Biomedical Engineering Reference
In-Depth Information
when the pacemaker was introduced. Since there is no interaction between the
pacemaker and the soft tissue, other than the need for a precise electrode site,
biocompatibility issues are not as critical compared with other implantable
devices.
1
The cochlear implant is a successful neuroprosthetic device. It is
routinely used in cases where the auditory neurons remain intact, e.g. when
hair cells are lost and the implant directly stimulates the auditory neurons. For
this application, microfabrication techniques generate electrode arrays (12-22
electrodes) using biocompatible materials with micron-size pores where
ingrown tissue develops, naturally fixing the prosthesis in place. Problems with
the implant arise approximately three weeks after surgery when fibrous tissue
grows to a thickness of about 400 mm, consequently increasing the local
electrical impedance. The impedance must be as low as possible so as not to
impede the transfer of the functional signal.
2
After three weeks, the adsorption
of non-specific tissue around the implant intensifies until vascular tissue with
well-developed capillaries envelopes the implant surface; nevertheless, the
prosthesis remains functional for a very long time.
3,4
The interaction between electrodes and brain tissue is critical and determines
the functional performance of the electrode. The electrode material should not
be toxic, it should be stable and should have superior electric properties. Metal
and silicon electrodes are commonly used for their electrical properties, but
their mechanical properties create a high strain field at the interface, which
maintains the inflammatory response of the tissue. Softer electrodes, made from
polyimide for example, pose insertion diculties since they lack the stiffness
necessary to penetrate the outer layers and buckling will only extend the trauma
to the tissue. Self-dissolving rigid coatings may be a solution for this problem
The size of the electrodes should be as small as possible to minimize tissue
damage, but the electrode impedance is inversely proportional to the surface
area and sometimes a larger area of neurons has to be stimulated or sampled,
so there is a design constraint regarding the dimension of the inserted
electrodes.
As we have implied previously, consideration of the surface properties
of devices for implantation is crucial. Surface modification using the extra-
cellular matrix (see Chapter 2) similar to that present in neurons—fibronectin,
collagen, laminin or peptide fragments from proteins such as RGD
(Arg-Gly-Asp) IKVAV (Val-Ala-Val) or YIGSR (Tyr-Ile-Gly-Ser-Arg)—
significantly improves cell adhesion, morphology and differentiation compared
with untreated surfaces. Surface topographical features such as grooves, ridges,
pillars and holes are much appreciated by the neuron cells, for reasons that are
still unclear.
Implanted electrodes are usually unable to sense consistent neuronal signals
for more than a few months. The loss of sensitivity is largely associated with the
continuous electrochemical processes taking place at the interface. Detected or
applied currents can harm living cells through such electrochemical reactions
occurring at the electrode interface. These processes further lead to protein
attachment and resultant tissue build-up on the electrode surface that masks
the input signals from neurons, either through an increase in electrical
d
n
4
t
3
n
g
|
2
n
3
.
