4.2 Why Organic Materials are Suitable for Bioelectronics

Before starting our discussion on the most used organic materials for bioelectronics, a

useful exercise is to identify their peculiarities and rationalize whether they match with

the requirements for bioelectronic applications. In this case, we reckon that an instructive

comparison is between organic and inorganic materials. To highlight the most prominent

differences, we select conjugated polymers and silicon as archetypical examples for or­

ganic and inorganic materials, respectively (Figure 4.1). The first difference lies in the

connectivity, with silicon exhibiting a strong network of covalent bonds, in which each

silicon atom shares valence electrons with the other four neighboring atoms. On the other

hand, conjugated polymers and, in general, organic solids are made of different molecular

blocks that however interact with each other via relatively weak non-covalent van der

Waals forces. This implies that organic materials are “softer” and more disordered than

their inorganic counterparts, in terms of the crystalline arrangement, excitation landscape,

and charge transport. However, such a disordered and soft nature can be seen as an ad­

vantage, since intermolecular interactions and non-covalent bonding are the tools that

nature uses to make life possible (DNA double helix, proteins folding/unfolding, and so

on). This is the first feature that makes organic semiconductors more similar to biological

matter than inorganic systems and, alongside their analogous chemical composition with

cells and tissues, renders them inherently biocompatible. It is worth saying that bio­

compatibility cannot be universally defined, as the same material can elicit different tox­

icological responses depending on the type of cells and tissues under investigation.

Other bioelectronically relevant properties of organic semiconductors arise directly and

indirectly from their softness. First, organic semiconductors usually offer a facile chemical

modification, due to the power and versatility of chemical synthesis. Electronic, optical, and

mechanical properties of organic semiconductors can be usually tailored via direct altera­

tion of the π-conjugated backbone, and/or via modification of their side groups. In this

regard, this latter aspect is extremely useful with the view to tune organic semiconductors’

FIGURE 4.1

Simplified schematic of inorganic and organic semiconductors. We selected silicon and conjugated polymers as

archetypical examples of the two classes of materials. Note that the size of hydrated ions is meant to be equal in

both cartoons (for silicon and conjugated polymers), while they appear to be smaller in the organic schematic

due to the different length scales.

Materials for Organic Bioelectronics

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