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International Journal of Bioprinting Bioprinting cell-laden protein-based hydrogel

the extrusion. Notably, a bioink’s viscosity relies on factors a short period of time in a bioprinted structure, but those
like temperature, concentration, molecular interactions, with high yield strengths can hold shapes much longer. The
and molecular weight [174] . yield stress is generated as a result of the viscosity peak, and

Owing to the interactions that occur between when viscosity is plotted against shear stress, yield stress
hydrophobic domains within the sequence of proteins, can be determined as the threshold value beyond which
[186]
shear forces moderately transform the liquid into the material begins to flow . Shape-retaining structures
solid structures (random coils to β-sheets) during the are typically produced by inks with high yield stresses;
extrusion process. Unfortunately, degradation occurs in accordingly, adding bulking agents and thickeners, such as
[187]
the production of proteins, affecting their rheological gellan, improves the PBHs’ yield stress . Moreover, high
properties and printing. Besides, bioprinting lacks the yield stresses can negatively influence cells when bioinks
[188]
solution’s flow-induced extensional stretching, which initiate flow . Another subject is that hydrogels with cell-
diminishes the hydrogel viscosity and makes maintaining laden networks should be able to self-recover following
a stable structure difficult [175-177] . printing because their physical crosslinking network is
broken by the shear stress [189] . For the rapid recovery of
Before printing, the distribution of velocity and the hydrogel’s viscosity after applying a shear rate, a sharp
shear stress across the cross-section of a nozzle is zero. decrease in the viscosity when the shear rate is applied
Additionally, a dispensing process cannot be carried out is indeed ideal [190] . In addition to being mechanically
without shear stress [173] . In fact, hydrogels are subjected strong, the extruded hydrogel filament must be capable of
to shear stresses, at the nozzle walls in particular, at the maintaining its shape subsequent to printing, as mentioned
time of printing. A number of parameters determine previously; thus, thixotropic properties are essential factors
the amount of shear stress exerted on the bioink and when evaluating a hydrogel’s suitability for bioprinting [164] .
embedded cells, such as printing pressure, nozzle diameter, From another viewpoint, for achieving optimum
and bioink viscosity [178] . In general terms, cells exposed to features of cell-laden printed structures, such as obtaining
low shear forces tend to survive longer; conversely, high a filament diameter that matches the diameter of the
shear stresses can decrease cellular viability [179] .
nozzle, strand uniformity, and accurate strand placement,
The shear stress generated in bioprinting does not pose printing parameters like pressure and dispensing speed are
an obstacle for small, globular proteins, but in the case of often varied; nevertheless, these factors can influence the
their larger, more fragile counterparts, structural integrity viability of extruded cells [191] . According to the scholars’
can be threatened [180] . As Nishioka et al. [181] argued higher investigation, increasing the applied pressure reduced the
compression rates that were employed to create droplets survival of encapsulated cells, regardless of whether the
resulted in more protein denaturation and biological bioinks were liquid or gel [192] . The nozzle’s diameter and
inactivity, and without the use of stabilizing additives, the shape should also be considered; within this context, a
enzyme activity was reduced under all printing conditions. study by Billiet et al. [193] reported that cells in gel-phase
It could have been possible to mitigate the adverse effects bioinks were more viable when loading bioinks in conical
of bioprinting if sugars like trehalose and glucose were nozzles compared with cylindrical ones. Furthermore,
added to help preserve the enzymatic activity. This the shear stress increases with the increment of nozzle
outcome raises some interesting queries regarding how length, so analyzing varying nozzle lengths is of crucial
proteins are denatured on the nanoscale as a corollary significance for drawing firm conclusions between using
to maintain their bioactivity while growing crystals. It tapered conical nozzles and cylindrical ones. Cell viability
is worthy to mention that a protein’s native secondary is also decreased when the nozzle diameter is reduced in
structure may be primarily determined by its amino acid cylindrical nozzles; to be more specific, the majority of cells
sequence if the protein is small and globular; however, are under highly stressful conditions, meaning that high
if the protein has deeply folded pockets functioning as pressure and a small diameter can cause necrosis rather
catalytic sites, such as most of the active enzymes, then than apoptosis, and the nucleus experiences morphological
this rule may not always hold true [182] . and irreversible damage [194] .
By the same token, when printing biomaterials, yield In addition, strand stretching and thinning can result
stress specifies the force required in order to permit from bioprinting with a high speed at a given pressure [195] .
smooth, continuous extrusion [87,183] and ensures the The polymer matrix probably introduces potentially
homogeneity of encapsulated cells within bioinks. In the undesirable tensile and compressive forces to the cells,
absence of forces, hydrogels with low yield stress leak out assisting with cell alignment along the bioprinted strand.
of nozzles or experience phase separation [184,185] . Of note, Depending on the cells’ location within a filament, cell
bioinks with high viscosities may maintain their shape for survival and morphology may differ [196] . For instance, in the

Volume 9 Issue 6 (2023) 483 https://doi.org/10.36922/ijb.1089