Tough, stretchable and compressive alginate-based hydrogels achieved by non-covalent interactions

Abstract

In this study, two alginate-based hydrogels with good mechanical strength, toughness and resilience were synthesized by hydrophobic interaction and coordination bonding. Sodium alginate/poly(acrylamide) semi-interpenetrating network (NaAlg/PAM semi-IPN) hydrogels were first synthesized through the micelle copolymerization of acrylamide and stearyl methacrylate in the presence of sodium alginate, then calcium alginate/poly(acrylamide) double network (CaAlg/PAM DN) hydrogels were prepared by immersing the as-prepared NaAlg/PAM semi-IPN hydrogels in a CaCl₂ solution. FT-IR and XPS results revealed NaAlg/PAM semi-IPN hydrogels and CaAlg/PAM DN hydrogels were successfully synthesized through non-covalent interactions. The tensile strength of CaAlg/PAM DN hydrogels could reach 733.6 kPa, and their compressive strengths at 80% strain are significantly higher than those of the corresponding NaAlg/PAM semi-IPN hydrogels, which is attributed to the alginate network crosslinked by Ca²⁺. The dual physically crosslinked CaAlg/PAM DN hydrogels can achieve fast self-recovery, and good fatigue resistance, which is mainly assigned to energy dissipation through dynamic reversible non-covalent interactions in both networks. The self-healing ability, swelling behavior and morphology of the synthesized alginate-based hydrogels were also evaluated. This study offers a new avenue to design and construct hydrogels with high mechanical strength, high toughness and fast self-recovery properties, which broadens the current research and application of hydrogels.

Scheme 1. Schematic diagram of NaAlg/PAM semi-interpenetrating network hydrogels and CaAlg/PAM double network hydrogels.

Fig. 1. FT-IR (a) and XPS wide-scan (b) spectra of SA, PAM, NaAlg/PAM and CaAlg/PAM hydrogels; C 1s (c) and O 1s (d) level spectra of SA, PAM, NaAlg/PAM and CaAlg/PAM hydrogels.

Fig. 2. The photographs of CaAlg/PAM DN hydrogel demonstrating the excellent mechanical behaviors: under stretching (a) without and (b) with central notch, compressing (c), slicing with a knife (d) holding a weight of 100 g (e).

Fig. 3. (a and d) Tensile stress–strain curves of CaAlg/PAM DN hydrogels; (b and e) tensile strength and elongation at break of CaAlg/PAM DN hydrogels; (c and f) compressive stress–strain curves of CaAlg/PAM DN hydrogels: (a, b and c) hydrogels with various SMA concentrations; (d, e and f) hydrogels with various SA concentrations.

Fig. 4. (a) Loading–unloading curves and (b) the corresponding toughness of NaAlg 0%/PAM 2% hydrogel, NaAlg 5.0%/PAM 2% hydrogel and CaAlg 5.0%/PAM 2% hydrogel; (c) cycle loading–unloading curves and (d) the corresponding toughness of CaAlg 5.0%/PAM 2% hydrogel at various strains.

Fig. 5. (a) Cyclic loading–unloading curves and (b) toughness recovery ratio and residual strain of CaAlg 5.0%/PAM 2% DN hydrogel at different resting time; the ten successive tensile loading–unloading curves of the (c) as-prepared samples and (d) recovered sample after 24 h resting time.

Fig. 6. Dynamic rheological behaviors of NaAlg 5.0%/PAM 2% and CaAlg 5.0%/PAM 2% hydrogels: (a) strain sweep by frequency of 10 rad s⁻¹ at 25 °C; (b) oscillatory frequency sweeps by 1.0% strain at 25 °C; (c) cyclic continuous step strain measurements in which the strain was switched from 1% strain for 100 s to 100% strain for 100 s; (d) cyclic continuous step strain measurements in which the strain was switched from 1% strain for 100 s to various larger strains (100%, 200%, 300% and 400%) for 100 s.

Fig. 7. Self-healing properties of NaAlg 5.0%/PAM 2% hydrogel: (a) digital photographs of the process to prepare healed sample; (b) digital photographs of healed sample that can withstand different deformations; (c) optical microscopy images of the sample after being healed for various time; (d) typical stress–strain curves of healed hydrogels; (e) healing efficiency of healed hydrogels.

Fig. 8. (a) Swelling behaviors of NaAlg/PAM and CaAlg/PAM hydrogels in pH = 7.4 buffer solution; (b) the mechanism of pH-sensitivity swelling behavior of CaAlg/PAM hydrogels; (c and d) equilibrium swelling ratios of CaAlg/PAM hydrogels with different compositions in different buffer solutions: (c) hydrogels with various SMA concentrations, (d) hydrogels with various SA concentrations.

Fig. 9. SEM images of alginate-based hydrogels: (a) NaAlg 5.0%/PAM 4%; (b) NaAlg 10.0%/PAM 2%; (c) CaAlg 5.0%/PAM 2%; (d) CaAlg 0%/PAM 2%; (e) CaAlg 5.0%/PAM 4%; (f) CaAlg 10.0%/PAM 2%.