Programmable Strain-Responsive Biopolymer Networks Adapt to High Magnitudes of Mechanical Loading

Biopolymer hydrogel materials typically exhibit relatively low range of programmable modulus less than 100 kPa, which limits their biomedical applications, such as in articular cartilage and synthetic joints, where tissues are cyclically loaded with high magnitudes of peak stress on the order of 10MPa, and applications in soft robotics require moduli across orders of magnitude from 1 kPa to 100 MPa. Here, we achieved a wide range of mechanical properties with double network biopolymer hydrogels that can sustain over 10-100 MPa peak stress under repeated axial unconfined compression. Previous systems use double-network to enhance hydrogel’s toughness and strength. Here, cryogelation generates a foam network that undergoes a rarefied to densified phase transition, which is reinforced with a second dissipative network to yield highly tunable properties across orders of magnitude of applied stress. The foam network is formed by cryogelation of a covalently crosslinked collagen-glutaraldehyde (GA) biopolymer network that can sustain repeated loading through phase transition of its porous foam structure. Interpenetrating ionically-crosslinked alginate biopolymers tune the final modulus to make the hydrogel programmable in a high range of mechanical performance. This dual-network composite hydrogel system also exhibits reversible properties, achieved by chelating ions to reduce ionic crosslinks or restoring crosslinks by supplying additional ions. Together, these data demonstrate a robust hydrogel composite system adaptable to wide ranges of mechanical loading.<br/>First, the mechanical properties of collagen-GA cryogel network were evaluated by the influence of gelation temperature, freezing rate, and GA concentration. Cryogel samples prepared under -20°C with 0.15% w/v GA and -2°C/min freezing rate exhibited the best balance of stiffness and flexibility. Detailed mechanical characterizations, including single axial compression and hysteresis testing, revealed nonlinear behaviors and significant phase transition dynamics under compression. A continuum theory of phase transition was employed to model the inelasticity, which showed the cryogel with 0.15% GA had 2 times higher peak stress and almost 10% lower transformation strain, compared to no GA one. This suggests covalent crosslinking during cryogelation leads to stiffer and less brittle structure. The micro-structure under axial strains of 0%, 50%, and 90% were characterized using second harmonic generation (SHG) imaging, which showed significant fiber reorientation and densification under increasing strain. Further, 20 loops of hysteresis tests revealed the cryogel's ability to withstand repetitive loading cycles, with mechanical properties stabilizing after 10 cycles.<br/>Next, the cryogel was pre-compressed at different axial strain and infiltrated with an ionically-crosslinked alginate network which “locked” this state to achieve enhanced mechanical properties. Reversibility was achieved by adding ion chelator, ethylenediaminetetraacetic acid (EDTA), which disrupted the ionic crosslinks in alginate matrix, allowing the hydrogel to return to an “unlocked” state. 20 loops hysteresis testing (0% to 90% strain) showed mechanical responses due to different pre-strain conditions, and higher pre-strain has higher peak stress and better mechanical properties with resistance to fatigue and stabilized properties after repeated loading cycles.<br/>In summary, the development of a double-network biopolymer hydrogel system with programmable mechanical properties offers significant advancements for biomedical applications and soft robotics. This system not only demonstrates robustness and adaptability through repeated loading cycles but also exhibits reversible and strain-responsive behavior by modulating ionic crosslinks. These characteristics highlight its potential as a versatile material for applications requiring precise mechanical performance and durability under dynamic conditions.