This paper presents a novel implantable, wireless,

passive pressure sensor for ophthalmic applications. Two

sensor designs incorporating surface-micromachined

variable capacitor and variable capacitor/inductor are

implemented to realize the pressure sensitive components.

The sensor is monolithically microfabricated using parylene

as a biocompatible structural material in a suitable form

factor for increased ease of intraocular implantation.

Pressure responses of the microsensor are characterized

on-chip to demonstrate its high pressure sensitivity (> 7000

ppm/mmHg) with mmHg level resolution. An in vivo animal

study verifies the biostability of the sensor implant in the

intraocular environment after more than 150 days. This

sensor will ultimately be implanted at the pars plana or iris of

the eye to fulfill continuous intraocular pressure (IOP)

monitoring in glaucoma patients

Chen, Po-Jui

Humayun, Mark S.

Rodger, Damien C.

Saati, Saloomeh

Tai, Yu-Chong

English

Caltech Authors - Main

Implantable parylene-based wireless intraocular pressure sensor

IMPLANTABLE PARYLENE-BASED WIRELESS INTRAOCULAR
PRESSURE SENSOR 
Po-Jui Chen1, Damien C. Rodger2, Saloomeh Saati3, Mark S. Humayun2,3, and Yu-Chong Tai1
1California Institute of Technology, Pasadena, CA, USA 
2University of Southern California, Los Angeles, CA, USA 
3Doheny Eye Institute, Los Angeles, CA, USA 
ABSTRACT 
This paper presents a novel implantable, wireless, 
passive pressure sensor for ophthalmic applications.  Two 
sensor designs incorporating surface-micromachined 
variable capacitor and variable capacitor/inductor are 
implemented to realize the pressure sensitive components.  
The sensor is monolithically microfabricated using parylene 
as a biocompatible structural material in a suitable form 
factor for increased ease of intraocular implantation.  
Pressure responses of the microsensor are characterized 
on-chip to demonstrate its high pressure sensitivity (> 7000 
ppm/mmHg) with mmHg level resolution.  An in vivo animal 
study verifies the biostability of the sensor implant in the 
intraocular environment after more than 150 days.  This 
sensor will ultimately be implanted at the pars plana or iris of 
the eye to fulfill continuous intraocular pressure (IOP) 
monitoring in glaucoma patients. 
1. INTRODUCTION
Intraocular pressure (IOP) monitoring is crucial in the 
management of glaucoma patients as it is known one of the 
most effective methods to evaluate the progression of this 
chronic eye disease [1].  Applanation tonometry using 
contact or non-contact approach is the current clinical 
technique for IOP recording.  However, it has difficulties in 
providing reliable and repeatable measurements and, 
particularly, in deployment for continuous tracking, which 
impedes prompt detection and appropriate treatment for IOP 
spikes considered as a separate risk factor to optic nerve 
damage.  Continuous IOP monitoring in glaucoma patients 
with high accuracy and high reliability is therefore a 
consistent need for ophthalmologists. 
Passive telemetric sensing is one of the viable methods 
for continuous and faithful non-contact IOP measurements.  
The concept as shown in Fig. 1 was first proved in 1967 using 
a sensor with resonant circuitry implanted to the anterior 
chamber of the eye [2].  By wirelessly interrogating the 
sensor through an inductive coupling link, an external reader 
can register IOP variations without indirect derivation and 
calculations as are used in applanation tonometry.  Enabled 
by microfabrication technologies, a miniaturized wireless 
sensor was later reported with its application for IOP 
monitoring as well [3].  The model has been more completely 
studied given the development of various pressure 
microsensors [4-7], with a needle-implantable flexible sensor 
for pressure monitoring of abdominal aortic aneurysms 
(AAA) as a specific example for biomedical applications.  
On the technology side, however, all these paradigms involve 
delicate multiple wafer fabrication with wafer bonding as the 
essential process.  In this work, with the knowledge of 
utilizing parylene in surface-micromachined capacitive 
sensors [8], we investigate a monolithically microfabricated 
parylene-based sensor in a form factor favorable for 
minimally invasive ocular implantation.  Featuring parylene 
as the biocompatible polymeric material, the pressure sensor 
is designed to be completely implantable in the intraocular 
environment so that long-term IOP monitoring of glaucoma 
patients can be fulfilled. 
Figure 1. Conceptual schematic of passive wireless IOP 
sensing.  The implanted sensor can register faithful pressure 
variations using corresponding electrical characteristic 
changes, which are measured from the external reader 
through a wireless inductive coupling link. 
2. DESIGN
Device Structure 
The proposed pressure sensor as shown in Fig. 2 
comprises a flexible diaphragm chamber integrated with 
parallel metal plates as an integrated capacitor and metal 
wires as a dual-layer planar inductor to create the resonant 
circuit that communicates with the external reader.  A 
deformable diaphragm with embedded electrical components 
enables gauge pressure sensing of the device.  For instance, a 
pressure sensitive variable capacitor can be realized by 
arranging the bottom plate on the substrate and the top plate 
in the diaphragm.  In addition to a fixed inductor surrounding 
the variable capacitor, a variable inductor can also be realized 
by embedding the top metal wire layer in the diaphragm to 
alter the mutual inductance between two planar inductors for 
further enhanced overall pressure sensitivity.  These two 
designs are implemented on a monolithic substrate with the 
use of surface-micromachining technology so as to have less 
fabrication complexity compared with wafer bonding 
technology.  All device structures including oxidized silicon 
substrate, titanium/gold metal lines and plates, and parylene 
diaphragm are made out of biocompatible materials to ensure 
the feasibility of device implantation.  Parylene C 
(poly-para-xylylene C) is selected as the diaphragm material 
because of its mechanical flexibility (Young’s modulus ~4 
978-1-4244-1793-3/08/$25.00 ©2008 IEEE MEMS 2008, Tucson, AZ, USA, January 13-17, 200858
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GPa), CMOS/MEMS processes compatibility, and 
biocompatibility (FDA approved USP Class VI grade).  Its 
room-temperature conformal deposition nature allows large 
diaphragm chambers to be easily and reliably formed on a 
monolithic substrate.  Additionally, its dielectric property 
(dielectric constant ~2.95 at 1 MHz) makes parylene C a 
straightforward insulator between metal layers. 
Figure 2. Sensor design: (top) Variable capacitor; (bottom) 
Variable capacitor/inductor. The deformable diaphragm 
chamber with respect to pressure changes realizes pressure 
sensitive electrical characteristics of the sensors. 
Circuit Analysis 
In the lumped model illustrated in Fig. 1, the equivalent 
impedance as viewed from the measurement instrument can 
be written as [4-7]: 
2
2
22 1
11
o
eq r
o s s
f
fVZ j fL k
I f fj
f Q f
?
? ?? ?
? ?? ?
? ?? ?? ? ?? ?? ?? ?? ?? ?? ?? ?? ?
where fs = [2?(LsCs)1/2]-1 is the sensor resonant frequency 
and Qs = Rs-1(LsCs-1)1/2 is the quality factor of the sensor at 
resonance, and k is the coupling coefficient of the inductive 
link.  This equation implies that a phase-dip technique can be 
applied to detect the resonant frequency of the sensor as the 
phase of the equivalent impedance drops to the minimum 
with an approximate dip magnitude: 
? ?1 2tan    when   s sk Q f f? ?? ? ?
As a result, the changes of resonant frequency with 
respect to pressure variations experienced by the sensor can 
be registered in situ by observing the shift of phase dip in 
frequency domain so that continuous environmental pressure 
monitoring can be accomplished.  Electrical characteristics 
including inductance, resistance, and capacitance of the 
sensor were calculated using theoretical equations [9][10], so 
that the overall sensing performance can be estimated 
meeting the specification in miniature pressure variation 
measurement (± 1 mmHg) for accurate IOP monitoring. 
3. FABRICATION
The fabrication process illustrated in Fig. 3 started by 
thermally growing and patterning 0.75 μm oxide on a 
double-side-polished silicon wafer as an insulation layer.  In 
frontside processes, e-beam-evaporated metals were used 
here for better material quality purposes while thick 
deposition was required to obtain low overall resistance from 
the metal wires.  A thick titanium/gold (200 Å/2 μm) layer 
was deposited and patterned using standard metal etching 
techniques to form the bottom half of the electrical 
components of the sensor.  A parylene diaphragm chamber 
was then created with photoresist as the sacrificial material.  
Gas-phase XeF2 silicon roughening was conducted for 
strengthened adhesion between the parylene and the substrate.  
Another thick titanium/gold (200 Å/0.5 μm) layer was 
deposited and patterned on top of the parylene to realize the 
variable capacitor and variable capacitor/inductor structures 
by having different patterns of the sacrificial photoresist 
layer.  For the variable capacitor, an additional parylene C 
layer was coated on the bottom metal before photoresist and 
parylene processes to reduce stray capacitance arising from 
the proximity of the two metal layers.  Devices were finally 
released after performing a deep reactive-ion etch (DRIE) on 
the backside of wafer followed by photoresist stripping with 
acetone.  Supercritical CO2 drying was used to eliminate 
stiction of the free-standing parylene/metal/parylene 
diaphragm.  Fig. 4 shows the microfabricated sensors with 
suture holes in 500 μm diameter at both ends for convenience 
in device anchoring during implantation. 
Figure 3. Fabrication process flow. 
Embedded dual-layer 
metal wires 
Flexible diaphragm 
chamber
Parallel metal 
plates
? ?2            2 1    when   r s sj fL jk Q f f?? ? ?
CL
R
.
59
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Figure 4. Device images: (left) Microfabricated sensors; 
(right) Micrograph of the sandwiched metal/parylene/metal 
structure showing the via in the variable capacitor design. 
In order to enable successful device release and 
packaging, a special two-step etch mask with oxide and 
photoresist materials was used for the backside DRIE etching.  
By controlling the heights of this two-step mask, a backside 
recess was able to be formed on the substrate along with 
through-wafer etches on photoresist release holes beneath the 
parylene diaphragm chamber, the end anchor holes, and the 
release boundaries of the device.  Therefore, a large cavity 
can be created inside the device after sealing the recess to 
encapsulate more air volume in addition to that in the 
surface-micromachined diaphragm chamber, which 
effectively increases pressure sensing capacity after 
packaging.  For simplicity, the fabricated sensor device was 
glued to another non-electronic dummy piece with identical 
geometrical dimensions in this work, but undoubtedly the 
overall device thickness can be trimmed by thinning down 
the silicon pieces during fabrication.  After packaging, a thin 
conformal parylene layer was finally coated on the device to 
enhance its biocompatibility.
4. RESULTS
Device testing was conducted using a 1.5-mm-diameter 
hand-wound coil connected to a HP4195A network analyzer 
to serve as the external reader for electrical measurements.  
Electrical parameters of the sensors were first obtained 
through lumped-model extraction by measuring the actual 
devices as well as several testing structures.  Table I 
summarizes the experimental results which are in good 
agreement with theoretical calculations. 
Table I Electrical parameters of the microfabricated sensors 
 Variable C Variable C&L
Resistance ~112 ?? ~72 ?
Inductance ~0.78 μH ~0.36 μH 
Capacitance ~8.5 pF ~3.1 pF 
Resonant frequency ~62 MHz ~150 MHz 
Quality factor at 
resonance ~3 ~5 
The wireless sensing distance is fundamentally limited 
by the quality factor of the device and the integrated coil size 
which affects inductive coupling efficiency with the reader 
coil.  Even though the sensors have low quality factors (Qs < 
10) and small structural dimensions, their responses were still 
wirelessly detectable as shown in Fig. 5, thus validating the 
feasibility of the phase-dip technique as the sensing scheme.  
The maximum sensing distance in which the phase dip ?? >
0.1°, defined by the noise floor from the testing system, was 
confirmed to be approximately 2 mm with sensors in both 
designs. Other than reader optimization, the following 
strategies directly related to device can improve the sensing 
distance and should be included in future development: (1) 
increase of integrated coil size (k ?); (2) increase of metal line 
thickness (Rs ?); (3) increase of insulating oxide thickness or 
modification of substrate material (Cs ?). 
Figure 5. Captured frequency scan example of equivalent 
impedance Zeq of the reader coil coupled with the sensor. 
After electrical characterization the sensors were tested 
on-chip to measure their pressure responses.  Pressure 
differences were generated by introducing pressurized air to 
the inside of the sensor diaphragm chamber through a 
customized packaging jig connected to a controllable 
pressure regulation system with constant environmental 
pressure outside the diaphragm chamber.  Experimental 
results as shown in Fig. 6 successfully demonstrate highly 
sensitive pressure responses on because of high compliance 
of the parylene flexible diaphragm and high resonant 
frequency of the sensors.  For sensors of the variable 
capacitor design, the measured data points were fit using the 
following relation with the assumption of small diaphragm 
deflection proportional to pressure difference: 
? ?
? ?
? ? ? ?1min 2
min
1
2
110
2
s s s
s s
L C Cf P
P
f P
L C
?
?
?
???
? ? ? ?
? ?
where ? is the fitting parameter.  The same procedures on 
normalized resonant frequency analysis were applied to 
sensors of the variable capacitor/inductor design under small 
deflection assumption with the relation between mutual 
inductance and separation distance in conductors [9]: 
? ?
? ?
1min 2
1 2
min
1 1 (1 )
0 1 1s s
s s
f P
P P
f P L C
L C
? ?
?
? ? ? ? ? ?
? ? ? ?? ?
Impedance phase
Impedance magnitude
f (MHz)
?Zeq (deg) 
|Zeq| (?)
fmin
4 mm x 2 mm 
(planar)
4 mm x 1 mm 
(planar)
Variable C&L 
Variable C 
50 µm
60
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where ?? and ?? are also the fitting parameters describing 
more sensitive pressure response due to the coupled 
capacitance and inductance variations.  Data analysis 
confirms that sensors of the both designs achieved excellent 
pressure sensing performance with high sensitivities (> 7000 
ppm/mmHg) and mmHg level resolution in a measurement 
range of more than 0-30 mmHg, which sufficiently covers all 
IOP variations of interest in practical monitoring. 
0 3 6 9 12 15 18 21 24 27 30
1.00
1.04
1.08
1.12
1.16
1.20 12min
min 0
(1 0.01499 )
P
fR Pf
? ?
? ? ? ?
0
Sensitivity 7495 ppm/mmHg
( ) P
R
P ? ?
?? ?
? ?
Fr
eq
ue
nc
y 
R
at
io
 fm
in
 / 
fm
in
(?
P
=0
)
 Measured data points
 Curve fit
Responsivity 468 kHz/mmHg?
Applied Pressure Difference ?P (mmHg)
0 3 6 9 12 15 18 21 24 27 30
1.00
1.04
1.08
1.12
1.16
1.20
1.24 ? ?
1
2 2min
min 0
1 0.01519 0.00004
P
fR P Pf
? ?
? ?? ? ? ? ? ?? ?
Responsivity 1.14 MHz/mmHg?
0
Sensitivity 7595 ppm/mmHg
( ) P
R
P ? ?
?? ?
? ?
Fr
eq
ue
nc
y 
R
at
io
 fm
in
 / 
fm
in
(?
P
=0
)
 Measured data points
 Curve fit
Applied Pressure Difference ?P (mmHg)
Figure 6. Pressure testing results of the on-chip sensors with 
variable capacitor (top) and variable capacitor/inductor 
(bottom) designs. 
In vivo testing was conducted using a live rabbit eye as 
the model to evaluate the bioefficacy and biostability of the 
sensor implant.  The device was implanted at a pars plana site 
to reduce the required sensing distance to the external coil.  
Its end anchor holes facilitate robust device anchoring with 
use of sutures during device implantation.  Results as shown 
in Fig. 7 indicate no surgical complications, inflammatory 
responses, or device encapsulation after more than 150 days, 
which verifies device biocompatibility in the intraocular 
environment.  Extensive animal studies including in vivo
pressure sensing demonstration and optimal device 
placement investigation will be performed in future work. 
Figure 7. In vivo device testing using a live rabbit eye model. 
5. CONCLUSION
A novel implantable wireless intraocular pressure sensor 
has been successfully developed featuring parylene as the 
biocompatible/implantable structural material.  Sensors with 
resonant circuitry involving pressure-sensitive variable 
capacitor and variable capacitor/inductor designs were 
implemented to facilitate wireless pressure sensing by 
creating an inductive coupling link with the external reader.  
A low-temperature multi-layer micromachining technology 
enables integrated sensor components fabricated on a 
monolithic substrate without the need of wafer bonding 
process.  Sensors with implantable planar dimensions (4 mm 
x 2 mm in variable capacitor and 4 mm x 1 mm in variable 
capacitor/inductor design) were microfabricated in a suitable 
form factor for intraocular implantation.  On-bench device 
testing achieved successful pressure sensing with more than 
7000 ppm/mmHg sensitivity and 1 mmHg resolution.  
Biocompatibility of the device implant was successfully 
verified through in vivo characterization in live animal study, 
which provides evidence that the sensors can be utilized for 
long-term continuous IOP monitoring in glaucoma patients. 
ACKNOWLEDGEMENTS
This work was supported in part by the Engineering 
Research Centers Program of the National Science 
Foundation (Award Number EEC-0310723) and part by 
Bausch and Lomb.  The authors especially thank Dr. Rajat 
Agrawal and Dr. Rohit Varma for their valuable comments 
on surgical procedures, and Mr. Trevor Roper for his 
fabrication assistance. 
REFERENCES
[1] M. Yanoff and J.S. Duker (Eds.), Ophthalmology, Second 
Edition, Mosby, St. Louis, 2003. 
[2] C.C. Collins, “Miniature Passive Pressure Transensor for 
Implanting in the Eye,” IEEE Transactions on Biomedical 
Engineering, BME-14(2), pp. 74-83, 1967. 
[3] L. Rosengren, P. Rangsten, Y. Bäcklund, B. Hök, B. 
Svedbergh, and G. Selén, “A System for Passive Implantable 
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[4] O. Akar, T. Akin, and K. Najafi, “A Wireless Batch Sealed 
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[5] A. DeHennis and K.D. Wise, “A Double-Sided Single- Chip 
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  Implant   Implant 
5 months after surgery 3 months after surgery 
61
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A Double-Sided Single- Chip Wireless Pressure Sensor,”

A Self-Resonant Frequency-Modulated Micromachined Passive Pressure Transensor,”

Flexible Wireless Passive Pressure Sensors for Biomedical Applications,” Proc.

Miniature Passive Pressure Transensor for Implanting in the Eye,”

Radio Engineers’ Handbook,

Surface Micromachined and Integrated Capacitive Sensors for Microfluidic Applications,”

The Design of CMOS Radio-Frequency Integrated Circuits, Second Edition,

http://authors.library.caltech.edu/19163/1/Chen2008p8557Mems_2008_21St_Ieee_International_Conference_On_Micro_Electro_Mechanical_Systems_Technical_Digest.pdf

Implantable parylene-based wireless intraocular pressure sensor

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