( A) Stress–strain curves of FiberGelMA prepared from GelMA fibers with diameters of 20 µm and 1 µm. Mechanical properties of GelMA fiber hydrogels. FiberGelMA with water content of 90 wt% was another extreme although it was very stretchable, it was also weakest ( Fig. 2 B).įig. FiberGelMA with water content of 50 wt% had much higher chance of intercrosslinked fibers, its UTS approached 2 MPa, but its stretchability was compromised, with a strain at break of below 75% ( Fig. 2 B). We adjusted the chances that fibers contacted and were crosslinked with each other by hydration ratios. Another important feature of FiberGelMA’s hierarchical structure is the crosslinks and welding between fibers. We also found that FiberGelMA prepared from fibers with a diameter of 20 µm was almost twice as strong as those with a diameter of 1 µm ( Fig. 2 A), showing that fiber architecture influences hydrogel mechanical properties. This is a striking contrast to traditional GelMA hydrogels (with the same water content as FiberGelMA), which had a UTS less than 0.05 MPa and could not sustain stretches with strains of over 50%. The FiberGelMA with 75 wt% water content reached a UTS of 1.1 ± 0.2 MPa and had a strain at break that was around 155 ± 41% ( Fig. 2 A). The microfiber architecture fundamentally improved the ultimate tensile strength (UTS) and stretchability of GelMA. We first studied the microarchitecture–mechanics relationship in FiberGelMA. To form hydrogels, the fibers were first hydrated in water containing photoinitiators, then molded and photocrosslinked into scaffolds by blue light. To mimic the microarchitecture of native tissues, we produced fibers with diameters from 1 µm ( Fig. 1 C) to 30 µm ( Fig. 1 D). In the meantime, concentration of MAA in methanol was kept below 1% and reaction time was less than 3 h to avoid MAA aggregation between fibers. To maximize the degree of functionalization, the weight ratio of MAA to gelatin fibers was above 2:1. They were then functionalized by methacrylic anhydride (MAA) in methanol to induce photocrosslinkable groups ( Fig. 1 B). The fibers were retrieved by dissolving PCL in acetone. After stretching, the PCL tube solidified when cooling to room temperature, and the gelatin inside the PCL tube was the as-formed fibers. We stretched the melting PCL tube to a total length of L ( Fig. 1 A and SI Appendix, Fig. The melting PCL is highly viscous and plastic, which allows large plastic deformations. The gelatin-loaded PCL tube was heated to the melting temperature of PCL, 65 ☌. We injected the gelatin into the tube and sealed two ends of the tubes. S1 A) and highly concentrated gelatin solutions (40 to 50 wt%). To fabricate the metastable GelMA fibers, we prepared a PCL hollow tube with an inner diameter of D 0 and length of L 0 ( SI Appendix, Fig. The integration of tissue-like mechanical properties and bioactivity is highly desirable for the next-generation biomaterials and could advance emerging fields such as tissue engineering and regenerative medicine. The fiber architecture also regulates cellular mechanoresponse and supports cell remodeling inside hydrogels. This hydrogel also resembles the biochemical and architectural properties of native extracellular matrix, which enables a fast formation of 3D interconnected cell meshwork inside hydrogels. As a demonstration, we create a gelatin methacryloyl fiber hydrogel with soft tissue-like mechanical properties, such as low Young’s modulus (0.1 to 0.3 MPa), high strength (1.1 ± 0.2 MPa), high toughness (9,100 ± 2,200 J/m 3), and high fatigue resistance (2,300 ± 500 J/m 2). Here, we present a thermomechanical approach to replicate the combinational properties of soft tissues in protein-based photocrosslinkable hydrogels. Although hydrogels prepared from synthetic polymers can be strong and tough, they do not have the desired bioactivity for emerging biomedical applications. By contrast, naturally derived hydrogels are weak and brittle. They have high water content but are still tough and durable. Load-bearing soft tissues normally show J-shaped stress–strain behaviors with high compliance at low strains yet high strength at high strains.
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