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Chen, et al.
A B

C
D

Figure 4. (A) Injectability of gelation colloidal gels (5, 7.5, 10, and 15 w/v % solid content) by measuring the compression force as a
function of time during injection of the hydrogels using medical syringes. (B) Photographs showing the injectability of 5, 7.5, 10, and 15
w/v% gelatin colloidal gels during the injection test. (C) The light microscopic images showing the strands formed by gelatin colloidal
gels (7.5, 10 w/v%) injected by syringes with different nozzle diameter (200 and 400μm). (D) Quantitative analysis of the expansion rate
calculated by the ratio of the hydrogel strands after extrusion as relative to the original nozzle size.

A C hydrogels. The relationship between deformation and
capacitance in a parallel-plate capacitor can be explained
by the equation C = εS/4πkd, where C is the capacitance,
ε is the dielectric constant, k is the electrostatic constant,
S is the effective area of the conducting layer, and d is the
thickness of the dielectric layer. Based on this theory, the
expansion of the cross-section area of a hydrogel ionic
B D skin can lead to increasing capacitance. Therefore, to
prepare a capacitor as ionic skin, we printed a layer of
electronic circuit array using DN hydrogels on the surface
of a dielectric PLA film followed by printing another
layer of electronic circuit array arranged orthogonally to
the first printed layer (Figure 6A). Another two layers
of PLA films were used as insulator layers to cover the
conductive hydrogels and prevent the moisture from
evaporation. To demonstrate the sensitivity of the ionic
Figure 5. (A and B) Photographs showing the patterned electronic
circuit printed by gelatin/polyacrylamide double-network (DN) skins, we pressed the microarray using finger-touch
hydrogel. (C) The ear-shaped construct was printed by DN or monitored complicated muscle movements. We
hydrogel. (D) Image showing the high flexibility and stability of used acrylic elastomer tape to adhesion the ionic skin
the printed patterned electronic circuit on a PLA film. to the finger. The two-layer ionic hydrogel’s contact

International Journal of Bioprinting (2021)–Volume 7, Issue 3 103