3D Bioprinting Tissue

A new 3D bioprinter has emerged from the University of Toronto with the shocking ability to print skin by making ample use of colligative properties as they relate to solutions. Although this printer is still in its early prototype stages, it has the potential to be used to print skin, organs, and even artificial food as discussed in an earlier blog post.

The printer works much like a normal printer, with seven reservoirs comparable to the color cartridges normally found. The main difference, of course, is that each of these reservoirs contains living cells instead of ink. This part is fairly standard for other 3D bioprinters, but what makes this different from the others is that it doesn't rely on the traditional layer-by-layer assembly often seen, which makes this less time-consuming and more readily available for applications such as burn dressing.

Each of these living cells from the reservoirs are released into a stream of a compound known as sodium alginate, pictured at left. This polymer is a derivative of algae and is thus biocompatible in addition to strong and flexible when in gel form, which makes it ideal for tissue engineering applications. However, sodium alginate is soluble in water, so in this form, the polymer cannot be used to make any tissues or organs. The next steps of the chemical process used in this bioprinter are therefore made to ensure this polymer can be changed into an insoluble form. Otherwise, the cells would do nothing more than merely sit in solution.

To solve this problem, the stream containing the cells and the sodium alginate flows into another reservoir containing calcium chloride. When sodium alginate comes in contact with this solution, most of the sodium ions exchange with the calcium ions to result in calcium alginate. The big difference between these two is that calcium alginate is insoluble. This is because the electrostatic force between the very positively charged calcium ions and the anionic polymer overcomes the hydrogen bonding and other solute-solvent interactions between the water and the alginate. The result is that the calcium ions crosslink the polymer by joining the strands together, and in the process, result in an insoluble gel. This is depicted in the figure below.

Using this gel, the printer can easily spin the result into organs and other tissues.