Microscopic defines anything too small to be visible by human eyes, and therefore, something that can only be seen with a microscope. Microscopic sensors combine both molecular electronics with nanoscale sensors, and they can be split into categories based on their signal and purpose.
Types of Microscopic Sensors
Microscopic sensors are divided into different classifications based on their functions. Sensor types include chemical, mechanical, biological, optical, electromagnetic, plasmonic, spectroscopic, magnetoelectronic and spintronics. The advancement of microscopic sensors allows for non-destructive in-situ measurements of absolute levels, as well as levels of change, and they are used in a variety of industries.
Environmental Sensors
Sensors equipped with bacteria can be used for environmental monitoring, as the bacteria can light up when exposed to specific toxic substances. The sensor works by using a nanophotonic chip which contains genetically modified coliform bacteria. The bacteria then emit a faint light when exposed to certain toxic substances.
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Microscopic chemical sensors have also been distributed into aerosol sprays as a way of tracking environmental and health hazards. The sensors are polymer chips overlaid with a circuit that contains atomically thin semiconducting materials. This circuit includes a photodiode that converts light into electric currents and becomes a chemical detector. This chemical detector conducts an electric current if the material binds with a specific chemical in the environment.
Microsensors can also be used to measure kinetic parameters such as net specific consumption, production rates, fluxes, and diffusion coefficients from concentration profiles. Micro profiles such as pH, redox potentials, ammonia, nitrates, monochloramines, phosphates, and dissolved oxygen can be monitored.
Microsensors can be used to look at water quality monitoring systems, to monitor chemical and biological reactions over large areas and to monitor algal blooms, chemical, and oil spill events. Biological aspects such as biofilm thickness, diffusion boundary layers, and stratification of biological reactions also can be determined using microscopic sensors.
Wearable Sensors
Wearable sensors are used as tactile sensors, artificial skins for soft robotics, to monitor human wellbeing and sports performance, and as pressure control of compression garments for wound healing.
Wearable patches can be used as sensors for toxic gases and harmful radiation. Skin patches can be attached to clothing discreetly and used while at work as a way of detecting dangerous leaks. Patches can also be used to identify the levels of ultraviolet radiation as a way of preventing dangerous exposure that could cause melanomas.
Microscopic sensors allow sport scientists to study and understand the locomotor demands of individual sports. Microsensors used include accelerometers, gyroscopes, and magnetometers that can be embedded within units to detect sport-specific movements.
In-Vivo Detection
In vivo sensors help us better understand the biology and treatment of diseases.
Electrochemical sensors are an established class of in vivo sensors that allows for the real-time measurement of analytes with implanted microelectrodes. The electrochemical sensors measure changes associated with biological events such as enzymatic activity by having a sensor embedded within the tissue to link to signal processing units and power supplies directly. An example of an electrochemical sensor is a glucose sensor.
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In the field of neuroscience, reliable electrochemical in vivo biosensors of neurotransmitters have allowed for a better understanding of how the brain works. In vivo monitoring of neurochemicals gives information on which networks are active. Fast-scan cyclic voltammetry provides real-time measurement of endogenous monoamine levels such as serotonin, dopamine, and norepinephrine.
In vivo, optical sensors can be used for both hypoxia and cancer detection. A typical cancer detection method looks at proteinase activity. Optical sensors of proteinase activity detect that proteinases cleave specific peptide sequences. A peptide linker that includes the proteinase cleavage site separates a fluorophore and a quencher, and local proteinase activity is visualized by an increase in fluorescence. In vivo, magnetic sensors can also be used to cancer from proteinase activity, as well as looking at cardiac damage from detection of apoptosis following heart attacks.
A sensor has also been developed that can measure radiation doses at individual cell levels in mixed radiation fields in cancer patients. The sensor allows doctors to estimate how much damage each cell has incurred following treatment. The measuring instrument contains microsensors placed alongside each other to form a sheet of sensors mounted on a silicon base. Dispersal across a given area enables the sensors to provide an image of the location within the cell that absorbs the highest levels of radiation.
The Future
Researchers from Johns Hopkins University have developed a new strong metal material that they believe could replace silicon as the main component of microelectromechanical systems (MEMS). The team attempted to make MEMS out of complex materials that are more resistant to damage and are better at conducting both heat and electricity. The new metal contains nickel and is a 29-micron peelable thin film. The new alloy films have a tensile strength three times greater than steel, can withstand high temperatures and is easily mouldable. The team say the next step of development is to shape the films into MEMS components
Researchers at Stanford University are looking into nanoparticles that emit light in response to a force stimulus using the digestive systems of nematodes. When illuminated with near-infrared light, the sensors emit visible light of certain colors and intensities depending on the forces applied. Nematodes have transparent bodies that allow the light emitted by the sensors to be recorded by a camera or microscope. The group is investigating the digestion in nematodes, as organs and cells exert strong forces at each step of digestion and so vivid color changes can be detected.
A team from MIT are looking into a wireless system inside the human body so that sensors can be implanted and then tracked to investigate tumors and dispense drugs. The method pinpoints the location of ingestible implants inside the body using low-power wireless signals. Their new technology is known as ReMix, and the system has been successfully tested in animals with the hope that the method can be used in the human body.
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