Alginate-based forensic blood substitutes mimicking whole blood clotting and drying properties in drip stains and...

Forensic Blood Substitutes (FBS) may be used in place of blood for training and research in bloodstain pattern analysis. They offer a longer shelf life and less stringent biohazard precautions than animal or human blood. The drying dynamics of FBS have not been previously investigated. This preliminary work compares the drying dynamics of two alginate-based hydrogel FBS formulations with whole bovine blood deposited as drip stains and pools on glazed tile and cotton jersey fabric with pools of 2, 10 and 500 µL of each liquid. The first formulation (FBS1), previously optimized and validated for impact and drip patterns, was modified by adjusting the ionic crosslinker (CaCl₂) and polymer (alginate) concentrations to create a second formula (FBS2) with more time-dependent viscosity to mimick the increasing viscosity of whole human blood during clotting. Rheometry was characterised with oscillation amplitude sweeps (0.1-150 % strain), flow sweeps (1-1200 s^-1) and time sweeps (3.5 % strain and 1 Hz for 2000 s). In single tests, blood pools formed from FBS2 appear to show drying characteristics closer to whole blood than FBS1. On non-porous tile, the FBS materials broadly replicated the speed and appearance of the early stages of drying of blood. It developed radial stress lines in the corona, but did not crack in the way blood does in the final stages of drying. It did not de-bond from the tile surface in the same way. In the limited tests performed, FBS1 appears to spread further than blood to form larger stains, whereas FBS2 may not. On cotton jersey, both FBSs wicked at a similar rate to blood. Both FBS1 and FBS2 dried lighter than blood, but with the same dark edge to the stain. Ultimately, we found that by changing the polymer and ionic crosslinker concentration, the physical fluid properties of the material can be manipulated to behave more like blood during stain formation. Thus, the current formulations of the FBS are suitable for the generation of blood stains for training and research, with FBS1 acting as an accurate mimetic of whole blood for drip stains, and FBS2 possessing the viscous behavior required to better mimic whole blood clotting in larger volume stains and pools. Further work with more replicates, applied volumes and substrates is needed.