Page 349 - IJB-9-5

International Journal of Bioprinting 3D printing of tough and self-healing hydrogels

systems, there remain some inherent dissimilarities and printable hydrogels, fabricated by triggering the in situ
between rigid electronic devices and soft biological polymerization of a polyaniline/poly(4-styrenesulfonate)
tissues, presenting a challenge to the seamless operation of (PANI/PSS) network which exhibits the dynamic and
human–machine interfaces [6,7] . Therefore, there has been reversible nature of the non-covalent crosslink. Similarly,
increasing interest in the development of soft, flexible, Jin et al. reported on conductive and adhesive cellulose
[20]
and stretchable electronics and robotics made from soft (CAC) hydrogel ink with self-healable and printable
materials to address these issues in bioelectronics. features mixed with tannic acid (TA) and various metal
ions. Although these multi-functional hydrogels provide
Hydrogels, which are three-dimensional (3D) self-healing, conductive, and printable properties, they
crosslinked polymer networks containing large amounts still possess poor printing fidelity with low resolution (over
of water, have gained considerable attention for their 0.6 mm) and have not been extended to printing in 3D
potential in bioelectronics [8,9] . The soft and flexible nature structures. Meanwhile, Wei et al. reported a super tough
[21]
of hydrogels reduces the mechanical mismatch with and printable agar/polyacrylamide (PAAm)-based double
human tissues, making them an attractive option for network hydrogel. The agar/PAAm hydrogel, owing to the
bridging the gap between electronics and human tissues alginate acting as a crosslinker, exhibited suitable viscosity
due to their biocompatibility and flexibility in controlling as a printable ink with good mechanical properties. Despite
their electrical, mechanical, and biological properties . the improved mechanical properties , low 3D-printing
[10]
[21]
However, there are still some challenges to overcome for resolution and the lack of self-healing capability limit their
their widespread use in bioelectronics. The mechanical practical use as 3D printable hydrogel inks.
weakness and brittleness of these water-soluble polymers
under large deformation and physical stress make them To achieve seamless human–machine interfaces in
unsuitable for most load-bearing physiological situations hydrogel-based bioelectronics, the development of multi-
and bioelectronics applications . functional hydrogels with all of the desired features,
[11]
including 3D printability with high resolution, toughness,
Furthermore, the lack of self-healing properties, which and self-healing ability, is urgently needed.
is a characteristic of human tissues, can lead to irreversible
collapse during bioelectronic operations . To address Herein, we present a novel multi-functional
[12]
these problems, self-healing hydrogels with reversible hydrogel ink that is 3D printable, tough, self-healing,
networks via breakage and reformation of bonds have been and conductive. The ink is composed of poly(vinyl
widely studied in recent years. Self-healing hydrogels that alcohol) (PVA), tannic acid (TA), and poly(acrylic acid)
can automatically repair themselves from external damages solution (PAA). PVA is used as the base material due to
have the potential to restore their original features , thus its biocompatibility and tissue-like softness, which is
[13]
integrating the self-healing ability into electrical devices achieved through hydrogen bonding (H-bond). However,
can extend the lifetime of the device [14,15] . pure PVA is mechanically weak and not printable. Thus,
TA is added as a crosslinker to form weak and reversible
In addition, for the successful application of hydrogel- H-bonds in the PVA hydrogel, and PAA is introduced
based bioelectronics, a fabrication method that can to form a double network by forming strong H-bonds.
produce individualized shapes and complex structures is This strong and weak crosslinking H-bond-based double
crucial . 3D printing presents a promising solution to this network-enabled PVA/TA/PAA hydrogel ink exhibits
[16]
requirement as it offers the ability to produce hydrogels 3D printability, mechanical toughness, and self-healing
with high precision and geometric freedom . However, properties. Since TA provided sufficient crosslinking sites
[17]
transforming bulk hydrogels into intricate designs and for the formation of reversible H-bonds, breakage and
patterns with high resolution (~100 μm) is still a challenge reformation of H-bonds could occur spontaneously even at
and remains in its early stages of research, hindering the high temperatures (over 80°C). This enabled the proposed
practical use of hydrogels [7,12] . To date, several attempts hydrogel inks to be 3D printable, as they exhibited
have been made to fabricate multi-functional hydrogels suitable viscosity for printing under heat and maintained
that possess 3D-printing capabilities, toughness, and self- a robust printed structure. The rheological behavior and
healing to improve their durability and practicability for tensile tests were evaluated to optimize the printable and
their practical use. However, it still continues to pose a mechanical properties of the PVA/TA/PAA hydrogel ink,
difficulty to fulfill all of those features, and in particular, and it was found that the optimized ink had shear-thinning
many researchers reported self-healing and tough hydrogel behavior, leading to excellent 3D-printing fidelity with
electronics but have not been incorporated with 3D high resolution (~100 μm). The hydrogel ink displayed
printability and high resolution of printing fidelity . For good toughness, with a tensile strength of ~45.6 kPa, an
[18]
example, Chen et al. represented stretchable, self-healing, elongation at break of ~650%, and Young’s modulus of
[19]
Volume 9 Issue 5 (2023) 341 https://doi.org/10.18063/ijb.765