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Double-network Hydrogels for 3D Printing Ionic Skin
a capacitor-based ionic skin, we first printed a hydrogel highly packed colloidal systems can eliminate the effect
layer on a poly(lactic acid) (PLA) film layer, and hydrogel resulting from the structural features of the particulate
was further crosslinked to form a polymer network by network on the mechanical properties of the resulting
365 nm UV light. Then, we further printed a hydrogel colloidal gels [22] .
layer on the top PLA layer and cured it with UV light, The gelatin/PAAm DN hydrogels not only
and the PLA film further covered as a dielectric layer and maintained the shear-thinning and self-healing properties
prevents evaporation of the hydrogel. of the colloidal network but also exhibited enhanced gel
To further test the capacitance of capacitor-based elasticity after photopolymerization (G’~4 kPa) 3 times
ionic skin, we use an LCR meter (TH2830) at an AC higher than before polymerization (Figure 2C). Further
voltage of 1 V and a sweeping frequency of 1 kHz. oscillatory frequency sweep tests revealed the frequency-
Capacitance changes were simultaneously detected with independent behavior of the DN hydrogels as well as the
various stimuli on the prepared devices. The printed ionic colloidal gels; the DN gels had higher gel strength than
skins were composed of five layers, of which the first, the colloidal gels. In contrast, 10 w/v% PAAm hydrogels
third, and fifth layers were PLA films (height: 50 μm), showed a more frequency-dependent behavior with the
and the second and fourth layers were printed hydrogels. network moduli increasing gradually as a function of
frequency (Figure 2D).
3. Results and discussion We further used conventional compression and

3.1. Mechanical properties of the DN hydrogels tensile tests to characterize the mechanical properties of
gelatin/PAAm DN hydrogels. On compression, the DN
The GNPs/PAAm DN hydrogels were prepared by hydrogels initially showed a linear region with the stress
directly mixing gelatin nanoparticles with flowable increased linearly as strain increased (Figure 3A). After
AAm monomer solution via multiple cycles of the linear region, the stress showed a sharp increase with
extrusion in a conventional medical syringe, followed increasing the compressive loading, with the compressive
by molding or printing into certain morphology that can modulus E and strength σ of 31.1±4.6 and 270.1±15.7
c
c
be further photopolymerized to allow the solidification kPa, respectively. Interestingly, we did not observe any
of the structures. Before triggering the polymerization yielding point even the compression strain was up to 95%,
of PAAm, we first evaluated the injectability and suggesting the considerable elasticity of the hydrogel
printability properties of pure nanostructured gelatin matrix resulting from the DN design. In comparison, pure
colloidal gels. By applying an increasing shear rate, the gelatin colloidal gels showed a similar elastic behavior
gelatin colloidal gels of different concentrations showed at rather small deformation with the E = 32.9±4.5 and
c
typical shear-thinning behavior as evidenced by a σ =15.7±1.8 kPa. However, gelatin colloidal gels can
c
linear decrease in viscosity upon shearing (Figure 2A). only resist 35% strain followed by an elastic fracture.
Moreover, gelatin colloidal gels also showed a high Moreover, PAAm hydrogels showed a significantly
degree of mechanical recovery after severe network weaker compressive modulus with E = 3.1±0.3 kPa as
c
destruction (or so-called self-healing behavior). As compared to both DN hydrogels and the colloidal gels.
shown in Figure 2B, in the initial low strain region Further tensile tests revealed almost purely elastic
(0.5%), the colloidal gel showed the formation of a properties followed by an elastic fracture for the gelatin/
stable gel network as reflected by a higher value of PAAm DN hydrogels, evidenced by the highly linear stress-
G’ than G”. Subsequently, a higher shear strain (1 – strain curve without showing any plastic deformation
1000% for 1 min) led to network destruction and the (Figure 3B). The tensile modulus E = 15.1±0.6 and
t
transformation from a solid gel to a liquid-like material tensile strength σ =27.8±1.3 kPa, respectively. Pure
t
as reflected by G” higher than G’. On release of the gelatin colloidal gel displayed a comparable tensile
destructive shear, gelatin colloidal gel immediately modulus E =14.1±3.1 but a considerably lower tensile
t
showed more than 70% of recovery of G’ value as strength σ =1.3±0.3 kPa as compared to DN hydrogels.
t
relevant to the initial G’. This can be attributed to the In contrast, PAAm hydrogels showed the weakest
cohesive interactions between gelatin nanoparticles. mechanical properties with E = 2.3±0.4 and σt =2.5±0.2
t
Such shear-thinning and self-healing behavior rendered kPa, respectively. Noticeably, the DN hydrogels also
the colloidal gels printable and capable of blending exhibited higher tensile fracture strain up to 1.74, slightly
with different components such as other types of higher than PAAm (fracture strain of 1.21), but more than
micro-/nano-particles or flowable precursor solutions. one order of magnitude higher than pure gelatin colloidal
To obtain a mechanically stable colloidal network, we gels with yield strain of 0.11.
prepared hydrogels containing gelatin nanoparticles We further performed cyclic compression/tensile
with mass fraction ranging from 10 to 12 w/v% tests to explore the capacity of the DN hydrogels for
(corresponding to volume fraction above 0.5). Such anti-fatigue performance and energy dissipation on

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