9.4 Implantable Neural Interfaces

Biosignals are transmitted from prosthetic limbs to the central nervous system. The sig­

nals transmitted to the brain enable the body to respond to the prosthesis. The process is

simple, yet there is a need to reduce the complex interaction of chemical and electrical

signals of neurons in the nervous system. The function of the human brain is complex that

can be understood by invasive and non-invasive methods in which electronic skin acts as

a neural interface. Traditionally, a patch-clamp has been used that records the brain

signal. It contains electrodes on the surface of the head. The data obtained is used to attain

the intentions of the user and stimulation external factors such as face recognition and

muscle movement. Many diseases and/or conditions such as depression, dementia, and

Parkinson’s disease are now treated with memory implants and brain pacemakers [33].

Electrodes are placed in the target location of the brain. Then electrical signals are sent to

the electrodes by the planted brain pacemaker. These devices are not biocompatible

due to their limited battery life. Material technology can help us to record and stimulate

the neurons to help patients with many diseases such as epilepsy, sclerosis, and tetra­

plegia [34]. This section covers neuroprosthetic applications of the probes and flexible

electronic skin. Here we will also demonstrate stimulation methods by probes that use

optical, electrical, and microfluidic delivery.

9.4.1 Stimulation Methods

Microelectrodes are the best neural interfaces that are used for bidirectional commu­

nication. These are used for listening and talking with neurons that are inserted in the

brain area. Each electrode performs two functions: stimulates and records the electrical

activity of a group of a neuron. There are some important factors for designing the neural

interface. These include size and quantity of stimulation channel, means to use interface,

biological response of cells, electrical requirements, and structure of the intended de­

vice [35]. Electrophysiological methods are used for stimulation and recording of neural

activity. Penetrating multi-electrodes in the brain is one of the best electrophysiological

methods. Electrically stimulated depolarized membranes of cells in the brain are used for

the initiation of functional response. Microelectrodes are used for improving selectivity

and resolution. Charge injection density can increase the performance of microelectrodes.

9.4.1.1 Flexible Stimulation Probes

For the development of penetration and comfortable electrodes, material chemistry has

achieved great intentions. The material includes the polymers such as polyimide, par­

ylene, and polydimethylsiloxane. These materials are used with metal electrodes as in­

sulation layers and substrates. Conducting polymers as the outermost layer provides the

electrical insulation and these do not need any additional coating. The probe based on

polyimide performs the stimulation and recording in the deep brain. The device record

the neural spike signal at the depth of 7 mm in the brain of a rat [36]. However, the

insertion of a soft tungsten guide stick is needed in the brain to record the signal.

Extraction of the neural probe can damage the tissues surrounding the brain. To minimize

this risk, electrodes of soft stimulation and freestanding materials like conducting poly­

mers and elastomers have been synthesized. These electrodes are constructed with two

layers; the outer layer of parylene and the inner layer of poly (ethylene dioxythiophene)

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Bioelectronics