Principal component analysis of roughness, wettability, and surface free energy parameters. This graph represents the Principal Component Analysis (PCA) of roughness (Ra), wettability, and dispersive and polar parts. PCA is based on the specific variance of the three variables (roughness, wettability and surface free energy) and extracts a minimum number of factors (here two factors: F1 and F2) that explain the greatest possible proportion of the specific variance. The almost right angle formed by wettability and roughness indicates that these two variables are independent of each other. Credit: Recipon, M., Agniel, R., Kunemann, P. et al.
Touch DNA, which can be found at crime scenes, consists of invisible biological traces deposited through a person’s skin’s contact with an object or another person. Many factors influence touch DNA transfer, including the “destination” substrate’s surface. The latter’s physicochemical characteristics (wettability, roughness, surface energy, etc.) will impact touch DNA deposition and persistence on a substrate. We selected a representative panel of substrates from objects found at crime scenes (glass, polystyrene, tiles, raw wood, etc.) to investigate the impact of these characteristics on touch DNA deposition and detection. These were shown to impact cell deposition, morphology, retention, and subsequent touch DNA genetic analysis. Interestingly, cell-derived fragments found within keratinocyte cells and fingermarks using in vitro touch DNA models could be successfully detected whichever the substrates’ physicochemistry by targeting cellular proteins and carbohydrates for two months, indoors and outdoors. However, swabbing and genetic analyses of such mock traces from different substrates produced informative profiles mainly for substrates with the highest surface free energy and therefore the most hydrophilic. The substrates’ intrinsic characteristics need to be considered to better understand both the transfer and persistence of biological traces, as well as their detection and collection, which require an appropriate methodology and sampling device to get informative genetic profiles.
At the scene of an offence, investigators are confronted with traces of different kinds on a wide range of substrates. Visible traces such as unwashed blood can be collected and analyzed, but what about invisible traces such as touch DNA?
Touch DNA, which refers to a loss of biological material transferred from a donor to an object or another person during physical contact is a real challenge for officers who have to collect it using a best assumption. These biological traces are invisible to the naked eye, cannot be detected using the usual tools (polychromatic lights like Polilight®, or chemiluminescence like Luminol/Bluestar®), and consequently may be poorly collected, even though they represent the majority of traces analyzed in forensic laboratories. The composition of touch DNA is still not clearly defined in the literature. According to Burill, et al., these traces may come from shed skin cells (corneocytes) and other biological material (body fluids, saliva, nasal secretions, sebum, epithelial cells, etc.), but van Oorschot revealed as early as 1997 that it is possible to obtain genetic profiles from manipulated objects.
Several factors influence the deposition of touch DNA, such as the nature of the contact (for example, the amount of DNA deposited increases with increasing finger pressure), the donor (the amount of DNA deposited may vary from one donor to another), and the surface specificities of the substrate on which the trace is deposited. In fact, depending on the type of substrate, the latter can have an impact not only on the deposition of the trace, but also on its persistence, detection, and collection.
A few studies have focused on the detection of touch DNA as a function of surface substrate type. Champion, et al., attempted to assess the relevance of detection using Diamond™ Nucleic Acid Dye (Promega, USA) on a range of substrates in order to provide recommendations on the use of Diamond. However, despite the positive effects of Diamond™, negative interaction states during direct PCR have been reported. In another study, our team developed a proof-of-concept based on a set of markers for the optimal detection of touch DNA combining cellular targets (DNA, keratin, laminin) and molecular targets such as carbohydrate patterns present in human cell glycoproteins (mannose, galactose, N-acetylglucosamin, and fucose) in order to improve their subsequent collection and success of genetic profiling. This innovative proof-of-concept made it possible to detect touch DNA, notably on a mobile phone, but it remained to be tested on several types of substrate.
Further research into the physicochemical composition of substrates and their impact on the deposition, persistence, detection, and collection of touch DNA remains essential for the forensic world’s ability to enhance the editing of robust genetic profiles, allowing for better decision-making during criminal investigations in keeping with the 2020–2025 European Union Security Strategy for fighting organized crime. In this way, this study has attempted to respond to this challenge by:
- Developing in vitro touch DNA models on a panel of crime-scene substrates: reconstruction of touch DNA from characteristic cells (keratinocyte cells derived from a skin and fingermark touch DNA model) on different substrates that mimic those typically found in the field.
- Analysis of physicochemical characteristics on substrate surfaces likely to modulate the deposition and persistence of touch DNA biological traces.
- Detection of biological traces on several substrates, their persistence in time, and obtaining informative DNA profiles after their collection.
The development of these in vitro touch-DNA models, using several on-field substrates, has enabled us to get insight into the impact of physicochemical characteristics on the editing of a genetic profile. These models have enabled to apply the proof-of-concept of touch-DNA detection using cellular-derived markerson a panel of substrates of different typologies.
Numerous forensic studies have traditionally used glass, polystyrene, or metal as standard substrates. Having taken into consideration the literature and actual crime scenes, we selected 10 types of substrate that are representative of materials found at a crime scene, namely: glass, polystyrene, two types of ceramic tiles C1 and C2, PVC floor covering, sticky adhesive tape, non-sticky adhesive tape, metal, varnished wood, and raw wood.
While the characterization of material surfaces is commonplace in the study of the cell colonization of biomaterials, it remains less explored in the forensic field, particularly in the study of cell–substrate interactions. The physicochemical characteristics and nature of a substrate influence cell deposition and behavior. It should be noted that, for the purposes of our study, the substrates were measured in their raw state in order to be as faithful as possible to crime scenes.
The physicochemical characterization enabled us to classify the substrates into three main groups:
- Group 1: Roughest substrates (PVC floor covering, raw wood).
- Group 2: Substrates with the highest surface energy and therefore the most hydrophilic (glass, C1, C2).
- Group 3: Substrates with the highest hydrophobicity and therefore the lowest surface energy (sticky adhesive tape, non-sticky adhesive tape, varnished wood, polystyrene, metal).
Once characterized, these substrates were tested by developing in vitro models of biological traces, which were similar to the reality in the field, by means of a calibrated deposition of cells characteristic of skin keratinocyte cells and fingermarks.
This choice of cell models was motivated by the fact that touch DNA derives notably from corneocytes desquamated from the surface of the skin. However, the latter are derived from keratinocytes that have undergone terminal differentiation, which leads to a flattened, anucleate cell morphology (with free DNA which could be used to edit a possible genetic profile), devoid of organelles and rich in keratin. It is for this reason that these cell models are best suited to our study. In the forensic literature, very few in vitro models have been developed using keratinocyte cells, except those in the study by Lee, et al., which allowed them to study the persistence of DNA for up to 85 weeks on different substrates (drink containers, wristbands, paper, aluminum, glass, plastic). In vitro models are generally blood cell deposits, mouth cells, and sperm that are generally modelled. In our study, we were able to show that cells deposited on substrates of different typologies were present, could be detected using our detection strategy, and persisted over time for up to 2 months. Touch DNA detection was successful on most types of substrate. However, in the case of raw wood, the cells were difficult to detect as they may have penetrated into the fibers and no longer been on the surface. So, despite the heterogeneity of substrates, we have developed a universal detection strategy based on cellular-derived markers, where the type of substrate has no impact on this strategy. This detection strategy can be applied to a wide range of substrates, enabling touch DNA to be localized, better sampled, and optimized for genetic analysis.
This study has enabled us to develop in vitro touch DNA models, as close to reality as possible, in order to better understand the interactions between a substrate and a biological trace. We were thus able to show that it is important to consider physicochemical characteristics in order to understand the adhesion properties of the traces, and thus to collect them as effectively as possible in order to obtain a usable genetic profile. The physicochemical properties of the different substrates are essential not only to a better collection of traces, but also to explaining the qualitative and quantitative differences in the analysis of traces in justice.
Whatever the substrate, touch DNA could be located and detected thanks to a detection strategy based on cellular and molecular targets, without any impact from the nature of the substrate. This is a new strategy in the forensic world for the in vitro and in situ detection of touch DNA.
Republished courtesy of Nature.