Biosensing Detection - Bioinspired Approaches for Human-Centric Technologies - page 83

OECTs have raised interest in the community first of all as biosensors (Lin and
Yan 2012 ; Lin et al. 2010 ), being employed, for instance, in DNA (Yan et al. 2009 ),
enzymes (Zhu et al. 2004 ), and cell attachment sensing (Bolin et al. 2009 ). More
recently, R. Owens and co-workers (Jimison et al. 2012 ) have demonstrated that it
is possible to use an OECT structure in order to detect, in situ, minute disruptions in
barrier tissue functions. In particular, they reported unprecedented temporal reso-
lution and sensitivity in measuring variations of paracellular ionic fluxes induced by
toxic compounds. New applications of organic semiconductors in toxicology, drug
development, and disease diagnostics are therefore expected in the next future.
The possibility to realize OECT arrays able to direct interfacing with liquid
electrolytes by lithographic processes was first demonstrated in 2011 (Khodagholy
et al. 2011b ). The transistors operated at low voltages and showed a response time
in the order of 100

s, thus compatible with biological processes recording. Highly
conformable OECT arrays based on PEDOT:PSS were then used in vivo, in
electrocorticography experiments for recording epileptiform discharges
(Khodagholy et al. 2013b ). Impressively, flexible transistor arrays were positively
compared with surface electrodes, showing superior signal-to-noise ratios, and even
with conventional penetrating electrodes. The observed differences rely on the key
difference between the transistor arrays and the electrode arrays: in fact, the OECT
locally amplifies the signal; conversely, in conventional electrode recordings the
signal can be preamplified only outside the head of the animal, thus amplifying the
noise generated by leads and interconnections as well.
Another interesting device capable of in vivo operation has been reported by
Berggren and his collaborators (Larsson et al. 2013 ). Called the organic electronic
ion pump (OEIP), it is essentially a variation on theme of the OECT principle
(Fig. 3.1c ). In the OEIP device structure, two PEDOT:PSS electrodes are patterned
on a plastic substrate and are connected by a channel. The channel is made by over-
oxidized PEDOT:PSS, and, while electronically it is an insulator, it preserves the
capability of conducting ions. The source electrolyte contains the positive ions to be
delivered into the target electrolyte; oxidation of the source electrode (anode) forces
ions to enter the anode itself from the source electrolyte. Since the channel is
electronically insulating but allows ionic conductivity, ions will be pumped toward
the cathode and finally delivered to the target electrolyte. Since ionic charges
transported through the channel are fully or partially compensated by electronic
charges flowing between the two electrodes, the current measured in the external
circuit is directly proportional to the delivery rate of cations in the target electrolyte.
It is important to note that, at variance with systems based on electrical stimulation,
in the OEIP electrical signals are translated to delivery of specific chemical
messengers, allowing to specifically target only the neurons expressing the cognate
receptors. Interestingly, it is possible to exploit the OEIP concept not only for
transporting metal ions but even for delivery of biomolecules, relevant for neuronal
cell signaling. Some notable examples reported in in vitro studies include acetyl-
choline, aspartate, GABA, and glutamate (Isaksson et al. 2007 ). OEIPs were also
realized in flexible geometries, suitable for surgical implantation. The controlled
delivery of neurotransmitters was then assessed in the cochlear system of guinea

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