A biosensor is a biological sensor. In simple words, a biosensor is an analytical device that can translate changes in the biological process to an electric signal. It is a combination of a biological sensor like enzyme, tissues, cells, acids and microorganisms and a transducer that will convert the changes in the biological sensor to an electrical signal. It also has a signal controlling unit, a microprocessor and a display unit. The biological element generates a biological response which is transduced to an electrical signal. This signal is amplified and filtered by the signal processing unit. The output is an analogue signal which is equivalent to the measured biological quantity. The analogue signal is passed to a microcontroller which converts it into a digital signal that is displayed on the LCD screen. In this article, we will have a deep insight into biosensors and saliva biosensor.
Characteristics of Biosensors:
Some of the key characteristics of biosensors are:
- It should be small in size and bio-compatible.
- It should be low in cost.
- It should give quick results.
- It should be very easy to use and capable of repeated use.
- It should be highly specific in detecting the analyte.
- The performance should be independent of temperature, pH, stirring, etc.
- The response should be linear for a wider concentration range.
Types of Biosensors:
The biosensors can be classified based on the biological element used and based on the transducer employed.
Based on the biological element used, the biosensors may be:
- DNA based
- Enzyme based
- Microorganism based
- Cell receptor-based
- Phage based
Based on the type of transducer used, biosensors may be:
- Optical based
- Fibre optics-based
- Surface plasmon based
- Raman and FTIR based
- Electrochemical based
Potential applications of Biosensors:
Biosensors have potential use in various fields like:
- Food industry (Example: monitoring acid and alcohol content)
- Agriculture industry (Example: monitoring of pesticides)
- Environmental monitoring (Example: monitoring pollutants from water)
- Industrial applications (Example: monitoring the generation of fermentation products)
- Medical, clinical and diagnostic application (Example: monitoring of glucose)
Blood has been the gold standard body fluid for various diagnostic purposes to date. However, the use of oral fluids like saliva and gingival crevicular fluids provide a non-invasive method of a diagnostic test. With oral fluid, the collection of samples is easy, it is safe to handle and store, they require smaller volumes, repeated sampling is easy and painless, and they have better sensitivity. Saliva is a watery liquid secreted by the salivary glands into the mouth and is often called a “mirror of the body”. Any changes in the composition of saliva are known to reflect the wellness of the individual as it has numerous analytes important for disease diagnostics. At least 1166 proteins have been identified from human saliva. Markers like matrix metalloproteinases, cytokines, interleukins, transferrin, tumour necrosis factors, etc which are associated with several types of cancers have been found in saliva. It is also a powerful body fluid for the diagnosis of HIV, hepatitis A, hepatitis B, hepatitis C, etc. Nanotechnology and microfluidics approaches have further increased the sensitivity of saliva-based assays and have enabled detection using multiple biomarkers for better and efficient disease detection. Saliva-based biosensors are used for the diagnosis of dental caries, periodontitis, oral cancer, diabetes, HIV, psychiatric disorders, sexual hormone disorders, cardiovascular diseases, obesity, drug abuse, and cancer. Despite the several advantages of saliva-based biosensors, the lack of their market readiness and the awareness among the people have restricted their use. However, it is certain that with further advancements, saliva-based biosensors will outperform the use of blood-based biosensors.
Examples of Saliva Biosensor
In 2015, the first saliva-based glucose biosensor was made and was tested with the saliva of healthy volunteers. The results were very rapid with high sensitivity and repeatability. This was a particularly important development as the conventional method of glucose detection is the invasive finger-prick method which is often uncomfortable and painful. However, saliva-based biosensors are a non-invasive, easy and safer alternative to the traditional use of blood glucometers. Furthermore, saliva-based biosensors may also enable continuous monitoring of blood glucose for a long period. As the glucose concentration in saliva will be less compared to blood, such biosensors need to be more sensitive and should accurately measure the glucose level. The saliva-based glucose biosensors available currently in the market show a linear range of glucose measurements at concentrations that are 100 times less than the glucose concentration in the blood. Several reports have shown a very strong correlation between blood-based and saliva-based glucose estimation.
Another example of a saliva-based biosensor is for the estimation of cortisol, which is a steroid released under conditions of physiological stress, Addison’s disease and Cushing’s syndrome. Salivary-cortisol biosensors enable immediate detection of salivary cortisol. Such kind of biosensors enables easy sample collection, storage and handling.
Several studies have focused on the development of saliva-based biosensors for general care, cancer, dental care, pharmacological studies, and so on. Such biosensors make use of biocompatible materials to minimize the toxic effects of the sensors when it is worn in the oral cavity. With increasing technological advances, more and more biomarkers can be studied for the detection of a multitude of medical conditions. Future advances may be focused on the miniaturisation of such biosensors and their integration into medical devices. The effect of the bio-material on oral health and the effect of the oral microbiome and the ingested food on the biosensor function has to be studied thoroughly. The life span of the implantable biosensors should be determined to enable the timely replacement of the biosensors. Wireless transmission of the data with blue tooth devices, continuous monitoring of analyte levels and easy data transfer are some other areas to focus on.
- Liu X, Hsu, HPC, Liu WC, et al, 2019. Salivary electrochemical cortisol biosensor based on Tin disulfide nanoflakes. Nanoscale Research Letters 14, 189. https://doi.org/10.1186/s11671-019-3012-0
- Lakshmi KR, Nelakurthi H, Kumar AS, Rudraraju A, 2017. Oral fluid-based biosensors: A novel method for rapid and noninvasive diagnosis. Indian Journal of Dentistry 9: 60-66.
- Zhang W, Du Y, Wang ML, 2015. Non-invasive glucose monitoring using saliva nano-biosensor. Sensing and Bio-sensing research 4: 23-29.
- Ilea A, Andrei V, Fuerdean CN, et al., 2019. Saliva, a magic biofluid available for multilevel assessment and a mirror of general health – A systematic review. Biosensors 9(1): 27.