A bioink is a hydrogel biomaterial that can be extruded through a printing nozzle or needle into filaments that can maintain shape fidelity after deposition. It is also critical that these bioinks maintain the viability of mammalian cells blended within the bioinks during and after the printing process. Bioinks provide support to cells while they produce their own extracellular matrix, and manipulate and reorganize the bioink toward the ultimate generation of a functional tissue.
Bioinks based on biopolymers, such as collagen, gelatin, hyaluronan, silk, alginate, and nanocellulose, are known for their favorable biocompatible properties and are attractive biomaterials for cell encapsulation and 3D bioprinting. These bioinks provide an aqueous 3D environment with biologically relevant chemical and physical signals that mimic the natural extracellular matrix environment and support cell proliferation and differentiation. Bioink formulations can be extremely diverse due to the myriad of cell populations, tissue compositions, and microstructures within the human body. So what does this mean? Where does one start if they want to engineer a tissue utilizing bioprinting? What choices does a researcher have to build a tissue? We are going to break it down for you by explaining in simpler terms the 5 types of bioinks, what process one may go through developing a construct, and how all types of bioinks play a role in the generation of a functional bioprinted tissue.
- Structural Bioinks
Lets imagine you want to build a house. You need to decide how big of a house you want, and what you want the house made out of. You hire a contractor, and you tell them that you want a brick house with 3 bedrooms, 2.5 bathrooms, and 1 kitchen. You are also really excited about wood paneling on the inside. To your surprise you show up to the job site and there is your house and no way to get in! And they trapped Bob, Bill, and Brad, the wood panel installers, on the inside with no air channels!
Structural bioinks can be thought of as the ‘framework’ of the bioprinted construct. Like a house, you have to determine what is the building material you want and how you want to structure it. Many types of materials can be utilized in the fabrication of bioprinted constructs. These materials can include alginates, chitosans, collagens, gelatin, hyaluronic acids, decellularized ECMs, and many others. However, simplify depositing only structural bioinks is not sufficient for the generation of a functional tissue. Regardless, bioprinted constructs fabricated from one or several structural bioinks are invaluable in the development process. Important results can be gathered from these constructs, included baseline mechanical and degradation properties, construct shape and size, and cell survivability (which may be the most important of all).
- Sacrificial Bioinks
I order to save Bob, Bill, and Brad you have a cut a hole in the wall. While you are at it, you cut holes in all the walls of the house. Now you have windows and doors, a way to enter and leave the house.
Like a house, constructs need areas where stuff can enter and leave. The incorporation of conduits and other channels within a printed construct may be key for the generation of vascular networks, cell migration into the construct, and nutrient diffusion. The incorporation of these void regions is key for improving cell viability through the free diffusion or convection of nutrients and waste. These conduits or void regions can be generated through the incorporation of sacrificial bioinks within a construct during the printing process. Often these sacrificial bioinks are soluble in water, under specific temperatures, or rapidly degrade to allow their removal from the construct. Sacrificial bioinks include non-crosslinked gelatins, pluronics, and other materials that can be easily removed.
- Functional Bioinks
Quickly you realize that your house is quite boring. It is has no electricity, no water, no heat or air conditioning. You can’t even watch the 63” TV you just bought. You run to the contractor and ask them why this is. They say, that you did not specify that you wanted all this functionality to the house. And if you want it, you need to start over and build a new one.
Similar to your house, bioprinted constructs require specialized bioinks to impart functionality to the construct. The incorporation of these specialized bioinks are necessary to guide the formation of the desired target tissue. Functional bioinks possess specific compositions that direct cell differentiation and guide behavior toward specific phenotypes or deposition of matrix. For example, these materials may contain growth factors and other biological cues that stimulate differentiation toward certain tissues. Additionally, functional bioinks may sequester and release growth factors or other molecules either provided by the researcher or released by the cells to recapitulate the native ECM. Other components can include mineral microparticles, or conductive molecules to guide differentiation toward osteogenic or muscular tissue for example. Beyond molecular cues, functional bioinks can provide different topographical cues to the cells to directly influence their organization and morphology. These specialized functional bioinks are necessary for the maturation of a bioprinted construct into a functional tissue.
- Supportive Bioinks
You now have your house and it now has electricity, water, and most importantly internet. But how you do ensure that it is still around in the future? You have to get your money back on your investment somehow? If you had built your house on the rocky, western coast of Sweden you may not have to worry about your house shifting over time. However, if you had built your house on the sandy shores of Florida, how your house is supported is something you need to consider. You better instruct the contractors to build some pilings!
Bioprinted constructs must be supported in a similar way. Depending on their desired application in the body, bioprinted constructs may possess insufficient mechanical properties until tissue development can occur. Until that point, the constructs must be protected, particularly if they are implanted in vivo. Supportive bioinks can fulfill this role. Protective lattices (fabricated from materials such as PCL, PLGA, etc) may be printed around softer bioprinted constructs for applications in cartridge and bone to withstand forces while development occurs. Additionally, these supportive bioinks act as structural supports that provide rigid structure to complex and intricate constructs that contain complex shapes, overhangs, and vessel networks, etc. until development progresses enough to support their own weight. These support bioinks can also be integrated within printed constructs to assist in construct integration into bioreactors and other systems that can accelerate tissue development. These rigid regions provided by support bioinks can resist construct deformation and compression due to clamping or suturing that may be necessary for bioreactor system integration.
- 4-Dimensional Bioinks
After all this hassle, you finally have a house you can live in, watch TV, and it won’t shift. You deserve some cool features, such as an automatic garage door, self-tinting windows, and automatic A/C. Perhaps, eventually you can upgrade to a smart house. However, that is the future but you are excited for it! If only it could come sooner.
4-Dimensional Bioinks are the next frontier for bioink development. These bioinks build on 4D biomaterials that are defined by their sensitivity to external stimuli. For example, the shape, structure, function, and other behaviors of these biomaterials chance when they are stimulated. These futuristic bioinks could be used in several applications such in muscle tissue. For example, the application of a repeatable electric stimulation to the construct can result in the contraction of the bioink. Perhaps this stimulation can be applied by bioprinted nerve! Other applications of 4D bioinks include self-assembly of the construct where a construct is printed and cultured in a ‘flat’ structure that is easier to provide with nutrients and at the appropriate time it ‘folds’ into a 3D construct suitable for implantation. More complex possibilities may include the release of growth factors or other molecules to influence cell behavior in specific cases. For example, these materials can be sensitive to the proximity of specific cell phenotypics types through the release of the growth factors only after specific cells adhere to it or when a phenotype-specific protein reaches a certain concentration. The possibilities are endless and promise to revolutionize bioinks and other biomaterials!