Platelet aggregate force and area in response to shear flow. Reconstituted whole-blood samples with fresh (green triangle), RT-stored (red circles), or 4°C-stored (blue squares) apheresis platelets were perfused through a microfluidic device to measure the force and area of platelet aggregates formed under shear flow. (A) Each microfluidic channel contains multiple sets of block and post force sensors. While under flow, platelets attach and aggregate to form a plug-like structure (green) that encapsulates the block and post. Aggregated platelets are able to produce contractile forces (F) that pull the flexible post toward the rigid block. Force is calculated from displacement of the tip of the post (δ) using Hooke’s law: F = k δ, where k = 3π E d⁴/64 L³, and E is the modulus of elasticity, d is the diameter, and L is the length of the post. (B) Representative images of platelet aggregate area 15, 60, and 300 seconds after blood enters the channel. (C) Mean force of the platelet aggregates over time (n = 5; shaded regions represent standard error of the mean [SEM]). (D) Force of the platelet aggregates at 60 seconds (n = 5; shown as mean ± SEM). (E) Force of the platelet aggregates at 300 seconds (n = 5; shown as mean ± SEM). (F) Mean area of the platelet aggregates over time (n = 5; shaded regions represent SEM). (G) Area of the platelet aggregates at 60 seconds (n = 5; shown as mean ± SEM). (H) Area of the platelet aggregates at 300 seconds (n = 5; shown as mean ± SEM). *P = .0103 for fresh and RT (H) , **P = .0032 for 4°C and RT (G).