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Three-dimensional (3D) printing has gained traction in recent years and is now used across a wide range of industries for the quick and easy fabrication of complex materials. 3D printing is now set to revolutionise the medical industry, especially in regenerative medicine, as it enables cells, tissues and organs to be printed on demand. These biological components are traditionally cultured for long periods of time, using various gaseous and chemical environments so that they grow.
Aside from the timescales, standard culturing methods can easily go awry if there is even a small, and often accidental or unavoidable change in the growth process. Whilst 3D bioprinting of living tissue still requires the cells to be cultured and grow, the printing process provides a new approach to fabricating tissues and organs with complex geometries.
3D bioprinting allows for the distribution of different cells to be built up, in a complex way, to produce the microarchitectures shown in both tissues and organs. In 3D bioprinting, the organic materials are built up through a bottom-up, layer-by-layer (LbL) precise position approach that often includes some element of self-assembly from the internal components.
Biological materials, biochemicals and living cells can all be built up, whilst maintain an accurate spatial control. There are many different approaches to 3D bioprinting, but some of the most common are biomimicry, autonomous self-assembly, inkjet bioprinting and mini-tissue building blocks.
3D bioprinting is currently being explored for many tissue engineering applications, where the fabrication of biologically and mechanically sound, functional and implantable human organs is possible.
Aside from biological materials themselves, 3D bioprinting can also be used to fabricate biocompatible scaffolds for the regeneration of cells, growth of cells and implanting of stem cells into an area requiring regeneration. These methods, whether they are 3D printed or not, form the basis of tissue engineering applications.
There are currently some challenges facing 3D printing, but most research has adapted well to them. Standard 3D printing machines are generally designed for the printing of plastics and metals.
So, in most cases, a 3D printer needs to be adapted by the researchers to enable the printing of biological matter, complex living architectures, and multiple cell types, whilst maintaining the sufficient resolution to recapitulate the biological function of the printed material.
Applying 3D Bioprinting to Tissue Engineering
Tissue Engineering encompasses a mixture of materials science and engineering-based methods, applied to biological cells, to replace or grow tissues.
The formation of new tissues is generally through a scaffold, which can either be implanted into a living organism to grow new cells on top of the existing tissue and essentially repairing it; or through being separately grown in the laboratory as a separate tissue which can later be implanted.
One of the most common and widely studied areas where 3D printing is used in bone tissue engineering. Many teams of researchers have 3D printed hydroxyapatite scaffolds for the growth and regeneration of bone tissue.
Some researchers have created porous ceramic scaffolds using a modified hydroxyapatite powder and a polymer binder. The scaffolds were printed using a Generis GmbH 3D printer with two z-pistons, and x-y plotter, a recoating unit and electronic control devices.
These printers produced scaffolds with inner channel dimensions as low as 450 µm and a wall thickness of 330µm. The scaffold was found to possess a high mechanical strength, biocompatibility and can house cells up to 30µm in size.
More recent research has produced 3D printed hydroxyapatite scaffolds, for bone tissue engineering, using a ZCorporation Inc. ZPrinter 310 (now owned by 3D Systems Inc).
These researchers used a mixture of hydroxyapatite and poly(vinyl) alcohol to produce a mechanically stable and porous scaffold. The researchers tried a few options, and the scaffolds with an average total porosity of roughly 55% are the most promising and are able to hold bone cells. These scaffolds, with a little optimisation, have the potential to be used in the future for bone regeneration through osteoconduction and osteointegration mechanisms.
The most recent advancement is though the 3D printing of induced pluripotent stem (iPS) cells in a nanofibril cellulose/alginate (NFC/A) bioink to fabricate cartilage tissue.
The researchers used a RegenHu 3D Discovery printer with a 300µm nozzle and BioCAD software (Biomedical Modeling Inc) to produce the cartilage tissue. The bioink was easily printed and the cells were cultured for 5 weeks to produce a tissue that could be used as cartilage.
The cells were co-cultured with irradiated chondrocytes and growth of the cells was found to be impressive, and after 1 and 7 days only, the growth of viable cells produced were 73% and 86%, respectively.
Different ratios of NFC/A were trialled and the 60/40 ratio performed the best and kept their pluripotency after printing. Through producing a large number of functional cells, the researchers hope that the method will be used as a future tissue regeneration treatment to repair damaged cartilage in joints.
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