Page 90 - IJB-9-1

International Journal of Bioprinting Progress in bioprinting of bone

ceramic frames are usually 3D printed. The incorporation 2.8. Dynamic culture
of thermoplastics not only enhances mechanical properties Mechanical stress continuously remodels bone in vivo,
but also enables the creation of large-scale structures with and it has been postulated that such stresses are mainly
very high fidelity and a high fiber resolution [89,90] . Using transferred to bone cells through fluid shear stresses [103] .
FDM of PLA to mimic the Haversian system of bone and During the loading of a bone, interstitial fluid flows
SLA of GelMA to simulate the blood vessels, a scaffold through the pores in the bone, causing a shear stress to
was engineered that exhibited a similar mechanical be sensed by osteocytes, which are then communicated
strength to native bone, as indicated by the compressive to osteoblasts and osteoprogenitor cells through their
modulus of ~0.38 GPa, while the elastic modulus of the neighboring ECM. During in vivo loading, bone cells
vascular region was 10 – 30 kPa, offering an appropriate experience shear forces ranging from 8 to 30 dyn/
microenvironment for cell encapsulation . Using PCL or cm 2[104,105] . Cells can be cultivated in bioreactors with a
[53]
PCL/TCP structures as support for cell-laden hydrogels, dynamic environment that mimics the growth conditions
a compressive modulus of ~30 – 45 MPa was obtained of bone, enhancing nutrient transport, exposing cells to
using various PCL: TCP ratios . In contrast to printing fluid shear stresses, and ultimately promoting cell seeding
[91]
hydrogel and supporting frame separately, core/shell efficiency and differentiation [106,107] . Bioreactors have been
scaffolds were constructed, consisting of calcium-deficient shown to benefit bone differentiation and mineralization
hydroxyapatite (CDHA) core and cellular-laden alginate through mechanical stimulation induced by fluid shear
(shell) . The integration of the CDHA core resulted stresses [107-109] . In a study of bone bioprinting, hMSCs were
[92]
in a significantly higher compressive modulus (7 MPa) shown to express significantly more COL-I and VEGF
than alginate-only scaffolds and preserved the structural when exposed to culture media flowing at a rate of 5 mL/
integrity in vitro for 35 days. min than when exposed to static culture media . The
[53]
results of dynamic culture showed superior Ca deposition,
2.7. Hypoxic culture an indication that shear stress aided osteogenesis and
Hyperoxia is another factor that plays a significant role mineralization. The considerations for bone bioprinting
in bone development through the hypoxia-induced are depicted in Figure 3.
transcription factors (HIF), although there is debate about
the impact of hypoxia on bone regeneration . Studies 3. Up-to-date progress of bone bioprinting
[93]
have indicated that hypoxia enhances the osteogenesis 3.1. Extrusion-based bioprinting of bone
of BMSCs [94-96] , whereas others have suggested that
hypoxia inhibits the growth and bone-forming ability 3.1.1. Alginate-based composite bioinks
of osteoblasts, such as the differentiation of MSCs into As a popular biomaterial for bone regeneration, alginate-
osteoblasts [97,98] . Hypoxia and HIF may have multiple roles based composite bioinks have been extensively explored for
in osteogenic induction, as suggested by the paradoxical bone bioprinting. Fedorovich et al. conducted an early
[71]
conclusions. In addition, it is well known that hypoxia study in 2008 to examine the bioprinting of Lutrol F127,
®
increases the expression of angiogenic factors in MSCs, agarose, alginate, and methylcellulose hydrogels with an
such as VEGF . An initial period of hypoxia is present EBB bioprinter and observed that the applied extrusion
[94]
during the process of bone regeneration in the body, conditions did not reduce the survival and differentiation
which stimulated the deposition of several factors, such as capacity of BMSCs. This process was capable of developing
VEGF and IL-6, and promoted vascularization later on . bone constructs containing multiple cell types indicated by
[99]
It has been observed that short-term (7 days) hypoxic bioprinting of two fluorescently labeled cell populations
conditioning did not retard osteogenic differentiation within a single scaffold. The authors subsequently created
of stromal vascular fraction-derived cells (SVFC) in porous constructs which encapsulated two types of cells,
bioprinted constructs, but it enhanced the vascularization namely, EPCs and MSCs . A rectangular 10-layer scaffold,
[58]
of SVFC as indicated by increased expression of VEGFA which consisted of two parts (EPC-loaded Matrigel and
and HIF1A . Given the fact that the hypoxic environment MSC-loaded Matrigel with the addition of biphasic calcium
[27]
is beneficial for capillary formation, but may also inhibit phosphate [BCP]), demonstrated that cell distribution
the differentiation of MSCs into osteoblasts [100-102] , an could be maintained after 2 weeks of culture. Furthermore,
ideal design for bone bioprinting is to provide a hypoxic the MSC/BCP-laden Matrigel part demonstrated apparent
environment with a controlled oxygen diffusion for bone formation in immune-deficient mice after 6 weeks of
the embedded human umbilical vein endothelial cells implantation, which demonstrated by Goldner’s trichrome
(HUVECs) so that the oxygen supply to the bone region is and COL-I staining, while cartilage formation was evident
not compromised . in the MSC/Matrigel part as determined by Safranin-O
[53]

Volume 9 Issue 1 (2023) 82 https://doi.org/10.18063/ijb.v9i1.628