What is exfoliated cellular DNA?
Exfoliated cell DNA, also known as contact trace DNA, follows the "Lockard material exchange theory" and refers to the traces left on the surface of the object after the human body comes into contact with it. Human epithelial cells are obtained through effective extraction methods, and DNA testing was performed on these cells.
The human body sheds about 400,000 epithelial cells every day, but these epithelial cells are the human tissue with the lowest success rate for DNA typing. Due to everyone's different lifestyles and differences in human metabolic functions, the left behind after everyone comes into contact with items The amount of exfoliated cells varies greatly. Since exfoliated cells are very small, in the micron range, and invisible to the naked eye, how to effectively solve the current problem of blind extraction of exfoliated cells is the key to improving the DNA detection rate.
In 2018, a research team composed of Adrian Linacre and others from Flinders University in Australia studied a technology that uses DD nucleic acid combined with fluorescent dyes to discover potential shed cells on physical evidence. The team organized a number of volunteers. After washing their hands, they long-pressed a variety of different carrier objects such as SIM cards, credit cards, cartridges , etc., and treated them with DD nucleic acid dye, and then used 495 nanometer blue light to excite fluorescence and imaging devices. Observed at 50× and 200× magnification respectively, the fluorescence emitted by the exfoliated cells can be clearly observed and is specific. After extraction, complete STR typing can be obtained. The following picture shows the relevant research results carried out by the research team:
The research team's research proved that using nucleic acid dye staining and fluorescence excitation is an effective method to discover exfoliated cells . However, since the DD dye stains nucleic acids after penetrating the cell membrane, the laboratory process is relatively long and requires complete immersion. At the same time, specific traces of contact must be known. Therefore, it will still take time for this method to be applied to actual cases. . Nonetheless, these research methods still provide an effective technical means for discovering exfoliated cells.
In addition, with the development of today's optical technology, biological technology and fingerprint display technology, although it is impossible to clearly observe the exfoliated cells, it is also an effective method to accurately extract exfoliated cells through the positioning of contact traces. technical means. It has been proven effective in actual cases and DNA typing has been successfully obtained. The relevant technical means are briefly described below.
Vacuum coating display technology
Vacuum coating is a method of depositing metal on physical evidence by evaporating or sputtering it in a high vacuum environment. In addition to being used to reveal difficult fingerprints, vacuum coating can also be used to locate contact traces on physical evidence, thereby enabling the extraction of exfoliated cells. Since vacuum coating is not only suitable for non-permeable objects, but also for semi-permeable and permeable objects, the contact traces that appear are the " negative " effect, that is, the metal coating will not cover the exfoliated cells and will not damage DNA , therefore many foreign criminology laboratories and domestic public security units have applied this technology to case handling.
For example, the Public Security Bureau of a certain domestic city used the Ruiying VMD900 large-scale vacuum coating fingerprint display system to detect the case in which a woman named Li was killed at home. After case study, it was discovered that the body had been dragged. To this end, vacuum coating technology was used to successfully locate the drag marks on the victim's clothing. Based on the positioning, the suspect's DNA was successfully typed after extracting exfoliated cells.
The use of vacuum coating technology can reveal the scratching and pushing marks on the cloth . It can not only locate the contact with DNA, but also play a key role in determining the nature of the case. The current representative product of
vacuum coating equipment is the Ruiying VMD1260/560/360 fingerprint display system , which is specially designed for the processing of criminal technical evidence. It has the characteristics of easy operation, accurate vacuum control in the cabinet and does not damage DNA. It has been used in the country The public security units have nearly 100 pieces of equipment.
502 Fumigation technology to reveal fingerprints
502 The principle of fumigation to reveal fingerprints is that the ethyl-cyanoacrylate (ECA) in the fumigation glue reacts with the fingerprint lines to form a milky white polymer, thereby turning the fingerprints into manifest.
This process is a physical process and will not destroy the exfoliated cells on the fingerprint lines. It should be noted that since the milky white polymer generated during the visualization process will cover the exfoliated cells, it is necessary to use a cotton swab to wipe them vigorously when extracting the exfoliated cells. The 502 fumigation method is easy to use and does not damage DNA. Currently, many domestic public security units are already using 502 fumigation cabinets, which organically combine 502 fumigation and exfoliated cell extraction, providing important technical support for case detection. The key to
502 fumigation technology is to use low-viscosity, high-volatility fumigation glue as much as possible, and accurately fumigation conditions in the control cabinet to achieve the purpose of quickly displaying fingerprints. When using low-viscosity fumigation glue, the fingerprints after displaying will not only have clear lines, but also be easy to wipe, which will facilitate the extraction of exfoliated cells. At the same time, it should be noted that before using the fumigation cabinet for evidence processing, 254 nanometer short-wave UV should be used to inactivate the DNA remaining in the cabinet to prevent cross-contamination of DNA.
At present, Ruiying MVC series is the representative product of 502 fumigation display cabinet. MVC fumigation display cabinet adopts quantitative control technology and uses low viscosity and high volatility fumigation glue to achieve clear display of fingerprints. The fingerprints after fumigation using the MVC fumigation display cabinet have clear lines and are easy to wipe and extract exfoliated cells. At the same time, the MVC fumigation display cabinet can be equipped with 254 nanometer short-wave components to inactivate DNA in the cabinet and prevent DNA cross-contamination.
Extraction of exfoliated cells from paper
After the suspect comes into contact with the paper, there must be exfoliated cells left on the paper. Through appropriate display methods, the suspect's contact traces can be accurately located for subsequent extraction.
Traditional chemical visualization methods include nindione, DFO and ninhydrin. These methods have been experimentally verified and will not cause damage to DNA; however, due to the chemical treatment process, especially immersion in the prepared solution or using The spraying of ninhydrin reagent through sprinkler irrigation will cause certain damage to exfoliated cells, thereby affecting the detection rate of exfoliated cells.
Thermal imaging technology currently widely used uses physical methods to quickly reveal fingerprints on paper. After visualization, fluorescence is stimulated by a special light source to accurately locate the remaining fingerprints and extract exfoliated cells.
Ruiying TFD2 is a device that quickly displays fingerprints left on paper through thermal imaging technology, and can excite the fluorescence of the fingerprints through special high-purity blue light and blue-green light, thereby accurately determining the location of the fingerprints left behind for subsequent extraction of exfoliated cells . According to tests by the manufacturer and domestic public security units, this method is safe and reliable. does not damage DNA, and the fingerprints that appear can be observed with the naked eye and extracted by photography. Domestic public security units have installed more than 100 units.
Targeting reagent technology
Currently, some domestic companies have launched targeting reagents that use fluorescent labeling solutions or other improved formulas based on indenedione, and use laser to excite fluorescence to detect potential traces of contact left on physical evidence. position.
The main advantage of this method is that it targets the appearance of fingerprints left on permeable objects, such as clothing, bricks, bags, wood and other objects, and extracts exfoliated cells through positioning. If uses the improved targeting reagent of indenedione, you need to wait for about half an hour after spraying and then use the laser to stimulate the fluorescence of the handprint, but the fluorescence difference caused by individual differences is also relatively large . At present, some domestic units are also trying to apply it to handling cases. There are many reagents of this type, but the principles are basically similar. You can also check out these .
exfoliated cell extraction technology
exfoliated cells Due to their inherent physical properties and the influence of different load-carrying objects, flocked cotton swabs, ordinary cotton swabs and dippers have been widely used on site. According to statistics from authoritative foreign case-handling agencies and the experience of domestic public security users, the use of flocked cotton swabs can effectively improve the detection rate of exfoliated cells. The key to the technology is that compared with traditional cotton swabs, flocked cotton swabs can not only extract more exfoliated cells, And in subsequent laboratory testing, more than 90% of eluted can be achieved.
Italian COPAN company uses nylon fiber flocked cotton swabs designed with patented technology. The top is a nylon fiber structure perpendicular to the base. This structure can maximize the DNA collection rate and ensure rapid and complete elution and release. . According to statistics from many domestic public security units, using nylon fiber flocked cotton swabs can increase the detection rate of exfoliated cells by more than 30% compared with traditional cotton swabs.
Forward-looking technology
It is reported that the Raman spectroscopy technology currently designed and developed by a company in Silicon Valley in the United States uses the short-wave Nd-YAG laser provided to the United States NASA for rapid positioning and display of handprints.
uses the unique advantage of laser monochromaticity, especially in the short-wave ultraviolet range, to stimulate the inherent fluorescence of fingerprints, to achieve the positioning of fingerprints on the surface of the object, and achieve good results. Compared with traditional 445 nm or 532 nm lasers, short-wavelength lasers can more effectively suppress the interference of background fluorescence, thereby obtaining Raman spectrum signals with a better signal-to-noise ratio. The company's applied scientists found that can produce two characteristic peaks of 330 nanometers and 440 nanometers when excited by a short-wave Nd-YAG laser, and the peak heights of the two peaks change in intensity over time. This intensity change is closely related to fingerprints. Ingredients related to .
Researchers from this company believe that changes in the potential fluorescence intensity of fingerprints can provide a reference for the time when the fingerprints were left behind. The following is the rendering of the same fingerprint through short-wave laser excitation and ninhydrin display:
The above-mentioned various technical means also need to be flexibly selected by the inspector according to the characteristics of the inspection material to achieve better results. Using physical methods first and then using chemical methods has always been the principle for carrying out relevant physical evidence examination work.
We believe that with the continuous development of science and technology, more effective technical means will continue to be introduced, thereby further improving the discovery and extraction capabilities of trace biological testing materials and assisting in the detection of cases.
