Systems and methods are disclosed herein for use in transducing, activating, and otherwise treating cells. Cells are introduced into an inner layer of a multi-layered stack that defines at least one flow chamber and a plurality of cell entrainment regions. Vertical flow through the stack entrains the cells in the cell entrainment regions along with genetic information introduction agents or other additives, before the cells are washed using a reverse vertical flow and are collected from the device.