Electromagnetic sensor technology for biomedical applications

Abstract

Magnetic bio-detection constitutes a large area of research and development driven by its potential to provide versatile diagnostic tools in biology and medicine. Specific sensing technology is used depending on the applications which can be subdivided in two main groups: measuring a magnetic field from people and detecting magnetically labelled bio substances. The human body is mostly composed of what is normally regarded as nonmagnetic materials. In reality, every substance has some magnetic sensitivity, however small, being paramagnetic or diamagnetic. Their response is greatly limited by thermal fluctuations. In addition to this there is a further source of a magnetic field due to the neural activity which operates continuously throughout a living body. This neural activity involves movement of electric charges and, as such gives rise to magnetic fields. In principle, these fields represent a description of the neural activity and can be studied to help understanding the workings of the human body as well as provide an aid to diagnosis. On the other hand, they can be predicted and quantified by the fundamental laws of electromagnetic. In the other stream of applications, the use of magnetic labels allows the detection of various bio molecular reactions in immunoassays. It also results in a number of additional functionalities, such as transport of bio-molecules to a specific location, on-chip magnetic immuno-separation and testing or accelerating bio-molecular binding events. The magnitudes of the magnetic fields involved are very small, being in the sub-nanoTesla region and their detection requires very sensitive instrumentation. We have extremely sensitive magnetic technology: SQUID magnetometer (superconducting quantum interference device). The noise level of the SQUID detection is in femto-Tesla so it could become an ideal instrument for studying magnetic fields from biological subjects (see, for example, Sternickel & Braginski, 2006). However, the cost involved and the complex cryogenic technology present huge hurdles that have prevented SQUID (including high transition temperature requiring liquid nitrogen) from becoming widely used. Several field measurements from various parts of the body were published and summarised in (Wikswo , 1999). Figure 1 presents comparison of some common magnetic fields and those generated by different parts of the human body. The signals from the brain are at about 1 picoTesla or less but from other parts of the body (such as the eyes and the stomach) are at levels an order of magnitude larger. One of the largest signals results from the heart which is at the level of about 25pT. The detection of this level of a magnetic field does not require all the