A Wireless Pressure Sensor Based on Surface Trapped Ferrofluid Plug

Albert Kim, Babak Ziaie School of Electrical and Computer Engineering

Purdue University West Lafayette, Indiana, USA

kim126@purdue.edu, bziaie@purdue.edu

Girish Chitnis School of Mechanical Engineering

Purdue University West Lafayette, Indiana, USA

gchitnis@purdue.edu

Abstract—We present a new inductive wireless pressure sensor based on the change in the shape of a ferrofluid plug/droplet trapped in a hydrophilic island and sandwiched between two superhydrophobic polymeric membranes one of which contains a flat coil. When external pressure deflects one of the membranes, it squeezes the droplet and changes the distribution of high-permeability ferrite microparticles around the coil. This results in a change in the inductance of the coil which also affects its self-resonant frequency that can be monitored wirelessly. When pressure is released, the ferrofluid plug/droplet regains its original shape due to the superhydrophobic nature of its surrounding surface. The sensor is fabricated using polyethylene terephthalate (PET) film and aerosol based superhydrophobic spray-coating. The sensor has a pill-shape design (thickness = 1.23 mm, diameter = 15 mm) with a sensitivity of 12 kHz/mmHg (0-15 mmHg).

I. INTRODUCTION

Physiological pressures at locations such as brain, heart, eye, bladder etc. can provide vital information about patient’s health that can be used to prevent, diagnose, and treat many diseases. Most of the techniques currently used in clinics (e.g., ocular tonometry, catheter-based hemodynamic and intracranial pressure monitoring, etc.) are percutaneous and typically used in acute settings. Alternatively, implantable pressure sensors can provide long-term chronic measurements. MacKay’s gastrointestinal pressure sensor [1] was the very first example of an implantable/ingestible wireless transponder comprised of a battery powered single transistor oscillator. Later, Collins [2] presented an intraocular sensor, which operated based on change in resonant frequency of a passive LC transponder. Due to its simplicity, passive LC schemes became popular with researchers and over subsequent decades many such sensors were developed [3-6]. Advances in MEMS fabrication techniques significantly aided the development of batch-fabricated miniature LC transponders. Most of these attempts have focused on pressure sensitive capacitors connected in parallel with a fixed inductor. Such capacitive sensors demand a complicated fabrication technique, which

increases the overall cost and complexity. A simpler approach presented here uses only a single inductor whose self-resonant frequency is modulated by pressure; eliminating the need for any additional capacitor (stray capacitance of the coil is sufficient to create the necessary resonant circuit). An earlier attempt reported by our group operated based on the change in the position of a solid ferrite core with respect to a planar inductor [5]. Here we present an alternative fluidic design using a single droplet of ferrofluid where the change in inductance is achieved by a change in shape of the droplet.

II. DESIGN AND MODELING

Figure 1 depicts a schematic diagram of the wireless pressure sensor based on surface trapped ferrofluid droplet. It consists of 1) a polyimide flat coil, 2) two bonded superhydrophobic PET substrates with a hydrophilic island in the center, and 3) a ferrofluid plug/droplet sandwiched between the PET films at the center of the coil. Basic working mechanism is based on the higher magnetic permeability of the dispersed ferrite microparticles in ferrofluid. The ferrofluid

Pressure

Figure 1. (a) Schematic diagram of pressure sensor design. (b) pressure squeeze down ferrofluid

(a)

(b)

Ferrofluid Superhydrophobic coating

Planer coilSpacer

Hydrophilic surface

978-1-4673-4642-9/13/$31.00 ©2013 IEEE

droplet is secured in place by the superhydrophobic-hydrophilic boundary. The pressure transduction of the sensor relies on two parameters; magnetic permeability of the ferrofluid and deflection of the polymeric membrane. When positive pressure is applied, the top PET membrane deflects and squeezes the entrapped ferrofluid droplet. This change in shape causes the droplet to cover larger area over the planer coil allowing more magnetic flux to pass through ferrofluid. As the membrane deflections increases, more ferrofluid gets closer to the coil and increases effective magnetic permeability. Change in thickness of the drop would also affect the magnetic field, but we expect that effect to be significantly smaller. Hence, as the pressure rises, total inductance increases and reduces the self-resonant frequency of coil. When applied pressure is released, the ferrofluid droplet regains its original shape due to the superhydrophobic nature of its surrounding surface. Assuming ferrite microparticle density in ferrofluid stays constant and stray capacitance of a planer coil is fixed, the self-resonant frequency of LC tank reduces as a function of applied pressure.

For a clamped circular membrane pressure, P, needed to achieve deflection, d, can be given as [7],

. (1)

where, = radius of the membrane = elasticity of the material = thickness of the membrane = Poisson’s ratio = applied pressure

For deflections significantly smaller than the membrane

thickness this expression predicts a linear behavior. However for the physiological pressures (0-100 mmHg) we expect larger nonlinear deflections. For a small change in droplet height, we assume a linear relationship between change in droplet height and resonant frequency. Hence, overall we expect nonlinear (cubic) correlation between applied pressure and resonant frequency. Note that geometry of pressure sensor, membrane thickness, ferrofluid volume, and ferrite microparticle concentration in ferrofluid could be altered for specific sensitivity and pressure range.

III. FABRICATION

In current design, we used polyethylene terephthalate (PET) sheet (thickness117±12 µm) as membrane and a polyimide based spiral planar coil (OD: 10 mm, 20 turns, L = 10 µH) as the inductor. The ferrofluid was obtained from Bangs Laboratory (ProMagTM) having ferrite embedded polymeric microparticles (mean diameter of 0.97 µm) with a concentration of 1.33 g/cm3. PET film was made superhydrophobic using HydrobeadTM spray coating system. Figure 2 shows the fabrication process.

The fabrication started with two sheets of polyethylene terephthalate (PET) films (117±12 µm thickness). Films were laser-cut to a specific size, in this case, 5 mm radius circular shape, Fig 2.a, using a CO2 laser system. Both PET films were then coated with superhydrophobic spray, Fig 2b. Two light coats were used to create a reliable superhydrophobic surface (35.56 µm coating thickness). This coating was then mechanically removed from the center to create a hydrophilic island (1.5 mm in diameter confining 5 µL of ferrofluid, Fig 2c). Hydrophobic coating near the perimeter was also removed to ensure a good bonding between the films. A polyimide flexible coil (L = 10 μH) was then epoxy-bonded to the bottom of one of PET films. The polyimide flexible coil was designed in house and out sourced for fabrication (Parlex Inc.). A PET spacer (780±10 µm thick) was also attached to the PET film, Fig. 2d. A ferrofluid droplet (5 µL) was deposited in the central hydrophilic island, Fig 2e. Finally, top membrane fabricated in step (c) was bonded to close the chamber. The final device was 15 mm in diameter and 1.23 mm in thickness.

Figure 3 shows optical photographs of prototype wireless

pressure sensor based on surface trapped ferrofluid plug. The final device, a fully assembled pressure sensor, is shown in the left. The middle picture shows the pressure sensor with the ferrofluid droplet deposited at the center before closing the chamber. Top and bottom membranes are shown at the right picture. Top right inset shows the hydrophilic central island with the ferrofluid droplet.

Figure 2. (a) PET film, (b) coat with Hydrobead® to create superhydrophobic surface (c)mechanical removal to create hydrophilic surface in the center and outer ring for spacer, (d) assemble a spacer and attached a planer coil at bottom, (e) drop 5μL of ferrofluid on top layer center (f) attach top membrane

(a) (d)

(b)

(c)

(e)

(f)

PET film

Planer coil

Superhydrophobic coat Ferrofluid Epoxy glue

Copper

IV. MEASUREMENTS

The sensor was characterized in vitro using a water column pressure modulator, Fig. 4. The pressure sensor was submerged in a sealed water tank connected to a water column while a syringe pump was used to modulate the input positive pressure. At various applied pressures, self-resonant frequency of sensor was measured wirelessly using a readout coil connected to a network analyzer. The sensor was placed close proximity to a readout coil (~3 mm). When water was pumped into the sealed water tank, self-resonant frequency of the sensor was shifted. Self-resonant frequency was measured by the phase-dip method. Applied pressure was varied from 0 to 60 mmHg in increments of 5 mmHg.

V. RESULTS AND DISCUSSION

Figure 5 shows the phase-dip measurement of sensor under different pressures. As the pressure is increased, the dip shifts towards lower frequencies. Location of the minimum phase, which is also the resonant frequency of the sensor, is extracted from the phase response. Figure 6 shows the self-resonant frequency plotted against applied pressure. The

sensor shows a nonlinear response with a sensitivity of ~12 kHz/mmHg at lower pressures (0 to 15 mmHg). Also, note that the sensor shows an almost linear response at lower pressures where membrane deflection is smaller.

VI. CONCLUSIONS

In this paper, we presented a new pressure-sensing scheme that utilizes higher magnetic permeability of a ferrofluid plug trapped on a hydrophilic island at the center of a superhydrophobic film. The displacement of the ferrofluid upon applied pressure modulates the self-resonant frequency of a planar coil thus allowing wireless measurement of the pressure. A prototype device was designed, fabricated and tested in vitro. Measurements showed a nonlinear response

Figure 6. Pressure vs. self-resonant frequency. Sensitivity is 12 kHz/mmHg in the initial linear region (0 to15 mmHg)Hydrophilic surface

Figure 5. Phase-dip measurements of the ferrofluid based super-hydrophobic-surface pressure sensor

Syringe

Water column

ImpedanceAnalyzer

Sensor

Readout coilFigure 4. Schematic diagram of the experimental setup.

Figure 3. Optical photographs of the ferrofluid based super-hydrophobic-surface pressure sensor. From left, final device, bottom layer with a coil, and top membrane.

Ferrofluid

Super-hydrophobic surface

Spacer

Top membrane

Pressure sensor

Hydrophilic surface

Bottom layer with a coil

between applied pressure and inductance change. The initial linear region showed a sensitivity of 12 kHz/mmHg.

REFERENCES [1] R.S. Mackay and B. Jacobson, “Endoradiosonde,” Nature, vol. 175, pp.

1235-1240, lune, 1957.

[2] C. C. Collins, “Miniature passive pressure transensor for implanting in the eye,” IEEE transactions on bio-medical engineering, vol. 14, no. 2, pp. 74-83, Apr. 1967.

[3] A. V. Chavan and K. D. Wise, “Batch-Processed Vacuum-Sealed Capacitive Pressure Sensors,” IEEE Journal of MicroElectroMechanical Systems, pp. 580-588, December 2001.

[4] O. Akar, T. Akin, And K. Najafi, "A Wireless Batch Sealed Absolute Capacitive Pressure Sensor," Sensors and Actuators A: Physical, 95 (1), pp. 29-38, 12/2001

[5] A. Baldi, W. Choi, and B. Ziaie, “A Self-Resonant-Frequency-Modulated Micromachined Passive Pressure Transensor,” IEEE Sensors, Vol. 3, pp. 728-733, Dec 2003.

[6] G. Chitnis, B. Ziaie, “A Ferrofluid-based Pressure Sensor for Biomedical Applications” Solid-state, actuator and microsystems workshop, Hilton Head Island, pp133-136, 2012.

[7] S. Timoshenko and S. Woinowsky-Krieger, Theory of thin plates and shells, 2nd ed. New York: McGraw-Hill Book Company, 1959.