Many bioinks contain additional additives, including thickening agents. Thickening agents can improve the printability of a bioink and enhance the bioprinting process.
A thickening agent is a material that increases the viscosity or changes the rheometric
properties of another substance (in this case a biomaterial) without significantly changing its other characteristics. Additionally, theses thickening agents can tabilize suspensions and emulsions. These thickening agents are common in food, cosemetics and are becoming more and more commin in bioprinting.
Over the past 30 years, low viscosity biomaterials such as collagen, fibrogen and matrigel have been developed for pipetting and have similar viscosites to water prior to crosslinking or self-assembly. These materials are not bioprintable on their own because they can not form continuous filaments, that is why thickening agents must be implemented to stabalize a bioink.
Common thickening agents and emulsifiers in bioprinting include:
- Nanofibrillar cellulose
- Nanocrystalline cellulose
- Bentonite clay
While it is possible to simply increase concentration of your desired biomaterial, using these agents instead can great reduce costs because you will be able to use lower quantities of the expensive biomaterial and yield the same results. Another thing to consider is how high concentrations of these biomaterials can cause the self-assembly kinetics and crosslinking properties to change, leading to rapid self-assembly and the entrapment of cells before they can adhere and spread.
These thickeners can either be permanent components of a bioprinted construct or leach and degrade away after bioprinting. An ideal filler bioink would be metabolized or degraded by the cells. Other thickeners may serve to soften or stiffen the resultant bioprinted filament by disrupting the assembly or crosslinking process or integrating into it. This can allow modulation of mechanical strength with components that won’t affect cellular activity. Additionally, certain thickeners can reduce the sensitivity of a bioink to temperature changes or drying out, improving the effective printability window.