The development of sensitivity tests for the creation and use of synthetic blood substitutes in bloodstain pattern analysis

Abstract

There is a growing interest within the bloodstain pattern analysis community towards the development of synthetic blood substitutes (SBSs) for use in forensic training and research. SBSs are attractive alternatives to using human and animal blood because they are safe, cost-effective and standardized. A robust substitute must comply with the relevant fluid behaviours of common blood-letting events found at crime scenes. These were identified as fluid spreading, splashing, impact and transfer dynamics. This thesis sets standards for future studies to critically analyse candidate SBSs in dynamic drip, impact and transfer pattern formation. It presents a set of highly discriminatory tests that has captured a substantial dynamic range of fluid-dependent stain and pattern characteristics. This central strategy was achieved by developing discriminatory tests that showed maximum differences between water and blood, which are two very rheologically different fluids. These tests were used to evaluate the performance of candidate SBSs and included measuring and comparing the (i) final drip stain diameter, (ii) total number of observable spines and scallops of a drip stain, (iii) calculated angle of impact of a drip stain, (iv) number of spatter stains found in the upper regions of an impact pattern and (v) resolution of a transfer pattern. Spreading and splashing of passively dripped bloodstains is more Newtonian than previously thought. By applying the concept of effective viscosity to passive dripping models, the shear-thinning properties of blood were not found to influence the final spread diameter and number of observable spines and scallops of a resultant stain. These findings encourage creating water-based fluids as candidate SBSs. The angle of impact of passive drip stains alone is not a suitable test to assess the performance of a blood substitute. All sensitivity tests were validated using human blood. Fifty-one natural, commercial and synthetic products have been evaluated and presented. Spatter Blood, a current commercially available forensic SBS, failed its sensitivity test evaluation and produced more water-like passive drip stains and impact patterns. To date, waterbased mixtures of 50 % v/v honey and 30 % w/v glycerol with 10 % w/v starch produce patterns with the most blood-like characteristics.