We investigated a label-free optical biosensor for analyte measurement in wound fluid based on a Resonant Mirror (RM) device and 3D printed instrumentation. This work builds on our previous report of a novel RM device (DOI: 10.1039/c3an00263b), which consisted of silica sol–gel spacer and polystyrene waveguide layers spin-coated on a standard microscope slide glass substrate. Unlike earlier RM ... [Show full abstract] devices, which were produced using vacuum deposition techniques on high index substrates, the devices described here were produced on microscope slide substrates at room temperature and pressure. Previously we demonstrated that the sensor could detect 10 nM IgG, which was used as an exemplar analyte. To improve the performance of the device, we have performed modelling using an in-house transfer matrix program to optimise the layer thicknesses and refractive indices. This modelling has directed the use of different materials as spacer and waveguide layers to improve the sensitivity and limit of detection of the device. Subsequently, we measured Ferritin using the RM integrated with a microfluidic flowcell which was interrogated using both lab-based and portable 3D printed instrumentation.