MEMS ELECTROCHEMICAL PATENCY SENSOR FOR DETECTION OF
HYDROCEPHALUS SHUNT OBSTRUCTION
Brian J. Kim, Willa Jin, Lawrence Yu, and Ellis Meng
Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, USAABSTRACTDESIGN AND OPERATION

Currently there are no practical methods to definitively
diagnose the obstruction failure of hydrocephalus shunts
without surgical intervention, resulting in suboptimaltreatment and needless patient suffering. We present the first
MEMS electrochemical patency sensor for direct andquantitative tracking of shunt patency and obstruction. Theimpact of electrode size, temperature, flow, and hydrogenperoxide (H2O2) plasma sterilization on sensor function wasevaluated and sensor performance in the presence of staticand dynamic obstruction was demonstrated. Electrode size
was found to have a minimal effect on sensor performanceand increased temperature and flow resulted in a slightdecrease in the baseline impedance (~8.5%) due to anincrease in ionic mobility. H2O2 plasma sterilization also had
no effect on sensor performance. This low power and simple
format sensor was developed with the intention of futureintegration into shunts for wireless monitoring of shunt stateand more importantly, a more accurate and timely diagnosisof shunt failure.

The sensor consists of two electrodes, one positioned oneach internal and external surface of the catheter, such thatthe catheter ports establish an ionic conductive path between
them. When measuring the electrochemical impedance at a
sufficiently high frequency (fm) to isolate the solution
resistance, any disturbances in the volumetric conduction
path between the two electrodes (i.e. port blockage) willregister as impedance changes. Thus, upon partial orcomplete blockage of the catheter ports by tissue, the fluidic
conduction pathway between the internal and external
electrodes is perturbed and an increase in the electrochemical
impedance is measured, similar to the mechanism of theCoulter counter (Figure 1).

INTRODUCTION
Hydrocephalus is a chronic, incurable condition
characterized by the accumulation of excess cerebrospinal
fluid (CSF) within the ventricles of the brain, resulting inelevated intracranial pressure. Currently, hydrocephalus istreated by implanting a shunt, a multi-holed silicone
catheter and valve system, into the ventricles to drain theexcess CSF. Though effective, shunts fail at an alarming
rate of 40% within the first year [1]. There are many causesof shunt failure, including mechanical issues [2] andinfection [3], but the most common is blood and tissue
obstruction of the drainage ports [2].
Though these failure rates are high, there are currently
no reliable and convenient methods to detect shuntobstruction failure. Obstruction can present itself in only
vague symptoms, such as headaches and nausea, which canbe confused with other medical conditions. There is a needfor a non-invasive method to periodically assess shunt
patency so that treatment can be improved through thetimely and accurate diagnoses of shunt failure.
We developed a Parylene C-based electrochemical
patency sensor capable of providing periodic measurementsof shunt obstruction. This sensor utilizes a very simple
transduction scheme to assess the degree of shunt patency bymeasuring changes in electrochemical impedance in the
conductive path through the drainage ports in the shuntcatheter. As the sensor is constructed on a flexible, thin film
Parylene C substrate, it is easily integrated with current shunt
systems either as a modular add-on or by direct integration
into the shunt.

978-1-4799-7955-4/15/$31.00 ©2015 IEEE

Figure 1: (a) Conceptual cartoon of impedance sensing
mechanism of patency sensor. Two electrodes on the internaland external surfaces of the catheter are fluidicallyconnected via the drainage ports. Obstruction of these ports
impedes the ionic conduction path between the electrodes,and (b) the electrochemical impedance between theelectrodes increases for measurements above a certain
frequency (fm).
The patency sensor was constructed using standard
surface micromachining processes for Parylene MEMS
devices. Platinum electrodes (2000 Å) were deposited andpatterned using the liftoff method on a Parylene C substrate(15 µm) supported by a silicon carrier wafer. Platinum waschosen as it has shown outstanding inert performance for invivo and electrochemical sensing applications [4]. The
electrodes were insulated using Parylene C (2 µm) andelectrode sites were exposed via oxygen plasma etching. In
the current design, four electrodes having different surface
areas were fabricated on a single device to evaluate effects ofelectrode size on sensor performance (Figure 2a). Devices
were released and electrically packaged using a zero
insertion force (ZIF) connector and flat flexible cable (FFC)
used for Parylene devices [5] before further assembly into themodule (Figure 2b).

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MEMS 2015, Estoril, PORTUGAL, 18 - 22 January, 2015

Figure 2: (a) Optical micrograph of fabricated device withfour electrodes (E1-E4) of varying electrode surface areas
for design optimization. (b) Image of packaged device usinga ZIF connector and FFC for assembly into module.

Figure 4: (a) Mock silicone catheters with varying numberof holes. (b) Magnified image of manually punched hole of 1mm diameter.
The catheter was then placed within a beaker of aCSF
and connected to the sensor module. The assembly was filled
via a syringe or peristaltic pump (for static and flow
conditions, respectively) prior to testing (Figure 5). A
platinum wire electrode was placed within the beaker to closethe circuit and complete the sensing setup. Impedance
measurements were acquired using a Gamry R600
potentiostat for experiments requiring frequency ranges or anAgilent e4980 LCR meter for a measurement at a single
frequency.

EXPERIMENTAL METHODS
The sensor platform was assembled into different
luer-lock modules designed for early acute validation studies
in human using an external ventricular drain system: a cap
and inline module (Figure 3). To form the cap module, theParylene device was first affixed within a slit of a rubber
stopper on top of the cap housing, which was then filled withartificial CSF (aCSF); a 3-way valve system allowed for
attachment of the cap module to the rest of the testing system.
The inline module was formed by affixing the sensor
platform within a milled slit in a luer-lock adaptor. The inline
design removed the need to prefill the module and also
allowed for simpler integration with the drainage system,with necessitating an extra 3-way valve component. In bothmodules, the FFC was used as the connection scheme to theimpedance measurement system.

Figure 5: Cartoon schematic of experimental setup.
Experiments were conducted using either the (i) cap or inline
module with a (ii) syringe or peristaltic pump for static orflow conditions, respectively. Impedance was measured
between the sensor electrode and platinum ground electrodein the beaker.
First, the fm that isolates the solution resistance within
the sensor’s impedance response was determined so as tohave the optimal sensing performance for patency. Thensensor sensitivity measurements were performed out toassess the relationship between measured electrochemical
impedance and number of holes (i.e. percent shunt blockage).
These experiments were conducted using all four electrodesizes to evaluate electrode size effects on sensor
performance.
As these devices are to be used within the body, testingat body temperature (37°C) and in the presence of clinically
relevant catheter drainage flow rates (0.3 ml/min [6]) wasconducted. The effect of temperature on sensor performancewas evaluated by using a water bath to maintain the beaker ofaCSF at a constant 37°C; flow was generated in the system

Figure 3: Assembled (a) cap and (b) inline modules for
simple integration of the Parylene-based patency sensor
with external ventricular drains within the clinic using
standard luer-lock connections.
In benchtop tests, blockages of the catheters weresimulated by mock catheters (silicone) of varying numbers ofholes, with 16 holes simulating 100% open (a 4-holedcatheter would be classified as 75% blockage, 8 holes as50%, etc.) (Figure 4a). To construct the mock catheters, one
end of a silicone tube (1.5 mm ID) was plugged and holes
were manually punched using a 15 gauge coring needle tocreate 1 mm diameter holes (Figure 4b), similar to those used
in a proximal catheter of a hydrocephalus shunt.

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using a peristaltic pump. Also, for use of this device inclinical applications, it is vital that the sensor modules aresterilized to eliminate any source of infection. Considering
this, the functionality of the devices following hydrogenplasma (H2O2) sterilization, a commonly used sterilization
technique used by hospitals, performed with a Sterrad 100
NX system was also assessed.

RESULTS AND DISCUSSION
Sensor Characterization
Initial experiments were conducted at fm between 0.1 Hz– 1 MHz and the measured impedance over certain frequency
ranges (>fm, corresponding to where the solution resistancedominates the impedance response) correlated well withcatheter blockage over all electrode sizes and types (data notshown). An optimal measured fm was determined for eachsensor size (Table 1).

Figure 6: Representative calibration curve for E1 sensor
electrode indicated that the response of the sensor was2
inversely proportional (R = 0.89967) to the number ofholes.
Temperature and Flow Characterization
Both an increase in fluidic temperature from room
temperature (20°C) to body temperature (37°C) and theaddition of flow in the system decreased the baseline
impedance measured (~8.5% for both cases), but stillretained a similar inverse relationship and sensitivity(Figure 7). The decrease in baseline impedance is due to anincrease in ionic mobility that occurs at elevatedtemperatures and increased fluidic flow as observed within
literature [7, 8].

Table 1:
Obtained optimal impedance measurement
frequencies and sensitivities for electrodes of theParylene-based EC-MEMS patency sensor.
Electrode Surface Measurement
Sensitivityfrequency
designarea(% Z /2(kHz)(μm )%Blockage)
E1300,000 100.183
E220,000
300.157
E320,000
300.168
E417,320
300.161
By analyzing the data for each electrode at its
corresponding optimal measurement frequency, calibration
curves were formed and results indicated that the impedance
varied inversely with the percent blockage of the catheter(Figure 6). The inverse relationship follows from theequation for solution resistance (Rs), where ρ is theconductivity of the solution, l, the distance between theelectrodes, and A, the cross-sectional area between theelectrodes:
ρlRs ൌ A(1)

The results suggest that hole blockages alter the area term ofequation 1. Though all electrode sizes were similar inperformance, the sensitivity of the largest electrode (E1) was
slightly higher than the others (0.183 % Δ impedance/%blockage), and thus was chosen for further sensor
development.

Figure 7: Sensor performance at (a) elevated temperatureand (b) with flow conditions demonstrated a decrease in thebaseline impedance due to increased ion mobility, but thefunctionality of the sensor was maintained.

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integration of the sensor with current catheter systems wasdeveloped for testing, and percent shunt blockage was foundto correlate inversely with electrochemical impedance
measured at an optimal measurement frequency varying
with electrode size. The integration of patency sensors into
shunts will enable quantitative monitoring of shunt
performance and more importantly, provide accurate andtimely diagnosis of impending failure to improve treatment
for hydrocephalus patients.

Sterilization Characterization
H2O2 sterilization of inline module packaged devices
did not alter sensor performance (Figure 8). Electrodecharacterization
usingelectrochemical
impedance
spectroscopy and cyclic voltammetry also indicated nochanges in electrode surface area or properties followingsterilization (data not shown). Further studies are underway
to assess the efficacy of the method in sterilizing themodules.

ACKNOWLEDGEMENTS
The authors thank Dr. Eisha Christian of Children’s
Hospital Los Angeles for clinical consultation during device
design, Dr. Donghai Zhu of the Keck Photonics Laboratoryfor help with fabrication, and members of the Biomedical
Microsystems Laboratory of USC for their assistance. AnOAI model 30 light source was used for processing ofParylene devices.
This work was funded in part by the NSF under award
number EFRI-1332394 and the University of Southern
California Provost Ph.D. Fellowship (BK).
Figure 8: Plots of sensitivity pre and post sterilization,
indicating no change to devices following hydrogen
peroxide sterilization.

REFERENCES[1] J. Drake, et al., "CSF shunts 50 years on–past, present andfuture," Child's Nervous System, vol. 16, pp. 800-804,
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[3] A. V. Kulkarni, et al., "Cerebrospinal fluid shunt
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[5] C. A. Gutierrez, et al., "Epoxy-less packaging methods
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[6] M. E. Wagshul, et al., "Amplitude and phase of
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[7] H. E. Ayliffe and R. Rabbitt, "An electric impedance
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[8] C. A. Gutierrez, "Development of Flexible
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Transient Patency Measurement
The ability of the sensor to measure dynamic blockagesin the benchtop system was also assessed. A 16-holedcatheter was used, and transient blockages were simulated bysheathing/unsheathing the drainage ports using larger
diameter tubing. Results indicated that the sensor wascapable of repeatedly measuring blockage events over time
(Figure 9).

Figure9:Transientblockageexperiments
ofsheathing/unsheathing a 16-holed catheter to simulate
dynamic blockage illustrated the real-time measurement
capabilities of the device. Obstruction events are labeledwith an X.

CONCLUSIONCONTACTWe designed, fabricated, packaged, and characterized aMEMS electrochemical patency sensor to detect obstructionof hydrocephalus shunts. A cap and inline module for*E. Meng, tel: +1-213-8213949; ellis.meng@usc.edu

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