We investigate in this work the formulation of composite resistive pastes based on epoxy resins and graphite for micro-heater manufacturing via thick-film technology. These resistive pastes are designed for screen-printing onto a printed circuit board (PCB) substrate; further coating with expandable polydimethylsiloxane (PDMS), a composite based on an elastomeric matrix and expandable microspheres, results in one-shot thermal actuators allowing pumping and sealing in disposable micro-fluidic lab-on-a-chip devices. The resistive paste must therefore have controlled rheology and resistivity, and exhibit good temperature stability in order to allow high heating power densities. This paper details the formulation and characterisation of suitable epoxy–graphite resistive composites, and the control of their properties through solvents.