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

C D

E F

Figure 6. (A) Schematic design of the ionic skin developed in the current study. (B) The printed hydrogel sensors attached to a puppet hand
and the real-time capacitance signals when different gestures were posed. Inserted images showing the corresponding gestures of the puppet
hand. Real-time capacitance signals when the tester who wore the device (C) laughed, (D) said “thank you,” and (E) took deep breaths. (F)
Real-time capacitance signals of pulse when the sensor attaches to the tester’s wrist.

area increased when the finger bending, leading to an panel (7×7 sensors, 5×5 mm square per sensor unit)
2
increase in capacitance (Figure 6B). When the ionic was fabricated to map the location of applied pressure
skin was attached to the throat, it exhibited repeatable with high resolution (Figure 7A). When one finger or
and characteristic signal peaks as the tester laughed two fingers touch the screen, the corresponding pressure
or spoke (Figure 6C and D). The ionic skins also can peak mapping can be generated by measuring all the
distinguish between breathing and deep breathing by capacitance of the subunits (Figure 7B). Moreover, by
the degree of deformation on being worn on the chest placing cross- or box-shaped plastic devices on the touch
of the tester (Figure 6E). Moreover, we also proved the screen panel’s surface, we can locate the position and
application of this ionic skin as a sensor for wrist pulse. shapes of the devices placed.
The capacitance changes showed periodic fluctuations For the development of a human-computer
according to the rhythm of pulse beating (Figure 6F). interaction monitor, a set of miniaturized functional
All these applications confirmed that these DN hydrogel- chips were utilized, including printed ionic skin, data
based ionic skins were promising candidates as highly collector, control system, data transition, main control
sensitive pressure and strain sensors for wearable devices. board, and output device (Figure 8A). The capacitance
To further evaluate the applications of the printed ionic value of ionic skin will increase under external pressure,
skin in the flexible smart devices, a flexible touch screen thus we can transfer the capacity signal to other signals,

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