HIGH EFFICIENCY WIRELESS ELECTROCHEMICAL ACTUATORS:DESIGN, FABRICATION AND CHARACTERIZATION BY
ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY
R. Sheybani and E. Meng
University of Southern California, Los Angeles, CA, USAABSTRACT

A wirelessly-actuated high efficiency Nafion®-coated
MEMS electrochemical actuator is presented. For the firsttime, high flow rates were achieved (up to 141μL/min,13mA) under wireless operation and high efficiency
pumping (up to 97%) was obtained with Nafion®-coatedelectrodes; coating further prevented previously reported
current-induced damage to electrodes [1]. Electrochemical
impedance spectroscopy (EIS) was used for electrodecharacterization, electrochemical cleaning, as well as
investigation of delamination, electrode polymer coating
and surface activation. Parylene and PEEK were
investigated as flexible substrate substitutes for soda limeglass. As a result of the combined effect of the rough PEEK
surface (increased electrode surface area) and the Nafion®
coating, higher efficiency was achieved compared to glass.

Constant current applied to an electrode pair results inwater electrolysis and subsequently, a pressure increase thatcan be harnessed for actuation. For example, the generatedpressure can be used to deflect a flexible membrane whichacts on the fluid to be pumped or directly pump theelectrolyzed fluid through an opening in a rigid chamber(Fig. 1).
(a)(b)

Figure 1: Electrochemical actuator: (a) operation conceptand (b) fabricated interdigitated electrodes.

INTRODUCTION
Electrochemically driven mechanical pumps feature
low power consumption, low heat generation, accurate flowcontrol, and large driving force. These factors, combinedwith the potential for biocompatible construction andwireless actuation, enable many interesting biological andmedical applications [1].
Previously, we reported a microelectrochemical
actuator capable of flow rates up to 6.5µL/min (constantcurrent operation, 1mA) [1]. The electrolysis actuatorconsisted of interdigitated Pt electrodes on a glass substrate.
Efficiency up to 90% was achieved by increasing electrodesurface area by electroplating; however, due todelamination, repeatable and reliable operation was notachieved. Delamination of Pt electrodes on flexible Parylene
films was also reported in [3]. Here, we present Nafion®coated and wirelessly-operated actuators that aremechanically robust and exhibit high efficiency actuation
for high flow rate in vivo drug delivery applications.
Electrochemical impedance spectroscopy has been used
to characterize Pt electrodes and to investigate corrosion,
surface activation, and the metal-polymer interface for fuelcell applications [2]. EIS is applied here to characterize Pt
electrodes used as electrolysis actuators. The impedance
spectra can be used to calculate equivalent circuit
parameters (double layer capacitance, polarization
resistance, and electrolyte resistance) that provide valuable
information about the electrodes. For instance, the double
layer capacitance is directly proportional to the effective
surface area. EIS could also be used to estimate thethickness of the Nafion® coating which acts as a dielectric in
the double layer capacitance.

978-1-4244-9633-4/11/$26.00 ©2011 IEEEDESIGNIn our previous electrolysis actuator [1], generated gas
bubbles could occlude the active electrode surface andprevent contact with the electrolyte (water) which reducespumping efficiency. Thus, Nafion® coating of electrodes isintroduced here to prevent bubble occlusion of the electrodeand improve efficiency without interfering with electrolysis
operation. The polymer electrolyte coating possesses highgas solubility allowing rapid diffusion of bubbles away fromthe catalyst surface and leads to a 20% increase in current
density [4].
It is important to note that the effective surface area ofNafion® coated Pt electrodes is reduced compared to that of
the native, uncoated Pt electrode (86% of original area). The
smaller value suggests that part of the Pt surface is blocked
by the electrochemically inactive hydrophobic fluorocarbon
backbone of Nafion® [5]. Optimal performance wasreported for coatings composed of less than 9 wt. % Nafion®
[6].

FABRICATIONInterdigitated Pt electrodes were fabricated on soda
lime substrates by liftoff [1]. A dual-layer photoresist
process was used to create an undercut sidewall profile tofacilitate metal liftoff. First, AZ1518 photoresist (AZElectronic Materials, Branchburg, NJ) was spun at 4 krpm
followed by global exposure. Then, AZ4400 photoresist(AZ Electronic Materials, Branchburg, NJ) was applied at 4
krpm and patterned. Following a short descum in oxygen
plasma, a Ti/Pt film (300 Å/2000 Å) was e-beam evaporated
and then liftoff was performed.

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MEMS 2011, Cancun, MEXICO, January 23-27, 2011

Flexible electrodes were fabricated using the same
liftoff method. For electrodes on thin film Parylene C(Specialty Coating Systems, Indianapolis, IN) membranes(5 μm thick), the Parylene was first deposited on clean
silicon substrates. Then electrodes were patterned.
Following removal from the supporting Si substrate bycareful peeling, electrodes were mounted on a flexible
polyetheretherketone (PEEK) substrate using adhesive, asreported in [3]. For electrodes on PEEK, 0.020" thick
PEEK™ Film (CS Hyde Co., IL, USA) was first cleaned
and then electrodes were patterned.
Prior to Nafion® coating, the electrodes werepotentiostatically cleaned at ±0.5V (Gamry Reference 600
Potentiostat, Warminster, PA) in 1X phosphate bufferedsaline. Nafion® (Dupont DE521 Solution, Ion Power, INC,New Castle, DE) was applied to the electrodes by dip
coating 2 times which produces a 1µm thick coating.

EXPERIMENTS AND RESULTS
Polymer Coating on Glass Electrodes
EIS (Gamry Reference 600 Potentiostat) was performed
in 1X phosphate buffered saline on uncoated and Nafion®coated soda lime electrodes (Fig. 2). The Nafion® dielectric
film contributes to the double layer capacitance asrepresented in the equivalent circuit of the electrodeelectrolyte interface (Cdl = double layer capacitance, Rp =polarization resistance, Rs = electrolyte resistance). The EIS
data demonstrates that the double layer capacitancedecreases with increasing coating thickness, suggesting adecrease in active surface area and blocking of some
activation sites on the electrode by the hydrophobic
backbone of Nafion® [5]. Therefore, the usable coating
thickness is limited. However, dip coating once, twice and 3
times did not increase the thickness enough for this effect tobe observed (data not shown).
Actuator Characterization
The flow rate was determined by clamping the actuatorin a test fixture filled with water (Fig. 3). The accumulated
pumped water was weighed over 5 minutes of constantcurrent application.
High flow rates were achieved up to 141±0.4μL/min
and 80.25 ± 1.3μL/min at 13mA for coated and uncoatedelectrodes, respectively (Fig. 4). The results clearly show
the increase in flow rate at all current values tested for theNafion® coated electrodes.
Also, the solid polymer
electrolyte acts as a barrier between the gas bubbles and theactive electrode surface which increases the sites for gas
bubble formation and results in greater linearity in the flowrate vs. current relationship.
Flow rate results were compared for electrodes dipped
once, twice and three times in Nafion® (data not shown).
While the coating thicknesses examined did not exhibit thesignificantly different flow rates, a single coating resulted inloss of coating integrity after repeated operation at highcurrent. Sufficient robustness was achieved for electrodesthat were dip coated twice.

Figure 2: EIS data (impedance and phase) before and afterNafion® coating: drop in double layer capacitance (slope ofplot) suggests decrease in surface area (coating thickness =10460 Å). Inset: schematic diagram of equivalent
electrochemical circuit.

Figure 3: Flow rate measurement test setup.

Figure 4: Current controlled flow delivery results with andwithout Nafion®-coating for electrodes on glass substrate.
The efficiency (η) associated with the electrolysis
reaction is expressed as:

η=V experimental
Vtheoretical(1)

where Vexperimental is the total volume of the generatedhydrogen and oxygen gases and Vtheoretical is the theoreticalvolume of the generated gas bubbles [7]. Vtheoretical iscalculated from:1234

⎛3 i⎞
Vtheoretical = qtheoretical t = ⎜Vm ⎟ t4F⎝⎠(2)

where qtheoretical is the theoretical gas generation rate (inm3/s), t is duration (in sec) over which the current is applied,
i is current (in A), F is Faraday’s constant (96.49×103C/mol), and Vm is the molar gas volume at 25°C andatmospheric pressure (24.7×10−3 m3/mol). Efficiency
increased with current, reaching 94% for Nafion®-coatedelectrodes compared to 53% for uncoated (13mA) (Fig. 5).

Figure 5: Pump efficiency with and without Nafion®-coatingfor glass substrate.
Flow rate dependence on actuator orientation wasinvestigated with and without Nafion® during constant
10mA current application by changing the angle of anactuator clamped in a test fixture (Fig. 6). The Nafion®
coating results in a stable flow rate regardless of actuator
orientation; the effect is most pronounced at 180°
orientation. This is likely due to the prevention of gas
bubble occlusion of the electrode surface.

Figure 6: Flow rate vs. angular orientation for coated anduncoated electrodes operated at 10mA.
Investigation of Delamination of and Electrodeposition
on Uncoated Electrodes on Flexible Substrates
EIS was performed in 1X phosphate buffered saline andresults showed noticeable shifts in the impedance curves
following current application on uncoated electrodes
fabricated on the flexible Parylene and PEEK substrates. For

Parylene electrodes, a decrease in solution resistance wasobserved which correlated to electrode delamination
verified by visual inspection (Fig. 7). For PEEK electrodes,
the double layer capacitance increased which correlatedwith increased surface area from the rough substrate and thepresence of an unknown electrodeposited material following
current application (Fig. 8). Electrodeposition was more
pronounced on the cathode. The nature of thiselectrodeposited layer is being further investigated.

Figure 7: EIS results for electrodes on Parylene before andafter current application (5mA). Inset: photo of device after
current application showing delamination of electrodeelements.

Figure 8: EIS results for electrodes on PEEK before andafter current application (5mA). Inset: photo of device after
current application showing electrodeposition (black spots).
Polymer Coating on Flexible Electrodes
Parylene electrodes were coated with 1µm of Nafion®
and operated over a range of currents (results not shown).
While the coating appeared to delay delamination, it did notprevent it at sufficiently high currents (> 5 mA). Therefore,
we conclude that Parylene may not be a suitable material forour application without further treatment to improve theadhesion of Pt. The PEEK electrodes however, once coated1235

with Nafion®, showed no signs of the electrodepositedmaterial present in the uncoated electrodes following current
application. Moreover, as a result of the combined effect of
the rough PEEK surface (increased electrode surface area)
and the coating, higher efficiency of up to 97% (13mA) wasachieved compared to electrodes on soda lime glass (Fig. 9).
Figure 10: (left) Class D wireless system showing primaryand secondary circuits, (right) voltage output trace.

Figure 9: Comparison of Nafion®-coated glass and PEEK
electrodes: current controlled flow delivery. Inset: photo ofPEEK electrode after current application. Coating protects
electrodes from delamination and electrodeposition.
Wireless Performance
A class D wireless system was designed andimplemented to provide constant current operation between
1mA-10mA (Fig. 10). Uncoated and coated electrodes wereoperated wirelessly with the constant current output set to
10 mA. Flow rates were slightly larger than those for wired
constant current operation due to noise in the current outputof the wireless system (Fig. 11).

CONCLUSION
We demonstrated high efficiency actuators compatible
with wireless operation, capable of delivering flow rates ofup to 141μL/min at 13mA. Coated and rough electrodes
significantly improved efficiency, enabling futureapplication of low-power actuators in wireless drug delivery
pumps. Flexible substrate actuators further enable in vivo
and other diverse applications.
Future work includes integration of actuators with ainto a drug delivery pump for in vivo drug addiction studies
in freely behaving mice.

ACKNOWLEDGEMENTS
This work was funded in part by a Wallace H. CoulterFoundation Early Career Translational Research Award andNIH/NIDA under award number R21DA026970. Theauthors would like to thank Dr. F. Mansfeld, Dr. A. K.
Manohar, Dr. D. Holschneider, Dr. J.M. Maarek, and themembers of the USC Biomedical Microsystems Laboratory
for their assistance with this project.

Figure 11: Wireless flow delivery for coated and uncoatedelectrodes.

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