’ (ionic conducting polymer films) to create novel actuation

2. Polymer Micro Actuators as Tactors for Tactile Display The proposed tactile-displaying array is shown in Figure 1. Our initial prototype will consist of a 5x6 array of actuators each with dimensions of 2.5mmx200pmx200pm. These actuators will be fabricated within a 1.5cmxlScm area of polymer film. Each actuator can be electrically addressable by wires patterned on top of the polymer film as shown in the figure. This Nafion chip (on a Si substrate) can then be

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An exception to this is the micro-balloon actuators which may deflect up to 2mm while holding 7psi of pressure. However, they must be pneumatically driven, which makes the system bulky and slow in time response [7]. Oguro’s group has called the Nafion and metal thin film stacks Ionic Conducting Polymer Films as oppose to Shahinpoor’s Ionic Polymer-Metal Composites.

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Commercial Nafion solution from Dupont (SE5012) was used to fabricate the ICPF micro actuators in our research. The actuators were made of Au/Nafion/Au layers with the Nafion film thickness controlled by a spin-on process. Several designs of ICPF actuators were batch fabricated on a 4-inch silicon wafer using surface micromachining technology. The basic process flow is shown in Figure 3. Details of the fabrication process are given in [15]. The major difficulty in this work was to develop a process to spin-on Nafion thin films to create MEMS structures. This task was successfully accomplished by a specialized spun-on-and-cured process as described in [15]. In order to generate a relatively uniform thin film, we used -0.2pm thick Nafion and -0.1pm thick gold layers as electrodes in our process.

packaged as shown in Figure 2 in a ceramic mount and a top protection frame is then placed over the chip. Since our polymer micro actuators can typically be driving from 2 to 5 volts, a CMOS controller chip can potentially be bonded to the Si supporting substrate using flip-chip bonding. More detailed fabrication processes for the polymer actuators are given in the following sections. Their actuation performance will also be presented.

B. Experimental Results The testing of the polymer actuators was carried out using a micro probe station. The samples were placed in a Petri dish filled with DI water. (DI water was needed for actuation of the Nafion and PANI actuators because they actuate base on ion or proton exchange processes; see [ 161 and [ 171 for explanations on Nafion and PANI actuation principles, respectively). Voltage was applied to the samples through two micro probes. The image of the actuator deflection was captured through a microscope with a CCD camera linked to a computer terminal. A sequence of motion of a 2-finger actuator actuated in DI water is shown in Figure 4 (2-D top-view under microscope). The actuator (each finger w 1 0 0 pm,1=1200 pm, t-O.4pm) started to deflect at -5V. It reached full deflection, i.e., 90" change of tip direction when the voltage was -7V. Gas bubbles due to electrolysis were generated from both electrodes. After the voltage was removed, the beams returned to their original positions. In order to understand the electromechanical property of the ICPF microactuators, the current-voltage property of some actuators were measured. Input I-V characteristics of some actuators were obtained which show the power consumption of these micro ICPF actuators to in the order of 50mW (i.e., for a single finger structure with w 3 0 0 pm, 1=1200 pm, and g0.4pm). A sample I-V dynamic response for an actuator actuated at 9Hz is shown in Figure 5.

Fig. 1. 5x6 Nafion actuator array on a 1Sx1 Scm2area.

Fig. 2. Illustration configuration.

3. Micro Fabrication of Nafion ICPF Actuators A. Fabrication of MEMS Nafion ICPF Actuators

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Aluminum

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Nafion

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acid, the polymer will be partially protonated, primarily at the imine nitogens. This reaction is reversible by deprotonation with base. Basically, protonation means adding extra hydrogen ions (protons), so PANI will increase its size due to protonation; while under deprotonation, it will decrease its size. In a solution abundant of hydrogen ions (acid), protonation occurs; in a solution lack of hydrogen ions (alkaline), deprotonation occurs [17]. As PANI can undergo protonation and de-protonation reversibly and repeatable, it can be used as an actuator under different pH medium.

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Fig. 3. Illustration of the process steps to fabricate micro Nafion actuators. (a) Deposition and etching of sacrificial aluminum on oxidized Si. (b) Deposition and etching of adhesion promoting chromium layer; parylene coated and pattemed. (c) Deposition and liftoff of bottom gold layer. (d) Deposition of Nafion by spin-on; deposition and etching of top gold layer; etching of Nafion by plasma. (e) Waterproof parylene layer coated and patterned. (0 Removal of sacrificial layer.

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=p -1.00 >” -2.00 -3.00 Time (s)

Fig. 5 . Dynamic I-V characteristicsof a micro Nafion actuator actuated at 9Hz with 3V. On the other hand, it is known that electrical potential can also drive this electrochemical reaction Error! Reference source not found., with potential ranging from -0.4 V to 1.OV. Recent research shows that PANI can actuate upon electric field in wet condition with a gilded gold film Error! Reference source not found., and in dry condition with a gel-like electrolyte Error! Reference source not found..

Fig. 4. A micro Nafion actuator under 5V and 7V DC voltage input. (a), (b),(c) are 2-D top views under 5V. (d) is a 3-D picture of actuator under 7V to reach full closure of gripper. The bubbles were caused by electrolysis of the aqueous medium. Full characterization on the performance of the micro Nafion actuators is underway in our lab.

A. Micro Fabrication PANI Actuators We have also demonstrated some initial success in making PANI actuators in the micro scale using MEMS technology. We found that as a polymer, PANI cannot stick well on normal metals and is insoluble to most of the acids. Thus, wet etching cannot be applied to define PANI. In our lab, we use dichloroacetic acid (DCA) plus formic acid (FA) as the solvent, thus the PANI can be formed as solvent-casting films. The film thickness can be experimentally varied from 0.1-OSpm (spinning at 500rpm to 4000rpm). Using silver as the conductive and structural layer, we have shown that the bi-layer strip bends toward the side

4. Micro Fabrication of PANI Actuators Polyaniline (PANI), a kind of conductive polymer, will undergo dimensional changes upon a reversible electrochemical oxidation-reduction (redox) reaction. This redox reaction is pH dependent, and can be activated by supplying abundant electrons. These properties make PANI one of the potential materials for making polymer actuators for applications where large deflection is required, e.g., tactors for tactile display. In

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of silver when it acts as the anode, which indicates that PANI expands upon oxidation. This reaction is reversible with a different polarity. The layer composition of the most successful PANI actuators fabricated in our laboratory is shown in Figure 6 below. The basic fabrication process is:

1. Deposition and pattern of bottom conductive Au Layer. 2. Pattern sacrificial PR. 3. Spin on PANI. 4. Deposition and pattern styctural Au layer. 5. Use plasma to pattern PANI. 6. Sacrificial release.

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Fig. 7. Microscope picture of an array

of PANI actuator in HC1 solution (top view).

B. Experimental Results The prototype actuators (50um x 200um actuator array, as shown in the microscope picture in Figure 7) were tested in a 0.5M HCl solution with 2.5V actuating potential. One electrode was connected to the pad and the other was dipped inside the solution. The actuators bend downward when a positive potential was applied (Figure 8a). They stayed in position after the electrical supply was removed, indicating that proton exchange stopped when the electrical supply was cut off. The actuator bend upward (2.5V) when a negative potential was applied (Figure 8b). Again, they stayed in position after the electrical supply was removed. A summary of our experimental observations for the PANI actuators is shown in Table 1.

Fig. 8. (a) Actuators bend downward (2.5V) and stayed in position after the electrical supply was removed. (The blur region is due to out of focus of the microscope. Only the center region is focused) (b) Actuators bend upward (2.5V) and stayed in position after the electrical supply was removed. . (Again, the blur region is due to out of focus.)

Structural and conductive\ I

I Gold I PANi

Substrate

Fig. 6. PANI actuator layer compostion.

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Table 1. actuators.

Summary of performance for the PANI

Material Actuating voltage Motion Mechanism

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Yield Rate -20% Actuator Size 50um by 250um Electrolyte 0.5M HCI

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[7] C. Grosjean, G. Lee, W. Hong, Y.C. Tai and C.M. Ho, Micro Ballon Actuators for Aerodynamic Control, 1998 The Eleventh Annual International Workshop on Micro Electro Mechanical Systems, Heidelberg, Germany, Jan. 25-29,1998. [SI M. Shahinpoor, Spring-loaded polymeric gel actuators, US Patent 5 389 222, February 1995. [9] K. Oguro, K. Asaka and H. Takenaka, Polymer Film Actuator Driven by a Low Voltage, Proceedings of 4th Intemational Symposium on Micro Machine and Human I Science (MHS93) at Nagoya (1993). [IO] M. Shahinpoor, Y. Bar-Cohen, J. 0. Harrison and J. Smith, Review Article Ionic Polymer-metal Composites (IPMCs) as Biomimetic Sensors, Actuators and Artificial Muscles- a Review, Smart Mater. Struct.7 (1998) R15R30. [ l l ] Y. Bar-Cohen, S. Leary, J. 0. Harrison and J. Smith,

5. Summary This paper proposes to develop an integrated tactors-on-chip system for tactile display applications in virtual reality and tele-operatioins. From a survey of the existing technologies for tactile display, it was concluded that current systems are too bulky and are slow in dynamic response, and hence no one single system is universally accepted by researchers and experts in the tactile display communities. MEMS polymer actuators are proposed by our group to be used as tactors because they have large displacement when compared to conventional silicon based micro actuators. Two different polymer materials, i.e., Nafion and PAM, are proposed and are currently under investigation in our group to eventually develop a fully integrated tactors-on-chip system. Thus far, we have successfully demonstrated that these polymer materials can in fact be used to fabricate MEMS-scale actuators with actuation voltage of less than IOV. Work is underway to build a practical tactors-on-chip based on our developed fabrication process.

Electroactive Polymer (EAP) Actuation of Mechanisms and Robotic Devices, Transducers '99 June 7-10, 1999

Sendai, Japan. [ 121 S. Tadokoro, T. Murakami, S. Fuji, R. Kanno, M. Hattori,

T. Takamori, and K. Oguro, An Elliptic Friction Drive Element Using an ICPF (Ionic Conducting Polymer Gel Film) Actuator, Proceedings of the 1996 IEEE

Intemational Conference on Robotics and Automation Minneapolis, Minnesota-April 1996. [13] Jennifer W. L. Zhou, Wen J. Li, Ning Xi, and Shugen Ma, "Development of force-feedback controlled Nafion micromanipulators", Electro-Active Polymer Actuators and Devices, SPIE 8th Annual Intemational Symposium on Smart Structures and Materials, New Port Beach, CA, USA, 4-8 March 2001. [14] Jennifer W. L. Zhou, Wen J. Li, and Ning Xi, "Development of a force-reflection controlled micro underwater actuator", IEEE/RSJ IROS 200 1, Hawaii, USA, October 2001. Jennifer W. L. Zhou and Wen J. Li, "MEMS-Fabricated ICPF Grippers for Aqueous Applications", The 12th Intemational Conference on Solid-state Sensors, Actuators, and Microsystems (Transducers 03'), Boston, USA, June 8-11,2003. R. Kanno, S. Tadokoro, T. Takamori, and K. Oguro, 3Dimensional Dynamic Model. of Ionic Conducting Polymer Gel Film (ICPF) Actuator, 0-7803-3280-6/96 @, 1996 IEEE. W. Lu, E. Smela and B. R. Mattes, "Electrochemical actuation of gilded Polyaniline bilayers in aqueous acid solutions", Smart Structures and Materials, 200 1, pp. 505.

6. Acknowledgement The authors would like to Prof. Shuxiang Guo of Kagawa University, Japan, for his technical insights on the operational principles of Nafion actuators. ,

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7. References

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[l] P. M. Taylor, A. Hosseini-Sianaki, and C. J. Varley, An Electrorheological Fluid-based Tactile Array for Virtual Environments, Proc. Of the 1996 IEEE ICRA,

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Minneapolis, 1926, Pp..l8.-23. [2] R. D. Howe, W.'J.,Peine, D. A. Kontarines, and J. S. Son,

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IEEE Engineering .in Medicine and Biology Magazine, 14(3):318-323, 1995: ' [3] D. A. Kontarinis, J. S. Son, W. Peine, and R. D. Howe, A Tactile Shape Sensing and Display System for Teleoperated Manipulation, IEEE International

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Conference on Robotics and Automation, 1995. [4] C. Hasser and .J; M. Weisenberger, Preliminary Evaluation of a Shape Memory Alloy Tactile Feedback Display,. , Symp. Haptic Interfaces Virtual Env.

Teleoperator Systems, ASME Winter Annual Meeting, 1993, New Orleans. [5] M. B. Cohn, M. Lam, and R. S. Fearing, Tactile Telemanipulator Feedback for Teleoperation, Technology Conference, Proc. SPIE vol. -1833,H. Das, ed., Boston, MA, Nov. 15-16, 1992. [6] J. C. Bliss et al., Optical-to-Tactile Image Conversionfor the Blind, IEEE Transaction on Man-Machine Systems, 1970, MMS - 11(1), pp. 58-65.

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