adjust electronic devices to biological systems, increasing their compatibility and effec­

tiveness [12]. Besides, organic materials can be chemically modified and their manu­

facturing processes adapted to obtain the physical characteristics according to the

application, therefore the organic bioelectronics material can have different displays as

coatings, films, hydrogels, nanoparticles, etc. Another of their main advantages is that

organic bioelectronics materials can be in direct contact with biological environments

without suffering degradation or oxidation, which keeps the interface free of con­

taminants and extends the useful life of the devices [2]. Furthermore, unlike other systems

in these, the transfer of charges not only occurs on the surface but also involves a three-

dimensional character, which is given by the interaction of the charges with the polymeric

network that can also swell facilitating the electronic transport. These characteristics

make them tempting materials for the fabrication of biosensors and bioactuators.

Organic bioelectronics materials need the ability to transport charges, whereby molecules

or polymers with high conjugation are generally used, which allow the mobility of charges

through their electronic cloud formed by delocalized π electrons. Furthermore, these

structures may increase their conductivity by mixing with agents that oxidized or reduced

the conjugate bonds or doping them with p- or n-type conductors. Based on the density of

transported charge and the material morphology, electronic organic materials can classify

as semiconductors or conductors [13]. Polymers are particularly outstanding since allowing

migration not only of electronics but also of ions due to their porosity and high flexibility, in

addition, polymers might have charged groups in their structure, the so-called polyelec­

trolytes, which allows them to act as a transport channel of ions and compensate coun­

terions that migrate under the action of an electric field. Polyelectrolytes can be polyanions

or polycations being selective in terms of the charge of the transported ion; these char­

acteristics make them useful in the manufacture of electrochemical membrane devices,

electrochemical cells, organic electronic ion pumps, and ion bipolar transistors [14].

The mixed conducting polymers stand out within the organic bioelectronics materials

because of can transport both electrons and ions. The former are transported by dis­

placements in the delocalized electrons cloud, and the latter by diffusion between the

polymer chains, which is improved by the swelling of the material. To exploit the full

potential of this type of material, a strong ionic-electronic coupling so that the currents

mutually induce each other is necessary; this behavior might be achieved by a redox

process of electrochemical doping, where the mobile electronic charges are stabilized by

the ions; organic bioelectronics materials are regularly used for the manufacture of or­

ganic electrochemical transistors that can translate amplified neuronal signals [15]. One of

the common forms of polymers in bioelectronics devices is the hydrogels, which are

three-dimensional networks of hydrophilic polymers that can swell in water and hold a

large amount of water while maintaining the structure due to chemical or physical cross-

linking of polymeric chains. This affinity with the aqueous systems and their excellent

mechanical properties (such as rigidity, torsional vibration, and hardness) together with

their conductivity properties make conductive hydrogels an excellent alternative for

implantable bioelectronics and tissue engineering [16].

In contrast to inorganic bioelectronics materials, organic bioelectronics polymers have a

lower overshoot that allows safe electrical stimulation of tissues. In addition, when these

polymers are used together with electrical responsive materials can work as drug carriers or

influence cell functions [17]. Nowadays, the most researched polymers in bioelectronics are

poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT: PSS), polypyrroles

(PPy), and polyanilines (PANI). Figure 2.2 show the molecular structures of these polymers.

PEDOT: PSS is a mixed conducting copolymer where PEDOT is responsible for electronic

Materials and Their Classifications

23