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Chen, et al.
A B

C D

Figure 2. Viscoelastic properties of gelatin/polyacrylamide (PAAm) double-network (DN) hydrogels. (A) Shear-thinning behavior of the
nanostructured gelatin colloidal gel of different concentrations (5, 7.5, 10, and 15 w/v%). (B) Evolution of modulus of the colloidal gel of
different concentrations by applying the destructive shearing (oscillatory strain sweep with increasing strain from 1% to 1000% with a fixed
frequency of 1 Hz) and the recovery (oscillatory time sweep at 0.5% strain and a frequency of 1 Hz for 200 s) on the release of destructive
shearing. (C) Triggered photo-polymerization of gelatin/PAAm composite gels (time sweep at 0.5% strain and 1 Hz frequency), as reflected
by the sharp increase of G’ and G” values. (D) Frequency dependence of storage and loss modulus of pure gelatin colloidal gel (10 w/v%
gelatin nanoparticles), PAAm (10w/v%), and gelatin/PAAm composite gel (10 w/v% gelatin nanoparticles, 10 w/v% PAAm).

loading. The gelatin/PAAm DN hydrogels can withstand non-covalent network to allow energy dissipation ,
[14]
the repetitive applications of the compressive stress of thereby realizing gel mechanics outperforming what
~200 kPa and a compressive strain up to 0.75, and can they can achieve individually. However, the preparation
rapidly recover to the original shape upon unloading during of conventional DN hydrogels was normally time-
the cyclic compression test (Figure 3C). For the cyclic consuming and complicated, which restricted the precision
tensile tests, the DN hydrogels also showed a wide-range manufacturing and wide-spreading applications .
[13]
linear elasticity and capability to resist repetitive tensile Alternative strategy of introducing nanoparticles as the
stress of ~15 kPa and a tensile strain of 1 (Figure 3D). reinforcement components to the continuous polymer
Similarly, we observed almost complete recovery to the network was widely-used to prepare strong hydrogels as
original state on unloading and overlapped hysteresis compared to the DN design, which normally confronted
loops even after 50 cycles of loading/unloading. These with the issue of network homogeneity of the disperse
findings suggest that the DN hydrogels are highly elastic and continuous phases, and subsequent compromised
and anti-fatigue, which are of significant importance for mechanical properties . In comparison, our DN design
[25]
the development of wearable devices that adapt to body based on colloidal and polymeric networks combined the
movements [23,24] . superior properties of both components, including ease
We compared the current colloidal-based DN of preparation, injectability, or moldability rendered by
hydrogels with previously reported highly strong or tough the reversibly crosslinked colloidal network, and high
hydrogels. Typical DN hydrogels achieve high robustness network mechanics and structural integrity resulting from
or toughness values through the combination of a the covalent PAAm network. Such combination allows
permanent covalent network with a reversibly crosslinked us to fabricate these hydrogels into micro-meter scale

International Journal of Bioprinting (2021)–Volume 7, Issue 3 101