dopants [24]. Note that PSS is an insulator, hence most research efforts have been devoted

to minimizing PSS interference in the PEDOT interchain π-π interaction, whose accounts

for the overall PEDOT:PSS conductivity via holes polarons/bipolarons hopping. For this

reason, most PEDOT: PSS-based materials are prepared as composites with additives,

which permit to maximization of the formation of hydrophobic fibers and, consequently,

PEDOT:PSS conductivity. For instance, sodium dodecyl sulfate has been used to remove

the insulating PSS, an effect that enhances interconnectivity between PEDOT chains [25].

The versatility of PEDOT:PSS to be mixed with a variety of materials has been exploited for

a range of applications. For instance, Bonfiglio and collaborators have built up conductive

textile based on PEDOT:PSS electrodes that can be employed as a wearable electro­

cardiogram system [26]. Furthermore, PEDOT:PSS can be fabricated as a thin film, which

can be transferred onto elastic substrates, such as PDMS. This can preserve the high con­

ductivity of pristine PEDOT:PSS, while ensuring the mechanical stability offered by the

presence of the substrate. Moreover, to enhance the adhesive properties of PEDOT:PSS,

Gan et al. have synthesized nanosheets by blending them with polydopamine-reduced

sulfonated graphene oxide [27], which are redox-active due to the abundant catechol

groups and can be implanted for biosignals detection in vivo. Besides its good ion and

electron mobility for organic electrochemical transistors and all the above-mentioned

properties, PEDOT:PSS can be used effectively as a coating layer for metal electrodes, an

effect that reduces interface capacitance, enhances tissue integration, and increases surface

area. In this regard, it is useful reporting the review article by Green et al. on the use of

conductive polymers to modify metal electrodes for developing effective long-term im­

plants for neurostimulation [28].

4.3.4 Polythiophene

Polythiophene derivatives have been the work-horse organic semiconducting materials in

solar cells for at least two decades. The photophysical/chemical properties that have

determined PThs success as photovoltaic materials have also driven its broad application

in bioelectronics as a photo transducer for cells stimulation. For these peculiar reasons,

in this section, we mostly focus on these aspects. The coexistence of different stimulation

mechanisms in PThs and, in general, π-conjugated materials stems from the deactivation

pathways occurring upon photoexcitation to the excited states, namely: i) radiative and

non-radiative recombination of excitons, with the latter effect contributing to the release

of thermal energy; ii) exciton dissociation into free charges leading to the generation of

polarons. These long-lived species (µs up to ms) can contribute to either the build-up of

an electrical polarization at the abiotic/biotic interface, thus leading to a photocapacitive

effect, or electrochemical faradaic phenomena. For instance, nanoparticles consisting of

the prototypical organic photovoltaics polymer poly(3-hexylthiophene) (P3HT) have been

used as light transducers in eyeless animals, namely, Hydra vulgaris [29]. Provided that at

the light intensities used in this study the authors could rule out the involvement of any

thermal effect, the formation of a stable population of polarons with consequent electrical

effect appears to be the most probable scenario. In particular, polymer chains are closely

packed in nanoparticles, allowing interchain interactions that, in turn, can favor deloca­

lization and stabilization of charged species. Conversely, in P3HT thin films that offer a

much larger coupling surface, the stimulation mechanism has been also ascribed to a

convolution of thermal and electrical phenomena [30]. For instance, our group has re­

cently exploited such an effect to optically enhance the contraction rate of a human and

patient-specific cardiac in vitro cell model [31].

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Bioelectronics