3D bioprinting can be grouped into three main stages: the preparation of bionks, the printing process itself, and the crosslinking of the cells into the scaffold material after the print is made.
3D bioprinting begins with the material that is being bioprinted. In extrusion-based bioprinting, this material is known as the bioink and consists of the specified cell culture, a jelly-like medium to encapsulate the cells (called hydrogel), and additional nutrients needed to maintain cell viability.
The composition of the bioink is one of the most important considerations of bioprinting, and there has yet to be a standard protocol in which scientists have agreed upon to be the most effective.
This is because there are many variables to consider when choosing the right combination of ingredients for the bioink. These include: type of cell, scaffold, printing and crosslinking methods; how these choices affect the viability of the cell culture before, during, and after the printing process; and general considerations such as cost, availability, and potential applications within medicine, genetic modifications, and industrial use. As one might conclude, many of these variables are interrelated tradeoffs.
There is an art and science to concocting the perfect solution of bioinks for 3D bioprinting.
There are various types/methods of bioprinting. The most common is an additive method known as extrusion-based printing (left 2 images). This is most similar to what most people think of when they think of a 3D printer. Extrusion-based printing extrudes material from a nozzle using pneumatic pressure from a pump and creates three-dimensionality through layering material on top of itself. There are different methods for hardening the material before the next layer is placed. Some printers use a piezoelectric actuator to soften the bioink before printing and relies on cooling for hardening the material. Again, this is largely dependent on the type of bioink being used and its properties.
The other 2 on the right of the above diagram use lasers to guide and harden the material. The printing method 3rd from the left is known as “laser-induced forward transfer” bioprinting, and uses a pulsed laser to knock out patterns of droplet pockets from the absorbing layer of bioink and onto a substrate below. The last method on the right is known as projection stereolithography. This method uses a projected system to shine a pattern onto the photosensitive material below.
Crosslinking is the process of hardening the 3D printed structure into a viable construct. There are various methods for doing so, many of which involve the addition of a chemical solution to the cell culture after it is printed. Again, the method and solution depend on the composition of the bioink being printed.