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Polymer films in sensor applications
Polymer films in sensor applications
The AuthorGÿbor Harsÿnyi is Associate Professor in the Department of Electronics Technology,Technical University of Budapest, 1521 Budapest, Hungary
Keywords: Polymers, Sensors, Microelectronics
Synthetic polymers are among the most widely used materials of the twentieth century. Although their ancestors, naturally occurring macromolecular materials such as natural rubber, cotton and hardened oils, have been known and used for much longer, synthetic polymers were produced first in the nineteenth century. These materials now dominate so many application fields that it is hardly possible to estimate their importance here.
At present, there is also a considerable interest in the use of polymers as components of microelectronic systems. The particular features of polymers that make them attractive for these applications are their mouldability, conformability, and extreme ease of deposition in thin or thick film form. Thin polymeric films are excellent interlayer dielectrics in multilayer metallisation structures, and can therefore be used as insulating layers. However, it has become clear in the last 20 years that some polymers can be processed to have semiconductor or even metallic behaviour. A relatively new application is also the field of optical light guide devices.
Integrated circuits have also changed many aspects of our life. Moreover, while the microelectronic revolution has been spreading, the IC technologies have been gradually transforming the way of thinking of today's engineers and scientists regarding sensing devices.
In the 1960s, the technique of polymer based chemical sensors developed rapidly and gave a possibility for the direct detection of various ion- and molecule-types with certain selectivity limits. The research and development of conventional macrosensors (like polymer based ion-selective electrodes) turned soon in the direction of microsensors (like ion-selective FETs), in consequence of the miniaturization in microelectronics and of the expanding applications in biology, medicine, safety, and environment protection. Meanwhile, industrial automation and especially automotive electronics required the development of various low-cost and reliable physical and chemical sensors.
Various material structures are applied in sensors depending on the nature of the quantity to be measured. Their spectrum ranges from single crystals to amorphous polymers. In recent years, polymeric materials have been developed for many fields of sensing technology. The sphere of various sensor types where they are applied is a broad spectrum ranging from mechanical and acoustic sensors to electrochemical, chemical and biosensors, including conventional electric sensing devices and also the new family of fibre-optic sensors.
One important reason for this spreading use is the remarkable progress that has been made in the field of disordered polymeric materials in both theoretical and experimental applications and also the great advances made in polymer film preparation technologies. From another point of view, the progress is also accelerated by microelectronics, its devices and techniques, which tend to force sensor technology to employ low cost materials compatible with electronic technologies.
Although the practical application of polymer-based sensors has been developing rapidly, the theoretical backgrounds are often clarified only partly or not at all. There are debates about the signal excitation mechanisms, signal condition methods, and the interpretation of practically measurable and theoretically accepted results.
When I started to deal with this subject, I could not realize how enormous this area is. I have been working and teaching sensor technology for more than one decade (including also polymer based sensors), but I was surprised by the various application possibilities of polymers in this field. When a search of abstracting journals was done for the purpose of getting a survey, a lot of information remained hidden because the authors of the articles often did not emphasize the application of polymers. Only the detailed analysis of the papers could provide enough information for a general overview. A surprising result was then discovered: almost half of the promising chemical and biosensor types are nowadays based on some kind of polymers. The great variety of properties, which can be altered and controlled with different additives in polymers, is the basis of their application for various sensor purposes.
Polymers offer a lot of advantages for sensor technologies: they are relatively low cost materials, their fabrication techniques are quite simple (there is no need for special clean-room and/or high temperature processes), they can be deposited on various types of substrates, and the wide choice of their molecular structure and the possibility to build-in side chains, charged or neutral particles, and even grains of specific behaviour into the bulk material or on its surface region, enable films to be produced with various physical and chemical properties including sensing behaviour.
The wide range of sensors employing sensing polymers can be grouped according to the quantity to be measured as follows: temperature sensors, mechanical sensors (touch switch devices, deformation sensors, tactile sensors, pressure sensors, accelerometers, etc.), acoustic sensors (microphones, ultrasonic sensors, hydrophones, etc.), radiation sensors, humidity sensors, gas sensors, ion-selective sensors, biomedical sensors, and others (liquid component sensors, material identification, etc.). More recently the application of polymers has gained considerable attention also in actuators.
As a conclusion, it can be recognized that:
Sensors and actuators extend intelligent electronic systems to enable self-controlled communication with their environment; thus, they might be key devices in building the information society during the upcoming century.
Polymers and semiconductors are the materials, and their fabrication techniques are the technologies of the twentieth century. Semiconductor devices are the basis of "intelligent systems" and polymers represent "intelligent materials".
If these are combined then great new horizons may be opened.