A stage heater for inverted optical microscopy was developed as an add-on to TU Delft portable integrated microfluidic platform. The platform was modified to gain additional functionality and portability. The stage heater was designed to work with custom PDMS microfluidic chips and commercial polymer chips which highlights the adaptability of the setup. The on-off temperature control was evaluated via an external thermocouple sensor, and the standard deviation of temperature in the cell chamber did not exceed 0.3 °C. An intermittent flow mode (flow for 1 minute at 10 µl/min, pause for 10 minutes) was introduced to match the requirements of an experiment involving T-cell and tumour spheroid interaction. The stage heater was deemed compatible with fluorescent and transmission imaging modes for up to 20x magnifications when equipped with a PET ITO-coated bottom heater with a window underneath the cell chamber. An increase in T-cell activity was observed with enabled heating. The process of T-cells attacking a spheroid via a time series of images was captured.