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Bioprinting of osteochondral tissues: A perspective on current gaps and future trends
[71]
of mechanical loading on the subchondral bone. Another zone) to 15 MPa in deep zone . On the other hand, the
important function of articular cartilage is lubrication modulus values of subchondral bone range higher than
of the joint. Lubricants, such as proteoglycan 4, reduce values obtained with biofabricated constructs, which
friction between contacting surfaces, thus minimizing mostly lie in 30–3,000 kPa range [72,73] . In this aspect,
wear and tear of the joint [68] . One of the hallmarks of though hydrogel materials have been the preferred choice
the osteochondral interface is the zonal variations in for cartilage bioprinting, the mechanical properties of
the structure, property of articular cartilage and the the subchondral bone demand a more robust support
subchondral bone, which makes the design of tissue structure such as PCL.
engineering scaffolds challenging. Chondrocytes or- To mimic the zonal compositions, a Fab@Home 3D
ganize extracellular matrix in to unique and highly printer has been shown to effectively deposit PLGA–
specialized tissue. Articular cartilage can be divided PEG microspheres co-printed with alginate-cell sus-
[66]
into the superficial zone, transitional zone and middle pension in multilayered structures . Present bio printing
(radial) or deep zone, and calcified cartilage zone. It capabilities are adequate to obtain scaffolds with mi-
varies in composition of primary constituents viz. water, metic osteochondral mechanical, biochemical and
collagen, proteoglycans, chondrocytes and some other porosity gradients. Custom-developed 3D bioprinters
minor proteins. The superficial-zone takes up to 20% have been used to create multilayered os teo chondral
of the total cartilage thickness and cells in that zone tissue constructs by bioprinting human tur binate mesen-
secrete lubricants. It contains densely packed collagen chymal stem cells (htMSC) on a slowly degrading
fibers in parallel to the articulating surface to resist PCL frame [74,75] . Using this approach, htMSCs with
shear stress and to protect the joint. The deeper zones, atelocollagen and recombinant human bone morpho-
including middle-zone, deep-zone and calcified zone, are genetic protein (rhBMPs) have been bioprinted over the
relatively less in cell density and have thicker collagen PCL layer, creating a layer with a thickness of 4 mm to
bundles, which are perpendicular to the articulating mimic the subchondral bone tissue. This was followed by
surface. Deeper zones help articular cartilage to resist bioprinting of htMSC-HA-TGF-β at 1-mm thickness on
compression force. The subchondral bone, on the part, the subchondral bone structure to mimic cartilage tissue.
is composed of concentric lamellar layers around the The constructs showed promising results in the repair of
osteons and flat layers representing new bone formation. rabbit knee joints. Recently, an EBB platform has been
The peripheral bone is largely avascular, while the endo- combined with a multi-nozzle electrospinning technique
[69]
steal bone abuts directly on calcified cartilage . to fabricate gradient constructs with differential release
The unique anisotropic arrangement is formed due rates of gentamycin sulfate (GS) and desferoxamine
[76]
to the external loads over time, which is transmitted (DFO), which can be extended to co-print cells .
through the matrix of the tissue and converted into a Despite some success of 3D bioprinting as a tool
biochemical signal, alerting cells to either produce more for osteochondral tissue regeneration, developing an
[5]
or catabolize existing ECM . Scaffold-based tissue integrated construct closely mimicking the heterogeneity
engineering approaches interrupt this transmission as and anisotropy of articular cartilage, subchondral
the scaffold material confines the cells and shields cells bone and the soft–hard interface remains a critical
from this mechanotransductive signaling cascade [70] . challenge. Solving this challenge plays a crucial role
Thus, novel scaffold-free tissue engineering approaches in improving the osteochondral tissue regeneration
are needed to help preserve the natural balance between process and graft integration with host tissue [77] . Many
external mechanical loading and the maintaining of of the limitations for traditional osteochondral tissue
zonal microenvironments for chondrocytes to adapt engineering approaches can be attributed to the inability
and regulate their biosynthetic activities in order to of precise spatiotemporal and temporal control of
produce zonally-stratified characteristics of cartilage. biomechanical and biochemical cues for direct cell mi-
Moreover, at the osteochondral interface, chondrocytes gration, differentiation and cell–cell interaction. 3D
from the calcified cartilage zone and cells from sub- bioprinting-based approaches to engineer osteochondral
chondral bone differ in their differentiation status and tissue can provide precise spatial control of bioactive
metabolic activities, making any tissue engineering compounds and biomaterials to mimic the gradients of
strategy to recapitulate this interface very challenging. biologic and mechanical signals along the osteochondral
The heterotypic cell-specific differentiation should not axis. For example, growth factors for chondrogenesis
compromise the mechanical integrity of the interface. In and osteogenesis, and plasmid DNA encoding osteo-
general, the compressive modulus of articular cartilage and chondrogenic genes and siRNA modulators of
increases from superficial layer (0.079 MPa) to 2.10 MPa differentiation, can be integrated into “bioinks” made
in the deepest zone, while the tensile modulus varies of biomaterials with different mechanical properties for
in inverse direction, reducing from 25 MPa (superficial cartilage and bone tissue, respectively. Several miRNAs,

114 International Journal of Bioprinting (2017)–Volume 3, Issue 2