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speed was 5 Hz and the printing pattern consisted of an array of drops spaced 1.5 mm apart. The

pattern was printed in a sequential row-wise pattern, starting with a horizontal line of droplets,
followed by a vertical offset to print the next row below, and continuing in this alternating

sequence. The z-position of the robotic arm was continuously updated using a distance tracking
system to maintain the printing head at a fixed distance from the moving substrate. The distance

tracking system automation is based on a homemade spectral-domain common-path Optical
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Coherence Tomography (OCT) system that has been described in detail elsewhere . For this
work, a miniaturized (125 µm in thickness) fiber-based probe was attached to the printing head,

emitting light towards the moving substrate. The backscattered light was collected by the same
fiber probe and directed to the readout system. A single mode fiber (SM800-5.6-125, Thorlabs)

was used for the probe. To optimize light emission and collection, we spliced a 272-µm GRIN

fiber (GIF625, Thorlabs) to the distal end of the fiber, resulting in a non-diverging light beam
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emission . The axial resolution of the system was experimentally measured to be 18 µm.

A Python-based algorithm was used to control the entire automation, including processing the
tracking system readout and controlling the robotic arm. The entire motion-tracking automation

system operated at a frequency of 5 Hz. Real-time Z-axis correction required stepwise movement

execution, with all coordinates updated at each step. The X and Y coordinates were predefined to
follow the sequential row-wise scanning pattern described above, while the Z coordinate was

dynamically calculated. Specifically, at each step, the Z-axis control logic compared the actual
printing distance to the target distance (3 mm). If the deviation was less than 50 µm, the Z position

remained unchanged; if the deviation exceeded 50 µm, the Z position was adjusted to offset the
difference. While the fiber-based sensor operated at 50 Hz, the Python-based control script queried

the sensor at 200-millisecond intervals (5 Hz). This update frequency was chosen as a compromise

between minimizing vibrations in the robotic arm and ensuring sufficient sampling to track the
substrate movement.

At a fixed standoff distance of 3 mm, droplet arrays were printed at both 2 Hz and 5 Hz to
investigate the effect of robotic arm vibrations on printing quality.

2.4. Statistical analysis

All reported values in this work represent the mean and standard deviation from three independent
printing experiments. Data in Figure 3 were analyzed using one-way ANOVA statistical tests.

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