Heart Tissue Grown Using Carbon Nanotubes

You have all probably noticed that hydrogels are a huge topic of interest in our blogs. Hydrogels are very important in the field of tissue engineering, especially due to their flexibility. Recently, a hydrogel was created using carbon nanotubes that would serve as a scaffold for cardiac tissue that would "beat spontaneously."

In heart attacks, cardiomyocytes, or muscle cells, are attacked. If artificial heart tissue is made, it must follow the structure and function of the heart. The heart is an organ that transmits signals to cells for the regulation of muscle contractions (heart beating). Heart tissue that is lab-grown must have a scaffold that is electrically conductive so it can also do this. 

Cardiomyocytes have been shown to grow on alginate or gelatin scaffolds in the past. Alginate is a polysaccharide made up of α-L-guluronic acid and β-D-mannuronic acid. Because alginate is found in seaweeds, its structure is very gel-like and flexible. When alginate interacts with metal ions, such as calcium ions, hydrophilic gels are formed. In terms of structure, both poly guluronic acid and poly mannuronic acid bind to calcium ions. However, poly guluronic acid interacts more strongly to calcium by cross linking, and it also has more hydrogen bonding between the carboxyl groups and OH groups. Alginate structures with a greater amount of poly guluronic acid thus have stronger structures. 

Sodium polymannuronic acid

Sodium polyguluronic acid

However, the combination between polyguluronic acid and calcium does not have a high specific heat, making it a poor conductor and unacceptable for an electrically conductive hydrogel. Another material, gelatin, is also not a very good conductor, but when bonded with methacylate monomers, a more structurally stable molecule is formed.  Methacrylate, C5H8O2, adds more covalent bonds to the overall structure.

The carbon nanotubes had a crosslinked methacrylated gelatin film on it, allowing the hydrogel to be formed. This new hydrogel imitated the Punkinje fibers in the heart that have conduction capabilities. It was also seen that these new cells beat at a faster rate than tissue that contained only gelatin and could use an 85% weaker electric field to beat.

In the future, these nanotube-grown cells will be continually used in cardiac tissue applications, as well as in other organs of the body that require the withstanding of contractions. 

http://cen.acs.org/articles/91/web/2013/02/Carbon-Nanotubes-Help-Grow-Beating.html

Spray-On Skin Kit

Avita Medical has created a new spray-on skin kit known as the ReCell Kit that does exactly what it sounds like - generates new skin for you after a simple spraying mechanism. It can be used to treat burns, wounds, and even correct blemishes for cosmetic purposes, such as altering abnormally pigmented skin or improving the appearance of scars.

Although this kit does seem to work wonders, its basis lies strictly in the scientific realm. In short, the keratinocytes and melanocytes of the patient are harvested and then suspended in a solution in which they can multiply. It is this solution that is sprayed onto the patient and results in the growth of new skin. Because the harvested cells come directly from the patient, any risks of the immune system rejecting the treatment are eliminated.

The reason these two types of cells are used in the solution is because they are the building blocks of skin. Keratinocytes in particular are of utmost importance because they are responsible for the production of keratin, a structural protein without which much of our bodies would fail to function. One of the most crucial of keratin's properties is its strength even at very high temperatures.

This is because its toughness comes from its supercoiled nature in which the polymer is kept together through cross-linking brought on by both intermolecular forces and covalent bonding, as seen in the diagram. Covalent bonds form between sulfurs to result in disulfide bridges, which kept the protein rigid. However, even at the high temperatures of the body, the bonds stay as they are. Thus, the dissociation of the bonds is nonspontaneous, which means the value of change in Gibbs Free Energy is positive.

Furthermore, since such dissociation would result in more positional arrangements, entropy would increase and thus change in entropy is positive as well. This means that the sign of enthalpy must be positive or else the change in G would be negative, so the bond dissociation is endothermic. Eventually, at a high enough temperature, the bonds will dissociate because of this fact.

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.