AccScience Publishing / ESAM / Online First / DOI: 10.36922/ESAM026180008
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ORIGINAL RESEARCH ARTICLE

Shape morphing of extrusion-printed thermoresponsive hydrogels regulated by infill topology and layer design

Xi Yin1 ,  Shishuo Liu1 ,  Yanglong Lu1*
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1 Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong SAR, China
ESAM 2026, 2(2), 026180008 https://doi.org/10.36922/ESAM026180008
Received: 30 April 2026 | Revised: 4 June 2026 | Accepted: 5 June 2026 | Published online: 12 June 2026
© 2026 by the Author(s). This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 International License ( https://creativecommons.org/licenses/by/4.0/ )
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Abstract

Four-dimensional (4D) printing offers an innovative method for fabricating thermoresponsive hydrogel structures with programmable shape changes. Although structural regulation has been explored in some stimuli-responsive shape-morphing materials, the influence of infill topology on thermally induced deformation in extrusion-printed thermoresponsive hydrogels remains insufficiently clarified. As a programmable structural feature in extrusion-based printing, infill topology can provide a practical route for regulating thermally induced deformation while retaining the potential advantages of infill-based lightweight design. In this study, a poly(N-isopropylacrylamide) (PNIPAM)-based thermoresponsive hydrogel ink was used as active ink, while alginate/laponite-based hydrogel ink was used as the passive constraining ink. These inks were used to fabricate hydrogel structures with different internal architectures, and their temperature-dependent deformation behaviors were systematically compared. Structures printed entirely from the active ink showed no obvious macroscopic bending during heating, although both cell area and overall shape area decreased with temperature. This suggests that the response of structures with only the active material was mainly dominated by thermal deswelling. In contrast, structures with an active–passive architecture exhibited clear temperature-dependent bending, showing that the active–passive mismatch was essential for converting material response into macroscopic shape change. Within this system, infill topology strongly affected bending level, with rectilinear patterns producing the strongest response and grid patterns the weakest. Under a fixed honeycomb topology, increasing active-layer infill density further enhanced bending. In addition, a heterogeneous active-layer design that combined two topologies in a single structure preserved local deformation differences, while the final bending response was governed by mechanical coupling throughout the structure. Overall, these results show that active–passive layer architecture, infill topology, and layer-specific infill design can regulate thermally induced shape morphing in printed hydrogel systems. This study guides the design of thermoresponsive hydrogel structures for soft actuation and related programmable devices.

Graphical abstract
Keywords
4D printing
Thermoresponsive hydrogel
Infill topology design
Active–passive layer design
Programmable shape morphing
Funding
This work was supported by Guangdong Basic and Applied Basic Research Foundation (2026A1515011635), Research Grants Council of Hong Kong (No. 26205925), and National Natural Science Foundation of China (No. 52505579).
Conflict of interest
The authors declare they have no competing interests.
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