Development of a bioink: key considerations for biocompatible and printable materials
With all the benefits demonstrated by 3D cell culture techniques, the demand to accurately deposit such a cell-laden matrix naturally follows. For this reason, 3D bioprinting has emerged as an area of interest for those active in the fields of tissue engineering, regenerative medicine, and high-throughput screening, among others. The process itself relies upon additive manufacturing of 3D structures by deposition of a suitable cell-laden material (the bioink). Thanks to the growing availability of affordable pneumatic extrusion printers, the cost of entry to 3D bioprinting research is now lower than ever before, and so the focus turns to the types of bioink available to support biofabrication.
The two crucial elements which must be present in a viable bioink are printability and biocompatibility.
On Printability –The print fidelity required will be defined by the final bioprinting application, and this can often be improved by increasing the viscosity and final stiffness of a bioink. However, this generally involves increasing the concentration of the base material or increasing the extent of cross-linking between the nanofibersin the matrix, both of which may affect cell viability. In terms of biocompatibility, it is best to develop a bioink using a known biocompatible base material which may then be optimized for printability. In many cases, this results in the use of naturally-derived base materials, however, these may suffer from batch-to-batch variability as discussed previously.
Another factor for consideration is the shear stress experienced by the cells in a bioink during the printing process. While there is an inevitable stress placed on cells due to their extrusion under pressure through the nozzle of a print cartridge, this may be mitigated by using bioink materials which exhibit shear-thinning behavior, i.e. the viscosity decreases under increased shear. This affords some protection to the cells as the effective viscosity of the bioink is lower at the points of higher shear stress. In addition, the conditions required to formulate, print and gelate the bioink should be compatible with the cells contained within the printable material. Commonly used bioinks require careful temperature control, UV-initiated radical generation or addition of reactive cross-linking reagents in order to produce a fully gelated 3D structure following bioprinting, and these conditions are not necessarily agreeable for the cells contained within.
On Biocompatibility– In the body, cells are present in an extracellular matrix (ECM) consisting of a complex 3D architecture, and they adapt to their surrounding environment by responding to physical and chemical cues, which have critical implications for cellular function. Materials utilized for 3D printing must be biocompatible and should incorporate biological cues to mediate tissue formation and aid in guiding cell adhesion and proliferation. There are several differences between natural and synthetic materials worth considering: Biomaterials of natural origin are often based on extracellular matrix (ECM) components such as collagen, hyaluronic acid and fibrin. They also include other naturally derived materials like alginate or silk. Natural materials might provide essential cues and contain adhesion sites that mediate biocompatibility, but they usually lack the mechanical tunability of synthetic scaffolds, which is often important for researchers wishing to better tailor their 3D models. A more important limitation is based on their intrinsic ‘natural’ origin which increases lot-to-lot variability and makes these materials very inconsistent and not as reproducible, which is an essential requirement for reliable research studies and drug testing.
To provide appropriate biomimetic functionality, Biogelx hydrogels are functionalized with specific peptide sequences that are relevant for different ECM proteins. These include BiogelxTM RGD (Fibronectin), BiogelxTM GFOGER (Collagen), BiogelxTM IKVAV (Laminin) and BiogelxTM YIGSR (also Laminin). These short peptide sequences and are a way of increasing the functionality of the matrix, enhancing cell attachment and other key cellular and molecular processes, while maintaining a model that is simple and defined. This allows for more control of the experimental system and permits focusing on the results, which are reproducible and consistent.
Using the non-animal derived and biocompatible Biogelx base technology, Biogelx have developed a new range of bioinks to allow printing of 3D structures into air under mild conditions. These materials exhibit shear-thinning behavior, are printable at physiological or room temperature and at neutral pH, and gelate upon incubation with cell culture media. As with the standard range of Biogelx hydrogels, these bioinks include incorporation of biomimetic peptide sequences to encourage cell adhesion. These new materials represent an exciting alternative for researchers who are active in the field of biofabrication and will be formally launched in the coming months.
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