Article authored by: Stephen Michielsen, Michael Taylor, Namrata Parekh, Feng Ji
Abstract
Bloodstain pattern analysis, BPA, on hard surfaces (such as walls, tables, appliances, hardwood floors, etc.) has grown into a science-based investigative tool that can help determine scenarios that are consistent with or counter to the events described by witnesses or suspects. At the vast majority of crime scenes involving a bloodletting event, textiles are present as apparel, household textiles (sheets, towels), upholstery, carpets, and so forth. Yet, the science of BPA is not able to render the same level of confidence in the analysis as on hard surfaces due to the complex structure of textiles and their ability to wick liquids. In the work described herein, a detailed examination of factors that affect BPA on two textile fabrics, an unbalanced 130 x 70 plain woven 100% cotton bed sheeting fabric (often referred to as a 200 thread count bed sheet) and a 100% cotton jersey knit T-shirt fabric.
During this study, both porcine blood and several synthetic blood recipes were used. The dynamic impact tests (time after impact < 100 ms) used porcine blood, while most wetting and wicking experiments employed synthetic blood (time after impact > 100 ms). Most of the synthetic blood recipes examined performed badly. Either they would not dry or they did not wick into the fabrics, but remained on the surface. A synthetic blood recipe from the American Society for Testing Material (ASTM test method F1819-07) performed well, but its viscosity and surface tension were both lower than typical human blood. Thus, this recipe was modified to lie within the range of surface tension and viscosity of human blood. It was used for the majority of wicking and wetting experiments. In a preliminary comparison, it was found that synthetic blood SB5 behaved similarly to porcine blood in many aspects, but the SB5 stains were significantly larger than the porcine bloodstains. We attributed this difference to the presence of red blood cells, which behave as particles, as well as plasma, which behaves as a liquid, in porcine blood. SB5 is an aqueous solution and behaves entirely as a liquid.
Using porcine blood, it was found that bloodstain development had two clear timescales, unlike many hard surface events. For times less than 100 ms after impact, the dynamics of impact and the interaction of the spreading drop led to distinctive bloodstain patterns, similar to bloodstains on hard surfaces but with characteristic differences attributed to the surface texture of the fabric and due to the energy absorption by the fabric and the presence of any supporting material in contact with the fabric. Using synthetic blood, it was found that if the fabric was lying on a hard surface, the initial pattern resembled a bloodstain on a hard surface, but more extreme. On the other hand, if the fabric was lying on a soft surface, much of the impact energy was absorbed and only a small (synthetic) bloodstain pattern occurred. In a third scenario, the fabric was suspended across two objects or held tautly between them. If suspended, the fabric absorbed much of the energy and only the highest impact velocities led to a stain with spines or satellite stains. However, if it was held taut, it acted as a drumhead and tossed the drop back off the surface, which led to stains with irregular geometries. These findings indicate that where there are bloodstain or bloodstain pattern evidence located on fabrics, it is important for the BP analyst to This document is a research report submitted to the U.S. Department of Justice. This report has not been published by the Department. Opinions or points of view expressed are those of the author(s) and do not necessarily reflect the official position or policies of the U.S. Department of Justice. 2 note and document any surrounding or backing textile surfaces that may have come into contact with the bloodstains or bloodstain patterns that were observed. Examples include a bloodied bed sheet lying on top of a bedding fabric at a crime scene or a bloodied shirt over a T-shirt worn by the suspect. In both these scenarios, the shape, size and distribution of the observed bloodstains or bloodstain patterns may be distorted and the final appearance which is observed by the BP analyst will look different compared to if these stains are deposited onto a hard surface.
After the initial impact, the wicking within the fabric takes over. Asymmetry in the fabric structure can lead to an asymmetric synthetic bloodstain patterns. These asymmetric patterns can lead to a stain that appears to come from an angle, even when a drop impacted the surface at right angles. For a drop that impacts the fabric from a known angle, it can lead to a pattern that appears to originate from a different angle or even from a different mechanism. This might lead to misinterpretation, for example, a passive drop falling onto an unbalanced woven fabric may be wrongly classified as a projected bloodstain. Furthermore, different fabric constructions and even different yarn manufacturing processes can lead to very different wicking behavior. This is exemplified in a synthetic blood transfer experiment that showed that when synthetic blood was applied to a woven fabric in a stack with a knit fabric, the larger stain occurred on the knit fabric.
In addition to these factors, it was observed that wicking resulted in a much larger stain than the initial drip stain that occurs immediately upon drop deposition. As the stain grew over the next 2 – 5 minutes, it spreads and masks the initial bloodstains, eliminating any of the initial spines and many of the nearby satellite stains.
Based on these studies, a bloodstain analyst should approach the interpretation of bloodstains and bloodstain patterns on textiles with great caution and be aware of the textile factors that can influence the appearance of blood on fabrics; as what they observe may not resemble the initial stain or the actual mechanism of deposition of the blood. This study has demonstrated that fabric may interact with, distort and alter the stain in many different and complex ways. Considerable additional research is needed to bring BPA on textiles up to the standards expected for BPA on hard surfaces.
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