Neural probe insertion methods have a direct impact on the longevity of the device in the brain. Initial tissue and
vascular damage caused by the probe entering the brain triggers a chronic tissue response that is known to attenuate
neural recordings and ultimately encapsulate the probes. Smaller devices have been found to evoke reduced
inflammatory response. One way to record from undamaged neural networks may be to position the electrode sites away
from the probe. To investigate this approach, we are developing probes with controllably movable electrode projections,
which would move outside of the zone that is damaged by the insertion of the larger probe. The objective of this study
was to test the capability of conjugated polymer bilayer actuators to actuate neural electrode projections from a probe
shank into a transparent brain phantom.
Parylene neural probe devices, having five electrode projections with actuating segments and with varying widths (50 -
250 μm) and lengths (200 - 1000 μm) were fabricated. The electroactive polymer polypyrrole (PPy) was used to bend
or flatten the projections. The devices were inserted into the brain phantom using an electronic microdrive while
simultaneously activating the actuators. Deflections were quantified based on video images.
The electrode projections were successfully controlled to either remain flat or to actuate out-of-plane and into the brain
phantom during insertion. The projection width had a significant effect on their ability to deflect within the phantom,
with thinner probes deflecting but not the wider ones. Thus, small integrated conjugated polymer actuators may enable
multiple neuro-experiments and applications not possible before.
Most implantable chronic neural probes have fixed electrode sites on the shank of the probe. Neural probe shapes and
insertion methods have been shown to have considerable effects on the resulting chronic reactive tissue response that
encapsulates probes. We are developing probes with controllable articulated electrode projections, which are expected
to provoke less reactive tissue response due to the projections being minimally sized, as well as to permit a degree of
independence from the probe shank allowing the recording sites to "float" within the brain. The objective of this study
was to predict and analyze the force-generating capability of conducting polymer bilayer actuators under physiological
settings.
Custom parylene beams 21 μm thick, 1 cm long, and of varying widths (200 - 1000 μm) were coated with Cr/Au.
Electroplated weights were fabricated at the ends of the beams to apply known forces. Polypyrrole was
potentiostatically polymerized to varying thicknesses onto the Au at 0.5 V in a solution of 0.1 M pyrrole and 0.1 M
dodecylbenzenesulfonate (DBS). Using cyclic voltammetry, the bilayer beams were cycled in artificial cerebrospinal
fluid (aCSF) at 37 °C, as well as in aqueous NaDBS as a control. Digital images and video were analyzed to quantify the
deflections. The images and the cyclic voltammograms showed that divalent cations in the aCSF interfered with
polymer reduction.
By integrating polypyrrole-based conducting polymer actuators, we present a type novel neural probe. We demonstrate
that actuating PPy(DBS) under physiological settings is possible, and that the technique of microfabricating weights onto
the actuators is a useful tool for studying actuation forces.
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