DEEP BRAIN TARGETING STRATEGY FOR BARE PARYLENE NEURAL PROBEARRAYS
A. Weltman1, H. Xu2, K. Scholten1, T.W. Berger2, D. Song2, and E. Meng1*
University of Southern California, Viterbi School of Engineering, Los Angeles, California, USA
2
University of Southern California Center of Neural Engineering, Los Angeles, California, USA

1

ABSTRACT
We present a Parylene neural microprobe array and the first
demonstration of deep brain (> 5 mm) implantation of barepolymer probes, enabled by a novel micropackaging technique.
Targeting brain sites below the surface of the cortex with softpolymer probes typically requires bulky overcoats or insertion
shuttles, which greatly increase probe dimensions and tissue
damage, negating advantages of the flexible polymers. We
achieved deep brain implantation of bare polymer probes bytemporarily shortening their effective length using a biodegradable
brace, thereby achieving the required transient increase in stiffness(Figure 1). This surgical approach solves a key technical obstacle
preventing the widespread adoption of polymer microprobes.
Here, we present mechanical, histological, and electrochemical
evidence to support this approach.

𝐹𝑏𝑢𝑐𝑘𝑙𝑖𝑛𝑔 =𝜋 2 𝐸𝐼(𝑘𝐿)2

Figure 1: (a) Buckling force threshold of bare probes is lower
than that required for insertion, resulting in deformation. (b) PEG
brace reduces the effective length of probes, increasing thebuckling force threshold and enabling successful implantation.
Buckling force F is modeled by the equation above in which k is
the column effective length factor, E is the Young’s modulus, I is
the moment of inertia, and L is the probe’s length. (c) Exposed
probes are inserted to brain, PEG brace is dissolved in saline, andprobes inserted to desired depth (5.5 mm).

INTRODUCTION
Soft polymers offer an appealing alternative to silicon, glass,or metal materials for the creation of neural probes. Rigid probes
suffer inevitable signal degradation over time as chronic tissue
irritation leads to an immune cascade that eventually may wall offof the implant [1, 2]. Rigid probes have successfully achieved
high-quality recordings on the scale of a few months, but this falls
short of long-term goals desired to produce effective, life-longbrain machine interfaces [3, 4]. Whereas metals and silicon have
Young’s moduli on the order of hundreds of GPa, the stiffness ofbrain tissue is orders of magnitudes softer, at around 10-6 GPa.
This disparity between the stiffness of brain tissue and implantableneural probes can lead to tissue damage as micro-motion of thebrain causes chronic tissue irritation. It is speculated that the useof soft probes might mitigate this damage and attenuate theimmune response and the glial cell sheath that impact achievablesignal-to-noise ratios over time [5]. In one study, soft Parylene
probes were shown to induce only a 12-17% neuronal loss aroundthe implantation site compared to rigid silicon probes which
incurred 40% neuronal loss at 4 weeks post-implantation [6].
The reduced stiffness of polymers, however, presents atechnical challenge for surgical insertion into brain tissue.
Polymer probes must be temporarily stiffened in order to penetratebrain tissue and for accurate surgical placement, typically via
bulky biodegradable overcoats or insertion shuttles [1, 7, 8] which
can increase probe cross-section by 1.2-3,900x’s thereby
significantly adding to acute tissue injury [1, 9]. The use of
coatings or stiffeners can lead to other difficulties as well.
Coatings may dull probe tips [9], cover electrode recording sites
during surgery [7], or cause the probes to curl as the coating dries[10]. Retraction of a stiffener post-insertion may lead to the
movement of a probe away from its target site, interfering withproper probe placement [8]. The need for temporary stiffening ofprobes during implantation has limited development of polymer
probes to designs with short shanks (typically 1-2 mm), as shorterprobes have larger buckling force thresholds, and has constrained
recording sites to superficial cortical structures [1, 11, 12]. Our
strategy overcomes these issues, and opens the door for the large
scale acquisition of neural recordings from deep brain structures
such as the cornu ammonis (CA1, CA3) and dentate gyrus (DG) inthe hippocampus.

METHODS
Fabrication and design of Parylene neural probe array
Parylene microprobe arrays, consisting of a Parylene-metalParylene
sandwich,werefabricatedusing
standard
micromachining techniques. A 10 µm layer of chemical vapor
deposited (CVD) Parylene forms the base of each probe. Platinum(Pt) electrode recording sites, traces, and contact pads arelithographically patterned on top of the base layer using sputter
deposition at a thickness of 2,000 Å, followed by lift-off. The
metal layer is then encapsulated by a 10 µm thick vapor deposited
Parylene insulation layer. Electrodes and contact pads are exposedby deep reactive ion etching (DRIE) and the array outline is cut out
from the substrate by means of DRIE as well. Devices are
removed from the silicon carrier wafer by wetting them with waterand gently lifting them with tweezers.
The microprobe array comprises eight probes with 250 µm
center-to-center spacing (5.5 mm long, 20 µm thick, and 110-150µm wide shanks; Figure 2), each with eight platinum (Pt)electrodes (30 µm exposed diameter).
The electrodes areconnected to contact pads by 5 µm wide traces, with 5 µm spacing.
The array is designed to span a 2,000 µm anterior-posterior
distance along a rat’s hippocampus, with electrode groups
positioned for simultaneous recording from two of the CA1, DG,
or CA3 regions. Sham arrays lacking Pt electrode recording sites,
traces, and contact pads were used for mechanical, in vitro, in vivo,and histological testing.

allows the buckling force threshold of the probe to surpass theforce required to penetrate brain tissue. For our probes, modelled
as beams that are clamped on one end and pinned on the other (k =0.7) with a constant width of 150 µm and a Young’s modulus of
3.13 GPa (data previously collected by lab), the calculated
buckling force threshold of each probe is 0.21 mN and 0.83 mN
with and without a PEG brace respectively. End supports of theprobes were modelled as clamped by the PEG brace on one end
and pinned by friction with the agarose or brain on the other end.
𝐹𝑏𝑢𝑐𝑘𝑙𝑖𝑛𝑔 =Figure 2: (a) Schematic showing dimensions of neural probe. Top
group of four electrodes targets CA1 and the bottom group targets
CA3 in the hippocampus (b) SEM of array (c) close-up of probe
tips (d) micrograph close-up of fully fabricated array with Pt
electrodes and traces.
Micropackaging of probes for insertion
Prior to surgical insertion, the array is partially encapsulated
in a biodegradable polyethylene glycol (PEG) brace to halve theeffective length of the probes and increase probe resistance tobuckling (Figure 3). The PEG brace is created by sandwiching theprobe array in between a two-part polydimethylsiloxane (PDMS)
mold, which is then heated in a 50⁰ C oven for 90 seconds. Molten
PEG is injected into the cavity of the mold, and the mold is cooled
for five minutes before the PEG brace encapsulated array isreleased from the PDMS mold.
The buckling force threshold of an individual probe can bemechanically modelled as a beam, and is directly proportional to
the width (w) and thickness (t3) of the probe, and inversely
proportional to the probe length (L2), where E is the Young’s
modulus of the probe material. (Eq. 1) Halving the effective
length of the probe theoretically increases its buckling force
threshold by a factor of four, which, when designed appropriately,

Figure 3: Fabrication of PEG brace: (a) probe array and siliconholder placed in bottom mold, (b) top mold placed over array, (c)heated PEG injected into mold, and (d) holder and braced arrayreleased.

𝜋 2 𝐸𝑤𝑡 3
5.88𝐿2(1)
Experimental design
In initial testing, the buckling force was directly measured forsham devices with and without the PEG encapsulation. Devices
were driven into a metal plate using a computer controlled stepper
motor until they buckled, while load was measured with a 50 gload cell (Omega, LCFA-50g). Buckling force was measured forarrays of 8 probes in addition to single probes. Values werecompared to theoretical values calculated from beam theory andliterature values (~1 mN) of required force for in vivo neural probe
penetration [13].
In preparation for in vivo implantation, benchtop testing was
conducted using brain phantoms (0.5% agarose gel solution;
Figure 4) to simulate the consistency and density of brain tissue.
PEG encapsulated sham arrays were inserted into gel samples
using a stereotaxic stage and the following technique (Figure 1):
the exposed tips of the probes were slowly driven into the agarose
up to an approximate depth of 2.75 mm, at which point the PEG
brace was dissolved with saline, and the remainder of the probe
advanced to an ultimate depth of 5.5 mm. Probe array weremonitored during implantation for signs of buckling anddeformation.
In subsequent in vivo rat studies, sham arrays encapsulated in
the PEG brace were inserted into a male, Sprague-Dawley (SD) rat
weighing between 300 g to 400 g. Animal experiments werereviewed and approved by the Institutional Animal Care and Use
Committee (IACUC) as well as the Department of Animal
Resources of the University of Southern California (USC). During
surgery, the superficial cortex above the dorsal hippocampi was
exposed by removing a 2×4 mm piece of cranium from each
implantation site and carefully removing dura with forceps. Threescrew holes were drilled into the cranium to serve as a port foranchor screws. The animal was anesthetized throughout theprocedure, as confirmed by negative toe pinch withdrawal reflexes.
The neural probe array was then inserted using the techniquedescribed above at a speed of 10 µm/s, then the subject was
perfused with paraformaldehyde and sacrificed immediately post-

Figure 4: PEG brace forshortening effective length ofprobe, (a) top view and (b) sideview.

Figure 5: Braced array
inserted into 0.5% agarose
brain phantom :(a) front, (b)side, and (c) top view.

surgery. Histology was performed following implantation andhippocampal slices (50 µm thick) were Nissl stained and examinedby light-field microscopy to determine the acute damage incurredto neural tissue by implantation of the probe array.
Electrochemical testing of functional devices was performed
via cyclic voltammetry to confirm electrode impedance values
were sufficiently low for the resolvable acquisition of neuralrecordings.

RESULTSThe full array of eight probes, unbraced, buckled under a meanload of 1.26 ± 0.19 mN, and were observed to consistently buckle
during in vitro insertion tests with brain phantoms. The buckling
threshold of a single probe could not be accurately recorded within
the resolution of our load cell, but by extrapolating based on
further experimentation, we estimated a single shank will buckle
under a load of approximately 0.65 mN. As expected this is well
below the loading anticipated during insertion [13], and consistent
with the buckling witnessed during in vitro testing. Full arrays
supported with a PEG brace had an average buckling force
threshold of 4.24 ± 0.24 mN, and for single braced-shanks
buckling force threshold was measured directly as 2.14 ± 0.25 mN.
Figure 6 displays load measurements of repeated trials with asingle shank supported by a brace and Figure 7 displays analogous
data for full arrays with and without the brace.
Insertion of braced probes into 0.5% agarose brain phantoms
during in vitro insertion tests confirmed what was expectedfollowing mechanical buckling measurements—that temporarily
shortening shanks with the PEG encapsulation technique increased
the buckling force threshold of probes and allowed for successful
penetration of probes into agarose. During in vitro experiments
arrays were inserted as described above and PEG braces dissolved
within five minutes with repeat saline rinsing. Arrays wereinserted into agarose without bending or buckling and could beplaced accurately (Figure 5). The footprints made by the shanks inthe agarose (stained blue in Figure 5, panel C) were no larger thanthe cross-sectional dimensions of our probes. This is a significant
advantage compared to the enhanced cross-sectional insult to tissuecaused by bulky overcoats and stiffeners.
Following benchtop demonstration, braced arrays were
implanted into rat hippocampi. Braced arrays penetrated rat brain
without buckling. One braced array experienced PEG softening
upon exposure to ambient moisture causing the unsupported probes
to buckle upon insertion, as visible in Figure 7. Thus, we directly
observed that braced devices enable the successful penetration of
rat brain, since unbraced devices buckled during insertion attempts.
Successful in vivo placement of the probe array in deep
hippocampal layers was confirmed in Nissl stained histologicalslices (Figure 9). The stab wounds match probe cross-sectional
dimensions and minimal damage to surrounding tissue wasobserved. This is a significant advantage over current insertion
techniques which rely on bulky insertion aides add dramatically tothe physical footprint of the neural probe.
Electrical impedance spectroscopy measurements fromfunctional devices confirm low electrode impedance requisite forneural recordings (< 200 kΩ at 1 kHz) (Figure 8).

DISCUSSSION
Arrays encapsulated with a PEG brace were shown tosuccessfully insert into a 0.5% agarose brain phantom as well as
bilateral hippocampal sites in an anesthetized rat. This confirms
the feasibility of our technique of temporarily shortening neuralprobes to achieve successful penetration of probes with longer
shanks. Theoretically, this same strategy can be applied to probe

Figure 6: Load exerted on a single probe with PEG brace during
induced buckling, three trials. Average buckling force threshold is
2.14 ± 0.25 mN (standard deviation) which exceeds the 1 mN topenetrate brain tissue reported in literature.

Figure 7: Load exerted on an 8 probe array with (solid curves)
and without (dotted curves) PEG brace, three trials each. Arrays
without PEG brace had an average buckling force threshold of
1.26 ± 0.19 mN and buckled during in vivo insertion, arrays
supported with a PEG brace had an average buckling force
threshold of 4.24 ± 0.24 mN and inserted without buckling.
shanks of any length, using an incremental PEG dissolution
scheme if necessary to sequentially expose short probe segments.
Calculated and experimental buckling force measurements
differed quite dramatically for probes with and without PEG braceencapsulation. A single, braced probe was estimated to have abuckling force threshold of 0.83 mN, but according to mechanicaltesting, had a higher threshold of 2.14 mN ± 0.25. This
discrepancy is curious, especially since the probe wasapproximated as a beam of constant width, which would have led
to a higher estimate of the buckling force threshold. Choosing
alternate end conditions for the buckling model (for example,
modeling the beam as fixed on one end and free on the other end)
would also only further decrease our estimation of buckling force
threshold. Further study into this mismatch between expected andexperimental buckling forces are warranted.
While PEG braces were created effectively using the two-part
PDMS mold, heating of the PDMS in the oven sometimes resulted
in contraction of the two thin PDMS layers, bending the probes in
the mold. This difficulty was eliminated by lowering the PDMS
heating time to 30 seconds instead of 90 seconds in order toprevent distortion of the PDMS layers.

hybralign series 200 mask aligner was used for processing ofParylene devices.

REFERENCES

Figure 8: Electrode impedance spectroscopy (EIS) of Pt recordingsite. At 1 kHz, electrode has an impedance magnitude of 163 kΩ
and a phase of -65°.

Figure 9: Nissl stained transverse slices of sham array insertion
into left rat hippocampus and sacrificed immediately post-surgery.
Slice depth 3.82 mm.
Fully fabricated devices are being tested in insertion studies
with the biodegradable PEG brace technique. Complete arrayswith microfabricated metal electrodes, traces, and contact pads canexperience curling do to internal material stresses upon release of
the arrays from the silicon substrate. Such curvatures interfere
dramatically with PEG encapsulation and insertion trials, as curved
probes cannot be straightened during the PDMS mold, and enter
agarose models at an angle, thereby missing their target destinationand making precise positioning of electrodes in the hippocampal
layer difficult. Different strategies to straighten the curved
devices, including heat treatments and material etching to thin theprobes in the direction of curvature, are being evaluated.
In on-going work, electrodes will be cleaned via cyclic
voltammetry and checked with EIS to ensure intact traces prior to
implantation. Arrays packaged with external connections will thenbe implanted in rats and the quality, chronicity, and density ofelectrophysiological recordings from active rat hippocampi will be
evaluated and compared to their rigid, penetrating neural probe
counterparts.

ACKNOWLEDGEMENTS
This work was funded by the NSF under award number CBET1343193. The authors would like to thank Dr. D. Zhu and themembers of the USC Biomedical Microsystems Laboratory for
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CONTACT
*E.
Meng,

tel:

+1-123-740-6952;

ellis.meng@usc.edu