*Forensics*

In the realm of forensic science, there is an increasing need for new technologies to aid investigators and lab scientists in the pursuit of gathering worthwhile information from evidence. There are many subsections of evidence that are pertinent in the forensic field, but recent research has focused on three: Fingerprints, blood samples, and sweat-based field testing for ethanol and other drugs.

In the history of forensics, fingerprints have been essential in addition to being a widely emblematic feature of the forensic field in pop culture. However, fingerprints are mainly used in the field as a comparative means of identification [22], and if a print cannot be utilized for this pictorial comparison based on ridge structure, size, and shape, it is treated as exclusionary evidence [23]. This can be viewed as a large limitation on the amount of data that one can gain from this relevant piece of evidence. By analyzing the content of a print, namely the amino acids, one can gain some understanding of who the donor of that print was and be able to narrow down the search for the investigators. This is due to the metabolic [24] di fferences [25–28] in people due to their gender, age, medications, and lifestyle. By analyzing these types of biomarkers in a fingerprint sample, it would not only allow for one to gain much needed information that would provide additional context for investigators, but would also lead to the reduction in the need to wait for the lengthy analysis of DNA that causes a backlog [29], if any was recovered. According to the NIJ, a backlog is defined as any evidence that was not analyzed for at least 30 days after submission to a laboratory. By analyzing the content of a fingerprint instead of the pictorial fingerprint commonly relied on, it allows for smudged or partial prints, the prints that would not provide ample evidence for comparison, to have value for investigators. The chemical content of fingerprints has been examined as well, mainly focused on laboratory-based equipment such as mass spectrometric (MS) techniques that focus on total fingerprint content [24,30,31], drugs of abuse [32,33], and fatty acids to di fferentiate individuals based on age [34]. In addition to MS, there were optical techniques, as well using spectrophotometric instrumentation with age di fferences based on lipids [35], visual representation [36], and explosive content found on prints [37], as well as combined techniques such as desorption electrospray ionization (DESI) and direct analysis in real-time mass spectrometry (DART-MS) for total content [38–40] and for pictorial [41–43]. Further analysis of these techniques can be found in a review in Trends in Analytical Chemistry [44].

In addition to fingerprints, blood is another matrix that is commonly obtained as evidence for forensic investigators. In addition to the commonly known DNA matching with a database [45,46], bloodstains are also used for splatter analysis [47–49], and there are even techniques to di fferentiate if there are multiple overlapping stains [50]. An important quality relating to the bloodstain that was missing was the time since it was deposited onto the surface, utilizing a technique that is practical and did not require grea<sup>t</sup> sample prep or laboratory instrumentation. This is a vital piece of information as it would allow for corroboration of stories told by possible witnesses and would enable the reduction in irrelevant and unnecessary lab work to be done with blood that is too fresh or too old at a particular scene. Another lapse in analysis of bloodstains is the determination of the age of the person the blood is from, as this can be done with DNA but is a lengthy process [51]. Similar studies using biosensors were to find biological sex and ethnicity [52,53]. Other lab techniques were attempted to be able to perform age deduction; however, they were not applicable, due to serious flaws in the techniques [54].

The third forensic matrix is sweat. Sweat is a viable forensic sample for multiple reasons, as it contains a small amount of DNA [55–58], and other metabolites and compounds [59–61], and people leave traces of it upon contact of surfaces with their skin [62]. As fingerprints contain sweat in addition to other components, it is a comparable matrix to what has been discussed previously. Sweat can be detected in the field utilizing one of the main components in sweat—lactate [63–67]. Lactate is prominent in sweat samples and, even though there are techniques that can be used in order to detect it [68–71], they require complex laboratory instrumentation and are not viable methodologies for on-site deployment. Some of these techniques rely on a tattoo-like sensing device that consists of a potentiometric sensor with a wireless receiver [70,71], involving the use of a screen-printed electrode on tattoo paper with a microfluidic channel for sweat collection and detection of analytes. By being able to find sweat on surfaces directly at a crime scene, investigators would have additional samples of viable evidence. Building upon this, emerging research has also looked at analysis involving ethanol. Multiple studies have shown that ethanol is excreted into sweat [72,73] and that sweat cannot be

tampered with, similarly to how people "trick" breathalyzers. There are wearable technologies that allow for biosensing of metabolites in sweat [74,75]; however, they have a long delay, some up to two hours, that makes them useless for on-site Driving While Intoxicated (DWI) analysis. Similar research has looked into saliva for the identification of tetrahydrocannabinol (THC) [76] and an overall approach that analyzes multiple types of legal and illegal compounds in sweat [77].
