Electrochemical Detection - Waterborne Pathogens: Detection Methods and Applications - page 161

Composite materials can also be employed to fabricate flexible elec-
trodes for disposable biosensors by combining the advantageous proper-
ties of different materials. For example, titanium dioxide nanoparticles have
been associated on carbon paper with CNTs to enhance the biocompatible
properties of the latter. 101 Thus, different types of leukemia cells have been
electrochemically discriminated with a flexible biosensor. A 2D assembly
of gold nanoparticles has been integrated on a graphene sheet to further
increase electron transport. 102 With this system, hydrogen peroxide release
from living HepG2 cells has been monitored by conductometry. Other
composite materials have been patterned on rigid electrodes for the label-
free analysis of living cells: CNTs have been integrated in cellulose for leu-
kemia cells detection by impedancemetry, as an example. 103
6.8.3. Fabrication process
So far, integrated electrochemical biosensors have been mainly fabricated
on rigid substrates like silicon, or on optically transparent substrate as
glass or Pyrex. Much research now focuses on the use of flexible sub-
strates for electrochemical biosensors to integrate them into smart textiles,
to improve interfacial contact with tissue in-vivo, and mostly to reduce
biosensors fabrication cost. Interestingly, insulating polymers are fabri-
cated and processed at low cost. PDMS is extensively used to fabricate
whole or partially polymeric devices (often associated with glass, Pyrex,
or other rigid substrates). Nevertheless, PDMS has been criticized fre-
quently for its incompatibility with mass manufacturing, notably because
of its protein adsorption, its permeability causing liquid evaporation, its
natural hydrophobicity, and its aging. 104,105 Numerous other polymers
are being investigated currently for integrated biomedical sensors. Among
them, polyimide—a flexible dielectric with excellent thermal stability and
resistance to solvent, also employed for encapsulation purpose in micro-
electronics packaging 106 —has been often exploited. Indeed, it has been
used in-vivo for the implantations of microelectrodes for the intra 106 and
surficial 107 integrated monitoring of brain electrical activity by potentiom-
etry. Muscles and nerves action potentials measurements have been thus
also monitored in-vivo. 108 In-vitro extracellular cardiomyocyte recording
has been also carried out with an electrochemical biosensor made on a
polyimide substrate. 109 Parylene has been also used as flexible and dielec-
tric substrate to study neuron electrical activity with integrated devices,
in-vitro 110 and in-vivo 111 . COC is also an interesting polymeric substrate,
which has already been used for integrated electrochemical cell analysis. 84