The optical force of an evanescent field is useful for trapping individual proteins and molecules. We theoretically investigate the optical pressure exerted on an attached and wellspread cell on a waveguide in the vicinity of an evanescent field. To do this, the cell is modeled as a three-planar-layer (membrane-cytoplasm-membrane). These layers and a gap of water are used as a stratified cover medium for the waveguide. Then, the mode equation of a slab waveguide with a simple and semi-infinite cover medium is modified and solved graphically. The effects of the cell cytoplasm and membrane thicknesses and their refractive indices and the sample- waveguide separation distance on the optical pressure are numerically studied. The results show that the optical pressure increases when the cytoplasm thickness increases, but decreases when the cell membrane thickness is increased. Furthermore, the optical pressure does not significantly change when the sample film thickness increases to more than 0.2 of a wavelength. The optical pressure decreases when the waveguide-sample distance for a specific mode is increased, but can be attractive or repulsive depending on both the cytoplasm refractive index and the mode number.
