Medical sensors used for monitoring and diagnosis

Citation

Santha, H. (2005), "Medical sensors used for monitoring and diagnosis", Sensor Review, Vol. 25 No. 4. https://doi.org/10.1108/sr.2005.08725daa.002

Medical sensors used for monitoring and diagnosis

Medical sensors used for monitoring and diagnosis

Keywords: Medical sensors

Since health is considered to be of great value in ones life even high-tech and high-cost solutions can be welcome in this area. But the big business opportunity resides, as always, in mass producible home use devices (e.g. blood glucose testers, automated blood pressure meters), if possible, complemented with consumable products (test strips for glucose, cholesterol, etc.), thus at first, such solutions are able to reach the wider public and the market. However, technical revolutions of the field are hindered not just by technical challenges. Very strict ethical points have to be respected during development and validation of yet unproven technologies and according to the spirit of the Golden Rule – “do no harm”, the natural disapproval of physicians towards unproven technologies also impedes rapid advancements.

Diagnostic and monitoring data from the human body to help and support physicians in their decision making can be acquired through non-invasive or invasive ways. Thus, sensors in medical applications can be divided into two groups based on this property. Non-invasive methods practically never face revulsions raised by physicians, because this approach is in perfect harmony with the Golden Rule, so their development and validation is much simpler in this regard compared to invasive solutions and the bottleneck is usually the technical side. Technical difficulties rise from the limited and small scale interactions between living tissue not to be damaged and an outer sensing device working with electronic (e.g. ECG – electro cardiograph), optical (e.g. pulsoxymetry) or mechanical (e.g. ultrasound) signal transducer principals.

Apart from the method, the nature of data intended to be acquired from human body can be the basis of another division into two categories, namely data acquired by medical imaging and data concerning the biochemical state of the body. It means that physicians can be supported either with additional information on anatomic structure of organs, mucous membrane surfaces, ventricles, etc. or with detailed and updated knowledge about the functioning of certain organs by actual or chronic biochemical parameters of the body (e.g. concentrations of different analytes, macromolecules or cells in blood, extracellular fluid, urine, etc.)

Practically speaking, in the first case doctors would like to see specific internal body parts without undertaking surgery, and as far as possible, see things with a higher resolution (e.g. OCT – optical coherence tomography, promises sub cellular resolution with 1-2 μm) and with a better differentiation between several types of tissue (e.g. NMRI – nuclear magnetic resonance imaging and PET – pozitron emission tomography) than with their own eye. In the second case doctors would like to know more about the operation of certain organs by obtaining data about some specific material content of fluids, cells, breathing gases, etc. as fast and as accurate as possible. These latter needs generate the market for the so-called point of care (POC) tests.

Medical imaging equipments are not really considered as sensors by the wider public, although they usually comprise an array of sensors for optical (visible, infra red, X-ray and other frequencies of electromagnetic radiation), magnetic or ultrasound signal transduction. This phenomenon clearly indicates, that if a physician needs complementary data of an anatomic structure not available with traditional examinations but probably available with some invasive methods (surgery, biopsy, histologic analysis) one simply considers this as a demand for a better imaging system and not for a specific sensor.

Although this argument can be comprehended, the significant advancements in medical imaging since 1895, when Wilhelm Conrad Röntgen discovered X-rays by a co-incidence, must be respected, because of the millions of people whose health have been rescued by the occurring innovations in the field of medical imaging sensors and systems. During the last decades advanced methods and basically new imaging principles too have conquered the health care industry (CT – computerized tomography, NMRI, PET, SPECT – single photon emission computerized tomography, isotopic imaging techniques, 3D and 4D ultrasonic imaging, IR body mapping, OCT, several endoscopic devices, etc.) and nowadays this area keeps growing its market potential as advancements in technology and image processing make better-and-better devices to be available for lower-and-lower prices every year.

Sensors to determine not the structure but the functioning of certain body parts are usually considered as typical monitoring and diagnostic sensors of medicine. Based on the above they can be divided into invasive and non-invasive methods. Besides, they can be divided also in biosensor based and non-biosensor based solutions, because not every sensor applied in medicine is a real biosensor. A real biosensor incorporates a biological or biologically derived sensing element either integrated within or intimately associated with a physicochemical transducer. The usual aim of a biosensor is to produce either discrete or continuous digital electronic signals that are proportional to a single analyte or a related group of analytes which analytes act as the counterpart molecules of the immobilized biological sensing elements. Thus, biosensors are based on a molecular size key-lock effect.

Biosensors are always connected to sample taking processes which can be invasive (e.g. finger pricking, blood cupping, biopsy) or non-invasive (e.g. urine sampling, saliva sampling, through skin low current reverse electrophoretic sampling of extra cellular fluid). For the case the biosensor would be implanted in the human body the sample taking process should not penetrate the borders of the body, but such approaches would never overcome the limited lifetime of the immobilized bioreceptor molecules, thus, biosensors cannot be considered the real solution for implanted devices. Such earlier concepts are replaced yet by the idea of immunoisolation, by which whole cells having the required function are implanted with an immunoprotective encapsulation, in order to avoid tissue rejection. In general, most commercialized biosensor devices need at least one drop of blood as a sample and the biosensing surface is realized on a disposable single use test strip or fluidic cartridge which has a limited self life and must be obtained as a consumable. The four basic groups of biosensors can be distinguished by the type of immobilized biological sensing elements: enzymatic biosensors, immunobiosensors, DNA sensors, living cell based biosensors. There is a separated group for chemical sensors, which can be targeted to several small molecular analytes or ions and based on arteficial receptor surfaces or semipermeable/ionselective membranes, which surfaces/membranes mimic the key-lock effect of nature as much as possible in order to reach a high selectivity.

Biosensors and chemical sensors have to compete with traditional analytical laboratory and microbiological methods. Advancements in these emerging fields are derived on one hand from our growing knowledge in biochemistry, supramolecular chemistry, protein engineering, nanotechnology, etc. on the other hand from the advanced level of miniaturization of electronics, optics and fluidic structures, highly developed signal processing circuits and data processing softwares. In medical applications handheld rapid analytical devices for blood glucose started the revolution ca. 20 years ago and now handheld or at least portable devices are commercialized for several metabolic blood parameters (like cholesterol, triglycerides, lactate, D-dimer, etc.). Since 1970s the major efforts in the world have been done for blood glucose meters, because this is the biggest market potential, thus, a disposable test strip for glucose costs 0.25 Euros while a test strip for cholesterol costs more than 2 Euros. Apart from the mentioned automated home applications further rapid analytical devices are available for professionals and research labs for DNA sensing, pathogen detection or drug development, but these devices need experienced personnel in order to be properly operated.

Non-invasive sensors to determine the functioning of certain body parts have the highest scientific, public and economic interest. Such devices could be sold in high quantities, because the inherent risks and inconveniences are very low. However, such sensors cannot be based on too many principles. ECG and EEG – electro encephalograph, are not real sensors, because practically they comprise some specifically located metal electrodes to collect electrical potential changes of the mV and μV range usually from the skin. To check the operation of blood circulation ultrasound transducers exploiting the Doppler effect can be applied. Recently the monitoring of human motion by wearable accelerometers and angular rate sensors is an interesting topic without much medical significance.

The most successful non-invasive method is an optical one, namely the pulsoxymetry, which determines the oxygen saturation of blood and exploits that oxygenated blood has a light red color while deoxygenated blood has a brownish dark red color and with a sensitive setup it is possible to measure how light or dark is the blood in the tissue, and determine the proportion of oxygenated and deoxygenated blood in that area. Of course optical spectrometric methods have been investigated for other analytes too, first and foremost for glucose (Instrumentation Metrics^®, Optiscan^®), but no such technologies apart from pulsoxymetry could conquer the market so far.

Impedance spectroscopy was also a promising concept, after in April 1999 a medical doctor has published, that changes of concentration of Na⁺ ions in blood significantly correlate with changes of concentration of blood glucose. A device based on this principle (Pendra-The Watch^®) was already announced to come to the market at the end of 2004, but because of some unpredicted unreliabilities discovered by the last validation studies the commercialization has been withdrawn.

To summarize the current situation of sensors used for monitoring and diagnosis in medicine, one should see that novel groundbreaking principles are rare (e.g. optical coherence tomography, impedance spectroscopy for in-vivo non-invasive blood glucose monitoring, thermal excitation of glucose molecules for in-vivo non-invasive blood glucose monitoring (Optiscan^®)) and not always satisfactory but are still being generated and the main advancements towards widespreading of medical sensors come from developments in fabrication technologies, data processing technologies and miniaturization.

Hunor Santha Department of Electronics Technology, Budapest University of Technology and Economics, Budapest, Hungary