Key Takeaways & Executive Findings
- •• • The coaxial encapsulation strategy reduces mass loss to 2.5% after 60 days at 25 °C, and swelling rate to 1.5% after 60 days underwater, enabling long-term operational stability in humid or aqueous environments. • • The sensor achieves a gauge factor (GF) of 1.705, indicating high strain sensitivity suitable for precise motion detection and wearable diagnostics. • • Stable operation across -20°C to 120°C extends applicability to extreme thermal conditions, from cold-chain monitoring to high-temperature industrial settings. • • The PDMS hydrophobic shell provides resistance to underwater and solvent environments, overcoming the dehydration and swelling failure modes of conventional hydrogel sensors.
Abstract
Ionic conductive hydrogels have gained extensive attention in the field of intelligent sensing due to their good flexibility, tunable electrical conductivity, and multi-stimuli responsiveness. However, hydrogels easily freeze, dehydrate or swell in external environments, and thus losing their original structure and functions. Therefore, improving the environmental adaptability of conductive hydrogels remains a challenge. Herein, ionic hydrogels were encapsulated in real time via UV-assisted multi-material coaxial direct ink writing (DIW) 3D printing, and ionic conductive hydrogel sensors with array structures were prepared. The core ionic conductive hydrogel is isolated from the external environment by the hydrophobic photocurable polydimethylsiloxane (PDMS) shell resin. The PDMS shell resin isolates the core hydrogel from moisture and heat in the external environment, thereby significantly enhancing the sensor’s stability. After 60 days of storage at 25 °C, the 3D-printed coaxial array sensor exhibits only 2.5% mass loss; when stored underwater for 60 days, its swelling rate is merely 1.5%. This sensor exhibits high strain sensitivity with a gauge factor (GF) up to 1.705 and good cyclic stability, demonstrates stable operation over a wide temperature range of -20°C to 120°C, and can withstand underwater and solvent environments. It has been successfully applied in various scenarios such as human motion monitoring, underwater sensing, and temperature sensing. This research breaks through the environmental limitations of conventional hydrogel sensors and provides a simple, efficient method for developing flexible sensors with high environmental adaptability.
1. Introduction
Conventional ionic hydrogel sensors suffer from intrinsic environmental fragility: they dehydrate in dry air, freeze at sub-zero temperatures, and swell or dissolve in aqueous or solvent-rich media. These degradation pathways lead to loss of mechanical integrity, electrical conductivity, and sensing accuracy, severely limiting their deployment in real-world applications such as wearable health monitors, underwater robotics, and industrial process control. Existing protective strategies, such as lamination or post-hoc coating, often compromise sensor flexibility, add manufacturing complexity, or fail to provide a hermetic barrier.
This work introduces a UV-assisted coaxial direct ink writing (DIW) 3D printing strategy that encapsulates the ionic hydrogel core in real time with a hydrophobic photocurable PDMS shell. The coaxial nozzle enables continuous, scalable fabrication of core–shell filament architectures, while UV curing ensures immediate solidification of the shell, preventing core dehydration and external contamination. The resulting sensors demonstrate exceptional environmental stability—minimal mass loss and swelling over 60 days—and maintain high strain sensitivity (GF up to 1.705) across a wide temperature range (-20°C to 120°C). This approach directly addresses the bottleneck of environmental non-adaptability, offering a manufacturing route that is both simple and efficient for producing robust, flexible sensors.
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ZHANG Chen, SHI Guohong, MIAO Jiatao, LIU Ren (2026). UV-assisted coaxial DIW 3D printing: a strategy for fabricating environmentally adaptive ionic hydrogel sensors. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4285-2
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Frequently Asked Questions
What is the maximum strain range over which the gauge factor of 1.705 remains linear, and what is the fatigue life under cyclic loading?
The paper reports a gauge factor up to 1.705 and good cyclic stability, but specific linear strain range and fatigue cycle numbers are not detailed in the provided text. For industrial deployment, further characterization of strain limits and cycle life is recommended.
How does the PDMS shell thickness affect the sensor's response time and sensitivity?
The abstract does not specify shell thickness or its effect on response time. However, the encapsulation is designed to isolate the core from moisture and heat, which likely increases response time due to mechanical buffering. Quantitative data on response time is not provided.
What is the ionic conductivity of the core hydrogel, and how does it change after prolonged storage or under extreme temperatures?
The paper does not report the absolute ionic conductivity values. It only states that the sensor maintains stable operation over -20°C to 120°C, implying conductivity remains sufficient for sensing, but exact conductivity data is absent.
Can this coaxial printing method be scaled up for mass production, and what are the printing speed and resolution limits?
The paper does not provide quantitative data on printing speed or resolution. The method is described as simple and efficient, but scalability parameters such as throughput and feature size are not disclosed.
What is the adhesion strength between the PDMS shell and the hydrogel core, and does delamination occur under repeated bending or stretching?
The paper does not report adhesion strength or delamination behavior. The good cyclic stability suggests adequate interfacial integrity, but quantitative adhesion metrics are not provided.
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