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International Journal of Bioprinting New fibrillar collagen for 3D printing and bioprinting

Nowadays, tissue engineering (TE) can be defined as an important, since the presence of cells would determine
interdisciplinary field aiming to provide new strategies the 3D bioprinting conditions and so, the feasibility and
to repair, restore, maintain, or improve damaged tissues success of the engineered tissue. Considering that the final
and/or whole organs by applying engineering strategies scope of TE is to artificially reproduce a certain tissue or
to combine biological components (cells, growth factors), organ, the formulation of bioinks and biomaterial inks is a
drugs, and natural and/or synthetic materials. The resultant crucial step, since they will determine the properties of the
scaffolds or constructs obtained by the combination of construct (mechanical, rheological), the cell environment
these ingredients are intended to be implanted in the and, ultimately, its performance and integration with the
human body to regenerate and/or replace the damaged native tissue(s) after implantation. With these premises,
tissue, thus reducing (or even avoiding) the necessity of the usefulness of natural, biocompatible materials is easily
transplants. foreseeable. Just to mention a few of them, materials

From its inception, TE has made use of different such as gelatin, alginate, collagen, hyaluronic acid, or
techniques and approaches to shape the three-dimensional de-cellularized extracellular matrix (dECM) have
(3D) scaffolds, among which electrospinning and 3D been proven suitable as ingredients of TE 3D scaffolds.
bioprinting can be mentioned. Before the adaptation of Nonetheless, due to their origin and intrinsic properties,
3D printing to TE (3D bioprinting), the artificial tissues these materials usually pose issues mainly related to their
produced were limited to two-dimensional cell sheets, mechanical performance, extraction, and reproducibility.
which hindered their final performance . The potential 1.2. Collagen and 3D bioprinting
[1]
and usefulness of 3D bioprinting for this field of study is In 3D bioprinting, collagen is one of the most useful
undeniable, since it not only allows for the creation of cell- and promising ingredients since it is a natural, fully
laden, 3D structures (controlled deposition of materials, biocompatible material for TE, together with the fact that it
giving rise to precise shapes and scaffold dimensions), but is a ubiquitous protein in the ECM. It possesses high affinity
also offers remarkable versatility due to the great diversity of for adherent cells due to the presence of peptide sequences
3D printing techniques . In fact, the reviews of Ng et al. recognized by cell receptors. Moreover, the biodegradability
[3]
[2]
and Ashammakhi et al. , among others, gathered some of of type I collagen by metalloproteinases act as chemotactic
[4]
the most outstanding studies in this field. The 3D printing for cells such as fibroblasts, which further improves tissue
and bioprinting techniques can be classified according to regeneration . Nevertheless, it is an ingredient with
[16]
the ISO/ASTM 52900 for additive manufacturing. When important limitations, such as its extraction process and
[5]
it comes to TE, the 3D printing technique must be as non- the lack of batch-to-batch reproducibility, the influence
detrimental as possible both with the materials and with of the environmental conditions (e.g., temperature could
the cells present in most cases. Briefly, other variations modify collagen viscosity) during the 3D printing process,
of this technique include material extrusion (mechanical and the low mechanical properties in vitro of the printed
and pneumatic) [6,7] , material jetting (inkjet, microvalve, scaffolds [17,18] . At least 29 different types of collagens have
laser-assisted, acoustic) [8,9] and vat polymerization been reported, which are classified, according to their
(stereolithography, digital light processing, two-photon structure, into: striatum (fibrous), non-fibrous (network
polymerization) [10,11] . Extrusion, stereolithography, laser- forming), microfibrillar (filamentous) and those which
assisted, inkjet, and microvalve-based printers have been are associated with fibril . Type I collagen (fibrous) is the
[19]
the most used for 3D bioprinting in the last two decades . most common, primarily in connective tissue, in tissues
[3]
Extrusion are pressure-driving printing techniques, in such as skin, tendons, and bones. It consists of three
which the ink is propelled through a nozzle either by polypeptide chains, two of which are identical, which are
mechanical (axial piston or screw-driven) or pneumatic called chain α1 (I) and α2 (I) [20,21] .
forces (air flow). Extrusion 3D bioprinting is the most
prevalent approach due to its fast fabrication speed, ease of Most of the collagen inks available in the market are
use, and compatibility with a wide range of materials, such based on soluble collagen (limpid collagen solution, with
as collagen and cells. no fibers), which requires a fibrillogenesis process before,
after, or during the printing process, thus complicating
The difference between 3D printing and 3D bioprinting the procedure, and possibly hinders cell viability and
lies not only in the final scope of the 3D construct produced reproducibility . The collagen extraction methods are
[3]
but also in the composition of the so-called “ink” [4,12-15] . based on the solubility of this protein in neutral saline
Briefly, the concept “bioink” is used when cells are present solutions, acid solutions, and acid solutions with added
in the product to be printed. On the contrary, “biomaterial enzymes. The method of extraction selected together
ink,” “biomaterial,” or just “ink” can be used to make the with the processing parameters used through it highly
difference. To draw a line between these two concepts is influence the length of the polypeptide chains and final

Volume 9 Issue 3 (2023) 312 https://doi.org/10.18063/ijb.712