Biosensing: from cells to multitrophic systems

The very name of biosensing places this biotechnological application in the category of biosemiotic technology, or if you prefer, biosemiotic technology can be seen as an epistemological tool for devising complex biosensing. Even in the most mechanical conceptions of biosensing, where there is a physical mechanism using the specificities of organic compounds, and where the only biosensing entity is the human observer, there will be triadic logic involved. At its most simple technological level a biosensor is an analytical device incorporating a deliberate and intimate combination of a specific biological element (that creates a recognition event) and some kind of physical element that records and transduces the recognition event to the observer.

According to Fraser (1997) most of the novelty of biosensors comes from the "bio"

side since the transducers of the recognition event are mostly physical instruments which have already been widely used in “physisensing”.

Biosensors perform a diversity of sensing functions allowing the acquisition, capture, communication, processing, and distribution of information about the states of physical and biological systems. It is a characteristic feature of a biosensor that the device is tailored to the environment in which it is to operate. In the development and application of sensors to the field of process control, there is a trend that moves away from the use of measuring devices towards the use of sensing systems. This strategy seeks to decentralise measurements by focusing not only on product control but mainly on “on-line” process monitoring (Lading et. al. 2001:4). In other words, biosensors are involved in assessing qualities, not measuring quantities. That makes them semiotic devices. Quantities, i.e.: thresholds, can eventually be deduced from our knowledge about the metabolism of the biosensor.

When the biosensors that the human eye has to sense are organisms which in turn have sensed a difference (that makes a difference to the organism and then to the human observer) the web of semiotic networks to be analysed can increase

considerably if we are to consider, for example, multiple biosensors for monitoring complex processes in an organism’s physiology or in a multitrophic interactive system, whether at the level of a niche or ecosystem.

Biosensors, in their wider definition, have been used in many different modalities and applications, from the use, in the old days, of canaries to detect poisonous gases in coal mines, to the extensive use of antibody technology, up to the recent use of the most diverse and sophisticated genetic constructs. Today, biosensing represents a growing research area of its own and a complete biotechnology sector that develops tools and services for a great variety of applications. These devices may include nucleic acid sensors and DNA chips, immunosensors, enzyme-based biosensors, devices with natural and synthetic receptors, organism- and whole cell-based biosensors, etc. To this we can add the methodological tools for biodiversity and ecosystem monitoring that are based on sensible species, bioindicators or indicator species.

Since, at the physiological level, in vivo sensing is considered to be a priority for therapeutic purposes, biosensing research will meet increasingly complex

requirements for the design of “on-line” biosensing devices or organisms. For example, the most quoted case of a widely used physiological biosensor is that of

glucose sensing for the control of diabetes. Continuous glucose monitoring in diabetics has been attempted through a variety of invasive or non-invasive methods.

Despite much effort and some encouraging results, numerous obstacles remain, mainly due to poor sensor biocompatibility and fluctuating body chemistries. In vivo sensors are subjected to many obstacles once they are placed in their niches. But it is recognized that one of the hardest problems to overcome will be the recognition of the biosensor as foreign by the host, i.e., the cellular tissue responds, leading to

membrane fouling and sensor encapsulation by fibrous tissue (Fraser, 1997). The cellular and humoral defence mechanisms will do their best to eliminate the spy. It turns out that biocompatibility is also a communication problem and careful attention to the semiotic niche could provide some hints as to how to camouflage the sensor while it does it job, and at the same time how to control its invasive presence. The fluctuating chemistry of the body is another important aspect to be considered from the semiotic point of view.

The possibility of fast, on-line, real-time sensing opens up new perspectives in a variety of applications in microbiology, medical diagnostics, biocontrol, biosafety, agriculture, ecological monitoring and in the pharmaceutical and food industries. In the pharmaceutical and biotechnological industries, the progress of microbial fermentation can be controlled and optimised through the use of biosensors.

Biosensors measuring microbial growth and contamination of foodstuffs have been developed and are already in use (Lading et. al. 2001).

By relying on patterns, a web of biosensors could be conceived in the assessment of complex relations in a given hierarchical system. This could theoretically be achieved by combining different biosensing organisms or devices; by combining biomarkers within a single biosensor; or by linking one biomarker (e.g.:

bioluminescence) to the simultaneous occurrence of a mix of analytes or

environmental conditions (e.g.: a complex blend of volatiles). If the mixing of such regulatory elements is not feasible in the operon of the biomarker, then biosensors with different biomarkers would be required for such a job.

In physiological and ecological monitoring there are increasing expectations in connection with new broad-band array biosensors capable of classifying, assessing and gradient-tracking their dilute analyte targets in more or less complex and demanding physico-chemical and biological backgrounds. Because of their refined

capability for categorial perception, biosensors are capable of operating in “dirty”

samples and complex mixtures.