Tough supramolecular hydrogels with complex structures Digital light processing 3D printing ! Available for impact absorbing elements!
Digital light processing (DLP) 3D printing is an efficient additive manufacturing technology for polymer materials that enables complex structures to be obtained without expensive and time-consuming mold making procedures. Compared with point-by-point stereolithography and line-by-line extrusion ink writing (DIW), layer-by-layer DLP has the advantages of fast printing speed and high resolution of the printed structure. To date, material systems for DLP printing have been mainly limited to resins with fast curing speeds and high stiffness, in order to avoid gravity-induced shape changes when printing fine and high-fidelity structures . hydrogel has broad application prospects in tissue engineering, flexible actuators/robots, flexible electronics, etc. However, most gel materials have relatively low stiffness, making it difficult to maintain the designed shape during the printing process. Currently, DLP printing of high-resolution hydrogel structures remains a challenge. This problem greatly hinders the development of the preparation of hydrogels with complex structures and their applications. In recent years, researchers have used the relatively highly reactive polyethylene glycol diacrylate (PEGDA) and methacryloyl gelatin (GelMA) to develop hydrogel-printed structures with moderate stiffness. However, highly cross-linked hydrogels are brittle and have low mechanical strength, which limits their application under load-bearing conditions.
During the DLP printing process, solidification occurs at the bottom of the cylinder ("top-down" projection) or at the gas-liquid interface ("bottom-up" projection), and then the retractable platform is pulled upward from the liquid; in the " In the “top-down” approach, the liquid cylinder at the bottom not only serves as a supply source for the new prepolymer layer, but also provides buoyancy to support the printing structure. A recent work enabled DLP printing of flexible hydrogels with complex structures by using a dense medium to provide buoyancy to support compliant structures. This work successfully printed stretchable poly(acrylic acid) hydrogels using a "top-down" projection method. By changing the concentration of the cross-linker in the prepolymer solution, the Young's modulus of the printed hydrogel structures can be varied from 7 kPa to 260 kPa. In addition, during the DLP printing process, due to the possible swelling of the hydrogel and the density mismatch between the gel and the prepolymer solution, the current "top-down" projection method can only be limited to only a few in a hydrogel system. Although a variety of tough hydrogels have been developed in recent years, has few suitable hydrogels due to its slow reaction speed and/or preparation process of gradual gelation and toughening that is incompatible with fast and continuous printing processes. The gel system can be used for DLP printing . To this end, researchers have proposed some strategies to alleviate the above problems. For example, a post-toughening step is employed to enhance the mechanical properties of printed hydrogels. However, has difficulty transferring the fine structure of the weak gel and maintaining its shape fidelity before the toughening process . Therefore, it is still necessary to explore new material systems that can easily form tough hydrogels and are suitable for DLP printing to further expand the application range of gel materials.
In view of this, Zhejiang UniversityProfessor Zheng Qiang, Professor Wu Ziliang’s team developed a DLP printing method for tough supramolecular hydrogel structures. The printed water-based prepolymer is composed of commercial photoinitiator, acrylic and zirconium ions (Zr4+). Due to the in-situ formation of carboxyl-Zr4+ coordination complex, the prepolymer can form tough under digital illumination. Metal supramolecular hydrogels. The gel system's high stiffness and anti-swelling properties enable gel structures to be printed with high fidelity while being efficiently printed. After soaking in water, the mechanical properties of the printed hydrogels were further improved. The swelling-enhanced stiffness gives the printed hydrogels shape-fixing capabilities after manual deformation, which provides additional avenues for forming more complex structures. The printed hydrogel can be used to design impact-absorbing components or highly sensitive pressure sensors, bringing new opportunities for the application of gel materials in biomedical and engineering fields.The research was published in the latest issue of "Advanced Materials" in a paper titled "Digital Light Processing 3D Printing of Tough Supramolecular Hydrogels with Sophisticated Architectures as Impact-Absorption Elements".
[DLP-based printing process of tough supramolecular hydrogels]
Tough hydrogels with complex structures were prepared by DLP printing of prepolymer solutions using the “bottom-up” projection method (Figure 1a). The target 3D architecture is segmented into serial 2D images to guide the DLP of sequentially printed layers. The prepolymer solution contains acrylic acid (AAc), photoinitiator (V-50), Zr4+ ions and light absorbers. Polymerization of the prepolymer is triggered by digital UV light at room temperature. The poly(acrylic acid) (PAAc) chains produced by the reaction are simultaneously cross-linked by forming carboxyl-Zr4+ coordination complexes in situ, thereby producing metal supramolecular hydrogels in the UV-irradiated area. The printed hydrogel is mechanically stable with a Young's modulus of MPa level, which provides high enough stiffness to avoid gravity-induced shape changes during the bottom-up DLP printing process. The printed hydrogel structure is then further swelled and equilibrated in water to remove residues. The stiffness and toughness of the gel are further increased due to the structural arrangement of the coordination complexes. Among them, the feed concentration of AAc monomer and Zr4+ ions will affect the printability and mechanical properties of the hydrogel. This hydrogel-printed structure exhibits high resolution, good fidelity, and excellent mechanical stability. For example, a printed hydrogel (PAAc-5-0.2) with a Kelvin unit cell and cubic lattice structure has a resolution of hundreds of microns (Figures 1b and 1c). The hydrogel with a Kelvin unit cell structure can support a weight of 500 grams, which is 300 times the weight of the printed hydrogel.
Figure 1. (a) Schematic diagram of DLP-based tough supramolecular hydrogel 3D printing through in situ formation of carboxyl-Zr4+ coordination complex as physical cross-linking of the gel matrix. (b,c) Photographs of printed hydrogel structures of Kelvin units (b) and cubic lattices (c).
[Mechanical properties of DLP-based supramolecular hydrogels]
By controlling the feed concentrations of AAc (Cm) and Zr4+ ions (CZr4+), the mechanical properties of printed hydrogels can be controlled in a wide range. As shown in Figures 2a and 2b, Cm greatly affects the mechanical behavior of the printed gels. As Cm increases from 1 to 7 M, the tensile breaking strength, breaking strain, and Young's modulus of the printed gel (PAAc-Cm-0.2) first increase and then decrease. The reduced mechanical properties of printed gels with higher Cm may be due to two reasons. First, the increase of Cm leads to a decrease in the pH value of the prepolymer solution, thereby reducing the stability of the coordination complex. Secondly, the amount of Zr4+ ions is not enough to form a coordination complex with the higher Cm system and the PAAc chain produced by the reaction; the printed gel will undergo a certain degree of swelling during continuous processing, and reduce the strength and stiffness. Printed gels with medium Cm have the best mechanical properties, which may be due to the equilibrium density of polymer chains and physical cross-links. When Cm= 5 M, both printed and equilibrated hydrogels have excellent mechanical properties.
Figure 2. Mechanical properties of DLP-based supramolecular hydrogels.
Figure 3. Mechanical properties and thermal deformation behavior of supramolecular hydrogels under different soaking times.
[Applications of DLP-based supramolecular hydrogels]
The authors use two proof-of-concept examples to demonstrate the versatility of finely printed structures of hydrogels. The first example is the use of printed tough hydrogels as impact absorbing elements for and . The flexibility of the structural hydrogel depends on the modulus of the gel material and the geometry of the printed structure, which also determines the impact absorption capacity of the printed hydrogel. As shown in Figure 4a, the authors printed tough hydrogel lattices (PAAc-5-0.2) in different sizes and wall thicknesses. Different units have different resistance to mechanical force. Through compression test analysis, the authors found that with continuous deformation, the printed hydrogel showed a relatively low initial stiffness, but could resist large compressive forces under high strains.This hierarchically structured printed hydrogel is an effective impact-absorbing unit and can be further applied in designing structured soft cushioning. As shown in Figure 4b, the printed hydrogel has an overall kirigami structure, which can better deform and encapsulate objects with complex geometric shapes. Quail eggs were wrapped with this printed hydrogel and then swollen in water to fix the shape (Figure 4c). When dropped from a height of 1 meter (Figure 4d), the quail eggs wrapped with hydrogel buffer remained intact without any damage. In contrast, naked quail eggs dropped from the same height shattered into pieces on the ground (Fig. 4e). The impact energy absorption rate of hydrogel buffer is as high as 95%.
Furthermore, the authors designed a high-performance capacitive pressure sensor using printed hydrogels with structured surfaces. The capacitive sensor in a parallel plate configuration is assembled by sandwiching a printed hydrogel between a pair of conductive carbon fabric sheets, which acts as a dielectric layer. As the capacitive sensor is compressed in the thickness direction, the distance between the two electrodes decreases, causing the capacitance to increase. Printed hydrogels with a series of solid pyramidal protrusions deformed to a greater extent than flat hydrogel sheets under the same apparent pressure (Figure 5 g). The hollow pyramid-like protrusions on the surface further enhance the structural compliance of the printed hydrogel. The sensitivity of this capacitive pressure sensor is better than most existing hydrogel-based capacitive pressure sensors (Figure 5i).
Figure 4. Application of DLP-based supramolecular hydrogels.
[Summary]
This work reports a versatile and tough hydrogel system that can prepare fine three-dimensional architecture and tunable mechanical properties through DLP printing. Its prepolymer solution contains concentrated acrylic acid, efficient photoinitiator and appropriate amount of Zr4+ ions, which can be quickly solidified under digital light to form a tough metal supramolecular hydrogel. During the printing process, carboxyl-Zr4+ coordination complexes are formed in situ as physical cross-links of the poly(acrylic acid) chains. The fast gelation speed, excellent mechanical properties and swelling resistance of the printed gel in the prepolymer solution enable DLP printing to adopt a "bottom-up" projection method. During the swelling equilibrium process in water, the printed hydrogel further increases its stiffness and toughness due to changes in local pH and arrangement of coordination complexes. This feature can be used to repair the printed architecture after program deformation. This simple system suitable for DLP printing to prepare structural hydrogels with excellent mechanical properties will greatly broaden the applications of gel materials in deformable structures, impact-absorbing elements, hydrogel devices, etc.
Related reports:
Zhejiang University Zheng Qiang/Wu Ziliang/Zhao Peng "Angew": Magnetic orientation of magnetic double stacks of patterned anisotropic hydrogels with multi-response and modulated motion
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https ://onlinelibrary.wiley.com/doi/10.1002/adma.202204333
Source: Frontiers of Polymer Science
