Implantable Unpowered Parylene MEMS Intraocular Pressure Sensor
Po-Jui Chen1, Damien C. Rodger1,2, Ellis Meng3, Mark S. Humayun2, and Yu-Chong Tai1
1California Institute of Technology
1200 E. California Blvd., Pasadena, CA 91125 USA
Tel: +1-626-395-2254, Fax: +1-626-584-9104, E-mail: pjchen@mems.caltech.edu
2Keck School of Medicine, University of Southern California
Health Science Campus, University of Southern California, Los Angeles, CA 90033 USA
3University of Southern California
University Park Campus, University of Southern California, Los Angeles, CA 90089 USAABSTRACTThis paper presents the first implantable, unpowered, parylene-based micro-electro-mechanical-systems (MEMS) pressure
sensor for intraocular pressure (IOP) sensing. From in situ mechanical deformation of the compliant structures, this sensor
registers pressure variations without power consumption/transduction. Micromachined high-aspect-ratio thin-walled tubes indifferent geometric layouts are exploited to obtain a high-sensitivity pressure response. An integrated packaging method has
been successfully developed to realize suture-less implantation of the device. In vitro testing results have demonstrated that
the IOP sensor can achieve 0.67 degree/mmHg angular sensitivity with a spiral-tube design, 3.43 µm/mmHg lateral sensitivity
with a long-armed-tube design, and 0.38 µm/mmHg longitudinal sensitivity with a serpentine-tube design. This IOP sensor isdesigned to be implanted in the anterior chamber of the eye and anchored directly on the iris so that, under incident visible
light, the pressure response of the implant can be directly observed from outside the eye, which enables faithful andunpowered IOP monitoring in glaucoma patients.
Keywords: Implantable, Intraocular Pressure, Parylene, Pressure Sensor, Unpowered

1 INTRODUCTION
Glaucoma is a debilitating disease that results in loss ofvision for hundreds of millions of people worldwide.
Although measuring intraocular pressure (IOP) is a wellknown method to detect and monitor glaucoma in patients,
current tonometry techniques involve only indirect IOP
measurements [1][2]. They have difficulties in faithfullyrecording IOP values inside the eye, and are problematic in
deployment for regular (e.g., daily) monitoring of IOPfluctuations and treatment progress. Therefore, a direct andreliable IOP sensing technology is highly needed.
Recently, an all-mechanical unpowered IOP sensing
technology has been proposed [3]. By implementing microelectro-mechanical-systems (MEMS) technology, a spiraltube parylene pressure sensor was successfullymicrofabricated. It demonstrated a linear pressure response
without power consumption or external electronic/magnetic/thermal transduction. Nevertheless, its sensitivity was nothigh enough to detect pressure variations in the mmHg range.
The size of the sensor was unfavorably large so that
implantation in the eye remained a significant problem.

These two factors are the main challenges in using this typeof pressure sensor for practical IOP measurement.
In this work, the unpowered IOP sensor paradigm has beenfurther developed in order to provide a solution to theseissues.
The all-mechanical actuation mechanism ispreserved for power-free sensing, while delicate geometricredesign is implemented to achieve higher sensitivity inpressure sensor response. State-of-the-art micromachining
techniques are applied to construct high-aspect-ratio
rectangular thin-walled tubes with higher flexibility.
Furthermore, these smaller IOP sensors are packaged with
auxiliary securing components, in an appropriate form factor
to enable suture-less implantation. By employing thesemodifications, our novel IOP sensors permit IOP
measurements inside the eye.
2 DESIGN
2.1 IOP Sensor
The IOP sensing element is a high-aspect-ratio compliant
structure inspired by a Bourdon tube, essentially a free-standing curved thin-walled tube. They are systematicallydeformed by a pressure difference applied between thepressure encapsulated in the tube/channel and the ambientpressure, realizing direct pressure measurement throughobservation of tube deformation. Table 1 shows various
geometric layouts of the IOP sensor design to accomplish
different in-plane deformation trajectories. In addition to theprevious spiral-tube design for angular deformation, a longarmed-tube design that amplifies rotation and a serpentinetube design for linear deformation are extensively developedin this work to facilitate both high-sensitivity pressure
response and optimization of device dimensions for sutureless clinical implantation using small outer-diameter needles.

issues during assembly, an integrated packaging method has
been developed.
By implementing micromachining
technology, a fully-packaged device comprising thefollowing components can be fabricated on a monolithic
substrate (Fig. 1): (1) A sensor platform to support the IOP
sensor; (2) Bottom posts with an optimized “anchoring foot”
design to accomplish suture-less anchoring of implant on iris[4]; (3) A perforated flexible parylene membrane to protect
the sensor from damage during implantation. With thesefeatures, a packaged device can be implanted on the iris sothat the sensor faithfully registers IOP variations, which isobservable from outside the eye through the cornea andprotective parylene membrane.
IOP sensor

Table 1. Designs of compliant structures for the IOP sensor.
Curved Tube

Protective membrane
Platform

Poutside < Pinside

Poutside > Pinside(Basic Bourdon Tube)

Poutside < Pinside

Spiral Tube

Top View

Bottom View
Anchoring feet

(Angular Deformation)
Poutside > Pinside

Long-Armed Tube
(Amplified Rotation)

Serpentine Tube(Linear Deformation)

As the main component of the IOP sensor, the thin-walled
tubes are designed to be flexible to facilitate pressure
responses with high sensitivity. Higher aspect-ratio tubes ina specific shape (e.g. elliptical or rectangular cross-section)
are required to obtain larger in-plane deformations andstronger out-of-plane resistance to environmental
interference. Parylene C (poly-para-xylene C) is selected as
the material for the tube because of its mechanical flexibility(Young’s modulus ~ 4 GPa), chemical inertness,
biocompatibility (USP Class VI grade), and conformality,
which enables fabrication of hollow tubes with a single
deposition. This material can be utilized to create flexible
and implantable biomedical devices.
2.2 Implantable Device
Because the IOP sensor is designed to be implanted inside
the eye for direct IOP measurement, packaging is necessary
to ensure that the IOP sensor is fully functional after
implantation. In order to avoid handling and alignment

Figure 1. Schematics of fully-packaged implantable device.
3 FABRICATION
The device was microfabricated on a monolithic substrate toachieve integrated device packaging.
The fabrication
process (Fig. 2) started with a 0.75 µm wet oxidation on adouble-side-polished silicon wafer. After patterning thebackside oxide, a backside two-step DRIE was performed todefine a 100-µm-thick sensor platform and 200-µm-high feetanchors. Fig. 3 shows micrographs of the fabricated feetstructures. After that, the frontside oxide was patterned,followed by a two-step complementary dry etching [5] andan 8-µm-thick parylene C deposition to create 180-µm-high,16.5-µm-wide thin-walled embedded channels with 5-µmthick parylene walls. Channels with an internal channelaspect-ratio of approximately 26 with quasi-rectangularsidewalls (sidewall angle ~ 90° ± 1°) were obtained forflexible mechanical responses.
During the parylene
deposition to form the channels, the backside structures weresimultaneously coated to promote their biocompatibility.
The tops of the embedded parylene channels were patternedby oxygen plasma etching to separate them and define thesensor tube features. Subsequent to frontside oxide removal,a sacrificial photoresist layer was spin-coated and the second
parylene layer was deposited and patterned to construct a 10µm-thick perforated membrane caging the channels with 20μm-high gap. Finally, the photoresist was stripped withacetone and the channels were released by an isotropic XeF2
gaseous silicon etch. Because the silicon surrounding thedevices was exposed, the entire fabricated microdevice wasseparated from the substrate after XeF2 etching without

dicing, while remaining intact due to protection fromparylene-filled trenches indicated in Fig. 2.

(a) Thermal oxidation
Backside oxide patterning
Backside two-step DRIE

The final implantable microdevices are shown in Fig. 5.
Although the spiral-tube sensors were too large to beimplanted using a needle, the long-armed-tube andserpentine-tube sensors were in such a form factor that they
could be accommodated inside a 19-gauge needle.

(d) Sacrificial PR coating and patterning
Parylene deposition and patterning(b) Frontside oxide patterning
Complementary HAR trench etching(e) Photoresist stripping
XeF2 silicon etching
Device release(c) Parylene deposition and patterning
Oxide removal

SiliconSiO2Parylene
Photoresist

Figure 2. Process flow for device fabrication.

19-gauge needle
Figure 5. Microfabricated implantable devices.
4 RESULTS AND DISCUSSION

200 µm(a)(b)
Figure 3. Backside anchoring foot structures: (a) Top view;(b) Side view (with 200 µm height).
Fig. 4 gives micrographs of the devices before the final XeF2
device release. Due to sufficient transparency of parylene C(low light absorption in the visible regime), the embedded
IOP sensors in Fig. 4(b) and Fig. 4(c) are clearly observablethrough the perforated protective membranes.

Three IOP sensors designs involving a large spiral tube (30turns, 1.2 mm x 1.2 mm size), a long-armed spiral tube (8turns, 0.4 mm x 0.4 mm size with 0.75 mm pointing tip), anda serpentine tube (15 folds, 0.35 mm x 2 mm size) weretested in vitro in water to characterize their performance. A
pressure regulation setup [3] was used to vary the pressurearound the sensors. The structural deformation of thesensors with respect to different positive pressures wasmeasured using a stereoscope and compared with numericalreadouts from a pressure gauge. Testing results in Fig. 6 andFig. 7 indicated 0.67 degree/mmHg angular sensitivity withthe spiral-tube design, 3.43 µm/mmHg lateral sensitivity
with the long-armed-tube design, and 0.38 µm/mmHg
longitudinal sensitivity with the serpentine-tube design, allshowing a continuous pressure response with high linearity.
These data further verified that different geometric layoutsof the tube result in different trajectories of tube deformation.

100
100μmμm(b)100 μm(c)100 μmFigure 4. Micrographs of fabricated devices (before XeF2
device release). Sensors in (a) and (b) have serpentine tubesand the sensor in (c) has long-armed tubes.

Observed Tip Rotation (degree)

(a)
120
100
8060
40
2000.00.5

1.0

1.5

2.0

2.5

3.03.5

Applied Pressure Difference (psi)Figure 6. Pressure response of the IOP sensor in the spiraltube configuration (angular trajectory) in water.

Observed Tip Movement (μm)

250
200
150

Long-armed tube
Serpentine tube

100
50
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Applied Pressure Difference (psi)Figure 7. Pressure response of the IOP sensor in the longarmed-tube configuration (lateral trajectory) and in theserpentine-tube configuration (longitudinal trajectory) inwater.
Fig. 8 shows a prototype device anchored to human skin.
The anchoring mechanism was confirmed to be robust evenunder vigorous shaking of the finger. This fact implies thatthe packaged microdevices can be secured on the iristhrough the use of such tissue anchors upon implantation.
Characterization of the devices in the intraocularenvironment is currently underway to characterize their invivo performance.

gauge needle for implantation. Testing results of the on-chip
IOP sensors have shown 0.67 degree/mmHg angularsensitivity with the spiral-tube design, 3.43 µm/mmHg
lateral sensitivity with the long-armed-tube design, and 0.38µm/mmHg longitudinal sensitivity with the serpentine-tube
design. These pressure responses indicate that detection of
IOP variations in the mmHg range is possible. A suite ofimplantation tests is currently underway to further
characterize the in vivo performance of the pressure sensors
in the intraocular environment. Using the merits of an allmechanical pressure response, the proposed sensing
paradigm featuring these implants can provide for direct,
faithful, and unpowered IOP monitoring in glaucoma
patients.
ACKNOWLEDGEMENTS
This work was supported in part by the Engineering
Research Centers Program of the National Science
Foundation under NSF Award Number EEC-0310723. The
authors especially thank Mr. Trevor Roper for his fabrication
assistance.
REFERENCES[1] M. Sherwood, in Ophthalmology, M. Yanoff and J. S.
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gravity
Figure 8. A device prototype with anchoring feet structuresattached to human skin.
5 CONCLUSION
An implantable unpowered parylene-based MEMS pressure
sensor has been successfully developed as part of a newparadigm for intraocular pressure (IOP) sensing. Variousgeometric layouts for micromachined high-aspect-ratio
flexible thin-walled tubes including a spiral-tube, a longarmed-tube, and a serpentine-tube design have been
implemented to realize pressure sensing with different
deformation trajectories. These devices are designed in sucha form factor that suture-less implantation through a needle
is feasible. Fabrication and packaging of the microdevices
have been successfully demonstrated. The microfabricatedimplantable devices could be accommodated inside a 19-[4] E. Meng, P.-J. Chen, D. Rodger, Y.-C. Tai, and M.
Humayun, "Implantable Parylene MEMS for Glaucoma
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Microtechnologies in Medicine and Biology (MMB2005), Oahu, HI, USA, May 12-15, 2005, pp. 116-119.
[5] P.-J. Chen, D. Rodger, and Y.-C. Tai, "Fully-Dry
Fabrication of Monolithic High-Aspect-Ratio Embedded
Parylene Microchannels," Technical Digest, The Ninth
International Conference on Miniaturized Systems for
Chemistry and Life Sciences (MicroTAS 2005), Boston,MA, USA, Oct 9-13, 2005, pp. 181-183.