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.

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.