Recent advances in the field of neural prosthetics have demonstrated the thought control of a computer cursor. This capability relies primarily on electrode array surgically implanted into the brain as an acquisition source of neural activity. Various technologies have been developed for signal extraction; however most suffer from either fragile electrode shanks and bulky cables or inefficient use of surgical site areas. Here we present a design and initial testing results from high electrode density, silicon based arrays system with an integrated parylene  cable. The greatly reduced flexible rigidity of the parylene cable is believed to relief possible mechanical damages due to relative motion between a brain and its skull

Andersen, R.

Burdick, J.

Emken, J.

Huang, R.

Pang, C.

Tai, Y. C.

Ustun, C.

English

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Integrated parylene-cabled silicon probes for neural prosthetics

Caltech Authors - Main

INTEGRATED PARYLENE-CABLED SILICON PROBES FOR NEURAL 
PROSTHETICS 
R. Huang1, C. Pang1, Y.C. Tai1, J. Emken2, C. Ustun2, R. Andersen2 and J. Burdick3 
1Caltech Micromachining Group, California Institute of Technology, Pasadena, CA 
2Division of Biology, California Institute of Technology, Pasadena, CA 
3Department of Mechanical Engineering, California Institute of Technology, Pasadena, CA 
 
ABSTRACT 
 Recent advances in the field of neural prosthetics have 
demonstrated the thought control of a computer cursor.  
This capability relies primarily on electrode array surgically 
implanted into the brain as an acquisition source of neural 
activity.  Various technologies have been developed for 
signal extraction; however most suffer from either fragile 
electrode shanks and bulky cables or inefficient use of 
surgical site areas. Here we present a design and initial 
testing results from high electrode density, silicon based 
arrays system with an integrated parylene cable.  The 
greatly reduced flexible rigidity of the parylene cable is 
believed to relief possible mechanical damages due to 
relative motion between a brain and its skull. 
 
1. INTRODUCTION 
 Regardless of the particular technology used to 
accomplish neural prosthesis, an ideal wired multi-electrode 
array would consist of: (1) biocompatible materials, (2) high 
density electrodes with high signal-to-noise ratio (3) 
reliable, high density cables and a percutaneous interconnect 
to communicate with peripherals and (4) ability to 
accommodate integrated circuitries for on-probe signal 
amplification.  Current state technologies have yet to 
balance the trade-offs between these criteria.  A few have 
well controlled metal electrode fabrication and circuitry 
integration techniques but have designs that are brittle and 
difficult to handle.  On the other hand, some have reliable 
interconnect cables but are weak on IC expansion capability 
[1 - 3]    
 In this work, we present a parylene coated silicon probe 
that has the potential to overcome these difficulties. A major 
improvement of the current device over its predecessor [4] 
is the complete encapsulation of the array with parylene. We 
recall that the previous generation consisted of a silicon 
substrate that was exposed on all sides except the front side, 
significantly reducing the biocompatibility of the device. As 
before, we have chosen to use parylene C (poly-para-
xylylene C) due to its insulative capability, flexibility, 
biocompatibility, and its prior use in medical applications.  
 Furthermore, our flexible parylene-based cable has a 
cross section that is 500 µm wide and 15 µm thick and is 
designed to route the signals acquired with a 32 channel 
array. It is a major feature of our device that the cabling is 
not a component that is later attached to the array but that it 
is simply an integrated extension of the top parylene 
structure of the array itself. The resulting manufacturing 
simplification is expected to significantly improve the 
reliability of our device.  Finally, this monolithically 
micromachined cable – probe system is able to 
accommodate future chip integrations through the addition 
or modification of the silicon pieces on the silicon probe.  
As a result, we are able to fabricate silicon probe devices 
that are highly customizable, fully biocompatible, and easy 
to mass fabricate which will facilitate future research in 
developing a neural prosthetic system.  
 
2. DESIGN 
 A flexible parylene lift-off technology [5] allows us to 
fabricate 2-D 32 channel flexible cabled electrode array 
devices (Figure 1).  These devices can be expanded to 3-D 
32N channel structures by probe stacking of N number of 
2D probes.  Platinum electrodes on each of the eight silicon 
shanks can be spaced at configurable intervals and two 
reference electrodes are located on two of the longer shanks.  
The shank lengths range from 3 mm to 12 mm and are 150 
µm thick and 60 µm wide towards the tip of the shank. The 
tip of the shank forms a 10° angle to facilitate the insertion 
into the brain.  
 
 
Figure 1. SEM picture of the electrodes on the silicon shanks 
 
 Figure 2 - 4 show the unpackaged cabled silicon probes 
and their bonding interface.  The circular platinum rings 
arranged in a 60° Y shape pattern on the end of device are 
used to electrically bond to commercial available connectors 
with conductive epoxy on a circular PC board.  The 
parylene cables (7 cm long) are 15 µm thick and have 34 
traces lines of 10 µm wide and spaced 10 µm apart connect 
the distal electrodes directly to external connectors [6] 
(Figure 5). 
 
 
Figure 2. Released silicon probe with parylene flexible cable.  
978-1-4244-1793-3/08/$25.00 ©2008 IEEE MEMS 2008, Tucson, AZ, USA, January 13-17, 2008240
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 Figure 3. Proximal bonding of commercial connector with 
parylene cable connector pads. Left: before bonding. Right: 
after bonding with conduction epoxy. 
 
 
Figure 4. Bonding interface of the connector pads on the 
silicon probe device and the commercial connector through 
a custom PC board. 
 
Figure 5. Schematic of the packaged silicon probes.  The 
design shows a 96 channel device consisting of three 32 
channel devices.  The legs of the titanium pedestal are 
secured to the skull with bone screws. 
 
3. FABRICATION 
 The high density silicon probes are fabricated on double 
side polished wafers with DRIE (Deep Reactive Ion 
Etching) technology. Figure 6 shows the fabrication process 
[4].  First, 0.5 µm of parylene C is deposited on one side of 
the double side wafer and is melted in an oven at 350°C, 
which is then patterned by oxygen plasma to leave areas for 
adhesion enhancement between the parylene / silicon 
interface. A sacrificial photoresist layer of 1 µm thickness is 
then coated to facilitate the final release of the device.   A 
bottom layer of parylene C (6.5 µm) is then deposited. This 
is followed by a layer of lift-off electron-beam evaporated 
platinum (0.25 µm) to define the trace lines, the electrodes 
and connector pads. The top layer of parylene C (6.5 µm) is 
deposited to complete the parylene-metal-parylene sandwich 
structure (Figure 7).  Electrode sites and the device 
definition are then etched by a two step RIE with O2 Plasma 
process. Silicon probe shanks are subsequently defined and 
etched by DRIE from both sides of the wafer.  The bottom 
DRIE etching duration defines the thickness of the probe; 
by depositing and painting protective photoresist die by die, 
the thickness of every probe on the wafer can be controlled 
very precisely.  The devices are then released in photoresist 
stripper (Figure 8), and coated on all sides.  Finally, the pads 
corresponding to the electrodes are opened and the device is 
annealed at 200°C for two days to bond the sandwiching 
parylene layers.  
 The total coating of the probe and the cable was done 
by covering the connector pads with hand-painted 
photoresist and conformal coating 1 µm of parylene C over 
the entire device. The device is covered with photoresist 
again to protect the cables and the rest of the device, leaving 
the silicon probe section exposed.  This structure is then 
etched directionally with RIE to re-open the parylene coated 
electrodes (Figure 9). This device is later released in acetone 
and photoresist stripper solution.  The silicon probe was 
dipped into a silicon etchant after the RIE etching to test for 
total coating.  The silicon was not etched after 1 minute of 
dipping, which validates the coating is still intact and thus 
full biocompatibility is achieved. 
 
Figure 6. Fabrication process steps  
  
 In previous versions of the device, XeF2 was used to 
roughen the surface of silicon substrate to enhance the 
mechanical adhesion of the parylene / silicon interface.  
However, due to the isotropic nature of XeF2 etching of 
silicon, the depth, undercut and the shape of the resulting 
241
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surface becomes hard to control.  This may result in broken 
metal lines as well as thermal stress cracks on the border 
lines during the high temperature electron beam evaporation 
of platinum.  Thus, the melted parylene alternative was used 
to enhance the adhesion of the desired area.   
 
  
Figure 7. Process wafer after top layer parylene deposition 
 
 
Figure 8. a) Tapered 5 mm length silicon array after 
releasing from the wafer.  b) Platinum electrode pads under 
microscope 
 
 
Figure 9. a) SEM picture of electrode.  b) Sidewall etching 
after the final opening of the electrodes after total coating. 
 
4. EXPERIMENTS AND RESULT 
 Device testing was performed in three phases: in-vitro 
measurements in saline solution, in-vivo probe penetration 
testing and accelerated life testing.   
 In the first phase, we used the setup shown in Figure 10 
to conduct in-vitro impedance tests with channel electrodes 
having mean dimensions 22 µm x 22 µm. The measured 
distribution of areas of these electrodes was determined to 
lie within ±9.1 µm2 of the nominal value under SEM 
measurements. Impedance measurements of channel 
electrodes resulted in values of 670 kΩ ±33 kΩ at 1 kHz 
whereas the reference electrodes with areas approximately 
25,000 µm2 resulted in impedances in the range of 20 kΩ.  
To test the electrode’s ability to pick up signal different 
signal types, sine waves and pre-recorded action potentials 
were applied across a saline solution and were successfully 
recorded with the fabricated electrodes (Figures 11).  
Impedance tests were also performed on the parylene 
flexible cable to determine the stray capacitances.  Results 
show a clear linear relationship on a log-log scale in Figure 
12, which allows us to derive the impedance of the cross-
talk and the leakage to be approximately 27.4 MΩ and 11.7 
MΩ respectively at 1 kΩ. 
 
 
Figure 10. In vitro testing setup.  The metal box on the left 
hand side is a pre-amplifier.  The x-y stage in the Faraday 
cage maneuvers the probe into the saline solution. 
    
 
Figure 11. a) Sine wave applied across a saline solution 
recorded with the silicon probe. b) Threshold aligned 
waveforms. The horizontal axis is the number of samples 
taken at the rate of 20,000 samples per second for a 
duration of 1.2 ms. The vertical axis represents the signal 
strength corresponding to a sinusoid with amplitude (above 
baseline) of 400 µV.  
 
 Secondly, the devices were inserted through the pia and 
into the cortex of live rats to test penetration ability.  Results 
show a full insertion of the probe was successful without 
any bending, buckling or breakage by observing the shanks 
under a surgical microscope during insertion. 
 Finally, the devices underwent accelerated life testing 
in saline to determine the mean time to failure.  Failure 
modes include the failure of the silicon to parylene adhesion 
as well as the delamination between the parylene layers.  
Soaking test results show that the parylene-silicon adhesion 
is able to withstand more than five weeks in 90°C saline 
(Figure 13) before delamination occurs.  Furthermore, the 
parylene cable quality is uncompromised for the four weeks 
of soaking in the same environment with a cable only device 
(Figure 14). 
5mm 
300 mm a) b) 
a) b) 
a) b) 
242
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 Figure 12. Stray Capacitance with cross talk (relative to 
neighboring traces) and leakage capacitance (relative to 
saline).  Only the cable was immersed into the solution 
 
 
 
 
Figure 13. Delamination occurrence after five weeks of 
soaking in 90°C saline solution.  Probe was still attached on 
the parylene cable after eight weeks of soaking.   
 
 
Figure 14. Parylene cable has same line resistance after 
four week of soaking in 90°C saline. 
 
5. CONCLUSIONS 
 We have developed and performance tested a fully 
parylene C encapsulated, high electrode density silicon 
based array with an integrated flexible cable.  A 3-D 
packaging scheme with commercial connectors and a 
percutaneous pedestal is also presented.  Electrode 
impedances were characterized and pre-recorded action 
potentials were recorded through these devices. In vivo 
insertion capability of the silicon probes have also been 
verified.  High temperature accelerated life time testing in 
saline indicates the stability of both the cable and the probe-
cable interface for more than four weeks in 90°C saline. 
From the testing results, this high density silicon probe 
shows promise as a neural probe to be incorporated into a 
future chronic cortical prosthetic. 
 Future work is underway to optimize the electrode 
placement and probe shank geometry to accommodate rat 
anatomy for in-vivo chronic experiments.  
 
ACKNOWLEDGEMENTS 
 This work is supported by Defense Advanced Research 
Projects Agency (DARPA) (Award # 908165) and by 
National Institute of Health (NIH) moveable probe project 
(Award # R01EY015545). The authors also would like to 
thank Mr. Trevor Roper, Agnes Tong, other members of the 
Caltech Micromachining Laboratory and members of the 
Andersen lab at Caltech for their advice and help.  
 
REFERENCES 
 [1] K. Jones, P. Campbell, R. Normann, “A Glass / Silicon 
Composite Intracortical Electrode Array”, Ann Biomed 
Eng. 1992;20(4):423-37 
[2] K.D. Wise, D.J. Anderson, J.F. Hetke, D.R. Kipke, K. 
Najafi “Wireless Implantable Microsystems: High-
Density Electronic Interfaces to the Nervous System”, 
Proceedings of the IEEE, Vol. 92, No. 1, 2004.   
[3] S. Musallam, M.J. Bak, P.R. Troyk, R.A. Andersen, "A 
floating metal microelectrode array for chronic 
implantation." Journal of Neuroscience Methods, 160 
(2007) 122–127 
[4] C. Pang, S. Musallam, Y.C. Tai, J. W. Burdick, and R. 
A. Andersen, "Novel Monolithic Silicon Probes with 
Flexible Parylene Cables for Neural Prostheses," IEEE-
EMBS MMB, Okinawa, Japan, May 9-12, 2006. 
[5] W. Li, D.C. Rodger, J.D. Weiland, M.S. Humayun, Y.C. 
Tai, “Integrated Flexible Ocular Coil for Power and Data 
Transfer in Retinal Prostheses”, Proc. 27th Eng. in Med. 
and Biol. Soc.,2005  
[6] D. Rizzuto, S. Musallam, C. Ustun, J. Emken, C. Pang, 
R. Huang, Y.C. Tai, R. Andersen, The Caltech Neural 
Prosthetic: Restoring function to paralyzed patients. 
Society for Neuroscience, 2006  
 
Delamination  
Delamination  
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Recent advances in the field of neural prosthetics have demonstrated the thought control of a computer cursor. This capability relies primarily on electrode array surgically implanted into the brain as an acquisition source of neural activity. Various technologies have been developed for signal extraction; however most suffer from either fragile electrode shanks and bulky cables or inefficient use of surgical site areas. Here we present a design and initial testing results from high electrode density, silicon based arrays system with an integrated parylene cable. The greatly reduced flexible rigidity of the parylene cable is believed to relief possible mechanical damages due to relative motion between a brain and its skull. 1

R. Huang

C. Pang

Y. C. Tai

J. Emken

C. Ustun

R. Andersen

J. Burdick

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A floating metal microelectrode array for chronic implantation.&quot;

Integrated Flexible Ocular Coil for Power and Data Transfer

Najafi “Wireless Implantable Microsystems: HighDensity Electronic Interfaces to the Nervous System”,

Novel Monolithic Silicon Probes with Flexible Parylene Cables for Neural Prostheses,&quot;

The Caltech Neural Prosthetic: Restoring function to paralyzed patients. Society for Neuroscience,

http://vis.caltech.edu/Papers/PDFs+of+journal+articles/conferences/MEMS2008.pdf

Integrated parylene-cabled silicon probes for neural prosthetics

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