Ionic conductive hydrogels play an indispensable role in energy storage, soft robotics, biosensing, and other fields. However, the low-precision molding process and fragile mechanical properties limit the practical application of ionic conductive hydrogels. Digital light processing (DLP) 3D printing technology, which has the characteristics of fast printing speed and high precision, has shown incomparable advantages in the preparation of a variety of complex hydrogel structures, including ionic conductive hydrogels. At the same time, attapulgite is an inorganic ore with a chain layered silicate structure, which has excellent adsorption capacity and good ion exchange capacity, and can be enhanced by different surface treatments. However, the organic modification of attapulgite often destroys its water dispersibility and causes it to settle in water prematurely, resulting in the modified attapulgite not meeting the needs of 3D printing ink stability, which hinders the enhancement effect of attapulgite on 3D printing ionic conductive hydrogel.
In order to solve the above problems, Zhang Biao's research group of Northwestern Polytechnical University and Associate Professor Hu Guang of Jiangsu Provincial Key Laboratory of Attapulgite Resource Utilization of Huaiyin Institute of Technology jointly developed a γ-methacryloyloxypropyltrimethoxysilane (MPS)-modified attapulgite (M-ATP). Combined with the filling of M-ATP and the double-network enhancement strategy of polyvinyl alcohol (PVA) microcrystallization, the hydrogels with high mechanical strength and good ionic conductivity were obtained, and the tensile strength was increased by 5 times. The study was published in the latest issue of Advanced Functional Materials under the title "Attapulgite Reinforced Robust and Ionic Conductive Composite Hydrogels for Digital Light Processing 3D printing". Guo Yunlong, a doctoral student at Northwestern Polytechnical University, is the first author of this paper, and Associate Professor Zhang Biao of Northwestern Polytechnical University and Associate Professor Hu Guang of Huaiyin Institute of Technology are the co-corresponding authors of this paper.
The authors used a water-soluble cationic acrylate, acryloyloxyethyltrimonium chloride (AEtMACl), as an ionic conductive photomonomer, and added polyethylene glycol diacrylate (PEGDA, molecular weight = 700) as a crosslinker, polyvinyl alcohol (PVA) as the enhancing phase, Li-TMPP as the photoinitiator, and a small amount of quinoline yellow as the photoabsorber (Fig. 1b). In addition, the modified attapulgite (M-ATP) is evenly and stably dispersed in water by ultrasonic, and together with other components it forms the printing ink.
Figure 1. Attapulgite reinforced ionic conductive hydrogel composition and 3D printing display.
Acrylication of attapulgite (ATP) requires two steps: hydrochloric acid acidification and MPS modification. Under the action of hydrochloric acid, the carbonate impurities in the attapulgite crystals are corroded, and the attapulgite crystal bundles are dissociated, increasing the reaction sites on the surface of the attapulgite soil. In the MPS modification process, by controlling the alkalinity of the reaction, M-ATP can form a homogeneous dispersion in water, and will not settle for at least 60 min after dispersion at a concentration of 5wt%. The C=O peaks at 1721 cm-1 and 1701 cm-1 in the infrared spectrum demonstrated the successful grafting of the acrylate group, and the grafting amount of MPS was calculated to be 6.74% according to the thermogravimetric (TGA) curve.
Figure 2. Modification and characterization of attapulgite soil.
The authors investigated the mechanical properties of AEtMACl ionic conductive hydrogels from four perspectives: M-ATP content, PVA content, M-ATP content in the presence of PVA, and moisture content. The results showed that the tensile strength of AEtMACl hydrogels was increased by 5 times under the optimal ratio. The authors observed the torsion and orientation of M-ATP in the hydrogel matrix by scanning electron microscopy, and revealed the mechanism of attapulgite enhancement of AEtMACl hydrogel.
Figure 3. Attapulgite enhances the mechanical properties of hydrogels.
The printed ionic hydrogel has good ionic conductivity (up to 45.6 mS/m), and the maximum conductivity in other formulations is 211.9 mS/m. The change in resistance of ionic conductive hydrogels during tensile has a good linear relationship with strain, so they can be used in strain sensors and can detect changes in the position of the Adam's apple during speaking or swallowing. In order to further overcome the problem of dehydration of hydrogels in practical use, the authors used a light-curing, hydrophobic and transparent polyurethane-based elastomer as the encapsulation material of hydrogels. The encapsulant material can form a covalent bond at the hydrogel interface, avoiding premature peeling of the hydrogel layer during the stretching process. In addition, 3D printing provides a wealth of freedom for the design of hydrogel structures, and the authors use a sin curve structure to further break through the limitation of strain in hydrogel materials.
Fig.4 Encapsulation and sensing structure design of attapulgite-reinforced hydrogel.
Summary: The authors have developed a modified attapulgite-enhanced ionic conductive hydrogel suitable for DLP 3D printing. Through the encapsulation technology and structural design, a stable sensor structure beyond the strain limit of hydrogel was fabricated, which provided a reference for the application of ionic conductive hydrogel in flexible electronic devices.
This work was supported by the National Natural Science Foundation of China and the Open Fund of the National and Local Joint Engineering Research Center for Deep Utilization of Mineral Resources.
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Source: Frontiers of Polymer Science