One potentiometer coordinates an 8-bit PWM signal to the DC motor controller, with an effective maximum and minimum translational velocity of the lead screw assembly being 42.86 mm/s (255 PWM signal) and 14.77 mm/s (50 PWM signal—the minimum signal to power the DC motor). To ensure the velocity of the external magnets corresponds to the stir bar magnet velocity, an evaluation was performed. Stir bars were placed into separate syringes containing Hank’s Balanced Salt Solution (HBSS, Thermo Fisher Scientific) and alginate (LF200, FMC Polymer) media, representing different viscosities and resistances to translation of the stir bar magnet. Syringes were mounted to the Harvard Apparatus 33500TM syringe pump and the agitation system was activated at varying speeds. The stir bar motion within the syringe barrel was captured by video recording for each agitation condition. Captured videos were analyzed using a customized image analysis algorithm written in MATLAB (version 2015, MathWorks) for semi-automated measurement of kinetic linear velocity. The algorithm first auto-calibrates the spatial resolution of a video using the known stir bar principal dimension (5 mm) and then tracks user-inputted mouse clicks as frames are manually advanced. Each mouse click corresponds to an X-Y location in the frame of a defining feature for the stir bar (e.g., its lower-right corner) that remains consistent across frames and translation. Mouse clicks are tracked across frames to measure a linear distance and calculate a velocity. Six linear motions were tracked for each condition/video, with an average and standard deviation of velocities being calculated. The velocities correspond to kinetic motion of the stir bar after static friction was overcome.
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