One such technology is called "Touch DNA" or "Contact Trace DNA." Touch DNA refers to the DNA that is recovered from skin (epithelial) cells that is left behind when a person touches or comes into contact with items such as clothes, a weapon, or other objects. A person sheds about 400,000 skin cells per day, but it is the lower skin cells that will provide the best DNA profile. These cells are typically recovered when force is used such as on the victim's clothes or at a crime scene after a struggle has occurred.
Skin cells are dead when they reach the surface & shed.
Basically, they start as normal cells in the stratum germinativum and then start migrating toward the surface. In the process, they die and get emptied of most of their biological apparatus. Their nucleus is degraded; and the cells fill up with keratohyalin that dehydrate them and cross-link them with long chains of keratine molecules.
By the time they arrive to the stratum corneum they are mostly inert interlinked bricks of keratin.
They might still contain DNA but it is no longer complete of functional. Might still be enough for some forensic, though.
There is the risk of cross-contamination at a crime scene from the tester's gloves, in that cells deposited in one location can stick to the gloves & be transferred to another surface if the tester doesn't change gloves.
The PJ CSI team did test Madeleine's bed with a specialist light.
http://www.mccannpjfiles.co.uk/PJ/5A_FORENSIC_4_5_7.htm"A search was also made for possible biological traces and fibres on the single bed from where the minor disappeared, using a variable-wave light source appropriate for the task, the result obtained having been negative."
IMO the most logical deposits should have been clothing fibres as a potential abductor's arm came into contact with the sheets & pillow if Madeleine was lifted from the bed
