Page 140 - IJB-10-6

International Journal of Bioprinting Fluid mechanics of extrusion bioprinting

and cell-containing solutions, followed by their the nozzles and accurate control of the bioink flow are
simultaneous deposition. For example, coaxial printing necessary when changing printing heads to maintain
involves minimal contact between precursors as they exit structural integrity. To address the slow printing
130
the nozzle and to deposit on the printing stage, where issue, some bioprinters incorporate multiple arms with
crosslinking solidifies them. Other methods, like the head independent motion paths and heads. However, this
131
sweeping technique, deposit different bioinks separately. approach introduces challenges in aligning the arms
In contrast, multi-material bioprinting with mixing, such accurately, which can potentially impact the overall
as using micromixer-equipped heads, involves contact integrity of the printed scaffolds. 131
between precursors before deposition. Depending on
the design and application, the extent of mixing can vary 4.1.2. Coaxial printing
from minimal (separated) to homogeneous. Most of Coaxial printing is an emerging technique in extrusion
these methods are designed to maximize the mixing of multi-material bioprinting 132–136 that uses coaxial needles
precursor biomaterials and cells, enabling the deposition for printing core–shell fibers. This method involves
of a homogeneous mixture with precise control over inserting small-diameter needles into larger-diameter
composition during the printing process.
needles, allowing for the fabrication of structures with
24
4.1. Extrusion multi-material bioprinting core–shell or hollow fibers. Depending on the specific
without mixing application, different biomaterials and cell types can be
extruded as the core or sheath layer of the filament. 11
4.1.1. Swapping the dispensing heads
The simplest approach for multi-material bioprinting Coaxial bioprinting can significantly improve the
is swapping the dispensing heads on the bioprinter. mechanical properties of biomaterials by combining
This method enables hierarchical printing of different them with stronger biomaterials. For example, core–shell
bioinks 62,125–127 by loading different biomaterials onto polyethylene glycol diacrylate (PEGDA)/alginate filaments,
separate printing heads, which are sequentially mounted with alginate as the core and PEGDA as the shell, exhibit
on the printing arm. This allows for specific parts of higher tensile moduli and strengths compared to alginate.
130
the scaffold to be printed in a predetermined manner. This method allows for printing a cell-laden hydrogel as
Hydrogels are typically pre-mixed with cells and facilitate the core, while the shell provides structural integrity and
the simultaneous printing of multiple bioinks. For protection for the cell-laden hydrogel. Furthermore,
27
137
example, a multi-head system can be utilized to print coaxial printing enables the creation of advanced scaffold
cell-laden scaffolds using decellularized extracellular 28,138
matrix (dECM)-based bioink and polycaprolactone structures, including vessel-like channels. However, a
(PCL). This method has also proven successful in major challenge in coaxial printing is the difficulty in using
128
creating hybrid scaffolds that combine alginate and PCL, multiple coaxially assembled needles to produce small-
exhibiting improved mechanical properties compared to diameter fibers, due to the requirement of larger-diameter
pure alginate scaffolds. 125,129 However, this method can outer needles to accommodate the internal small-diameter
only print one bioink at a time, resulting in a relatively needles. Despite this, coaxial heads can print fibers with a
36
slow printing process. Additionally, precise alignment of core–shell structure without mixing the materials.

Table 4. Rheological characteristics and models for common biomaterials

Biomaterials Newtonian/non-Newtonian Models Ref.
Alginate Non-Newtonian: (i) shear-thinning; (ii) thixotropy; (iii) Power-law; cross; viscoelastic 112–116
viscoelastic (PTT)
Alginate/gelatine Non-Newtonian: (i) shear-thinning; (ii) viscoelastic Herschel-Bulkley 117
Agarose Non-Newtonian: (i) shear-thinning; (ii) viscoelastic Power-law 118
Carboxymethyl cellulose (CMC) Non-Newtonian: (i) shear-thinning; (ii) thixotropy; (iii) Power-law; cross 119
viscoelastic
Chitosan Non-Newtonian: (i) shear-thinning; (ii) thixotropy; (iii) Carreau-Yasuda 120,121
viscoelastic
Collagen Non-Newtonian: (i) shear-thinning; (ii) viscoelastic Power-law; Carreau 122
Gelatin Non-Newtonian: (i) shear-thinning; (ii) viscoelastic Power-law 123,124

Volume 10 Issue 6 (2024) 132 doi: 10.36922/ijb.3973