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In recent years, two programs of the US Defense Advanced Research Projects Agency (DARPA) have caused a sensation, in which technical sensor systems are to be supplemented or replaced by natural ones. In general, the concept of living sensors on which these considerations are based aims to use the natural sensory performance of animals or the natural stimulus-response mechanisms of plants and bacteria in order to develop new types of sensor systems. How great the progress that can be achieved with this actually can be in practical use must be critically examined in each individual case.

There are basically three approaches to living sensors. The first two use the natural sensory capabilities of animals. On the one hand, their natural behavior is observed and analyzed in order to then use sensors to detect deviations from normal behavior, which provides information about possible dangers, such as B. natural disasters, supplies. On the other hand, the animals' extremely powerful sense of smell is trained in such a way that they are conditioned to detect hazardous substances, addictive substances or diseases. In the third approach, bacteria and plants are specifically programmed in such a way that they are able to detect certain pollutants such as explosives or biological and chemical agents. This is done through genetic modifications. Another possibility is the introduction of nanoparticles or nanostructures into the plant tissue, whereby plants with new functionalities (nanobionic plants), e.g. B. with the ability to detect hazardous substances to be developed.

A new approach in behavior analysis is to use the natural behavior of marine life as a warning system for underwater vehicles. With the help of hydrophones, acoustic transmitters, sonar, cameras and other sensors, the noises in a reef and the movements of marine life will be continuously monitored and analyzed. The reactions of marine microorganisms to magnetic signatures should also be recorded. Deviations from normal behavior are intended to provide information about submarines or unmanned underwater vehicles.

The use of the extraordinary olfactory sense of sniffer dogs to detect explosives or addictive substances has long been part of everyday life, and studies have shown that dogs are even able to sniff out diseases such as cancer or malaria. It is currently being investigated whether medical detection dogs can be trained to detect people with COVID-19. Rats have also been used for many years to detect land mines or to examine spit samples for tuberculosis bacteria. But insects, especially bees, have already been successfully trained to detect explosives, addictive substances or diseases. The bees are exposed to the smell of a certain substance and then rewarded with sugar water. Successfully trained bees now extend their proboscis in the presence of the target substance, which can be monitored using cameras.

Reprogramming bacteria genetically to act as a sensor is relatively easy today. The greatest challenge is to ensure their survival and functionality outside of the laboratory. In addition, the cells must remain within the material so that the biosafety with regard to the release of genetically modified organisms is ensured. A stretchable, tear-resistant hydrogel-elastomer hybrid has already been created, into which live E. coli bacteria were injected, which were genetically modified in such a way that they produced a green fluorescent protein when they came into contact with certain chemicals. From this living material z. B. developed rubber gloves and bandages that could be used for forensic examinations or in medical diagnostics to detect viruses on the skin.

In order to be able to use a plant as a hazard detector, extensive interventions in the genetic material of the plant are necessary. The model plant Arabidopsis thaliana was z. B. genetically modified so that in the presence of TNT it breaks down the leaf pigment chlorophyll and turns it green.

A completely new approach is plant nanobionics. In the leaf tissue of spinach plants, carbon nanotube-peptide complexes have been smuggled into, which react to nitroaromatic compounds, which are released from landmines to the ground and taken up by the plants with the water. The presence of these compounds is indicated by light emission in the near infrared range when the plants are illuminated with a laser.

Living sensors are often touted as being faster and more sensitive than the corresponding conventional technologies. They are ubiquitous, reproduce and keep themselves alive, making them cheaper and energy-independent. However, the need for downstream technologies, such as sensors or cameras, data transmission, image processing and evaluation algorithms, in order to be able to perceive and interpret the signals from the living sensors at all, is ignored. And how this monitoring and evaluation can be implemented is still partly unclear. Further hurdles when transferring the results obtained in the laboratory or field test to real conditions are the sometimes insufficient lifespan of the living sensors (bacteria only survive in the hydrogel elastomer mentioned for a few days), the reaction time sometimes being too long (the process of de-greening the plants two hours), the strong dependence of the organisms on the weather and the handling of genetically modified organisms, which, according to the current legal situation, is usually only allowed in the laboratory.

Dr. Britta Pinzger