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Removable Silicon Insertion Stiffeners for Neural Probes Using
Polyethylene Glycol As a Biodissolvable Adhesive
Sarah Felix, Member, IEEE, Kedar Shah, Diana George, Vanessa Tolosa, Angela Tooker, Heeral
Sheth, Terri Delima, Satinderpall PannuAbstract- Flexible polymer probes are expected to enable
extended interaction with neural tissue by minimizing damage
from micromotion and reducing inflammatory tissue response.
However, their flexibility prevents them from being easily
inserted into the tissue. This paper describes an approach for
temporarily attaching a silicon stiffener with biodissolvable
polyethylene glycol (PEG) so that the stiffener can be released
from the probe and extracted shortly after probe placement. A
novel stiffener design with wicking channels, along with flip-
chip technology, enable accurate alignment of the probe to the
stiffener, as well as uniform distribution of the PEG adhesive.
Insertion, extraction, and electrode function were tested in both
agarose gel and a rat brain. Several geometric and material
parameters were tested to minimize probe displacement during
stiffener extraction. We demonstrated average probe
displacement of 28 9 pm.
I. INTRODUCTION
Microelectrode neural interfaces are an essential tool in
neuroscience as well as emerging clinical applications such
as prosthetics. In particular, penetrating micro-electrode
probes enable stimulation and recording of neuronal activity
in the brain and spinal cord. While conventional micro-
electrode probes made of material such as silicon or tungsten
are adequate for acute studies, stimulation performance and
recording signals typically degrade over time [1]. Modeling
and experimental studies of the interaction between
microelectrode probes and neural tissue reveal several
mechanisms for this degradation, including micro-tearing
and immune response [2-4]. It has been widely concluded
that flexible probes that match more closely the bulk stiffness
properties of neural tissue can minimize relative
micromotion that can cause damage [3]. In addition, probes
with a smaller overall size reduce the extent of tissue
inflammation [4]. To achieve these characteristics of
flexibility and minimal size, biocompatible polymers such as
polyimide and parylene have been adopted as an ideal
substrate for microelectrode probes [5-7].
A major challenge with flexible polymer probes is that it
is difficult to insert them into the neural tissue. Flexible
microscale probes tend to buckle upon insertion [8].
*Research supported by National Institute of Deafness and Other
Communication Diseases.
S. Felix is with Lawrence Livermore National Laboratory, Livermore,
CA 94550 USA (phone: 925-423-4921; fax: 925-422-2373; e-mail:
felix5 @llnl.gov).
K. Shah, D. George. V. Tolosa, A. Tooker, H. Sheth, T. Delima, and S.
Pannu are with Lawrence Livermore National Laboratory, Center for
Micro- and Nano-Technology, Livermore, CA 94550 USA (e-mail:
shah22@llnl.gov, george27@llnl.gov, tolosal@llnl.gov, tookerl@llnl.gov,
sheth2 @llnl.gov, delimal @llnl.gov, pannul @llnl.gov).Researchers have attempted to develop methods to facilitate
insertion of flexible probes. Early studies reporting results
with polymer probes achieved implantation by permanently
integrating the probe with a stiff material such as silicon or
tungsten [9]. However, this negates the flexibility originally
intended for the polymer probe. More complicated designs
modify the polymer probe geometry to incorporate ribs or
layers of other materials such that the device has increased
stiffness in certain sections or axes, while maintaining
compliance in other parts [10, 11]. Yet another approach
integrates a 3-D channel into the polymer probe design that
is filled with biodegradable material [12]. This probe can be
temporarily stiffened, and after insertion the material in the
channel dissolves and drains out. All of the approaches just
mentioned permanently modify the geometry of the final
implanted device, compromising the desirable features of the
flexible planar probe.
Methods that do not alter the geometry or function of the
probe itself are more advantageous. One method to
accomplish this is to encapsulate the polymer device with
biodegradable material to temporarily stiffen the device
[8,13]. However, typical biodegradable materials have
Young's moduli orders of magnitude smaller than that of
silicon and would consequently require larger dimensions to
achieve the same stiffness. Also, coating the probe leads to
rounding of the tip, making insertion more difficult. One
promising method reported is to coat a stiffening shuttle with
a self-assembling monolayer (SAM) to customize the surface
interaction between the shuttle and the flexible probe [14].
When dry, the probe adheres to the coated shuttle
electrostatically. After insertion, water migrates onto the
hydrophilic surface, separating the probe from the shuttle so
that the shuttle can be extracted. While shuttle extraction
with reduced probe displacement was demonstrated (85pm),
this method has several limitations. With only electrostatic
interactions holding the probe to the shuttle, there is a risk of
probe slippage relative to the shuttle during insertion. For the
same reason, it is not possible to have any part of the
polymer probe overhanging the stiffener. This means that
alignment is critical unless the stiffener is oversized to
contain the footprint of the probe while tolerating the
significant misalignment that results from manual assembly.
Moreover, this limitation eliminates designs that could
minimize the cross-section of the stiffener by making its
footprint smaller than that of the probe.
We have developed a method in which the flexible
probe is attached to a stiffener with a temporary
biodissolvable adhesive material that securely holds the
probe during insertion. Once inserted into the tissue, the
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Felix, S. H.; Shah, K. G.; George, D. M.; Tolosa, V. M.; Tooker, A. C.; Sheth, H. J. et al. Removable silicon insertion stiffeners for neural probes using polyethylene glycol as a biodissolvable adhesive, article, March 28, 2012; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc828119/m1/3/: accessed May 9, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.