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. 

Gritty Graphene

Graphene: The gritty nanomaterial

Over the past few weeks, we have looked at various different nanomaterials that can serve as possible candidates for implants; from hydroxylapatite to silk, all these materials can be used to make significant upgrades on the current titanium implants in the medical field today. In this post, we will be looking at another nanomaterial - this time however, the nanomaterial is completely composed of carbon. Graphene will be the focus of this post today, as graphene is an extremely strong yet extremely thin material, as mentioned in the blog Graphene Mining. Graphene Mining goes on to mention that graphene is about 10 times stronger than titanium - you can check that out, along with some other interesting facts about graphene over here. Due to this fact and its size, graphene presents itself as another innovation in the field of implants.

An interesting article found by Graphene Mining talks about Mark Cheng, a scientist who wants to study the use of graphene as a material of implants.

New research aims to use Graphene to build better neural implants to treat blindness, spinal cord injury, etc. bit.ly/TBYLUE #greg
— Graphene Mining (@Chemblue1) December 2, 2012

Cheng is researching the possibility of neural implants made of graphene. The one important aspect of neural implants is the ability to send and receive electrical impulses, as neural implants are going to be used in the brain. Current implants, which are made of a combination of titanium and plastic as mentioned here (link), do not conduct electricity very well. Graphene, however, is an extremely good conductor of electricity, and this can be seen with some EChem (or electrochemistry). At the Harbin Engineering University, researchers have demonstrated the conductivity of graphene by producing a manganese dioxide-graphene composite through the following redox reaction.


The redox reaction used in the experiment.

Along with dissipating greater charge, graphene is also extremely thin and small, making it a lot easier to implant. With every innovation, there is a setback, and in this situation, graphene’s flexibility impedes it from being extremely effective. Graphene’s flexiblity makes it more difficult to implant into the cranium. Cheng, however, has devised an interesting solution; he suggests the use of a silicon backbone to make it easier to implant the graphene into the brain.

Graphene presents itself as an interesting alternative for neural implants. This follows from its flexibility, strength, conductivity, and size. This post, of course, could not have been possible without the blog Graphene Mining. Once again, take a look at their blog over here. Also, stay tuned for a post on their blog about nanoimplants as well. See you soon!

Implantable Silk Optics

In one of our earlier posts, we introduced you to the work of Fiorenzo Omenetto and a TED Talk he gave about the potential applications of silk in the medical industry and how silk can be used to fabricate better medical implants. Now, Omenetto and his team of researchers at Tufts University have created an silk optical implant.

A microscopic image of a silk optical implant embedded with gold nanoparticles

The optical implants are made of a purified silk protein and arranged in microprism array by pouring the solution of silk protein into molds of multiple micro-sized prism reflectors. After the cast solution dries, a silk sheet is formed that looks very similar to reflective tape. The benefit of fabricating silk sheets in the MPAs is that when the silk films are implanted into the body, reflect back photons that are ordinarily lost with reflection-based imaging technologies, which allows for enhanced imaging even in deep tissue. 

These silk optical implants, which are an improvement to current tissue imaging techniques, can do much more than simple providing imaging for doctors. The devices can be engineered to also administer therapeutic treatments to patients. The researchers are able to embed both gold nanoparticles and the cancer drug doxorubicin into the silk sheets to treat targeted parts of the body were the silk sheets can be implanted. The benefit of this is that it allows for the precise delivery of drugs while also providing a mechanism with which to monitor the body with. When silk sheet is no longer needed, it dissolved into the body, doing no harm because it is biocompatible.

For more information visit Fiorenzo Omenetto's website.