PLATINUM-IRIDIUM COATINGS FOR INCREASING SENSITIVITY OF
IMPEDANCE-BASED POLYMER MICROFLUIDIC SENSORS
A.B. Baldwin1, C.D. Lee2, A. Petrossians2, J. Weiland3, and E. Meng1
1
University of Southern California, USA
2
Platinum Group Coatings LLC, USA
3
University of Michigan, USA
ABSTRACT
We present the first use of electrodeposited platinum-iridium (Pt-Ir) coatings for improving sensitivity and measurement
range of impedance-based MEMS fluidic sensors. Pt-Ir deposited onto Pt microelectrodes increased electrochemical surface
area (ESA) and decreased electrochemical impedance, enabling transduction at significantly lower frequencies (from 1 to 100kHz) and increasing sensitivity. MEMS patency and flow sensors were characterized before and after Pt-Ir electrodeposition.
Patency sensing utilizing larger electrodes exhibited moderate sensitivity gains (~3-11%), while flow sensing utilizing
smaller electrodes improved significantly (61% sensitivity increase and 2x improvement in resolution (39 to 19 µm/s)). Pt-Ir
coating reduced drift by an order of magnitude in flow sensors over a 12-hour period.

KEYWORDS:Platinum-Iridium, Parylene C, Electrochemical Impedance, Microfluidic Sensors, Hydrocephalus

INTRODUCTION
Impedimetric sensors monitor the electrochemical impedance between a pairof electrodes exposed to an aqueous solution, and are a promising technology forchronic in vivo medical devices because the sensors interact directly with physiologicalsolutions, without needing encapsulation or protection from corrosion. Microfabricatedimpedimetric sensors made out of only biocompatible metals and polymers have beenreported for measuring flow rate[1], fluidic pressure[2], or catheter patency[3] in thebody, and we have recently developed a multi-sensor module designed for chronic
implantation in hydrocephalus shunts. The frequency range available for impedimetrictransduction is limited on the high end by parasitic capacitance (~100kHz) and on the
low end by electrode’s double-layer capacitance. For microfabricated electrodes, thesefrequency limits often overlap, reducing sensitivity or preventing signal transduction
entirely. Increasing microelectrode ESA, such as by coating a Coulter counter with Pt
black[4], has been reported to improve sensor response, and recently electroplated PtIr coatings have emerged as an effective way to increase ESA in polymer neuralprobes[5, 6]. Here, we explored the ability of 60-40% Pt-Ir electrodeposited coatings
to decrease baseline impedance, increase sensitivity and resolution, and extend
measurement frequency range of an impedimetric multi-sensor module forhydrocephalus shunts.

Figure 1. (A) The impedimetricmulti-sensor module after Pt-Ircoating. Each module contains aflow sensor, which consists of a
microfabricated heater and pair ofimpedance electrodes, and twoelectrodes which serve as apatency sensor. (B) Micrograph ofa Pt flow electrode before coatingand (C) after Pt-Ir coating.

METHODS
Platinum-Iridium Coating
Electrodeposition of Pt-Ir was performed by Platinum Group Coatings, LLC using
previously reported methods involving ultrasonication and voltage cycling in solution
containing Na3IrCl6 and Na2PtCl6[5]. Electroplated Pt-Ir exceeds the performance of Pt black;
Pt-Ir coatings are highly fractal and exhibit pseudo-capacitive reactions, dramaticallyincreasing ESA and significantly improving neural electrodes. Pt-Ir’s biocompatibility androom-temperature coating process makes it a promising candidate for polymer sensors,
implantable devices, and microfluidics.
Figure 2: Representative
SEM image of an electrodecoated with electroplated
Pt-Ir.

Sensor Design
We applied Pt-Ir coatings to multi-sensor modules containing microfluidic patency[3]and flow[1] sensors. These flexible sensors are fabricated from 2000Å Pt sandwiched between
two 10µm Parylene C layers and are intended to transduce blockage and flow throughhydrocephalus shunts. Sensors utilize electrochemical impedance between electrode pairs
exposed to cerebrospinal fluid. The flows sensor operates by delivering a heat pulse to physiological fluid using a
microfabricated heater and measuring the pulse’s time of flight as it travels past one or more pairs of impedance electrodes;the patency sensor directly measures the impedance across electrodes inside and outside a catheter, and partial catheter
occlusion will cause the impedance magnitude to increase. A pressure sensor is also included on the multi-sensor module, butpressure sensor tests after Pt-Ir coating are still ongoing.
Benchtop Testing
Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on electrodes before
and after coating. For benchtop testing, devices were packaged into modules compatible with standard hospital catheters (ID3.25 mm). To test the patency sensor, catheters with varying number of holes (simulating progressive blockage) wereattached and filled with saline. EIS was performed through each catheter between the coated electrode and an uncoated Pt
wire placed external to the catheter. For flow testing, a syringe pump was used to control saline flow, and impedance wasrecorded using an Agilent LCR meter.

RESULTS
Robust Pt-Ir coating was achieved on multiple devices containing flow and patency sensors. CV and EIS revealed adramatic decrease in electrode impedance magnitude and absolute value of phase (81x magnitude decrease and 65° phaseincrease at 100Hz), and a 17.9x increase in charge storage capacity in saline. Patency sensor testing revealed only a small
decrease in baseline impedance, due to patency impedance consisting mainly of resistance through the catheter which is notdependent on electrode surface area, and an increase in sensitivity (3.59% for electrode 1 and 11.1% for electrode 2) aftercoating. Flow sensor testing revealed a dramatic 61.2% increase in sensitivity after coating and an improvement in 2σresolution from 38 to 19 µm/s. Furthermore, the frequency range available for flow transduction was extended down to 1kHz, while 100 kHz was necessary for uncoated sensors. Pt-Ir also increased sensor stability and reduced flow sensor drift
over a 12 hr period from 2.1%/hr pre-coating to less than 0.16%/hr post-coating.
(B)

(A)

(C)

(D)

Figure 3. (A) Cyclic voltammetry before and after Pt-Ir coating reveals a 17.9x increase in charge storage capacity inphosphate-buffered saline (0.21 to 3.7 µC). (B) Flow sensors coated in Pt-Ir increased sensitivity 61.2%, from -1.4x10-4 to-2.3x10-4 %/µm/s, 2σ resolution improved from 38 to 19 µm/s, and flow transduction was possible at lower frequencies(down to 1kHz) for the first time. (C) Patency sensor improvement after Pt-Ir coating was minimal, due to the already large
size of the patency electrodes. (D) Sensor drift also improved, dropping from nearly 2% per hour to negligible.

CONCLUSION
Pt-Ir coatings significantly improve MEMS fluidic sensors and will enable further miniaturization of polymer MEMS
sensors and implantable devices. Future work will evaluate additional impedance-based sensors as well as Pt-Ir longevity andin vivo use.

REFERENCES
1.
2.
3.
4.
5.
6.

Baldwin, A., L. Yu, and E. Meng, An Electrochemical Impedance-Based Thermal Flow Sensor for PhysiologicalFluids. Journal of Microelectromechanical Systems, 2016. 25(6): p. 1015-1024.
Yu, L., C.A. Gutierrez, and E. Meng, An Electrochemical Microbubble-Based MEMS Pressure Sensor.
Microelectromechanical Systems, Journal of, 2016. 25(1): p. 144-152.
Kim, B.J., et al., Parylene MEMS patency sensor for assessment of hydrocephalus shunt obstruction. Biomedical
microdevices, 2016. 18(5): p. 87.
Zheng, S., M. Liu, and Y.-C. Tai, Micro coulter counters with platinum black electroplated electrodes for humanblood cell sensing. Biomedical microdevices, 2008. 10(2): p. 221-231.
Petrossians, A., et al., Electrodeposition and characterization of thin-film platinum-iridium alloys for biologicalinterfaces. Journal of The Electrochemical Society, 2011. 158(5): p. D269-D276.
Petrossians, A., et al., Improved Biphasic Pulsing Power Efficiency with Pt-Ir Coated Microelectrodes. MRS
Proceedings, 2014. 1621: p. 249-257.