When two different materials rub against one another, there is a transfer of electrons across the interface. The amount of transfer depends on the energy needed to remove an electron from the surface of a material to infinity, known as its work function. The electrons move from the material with a lower work function to the adjacent material with a higher work function, thus creating a negative charge in the latter while the former becomes positively charged. This effect is known as contact electrification (or contact charging).
The triboelectric series orders common materials according to their tendencies to gain or lose electrons. On this list, materials such as human skin, fur, wool sit on the top with high tendency of gaining a positive charge (loose electrons) whereas synthetic materials, such as Teflon, polyester and plastics feature at the bottom of the list with a high tendency of gaining electrons and becoming negatively charged. This explains why when our bodies rub against a polyester top or a winter jacket all day, a deficit of electrons is created on our bodies while the polyester gets a negative charge.
In this way, humans in contact with synthetic materials become potential sources of static electricity that, if managed carefully, can become a resource for designing e-Textiles.