*Biometrics*

Currently, biometrics is another avenue of interest for biosensors [149]. This paper from Hair et al. uses the levels of three metabolites found in sweat in order to di fferentiate people. This analysis is performed using three enzymatic assays that each target the metabolites: Lactate, urea, and glutamate. The assay for lactate is the same one that was outlined in the sweat paper involving the paper strips. The assay for urea involves the enzymes urease and glutamate dehydrogenase. The final assay that is used for glutamate involves only glutamate dehydrogenase. These assays could be measured spectrophotometrically using either a redox dye in LOx/HRP or with conversions involving NADH and NAD<sup>+</sup> in the other two. First, 50 mimicked sweat samples were run and compared, which showed that this method was viable as there was no overlap between the samples. Additionally, 25 authentic samples were analyzed, where the sweat was collected according to the same procedure as the Gibson and Cooke method mentioned above [148]. A multivariate analysis of variance statistic test (MANOVA), was performed for both the authentic and mimicked sample sets to determine if the combination of the three analytes were truly unique. Both sets produced *p*-values of <0.001 each. In addition, six ANOVA tests were performed between each analyte for the mimicked and authentic samples. All six of these tests also produced *p*-values of <0.001. These statistical values show that there is not only a significant statistical di fference between individuals in the combination of the three analytes, but also a significant di fference between each individual for a single analyte. The analysis shows that these three metabolites can be used in order to produce an individual's "sweat profile," enabling di fferentiation between individuals. The implications for this research are multidisciplinary as there are many forensic, cybersecurity, and point-of-care diagnostic applications that would benefit from this method.

This use of sweat content for biometric purposes is progressing but further research needs to be done for biometrics to be a reliable form of authentication using biosensor methodologies. The main future aspect that would need to be studied is a long-term study relating to the monitoring of the levels of the chosen metabolites in people and how the levels fluctuate over time relating to di fferent factors such as stress, diet, and other day-to-day habits. For higher security when used for authentication, especially for higher-security systems and cybersecurity, additional metabolites would need to also be concurrently measured as well. This monitoring process would not only assist in the future of biometrics but also in existing disciplines such as clinical diagnostics.
