devices due to they are chemically stable, flexible, biocompatible, and have a high surface

area and electrochemical versatility. Graphene (Figure 2.3) has become one of the most

used 2D-nanomaterials in medicine, electronic engineering, and other fields. There are

several graphene-derived materials such as graphitic carbon nitride (g-C3N4), boron ni­

tride (BN), transition metal dichalcogenides (TMDs), transition metal oxides (TMOs),

which are promissory to be applied as a new family of engineered nanomaterials for

electronic devices [22]. Graphene presents a single or double layer of carbon, which is

linked by different sp² hybridization and van der Waals interactions, its electric prop­

erties are given by the delocalized electrons, and its surface properties can be modified by

the use of condensation or addition reactions, nucleophilic substitution among others.

Graphene films may be used in combination with metallic microelectrodes, generating a

synergistic effect that improves the electrochemical performance of the components; for

example, Lee et al. developed patches for diabetes control based on a therapeutic feed­

back system that monitors glucose in sweat and uses a bilayer made up of a gold mesh

and a gold-doped graphene film to transfer the electrical signal to stimulate the drug

release, which showed the formation of a more efficient charge transfer interface than the

individual layers [23].

CNTs are peculiar materials that are well ordered with a high aspect ratio and may be

visualized as rolled-up structures of single or multiples sheets of graphene (Figure 2.3).

Since their discovery, CNTs have been applied for applications in both biological and

biomedical fields. In general, CNTs are one-dimensional (1D) tubular forms of sp² carbon

networks, which also contain concentric graphitic shells and are typically 1–50 nm in

diameter and micrometers in length [24]. Because of their π-delocalized electrons, elec­

trical properties have been recognized in comparison with other reported materials.

Although their low solubility in fluids has been a barrier for biomedical applications, the

modification of CNTs’ surface with hydrophilic organic/inorganic groups has made them

facile to manipulate in physiological environments. Modification of CNTs’ surface can be

achieved by different pathways that include the typical impregnation (incipient/wetness

physio-adsorption) and covalent attachment. CNTs also increase cell growth and adhe­

sion, and they have been studied in transistors, self-repairing skins, and implantable

microfibers due to their flexibility. Vitale et al. used CNT fiber electrodes for neuronal

monitoring and stimulation, both in in-vitro and in-vivo tests; CNT electrodes showed a

significant decrease in contact impedance with neurons in comparison with metallic

electrodes; besides tests with Parkinsonian rodents, CNT electrodes showed the same

stimulation efficiency as metal electrodes but with a lower inflammatory response [25].

FIGURE 2.3

Carbon nanotubes and graphene-derived materials.

Materials and Their Classifications

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