3C1.4

HIGH-DENSITY FLEXIBLE PARYLENE-BASED MULTIELECTRODE ARRAYS FORRETINAL AND SPINAL CORD STIMULATION
D.C. Rodger1,2, A.J. Fong1, W. Li1, H. Ameri2, I. Lavrov3, H. Zhong3, S. Saati2, P. Menon1,E. Meng2, J.W. Burdick1, R.R. Roy3, V.R. Edgerton3, J.D. Weiland2, M.S. Humayun2, and Y.C. Tai1
1
California Institute of Technology, Pasadena, CA, USA(Tel: +1-626-395-8477; E-mail: dcrodger@mems.caltech.edu)
2
University of Southern California, Los Angeles, CA, USA
3
University of California, Los Angeles, Los Angeles, CA, USA

TRANSDUCERS & EUROSENSORS ’07

The 14th International Conference on Solid-State Sensors, Actuators and Microsystems, Lyon, France, June 10-14, 2007

Abstract: Novel flexible parylene-based high-density electrode arrays have been developed forfunctional electrical stimulation in retinal and spinal cord applications. These electrode arrays aremicrofabricated according to single-metal-layer and, most recently, dual-metal-layer processes. A newheat-molding process has been implemented to conform electrode arrays to approximate the curvature ofcanine retinas, and chronic implantation studies have been undertaken to study the mechanical effects ofparylene-based prostheses on the retina, with excellent results to date. Electrode arrays have also been
implanted and tested on the spinal cords of murine models, with the ultimate goal of facilitation of
locomotion after spinal cord injury; these arrays provide a higher density and better spatial control ofstimulation and recording than is typically possible using traditional fine-wire electrodes. Spinal cord
stimulation typically elicited three muscle responses, an early (direct), a middle (monosynaptic), and alate (polysynaptic) response, classified based on latency after stimulation. Stimulation at different
rostrocaudal levels of the cord yielded markedly different muscle responses, highlighting the need forsuch high-density arrays.
Keywords: BioMEMS, Neural prosthesis, Parylene C, Retinal prosthesis.
electrode array and a high-lead-count cable toallow for high-resolution vision. We present thefirst flexible parylene-based multielectrode arrays(MEAs) designed for functional electricalstimulation (FES) in retinal prostheses, and theextension of this technology to enable stimulationof central nervous system structures after spinal
cord injury. In addition, we present chronic
biomechanical stability and acute neuralstimulation results. We address the importance ofa novel dual-metal-layer fabrication methodologythat enables complex electrode arrangements
while alleviating the traditional problems ofelectrode crowding due to wire-routing. The
advantages of the use of parylene C as thestructural material for such neuroprostheses, whencompared with technologies based on the use ofother materials such as polyimide [2] and silicon[3], include parylene’s pinhole-free conformality,
its low water permeability, its United States
Pharmacopoeia (USP) Class VI biocompatibility,
and its high flexibility and mechanical

1. INTRODUCTION
Prototype low-density retinal prostheses haveshown great promise, enabling subjects blind fromsuch diseases as retinitis pigmentosa and agerelated macular degeneration to perceive visual
data [1]. The next-generation retinal prosthesis(Fig. 1), however, requires a high-density flexible

Fig. 1 Placement and components of nextgeneration retinal prosthesis.

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3C1.4TRANSDUCERS & EUROSENSORS ’07

The 14th International Conference on Solid-State Sensors, Actuators and Microsystems, Lyon, France, June 10-14, 2007

Fig. 2 Fabrication process for dual-metal-layer electrode arrays.
strength (Young’s modulus ~4 GPa).

days at 200°C in a vacuum oven with nitrogen
backfill to optimize parylene-parylene adhesion.
In order to maintain the planarity of the spinal
cord arrays, they are clamped between two flatpieces of Teflon or glass slides coated withaluminum foil. The retinal arrays, on the otherhand, are shaped using a custom 6061 aluminum
mold comprising a recessed concave region and amating stainless steel sphere that approximates thecurvature of the retina.

2. MICROFABRICATION
Single-metal-layerelectrodearrays
arefabricated as has been previously described [4].
Dual-layer electrode arrays are fabricated asshown in Fig. 2. Approximately 8 µm of parylene
C is first deposited on a silicon wafer with an
optional photoresist sacrificial layer, forming theunderside of the electrode array. A platinum ortitanium/platinum metal liftoff process is used todefine traces with 16 µm pitch and 200 nm to 300
nm thickness. A second parylene deposition (~1µm) forms the insulation between the two metallayers. At this point, 6 µm × 6 µm vias arepatterned in the insulation layer over the ends ofthe traces using an oxygen plasma reactive-ionetch. A second step-coverage optimized liftoff
process is used to define a second metal layer
comprising electrodes and traces, while at thesame time achieving electrical continuity between
the underlying traces and the overlying electrodes.
A final parylene coating approximately 7 µm thickforms the top insulation. The electrodes areexposed and the overall geometry of the implant isdefined in a final set of reactive-ion etches using athick photoresist etch mask. Finally, the arrays arepeeled from the wafer in a water bath or released
through removal of the sacrificial photoresist inacetone.
The implants are typically annealed for two

3. RESULTS
3.1 Fabricated devices
Retinal arrays consisting of 1024 75-µmdiameter electrodes arranged in a complex
biomimetic pattern that closely mimics the densityof ganglion cells in the human retina [5] werefabricated according to the dual-layer process(Fig. 3a), with 60 of the electrodes connected via
two traces each to facilitate electrical conductivity
verification. The strength of metal adhesion wasverified using a Scotch tape test. Electrical testing
demonstrated
atypical
impedanceofapproximately 5 kȍ, which includes two 8-µmwide traces of 20 mm length, as well as two via
step junctions (vias are shown in Fig. 3b, ascanning electron micrograph of a single
electrode). Each via has an impedance of less than
12.5 ȍ. These arrays were successfully molded tothe approximate curvature of the canine retina(Fig. 3c), and sterilized using ethylene oxide gas

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3C1.4prior to surgical implantation.
Spinal cord arrays, consisting of five or ten
electrodes of 250 µm diameter (Fig. 4), werefabricated using the single-layer process.
Interelectrode spacing was controlled so that eacharray covered four to five segments of thelumbosacral spinal cord.

TRANSDUCERS & EUROSENSORS ’07

The 14th International Conference on Solid-State Sensors, Actuators and Microsystems, Lyon, France, June 10-14, 2007

3.2 Retinal chronic implantation
Our biomimetic arrays were implanted in theright eye of two canines through a 5-mm pars
plana incision and were affixed to the retina using
a retinal tack modified by the addition of a PDMS
washer to account for the thin nature of theparylene-based arrays. They have remained
chronically implanted for five and four months,
respectively, and fluorescein angiography (FA)and optical coherence tomography (OCT) havedemonstrated that the arrays are less than 50 µm
away from the ganglion cell layer at the center andonly slightly more raised at the edges, with normal
retinal blood vessel filling underneath (Fig. 5).

Fig. 4 Flexible electrode array for spinal cord
stimulation and recording.
mice. The electrodes were oriented linearly along
the rostrocaudal extent of the cord.
To test the capability of the electrode array toact as a multi-channel stimulating device forgenerating hindlimb movements, constant current
monophasic stimulus pulses (amplitude: 50850 µA, frequency: 0.3-10 Hz, pulse duration:
0.5 ms) were applied to the spinal cord via each of
the array electrodes, while muscle activity wasmonitored
usingelectromyogram(EMG)
recordings of the tibialis anterior and medial
gastrocnemius muscles. Stimulation generated atypical three-component EMG action potentialconsisting of an early (direct motor), a middle

3.3 Spinal cord electrical stimulation
Under isoflurane anesthesia, the spinal cord
electrode arrays were implanted epidurally on
spinal cord segments L2-S1 in nontransecteda)b)c)Fig. 3 a) Fabricated biomimetic electrode arraywith 60 of 1024 75-µm-diameter electrodesconnected through dual-layer process; b)scanning electron micrograph of electrodehighlighting vias to underlying traces; c) heatmolded and annealed retinal electrode array.

Fig. 5 a) Fundus photograph showing arraytacked to the retina; b) FA showing normal bloodflow under array and no leakage at the perimeter(no signs of retinal trauma); c) OCT showing bareretina at left and array in close apposition atright.

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TRANSDUCERS & EUROSENSORS ’07

The 14th International Conference on Solid-State Sensors, Actuators and Microsystems, Lyon, France, June 10-14, 2007

(monosynaptic), and a late (polysynaptic)
response, classified by post-stimulus latency (Fig.
6). The appearance and magnitude of each of theseresponses was correlated with the choice ofelectrode position and stimulation parameters(Fig. 7).
Recording capability was assessed by using theelectrode array to record spinal cord potentials
evoked by tibial nerve stimulation. At different
spinal cord levels, two or three distinct responses
were observed. These findings closely mirror
results reported previously in a study using
conventional spinal cord recording electrodes [6].

Fig. 7 Medial gastrocnemius EMG showing
varying levels of activation caused by stimulation
at different rostrocaudally located electrode sites.
5. ACKNOWLEDGMENTS

4. CONCLUSION

This work was supported in part by theEngineering Research Centers Program of theNational Science Foundation under NSF Award
Number EEC-0310723, and by a fellowship fromthe Whitaker Foundation (D.R.). The authors
especially wish to thank Mr. Christian Gutierrezand Ms. Ronalee Lo for their help with thefabrication of the PDMS washers used with theretinal tacks, Mr. Andrew Pullin for his help withfabrication of the heat-forming mold, and Mr.
Trevor Roper for his always valuable assistance.

These revolutionary parylene-based platinum
multielectrode arrays have demonstrated theability to stimulate and record from neural tissue,and have shown excellent biostability whenchronically implanted in contact with canine
retinas. The dual-metal-layer process enables
increased electrode density while obviating manyof the issues typically associated with single-layer
arrays, such as those of electrode crowding due toincreased electrode density and wire-routing. It isa simple matter to extend this process to increase
the number of metal and parylene insulation
layers. Future work includes the stimulation ofretinal tissue using parylene-based arrays,
development of two-dimensional arrays forbilateral spinal cord stimulation, and thefabrication of a completely integrated wirelessstimulation system in a parylene substrate.

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Fig. 6 Typical medial gastrocnemius (leg) EMG
recording showing early, middle, and late
responses after stimulation of spinal cordsegments with parylene multielectrode array.

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