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Double-network Hydrogels for 3D Printing Ionic Skin
mostly based on chemically cross-linked hydrogels, such as avenue for the design and fabrication of degradable,
polyacrylamide (PAAm). These hydrogels were inherently implantable, and biocompatible smart electronics.
elastic, lack of stretchability and durability, and cannot adapt
to the local movements of the human body . Therefore, 2. Materials and methods
[5]
physically cross-linked hydrogels with self-healability and 2.1. Preparation of PAAm hydrogels
flexibility that can adapt to dynamic surfaces have been
recently developed. However, these systems showed poor Acrylamide (AAm, 99%, Aladdin), N,N-
structural integrity, insufficient anti-fatigue properties, methylene bis(acrylamide) (MBAA, 99%, Sigma-
and plastic deformation under external force, which can Aldrich), and 2-hydroxy-4’-(2-hydroxyethoxy)-2-
compromise the materials stability and functionality of the methylpropiophenone (2959, >99%, Aladdin) were
ionic skins [8-11] . Therefore, it is still a challenge to develop dissolved in 2 M NaCl solution at room temperature to
hydrogel-based electronic sensors as ionic skins that feature obtain clear solution with different concentration of AAm
the combined attributes of adaptability, high elasticity, and (5 w/v%, 10 w/v% and 15 w/v%). The solution was
stretchability. further cross-linked by 365 nm ultraviolet (UV) lights
Gong et al. proposed double-network (DN) (50 mW/cm , 60 s) to form PAAm hydrogels.
2
hydrogel, which typically combines a stable covalent
network with a reversible non-covalent network, and 2.2. Preparation of colloidal hydrogels
has shown mechanical strength and toughness values Gelatin nanoparticles were prepared based on a previously
orders of magnitude greater than what the can achieve reported two-step desolvation method . Different mass
[17]
separately [12-15] . Inspired by this concept, our group has fractions of gelatin nanoparticles (5%, 7.5%, 10%, and
developed DN hydrogels based on reversibly crosslinked 15%) were dissolved in 2 M NaCl solution to obtain
colloidal network combined with a continuous phase of the gelatin colloidal gels.
permanently crosslinked network by covalent bonds. These
hydrogels have shown significantly enhanced mechanical 2.3. Preparation of DN hydrogels
behavior, a high degree of stretchability and adaptability to For preparing the DN hydrogels, gelatin nanoparticles
local deformation or shear force . Specifically, colloidal with a mass fraction of 10% were fully mixed with the
[16]
gels are based on a bottom‐up assembly of micro‐ or solution which contained AAm as a monomer, MBAA
nano-sized particles to form a porous but interconnected as a crosslinking agent, and irgacure 2959 as a photo-
particulate network [17,18] . By introducing interparticle initiator to obtain the colloidal gel. The AAm monomer in
interactions such as electrostatic forces , magnetic colloidal gel further was crosslinked by 365 nm UV lights
[19]
forces , or hydrophobic interactions , and colloidal (50 mW/cm , 60 s). The detailed parameters of hydrogels
[20]
[21]
2
gels can develop shear-thinning and self-healing behavior, as listed in Table 1.
which are advantageous to applications in bioprinting,
injectable scaffolds, or drug carriers. 2.4. Scanning electron microscope (SEM)
Based on this design rationale, we hereby propose
a DN colloidal hydrogel for the fabrication of ionic skins. The microstructure of hydrogels was observed by
Particularly, a binary composite hydrogel system composed scanning electron microscopy (FEI, Quanta 450, USA).
of PAAm as the covalent network to maintain long-term Specifically, the DN hydrogels were frozen and fractured
material integrity in combination with a shear-thinning at −80°C, and then freeze-dried to remove water. Then,
and self-healing gelatin colloidal network to improve the the cross-sections of the hydrogel samples were coated
injectability and printability was developed, which allowed with gold to increase the conductivity.
high-resolution fabrication of ionic skin with microscopic 2.5. Rheology
patterned microstructure by three-dimensional (3D) printing.
The resulting DN ionic skin sensor can be rendered with high The hydrogels were tested for rheological behavior using
stretchability and elasticity, as well as enhanced sensitivity to a discovery hybrid rheometer (DHR, TA instrument). The
forces and deformations, outperforming conventional bulk measurement was performed using a parallel plate fixture,
hydrogel-based ionic skins due to the specific microscale and the gap distance was set to 1000 μm. The shear-
micro-architecture. The printed microarray in the current thinning behavior of the colloidal gels was evaluated by
ionic skin devices can enable the sensing of the location a stepped flow test (shear rate was 0.1 – 100 1/s). The
where the external stress was applied with a resolution self-healing behaviors were quantitatively characterized
of millimeter-scale resolution (Figure 1). In general, we by the change of storage modulus of the gels before/
provide a novel approach for high-resolution fabrication after destructive shearing (oscillatory strain sweep with
of a new generation of ionic skin sensors with substantially an increased strain from 1 to 1000% and a frequency
higher stretchability and elasticity, which opens up a new of 1 Hz for 200 s). The hydrogel samples were further

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