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

Tough hydrogels that are capable of efficient mechanical energy dissipation and withstanding large strains have potential applications in diverse areas. However, most reported fabrication strategies are performed in multiple steps with long-time UV irradiation or heating at high temperatures, limiting their biological and industrial applications. Hydrogels formed with a single pair of mechanisms are unstable in harsh conditions. Here we report a one-step, biocompatible, straightforward and general strategy to prepare tough soft hydrogels in a few tens of seconds under mild conditions. With a multimechanism design, the network structures remarkably improve the mechanical properties of hydrogels and maintain their high toughness in various environments. The broad compatibility of the proposed method with a spectrum of printing technologies makes it suitable for potential applications requiring high-resolution patterns/structures. This strategy opens horizons to inspire the design and application of high-performance hydrogels in fields of material chemistry, tissue engineering, and flexible electronics.


Supplementary Note 1
To study the feasibility of this strategy for fabricating THVMD hydrogels via the proposed orthogonal photoreactions with the catalysis of Ru(II)/S2O8 2-. Three phenolcontained natural polymers, including gelatin, silk fibroin, and bovine serum albumin were used for constructing Ph-N networks of tough hydrogels via the phenol coupling reaction. AAm and MBA were used for building P-N networks via radical polymerization. Sodium alginate was used for creating M-N networks via the ioniccrosslinking reaction. Supplementary Figure 2a showed that these hydrogels were obtained in one step with short light irradiation, and presented good mechanical strength and stretchability. Moreover, the cyclic tensile tests in Supplementary Figure 2b indicated that there were obvious hysteresis during all stretching/release processes. The achieved results indicated that these THVMD hydrogels were tough, and can dissipate the mechanical energy when tensile strains were applied.

Supplementary Note 3
The stress-strain curves in Supplementary Figure 5a showed that different THVMD hydrogels were successfully prepared by introducing different monomers and metal ions to construct corresponding P-N and M-N networks. These hydrogels are stretchable and have good mechanical strength (150~250 kPa) with strains of 3~24.
Moreover, the cyclic tensile tests showed that there was obvious hysteresis in all samples ( Supplementary Figure 5b), which indicated the mechanical energy was efficiently dissipated via the disrupt of rigid networks in these hydrogels when releasing the strain from 2 to 0. On the basis of these achievements, we can conclude that this new strategy is general, straightforward, and is readily applicable for preparing THVMD hydrogels. The mechanical property and toughness are easily tuned by varying synthetic polymers and metal ions in hydrogels.

Supplementary Note 5
The 2-hydroxytoluene-contained control experiment was designed to detect the possible reaction between P-N and Ph-N. As shown in Supplementary Figure 7a, this phenol derivative supplies one reaction site to modify the polymer and themselves. The diphenol products are soluble in methanol that can be easily removed from the precipitation. Finally, only the PAAm-contained polymer was obtained. Therefore, the possible reaction between 2-hydroxytoluene and PAAm can be easily studied by using UV-vis and 1 H-NMR spectra. In detail, AAm, MBA, Ru(II), S2O8 2-, 2-hydroxytoluene was mixed in water with the pre-determined concentrations. After light irradiation for 100 s, the solution was first dropwise added into methanol to precipitate the PAAmcontained polymer. Then, the solid product was collected, re-dissolved into an aqueous solution of NaOH (1 wt%), and precipitated again in methanol. After repeating the same procedure three times, the product was dried in a vacuum. Figures S7b and c showed that there was no obvious characteristic absorption and peaks of phenol groups observed in the UV-vis and 1 H-NMR spectra. This result indicated that 2hydroxytoluene did not react with AAm, and no phenol groups were grafted onto the PAAm chains. As for the hydrogels, the possible reaction between networks of P-N and Ph-N did not proceed via PAAm (in P-N) and phenols (in Ph-N). Therefore, we can speculate that three networks of P-N, M-N, and Ph-N independently formed even under the same light irradiation in one pot.

Supplementary Note 6
To further understand the formation mechanism of THVMD hydrogels, another Ru(II)/DMA photoinitiator system was employed to construct the P-N network via the radical polymerization of typical monomers. When exposing the aqueous solution to visible light irradiation, Ru(II) is reduced to Ru(I) by DMA. Meanwhile, DMA is oxidized to the amino radical that can trigger the polymerization reactions. In this study, the experimental conditions were performed at the same conditions, but the initiator systems were different. The reaction processes were monitored by observing the solgel transition and in situ FT-IR characterization. As shown in Supplementary Figure 10, we can observe that the hydrogel was formed in 120 s in Ru(II)/S2O8 2-, while the sample was in the liquid state in Ru(II)/DMA with long irradiation. This difference was strongly supported by the FT-IR characterization. The characteristic absorption peak of AAm at