Mechanical properties of an interpenetrating network poly(vinyl alcohol)/alginate hydrogel with hierarchical fibrous structures

Abstract

Bioinspired hierarchical fibrous structures were constructed in an interpenetrating poly(vinyl alcohol, PVA)/alginate hydrogel network to improve its mechanical properties. The interpenetrating hydrogel network with hierarchical fibrous structures was prepared by combining the confined drying method and freeze-thaw method. First, Ca²⁺ cross-linked alginate formed a nano-micro hierarchical fibrous structure via the confined drying method. Then, PVA that was uniformly distributed among the Ca²⁺-alginate chains was cross-linked by hydrogen bonding via the freeze-thaw method, further dividing the hierarchical fibers into finer fibers. The results of a tensile test demonstrated that both the tensile stress and fracture energy improved by more than double after the introduction of 2 wt% PVA, achieving a combination of high strength (∼12.9 MPa), high toughness (∼13.2 MJ m^-3) and large strain (∼161.4%). Cyclic tensile tests showed that a hysteresis loop existed on the loading-unloading curves of the hydrogel along the fibrous directions, and a good self-recovery property emerged after resting for a period of time. The hydrogel with hierarchical fibrous structures constructed by alginate and PVA can be employed in biomedical applications in the future.

Fig. 1. Schematic diagrams of the preparation process for the interpenetrating network hydrogel with hierarchical fibrous structures: randomly distributed alginate chains in the PVA/alginate solution were crosslinked by Ca²⁺ (a), alginate hierarchical fibrous structures were formed by the confined drying method (b), and the interpenetrating network hydrogel with hierarchical fibrous structures was formed after the freeze–thaw method (c).

Fig. 2. Typical FTIR spectra of alginate, PVA and the interpenetrating network PVA/alginate hydrogel with hierarchical fibrous structures.

Fig. 3. Typical SEM images of the interpenetrating network PVA/alginate hydrogel with hierarchical fibrous structures containing 0 wt% (a) 1 wt% (b), 2 wt% (c), 4 wt% (d) of PVA.

Fig. 4. Typical high magnification SEM images of the interpenetrating network PVA/alginate hydrogel with hierarchical fibrous structure containing 0 wt% (a) 1 wt% (b), 2 wt% (c), 4 wt% (d) of PVA.

Fig. 5. Mechanical properties of the interpenetrating network PVA/alginate hydrogels with different mass fractions of PVA along (‖) and perpendicular (⊥) to the fibrous directions: tensile stress–strain curves (a), tensile stress (b), tensile strain (c) and tensile fracture energy (d).

Fig. 6. Cyclic loading–unloading curves, dissipated energy and maximum stress for fifty successive loading–unloading cycles of the interpenetrating network PVA/alginate hydrogels with 1 wt% (a and b), 2 wt% (c and d), and 4 wt% (e and f) of PVA.

Fig. 7. Dissipated energy (a) and elastic modulus (b) of a single loading–unloading test at a strain of 30% after different resting times for the interpenetrating network PVA/alginate hydrogel with hierarchical fibrous structures.

Fig. 8. Schematic diagrams of the alginate hydrogel (a) and the interpenetrating network PVA/alginate hydrogel (b) with hierarchical fibrous structures.