Visible-light-assisted multimechanism design for one-step engineering tough hydrogels in seconds

Tough hydrogels, possessing both high strength and the ability to withstand large deformations, hold immense potential across diverse fields such as wearable electronics, tissue engineering, and drug delivery. Their unique mechanical properties stem from interpenetrating networks of rigid and soft components. Upon strain application, the rigid network dissipates energy, preventing crack propagation, while the soft network facilitates recovery to the original shape. However, creating these multi-network hydrogels presents a significant challenge. Conventional approaches often involve multiple steps, including diffusion of monomers into pre-formed networks, followed by polymerization via heat or UV irradiation, or the sequential construction of rigid and soft networks. These methods are time-consuming (often requiring hours), use harsh conditions like UV or high temperatures which limit biocompatibility and industrial applications, and frequently result in hydrogels with unstable properties under harsh conditions due to the reliance on single energy dissipation mechanisms. The need for a general, rapid, biocompatible, and printable method for one-step fabrication of tough hydrogels remains a significant challenge. This study addresses this challenge by introducing a novel multimechanism, one-step approach for producing tough hydrogels under mild visible light irradiation within seconds.

Existing methods for fabricating tough hydrogels often rely on double-network designs, typically created through multi-step processes involving either diffusion of a second monomer into a pre-existing network followed by polymerization using heat or UV irradiation, or the sequential construction of soft and rigid networks via techniques like freezing or ion soaking. Although one-pot synthesis and in situ formation of secondary networks have emerged as more efficient alternatives, they still require reaction times exceeding an hour. Moreover, the reliance on long UV irradiation, gamma irradiation, or high temperatures compromises the biocompatibility and limits the applications of these hydrogels. These traditional methods often lack the precision required for creating high-resolution 2D and 3D microstructures needed for applications in tissue engineering and ionotronics. Additionally, hydrogels based on a single pair of energy dissipation mechanisms (e.g., ionic crosslinking) often prove unstable in harsh conditions, significantly reducing their overall toughness. Therefore, the development of a rapid, biocompatible, and versatile one-step method for fabricating tough hydrogels remains a critical unmet need.

This study introduces a visible-light-assisted multimechanism design (THVMD) for the one-step fabrication of tough hydrogels within seconds. This method leverages three orthogonal photoreactions: phenol-phenol coupling, radical polymerization, and ionic crosslinking, to simultaneously create three distinct interpenetrating networks. The precursor solution comprises tris(2,2'-bipyridyl)dichlororuthenium(II) (Ru(II))/ammonium persulfate (initiator), ethylenediaminetetraacetic acid-chelated metal ions (EDTA-M, metal ion source), phenol-containing polymers (e.g., silk fibroin, gelatin, bovine serum albumin), alginate (for metal-ion crosslinking), and acrylamide (monomer). Visible light irradiation (~452 nm) triggers the decomposition of the initiator, initiating polymerization and the simultaneous coupling of phenol groups. The release of metal ions from EDTA-M facilitates rapid ionic crosslinking with alginate. The use of difunctional phenol-modified alginate (mALG) ensures the stability of the network even if the ionic crosslinks are partially disrupted. The method's versatility is demonstrated using various metal ions (Ca²⁺, Al³⁺, Eu³⁺, Tb³⁺) and monomers (PEG, PAAm, PNIPAM). Real-time rheology was used to monitor the sol-gel transition, revealing individual network formation rates and crosslinking densities. The biocompatibility of the method was evaluated using cell encapsulation and proliferation studies. Mechanical properties were assessed using tensile, compression, and tearing tests. The influence of pH and salts on mechanical properties was investigated. Finally, the compatibility of the method with various printing techniques (shadow-mask lithography, laser-guided direct writing, and 3D extrusion printing) was demonstrated to fabricate high-resolution 2D and 3D structures. The fabricated hydrogels were also used to build an integrated electronic system including a capacitive pressure sensor and electroluminescent devices.

The THVMD method successfully fabricated tough hydrogels in a matter of seconds using visible light. The resultant hydrogels exhibited a remarkable combination of properties. The introduction of multiple networks significantly enhanced the mechanical properties compared to single-network hydrogels. Specifically, the critical strain at rupture increased by factors of 1.4 and 3.4 compared to the control hydrogels lacking the phenol network and both metal-ion and phenol networks, respectively. Fracture energies reached ~8000 Jm⁻², comparable to state-of-the-art tough hydrogels. The multimechanism design resulted in improved elasticity and reduced plastic deformation, even under cyclic stretching. The hydrogels demonstrated remarkable resilience in harsh environments, such as varying pH and the presence of metal-ion chelators like EDTA. Notably, even with the addition of salts, fracture energies remained substantial (2000-3000 Jm⁻²), 25%-45% of those under normal conditions, attesting to the robust nature of the covalently crosslinked networks. The rapid and controllable gelation, combined with biocompatibility, allowed for successful cell encapsulation and proliferation. The compatibility with various printing techniques enabled the fabrication of high-resolution (sub-100 μm) 2D and 3D structures, including intricate patterns, meshes, and tubes. The high conductivity and compressibility of the hydrogels facilitated the development of a capacitive pressure sensor with good responsivity and reliability. Finally, a fully stretchable and functional integrated electronic system was successfully constructed using THVMD hydrogels as transparent electrodes in electroluminescent devices, demonstrating the potential for applications in ionotronics.

The findings demonstrate the successful creation of tough, biocompatible hydrogels using a novel, efficient, and versatile one-step method. The use of visible light significantly improves the biocompatibility and simplifies the fabrication process. The multimechanism design, employing three interpenetrating networks, provides exceptional mechanical properties and high toughness even under challenging conditions. The compatibility with various printing technologies makes it highly suitable for diverse applications requiring precise patterning and complex 3D structures. The construction of a functional electronic system using THVMD hydrogels as electrodes underscores the significant potential of this approach in ionotronics, offering a route to robust and flexible electronic devices. The improved stability of these hydrogels over those relying on single energy dissipation mechanisms represents a major advancement in hydrogel technology.

This study presents a significant advancement in hydrogel fabrication by introducing a one-step, visible-light-assisted method for creating tough hydrogels with exceptional mechanical properties and biocompatibility. The multimechanism design and compatibility with various printing techniques greatly expand the potential applications of these materials in diverse fields, including tissue engineering, flexible electronics, and ionotronics. Future research could focus on exploring new combinations of polymers and metal ions to further tailor the properties of these hydrogels and investigating their long-term stability and biocompatibility in complex in vivo settings.

While the study demonstrates the potential of THVMD hydrogels, some limitations exist. The long-term stability of the hydrogels in various physiological environments requires further investigation. A detailed analysis of the degradation kinetics of the hydrogels under different conditions is warranted. Moreover, the scalability and cost-effectiveness of the method for large-scale production need to be thoroughly evaluated. Although the price of Ru(II) appears high, the amount used per sample is minimal. Finally, a more comprehensive investigation of the biocompatibility with different cell types and in vivo studies are necessary before widespread biomedical applications.