the invention disclosed relates to an agent capable of selectively binding lithium ions to form a lithium complex , employed in association with a biologically inert carrier or excipient . typically , the agent is included in a novel graft polymer , such as a cross - linked polystyrene polymer . this invention may also be applied to sensing lithium ion concentrations in aqueous solution and to the selective removal of lithium ions from an aqueous solution , including biological fluids .