9.3.3 Multiple Access Techniques for Signal Transmission ....................................149

9.3.3.1 Time-Divisional Access..........................................................................149

9.3.3.2 Event-Based Access ................................................................................149

9.3.3.3 Biomimetic Synaptic Access .................................................................149

9.4 Implantable Neural Interfaces.........................................................................................150

9.4.1 Stimulation Methods.............................................................................................150

9.4.1.1 Flexible Stimulation Probes ..................................................................150

9.4.1.2 Multifunctional Stimulation Probes ....................................................151

9.4.2 Recording Methods...............................................................................................151

9.4.2.1 Epidermal Recording Devices..............................................................151

9.4.2.2 Implantable Recording Devices ...........................................................152

9.5 Summary and Perspectives .............................................................................................152

References ....................................................................................................................................154

9.1 Introduction

Prosthetics have played an important role in the fields of information technology,

biology, materials, and mechanics. These are lowering the barriers in the life of those

people who have lost part/parts of the body due to disease, injury, or aging [1]. A high

barrier in the development of prosthetics is the computing hardware. However, the de­

crease in the size of electronic devices made it sophisticated. Signal analysis and digital

computing provided the advancements in prosthetics that are seen in the trials taken on

human and non-human models [2]. In the early days, the versatility of the amputees was

limited due to the poor performance of the prosthetic limb. However, neural interfaces

have provided possible solutions to restore the tactile solution. Specific areas of the so­

matosensory cortex in hand can be stimulated to evoke tactile sensations of pressure and

pain. Periphery nerves are helpful for the better movement of the prosthetics. Key nerves

can be rerouted to provide the required motions in the chest muscles. Instead of the

advancements in prosthetics, their wide use has been limited due to invasive surgery,

neural plasticity from repeated stimulations, and complexity of intent. Human skin

provides the inspiration of touch for these prosthetics [3].

Action potentials generated from the neurons enable the somatosensory feedback. These

electrical signals are transmitted through neurons to the brain and analyzed. A combination

of any of the five senses can generate neural activity through the stimulation of receptor

cells. Encoded information in the form of shape, quantity, frequency, and pattern in action

potentials are transferred with the speed of 1–100 m/s from distances of 0.1 mm to 2 m [4].

Neurotransmitters and chemical substances are released from the cells at the terminal of the

axon and bind to the receptors after diffusing across the synaptic cleft. There are three main

parts of sensory feedback: (1) transduction of stimuli into electrical signals, (2) signal

transmissions through neurons, and (3) neural stimulation of neurons in the brain. This

chapter discusses the materials as well as technological interfaces that create the artificial

sense of touch. Skin-inspired multifunctional interfaces make the bridge with external

stimuli for prosthetic limbs. The multifunctionality, sensory components, and stretchability

of the artificial interfaces are evaluated and reviewed. Accessing methods for artificial

signals, signal encoding, and transmission will be summarized along with the implantable

neural interfaces of the materials. These methods are inspired by analog to the digital

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