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

The study addresses the challenge of rapidly fabricating tough hydrogels that combine high strength, extensibility, and energy dissipation while remaining biocompatible and printable. Conventional tough hydrogels often rely on double or multiple networks formed via multi-step processes (e.g., sequential monomer diffusion and polymerization, freezing/ion soaking, or incorporation of microgels/fibers), which typically require long irradiation or heating times and limit patterning and 3D printing. Moreover, systems based on a single dissipation mechanism (e.g., ionic, supramolecular, or a single rigid network) can lose toughness under harsh conditions due to partial disruption of unstable networks. The purpose is to develop a general, one-pot, fast, visible-light-triggered approach to simultaneously construct orthogonal networks that yield tough, soft, biocompatible hydrogels with robust mechanical performance and compatibility with high-resolution patterning/printing.

Prior work on tough hydrogels includes classic double-network strategies and various energy dissipation mechanisms that combine rigid and soft networks to prevent crack propagation and enable recoverability. Methods include sequential polymerization to form DN hydrogels, thermal or UV-initiated processes, freezing and ion soaking to form rigid networks, and adding microgels/fibers for dissipation. One-pot syntheses and in situ secondary network formation have emerged but still require >1 h and often rely on non-selective thermal initiation or radiation such as 60Co-γ, limiting high-resolution printing and biocompatibility. Single-mechanism systems (solely ionic or supramolecular) can lose performance in varying pH or ionic strength. This study builds on these insights by integrating multiple orthogonal, light-triggered reactions to overcome slow, multistep processing and environmental instability.

Design: A visible-light-assisted multimechanism design (THVMD) constructs three interpenetrating networks in one pot: (1) phenol–phenol covalent coupling (Ph-N), (2) radical polymerization to form a soft elastomeric polymer network (P-N), and (3) metal ion–alginate ionic crosslinking (M-N). Difunctional phenol-modified alginate (mALG) provides both ionic crosslinking sites (guluronic acid units) and phenolic moieties for covalent coupling.

Chemistry and initiation: Under 452 nm irradiation, Ru(bpy)3^2+ [Ru(II)] and ammonium persulfate (S2O8^2−) form an initiation system. Visible light excites Ru(II) and decomposes S2O8^2− to sulfate radicals that initiate monomer polymerization, while Ru(II) is oxidized to Ru(III), triggering phenol coupling and releasing metal ions (Mn+) from EDTA–M complexes. Released Mn+ (e.g., Ca2+, Al3+, Eu3+, Tb3+) rapidly crosslink mALG to form M-N. Monomers (e.g., acrylamide; with crosslinker MBA) form P-N; phenol-containing components (mALG and phenolic polymers such as silk fibroin/gelatin/BSA) yield Ph-N.

Formulation: Precursor solutions contain Ru(II)/S2O8^2−, EDTA–M, mALG, monomers (e.g., AAm), and crosslinker. Components are commercially available or synthesized in one step (e.g., EDTA–M by 1:1 coordination and reflux; mALG by phenol modification). Typical light intensity is 15 mW cm−2 at 452 nm. Control experiments show all components and light are required for gelation.

Kinetics and network formation: Real-time rheology tracks sol–gel transitions. Single-network gelation times: P-N ~8 s, M-N ~40 s, Ph-N ~190 s; estimated crosslinking densities: P-N ~0.7 mM, Ph-N ~1.4 mM, M-N ~28 mM. Networks form independently under the same irradiation, yielding a hierarchical interpenetrating structure. EDTA–Ca serves as a homogeneous, rapidly photoreleased Ca2+ source, enabling uniform and rapid M-N formation versus CaSO4/CaCl2. Gelation is tunable (e.g., 15–112 s) by formulation and light exposure; stepwise increases in storage modulus (G′) occur under intermittent light.

Fabrication and scaling: Rapid gelation facilitates large-area films (e.g., 13.5-inch transparent films on PET after ~100 s irradiation) and uniform particle dispersion. Hydrogels exhibit high flexibility, stretchability, and compressibility, supporting loads far exceeding their own weight.

Printing/patterning: The light-controlled process is compatible with shadow-mask photolithography, laser-guided direct writing (LGDW), and 3D extrusion printing. Sub-100 μm resolution is achieved with shadow masks and extrusion; sub-1 mm with LGDW. 3D structures (pyramids, porous scaffolds, coaxially printed tubes) are formed layer-by-layer under concurrent irradiation.

Biocompatibility: Due to fast S2O8^2− decomposition under visible light, the process is non-toxic. L929 fibroblasts encapsulated during gelation show no acute death and ~260% proliferation at 48 h compared to initial; superior to UV-triggered controls (~86% viability reported for a similar system with ~10 min UV exposure).

Characterization: Rheology (Anton Paar MCR302, 1% strain, 10 Hz), tensile (100 mm min−1, dogbone/strip), compression (10 mm min−1), and tear tests for fracture energy per literature methods. Environmental robustness evaluated by adjusting water content to 80 wt% and exposing to different pH and salts (NaCl 0.35 M, LiCl 0.47 M) or EDTA (0.1 M, 1:1 to EDTA–Ca), followed by mechanical tests. Conductivity measured; capacitive pressure sensors and EL devices assembled to demonstrate function.

• One-step visible-light-triggered gelation completes in tens of seconds; single-network gelation times: P-N ~8 s, M-N ~40 s, Ph-N ~190 s; gelation tunable 15–112 s; stepwise G′ increases with intermittent light.
• EDTA–Ca photorelease yields rapid, homogeneous Ca2+ distribution and uniform M-N; large-area 13.5-inch transparent films (>85% transmittance at 550 nm) fabricated in ~100 s.
• Mechanical performance: critical strain at rupture up to ~28 with stress ~200 kPa (1.4× strain and 3.4× stress vs single P-N); fracture energy ~8000 J m−2. Hydrogels can bear ~1.2 kg (~4,000× their own weight) and show excellent flexibility, stretchability, and compressibility.
• Elastic recovery: After stretching to strain 2 and 30 min rest, reloading work recovers to ~60% (similar to w/o Ph-N). Introducing Ph-N reduces plastic deformation by ~20–40% and increases elasticity from ~25% to ~45% versus w/o Ph-N, without sacrificing toughness.
• Environmental robustness: Under varied pH (4.5, 8.6), salts (NaCl 0.35 M, LiCl 0.47 M), and EDTA (0.1 M, 1:1 to EDTA–Ca), stresses decrease at low strains due to M-N debonding, yet fracture energies remain ~2000–3000 J m−2 (25–45% of normal). Without Ph-N, EDTA exposure reduces fracture energy by ~95% to a few hundred J m−2 (near pure PAAm).
• Conductivity: Adding salts increases ionic conductivity by ~2.0–3.5×, enabling capacitive pressure sensors with responsivity S ~4 kPa−1 and reliable performance; a 3×3 sensor array records multiple signals.
• Printing/patterning: High-resolution 2D and 3D structures produced via shadow-mask, LGDW, and 3D extrusion (sub-100 μm to sub-1 mm features). Coaxially printed tough tubes achievable only with light during printing due to rapid gelation.
• Ionotronics: Fully stretchable EL devices assembled with 1 mm-thick hydrogel electrodes operate at low voltage (~110 V AC), are controllable via hydrogel-based sensor inputs, and function under folding, stretching, twisting, and rolling without damage.
• Biocompatibility: Cell encapsulation during gelation shows no acute cytotoxicity and ~260% increase in L929 cell number after 48 h.

The multimechanism, visible-light-initiated strategy addresses the core challenge of rapidly and controllably fabricating tough hydrogels in a single step while preserving biocompatibility and printability. By orthogonally forming M-N, Ph-N, and P-N networks, the hydrogels dissipate energy efficiently via reversible ionic debonding and maintain integrity and elastic recovery through covalent networks. This architecture minimizes plastic deformation without compromising toughness and preserves substantial fracture energy in environments that disrupt ionic crosslinks (pH shifts, high salt, chelators). Spatial and temporal control of light-triggered reactions enables large-area, uniform films and high-resolution 2D/3D structures, directly compatible with lithography and extrusion printing—capabilities typically inaccessible to multistep DN systems. Enhanced ionic conductivity with salts broadens utility in ionotronics; integrated capacitive sensors and EL devices demonstrate durable function under large deformations. Overall, the findings support that a light-assisted, orthogonal network design offers a general, scalable route to tough, soft, functional hydrogels suitable for bioencapsulation, tissue engineering scaffolds, and flexible electronics.

This work demonstrates a one-step, seconds-scale, visible-light-assisted method to engineer tough, soft hydrogels by simultaneously constructing phenolic covalent, polymeric elastomeric, and metal–alginate ionic networks. The resulting THVMD hydrogels combine high extensibility (strain ~28), strength (~200 kPa), and toughness (~8000 J m−2) with reduced plastic deformation and retained fracture energies (2000–3000 J m−2) under harsh conditions. The process is broadly compatible with lithographic and 3D printing techniques to achieve high-resolution structures and is biocompatible for cell encapsulation. Functional devices including capacitive sensor arrays and low-voltage EL units validate performance in ionotronics. Future work can leverage the generality of the approach by selecting environment-responsive monomers and metal ions to create smart hydrogels, exploring additional functional fillers, and integrating more complex bioactive architectures for tissue engineering and soft robotics.

• Ionic network sensitivity: Mechanical properties decrease with pH changes, added salts, or Ca2+ chelation (e.g., EDTA), reflecting the inherent lability of M-N; although Ph-N and P-N preserve significant toughness, fracture energy drops to ~25–45% of normal in harsh conditions.
• Printing constraints: Rapid gelation requires concurrent visible-light irradiation; without light (e.g., in dark conditions), structures like tubes cannot be formed due to slow gelation.
• Testing conditions: Mechanical comparisons under varying environments were controlled at 80 wt% water content to minimize swelling effects; performance may vary with different hydration states.
• Resolution limits: LGDW patterning demonstrated sub-1 mm resolution (coarser than shadow-mask/extrusion), which may limit some microfabrication applications.