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International Journal of Bioprinting Swelling–shrinking behavior of hydrogel

hydrogel 3D printing, maintaining levels above 70% to property, while thinner filaments are generally adopted
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prevent the shrinkage and collapse of the 3D-printed for smaller constructs to achieve a higher resolution.
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hydrogel structures. In such complex circumstances, identifying an optimal
Although prior studies have attempted to regulate humidity level for hydrogel 3D printing becomes
ambient humidity during hydrogel 3D printing, most increasingly challenging. Therefore, there is an urgent need
efforts—including the work by Yu et al. —have primarily to investigate the humidity-driven swelling–shrinking
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focused on the external environmental control of the behavior of 3D-printed hydrogel filaments.
printing space. Specifically, they proposed a humidity- In this study, the influence of ambient humidity during
controlled atmospheric enclosure and developed a fluid– the 3D printing process on the swelling–shrinking behavior
thermal–humidity coupled finite element method (FEM) of 3D-printed hydrogel filaments was investigated through
model to simulate the spatial distribution of humidity numerical simulations and experimental validation. A
within the printing chamber. This strategy successfully two-phase flow simulation model, coupled with heat
improved macro-scale environmental stability and transfer and water vapor transport, was developed based
reduced the likelihood of structure collapse caused by on FEM. The geometric variation of hydrogel filaments
low humidity. However, their approach only addresses the with various diameters under varying humidity conditions
ambient airfield and does not account for the behavior of was estimated, and the optimal humidity levels for hydrogel
individual hydrogel filaments under humidity fluctuations 3D printing were identified. Subsequently, a series of 3D
at the material level. In particular, the internal diffusion of printing trials was conducted using hydrogel materials
water within hydrogel filaments, the associated volumetric with and without the optimal ambient humidity. The
deformation, and the interface-driven geometric evolution results demonstrated that the shrinkage of filaments and
remain unmodeled. Consequently, their framework is the collapse of fabricated 3D architectures were effectively
incapable of predicting localized distortion, interlayer prevented by maintaining the humidity at an optimal level.
error accumulation, or structure failure due to filament-
scale swelling and shrinkage. 2. Materials and methods
However, the omission of humidity-driven swelling– This study proposed a 2D thermal–humidity–multiphase
shrinking behavior of 3D-printed hydrogel filaments in flow coupling field simulation model for humidity-driven
prior studies has critically limited the advancement of swelling–shrinking behavior of hydrogel filaments.
hydrogel-based 3D printing and hindered progress toward
reliable biomanufacturing. During the hydrogel printing In the 3D printing of hydrogel structures, well-
process, local humidity exerts a significant impact on the arranged filaments were smoothly deposited. The local
geometric variation of fabricated hydrogel models. To humidity within the printing space plays a significant role
preserve the shape of 3D-printed hydrogel constructs, in influencing the geometric variation of these hydrogel
the ambient humidity must be precisely regulated to an filaments. To accurately regulate the humidity field within
optimal level. the printing space, a novel configuration of an extrusion-
based 3D printer (SIA bioprinter PRO, Shenyang Institute
Existing research generally focuses on the invention
of novel humidity-controlled devices 29–31 to precisely of Automation, China) was employed, as demonstrated in
regulate the ambient humidity of 3D-printed structures. Figure 1. The feasibility and reliability of this setup were
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However, these studies typically rely on the trial-and-error validated in previous research. The 3D printing device
method to identify optimal humidity levels, a method comprises a temperature-controlled horizontal plate and
that is time-consuming, lacks reliability, and suffers from a humidity-controlled atmospheric enclosure. During
poor repeatability. Such limitations fall short of meeting the printing process, the print head operates within this
the requirements for manufacturing biomimetic 3D enclosure, allowing the humidity in the printing space to
architectures. be precisely maintained at a constant value.

The 3D printing of biomimetic scaffolds aims to 2.1. Finite element method modeling for the
produce a range of complex 3D architectures that printed filament
accurately replicate the natural structure of human tissues To estimate the humidity-driven swelling–shrinking
and organs. Given the tremendous difference in the sizes behavior of printed filaments, a two-phase flow simulation
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of various tissues and organs, hydrogel filaments of varying model coupled with heat transfer and water vapor
diameters are often required during the printing process. transport was developed using COMSOL Multiphysics
For example, 3D architectures with larger volumes usually 6.2 (COMSOL, USA). Mechanical components not
consist of thicker filaments to provide a better load-bearing directly interacting with the printed filament, such as the

Volume 11 Issue 4 (2025) 411 doi: 10.36922/IJB025220222