metal electrode coating, usually leading to better charge injection, higher surface area,

and lower electrode impedance than the separate constituents. For example, gold-doped

graphene has been employed as an electrode in a wearable patch for diabetes monitoring

and therapy [53]. Such a combination showed improved electrochemical properties than

graphene, as well as stable operation under various mechanical deformations. Graphene

has been also widely employed as bioelectronic material for building up devices. For

instance, Masvidal-Codina fabricated a graphene solution-gated field-effect transistor

that could map out ultraslow (< 0.1 Hz) cortical brain activity that is typical of neuro­

logical diseases [54]. In addition, graphene 3D foams can be used as conductive platforms

for neuron electrostimulation. In this regard, one relevant work is reported by Liu et al.

where high-density micro-electrode arrays of 3D porous graphene were employed for

efficient cortical neuromodulation and sensing with minimum invasiveness (Figure 4.7).

The excellent broadband optical transparency of graphene (> 90%) has enabled a range of

applications, such as simultaneous optical imaging, optogenetic stimulation, and elec­

trophysiology [20]. For instance, Duan and collaborators reported the fabrication of soft

graphene contact lens electrodes (GRACEs) with broad-spectrum optical transparency,

and their application in conformal, full-cornea recording of electroretinography (ERG)

from cynomolgus monkeys [55]. The authors found that the GRACEs give higher signal

amplitude than conventional ERG electrodes in recordings of various full-field ERG

responses, as well as high-quality topographic mapping of multifocal ERG under si­

multaneous fundus monitoring.

Furthermore, graphene displays excellent biocompatibility. In particular, it has been

reported that graphene can support neural growth without other biocompatible mate­

rials. Moreover, its biocompatibility and adhesion to cells and tissues can be further

enhanced via material post-processing, such as oxygen plasma treatment or functiona­

lization with poly-L-lysine [20].

4.5.2 Graphene Micro/Nanostructures

Graphene micro/nanostructures can be precisely synthesized for size and density, and

hold promise as an approach for next-generation bioelectronic materials [57]. The most

important advantage connected to the use of micro/nanostructure lies in the possibility to

implement complex device interfaces and establishing close contact with biological sys­

tems. One of the most relevant examples is represented by graphene flakes. For example,

Cohen-Karni et al. recorded extracellular field potentials from spontaneously beating

embryonic chicken cardiomyocytes using a graphene flakes-based field-effect transistor

(FET) [58]. The functioning rationale lies in the fact that extracellular field potentials

generated during electrical activity induce a change in the conductance of the FET

FIGURE 4.7

Schematic of the electrode array placed on top of the cortical surface during the recording. Adapted with

permission [ 56]. Copyright (2018) Springer Nature. Distributed under a Creative Commons Attribution License

4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/.

Materials for Organic Bioelectronics

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