A dog’s sense of smell is legendary. But our attempts at ‘engineering’ the canine olfaction system loses something in the process. Instead of mimicking nature by forcing a human-designed solution, do we need to adopt an holistic systems approach that retains naturally evolved characteristics?
Learning from nature seeks to develop bio-inspired sensing systems that may one day emulate the amazing sensitivity and specificity observed in the natural world. Nature’s capabilities have evolved often for specific tasks, providing the organism with an advantage in its ability to survive and prosper in its environment. Naturally evolved capabilities cover a wide range of sensing functions including vision, temperature, hearing, touch, taste and smell; and for many such functions, the capabilities of natural systems are vastly greater than those achieved by current engineered solutions.
Since natural systems have developed specifically to detect the sort of entities that are of interest in chemical and biological defence, it is not surprising that they could be the source of inspiration for improved capability. In fact it is possible to conceive of a complete bio-inspired security system concept which is likely to be radically different from more conventional approaches.
Terrorist activity is not confined to conventional means of attack, and the use of chemical or biological agents is a real possibility, as was seen in the attack on the Tokyo underground system in March 1995. With this increasing concern comes the need to provide ever more capable detection systems. The increased capability required does not just concern the ability to detect and recognise agents, but extends to many aspects of the whole system which may be required to operate outside conventional military environments.
Current engineered systems are produced in a totally different way from the process that leads to a natural biological system. In engineering, a specification is derived and a design developed to meet this specification. The designer will select materials and components that can meet the requirements in an efficient manner, and will also consider how the product will be manufactured in order to minimise the number of processes involved and the amount of waste energy and material produced.
In contrast, natural systems develop over long periods of time. What we observe is a system that has adapted to its environment to produce an optimised solution. However, the constraints that have driven this iterative ‘design’ process may be impossible to determine in any detail. Optimisation against one set of constraints can result in a lack of tolerance to new conditions, and this poses a challenge in assessing the suitability of a biologically inspired solution to an engineering problem. While there are undoubtedly difficulties in understanding why natural systems are constructed as they are, it is also true that natural systems possess elegant and effective designs from which inspiration can be drawn.
Natural systems have developed specifically to detect the sort of entities that are of interest in chemical and biological defence
In recent years there have been extraordinary advances made in biotechnology, and the technologies and techniques developed offer significant promise for sensing systems, especially for chemical and biological agent detection. A fundamental issue, however, is the ability of systems to operate in extreme environments rather than in a controlled laboratory.
The direct use of biological entities can seem attractive but brings the significant overhead of sample preparation, which can be important for the analysis of many agents. Samples have a limited lifetime which adds to the logistical burden associated with such detection schemes. Collection, concentration and purification are also critical steps for biological detection in practice, where agent levels can be low compared to a variable and sometimes high natural background material. Biological detection methods tend to rely on laboratory-based equipment which requires qualified personnel to operate it and which can take several hours to produce a result. Such long response times are unacceptable for field use.
The challenge, then, is to capitalise on the extraordinary capabilities of natural systems and to develop techniques for the detection of bacteria, viruses and toxins in a form that can be employed for field use. These techniques need to be immune to background material and provide rapid results, and they need to be part of an intelligent and autonomous system which requires minimum operator intervention or logistic support.
The Situational Awareness Sensing System
To help facilitate the application of biotechnology and bio-inspiration to sensors, a conceptual model can be developed to focus on key issues that need to be overcome. Whilst the model itself may never be fully realisable it can be used to generate a vision of what could be developed and also to assist in the identification of enabling technologies that are required. One such approach is the Situational Awareness Sensing System (SASS). It is not a design for a real system but rather a schematic of a concept to enable aspects of the overall system and its component parts to be explored.1
There are six key parts to the system: (i) sampling/collection, (ii) structure, (iii) receptor surfaces, (iv) transduction, (v) processing and (vi) power. This concept is not aimed at a single component, such as a sensor, but leads naturally to the consideration of a whole system concept. The colour coding of the various components indicates our view of the maturity of a particular item. For example, there have been significant advances made in the science of surfaces and receptors (coded light green) while the issue of power, especially to enable extended autonomous operations, still requires major advances to be made (coded dark green).
A critical part of any sensing system is the ability to collect and sample target material. For substances dispersed in air, well-established techniques can be used to capture and concentrate aerosols into liquids. But these devices are cumbersome, require user interaction and are inefficient at collecting trace materials. In contrast, some organisms have evolved to collect material even when distributed in low concentrations. An example is the Australian Thorny Devil (Moloch horridus), which has the ability to move water through inter-scalar spaces on its skin surface from its feet to its mouth for drinking.2 This ability seems to rely upon hinged joint walls which are covered by a complex arrangement of fractured surfaces with a large surface area. Semi-tubular joint channels and a jaw buccal cavity pumping mechanism work together to collect and transport drinking water harvested from infrequent rainfall. This enables the Thorny Devil to remove water from sand at just 3% concentration.
The physical structure of a SASS will need to protect and align components, regulate the internal environment and maybe also the physical aspects of locomotion. Natural systems can exhibit remarkable structural performance from materials with relatively low performance constituents. Bone, for example, has outstanding toughness which is at least partly explained by its structural hierarchy and the size scale of the bio-mineralisation.
Moving from the micro to the macro scale, it is possible that inspiration can be drawn from social structures such as a mound of termites (Macrotermitinae).3 The study of these mounds has shown that they are complex self-regulating structures and the way in which they are constructed and operate is the subject of current research. While there has been considerable work done in trying to understand the fundamental mechanisms behind the properties of natural materials, much remains to be done to convert this knowledge into synthetic materials that can be commercially produced. The need for efficiency will encourage the concept of multi-functionality to combine functions with structure.
Surfaces and receptors are in essence an extension of the structure of a material, where the properties of surfaces and receptors are directly related to their structure down to the molecular level. Much has been learnt from nature in this area. There are a large number of existing bio-receptors including antibodies, enzymes, olfactory binding proteins, DNA/RNA probes, synthetic ligands and cell surface receptors. These biomolecules bind firmly or reversibly to a range of target ligands and are used for ‘in vitro’ applications in sensor and detector systems of exquisite sensitivity and remarkable specificity.
Assay systems, for example, have employed enzymes for some time. An example is the use of firefly (Photinus pyralis) luciferase in bioluminescent assay formats in which the degree of bioluminescence is based on the level of adenosine triphosphate (ATP) present within a bacterial cell. However, encouraging biological molecules to behave naturally while in an unnatural and hostile environment is a considerable challenge. For example, spacing, positioning and orienting antibodies and enzymes are critical for ensuring maximum functionality.
The rapidly evolving threats to security demand that we consider all possible means to counter them; bio-inspired approaches offer one possible route, making use of nature’s own highly efficient design processes to protect itself
Considerable effort has also been undertaken to develop sensor transduction. These systems have been based upon a number of principles including optical, electromechanical, acoustic, piezoelectric, magnetic and mass spectrometric techniques. Although these are well established techniques, it may be possible to derive improved methods by studying nature. It is known that the detection of heat or infra-red radiation is used by some animals to identify prey and to enhance their survival. An example is the Jewel Beetle (Melanophila), which is reported to be capable of detecting forest fires from a distance of 50km. It is understood that infra-red radiation in the 2.5 – 4 µm wavelength region causes wax filaments in the front legs to swell and stimulate the nervous system.
An important aspect of any sensing system is how information is processed. Bio-inspiration has been employed in the processing for detection systems: the use of neural networks is a good example. Developments in neuroscience are leading to an improved understanding of how the brain works which may lead to new techniques for sensor data processing. Some organisms appear to have relatively simple brains which are optimised for the efficient processing of sensory inputs. For example, research on the fruit fly (Drosophila) glomerulus using fluorescence imaging shows that different regions of the glomeruli respond to differing odours.
The SASS concept is more than just a biologically inspired sensor: it is a complete system possessing some degree of intelligence expressed in the decisions it makes about its environment and mission as well as the sensor inputs. In the context of a SASS, the algorithmic element must be able to exploit and extract information from bio-inspired sensors which are likely to have quite different characteristics when compared to conventional devices. For example, an imaging device inspired by an insect eye with multiple optical detectors must be matched with suitable signal or data processing algorithms to extract useful information from the device. In general biological devices are strongly non-linear (and non-Gaussian) and so will require non-linear algorithms to make good use of their outputs.
Finally, a key component of any deployed sensing system is access to a reliable low mass source of power and energy. For extended periods of operation the available energy is likely to be the critical parameter, but if the system has to move about in its environment then peak power may also be crucial. Whilst battery technology has advanced significantly in recent years, it still has relatively low endurance and the logistics burden imposed by battery usage is a growing problem especially for military operations. Potential alternatives are being investigated, for example energy can be harvested from estuarine and marine sediments to power unattended sensing platforms in rivers or the ocean. Obtaining energy from tree sap or mammalian fluids has also been investigated. Energy might also be scavenged from the environment using solar, thermal, vibrational and other sources. Robotic systems that can be energy self sustaining are being developed in both land and water environments. A robotic system called Eco-Bot4 has been designed to power itself solely by converting unrefined insect biomass using on-board microbial fuel cells with oxygen cathodes.
Exploitation of Natural Systems
There is nothing particularly new about exploiting natural systems, and to some degree mankind has always done this, with many specific examples of where natural systems have been exploited for sensor applications. A good example is the use of caged canaries by miners to detect methane in coalmines.
Biologically inspired solutions, in contrast, seek to exploit underlying biological principles in synthetic systems. While this approach could provide a new range of possibilities, it is technically and intellectually challenging. The long-term potential for this approach is exciting, and is perhaps the key to overcoming some of the limitations described in exploiting biological systems in a SASS framework, and which could enable high performance devices to be made better suited to the constraints imposed by the conditions in which they must operate.
Demonstrations in response to operational requirements are needed to show that there are improvements to be exploited for significant gains in capability. The rapidly evolving threats to security demand that we consider all possible means to counter them; bio-inspired approaches offer one possible route, making use of nature’s own highly efficient design processes to protect itself.