poly (ethylene glycol) and silicone. The electrodes can perform the stimulation and re­

cording with the help of a counter electrode made up of stainless steel. These microwires

show enhanced properties like biocompatibility and a high level of electrode integration.

Polymers with flexible and transparent properties allow the stimulation of electrical

signals at high curvature surfaces like the retina and spinal cord.

9.4.1.2 Multifunctional Stimulation Probes

Recently, optogenetics and biochemical drug delivery including many other neural

transmission methods have emerged as an attractive multifunctional probes. In optoge­

netics, neurons are stimulated by using visible light of a specific wavelength. Opsins, also

called transmembranes, change the response as these are exposed to specific visible light

that excites them or inherent neural activity [37]. In optrodes, silicon microelectrodes are

combined with optical fibers for stimulation and recording of the signal. In microfluidics,

chemicals or analytes of interest are delivered to the specific area to check their response

at that area. This method can be used for the treatment of diseases like brain disorders.

Various multifunctional probes can facilitate the drug delivery, recording of neurons,

and stimulation of signals. These functionalities are achieved through silicon probes.

Microfabrication helps to concentrate the active metals in probes [38].

A probe designed for drug delivery consisting of micro-electro-mechanical systems of

40 μm thickness combined with microelectrode arrays and a microfluidic channel of

optical guide records the signal from different areas [39]. The probe successfully delivers

the drug in mice. The probes can be synthesized with different spacing, widths, and

depths having less cross-sectional area. These probes can be used for multifunctional

purposes. Metal electrodes and electrical wiring can be integrated with optical guides and

microfluidic channels into one device [37]. Neurons present near the electrode are sti­

mulated by light and response is recorded. Optical fibers used for telecommunication

over a long distance can also inspire this design. For rerecording and drug delivery, fibers

based on polymers are well preferred. Human hair-like thickness containing fibers made

up of polymers and metals is another attractive material [40].

9.4.2 Recording Methods

Electrodes can be implanted into the neuron or at the surface of the neuron to obtain the

best action potential. These electrodes receive the recording and provide the control of

prosthesis to patients for communication. These microelectrodes have a surface area

lower the 200 μm. Action potential has the amplitude of 100 μV. Impedance of electrodes

plays an important role for the noisy signal that depends on the distance between elec­

trode and neuron as well as the distance of tissue. Neural recordings of large scale can be

obtained by uniformity and low impedance. Various attempts have been made for im­

proving the signal-to-noise ratio through electronics and nano-electrodes by addressing

the issue of hemocompatibility, mechanical integrity, and biocompatibility [41].

9.4.2.1 Epidermal Recording Devices

The most widespread non-invasive technique for signal recording in bioelectronics is the

epidermal recording electrodes. In electroencephalography, a set of electrodes is kept on

the scalp of the subject and electrical rhythms are recorded. Brain waves have different

ranges and these define the functions of the brain and its state. These ranges are

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