The in vitro measurements took place either in filtered phosphate-buffered saline (FPBS) or bovine serum. The microneedle device was placed inside an eppendorf containing 160μL of either unspiked ("clean") or protein-spiked solutions using different concentrations for approximately 1 hour. The measurements were conducted using a fluorescence microscope with a 470nm LED and a 495-535nm filter (LEICA MD4000M Fluorescence microscope with LEICA DFC450 camera.).