Effect of yarn structure on wicking and its impact on bloodstain pattern analysis (BPA) on woven cotton fabrics

- Bloodstains from identical events on nearly identical fabrics can differ.
- Yarn structures can affect the size and shape of a bloodstain.
- Vertical impact can result in elliptical stains for some fabric constructions.
- Blood may wick faster in one direction of a fabric than another.
- The shape of the final bloodstain may bear little resemblance to the initial stain.

Bloodstain pattern analysis (BPA) of bloodstains on hard, non-porous surfaces has found widespread use in crime scene analysis and reconstruction for violent crimes in which bloodshed occurs. At many violent crime scenes, bloody clothing is also found and may be analyzed. However, to date, there are no definitive methods for analyzing bloodstains on textiles, even for simple drip stains. There are two major classes of textiles used for apparel and household textiles, weaves and knits. In this article, drip stains on two 100% cotton plain weave fabrics representative of bed sheets are analyzed. Since it is common practice in the manufacture of bed sheeting to use different types of yarn in the warp and weft direction to reduce cost, custom weaves were made from yarns produced by each of the three most common staple yarn production techniques to control this variable. It was found that porcine blood wicked into the fabrics made with ring spun yarn, but not into those made with open end or vortex spun yarns. The uneven wicking of blood into the different yarns resulted in elliptical-shaped stains on commercial bed sheeting that can be misleading when performing bloodstain pattern interpretation based on the stain morphology. This surprising result demonstrates that it is not sufficient to analyze the structure of the fabric, but one must also characterize the yarns from which the fabric is made. This study highlights the importance of a deeper characterization of the textile structure, even down to the yarn level, for BPA on textiles.