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fluid behavior, surface tension and capillary forces govern fluid dynamics at the

microfluidic level. In fluid dynamics, the Reynolds number (R ) quantifies the ratio of
e
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inertial to viscous forces within a fluid, defining its flow regime . It is expressed as:
R = uL/v
e
Where u is fluid velocity, L is characteristic length (e.g., channel hydraulic

diameter), and v is kinematic viscosity. Submillimeter microfluidic channels yield low

Reynolds numbers (Re ≪ 2300), indicating laminar flow. Under these conditions, flow

properties (e.g., velocity, pressure) remain temporally stable and exhibit gradual spatial

transitions. These force-dominated characteristics enable key passive functions:

microchannel fluid pumping, analyte filtration, selective capture, and droplet

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generation without external energy . Microfluidic devices are classified by actuation:
(1) Passive devices: Utilize engineered geometries to harness intrinsic forces

(interfacial effects, diffusion, secondary flows) for fluid mixing and particle control.

(2) Active devices: Employ external energy sources (magnetic/acoustic fields) to
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enhance fluid manipulation .

2.2 Characteristics of 3D Printing Technology

3D printing, or additive manufacturing, constructs 3D objects from CAD/digital

models by depositing, connecting, or curing materials layer-by-layer under computer

control (e.g., fusing plastic, liquids, or powders) 48,49 . Recent advances in 3D printing

have revolutionized the fabrication of microfluidic devices, offering unparalleled

flexibility in design, material selection, and functional integration. Among the most

widely adopted techniques are inkjet 3D printing (i3DP), stereolithography (SLA), two-

photon polymerization (2PP), and fused deposition modeling (FDM), each with distinct

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operational principles, strengths, and limitations . I3DP excels in multi-material
deposition, enabling the fabrication of heterogeneous tissue constructs with spatially

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controlled biochemical cues . This capability is particularly valuable for tumor models
requiring graded stiffness or embedded vasculature. However, i3DP is limited by

moderate resolution (~50–100 μm) and challenges in maintaining droplet uniformity

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