biodegradability, which can be modified via the insertion of specific chemical groups in the

conjugated polymer backbone [11]. Furthermore, one can exploit weak intermolecular

forces to control their arrangement at the molecular level. This enables the formation of a

wealth of morphologies and structures from the same material (i.e. from single crystals to

semicrystalline films and nanoparticles) that can show different bioelectronic perfor­

mances. Finally, their softness in terms of intermolecular forces enables their processability

from solution and at a low temperature. This property not only permits to fabrication of

bioelectronic devices in a large variety of substrates with high conformability and low

stiffness but also maintains the functionality of cells and tissues during device fabrication.

In this context, it is worth reporting the work of Torsi and collaborators, showing that the

organic active material can be deposited on top of biological materials without damaging

them [12].

Second, the large free volume provided by the non-covalent forces allows for ion pe­

netration and transport in the organic material. In addition, hydrophilic organic films can

swell upon the incorporation of water molecules, an effect that enhances ion permeation.

The possibility to transport both electrons and ions is indeed one of the most important

properties of organic semiconductors since it implies that the entire bulk of the material,

and not just its surface, can participate in the interaction with the biological environment.

To be more specific, the fact that organic materials usually exhibit oxide-free interfaces

represents an important asset since this effect increases further the contact between the

material and the biological environment and, consequently, ion exchanges. Again, this

property stems from the fact that organic semiconductors do not expose dangling

covalent bonds at the interface that can react with oxygen, while inorganics do. This

appealing feature can be exploited to develop effective biosensors and bioactuators, such

as organic electronic ion pumps (OEIP) [13] and organic electrochemical transistors

(OECT) [14]. In particular, this latter device was introduced by Mark Wrighton and

collaborators in 1984 [15] and relies on the use of an organic semiconductor as an oxide-

free channel layer. In OECT, ions from the electrolyte penetrate the volume of the

polymer film and change its conductance. In 2013, Malliaras and coworkers have

exploited this device configuration to record brain activity in a rat model for epilepsy [16].

Finally, in organic semiconductors excitations are strongly coupled to their lattice. For

instance, inorganic semiconductors the addition/removal of charges does not affect the

lattice appreciably, while in organics this leads to lattice distortion, with the corre­

sponding quasi-particles, which deform and polarize the surrounding lattice, called po­

larons. This also means that ionic doping from electrolyte solution can cause large

dimensional changes in the organic material, implying that doping has a strong influence

across the abiotic/biotic interface. As an example, Malliaras and coworkers have shown

that the red-ox state of a conjugated polymer can influence the fibronectin conformation,

which is an important cell adhesion protein [17], as a result, that can enable the pre­

paration of substrates for cells culturing exhibiting a different degree of stickiness that can

be tuned by the external bias.

However, organic materials come with some disadvantages, which have been or are

being tackled currently by the concerted action of chemists, physicists, and materials

scientists. We believe that one of the most important issues is material stability in bio­

logical environments since organic semiconductors show usually degradation upon ex­

posure to water and oxygen. In addition, some conjugated polymers are water-soluble,

and therefore they must be stabilized to avoid deformation or delamination upon im­

mersion in physiological electrolytes. This is a crucial aspect since the materials must

ensure their electrical stability for many cycles of operation to secure accurate electrical

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Bioelectronics