Hydrogels, as a hydrophilic three-dimensional network polymer, have shown tremendous application potential in cutting-edge fields such as tissue engineering, drug delivery, and soft robotics in recent years. However, traditional hydrogels often suffer from poor mechanical properties and limited molding methods, making it difficult to meet the demands of complex application scenarios. Frontal polymerization (FP), an efficient and self-sustaining polymerization method that rapidly converts monomers into polymers, has been widely used to prepare functional materials such as hydrogels and resins. However, existing FP typically requires polymerization to be initiated at high temperatures (above 80 °C), which can lead to the death of biological cells at around 50 °C and can easily damage temperature-sensitive components such as bioactive molecules, greatly limiting its biomedical applications. Meanwhile, with the rapid development of 3D printing technology, biohydrogels as printing inks have gradually gained attention, but there is still a significant lack of hydrogel systems compatible with various printing methods (such as DIW and DLP), which has become a key bottleneck restricting the popularization of biological 3D technology.
To address these challenges, Professor Su Chen and his team from Nanjing University of Technology designed a hydrogel precursor system composed of hydroxypropyl acrylate (HPA), N-vinyl pyrrolidone (NVP) monomer materials, and polycaprolactone (PCL). This system achieves the rapid construction of an interpenetrating network structure (IPN) through low-temperature frontal polymerization (FP) and exhibits excellent adaptability for multimodal microfluidic 3D printing. The team first constructed the first network structure using PCL, and then initiated the copolymerization of HPA and NVP through FP to form the second network, achieving dual physical-chemical crosslinking at the molecular level, significantly enhancing the mechanical strength and stability of the hydrogel. Through formulation optimization, the entire FP process can be initiated at a super low temperature of 45.4 °C, rapidly converting monomers into polymers within 6 minutes, providing a new pathway for constructing high-performance biohydrogels under mild conditions.
Figure 1. Microfluidic bio 3D printer (developed by Nanjing Beier Times Technology Co., Ltd.)
Figure 2. (a) Schematic diagram of the process route for preparing poly(HPA-co-NVP)/PCL IPN hydrogels via the flat FP method. (b) Synthesis scheme for forming IPN hydrogels from HPA and NVP through FP in the presence of PCL within 6 minutes. (c) Infrared thermal imaging shows the stable advancement of the reaction front from left to right during the synthesis of IPN hydrogels. (d) Temperature-time curve of the polymerization process of IPN hydrogels. (e) Position-time relationship diagram of the reaction front of IPN hydrogels with different PCL contents. (f) Relationship between the front speed and maximum temperature with the concentration of PCL in IPN hydrogels.
This hydrogel system not only exhibits a mild and efficient reaction process but also demonstrates significant advantages in processing performance. The team achieved adaptability of the precursor system to various microfluidic 3D printing technologies by adjusting the PCL content. Particularly in direct ink writing (DIW), the research team successfully achieved uniform extrusion and structural formation of the hydrogel on the DIW platform using a microfluidic bio 3D printer developed by Nanjing Beier Times Technology Co., Ltd.; at the same time, its excellent light-curing performance also made it perform outstandingly in digital light processing (DLP) printing. The multimodal adaptability of this system not only significantly broadens the manufacturing methods of hydrogels but also provides possibilities for the fine construction of functional devices. Additionally, thanks to its rich hydrogen bond structure, this hydrogel can self-heal to 91.0% of its original mechanical strength within 10 hours under ambient conditions, endowing the material with excellent durability and lifespan.
Figure 3. (a) FTIR spectra of HPA, NVP, PCL, and poly(HPA-co-NVP)/PCL hydrogels. (b) TG curve and corresponding DTG curve of poly(HPA-co-NVP)/PCL hydrogels. (c) Variation of water contact angle of poly(HPA-co-NVP)/PCL hydrogels over time. (d) Water swelling behavior of poly(HPA-co-NVP)/PCL hydrogels prepared with different PCL contents. (e) SEM images of poly(HPA-co-NVP)/PCL hydrogels prepared with 5 wt%, (f) 10 wt%, and (g) 15 wt% PCL.
Figure 4. (a) Photos of poly(HPA-co-NVP)/PCL hydrogels under tensile, knotting, and twisting deformations. (b) Stress-strain curves of poly(HPA-co-NVP)/PCL hydrogels prepared with different PCL concentrations. (c) Young’s modulus of poly(HPA-co-NVP)/PCL hydrogels prepared with different PCL concentrations. (d) Storage modulus G’ and loss modulus G” of poly(HPA-co-NVP)/PCL hydrogels as a function of frequency. (e) Relationship between tan δ and the frequency of poly(HPA-co-NVP)/PCL hydrogels. (f) Evaluation of the cytocompatibility of IPN hydrogels using the MTT method. (g) Assessment of the viability of L929 fibroblasts co-cultured with IPN hydrogels for 24 hours using live/dead fluorescence microscopy (scale bar: 200 μm).
Figure 5. (a) Schematic diagram of the application of inks in DIW microfluidic 3D printing. 3D printed patterns using DIW: (b) circle, (c) pentagram, (d) planar grid, (e) three-dimensional grid (scale bar: 1 cm). (f-i) Photos of grids printed at different extrusion rates using a 22G nozzle, and (j-m) photos of grids printed using different nozzle sizes (scale bar: 1 cm). The inset in the upper right corner shows the lines extruded from the nozzle during printing (scale bar: 1 cm), while the inset in the lower left corner shows a microscopic image of the line diameter distribution (scale bar: 500 µm). (n) Relationship between the viscosity of hydrogel inks and shear rate (PCL = 20 wt%). (o) G′ and G′′ of poly(HPA-co-NVP)/PCL hydrogels (PCL = 20 wt%) during strain scanning. (p) Time-dependent G’ and G” values obtained from the step strain test of poly(HPA-co-NVP)/PCL hydrogels (PCL = 20 wt%, high strain: 100%, low strain: 1%).
Figure 6. (a) Schematic diagram of DLP microfluidic 3D printing. Photos of structures printed via DLP 3D printing using poly(HPA-co-NVP)/PCL IPN hydrogels: (b) tower segment, (c) assembled whole tower, (d) rose segment, (e) assembled whole rose (scale bar: 1 cm). The green stem segment of the rose contains methyl green, and the red flower segment contains rhodamine b, with the inset showing red fluorescence under UV light. (f) Self-healing behavior of the hydrogel. (g) Microscopic infrared images of the hydrogel before and after healing at 3432 and 1740 cm−1. (h) Stress-strain curves showing the tensile behavior of original and healed hydrogels. (i) Relationship between the viscosity of hydrogel inks and shear rate. (j) G’ and G” of poly(HPA-co-NVP)/PCL hydrogels during strain scanning. (k) Time-dependent G’ and G” values obtained from the step strain test of poly(HPA-co-NVP)/PCL hydrogels (PCL = 10 wt%, high strain: 100%, low strain: 1%).
This research breaks the triple limitations of traditional hydrogels in preparation temperature, processing methods, and application performance, achieving an organic unity of rapid molding, self-healing recovery, and multimodal microfluidic 3D printing under mild conditions. Its outstanding structural control capabilities, process compatibility, and functional expansion potential provide forward-looking technical support and broad application prospects for cutting-edge fields such as biomedicine, tissue engineering, and soft robotics. The research results were recently published in the prestigious journal Chemical Engineering Journal. Professor Su Chen, Professor Cai-Feng Wang, and Associate Professor Jing Zhang from the School of Chemical Engineering at Nanjing University of Technology are co-corresponding authors. Master’s student Wei Zhao from Nanjing University of Technology is the first author. This project was supported by the National Natural Science Foundation of China, the National Key R&D Program, and the Jiangsu Province Higher Education Advantage Discipline Construction Project.
Original text (scan or long press the QR code to access the original page):
Interpenetrating network hydrogels via low-temperature frontal polymerization and 3D printing
Wei Zhao, Fucheng Li, Hao Li, Cai-Feng Wang, Jing Zhang, Su Chen
Chem. Eng. J., 2025, 519, 164775, DOI: 10.1016/j.cej.2025.164775
Mentor Introduction
Su Chen
https://www.x-mol.com/groups/su_chen
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