Abstract: A core for interacting with a fluid media flowing therethrough which includes a plurality of adjoining channels. Each of the channels is generally in the form of a hexagon in cross section and includes an internal surface area. The internal surface area is coated with an interactive material which interacts with the fluid media flowing through the channels to achieve one of heat transfer, mass transfer, catalysis, ion exchange, separation of the fluid media, removal of organic compounds from the fluid media, dilution or concentration of fluid components, treatment of fluid-carried organisms, and the capture and retention of hazardous materials. Examples of the use of the core include positioning the core within a rotary heat exchanger wheel or a rotary mass-transfer wheel, often used in desiccant air-conditioning systems.