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remained zero was 3 mm. Beyond this point, splatter coverage progressively increased, from

5.11 ± 2.03% at 6 mm to 37.36 ± 13.01% at 24 mm (Figure 2c). Secondary metrics of printing
quality, such as drop placement error, drop circularity, and main drop area followed the same trend.

Drop placement error increased from 181 ± 94 µm at 1.5 mm to 383 ± 227 µm at 24 mm (Figure
2d). Drop circularity declined from 0.85 ± 0.12 to 0.69 ± 0.18 (Figure 2e), while the main drop

area decreased from 1.29 ± 0.25 mm² to 1.08± 0.31 mm² (Figure 2d), primarily due to the

formation of satellite droplets.

Taken together, these printing quality metrics demonstrate that print quality degrades as the

printing head-to-target distance increases. A distance of 3 mm is identified as a practical upper
limit, where printing remains splatter-free.

3.3. Preventing printing quality loss in dynamic environments using a distance sensor

For dynamic printing environments where the printing target cannot be fixed (e.g., printing on

living animals), the distance between the printing head and the target must be dynamically adjusted

to maintain printing quality. We developed an optical fiber-based distance sensor using the OCT
principle and integrated it with the mobile printing head and robotic arm to mitigate quality loss

when printing on a moving substrate. The working principle of this automation is outlined in

Figure 3a and detailed in the Materials and methods section.

We employed a dynamic z-position adjustment approach to print our model ink (viscosity: 18 cP)

on a moving target that oscillated with a displacement amplitude of 12 mm (24 mm peak-to-peak
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displacement), representing the average chest expansion due to breathing . Figure 3b shows
optical microscopy images of arrays of model ink droplets printed at a constant energy per pulse

(230 µJ, corresponding to 1.15-times the printing threshold energy) under three conditions: (i) a
constant 3 mm printing head-to-target distance, (ii) target movement without compensation, and

(iii) dynamic compensation, where the target moves while the printing head actively maintains a
3 mm distance.

Figure 3c to 3f presents a quantitative analysis of the printing quality metrics under the three
conditions. We found that printing on a moving target significantly worsened most printing quality

metrics. Splatter coverage increased from 0.00 ± 0.00% to 34.17 ± 5.03% (p = 0.0003), drop
circularity decreased from 0.84 ± 0.02 to 0.72 ± 0.05 (p = 0.0016), and the main drop area was

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