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Chen, et al.
A B

Figure 1. Design principle of gelatin/polyacrylamide (PAAm) double-network (DN) hydrogels for the preparation of ionic skin. The
conductive layers were printed by gelatin nanoparticles in combination with AAm monomers, followed by photoinduced polymerization
of PAAm, to obtain an elastic and flexible ionic skin. The dielectric layer was polylactic acid (PLA) film. The schematic diagram showing
the mechanism of the formation of nanostructured gelatin colloidal network (A) and the DN hydrogels composed of gelatin nanoparticles
and PAAm (B). Scanning electron microscope photographs showing the microstructures of pure gelatin colloidal gel and gelatin/PAAm DN
hydrogels.

shaped by molding followed by secondary crosslinking based bioprinter. Thereafter, the printed constructs were
by photo-induced polymerization of PAAm, in which the exposed to UV light to allow polymerization of PAAm.
evolution of viscoelastic properties was determined by an The injectability of the colloidal gels with different
oscillation time sweep (frequency of 1 Hz and strain of concentrations was evaluated by injecting force during
0.5%). The inherent viscoelastic properties of the resulting the extrusion process by a universal testing machine (E43,
hydrogels were characterized by an oscillation frequency MTS instrument, USA). Specifically, gelatin nanoparticles
sweep (0.1 to 100 rad/s at a constant strain of 0.5%). were weighed and mixed with 2 M NaCl solution (pH =
7.0) in 5 mL medical syringes (BD Plastipak™, orifice
2.6. Mechanical test diameter of 400 μm) to obtain colloidal gels. After storing
Compression and tensile test were performed using at 4°C for 2 h, the syringe was fixed vertically under
dumbbell-shaped and cylindrical-shaped specimens the plate of the tensile bench, and a compressive force
designed according to ISO standards and evaluated by a was applied to the plunger of the syringe at a constant
universal testing machine (E43, MTS instrument, USA) velocity of 1 mm/min. The injection force was recorded
equipped with a 50 N sensor (25℃, 60% RH). The cyclic as a function of the plunger travel time. In addition, the
compression and tensile tests were carried out with a degree of material expansion after injection was evaluated
loading rate of 1 mm/min and samples were loaded to by immediately recording photographs of the printed
the initial deformation after being compressed to the filaments from the nozzle of different diameters.
strain of 0.75. The fracture energy (kJ/m ) was calculated 2.8. Fabrication of DN hydrogel-based ionic skin
3
by the area below the tensile stress-strain curve used to
characterize the work required to break a sample per unit devices
volume. The elastic modulus was defined as the slope of To generate an ionic skin device with high resolution, we
the initial linear region of the stress-strain curve. designed an electronic circuit with microarray printed

2.7. Printability and fidelity of DN hydrogels by gelatin/PAAm DN hydrogel. Specifically, we used
homemade 3D printing equipment, and 400 μm needle
To print the DN hydrogel, the hydrogel inks composed of to further print DN hydrogel, and each sensor unit area
gelatin nanoparticles and PAAm precursor solution were is 7 mm × 7 mm, printing a total of 1 (1 × 1), 4 (2 ×
used to fabricate microstructures by a homemade extrusion- 2), and 49 (7 × 7) units of the ionic skin. To fabricate

International Journal of Bioprinting (2021)–Volume 7, Issue 3 99