Investigation of secondary DNA transfer of skin cells under controlled test conditions.

There is a paucity of data on the relative transfer rates of deposited biological substances which could assist evaluation of the probability of given crime scene scenarios, especially for those relating to objects originally touched by hand. This investigation examines factors that may influence the secondary transfer of DNA from this source, including the freshness of the deposit, the nature of the primary and secondary substrate and the manner of contact between the surfaces. The transfer rates showed that both the primary and secondary type of substrate and the manner of contact are important factors influencing transfer of skin cells, but, unlike other biological fluids, such as blood and saliva, the freshness of the deposit in most instances is not. Skin cells deposited on a non-porous primary substrate transferred more readily to subsequent substrates than those deposited on a porous substrate. Porous secondary substrates, however, facilitated transfer more readily than non-porous secondary substrates, from both porous and non-porous surfaces. Friction as the manner of contact significantly increased the rate of transfer. The findings of this study improve our general understanding of the transfer of DNA material contained in fingerprints that is left on a surface, and assist in the evaluation of the probability of secondary and further DNA transfer under specific conditions.