Biosensing Detection - Bioinspired Approaches for Human-Centric Technologies - page 85

and only when stem cells were differentiated into neurons, it was possible to
measure electrical signals in the current following the stimulation.
Muccini et al. (Benfenati et al. 2013 ) demonstrated that a transparent organic
transistor based on an n-type organic semiconductor, namely, N ,
N 0 -ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13), is able to provide
both stimulation and recording of dorsal root ganglion primary neurons. Thanks
to the improved, efficient coupling between the semiconductor and the neuronal
cells, peculiar of the organic technology, it was possible to obtain a very good
signal-to-noise ratio, exceeding by 16 times that of standard microelectrode array
systems, without inducing any electroporation effect in the cell membrane. The
good properties shown by the transistor in multicell activity recording and stimu-
lation prompted the authors to be optimistic for the use of the device even in single
cell recording.
3.6 Biopolymer Interfaces for Cell Photostimulation
In all abovementioned applications, electrical and/or ionic conductivity of organic
semiconductors was exploited. Surprisingly, their most appealing properties,
namely, the absorption and emission of light in the visible spectrum, which
determined the flourish of organic optoelectronics with a plethora of devices
(light-emitting diodes, organic photodetectors, organic photovoltaic cells, light-
emitting transistors, light-emitting electrochemical cells), were not at all investi-
gated in bioorganic interfaces until very recently. Indeed, only in 2011, organic
semiconductors, working in a photodetector-like configuration, have been proposed
as photoactive materials for optical excitation of neural networks. The use of
organic photodetectors in medical and biological applications raises an important
issue in the design of bioorganic interfaces, since the liquid electrolyte challenges
the survival of organic device optoelectronic performances (capability of generat-
ing, transporting, and extracting charges), and degradation of the metal electrode is
greatly accelerated in a liquid environment. A solution to the latter problem is the
realization of devices in which the metal electrode is substituted by aqueous saline
solutions.
In these examples, the device structure comprises an anodic contact (usually ITO)
covered by the organic photosensitive layer and a saline electrolyte. It is clear that
the complex polymer-liquid interface plays a key role in this specific case, and a
number of studies aiming at characterizing such an interface have been reported
(Cramer et al. 2009 ; Eisenthal 1996 ; Svennersten et al. 2011 ). Several conjugated
polymers have been demonstrated to work as active layers in direct contact with
liquid electrolytes (Gautam et al. 2011 ; Antognazza et al. 2009 ; Lanzarini
et al. 2012 ; Gautam et al. 2010 ). Reported materials include regioregular poly
(3-hexylthiophene-2,5-diyl) (rr-P3HT); poly-[ N -90-heptadecanyl-2,7-carbazole-
alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole) (PCDTBT); P3OT, poly
(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT); MEH-PPV; poly[2-methoxy-5-