Introduction
Tissue engineering has had incredible advancements in the last ten years yet a significant problem still plagues the effectiveness of bioengineered tissues-vascularization. Mechanical diffusion of nutrients is insufficient to alone maintain cell viability in any engineered tissue thicker than a few hundred microns. The lack of a functional network of blood vessels in such tissues leads to the rapid depletion of oxygen and nutrients in the inner parts of these tissues leading to hypoxia and cell death.
This problem has long been the desire to be solved by traditional bioprinting techniques, which have, however, been partially successful, especially when it comes to the building of dense vascular networks incorporated into three-dimensional tissues. Most recently still, an even more refined technology referred to as Coaxial SWIFT, an acronym that stands Sacrificial Writing Into Functional Tissue, has been demonstrated as an extremely potent new method of creating perfusable channels in living tissue matrices. This article gives a comparison in detail between traditional bioprinting methods and Coaxial SWIFT, as to how they both tackle the key issue of vascularization, and also points out the strengths, weaknesses of the two, as well as areas of applications.
Why Vascularization is Essential in Tissue Engineering
Some of the functions which vascular networks play in biological tissues cannot be dispensed with. To start with, they act as a medium of transporting oxygen and vital nutrients to the cells. In the absence of these resources that keep life going, the cellular metabolism is not supported. Of equal significance is the elimination of carbon dioxide and waste products of metabolism that is taken care of by the venous drainage in the vascular system. Blood vessels are also important to immune surveillance, white blood cells are carried by the blood vessels to an area of infection or injury, and blood vessels are very important in the distribution of signaling molecules like hormones, within the body.
In engineered tissues, these functions have to be reproduced by fabricating an equally complicated vascular network. This is especially acute when it comes to building tissues that would be transplanted or implanted over a long period. Unless cells in the inner parts of a tissue construct have access to a vascular supply within 100 to 200 microns the normal diffusion distance in biological tissues, they will not be able to survive. Therefore, vascularization cannot be regarded as a mere improvement of engineered tissues, but as a biological requirement.
Traditional Bioprinting: Foundation and Shortcomings
Conventional bioprinting entails a number of different technologies with mechanism of operations. Inkjet bioprinting relies on the thermal or piezoelectric forces to precipitate minute drops of bioink on a substrate. This method can be fast and cost effective, but is usually restricted to low viscosity materials, which is not necessarily structural integrity ideal. In contrast, extrusion bioprinting relies upon pneumatic or mechanical pressure to eject a continuous thread of bioink through a nozzle. The method can be applied to a broader variety of materials, such as viscous hydrogel as well as cell-laden matrices. Laser-assisted bioprinting Laser-assisted bioprinting uses focused laser pulses to transfer drops of biomaterial between a donor surface and a target region with great precision and low levels of cellular destruction.
All these conventional techniques have been used to assemble tissues with some success or failure levels. They provide a compromise between spatial control and can print trivial tissue types like skin and cartilage. They however, fall short on the production of vascularized tissues which need to have blood vessels within them in order to promote metabolism.
In order to overcome the problem of vascularization, a number of strategies have been used in the traditional approaches. A popular method is the pre-patterning of microchannels through the hydrogel network which are afterwards seeded with endothelial cells. Alternatively, it is also possible to co-culture tissue-specific cells with endothelial cells and, over time, the latter will self-organize into vascular structures. The third approach involves the addition of angiogenic growth factors, e.g. vascular endothelial growth factor (VEGF) to promote the growth of blood vessels in the construct. Other investigators also print tissues in sacrificial or gelatinous support baths in order to generate embedded channels.
The latter strategies have proven concept, but are associated with significant limitations. The formation of vessels is also frequently not immediate, but days or weeks after printing the construct. In the process, cells that are distant to the surface might not get enough oxygen causing necrosis. Moreover, the resolution of the vascular channels created by the traditional methods is quite low so that it is hardly possible to replicate the complicated pattern of the branches observed in natural vasculature. These techniques also do not have the capacity to facilitate continuous perfusion particularly during the early phases of tissue development. Consequently, conventional bioprinting has regularly been unproductive in creating thick and completely vascularized tissues.
Coaxial SWIFT: A Targeted Approach to Vascularization
Coaxial SWIFT offers a radically different method of overcoming the limits of traditional bioprinting. The fundamental principle of the method is that it is a kind of extrusion through a coaxial nozzle of two different materials: a central core of sacrificial ink, most commonly a gelatin-based material, and an outer sheath of bioactive hydrogel, which contains living cells. Any printing is carried Matrix of Living Cells or tissue matrix which is highly packed and provides both structural support and biological activity.
Following printing, the tissue construct is heated or incubated with enzymes selective to the sacrificial ink, resulting in hollow microchannels after printing. Endothelial cells seeded in these channels can then be seeded to replicate blood vessels or it can be perfused directly with culture medium to supply oxygen and nutrients. Since the sacrificial ink is meant to be biocompatible and thermo-reversible, its dissolution does not kill the cells surrounding it and does not lead to any change in the mechanical stability of the construct.
Among the key benefits of Coaxial SWIFT is the fact that the technology allows the creation of perfusable channels upon printing. This aspect means that cells across the construct are well oxygenated and supplied with nutrients initially, thus chances of early necrosis are nullified. Moreover, using the coaxial nozzle, the geometry of the channels can be controlled accurately (the diameter of the channels, their branching structure, and their spatial orientation). This allows the reconstruction of the hierarchical structure of the natural blood vessels in an unprecedented accuracy.
Comparing the Two Approaches
Vascular Network Formation
Older techniques are angiogenesis or endothelial self-assembly, both of which are slow processes. These mechanisms are biologically delicate and erratic, so the vascular networks are frequently inconsistent or incomplete. By contrast, Coaxial SWIFT enables the creation of hollow, perfusable channels to be created intentionally and in real-time as part of the printing process. Not only does this enhance consistency it also allows early perfusion to take place which is crucial to the survival of embedded cells.
Structural and Spatial Precision
Nozzle resolution and print speed are commonly regarded as the limitations of the spatial control in traditional bioprinting. Although these techniques are able to create relatively simple patterns, they are not easily able to create the complex 3D vascular trees that are present in native tissues. Coaxial SWIFT in contrast offers high resolution control of lumen geometry and the capability to generate branched networks to resemble natural capillary networks.
Perfusion and Cell Viability
The most profound perhaps difference is the capability to support perfusion. Conventional structures tend to use passive diffusion until the development of new vessels. This constraint causes hypoxic areas in the interior of the tissue. Coaxial SWIFT constructs may be perfused immediately, greatly increasing oxygenation, waste clearance and total cell viability throughout the construct.
Scalability and Efficiency
Traditional methods are slow and unstable to all increasing degrees of size and complexity of the tissue construct. They are also prone to mechanical deformation due to their layer-by-layer structure. Coaxial SWIFT allows fast channel printing in pre-formed matrices, cutting print time and scaling up. This causes it to be more applicable to organ sized structures.
Material Requirements and Cost
Conventional bioprinters tend to be both less complex and cheaper than the dedicated instruments needed to do Coaxial SWIFT. They also go well with more biomaterials. Nevertheless, Coaxial SWIFT needs a more sophisticated insight in material characteristics, and especially the performance of sacrificial inks and sheath hydrogel, in order to preserve channel patency and cell health.
Practical Use Cases
Although Coaxial SWIFT demonstrates some definite benefits in vascularized tissue engineering, the conventional technique still has its value in certain options. As a point of example, skin grafts, cartilage, and corneal tissues do not havecomplex internal vasculature, and can be successfully printed in regular extrusion or inkjet approaches. The techniques are also useful in teaching, and in prototype development where expenses and ease are valued.
Coaxial SWIFT is, however, brilliant in more challenging situations. It has been used especially successfully in the formation of cardiac patches, where coordinated contractions and integrated vascular support are needed to be functional. It also has been employed in the fabrication of liver lobules possessing enhanced metabolic functionality and bone grafts having channels that are integrated which facilitate osteogenesis as well as vascularization.
Looking Forward
Integration of Coaxial SWIFT with other advanced technologies will most likely determine the future of vascularized tissue engineering. With computational models it is now possible to produce vascular tree designs that are optimized with respect to a particular organ geometry. They can be converted to printing instructions coaxial systems. Also, bioreactors that can simulate blood flow conditions are under development, to keep printed tissues alive and developing after fabrication.
There are still challenges, especially with standardization of material formulations as well as reproducibility between laboratories. Regulatory environment should also adapt to new techniques of biofabrication. However, the possibility of Coaxial SWIFT to break the vascularization bottleneck is getting more evident.
Conclusion
Summing up, traditional bioprinting has paved the way to biofabrication but cannot enable the development of complex and clinically relevant tissues due to its inability to vascularize constructs. Coaxial SWIFT overcomes this difficulty in a straightforward manner, providing a technique to incorporate accurate, perfusable vasculature channels into dense tissue matrices at the initial stage. Not alone does this improve cell viability and functionality, but it also paves the way to the production of large-scale, functional tissues that can be used as therapy.
Although Coaxial SWIFT implants require particular apparatus and exquisitely sophisticated system of materials, its advantages in terms of structural faithfulness, perfusion effectiveness, and biological realism make it a technique of predilection in the next-generation tissue engineering practices. This technology continues to develop and promises to transform the sphere of regenerative medicine even more and take us to the ultimate destination bioprinted organs that could be transplanted.
