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RESEARCH ARTICLE

Elastic and Stretchable Double Network Hydrogel as

Printable Ink for High-Resolution Fabrication of Ionic

Skin

Kaiwen Chen , Qiwei Ying , Xingxing Hao , Kai Sun, Huanan Wang*
†
†
†
Key State Laboratory of Fine Chemicals, School of Bioengineering, Dalian University of Technology, No. 2 Linggong
Road, High-tech District, Dalian, 116024, P.R. China
† These authors contributed equally to this work

Abstract: A hydrogel that combines both printability and adaptability, high elasticity, and stretchability can provide ideal
mechanical properties, and also render complex and accurate construction for ionic skin. However, it is extremely challenging.
Here, we propose a colloidal-based double-network (DN) hydrogel as printable inks for high-precision fabrication of ionic
skins. Particularly, polyacrylamide (PAAm), as the covalent network that can maintain the long-term material integrity, was
combined with gelatin colloidal network to improve the injectability and printability of the resulting DN hydrogels. The
DN design cooperatively provides the hydrogels with higher toughness values and deformability than what single colloidal
or PAAm network can achieve. Further design of ionic skin based on capacitor microarray was demonstrated to serve as a
sensitive and stable capacitor that can respond to external stimuli, thereby allowing to sense the body movements such as
finger bending, laugh, and wrist pulse by translating mechanical changes into electric signals. Therefore, this study provides a
novel strategy for the design and preparation of high-resolution ionic skins as the wearable sensor.
Keywords: Double-network hydrogel; Colloidal gel; 3D printing; Ionic skin; Capacitance microarray

*Correspondence to: Huanan Wang, Key State Laboratory of Fine Chemicals, School of Bioengineering, Dalian University of Technology, No.2
Linggong Road, High-tech District, Dalian, 116024, P.R.China; huananwang@dlut.edu.cn
Received: April 20, 2021 Accepted: May 25, 2021 Published online: June 25, 2021
(This article belongs to the Special Section: Bioprinting of 3D Functional Tissue Constructs)

Citation: Chen K, Ying Q, Hao X, et al., 2021, Elastic and Stretchable Double Network Hydrogel as Printable Ink for High-
resolution Fabrication of Ionic Skin. Int J Bioprint, 7(3):377. http://doi.org/10.18063/ijb.v7i3.377

1. Introduction biomimetic biosensor materials, in which mechanical signals
can be converted into electric signals by using the soft,
Recently, various soft electronics have been developed to biocompatible, and ionically conductive hydrogels as the
imitate skin’s unique characteristics, such as mechanical electronic conductors . Unlike electronic skins composed of
[5]
adaptability and sensory capabilities . These so-called organic polymers and inorganic conductive materials, ionic
[1]
“electronic skin” devices can convert external stimuli such skins typically consist of hydrophilic polymers dispersed in
as pressure , strain , and vibration into reliable electronic an aqueous salt solution that can transmit electrical signals
[2]
[3]
[4]
signals, thus serving as ideal materials for the development through ions instead of electrons, resulting in physiological
of wearable devices and soft robotics. Although these and mechanical properties comparable to natural skin
electronic skins can sense various signals, they still show tissues [6,7] . In general, the structure of the ionic skin is
insufficient biocompatibility and mechanical adaptability composed of a parallel plate capacitor and ionic hydrogel
which are caused by the potentially cytotoxic and rigid as the conductive layer. When the capacitor senses external
inorganic conductive materials. To this end, Whitesides stimuli (such as strain and pressure), the capacitance value
et al. recently proposed the concept of “ionic skin” as will change. However, currently available ionic skins were

© 2021 Chen, et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/
licenses/by/4.0/), permitting distribution and reproduction in any medium, provided the original work is cited.
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