Page 220 - IJB-9-5

International Journal of Bioprinting Hydrogels for 3D bioprinting

attapulgite (nano-ATP) on the printability and mechanical then crosslinked under ultraviolet (UV) light with the help
properties of SA-Gel bioinks. They fabricated sodium of a photoinitiator. The third strategy is the crosslinking of
alginate (SA)/gelatin (Gel) hydrogel scaffolds doped with gelatin, which is divided into two situations: (i) the prepared
different contents of nano-ATP via 3D printing. It was gelatin hydrogel can usually be mixed with a crosslinking
found that the compressive strengths and compressive agent for reaction, and (ii) the gelatin used for printing is
modulus of the composite hydrogel increased significantly used to immerse the printed stent in a crosslinking agent
with the increase of nano-ATP concentration. In addition, solution for curing.
as the nano-ATP amount increased, the swollen scaffolds
[57]
is able to better retain its shape and mechanical support. Yang et al. described several crosslinking agents
Thus, nano-ATP makes bioink more effective in inducing for gelatin. Commonly used crosslinking agents include
bone regeneration with the potential to repair bone chemical agents and enzymes, such as genipin (GP),
defects. Chen et al. adjusted the physicochemical and glutaraldehyde (GTA), 1-ethyl-3-(3-dimethylaminopropyl)
[51]
biochemical properties of the hydrogel by changing the carbodiimide (EDC)/n-hydroxysuccinimide (NHS),
concentration and crosslinking sequence of SA-Gel. They microbial transglutaminase (mTG), etc. They mixed a 4%
used Ca -crosslinked SA molecules and transglutaminase gelatin solution with four crosslinking agents respectively,
2+
(TG)-crosslinked gelatin molecules to construct the SA- and after gelation at 37°C, they were frozen at -20°C
Gel interpenetrating polymer network (IPN), which for 8 h and then freeze-dried for 48 h to obtain gelatin
provides the best interior for cell survival microstructure sponge scaffolds. Based on the different properties of
and environment. Ionic and covalently crosslinked SA- crosslinking agents, they found that (i) GTA was one of
Gel hydrogel is a material with great potential, not only the most commonly used crosslinking agents with certain
in tissue/organ printing but also in the field of drug cytotoxicity; (ii) EDC/NHS had low toxicity and good
screening models and pathological mechanism analysis. biocompatibility, but the degradation rate is excessively
Moreover, SA is usually mixed with other different quick; (iii) the mechanical strength of gelatin scaffolds is
biological materials, such as collagen, agarose, PEG, and weak after crosslinking. Hence, the scaffolds are insufficient
carboxymethylcellulose (CMC) [31,49,52,53] . Geevarghese to provide a suitable living environment for the cells,
et al. used a mixture of gelatin, CMC, and SA as a printing which in turn leads to cell death. Genipin has a slighter
[54]
bioink to prepare scaffolds for cartilage tissue engineering. toxicity compared to glutaraldehyde. But its crosslinking
As the concentration of CMC in the mixture increased, the effect is over powerful, which results in lower swelling
viscosity of the bioink also increased. Among them, 2% and high hardness of the gelatin sponges. Gelatin scaffolds
CMC has excellent printability and mechanical stability, crosslinked by genipin are not suitable for soft tissue
while the bioink containing 4% CMC is highly viscous and repair, while it is suitable for hard tissue repair. The authors
not extrudable. reported gelatin sponges crosslinked with transglutaminase
for the first time, and found that mTG sponge had the best
3.1.2. Gelatin performance compared to the other three crosslinkers:
Gelatin (Gel) has antigenicity and low immunogenicity, good internal and external biocompatibility, uniform
and cell adhesion motifs (RGD peptides). Therefore, pores, and resistance to degradation. Compared with the
gelatin can be used as cell adhesion and metalloprotease- other three crosslinking agents, it is the most suitable for
driven degradation sites, which can be effectively absorbed soft tissue repair. The required properties of soft tissues are
in the body without toxic degradation. Additionally, the different, so the corresponding concentration of gelatin
raw materials of gelatin products are easily available and and the crosslinking agent are also different. Besides, as a
inexpensive . In adipose tissue regeneration, gelatin is bioink for 3D bioprinting, it is necessary to consider factors
[55]
one of the most commonly used natural polymer materials such as the temperature and viscosity of gelatin. Most
for soft tissue repair . Yet, the fundamental problem that importantly, the crosslinking agents are not limited to the
[56]
needs to be solved urgently is maintaining the stability of list above, and there are some other chemical crosslinking
the gelatin scaffold structure after printing. Currently, there agents. Negrini et al. mixed gelatin with concentrations
[58]
are usually three strategies to resolve this problem. The first of 15% w/v and 25% w/v with different ratios of N-N’-
strategy is to mix the gelatin solution with other polymers. methylenebis(acrylamide) (MBA)/gelatin amino
To form a stable 3D structure after printing, researchers crosslinkers. Gelatin does not need to be mixed with other
use the properties of other materials to crosslink. In this polymers or chemically modified, and the crosslinking
case, since gelatin does not participate in the crosslinking reaction can be initiated by adding MBA crosslinking agent.
reaction, it is unstable and easily degradable. The second They consider that the suitable concentration of gelatin is
strategy is what we will introduce in detail in section 3.3. 15% w/v in terms of mimicking the mechanical properties
The gelatin is modified, grafted with methacrylate, and of adipose tissue. The concentration of MBA is 0.4% w/v,

Volume 9 Issue 5 (2023) 212 https://doi.org/10.18063/ijb.759