Cornell's Odd DNA Hydrogel

A research team at Cornell University recently created a new type of hydrogel. Opposed to the other hydrogels we have written about in this blog before, this hydrogel is made not of gelatin, but of DNA This hydrogel is particularly fascinating, to us and to Cornell University Professor Dan Luo as well, because the hydrogel consists of a solid shape when it is filled with water but loses this shape when the water is removed. Without water, the material behaves as a liquid, able to be poured into containers and retain the container's shape. Thus, it is referred to as "solid when wet and liquid when dry."

The effects of removing and reintroducing water to the DNA hydrogels

Chemical structure of DNA

The DNA in this hydrogel greatly contributes to its ability to changes shape. DNA, a polymer, cross-links with other DNA polymers to create a network. Numerous hydrogen carbon, and nitrogen atoms make up DNA's chemical structure, specifically forming a five carbon sugar, a phosphate group, and nucleotide bases. As we learned in chemistry, hydrogens can form hydrogen bonds if bonded with nitrogen, carbon or oxygen, which is why hydrogen bonds exist in the structure of DNA. These hydrogen bonds hold together the pairs of nucleotide bases. Cross-linking is the process of creating covalent bonds from the several polymer chains joined together. In this case, the DNA hydrogel is able to form a large network because of this cross-linking, enabling it to adsorb great quantities of water. When a material adsorbs another substance, most often gas, the substance spreads as a thin film on top of the solid material. By adsorbing different amounts of water, this hydrogel's properties change, which is what ultimately causes its uniqueness.

For those of you who are having trouble actually imagining what this hydrogel would look like as a "solid when wet and liquid when dry," it can be compared to the Terminator T-1000 of the movie, Terminator 2. This character, if you recall, was made of liquid metal and was able to "quickly liquify" and "assume different forms." Similarly, this DNA hydrogel, with the removing and addition of water, can quickly morph into a different shape. Although this hydrogel will certainly not be used for violent purposes such as Terminator T-1000, it has a variety of other applications.

Because of its chemical properties, the DNA hydrogel can be utilized for 3D tissue scaffolding and biomedical applications. In particular, DNA hydrogels are different from other hydrogels because their polymer networks, under specific conditions, are formed spontaneously. Just like we learned in class, spontaneous reactions occur without an external driving force; or in other words, these reactions can occur naturally. The polymer networks then form spheres that weakly-bond to one another. Although weak bonds, these bonds are what hold together the shape of the hydrogel. Without the bonds, the hydrogel, when wet, would simply be a mixture of DNA without a structure of some sort. What is specifically interesting to us is that when the water in this DNA mixture is removed, the DNA collapses into a liquid form, able to be poured into molds of certain shapes and to adapt to these shapes. Even when water is added back, the DNA hydrogel remembers this structure and retains this shape when more solid.

Although it is not certain how the DNA hydrogels are able to preserve their shapes, it is very useful. For example, applications such as injectable stents can utilize this behavior in order to be properly place into the body. Large quantities of water would allow the creation of the injectable stent's shape. When the liquid is removed, the injectable stent would be injected into a particular section of the body as it is a fluid, capable of traveling easily. When in the body part, the stent would revert to its original shape. To us, this is one of the most interesting inventions we have seen in the topic of hydrogels, and in terms of tissue engineering, in general.

http://www.gizmag.com/dna-hydrogel-solid-liquid-cornell-luo/25402/   

Shape-Memory Polymer Scaffolds

Researchers at Harvard University have recently come up with a method to create polymer scaffolding that can transport drugs and stem cells but doesn't have to be surgically implanted. Instead, it can be injected through a syringe and emerge in the body in the original shape it had because of shape-memory properties. They tested it using scaffolds in circle, heart, and star shapes, and found that it worked each time.

The actual chemistry behind the process of creating these scaffolds relates quite a bit to what was learned in class. The polymers undergo what is known as cryogelation, depicted in the figure below. First the polymer solution is prepared in the liquid phase. Since it is a solution, as we learned in class, the freezing point is going to undergo a depression so the mixture has to be frozen at a subzero temperature. Once frozen, polymerization occurs and ice crystallizes. Then the cryogel is left at room temperature, where it thaws and leaves behind a system with interconnected pores.

Note, however, that this polymerization only occurs in the first place due to the presence of intermolecular forces. The polymer being used is alginate, whose structure is shown below. Because of the hydroxyl groups, there are many polar oyxgen-hydrogen bonds, and the presence of ionized carboxyl groups means that hydrogen bonding will occur. This is what allows the polymers to cross-link so well and form the pores in the first place.

Without the fundamental concepts of intermolecular forces and solution composition as covered in class, these polymer scaffoldings would not even have been possible to make.

New Method to Revive Old Stem Cells

Stem cells are cells that are able to transform into cells of other types. This is particularly useful when trying to restore a part of the body internally, as they can continually divide in order to repair other cells. As stem cells grow older, however, their capability to maintain tissues declines. A new method was recently discovered that changes the stem cells so they behave more like younger stem cells, increasing their functioning ability. In particular, this method can help develop cardiac patches for those patients with damaged hearts from their own stem cells. This new procedure is specifically useful as the age of the patient will not affect the success of the stem cell restoration. 

However, in some stem cell therapies, such as those involving bone marrow, there is always a risk of patient rejection. As we discussed in one of our Diigo posts, scientists often use cells from a patient's own body to resolve this issue. We believe that this would also be helpful as patients would not have to wait around for another individual's organ or tissue donations; rather, they would have immediate access, as cells are from their own body. However, for older patients, the stem cells have become aged and function less than younger cells. Using this new technique, the stem cells from elderly patients can be utilized without being rejected.

This method was created by Milica Radisic and Dr. Ren-Ke Li, two researchers. They created conditions similar to the aged stem cells in tissue cultures, referred to a "micro-environment." In this micro-environment, which the heart tissue would grow in using stem cells from elderly patients. Various factors causing cell proliferation , or the increase in cell numbers, are added to the cell cultures. These infusions caused aging factors to be reduced and cells to be restored to younger versions. In particular, molecules such as p16 and RGN were infusions that caused this to occur. 

The molecule "p16" is a gene that is inactivated when cancers are present in humans. Similar to our topics in biology last year, genes can be inactivated or activated to cease or continue a function. In this case, c ells are usually limited in their growth through the activation of p16, but when inactivated, p16 actually causes tumors to form. Tumor suppression occurs through protein-protein interactions with p16. Specifically, p16 binds to CDK4, another gene, which causes it to be active.

The structure of p16

The bonding that takes place between p16 and CDK4 is of Intermolecular Forces (IMFs). In particular, when CDK4 bonds with other genes, hydrogen bonding networks are created. As we have discussed in class, hydrogen bonds are attractive forces between a hydrogen and an electronegative molecule., usually of a nitrogen, oxygen, or fluorine atom. The picture above shows the large number of hydrogen atoms in the structure of p16. Almost all of these hydrogens are attached to an N or an O, two atoms that are also abundant in p16's structure. This shows that hydrogen bonds exist in p16. P16 is not very structurally stable, particularly due to its "highly dynamic structure as measured by ANS-binding, NMR hydrogen-deuterium exchange, and fluorescence." Because of its weak structure, p16 also self associates and forms dimers. The Kd value of the molecule is only 270 microM for p16. Kd is the equilibrium dissociation constant of products over reactants in the reaction A + B ↔ AB. As discussed last year, it is a specific type of equilibrium constant, K, and is the reciprocal of Ka, the equilibrium association constant. 

In the future, this method can be utilized in many applications. When the stem cells in elderly patients start decreasing in function, they will be revived using this "fountain of youth" technique. Damage caused by heart attacks will be repaired, and defects such as aneurysms can be fixed as well. Although this discovery can be very useful, it might also have several drawbacks. There has always been debate over the moral issues of stem cells. With this discovery, scientists would be tampering with the naturally occurring chemicals, bonding, and genes in the body. Perhaps, for many people, this new method would not be a chemical wonder, but more of a manipulation of nature. 

http://www.newstrackindia.com/newsdetails/2012/11/28/373--Fountain-of-youth-technique-may-help-create-heart-patches-from-old-cells.html

Silk Hearts

Tying in with what we've learned in class, this article from Science Daily explains how silk from a Tasar Silkworm can be used as a scaffold for heart tissue. In other words, how they can use this silk to replace damaged heart cells that are unable to regenerate. They are hoping that this artificial tissue can restore total cardiac function in humans.

Penny-sized silk discs used for heart scaffolding.

These types of studies are so important because, throughout evolution, the heart has lost all regenerative abilities. So when people have heart attacks, all of the cardiac cells that die are lost and cannot be replaced by the heart itself. These studies are conducted to test different types of materials that can patch these dead cardiac muscles. One of their main issues is trying to reconstruct this three-dimensional structure.

Various other materials have failed to work because they were too brittle, rejected by the immune system, or did not allow the muscle cells to adhere with the fibers. Fortunately, researchers are starting to believe that this silk can be a viable material for this heart operation.

The silk has a protein structure that is able to adhere strongly to the heart muscle cells. The coarser material allows the cells to grow and form three-dimensional structures. The re-patched rat heart was able to beat as if it were healthy after being patched with the silk. This is certainly a promising sign for the future and is an indicator that silk can be very successful.

 As the figure to the right demonstrates, the structure of the silk in the Tasar silkworm is very similar to that of spider silk as discussed in a previous blog post, which is what lends it its adhesive properties.

The silk itself is composed of highly crystalline B-sheets crosslinked with less-ordered B-sheets. As discussed in class, the crystalline structure of the more ordered B-sheets allows the atoms to be more tightly connected in a pattern, whereas the less-ordered sheets have an amorphous structure. Furthermore, the pleated B-sheets exhibit hydrogen bonding. As we also learned in class, this is the strongest intermolecular force and results in the interlocking strands. It is because of these properties that the silk is so strong and so capable of adhering tightly to the human body.

Unfortunately, they have been unable to obtain the necessary amount of cardiac cells for the starting material in humans. All tests were conducted on rats and were generally successful. On humans, however, much work still needs to be done. Scientists are in the process of using stem cells in place of these cardiac cells, but trials are still in the early stages.

Tissue engineering is a very important field for improving the overall life of humans. In my opinion, the studies conducted to improve the heart are some of the more important due to the vital role that the heart plays, as well as the fact that heart issues are very common. Any way that scientists can help this problem would be very beneficial to all that suffer from heart conditions.

Injectable Shear Thinning Hydrogels

Hydrogels provide a promising new way to deliver drugs or other materials with which to regenerate damage tissue because of their flexibility and biocompatibility. Up until now, though, these could not be fully used because their solid structure tended to degrade under the harsh conditions of the human body, particularly our high body temperatures. To solve this problem, MIT researchers synthesized a special kind of shear thinning hydrogel.

Shear thinning hydrogels are notable because they can switch between a solid and liquid state depending on their surroundings. This makes them ideal as injectable gels because they're liquid when being pushed through a needle but become solid once they're inside the body. However, if the researchers left these gels as is, they would still be subject to stresses of a mechanical nature inside the body and might revert back to liquid.

To fix this, they created a self-healing, reinforcing network inside the gel that only activates when the temperature of its surroundings are body temperature. That way, their new structure would not interfere with the gel's functionality in the needle but would still be able to solve their problem of durability once inside the body.

They opted to create protein hydrogels instead of the more standard polyethylene glycol gels so that they would have better biological properties, and altered the structure so that they would have the desired characteristics. The proteins were loosely held together by links called coils to result in a rope-like structure, and then a second network was built inside that.

At higher temperatures, cross-linking results in a reinforcing network.

At low temperatures, the protein gels are hydrophilic and thus float freely within the gel. As soon as the temperature becomes high enough, though, the gel becomes hydrophobic and groups together, making polymers at the ends of the protein bind to each other. This results in cross-linking so that the gel is significantly more elastic and durable, able to withstand mechanical strength.

Painless Medical Tape

A research team at the Brigham and Women's (BWH) hospital recently created a new type of medical tape that can be removed from skin without pain. Medical tapes before this invention were very secure in terms of adhesion properties, but they damaged skin when being peeled. Current medical tapes have a lot of backing and support that allows it to stick strongly on a patient. However, when taken off, the tape used leaves some of the adhesive material on the skin. Although this does not affect all skin types, neonate skin, which is newer and more recently grown, does face more serious damage. 

Painless medical tape

The new medical tape created has three-layers which retains the strong adhesion properties of previous tape, but also adds a feature that allows the tape to be easily peeled off. There is one layer of tape that serves as the adhesive layer and another layer that serves as the backing support. A final layer exists in between these two, referred to as the anisotropic adhesive interface. The anisotropic feature means that the layer's physical properties depend on direction, opposed to an isotropic feature, which would refer to the properties of materials that are the same in all direction. For example, wood has anisotropic properties, containing lines that go in one direction. The wood contains more strength when the lines all go in one direction, causing wood to have anisotropic properties. 

Anisotropic features have advantages, being stronger in a certain direction. The scientists from BWH used laser etching and a release liner in order to make the anisotropic layer of the medical tape. This feature allows the medical tape to have a much higher shear strength as it is now dependent on direction. The release liner allows for the medical tape to be removed with little force. Solving the previous issue, if any adhesive remains of the skin, it can be gently rolled off using a finger. In other words, the medical tape would come off skin more easily. Release liners work by using silicone releasing agents, which have a low surface tension of wetting. Wetting refers to the characteristic of how a liquid deposited on another liquid or on a solid spreads out. Because silicone, a key component of release liners, has a low surface tension energy, it does not spread too much when on another layer of the medical tape and, ultimately, the skin. Thus, it can be pulled off easily without any damage to the skin. Frequently in the past, carbon-carbon backbones were utilized instead of silicone polymers, which caused a more rigid structure and, as a result, harm to skin. Silicone polymers, however, have methyl groups which interact only slightly with one another, producing little surface tension energy.  Additionally, silicone has a low adhesion in terms of sticking to the skin. When materials have diffusive adhesion, the molecules of each are soluble in the other material. They move around, and when joined, connect through diffusion. In particular, polymer chains demonstrate diffusive adhesion effectively, in which the ends of one molecule diffuses with another. 

Over 1.5 million injuries occur in the United States due to medical tape removal. Using this medical tape, this number can be reduced greatly, affecting numerous patients positively. 

http://articles.timesofindia.indiatimes.com/2012-10-30/health/34816599_1_tape-skin-release-liner 

Walking Bio-Bots

For the first time, researchers in the University of Illinois have created an autonomous robot made from plastic and living cells. With just hydrogel, heart cells, and a 3D printer, these scientists have created miniature "bio-bots" that are capable of walking by themselves, without the use of electricity.

The Bio-Bot is 7 mm long and can move on its own

Each bot has one long, flexible leg that rests on a short, stout leg for support. The long leg is covered with rat cardiac cells. It pulses like a heart and as it beats, the bio-bot propels forward. The rhythmic expansions and contractions pull the little robot forward in what looks like a swimming motion.

The hydrogel part of the robot was made by a 3D printer. This makes the production of robots easier because more shapes can be made faster and easier. Because it is easy to print these plastics, several changes can be made fairly easily. The researchers can use trial and error as much as they need to improve the design of the bio-bot.

The bio-bot's unique structure allows it to manipulate into different shapes that would not be possible if it were made of metal and hard plastics. The soft plastic and cells allow the robot to be flexible, as well as function without the use of electricity. Rashid Bashir, one of the head engineers, stated “As engineers, we’ve always built things with hard materials, materials that are very predictable. Yet there are a lot of applications where nature solves a problem in such an elegant way. Can we replicate some of that if we can understand how to put things together with cells?” So far, they have been successful in creating functioning robots.

The researchers hope that these bio-biots could be used to detect chemicals in water, climb walls, or function as a sensor to react to certain elements in the water. If the bio-bots can be made using cells that respond to different stimuli, like chemical gradients, then they can definitely be used in drug screening and chemical analysis. The way the bot's motion changes would indicate the reaction of the cells to the environment and whether or not a similar reaction may occur in humans. It is the researchers' hope that if they can get the bot to move toward chemical gradients, they could design something that can look for a specific toxin and attempt to neutralize it.

A depiction of the movement of the robot

The scientists plan to continue working on this discovery. They hope to improve the bots to develop control and function. For example, they plan to create robots that can respond to light, climb steps, and utilize different numbers of legs. They believe this is the first step of many in this exciting field that utilizes nature for beneficial purposes and functions.




http://news.discovery.com/tech/walking-bio-bot-made-with-cells-gels-121115.html
http://news.illinois.edu/news/12/1115bio-bots_RashidBashir.html