Статьи на перевод PVDF_P(VDF-TrFE) / Flexible Dome and Bump Shape Piezoelectric Tactile Sensors Using

Abstract—In this paper, a new mold-transfer method to pattern piezoelectric polymer has been developed and applied to fabricate innovative dome and bump shape polyvinylidene- fluoride-trifluoroethylene (PVDF-TrFE) films. The dome and bump shape PVDF-TrFE films have been successfully fabricated and characterized as a sensing component for flexible tactile sensors. The tactile sensors developed using these polymer microstructures show a high sensitivity which can measure as small as 40 mN force for bump shape sensors and 25 mN for dome shape sensors. The newly developed fabrication method provides a flexible way to pattern the piezoelectric polymer with different shapes and dimensions, including bump and dome shape piezoelectric polymer microstructures. In addition, a selective dc poling method for the PVDF-TrFE film has been developed for fabricating precisely located piezoelectric sensors with minimum crosstalk. The bump and dome shape PVDFTrFE films developed in this paper can have numerous applications for microcatheters or other minimally invasive biomedical

Index Terms—Bump shape, dome shape, flexible tactile sensor, mold-transfer method, piezoelectric devices, polyvinylidene- fluoride-trifluoroethylene (PVDF-TrFE) piezoelectric copolymer.

I. INTRODUCTION

THE SENSE of touch is of particular importance to physicians, particularly during minimally invasive medical procedures that enable diagnosis and treatment of many conditions and surgery to be performed through a small incision [2]. The minimally invasive procedures offer significant advantages over traditional open surgery, such as smaller incisions to heal, less physical pain, and faster recovery time. However, since catheterization only involves a small incision, the surgeon cannot gain a direct tactile access to the surgical site and is deprived of the sensory feedback that is normally available during the open surgery. Thus, there has been a large demand from the surgeons for a more effective sensory-feedback mechanism for in situ surgical tactile sensing and for an improved surgical safety operation. Until now, much attention has been directed

Manuscript received June 12, 2007; revised October 8, 2007. This paper was presented at the 20th IEEE International Conference on Microelectromechanical Systems (MEMS 2007), Kobe, Japan, January 21–25, 2007 [1], and is an expansion of the abstract as printed in the Technical Digest of this meeting. Subject Editor G. Stemme.

C. Li, P.-M. Wu, S. Lee, and C. H. Ahn are with the Microsystems and BioMEMS Laboratory, Department of Electrical and Computer Engineering, University of Cincinnati, Cincinnati, OH 45221-0030 USA (e-mail: leecc@ececs.uc.edu).

A. Gorton and M. J. Schulz are with the Smart Structures BioNanotechnology Laboratory, Department of Mechanical Engineering, University of Cincinnati, Cincinnati, OH 45221 USA.

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JMEMS.2007.911375

toward developing piezoelectric tactile sensors which generate voltage when touched, squeezed, or bent.

Piezoelectric polymers, the polyvinylidene fluoride (PVDF), and its copolymer, the [PVDF trifluoroethylene (PVDF-TrFE)], are strong candidates for tactile sensors because of their good mechanical flexibility, biocompatibility, and excellent sensitivity particularly in harsh and biological environments [3]. The PVDF is one of the well-known piezoelectric polymers and most widely used in the fabrication of the piezoelectric tactile sensors [4]. Unfortunately, inducing piezoelectric properties into this material requires mechanical stretching, which is not compatible with conventional microfabrication processes. In the PVDF-TrFE, the crystalline structure and, consequently, the piezoelectric and pyroelectric properties depend on the molecular proportion x of vinyledene fluoride in P(VDFxTrFE1−x). The presence of TrFE in the copolymer of the PVDF-TrFE film introduces significant features to the PVDF homopolymer. First, it increases the tendency to crystallize in the polar β-phase without the requirement of mechanical stretching to transform the nonpolar α-phase to the polar β-phase as in the case of PVDF, when 0.6 < x < 0.85 [5]. In addition, the PVDF-TrFE demonstrates a higher level of piezoelectricity than its predecessor [6]. Among the PVDF-TrFE copolymers, the copolymer at composition near 75/25 mol.% exhibits the highest ferroelectric responses [7]–[9]; hence, in this paper, the PVDF-TrFE copolymer with a molar ratio of 75/25 was used to fabricate flexible tactile sensors.

The ability to pattern the piezoelectric polymers into a desired shape and dimension will increase the range of its applications. Piezoelectric polymer film sheets have been typically used for the development of planar piezoelectric tactile sensors; thus, the microfabrication techniques on the film sheet have been well explored. In addition, various designs have previously been achieved by attaching different shapes on the planar PVDF film or by distorting the PVDF film [10]–[13]. The most common design concept among the piezoelectric tactile sensors involves bonding a flat PVDF film with patterned electrodes on a rigid substrate and then bonding patterned bump structures (silicon, glass, or sponge rubber) on the top of the PVDF film. The bump structure is included to form a good contact with or grasp an object. A different approach that directly spin coats a piezoelectric copolymer onto a curved substrate was demonstrated for the development of focused ultrasound transducers [14]. Methods, such as photoetching of PVDF using X rays [15], micropatterning of PVDF solution using temperature-controlled capillary micromolding [16], and micropunching of PVDF film [17], have been developed and extended to pattern the piezoelectric

Fig. 1. Conceptual drawing of a new mold-transfer method to pattern the PVDF-TrFE piezoelectric polymer film and the new dome and bump shape tactile sensor modules for smart microcatheter.

polymers. However, the difficulty and the limitation of forming and assembling the fabricated microstructures still limit the range of mechanical designs and practical applications in real systems.

In this paper, a new approach to pattern the piezoelectric polymer films by a mold-transfer method using standard MEMS techniques has been developed and then applied to a flexible tactile sensor. Innovative dome and bump shape PVDFTrFE films with different dimensions were successfully fabricated using a new mold-transfer method and tested as flexible tactile sensors. The developed sensors were assembled on a tip and around a Kapton tube [18] for catheter application. The sensors show high sensitivity and strong mechanical strength, and the selective dc poling method developed in this paper enables localized poling and minimizes the crosstalk between the sensors. Fig. 1 shows the conceptual diagram of the proposed devices in this paper.

II. DESIGN

Fig. 2 shows the design and working principle of the flexible dome and bump shape piezoelectric tactile sensors which were developed by the mold-transfer method. Both the dome shape and the square bump shape PVDF-TrFE microstructures were fabricated using the micromachined mold with diameters and widths of 500 µm, 1 mm, and 1.5 mm, respectively. The dc poling equipment used in this paper had a limited capability for poling only up to the film thickness of

Fig. 2. Design and working principle of the dome and bump shape flexible

40 µm; thus, the thickness of the sensors was designed to be 30 µm. A 1000 Å Al was deposited as top and bottom electrodes and the sensors were coated with 5 µm thick parylene film. Parylene is used as a protective layer to prevent the sensor from external damage, to enhance the mechanical stiffness, to be used as a biocompatible coating material, and to isolate the sensor from the environment so as to provide a sufficient thermal insulation to minimize transient pyroelectric effects.

The functionality of the piezoelectric-based tactile sensor can be attributed to the well-established piezoelectric effect which predicts that the amount of voltage generated on a piezoelectric material is proportional to the magnitude of the externally applied force. The output voltage V produced by a piezoelectric tactile sensor can be calculated using the following expression [19]:

V = F · g3n · d/A

where F is the applied force, g3n is the appropriate voltage output coefficient for the axis of the applied force, and d and A are the thickness and the surface area of the piezoelectric film, respectively. When a sinusoidal dynamic force is applied, the voltage produced from the PVDF-TrFE tactile sensor is sinusoidal with a frequency that is equal to the excitation and directly proportional to the magnitude of film deformation. The greater the deformation of the film, the higher the voltage will be.

III. FABRICATION

The dome and bump shape piezoelectric films can be fabricated using standard MEMS techniques. It is comprised of three steps such as fabricating the micromachined mold, spincoating and screen-printing piezoelectric polymer solutions over the mold, and, finally, detaching the film from the mold and assembling it to the devices.

A. Fabrication of Bump Shape Film

Fig. 3(a) summarizes the fabrication steps for the bump shape films. Aluminum was patterned on a silicon wafer as an electrode for the selective dc poling, and SU-8 photoresist was patterned as a mold to fabricate the bump shape patterns. Various solvents were used to investigate the solubility of PVDF-TrFE (75/25 molar ratio, MSI Inc.) and the solvent compatibility with SU-8 mold in this paper. Acetone and methyl ethyl ketone (MEK) have good compatibility with the SU-8 mold while the PVDF-TrFE is molten in a liquid phase for screen-printing process. Two different concentrations of the PVDF-TrFE solutions were made by mixing PVDF-TrFE pellets in the MEK solution. One solution was made by melting 20 g of PVDF-TrFE pellets into 100 mL MEK solution, and the other solution was made by melting 10 g of PVDF-TrFE pellets into 100 mL MEK solution. To get the desired bump shapes with smooth bottom surface, we did two steps of screenprinting. First, the solution with higher viscosity was screenprinted over the micropatterned well of the SU-8 mold and soft-baked at 90 ◦C for 15 min. Then, the solution with lower concentration was screen-printed over the whole surface and soft-baked again at 90 ◦C for 15 min. Then, the PVDF-TrFE film was annealed in a 135 ◦C oven for 5 h to promote uniform chemical and mechanical properties across the surface. Al was deposited and patterned as bottom electrodes. Then, the dc poling over the film was performed in a 90 ◦C oven with a

Fig. 4. Microphotographs. (a) Bump shape: Micromachined SU-8 mold on Si, transferred bump shape PVDF-TrFE film, and enlarged view of the bump shape (height = 30 µm). (b) Dome shape: Micromachined COC mold, transferred dome shape PVDF-TrFE film, and SEM picture of the cross section of the dome shape (thickness = 30 µm) from left to right, respectively.

70 MV/m electric field applied for 30 min and allowed to cool to room temperature before the applied voltage was removed. Finally, the film was carefully detached from the mold, and another Al layer was deposited using a passive mask for the top electrodes. The fabricated mold and the bump shape film were shown in Fig. 4(a).

Fig. 5. Microphotographs of assembled PVDF-TrFE flexible tactile sensors on the microcatheters. (a) Dome shape tactile sensors for the head tip.

(b) Bump shape tactile sensors over the wall of the microcatheter.

B. Fabrication of Dome Shape Film

Fabrication of the dome shape films was realized by spin-coating the PVDF-TrFE solution on the cyclic-olifen- copolymer (COC) lens mold from our previous work [20]. The detailed steps are shown in Fig. 3(b). Various solvents were used to investigate the solubility of PVDF-TrFE and the solvent compatibility with the COC polymer mold. Tetrahydrofuan (THF) has a good compatibility with the COC mold for micropatterning. To achieve the dome shape film with a good uniformity, 30 g PVDF-TrFE powder was melted in a 90 mL THF solution. Aluminum was patterned on the COC mold using a shadow mask to create an electrode for the selective dc poling. Then, the PVDF-TrFE solution was spin-coated on the mold, rested at room temperature for 15 min, and cured at 90 ◦C for 30 min. The top electrodes were deposited and dcpoled with the same condition described in the fabrication of the bump shape microstructures. To maintain the shape as a tactile sensor, finally, we filled the cavity with implantable grade silicone adhesive (MED ADH 4100 RTV). The microfabricated mold and the dome shape film were shown in Fig. 4(b).

C. Sensor Assembly to Test

The thicknesses of the developed bump and dome shape PVDF-TrFE films were measured using surface profilometer (KLA Profilometer). Using our fabrication methods and setup, we could achieve a thickness nonuniformity of ±4.5% for the 30 µm thick PVDF-TrFE films. One of the main problems involving the use of ferroelectric materials for sensor applications, specifically for the tactile sensors, is the separation of the pyroelectric and piezoelectric responses produced by the temperature and pressure variations. Thus, as a protection and thermal isolation layer for the sensor, a parylene film of 5 µm thickness was deposited on the sensors. The films with microstructures were cut into a chip to assemble on the catheter, and wires were bonded on the chip using silver epoxy. The dome shape films were glued at the head tip of the catheter, and the bump shape films were spirally rolled along the tube wall for experiments [21]. Both surfaces of the microfabricated PVDF-TrFE films were wired as a short circuit for at least one day to relax the excess charge before testing. Fig. 5 shows the assembled sensors for the catheter experiments.

Fig. 6. Measurement setup. (a) Schematic diagram of the apparatus for experiments. Dynamic load was supplied from the vibrator, and the output charge from the PVDF-TrFE tactile sensors was amplified by a charge amplifier and measured by an oscilloscope. (b) Photograph of the testing setup.

IV. EXPERIMENTAL RESULTS

A. Experimental Setup

Fig. 6 shows the experimental setup for the characterization of the PVDF-TrFE tactile sensor. It consists of a function generator (HP 8116A Pulse/Function Generator) which is the source of sinusoidal excitation, an amplifier (MB Dynamics Model SS250 Amplifier) for the function-generator output before sending a signal to the shaker, a dynamic shaker (MB Dynamics CAL 50 Exciter), a charge amplifier (ENDEVCO Charge Amplifier Model 2735), and an oscilloscope (Tektronix TDS 3052). A plastic probe with a diameter of 4 mm was attached to the calibrated piezoelectric force transducer having a sensitivity of 112.4 mV/N to measure the force input to the tactile sensor. The peak-to-peak voltage output from the force transducer was measured using the oscilloscope. The output charge from the PVDF-TrFE tactile sensor was converted into a voltage using the charge amplifier, and the peak-to-peak dc voltage output from each tactile sensor was recorded with the oscilloscope.

B. Poling Field Effect

The spin-coated and screen-printed PVDF-TrFE solutions over the micromachined mold directly crystallize into the polar β-phase. However, due to the random orientation of the unit cells in the semicrystalline film, the poling process is essentially required to induce piezoelectricity to the untreated films [22]. The magnitude of the poling field, the poling temperature, and the poling time will affect the piezoelectric activity of the PVDF-TrFE film. To find the optimal condition for dc poling in this paper, we fixed the poling temperature and the poling time as 90 ◦C and 30 min, respectively. Subsequently, the films were poled in a range of different electric fields, spanning from

Fig. 7. dc poling characteristics. (a) Experimental setup. (b) Dependence of the piezoelectric activity on the magnitude of poling field.

20 MV/m, as a minimum, up to 100 MV/m. The films were then tested as a tactile sensor, and the voltage outputs were measured to identify the best parameters for poling. Fig. 7 shows the dc poling setup and the effect of poling field on the piezoelectric activity. A saturation region for the remnant polarization was achieved when the applied electrical field was greater than 60 MV/m. By considering the thickness nonuniformity of

±4.5% for the 30 µm thick film, a 70 MV/m was chosen as the optimized electric field for poling with the conditions imposed.

C. Dynamic Response of the Dome and Bump Shape Tactile Sensors

In order to determine the dynamic sensitivity of each sensor, dynamic forces at a frequency of 5 Hz were applied directly to the sensor surface using the developed tactile sensing system. Catheters with assembled tactile sensors were placed on the x–y–z positioning stage and vertically aligned with the probe. The loading and the recording were repeated for each sensor several times for loads ranging between 20 mN and 1 N. When a sinusoidal force is applied, the voltage output from the sensor was also sinusoidal with a frequency equal to the excitation and with an amplitude proportional to the input excitation, as shown in Fig. 8(a).

The measured peak-to-peak voltages are plotted against the applied force for the bump shape tactile sensors, as shown

Fig. 8. Piezoelectric response characteristics of the PVDF-TrFE tactile sensors. (a) Waveform of sensors captured from the oscilloscope driven at a frequency of 5 Hz. (b) Measured peak-to-peak voltage versus added loads for the bump shape PVDF-TrFE sensors on the polymer tube’s outer surface.

(c) Measured peak-to-peak voltage versus added loads for the dome shape PVDF-TrFE sensors for microcatheter tip.

Fig. 9. Frequency response of the gains for the PVDF-TrFE flexible tactile sensors.

Fig. 10. Temperature response of the piezoelectric activity on the varied working temperatures.

in Fig. 8(b). The piezoelectric responses to the loads varied linearly, and a force increment of 40 mN can be successfully measured. The achieved sensitivities of the sensors are 0.81, 3.23, and 9.1 mV/N for the bump areas of 0.25, 1, and 2.25 mm2, respectively. The measured peak-to-peak voltages for the dome shape sensors are plotted against the applied force shown in Fig. 8(c). With the dome shape PVDF-TrFE sensors on the tip head of the microcatheter, the piezoelectric responses to loads varied linearly, and a force increment of 25 mN can be measured. The achieved sensitivities of the sensors are 1.1, 5.07, and 10.6 mV/N for the different diameters of 500 µm, 1 mm, and 1.5 mm, respectively.

D. Frequency Response

To determine the response speed of the tactile sensor, the frequency response of the sensor has been measured. By controlling the amplitude and the frequency of the oscillator, the output of the tactile sensor can be obtained from the oscilloscope. Fig. 9 shows the dependence of the sensor response on frequency. Gain is the attenuation ratio output signals with respect to the reference signal at 1 Hz, which is defined as

G(Gain) = 20 log(Vf /Vf1)

10

where Vf is the measured values at different frequencies, and Vf 1 is the reference value which is measured at 1 Hz. The gain of the PVDF-TrFE tactile sensor shows almost a uniform response from 1 to 100 Hz.

E. Temperature Effect

There are differences between the calibration temperature (i.e., a room temperature) and the working temperature (i.e., body temperature for microcatheter application). To check the stability of the PVDF-TrFE sensor under different working temperatures, the piezoelectric response of the tactile sensors were also recorded. Fig. 10 shows that the response of the sensor is not influenced by the temperature and remains constant at the

working temperature range from room temperature to 45 ◦C. The uniform signals at the variation of working temperatures indicate that the microfabricated tactile sensors are favorable to the application for microcatheters.

V. CONCLUSION

In this paper, the innovative dome and bump shape microstructures with PVDF-TrFE piezoelectric copolymer were successfully developed by a new mold-transfer technique, characterized, and assembled on the microcatheter as flexible tactile sensors. These sensors are easy to fabricate, miniaturize, and selectively pole by the standard MEMS techniques. The thicknesses of the dome and bump shapes have been controlled by the spin-coating and screen-printing PVDF-TrFE solutions over the micromachined mold. The uniformity of the PVDFTrFE film has been controlled less than ±4.5% at 30 µm thickness. The tactile sensors fabricated by the mold-transfer method can be realized in microstructures as well as arrays with various film thicknesses. As a result, the tactile sensors can be mass-produced at low cost in a disposable platform for numerous biomedical applications. The developed sensors have high sensitivity which can measure as small as 40 mN force for the bump shape sensors and 25 mN for the dome shape sensors. The dc-poled films present excellent piezoelectric properties, whereas the nonpoled films just present dielectric properties; thus, the selective dc poling method enables localized poling and minimizes the crosstalk between the sensors which are precisely located in array format. The bump and dome shape PVDF-TrFE films developed in this paper can have numerous applications for miniaturized biomedical devices which include microcatheters, minimally invasive surgical devices, and biomedical image systems.

ACKNOWLEDGMENT

The authors would like to thank J. Simkins and R. Jones of the Institute of Nanotechnology, University of Cincinnati

for their technical support in setting up the dc poling systems and taking SEM pictures. The authors would also like to thank A. Browne, M. Rust, and Dr. J. Do of the Microsystems and BioMEMS Laboratory, University of Cincinnati for their assistance and valuable discussion.

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Chunyan Li received the B.S. degree in electronics and computer engineering from Yanbian University of Science and Technology, Jilin, China, in 2001 and the M.S. degree in electrical engineering from the University of Cincinnati, Cincinnati, OH, in 2004. She is currently working toward the Ph.D. degree in the Microsystems and BioMEMS Laboratory, Department of Electrical and Computer Engineering, University of Cincinnati.

Her research interests include MEMS/BioMEMS, microsensors for in vivo catheter applications, pack-

aging of microdevices, and lab on a chip/µTAS.

Pei-Ming Wu was born in Kaohsiung, Taiwan, R.O.C., in 1977. He received the B.S. and M.S. degrees in electronics engineering from the National Cheng Kung University, Tainan, Taiwan, R.O.C., in 1999 and 2000, respectively. He is currently working toward the Ph.D. degree in the Department of Electrical and Computer Engineering, University of Cincinnati, Cincinnati, OH.

His research interests include integrated magnetic sensor systems.

Soohyun Lee received the B.S. degree in electrical engineering from Korea University, Seoul, Korea, in 2002 and the M.S. degree in electrical engineering from the University of Cincinnati, Cincinnati, OH, in 2005. He is currently working on the Ph.D. degree. His Ph.D. work is the development of disposable lab- on-a-chip for reverse transcription-polymerase chain reaction and portable analyzer.

Since 2006, he has been with the Korea Institute of Science and Technology, Seoul, Korea. His research interests include BioMEMS, lab-on-a-chip, polymer

waveguide, and piezoelectric resonator, especially in nanoscale.

Andrew Gorton received the B.A. degree (with honors) in acoustics and audio from Columbia College Chicago, Chicago, IL, in 2000. He is currently working toward the M.S. degree in mechanical engineering at the University of Cincinnati, Cincinnati, OH.

He is currently an Associate Consultant with the Papadimos Group, San Rafael, CA, working on signal processing and finite-element analysis for the design of research and medical facilities that require critically low levels of vibration and noise.

His research interests include vibrations, acoustics, sensors, and the synthesis, characterization, and application of carbon nanotubes.

Mark J. Schulz is an Associate Professor of Mechanical Engineering at the University of Cincinnati, Cincinnati, OH. He performs interdisciplinary research in the area of smart and nanoscale materials for structural and biomedical applications. He is working on continuous sensors, large sensor arrays, tiny sensors that can go inside the body and inside composite materials, and carbon nanotube thread for structural and electronic applications.