Sensory Systems/Neurosensory Implants/Future Directions

Electronic measurement of odors

Nowadays odors can be measured electronically in a huge amount of different ways, some examples are: mass spectrography, gas chromatography, raman spectra and most recently electronic noses. In general they assume that different olfactory receptors have different affinities to specific molecular physicochemical properties, and that the different activation of these receptors gives rise to a spatio-temporal pattern of activity that reflects odors.

Electronic Nose

Flight engineer Alexander Misurkin of Roscosmos works with the JPL Electronic Nose (ENose) experiment on board of the ISS.

E-noses are artificial odor sensing devices based on a chemosensor array and pattern recognition. They are used to identify and quantify substances dissolved in air (or other carrier substances). An e-nose consists of a sampling device (analog to the nose), a sensor array (analog to the olfactory receptor neurons) and a computing unit (analog to the brain).

Sensor arrays

Like in the animal noses, unspecific sensors are used. This is not only due to the fact that it is very hard to find very specific sensors, but one also wants to cover a huge range of possible compounds without a sensor for each of them. Furthermore it is more robust, precise and efficient if the processing is based on information of more than one sensor. Such sensors experience a change in their electrical properties (E.g. higher resistance) when they come in contact with a compound. This alteration leads to a voltage change that is digitized (AD Converter).

The most frequently used sensor types include metal oxide semiconductors (MOS), quartz crystal microbalances (QCM), conducting polymers (CP) and surface acoustic wave (SAW) sensors. Another promising technology is bioelectronic noses that use proteins as sensors. It is also possible to use a combination of different sensors to get a more precise result and to combine the advantages of several sensor types, e.g better temporal responsivity versus better sensitivity.

Example: working principle of a conducting polymer sensor

A conducting polymer sensor consists of an array of about 2-40 different conducting polymers (long chains of organic molecules). Some odor molecules permeate into the polymer film and cause the film to expand thereby increasing its resistance. This increase in resistance of many polymer types can be explained by percolation theory. Due to the chemical properties of the materials, different polymers react differently to the same odor.

Computation

The sensor signal has to be matched to an odorant mixture with a pattern recognition algorithm. It is possible to create a database of potential combinations and find the best match with multivariate statistical methods when an odor is presented or a neural network can be trained to recognize the patterns. Often also principal component analysis is used to reduce the dimensionality of the sensor data.

Applications

There are many applications for e-noses. They are used in aerospace and other industry to detect and monitor hazardous or harmful substances and for quality control. Possible applications in security are drug or explosive detection. E-noses may someday be able to replace police dogs. A very powerful application could be the diagnosis of diseases that alter the chemical composition of breath or the smell of excretions or blood, thereby potentially substituting invasive diagnostic techniques. It can also be employed to diagnose cancer, as certain cancer cells can be identified by their volatile organic compound profile. Cancer diagnosis by smell has already been found to work with dogs, flies, but practically suitable methods with high sensitivity and specificity are still under development. Another medical application is the treatment of anosmia (inability to perceive odor) by an olfactory implant on basis of an e-nose. This too is still in development. In contrast, e-noses are already in use for environmental monitoring and protection. In robotics, e-noses could be used to follow airborne smells or smells on the ground. Especially for robotics it would be very interesting to have a better understanding of the insect’s olfactory system, since, in order to use the smell to navigate or to locate odor sources the often neglected temporal stimulus information has to be used.

Insects can follow odors as they can react to changes within about 150 milliseconds, and some of their receptors are able to depict fast odor concentration changes that occur in frequencies above at least 10 Hz. In contrast, conducting polymer as well as metal oxide e-noses have response times in the range of seconds to minutes with only few exceptions reported in the range of tens of milliseconds.

Photostimulation of neurons

Photo-stimulation of neurons is an emerging field of research. Neuronal firing is achieved by shining a focused light source onto the nerve cell, causing it to depolarize. There are two major ways to approach this goal: irradiation of the neurons with a laser, inducing a local temperature gradient; and the introduction of light sensitive channels or receptors into the nerve cell making it sensitive to light, similar to rods and cones in the human retina. Advantages over the traditionally used electric stimulation are increased precision and less to no tissue trauma.

Electric vs Optic stimulation

Electric stimulation has inherent limitations compared to optic stimulation. To elicit reliable firing the electrodes have to be in physical contact with or in close proximity to the targeted tissue. Introduction of electrodes into the nerve tissue damages it and surrounding tissue.

In many cases the electrode array is introduced into electrically conductive tissue allowing for current spread, further decreasing the spacial resolution that can be achieved.

Measurement of the evoked neural activity is often contaminated by stimulation artefacts much larger than the measured neural activity. This is especially the case in measurements close to the excitation site.

In contrast, optic stimulation can reliably achieve excitation of single cells or small cell populations. It does not require direct contact to the target tissue, reducing tissue damage. Finally electrical recordings of neural response in close proximity are not contaminated by the excitation stimulus. Although electrical stimulation suffers from the above mentioned drawbacks it is still the most well established and reliable method for nerve stimulation in patients.

Infrared Stimulation

Infrared stimulation is based on an infrared laser inducing a local temperature gradient inside the neuron. It does not require any modification of the cells prior to stimulation. The low energy laser does not cause damage to the tissue and elicits an artefact free stimulation. The exact mechanisms that lead to neuronal discharge are not known. However studies have shown that this phenomenon is is most likely due to local photothermal processes. Thus the IR irradiation creates a temperature gradient confined to a small space which rapidly vanishes after irradiation ceases. The local temperature rise of up to 9°C is thought to cause conformational changes in molecules ultimately leading to neuronal firing. At high irradiation frequencies the heat becomes additive, causing the irradiated tissue to heat up gradually and ultimately damaging the cell.

Optogenetics

Optogenetics is the sensitisation of cells to light by the introduction of foreign genes, allowing temporal and spatial high resolution alteration of neural firing patterns. The genes can be expressed in genetic modification of animals or be introduced by vectors like viruses. Most light sensitizing genes used today were first discovered in unicellular organisms like algae or archaea. These genes can encode light sensitive ion channels or receptors producing various responses to optic stimulation.

Optical neural-firing manipulation techniques from left to right: ChR: blue light (480 nm) agitation of Channelrhodopsins (ChRs) leads to channel opening and Na⁺ influx. Sodium ion influx leads to depolarisation and evokes neural firing. HR: Halorhodopsins are activated by yellow light (570 nm). Open channels allow chloride ions to enter the cell, leading to hyperpolarisation and thus inhibition of action potential formation. Opto-XR: Animal-Rhodopsins (light-green) are the light sensitive part in the constructs. The intracellular loops (dark-green) are exchanged for signalling sequences of the targeted pathways. Opto-XR excitation leads to alteration in the signal transduction and thus influences the cells responses and metabolism. IR-irradiation: Cell irradiation with infrared laser-pulses locally induces thermal gradients and provokes neural firing.

For neuronal activation natural Channelrhodopsins (ChR) or engineered genetic variants thereof are commonly used. ChRs are light sensitive non-specific cation channels, which open when excited with blue light (480nm). In nerve cells ChR opening leads to sodium influx and membrane depolarisation. The light sensitive component is an all-trans-retinal which is also found in the human retina. Light induces a conformational change to 13-cis-retinal allowing cations to flow across the channel. Introduction of specific point mutations close to the retinal binding site can alter the kinetic properties and specificity of the channel. Linking ChR to other proteins allows for tools with diverse functionalities as in vivo monitoring of the introduced constructs.

Halorhodopsin (HR) are light gated chloride ion pumps used for light activated neuronal inhibition. Optical excitation by yellow light (570nm) in sensitized neurons leads to an import of chloride ions and hyperpolarisation. Like in ChR, the light sensitive molecule is also all-trans-retinal. Due to different stabilisation and thus wave length sensitivity differences of the retinal in HR and ChR, they can be used in the same cells and targeted separately. This allows for very close control of the activity in neural circuits.

For optical control of cell pathways the Opto-XR proteins were developed, where the X stands for the targeted signalling pathway. Opto-XRs consist of an animal-rhodopsin (bovine, rat etc.), with its intracellular domains exchanged for signalling sequences of the cell. This allows optic regulation of the cells signalling pathway. The signaling sequences can either be activated or deactivated by conformational changes induced by light falling on the rhodopsin. This allows for specific activation of certain receptor pathways like serotonin or adrenergic signalling.

Optic stimulation in Neuro-Prosthetics

Electric stimulation has long been used for evoking nerve firing in neuronal prosthetics. However, spread of electrical current and generation of electric fields limit the spatial resolution that can be achieved. This limits the fidelity of the transmitted signal. In the case of auditory prostheses a maximum of around twenty electrodes is feasible, leaving the sound quality achieved far off the desired goal. A switch to optic based technology could achieve activation of smaller areas, increasing the amount of potentially perceived tones. Recent development in optic stimulation techniques promise ways to overcome those obstacles and improve prosthetic devices and the quality of live for patients.

Cochlear Implants

Infrared stimulation of the cochlea as well as the auditory nerve have been tested in various animal models such as rodents and cats. The optic variant shows remarkable precision concerning the area stimulated by the laser, which is approximately the same size as that activated by a medium loudness tone. It has also been shown that using low energy IR irradiation, constant stimulation can be achieved without gradual heating and damaging of the tissue. This enables the usage of the implant throughout the day without damaging the cochlear system. A major drawback of the IR stimulation is a much higher energy consumption compared to electrical stimulation.

To overcome the energy problem described, researchers have begun to test an optogenetic approach in rodents. They genetically engineered mice to express Channelrhodopsins in spinal ganglion neurons. Sensitisation of the nerve cells reduced the energy required to induce firing compared to IR irradiation by a factor of seven (IR: 15 μJ, Optogen.: 2 μJ, Electric: 0.2 μJ). Stimulation was thus possible using μLEDs instead of lasers. In spite of this progress, implementation of this technology in humans in the near future is questionable. This is mainly due to the possible dangers of viral introduction of genetic material into an organism. Up to now only very few gene therapies have been approved. A save but still effective way of specifically infecting the cochlear organs would have to be implemented and approved.

First patents have been registered describing potential optical cochlear implants for humans. These implants function similar to the traditional electrical implants. But instead of the electrodes they have VCSELs (vertical-cavity surface-emitting lasers) that are driven by the implant's input device. VCSELs are laser emitting diodes that can be fit into the small tubing of the implant. The lasers are directed at the organ of Corti and can be much closer spaced than electrodes, more than doubling the amount of implant output channels. Laser diodes are used for higher pitch signalling, while electrodes drive lower amplitude nerve cells.

Vestibular Prostheses

Vestibular Prostheses aim to restore imbalance issues arising from dysfunction of the vestibular system. Since the semicircular canals are interconnected, current spread is a major problem in electrical stimulus delivery. Current spread can lead to additional stimulation of unwanted semicircular canals, resulting in incorrect balance signals sent to the brain. The possibility of using IR irradiation has been investigated. Irradiation of the ampullae did not evoke action potentials. The reason for failing stimulation might lie in an insensitivity of the hair cells to IR irradiation. Nevertheless, optical stimulation of the vestibular nerve might be feasible. It is as of yet unclear if separate stimulation of the nerves from different ampullae is possible in this manner.