Unraveling the Web of Mystery Surrounding Spider Silk

Until recently, there was so little we knew about the physical properties of a spider web. Spider silk is one of the most unique fibers in the world, being able to endure all kinds of abuse like stretching and soaking, and still being able to function as normal afterwards. It is stronger than steel and stronger than Kevlar, the stuff that makes up bulletproof vests, when you compare it ounce for ounce.

Now, we know a lot more about spider silk, after a team of scientists was able to measure the elastic properties of an intact spider web and this increased knowledge should help with further innovations about silk that we previously mentioned in our blog here and here.

The Stanford researchers utilized a technique known as Billouin spectroscopy, which shines a laser at the spider web and then records the scattering of light to measure the mechanical properties of spider silk. In short, it is a much more complicated form of the spectrophotometry done in freshman and sophomore labs to measure absorbance values, except Billouin spectroscopy measure the mechanical properties of a material.

Measuring mechanical properties of spider silk using Billouin spectroscopy

The researchers learned that although spider webs are made of uniform spider silk, the stiffness and elasticity of the silk varies between individual strands. They also discovered that the silk stiffens in conditions of 100% humidity to produce a tighter web, a behavior known as supercontraction. The second discovery that adjusting water content to alter the mechanical properties of spider silk is especially interesting and Kristie Koski, the lead researcher, say could lead to new and exciting advancements.

A Closer Look at Cellulose

A few weeks ago, we mentioned the promise of cellulose in medical devices due it being both biocompatible and a relatively accessible material. However, in this post we will take a closer look at the properties of cellulose that make it an ideal nanomaterial to consider, in particular how hydrogen bonding makes cellulose a very special molecule.

The orientation of glucose molecules in the chain causes the difference in strength between cellulose and starch, a strength that results from hydrogen bonding between adjacent strands of cellulose. Hydrogen bonds are very strong, which is why cellulose is an excellent fiber that is used from paper to hemp rope. The next time, you have a sheet of paper, try to pull it apart from the sides and you will notice how hard it is to do so.

Hydrogen bonding between cellulose chains also means that cellulose will not dissolve in water. Such a property both practical and useful. What this means in terms of using a medical device made of cellulose is that the medical device will not spontaneously dissolve after coming in contact with the water present in the human body. To explain the dissolving of cellulose in water in terms of free energy, the change in free energy of the reaction would be positive as the reaction is never spontaneous.  This also tells us that the enthalpy change of the reaction is positive and the entropy change of the reaction is negative. If you do not understand how one would arrive at the previous conclusion, consider the Free Energy equation:

where G is Gibbs Free Energy, H is enthalpy, S is entropy, and T is temperature.

Hopefully this post gave you greater insight about how cellulose serves as such a great nanomaterial.

An Interesting Visitor

In the span of three months since the Yellow Five blog's conception, we have received visitors from around the world. While we would like to believe our blog is being enjoyed in places like Finland and China, we know that these foreign views are most likely the result of random clicking. However, about one month ago, we received a very special visitor to our blog named Fiorenzo Omenetto. He has been featured on our blog here. Omenetto primarily does research with silk and its effectiveness as a nanomaterial. He has been named an Oppenheimer Fellow and a Guggenheim Fellow in 2011. About our blog, Omenetto commented saying:

"Yes, of course – there are so many interesting things out there and it's really nice to see you review these things and select the ones that you think are important or that strike your imagination. Keep going and put more!"

We are very thankful to Omenetto for taking the time to view our blog, and we wish him the best in his future research. To get more of Omenetto's research, check out his website here and his twitter over here. We interviewed him, and the contents of this interview can be viewed below. Once again, thanks for viewing our blog, and we hope to have another post up and running next week. See you soon!

The Interview:

Q: Can you introduce yourself?

Hi YellowFive! I'm Fio Omenetto a professor of Biomedical Engineering and a Prof. of Physics at Tufts University in Boston.

Q: What made you so interested in researching silk?

A chance conversation in the hallway with Dave Kaplan, who is now one of my closest collaborators. I didn't even know what silk was before coming to Tufts (OK, I did, but had not remotely thought about it as a photonics or electronic material, much less on the nanoscale. )

Q: What is the most exciting result of your research so far?

I think for me just the discovery of silk and the things that it can do at multiple scales, the favorable properties of the material, and how it can reinvent technology as we know it.

Q: Do you feel that the silk optics that you recently created and that we mentioned in our blog is a big accomplishment?

I think it is, but I'm biased… I think being mentioned in your blog is quite flattering! (thanks)

Q: What is the most interesting thing about being a researcher?

The fact that you have endless puzzles to solve, that you get to work with great people solving them, and that you get to imagine new things all the time (because, in fact, not all the things that you imagine or that you research work!).

Q: What do you think is the most interesting aspect about nanomaterials?

My first research love (and background) is in optics and physics – I have always liked the structural interaction of materials on the nanoscale with electromagnetic radiation – structural color, photonics band gaps and related things. I like how structure affects function, and specifically how nanostructures affect light propagation (and electromagnetic waves in general).

Q: What inspired you to look into silk and nanomaterials?

I was just trying to connect the dots between what I liked and something new.

Wood and Cellulose: Natural Nanomaterials

In our past few posts, we have primarily looked at naturally-occuring nanomaterial sources, such as a crab's claw and silk. Today, for a change, we present to you a potential source of nanomaterials that can easily be found in one's backyard - wood pulp. Before looking into the actual properties and benefits of the nanocellulose, we need to understand the physical properties of cellulose itself.

Cellulose, pictured to the right, with chemical formula (C6H10O5)n, is one of the main components of cell walls in plants. Its primary use in the industry is the manufacturing of paper; the cellulose obtained for this process can be extracted from wood pulp. Cellulose is a polar molecule, as evidenced from its various OH groups, which contribute a dipole movement. It has a high melting point of 773 K, which is a result of its IMF's. Cellulose also is a highly viscous fluid as a result; this poses a problem for manufacturers, as pulp flow is highly difficult. Despite its polarity, cellulose is insoluble in water, meaning that it does not dissociate at all.

Nanocellulose, on the other hand, has physical properties antithetic to those of cellulose. The wood pulp is taken, and gently beaten to reduce to cellulose to thin fibers. The cellulose then takes on a needle like crystalline structure, pictured to the left. The nanocellulose has its most impressive properties once it dries. It has eight times the tensile strength of steel, and is almost as stiff as Kelvar. The US Forest Service, which opened the first nanocellulose lab in the United States at Madison, WI, reports that nanocellulose can sell for just a few dollars per kilo. This has extreme economic implications; if nanocellulose is stronger and cheaper than steel, it could one day replace steel in the industry.

For more information about nanocellulose, check out the embed video right below this post. A few posts later, we will be publishing an interview we had with Fiorenzo Omenetto, Guggenheim Fellow, who worked with silk and its applications as a potential nanomaterial. Stay tuned for more posts.

Self-Healing Electronic Skin

Zhenan Bao and her team of researchers at Stanford University are striving to create electronic skin that can both register touch and heal itself the way natural skin can. Many materials scientist have worked on created flexible circuits while chemists have tackled the problem of self-healing polymers. Yet despite all the effort, no material has be discovered or created that can combine these two attractive properties. This is a big problem for prosthetics as many of these devices do not have a sense of touch, which makes many of its users complain about the clumsiness and clunkiness of current devices.  

Last year Bao's team was able to solve half of the puzzle, creating a flexible electronic skin that can sense pressure (video below), and this November, the team announced that they have come one step closer to a self-healing polymer.

In a paper published in Nature, Bao showcases her recent breakthrough regarding a self-healing electronic skin. By incorporating nickel atoms into a self-healing polymer, the Stanford team was able to arrive at a material that achieves both of the qualities needed for a self-healing electronic skin. The polymer is able to gain the electrical properties because through forces like pressure or twisting, the distance between adjacent nickel atoms changes. This affects how easily electrons can move from atom to atom, which changes the electronic resistance of the material. 

To demonstrate the effectiveness of the healing properties of the material, the researchers cut the material with a scalpel  They then help the two pieces together at the edges and after 15 seconds, electrical conductivity was restored with about 90% efficiency, and after a mere 10 minutes, the break was fully repaired and the material was once again flexible. 

John Boland of the Center for Research on Adaptive Nanostructures and Nanodevices (CRANN) at Trinity College Dublin called the development a breakthrough in an article on ScienceNOW, though he notes that scalpel cuts are very neat and the tearing and stretching of the material may affect its self-healing properties.  

Smart Materials have SMARTS

In the July 12 issue of Nature, a Harvard-led research team showcased a strategy for building self-thermoregulating nanomaterials that can be tailored to maintain a determined temperature, pressure, or any other measure by meeting the environmental changes with a compensatory chemical feedback response. 

This group of materials is called Self-regulated Mechano-chemical Adaptively Reconfigurable Tunable System, or SMARTS for short. As the name suggests, this group of innovative materials has offers a customizable way to automatically turn on and turn off chemical reactions in a way that mimics how biological systems naturally adjust to the dynamics of their surroundings. 

This development greatly benefits medical implants because it allow for more intelligent and efficient medical implants. In terms of its structure, SMARTS looks like a microscopic toothbrush with tiny fibers capable of either standing up or lying down, which results in creating or breaking contact with the later of the material that contains chemical 'nutrients'. Joanna Aizenberg, a lead author of the research, explains the structure of SMARTS as something similar to the hair on a person's arms. “When it is cold out, tiny muscles at the base of each hair on your arm cause the hairs to stand up in an insulating layer. As your skin warms up, the muscles contract and the hairs lie back down to keep you from overheating. SMARTS works in a similar way.”

At first glance, SMARTS may not seem like a big deal; there already exist glasses that dim and brighten depending on the intensity of light and piezocrystals that can convert vibrations into electrical signals, but one major downfall of these two examples is that they response to one specific stimuli and cannot self-regulate. 

A demonstration of the material can be seen in the video below. 

In the video, the stimuli used is temperature and a hydrogel is embedded with an array of tiny nanofibers, which cause the hydrogel to either expand or contract in response to the temperature changes. When the temperature drops, the gel swells, and the hairs stand upright and make contact with the ‘nutrient’ layer; when it warms up, the gel contracts, and the hairs lie down. The key aspect is that molecular catalysts placed on the tips of the nanofibers can trigger heat-generating chemical reactions in the ‘nutrient’ layer. 

Figure 1: Process of a homeostatic material maintaing constant temperature.

Aizenberg likens the process to homeostasis, saying, "The bilayer system effectively creates a self-regulated on-and-off switch controlled by the motion of the hairs, turning the reaction on and generating heat when it is cold. Once the temperature has achieved a pre-determined level, the hydrogel contracts, causing the hairs to lie down, interrupting further generation of heat. When it cools again below the set-point the cycle restarts autonomously. It’s homeostasis, right down at the materials level." 

Through additional refinement, the technique could eventually be incorporated into the material of medical implants to help stabilize bodily functions. Some examples include sensing and adjusting glucose or carbon dioxide levels in the blood. 

Supramolecular Bonds and Thermodynamics

Last week's post on supramolecular bonds facilitates itself for a new post about a topic that is taught in high schools nationwide - this post is going to be about thermodynamics, and its relation to supramolecular bonds and medical implants.

In last week's post, we said that the construction of the machines was due to the supramolecular bonds found in rotaxane. Check out this video showing macroscopic self assembly molecules.

There are many constraints, however, to the making of these machines, and this is due to the supramolecular bonds. These machines require great precision, and while one may think that this is due to their small size or delicate material, it is actually due to the thermochemistry behind these bonds. These bonds require very little activation energy for formation, like other non-covalent bonds. As a result, the making of these machines requires great precision and accuracy, as this process can be extremely spontaneous. The importance of thermodynamics becomes more pronounced in biological systems. Because these systems often operate with a narrow temperature range, it is important that the supramolecular bonds are stable so molecules stay in their conformations and molecular mechanics can continue to occur. You can check out some interesting papers about the thermochemistry of supramolecular bonds here and here. Thanks for following.

Nano-Machines Mimic Muscles

In a recent paper, scientists from France's National Centre for Scientific Research (CNRS) reported that they were successful in the development of an artificial muscle construct.This innovative work, headed by Nicolas Giuseppone, a professor at the Université de Strasbourg, and involving researchers from the Laboratoire de Matière et Systèmes Complexes (CNRS/Université Paris Diderot) uses tens of thousands of individual molecular nano-machines. The nano-machines work together to extend and contract a polymer chain over a length of 10 micrometers. The main compound in the polymer chain is rotaxane, which is a molecule that is commonly studied with regard to molecular machine construction. 

Rotaxane is an example of a mechanically-interlocked molecular architecture. What this means is that rotaxane is compound that is a combination of molecules that are not connected by bonding but rather by geometry. An analogy, which is explains this is that of a key and a key chain; the keys are not directly attached to the key chain, yet the keys cannot be removed from the chain until the chain is broken. In this case, the blue part of the rotaxane, which is a dumbbell shaped molecule, plays the role of the key while the cyclobis macrocycle is the key chain.

Crystal Structure of Rotaxane with a Cyclobis Macrocycle

The French researchers were able to extend and contract the chain by varying the pH levels of the environment. A change in pH causes the thousands of individual components to either compress or elongate by one nanometer, and these changes in size in the individual molecules add up to cause a drastic change in the size of the polymer chain. This movement in the polymer chain is significant enough and easily controllable so the chain can be used to mimic the function of real muscles and can also function as a nano-scale mechanical machine.

Image depicting the contraction and expansion of the polymer chain in a way that mimics human muscle movement.

The way scientists were able to connect thousands of rotaxane together to form a polymer chain is through supramolecular bonding.  A supramolecular bond is an interaction between different molecules that is not based on a traditional “covalent” chemical bond but instead on what are known as “weak interactions”, thereby constituting complex molecular structures. Supramolecular bonding can consist of a variety of interactions like hydrogen bonding and van der Waals forces. The strength of supramolecular bonds ranges from very weak like hydrogen bonding to very strong like covalent bonding.  It is the supramolecular bond that connects all the elements in the polymer chain together and makes the contracting and extending of the chain possible.