Platelet spreading is spatially regulated at the microscale. (A) Platelets (red, cell membrane stain) adhere and spread onto fibrinogen microdots (blue) fabricated via protein microcontact printing. On larger fibrinogen microdots (eg, 7 and 10 µm in diameter; “∞” denotes no geometric boundary), platelet spreading conforms to the microenvironmental geometric boundaries of the microdots with high fidelity. Decreasing the fibrinogen microdot size (eg, diameters of 2 and 5 µm), however, leads to platelet spreading beyond the geometric boundaries of the microdots. (B) This phenomenon is quantified by measuring the surface area of spread platelets on fibrinogen (blue) and collagen (green) microdots of different diameters. The gray line denotes the protein microdot area and diameters for reference. The scale bar = 5 µm. Error bars indicate standard error (SE).