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Double-network Hydrogels for 3D Printing Ionic Skin
A B

C D

Figure 3. The mechanical properties of double-network (DN) hydrogels tested using conventional compressive and tensile tests. The stress-
strain curves of (A) compressive and (C) tensile measurements at a strain rate of 0.021 1/s for the gelatin colloidal gel, polyacrylamide
(PAAm), and gelatin/PAAm DN hydrogels. The hysteresis curves of DN hydrogels on (B) cyclic compressive and (D) tensile loading and
unloading.

resolution through 3D printing meanwhile achieving the release of shear stress, for which we can compare the
outstanding gel mechanics through secondary crosslinking change of the diameters of the injected strands as relative
based on photo-polymerization of the polymer phase. to the original nozzle diameter to evaluate the capacity of
the inks to facilitate precise microfabrication. As a result,
3.2. Printability of the DN hydrogels inks containing 10 w/v% colloids showed an expansion
We further evaluated the printability of the DN hydrogels rate of ~120% when using a nozzle with a diameter of
as inks for 3D printing. Typically, we first assessed the 200 μm, which was half to that of the 7.5 w/v% colloids
injectability of the hydrogel inks by monitoring the (~240%) (Figure 4C and D). This can be explained by the
compressive force for the extrusion of the inks using tendency of the dilute colloidal components to stress-relax
conventional medical syringes (Figure 4A). It was after being released from the confinement from the nozzle.
shown that gelatin hydrogel in with different colloid To demonstrate high-resolution printing using
concentrations of 5, 7.5, and 10 w/v% can be easily gelatin/PAAm DN hydrogel ink, we fabricated a
extruded with a rather low compressive force (<20 N). connected circuit device using the hydrogel to form
In comparison, a higher colloid concentration (15w/v%) more than 49 patterned sub-units in a 50 mm × 50 mm
in the inks showed difficulty to extrude the inks out of area. A nozzle size of 400 μm was used, and resolution
the nozzle, with continuously increased compressive of the device preparation can reach up to 800 μm
force up to 35 N. Moreover, images showing the shapes (Figure 5A and B). Such high-resolution construction of
of the inks extruded out of the nozzles of the syringes microarray architecture can allow high-degree sensitivity
showing different flow patterns during the injection of the spatial location with the circuit area where pressure
(Figure 4B). Only inks containing 7.5-10 w/v% gelatin or deformation was applied. Moreover, we also printed
colloids still retained a noodle-like shape after injection, an ear-shape construct using our DN hydrogel ink and
indicating the formation of stable flow during shear- observed that the sharp and rounded corners can be
thinning process. In contrast, both inks containing lower precisely fabricated, confirming the fidelity of the gelatin/
or higher concentrations of gelatin colloids formed drop- PAAm DN hydrogel-based inks (Figure 5C and D).
like shapes on injection. The former condition can be
related to too dilute colloid concentration to allow fluent 3.3. 3D printed capacitance devices as wearable
injection, while the latter can be related to too densely devices using the DN hydrogel
pack colloidal network that jammed in the syringe and
led to poor injectability. We further observed the inks to Typically, the hydrogel-based capacitor senses the
expand when they were extruded out of the nozzle due to pressure by the change in the relative area of conductive

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