Page 231 - IJB-9-5

International Journal of Bioprinting Hydrogels for 3D bioprinting

cellulose nanocrystals (CNCs), which are principally used 4.4. Gold nanoparticle composite hydrogels
as reinforcement materials for other hydrogels. The second In the construction of bone and heart tissues, the poor
category is cellulose nanofibrils (CNFs), which serve as electrical conductivity of polymer-based hydrogel
potential carriers for functional ingredients like proteins. materials is one of the major challenges for their wide
The third category is bacterial nanocellulose (BNC), application, and this is where the addition of materials with
which shows exceptional potential, but one of the biggest electrical conductivity is needed to induce the formation
limitations is the technical problem of BNC production of new tissues and promote intercellular signaling. There
at present [131,132] . are several types of conductive nanomaterials such as
graphene, GO, and CNTs mentioned in carbon-based
The addition of cellulose nanofibers to the SA hydrogel nanomaterials. However, although CNT is a popular
can improve the rheological properties of its printing [31,49] . conductive material, its cytotoxicity is controversial.
Jessop et al. used biomass-derived cellulose nanocrystals Currently, among various conductive nanomaterials, gold
[31]
(CNCs, 3%), biomass-derived cellulose nanofibers (CNFs, nanomaterials are emerging as the best candidates due to
6%), and a unique mixture of the two (NCB, 3%) as the fact that they often exhibit several attractive properties,
extrusion. For the bioinks, they optimized the formula including good cytocompatibility, no cytotoxicity, easy
of the bioinks. By printing a single-layer square grid with preparation and sizing, high reproducibility, and easy
a height of 1.7 mm, they tested the resolution of three surface modification, as well as the ability to propagate
different formulations of nanocellulose and SA, which electrical signals efficiently [131,132] . It binds to various thiol-
showed high resolution. The results showed that the bioink containing biomolecules through gold–thiol bonds to
has good shear thinning characteristics and great shape promote cell proliferation and increase cell–cell signaling.
fidelity after printing. Among them, the transmission Overall, gold nanostructures are extremely promising
scanning electron microscope of NCB-AG (Figure 6C, materials for biomedical research, and researchers often
(C1)) shows the entanglement state between the nanofibers use conductive hydrogels containing gold nanorods
before the calcium chloride crosslinking, forming a sparse (GNRs) for cardiac tissues because of their excellent
and scattered structure. Moreover, the pores between electrical conductivity.
nanofibers are very large, resulting in an unstable structure.
Figure 6C (C2) is the state after crosslinking. It is obvious However, except for repair sites that are in need of
that the crosslinking effect of SA entangles nanofibers biomaterials with electrical conductivity, gold nanoparticle
and CNCs together, forming a dense and firm structure, composite hydrogels may not be as effective as other
and the arrangement is orderly. Therefore, they used the polymer-based hydrogels when applied to tissues such as
NCB–AG combination of bioinks to print several complex the urethra and skin, due to their lack of bioactivity and
shapes, such as hollow and solid cylinders, pyramids, and relatively uncertain biostability. Therefore, researchers
cubes, as well as human right ear models. They continued have often incorporated them into many cell-containing
to test the compatibility of human wing chondrocytes. The bioinks and biomaterials to enhance and expand their
results showed that bioinks provide a suitable environment functionality and printability for tissue engineering and
[134]
for cell survival and differentiation while maintaining the regenerative medicine applications .
shape and structure of the scaffold. Sultan et al. [133] used In the construction of bone tissues and heart tissues,
SA/gelatin hydrogel bioinks reinforced with CNCs to form materials with conductive functions need to be added
an interpenetrating polymer network structure through to induce the formation of new tissues and promote
a double crosslinking reaction of covalent and ionic signal conduction between cells. There are several
crosslinking. Because the crystals are oriented, they found types of conductivity nanomaterials such as graphene,
that when the orientation of CNCs is consistent with the GO, and CNTs, which are mentioned in carbon-based
printing direction, a scaffold with uniform pore size can be nanomaterials. Besides, gold nanoparticles are also
obtained. In short, CNCs not only improve the rheology of conductive and can transmit electrical signals [135,136] .
hydrogels but also make it easy to print controllable pore Although CNT is a popular conductive material, its
sizes and gradient pore structures. Besides, the scaffold is cytotoxicity is disputed. Gold nanomaterials exhibit
suitable for cell interaction, which once again proves that some attractive properties, including good cell
CNCs have great potential to be used in 3D bioprinting compatibility, non-cytotoxicity, easy preparation and
bioinks. Some studies on nanocellulose have found that size, high reproducibility, and easy surface modification.
nanocellulose has great potential as a bioink that can be It combines with various thiol-containing biological
used for bioprinting. In the future, more tests on printing molecules through gold–thiol bonds to promote cell
performance, mechanical properties, and cell compatibility proliferation and increase cell–cell signal transmission .
[36]
will be needed in this regard. Gold nanostructures are exceedingly promising materials

Volume 9 Issue 5 (2023) 223 https://doi.org/10.18063/ijb.759