Give me the best bioink! | A short guide to the currently available 3D bioinks
As with other forms of 3D printing, the properties of an ‘ink’ used for 3D bioprinting is vital to its success in forming 3D printed structures. In addition to this, there are vital properties which must be considered if a bioink is to support cell growth.
Let’s start with the basics
There are two main categories of bioink materials currently used in 3D bioprinting. One is used in a cell-scaffold based approach and the other in a scaffold-free cell based approach. In the first method, the bioink consists of biomaterials mixed with live cells, which are printed to develop 3D tissue structures. In the second method, the living cells are printed directly in a process which resembles the normal embryonic growth. In this case, the selected group of live cells forms the neo tissues which are later deposited in a specific arrangement to form functional tissue structures over time. In the case of the cell-scaffold based approach, an ideal bioink formulation should satisfy certain biomaterial and biological requirements. Essential biomaterial properties include printability, mechanical properties, biodegradation, and modifiable functional groups on the surface. Biological requirements include biocompatibility, cytocompatibility and bioactivity of cells after printing. The printability of the bioink depends on the viscosity and surface tension of the ink. The printing reliability and the live cell encapsulation highly depend on the hydrophilicity and the viscosity of the bioink solution. If the bioink formulation is highly viscous, then the pressure needed for the extrusion will be higher, potentially increasing the shear stress experienced by the contained cells. If the ink formulation has a low viscosity, it may affect the stability of the printed structure. The printed structure needs enough stiffness to retain the 3D structure as well as to support the direct cellular behaviours. Moreover, the biodegradation of the selected biomaterials should match the tissue of interest and allow the cells to grow and proliferate by replacing the bioink construct with their own regenerated extracellular-matrix (ECM) without creating any immunological response. These basic requirements are very important when selecting a successful bioink material for 3D bioprinting.
Bioinks available for 3D printing
There are many different biomaterials which are reported as bioinks for3D bioprinting. According to the requirements of the desired tissues and organs, the bioink should be selected and may require modification.
- Agarose-based bioinks
Agarose is a marine polysaccharide obtained from seaweed. It is one of the most commonly used biopolymers in the biomedical field for a range of diverse applications because of its excellent gel formation properties. Agarose consists of a linear polymer chain composed of disaccharides, namely D-galactose and 3,6-anhydro-L-galactopyranose. Although it has excellent gelation properties, its ability to support cell growth is limited. Therefore, researchers have begun to use blends of functional biomaterials along with the agarose gel. For instance, agarose-based blend material (consisting of collagen and fibrinogen) has been shown to form a stable 3D structure which supports endothelial and fibroblast cell growth.
- Alginate-based bioinks
Alginate is another natural biopolymer attained from brown algae. Alginates are negatively charged polysaccharides, which do not elucidate or provoke much inflammatory response when implanted in vivo. The aliginate polymers can entrap water and other molecules by capillary forces and allow it to diffuse from inside out. This characteristic makes alginate ideal for a bioink. Over recent years different polymers have been blended with alginate to form various 3D printed constructs for tissue engineering including polycarprolactone, poloxamer, hydroxyapatite, gelatin. For instance, the mixture of alginate, agarose and carboxymethyl-cyitosan were used to print cell-laden 3D constructs, which demonstrated in-situ differentiation of stem cells. Moreover, alginate was used as the bioink when induced pluripotent stem cells and human embryonic stem cells were bioprinted for the first time.
- Collagen-based bioinks
Collagen is a main component of the ECM, and is obtained from natural biomaterials. Collagen has been used as a bioink material in 3D bioprinting either alone or in combination because of its excellent biocompatible properties. This biopolymer can be crosslinked using temperature or pH change. However, the crosslinking or gelation of collagen requires a minimum of 30 mins for gelation at 370C. Hence, the usage of collagen directly in 3D printing is tough. Combining with different other materials may help to address this issue.
- Hyaluonic acid-based bioinks
Hyaluronic acid (HA) is also a natural ECM componentwhich is found in abundancein cartilage and connective tissues. HA is one of the most prominent biomaterials which are used in bioprinting for developing 3D structures. Despite this, hyaluronic acid has poor mechanical properties and slow gelation behaviour.
- Fibrin-based bioinks
Fibrin is a protein which is seen in the blood and helps in clotting. These hydrogels have excellent biocompatibility and biodegradation properties, but have weak mechanical properties.
- Cellulose-based bioinks
Carboxymethyl cellulose (CMC) is a semi flexible polysaccharide obtained from cellulose. CMC can be converted into an environment-sensitive hydrogel by altering its concentration, molecular weight, salt content and degree of methyl grafting appropriately. Anaqueous solution of CMC can form gels below 370C. The material shows good shape and size retention, and high cell viability after printing.
- Silk –based bioinks
Silk fibroin is a natural protein obtained from silk worm. Silk-based scaffolds are more frequently used in regenerative medicine and tissue engineering because of their exceptional properties. Recent research reported a silk-based material which, in a composition with PEG, has shown excellent printability with high resolution and supported MSCs viability for a longer period.
- Synthetic biomaterials
Even though natural polymers or hydrogels provide the desired microenvironment mimicking the native ECM for cell attachment and proliferation, the tuneable properties of these materials are low. While synthetic polymers may not promote cellular adhesion as natural polymers, they are promising candidates as they allow the user to tune the mechanical properties, printability, cross linking, etc. Hence, natural polymers are combined with synthetic polymers to obtain more stable structures with tuneable properties for 3D bioprinting.
In bioprinting, Pluronic and polyethyleneglycol (PEG) are the most commonly used polymers. However, there are a few other synthetic polymers which are used as bioinks in 3D printing applications, such as BiogelxTM-Ink.
The Future of Bioprinting is here
BiogelxTM-Inks are composed of synthetic peptide hydrogels which form a nanofibrous network, mimicking the extracellular-matrix which supports cell growth, signalling and proliferation. These bioinks are printable at room temperature and do not require exposure to UV light, extreme temperature or pH changes, which can be detrimental to the health and viability of the cell culture. The advantage of using BiogelxTM-Inks is to provide a biocompatible environment with tunable mechanical properties, offering the ability to balance printability with cell viability. These bioinks can be printed into air with good 3D fidelity without requiring the use of a support, or sacrificial or curing inks.
- Gopinathan et. al. “Recent trends in bioinks for 3D printing” Biomaterial Research (2018)
- Jammalamadaka et. al. “Recent Advances in Biomaterials for 3D Printing and Tissue Engineering” (2018)