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Wang, et al.
in 2PP is initiated by exciting the molecules through two- and high-throughput features of inkjet printing still
photon absorption. The process can occur in an extremely outweigh its drawbacks.
short period (mm/seconds of scanning speed) and fabricate
highly complex structure at any spatial position. However, 2. Bioinks
the application of 2PP in bioprinting is also limited by a Compared with the traditional methods, bioprinting can
few drawbacks, including the material degradation caused be used to design complex 3D structures. It prints bioinks
by high laser power and bubble damage . layer by layer to control the spatial distribution of cells and
[16]
1.2. Extrusion-based bioprinting is used to fabricate the specific structure of tissues. The
success of using bioprinting depends on printability and
The extrusion-based bioprinting is the most widely bioactivity of the bioinks . In ophthalmology, bioinks of
[26]
used technique because it has fast printing speed and either cell-free hydrogel or cell-laden biomaterials were
can work with a broad range of printable bioinks . In developed for different purposes.
[17]
general, the material can be extruded from the cartridges
either through pneumatic pressure or mechanical forces 2.1. Biomaterials
(piston-driven or screw-driven) (Figure 1B). The Biocompatibility, printability, and mechanical properties
extrusion-based bioprinting can print bioinks with high are the three main requirements for bioinks . To
[27]
cell densities (10 cells per ml) and print multiple types of formulate a highly biocompatible environment for the
7
cells at same time to fabricate a heterogenous structure. cells, decellularized extracellular matrix (dECM), and
However, it also has limitations on the cell viability due nature-derived or semi-synthesized hydrogels are widely
to the damage with shear stress and nozzle clogging. To
improve the performance of extrusion-based bioprinting, used in ophthalmic applications.
it is necessary to optimize the bioink design, select nozzle (1) dECM
of suitable size, and choose suitable materials. The
adjustment of printing parameters (e.g., pressure, speed, The ECM is important for cell nutrition, protection,
[28]
layer thickness) is required before printing to achieve a and tissue function . The ECM network is rich in
better performance. molecules of collagen, elastin, microfibrillar proteins,
adhesive glycoproteins and proteoglycans, providing
1.3. Jetting-based bioprinting support and crucial cues for cell adhesion, engraftment,
[29]
As shown in Figure 1C, the jetting-based bioprinting and functions . The dECM was developed for bioink
represents a big group of techniques, including inkjet bioprinting to mimic an optimized microenvironment for
[30]
bioprinting, microvalve bioprinting, laser-assisted specific tissue engineering ; for instance, cornea- and
bioprinting, and acoustic bioprinting . An advantage retina-specific dECMs were developed for ocular tissue
[18]
[31-33]
of the jetting-based printing technique is the drop-on- regeneration . In corneal engineering, dECMs present
demand (DOD) patterning of different types of cells and great advantage in maintaining the keratocyte morphology
biomaterials in a noncontact profile. Using this method, and transparency by preventing the transdifferentiation of
[34]
the droplets can be generated either by the heater- corneal fibroblasts . To prepare the cornea-specific dECM
vapored bubbles in a thermal style , the deformation bioink for bioprinting, Kim et al. decellularized the ECM
[19]
under the electrode pressure in an electrostatic style [20,21] , from the bovine corneal stroma and lyophilized the cornea-
[31]
the vibration of a piezoelectric actuator in piezoelectric derived dECM (Co-dECM) samples . When printing, the
style [22,23] , or the high voltage filed energy from the nozzle Co-dECM powder was solubilized in acidic solutions and
electrodes in electrohydrodynamic style [24,25] . Different adjusted to form printable gel. The removal of the cells helps
from the traditional continuous method, the DOD inkjet to reduce immune rejection response in tissue grafting.
bioprinting ejects droplets only when the signal meets the Moreover, the Co-dECM bioink has comparable levels of
demanded levels, which makes the droplet to formulate collagen and glycosaminoglycans as the natural cornea.
specific patterns as designed. Besides, as the diameter of The thin collagen fibrils in the bioink have a larger amount
inkjet nozzle is comparable to the size of a cell (50 μm), of proteoglycans, which make it possible to pass through
the inkjet printing could also be used for single cell the greater amount of light and maintain the transparency
printing. The high-resolution printing helps fabricate property as the native cornea. In addition, the Co-dECM
smaller tissues/organs, and the unique printing patterns bioink presented no toxicity in animal experiments and
[35]
could enhance the cell-cell and cell-matrix interactions. showed a good therapeutic potential in corneal diseases .
The inkjet bioprinting would contribute to cell damages (2) Hydrogels
by shear stress or pressure during printing or collision
with the substrate after ejection. However, the damage Hydrogels are structured networks of crosslinked polymers
only occurs to a low proportion of the cells; the efficiency that are capable of absorbing and retaining large quantities

International Journal of Bioprinting (2022)–Volume 8, Issue 2 149