Have you ever met someone who has only one leg or arm? In the USA alone, an estimated 185,000 people undergo amputation procedures each year, mainly due to vascular diseases. Thanks to advances in medical devices, some of the functions of lost limbs can be restored with artificial limbs or prostheses. People with prosthetic legs can easily walk, run, dance, and people with prosthetic hands can grasp well with the finger, while controlling in a coordinated manner.
However, current prostheses lack an important aspect of natural limbs and these are the tactile sensations of human skin. Scientists from Stanford University and Seoul National University have developed an artificial nerve. This artificial nerve can feel when touched and transmits the signal to control biological muscles. For people living with prostheses, this development means that they can regain lost sense of touch or gain control of disabled limbs.
Biological Sensory Nerves
Sensory nerves carry information from the outside world to the spinal cord and brain. In particular, the ability to perceive the sense of touch is provided by a type of sensory nerve in the skin called mechanoreceptors. When pressure is applied to the skin, mechanical receptors respond by changing electrical voltages (i.e., a measure of electrical energy). Voltages from multiple mechanical receptors are combined and transmitted to a single neuron or nerve cell. At a certain voltage threshold, the neuron produces repetitive electrical pulses that are transmitted to other neurons through junctions called synapses, ultimately reaching the neurons in the brain to record the sensation of touch.
The frequency with which electrical pulses are generated (measured in hertz, i.e. number per second) is determined by the applied pressure. Higher pressures produce electrical impulses at higher frequencies, while lower pressures produce pulses of lower frequency These electrical impulses are ultimately transmitted and processed to the brain to feel the pressure of the external stimulus relative to the pulse frequencies.
In summary, when a biological sensory nerve is applied pressure to the skin, the mechanical receptors change their voltages that are combined and transmitted to nearby neurons. When a voltage threshold is reached, these neurons send electrical impulses integrated with synapses, reaching the brain to record the sensation of touch. Higher pressures generate electrical pulses at higher frequencies.
Artificial Sensory Nerve
Artificial sensory nerves are at a very early stage of development and have not yet been tested in humans. However, these artificial nerves are designed in the hope that they will one day be safe and effective for use in humans. To mimic its biological counterpart, the artificial sensory nerve is created using three components: resistive pressure sensors, ring oscillators, and a synaptic transistor corresponding to biological mechanoreceptors, neurons, and synapses. To briefly explain these three components;
Resistive pressure sensors: Resistive pressure sensors consist of rubber pyramid structures filled with carbon nanotubes that conduct electric current to gold electrodes. These pressure sensors exhibit piezoelectric properties, that is, their ability to convert mechanical voltage into electricity. The increase in pressure applied to the rubber pushes more carbon nanotubes into the gold electrodes, providing a greater electrical current and greater voltage input to the ring oscillator.
Ring oscillators: Ring oscillators are devices that can generate electrical pulses at frequencies determined by the voltage input. Due to the greater pressure on the resistance sensors, a larger voltage input results in a higher frequency, similar to the way sensory neurons generate electrical impulses.
Synaptic transistor: Synaptic transistor receives and combines electrical pulses from multiple ring oscillators. The signals from the synaptic transistor can be recorded in a computer (similar to the brain) or used to drive biological muscle movements (as discussed in the next section).
The components of an artificial sensory nerve consist of an artificial sensory nerve, resistive pressure sensors, ring oscillators and a synaptic transistor corresponding to biological mechanical receptors, neurons and synapses, respectively. The signal from the synaptic transistor can be recorded in a computer (similar to a brain). Similar to the biological sensory nerve, the artificial sensory nerve receives pressure information in the range of its ability to sense human pressure (1 to 80 kilopascals, i.e. light touch a hard pressure) from pressure sensor clusters. Pressure information is then converted into electrical pulses matching the frequencies of sensory neurons (0 to 100 hertz) using ring oscillators, and a synaptic transistor integrates electrical pulses from multiple ring oscillators.
Development of Artificial Sensory Nerve Applications
Researchers who went to develop real-life applications for the artificial sensory nerve carried out two experiments to test and evaluate this new technology. One of these experiments is reading Braille letters and the other is to induce the movement of the cockroach leg. The first test demonstrated the technology’s ability to precisely interpret complex tactile sensations, the second showed that it was possible to interface the artificial nerve with biological muscles and simulate a realistic neural response circuit similar to a reflex. Both traits approach futuristic concepts such as detecting artificial limbs, precision robots, and animal machine hybrids.
Braille alphabet is designed for the blind. 26 letters created from 6 points in a 2 × 3 grid are represented by 26 different combinations of points. For example, the braille alphabet E is a combination of point 1 and point 5. In the first test, the authors produced 6 zones containing resistive pressure sensors located as 6 points, similar to the layout of the braille grid. Each pressure sensor zone is connected to a ring oscillator. The output from different ring oscillator combinations is used as input for synaptic transistors. When the dots representing certain braille letters are pressed, the corresponding synaptic transistors interpret the electrical signals from the ring oscillators and generate different signals for the different letters. These sensory nerves can thus be applied to improve the tactile abilities of robots.
Hybrid Bio Electronic Reflex
In the biological nervous system it receives and processes sensation from the spinal cord or brain sensory nerves. After receiving input from the sensory nerves, the spinal cord or brain can generate an output response through motor nerves (i.e. nerves that send motor signals from the spinal cord and / or brain to motor signals) to generate a mechanical response to sensation. The way motor nerves communicate is opposite to that of sensory nerves. For this response to be voluntary, it must interface between sensory and motor nerves, such as when the brain feels a situation that tickles and decides to actively move his arm and fingers to draw. On the other hand, when the body wants to respond to the stimulus as quickly as possible, the signal and response are directly coordinated by the spinal cord, directly bypassing the brain. An example of such an involuntary response is the knee-jerk reflex, the sudden extension of the leg in response to a connection under the kneecap. In this case, the stretch is controlled by signals sent from the spinal cord to the leg muscle and bypasses the brain altogether.
While real brain interfaces are currently unavailable, the artificial sensory nerve can already mimic a spinal cord reflex. In the second test of their technology, the authors connected the synaptic transistor of the artificial nerve to an excitatory electrode (i.e. metal wire) inserted into the motor nerve of a detached cockroach leg. The electrical pulse output from the synaptic transistor similar to the spinal cord output caused the leg to act like a reflex. An increase or decrease in the intensity of pressure applied to the resistive sensors increased or decreased leg extension force. This test demonstrated that artificial sensory nerves can communicate directly with the biological motor nerve and can influence the muscular response to an external stimulus.
In summary, an artificial sensory nerve can integrate sensory inputs with motor outputs. To test the ability of artificial sensory nerves to produce motor responses, the nerve was interfaced with a disjointed cockroach leg. Artificial sensory nerve output signals can control leg extension strength.
Recognition and Conveniences of Artificial Nerve
Artificial sensory nerves, which can feel pressure and interface with biological muscles, are an important step in helping people living with prosthetic limbs regain lost sensations. One day, they can give robots artificial skin that can sense the environment and react. However, in its current state, the artificial nerve is still far from having all the sensations of real skin. Specifically, the current artificial nerve can only generate neuron-like signals in response to pressure, while the real skin may feel vibration, texture, warmth, pain, and itching. In addition, it remains to be seen whether the human brain can process signals from artificial nerves that are key to recovering lost sense of touch.