reliable interface for spatiotemporal mapping and stimulating cardiac physiology

without interrupting the actions of cardiac muscle. In 2014, devices based on 3D in­

tegumental membranes were developed for high-density multipurpose recording, precise

measurements, and activation of organs like brain and heart [17]. 3D printing was used to

create a 3D heart structure with multipurpose electronics (pH, temperature, and strain

sensors) and optoelectronic components. These accomplishments in implantable devices

have considerable promise for biological and clinical research.

3.2.3 Electrical and Optical Properties

To design a bioelectronic device, the electrical properties of material play a vital role.

Among a variety of materials, graphene has demonstrated excellent electrical properties.

Even at room temperature, the graphene carrier mobility was found to be ~10,000 cm²/Vs

[18]. Some 2D materials emerged as intrinsic semi-conductors with carrier mobilities

~200 cm²/Vs and bandgap ~1.8 eV for monolayer MoS2 [19]. These materials are highly

suitable for the development of the digital transistor. The chemical diversity of 2D ma­

terials offers ease to tune properties for the desired application. This opened new research

avenues to design bioelectronic devices based on tunable 2D materials. Conley and co-

workers [20] released an indirect bandgap in multi-layered and direct bandgap in

monolayer 2D MoS2. The bandgap of 2D materials is strongly influenced by the number

of layers (Figure 3.4) [21]. Black phosphorous (BP) is also a 2D semiconductor and its

bandgap varies (0.2–2.1 eV) with the thickness [22]. Single-layer BP along the zig-zag

direction demonstrated higher carrier mobility (10,000–26,000 cm²/Vs) [23].

Additionally, some of the 2D materials act as semiconductors and insulators such as

MoS2. The flakes of MoS2 with odd layers possess piezoelectricity while MoS2 flakes with

even layers demonstrated no piezoelectric response [24]. Such materials with tunable

electrical properties are promising to develop next-generation smart bioelectronic de­

vices. To develop wearable bioelectronic devices, the optical properties of materials must

be considered. A material should exhibit high visible light absorption, zero bandgap, and

high carrier mobility. 2D semiconducting and insulating materials have shown a higher

absorption coefficient. The absorption coefficient of 2D materials varies with the number

of layers (Figure 3.5) [25].

3.2.4 Mechanical Properties

Mechanical properties are one of the significant and basic aspects of novel material

investigation, development, and design. Regarding bioelectronics, the mechanical

properties of material play a vital role. The material required for bioelectronic devices

not only sustain substantial deformation but is also flexible enough to be compatible

with tissues. In this scenario, 2D materials emerged as suitable candidates for bioe­

lectronic devices. These materials possess higher strength due to strong in-plane

covalent/ionic connections and good flexibility owing to their atomic thickness. Among

2D materials, graphene possesses the highest value of Young’s modulus and fracture

strength [26]. Other 2D materials have poor mechanical properties but are strong en­

ough to be used for the fabrication of bioelectronic devices [27]. The 2D materials

exhibit higher Young’s modulus and fracture strength in comparison to 3D materials.

The fracture strength of mono-layered 2D MoS2 (~23 GPa) is found to be higher than

steel [28]. This enables MoS2 to withstand ~10 times larger strain in comparison to steel.

The single crystalline structure of 2D materials at micro or nanoscale is responsible for

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Bioelectronics