We meet at 9:30 AM in AE 108 to hear Floyd give a talk titled "Signal
changes Over Time of Implanted Electrodes as an Indicator of the
Progression of the Foreign Body Response"

Introduction:

One of the principal challenges of the long-term implantation of
biosensors is that there are normal physiological responses of the
immune system which create a fibrotic capsule of scar tissue
surrounding the implanted sensor.  This tissue acts to separate the
device from the local environment which it is intended to sense, and
this separation causes a degradation of the signal.  We hypothesize
that this degradation in signal is itself an indicator of the
physiological responses and can be interpreted to track the
progressive stages of this physiological response to the implantation
of the foreign body.

Materials and Methods:

The micro-electrode arrays utilized as the biosensor are intended for
deep neural implantation.  These iridium-oxide electrodes have a
surface area of approximately 170-micron2 with a distance of
300-microns between nodes and are constructed on a silicon chip using
standard electronic microfabrication techniques. Some probes have a
thin coating of silicone polymer (pdms) and others are free of this
contamination.  We are applying the techniques of cyclic voltimetry
and impedance spectroscopy to track the signal changes over time.  We
have preformed experiments in vitro using a reservoir of phosphate
buffered saline with the controlled addition of selected proteins.  We
are now investigating biological encapsulation ex ova using the chick
chorioallantoic membrane model. This model has the advantage that
there are no challenges associated with a tethered small rodent or
other animal, but the common stages of the normal foreign body
response do occur.  Upon completion of these ex ova experiments, we
will demonstrate in vivo electrical signal changes by placing the
microprobe within the subcutaneous intramuscular fascia tissue of a
small rodent animal model in order to follow over time the changes in
complex electrical impedance, capacitance, current, and voltage versus
frequency.



Results and Discussion:

Analysis shows that we can easily differentiate which microelectrodes
had the thin silicone surface layer by the electrode dielectric
behavior being more like a 0.8-pf capacitor with about a 10 to 20
angstrom thick silicone layer between the pbs solution and the
312-micron2 active site.  This is within the expected range of
thickness measurement compared to our earlier time-of-flight secondary
ion mass spectroscopy investigation of the probe surface.

We performed trials with coating probes with collagen, egg-white and
fibrinogen followed by placing them in the reservoir.  In between
trials, probes were cleaned in mildly agitated SDS detergent, DI,
ethyl alcohol, and acetone.  All proteins were interpretable by the
EIS graphs having distinct different behaviors at 80 to 100-KHz from
uncoated.

We are currently performing the ex ova trials and report these results.

Conclusions / Summary:

There is a potential to develop a new type of useful biosensor as a
consequence of this project.  This will provide a tool for accessing
the biocompatibility of various coatings and surface treatments.  In
addition, current in vivo sensors such as blood glucose sensors have
the problem of signal degradation leading to sensor non-functionality.
This project may provide the means to address the problems common to
many in vivo sensors.