ALIGNED ELECTROSPUN PVDF-TrFE NANOFIBERS FOR FLEXIBLE PRESSURE SENSORS ON CATHETER

Tushar Sharma1, Jewel Langevine1, Sahil Naik1, Kevin Aroom2, Brijesh Gill2, John X.J. Zhang1

1The University of Texas at Austin, Austin, Texas, USA 2The University of Texas Health Science Center at Houston, Houston, Texas, USA

ABSTRACT

In the present paper, we report the development of electrospun PVDF-TrFE (polyvinyledenedifluoride-tetrafluoroethylene) nanofibers based sensor. In order to boost the sensitivity of the sensors, highly aligned nanofibers were directly deposited on a custom patterned flexible substrate.

The complete process from optimization of electrospinning parameters to fabrication of sensors and testing of the sensors has been described. Preliminary results showed that devices were capable of generating 300mV signal with 0.45s recovery time on manual mechanical deformation.

KEYWORDS

Aligned nanofiber, pressure sensor, catheter, flexible sensor INTRODUCTION

Current state-of-art in pressure measurement in a clinical setting employs a fluid-filled catheter coupled to remote transducers for pressure measurement. The fluid transmits the endovascular pressure to a piezoresistive sensor outside. Consequently, such a methodology for pressure measurement is highly inaccurate and not indicative of instantaneous blood pressures. Further, the catheter lumen geometry (diameter, length) and blood vessel size introduce undesirable resonance and artifacts to the output signal.

MEMS based capacitive and piezoresistive sensors offer great mechanical stability [1-4], but have several drawbacks. For instance, bulkiness, significant power requirement, large temperature dependence offset, nonlinearity, long-term stability in dynamic field conditions and the stiffness imparted to catheter prevent the widespread clinical acceptance of these pressure sensing technologies.

Figure 1: Schematic showing (a) placement of pressure sensors on either side of balloon on a catheter, and (b) highly aligned nanofibers based pressure sensor A reliable, highly sensitive, flexible pressure sensor, that is easy to fabricate, is highly desirable. Making the pressure sensor flexible along with compact would allow multiple of such sensors to be integrated simultaneously along the length of a catheter (Figure 1).

Previously, we have demonstrated the capability to fabricate nanoconfined thin film flexible pressure sensors from PVDF-TrFE membranes [5, 6]. In the present study, we tap the higher surface area to volume ratio and the higher electromechanical coupling coefficients [7] of PVDF nanofibers to fabricate more sensitive pressure sensors. We employ the technique of electrospinning to fabricate PVDF nanofibers on desired flexible substrates. Through the designs of mesh and rotating drum collector electrodes, we can optimize the control parameters critical to electrospinning highly aligned PVDF nanofibers on flexible substrate (Figure 1b). Further, we developed highly aligned electrospun PVDF-TrFE nanofiber based sensors, for multiple points of pressure measurement on an interventional catheter without consuming internal lumen space (Figure 1). PVDF-TrFE The key to achieving high piezoelectricity using PVDF polymer is forming β-crystalline phase, which significantly improves sensor and actuator performance [8]. However, untreated PVDF itself cannot have β-phase without delicate mechanical stretching and electrical poling processes. Electrospinning results in simultaneous electrical poling and mechanical stretching of electrospun fibers. Figure 2 shows atomic structure of PVDF, explaining α-phase, β-phase formation, and how dipoles are aligned for effective piezo-effect.

Figure 2: Atomic Structure of PVDF. TOP is the ‘trans-gauche’ conformation of α-phase PVDF; BOTTOM is the ‘all-trans’ conformation of β-phase PVDF [5, 6] FABRICATION Following protocol similar to reported before [5, 6], we started with patterning of bottom electrode using lift off process (Figure 3a). Copper (250nm) was deposited on kapton film and photoresist was removed. The kapton film was then stretched over the ground collector plate to pattern nanofibers directly between the two metal electrodes. Figure 3b,c shows the schematic and photograph of the devices with nanofiber patterned between the two electrodes.

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Figure 3: (a) schematic for fabrication of electrospun nanofiber based devices, (b) schematic of first generation devices; and (c) photograph of second generation device electrode pattern.

Figure 4: (a) Schematic and (b) photograph showing the Electrospinning setup for fabricating highly aligned fibers rotating drum as the ground collector electrodes Following the electrospinning of nanofibers on the patterned substrate, another layer of copper was deposited on top (250nm) using a hard shadow mask (Figure 3a). This additional layer of copper helped anchor the nanofibers to the electrodes. The sample was then annealed at 90ºC for 1h. Electrical wires were connected to copper tape using silver print (GC Electronics) and connection was sealed with epoxy. The devices were then coated with UV curable epoxy (NOA89, Norland optics) for passivation. Electrospinning is a simple technique of fabricating nanofibers from a polymer solution under the influence of high electric fields. Schematic for fabricating highly

aligned electrospun fibers is illustrated in Figure 4. Polymer solution was filled in syringes connected to 27 gauge blunt-tip needle. The needle was charged at 7kV using high voltage power supply. Resulting droplet at the tip of needle is pulled towards a ground collector placed 7cms away due to the voltage gradient. We optimized electrospining conditions for different PVDF-TrFE concentrations in MEK (methyl ethyl ketone) as the solvent. EXPERIMENTAL

Figure 5: Photographs showing ground collectors used in the present study: (a) Mesh-type, and (b) rotating drum assembly In order to fabricate highly aligned nanofibers, we tried Mesh-type (1x1cm2, stainless steel gauze on polypropylene support) current collector and rotating drum (1” diameter, 2mm diameter grounded copper wires parallel to drum axis, 1.5cms apart). Both the stainless steel gauze and copper wires were connected to electrical ground. The rotating drum was operated using external power supply (Figure 4) at a constant speed of 3500rpm.

Figure 6: Schematic showing the testing setup and electrical connections to measure the device response.

For the testing of devices, external electrical wires were directly connected to a charge amplifier (100nF feedback capacitance, 0.1-10Hz filter, 40db gain), which was fed to the data acquisition system as mentioned earlier [5, 6]. Fluid pressure chambers were used for the testing of the nanofiber based devices as described before. Additionally, strain based measurement of the devices was carried out as shown in Figure 6. RESULTS AND DISCUSSIONS Preliminary optimization of the electrospinning solution was carried out to determine the optimal PVDF concentration in MEK. Figure 7 shows the SEM images obtained with various solution concentrations on a plate

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ground collector along with the average fiber diameters observed.

Figure 7: Scanning Electron micrograph images showing the variation in average fiber diameters as a function of concentration (w/v): (a) 8%: 500 nm, (b) 12%: 450 nm, (c) 14%: 1.07 µm, (d) 16%: 1.6 µm, (e) 18%: 1.9 µm (f) 20%: 2.0 µm. These fibers were deposited on a stationary copper plate as the ground collector. From the SEM images (Figure 7) we found that increasing PVDF concentration above 14% (w/v) avoids bead formation in the electrospun nanofibers and found that 16% (w/v) of PVDF-TrFE in DMF gives best fibers.

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Figure 8: Plots showing the variation of electrospun fiber diameter as a function of (a) flow rate for 12% PVDF/MEK solution; and (b) the needle diameter (gauge) for 0.3ml/h flow rate on the syringe pump Further experimentation determined negligible difference in fiber diameters due to the flow rate and needle gauge (diameter) variations (Figure 8). Therefore, the PVDF solution concentration was found to have the most significant impact on the fiber diameters. Figure 9 shows the aligned nanofibers patterned using mesh type electrodes. From the SEM images, we observe that even though there was a general orientation of the fibers, there a lot of fibers that were not aligned. The fibers were electrospun from at 14% (w/v) PVDF solution in MEK, with flow rate of 0.5ml/h and 1kV/cm (8cms spacing) voltage gradient. The total duration of the electrospinning was 20mins. The average fiber diameters were found to be 1.45 ± 0.18µm.

Figure 9: Sanning electron microscope (SEM) images obtained using the collector for (a),(b) Mesh electrode; and (c)Rotating Drum electrodes. Fibers obtained using rotating drum setup were 737±75nm thick, implying that stretching of fiber takes place when rotating drum setup is used (Figure 9). SEM images of samples show that rotating drum setup works best for obtaining highly aligned nanofibers. Further, the density of nanofibers on the kapton film was found to be the highest directly above the copper wires (of rotating drum) and the area between the ground copper wires contained highly aligned nanofibers (Figure 9c). When used with mesh and rotating drum assemblies, 14% w/v PVDF solution did not result in bead formation.

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For the fabrication of the devices using aligned nanofibers, the patterned kapton film was so arranged that inter-device gap was present directly above the ground

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copper wires of the rotating drum, thus making sure that aligned nanofibers were stretched between the two electrodes of a device. For the fabrication of randomly oriented nanofibers, copper plate was used as the ground collector.

Dynamic bending measurements were performed on fabricated devices using the strain based measurement setup (above). Responses upon intermittent external stress applications were immediate. Figure 10 shows experimental results of output voltage from 10x4mm2 devices with amplitude of 300±5mV and 94% repeatability. Both, aligned nanofiber and randomly oriented nanofiber based devices show similar response. Similar performance was observed from devices when testing inside water chamber (Figure 11). The devices show good peak-to-peak correlation with the ambient pressures inside the chamber, determined by a commercial pressure sensor. Aligned nanofiber based devices were found to be extremely sensitive compared to thin-film based sensors or the commercially used pressure sensor inside fluid testing chamber [5, 6].

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Figure 12: Plot comparing raw data output from aligned nanofiber based PVDF devices sensitive enough to detect changes in water flow rate The high sensitivity of the aligned nanofiber based devices is noticeable from Figure 12. Aligned nanofiber based devices were capable of detecting minor changes in the water flow rates inside the fluid chamber (indicated by the arrows), which went unnoticed by the commercial pressure sensor. CONCLUSION The fiber mat density of the aligned nanofibers was lower than randomly oriented nanofibers, however. Recovery time of nanofiber based devices was found to be 0.45±0.004s, which is comparable with thin-film devices (0.34s) [7], even after substantially reduced piezoelectric material volume. The demonstrated ease of fabrication, device response, biocompatibility, and compact form factor show the great potential for development of a highly sensitive pressure sensor for biomedical implantation application.

ACKNOWLEDGEMENTS This work was supported in part by an internal grant from University of Texas Health Science Center at Houston. We also appreciate Microelectronics Research Center (MRC) and National Nanotechnology Infrastructure Network (NNIN) staff for their continuing support. REFERENCES [1] S. Romagnoli, et al., "Dynamic response of liquid-

filled catheter systems for measurement of blood pressure: precision of measurements and reliability of the Pressure Recording Analytical Method with different disposable systems," Journal of critical care, vol. 26, pp. 415-422, 2011.

[2] S. Wassertheurer, et al., "Invasive assessment of aortic pressure waves: comparison between pressure wire and fluid filled catheter," Artery Research, vol. 3, pp. 161-162, 2009.

[3] W. Hasenkamp, et al., "Polyimide/SU-8 catheter-tip MEMS gauge pressure sensor," Biomedical Microdevices, pp. 1-10, 2012.

[4] L. Murtha, et al., "Epidural intracranial pressure measurement in rats using a fiber-optic pressure transducer," Journal of visualized experiments: JoVE, 2012.

[5] T. Sharma, et al., "Patterning piezoelectric thin film PVDF–TrFE based pressure sensor for catheter application," Sensors and Actuators A: Physical, vol. 177, pp. 87-92, 2012.

[6] T. Sharma, et al., "Flexible Thin-Film PVDF-TrFE Based Pressure Sensor for Smart Catheter Applications," Annals of Biomedical Engineering, pp. 1-8, 2012.

[7] C. E. Chang, et al., "Direct-Write Piezoelectric Polymeric Nanogenerator with High Energy Conversion Efficiency," Nano Letters, vol. 10, pp. 726-731, 2010.

[8] H. S. Nalwa, "Ferroelectric Polymers: Chemistry," Physics, and Applications (Marcel Dekker, New York, 1995), 1995.

CONTACT Tushar Sharma, Department of Biomedical Engineering, University of Texas, Austin TX 78758, U.S.A.; Tel: +1-512-487-0639; Email: tusharsharma@utexas.edu

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