By Yagmur Kavukcuoglu
A nanofiber is a substance made up of a polymer and is about 50 to 500 nanometres in diameter. The image on the left shows a strand of hair, a mere piece of pollen in comparison to the extensive network of nanofibers in the background. Scientists and engineers often use different polymers to form different types of nanofibers, each used in different situations. This can include batteries, fuel cells, regenerative bio-tissue, and more. As for the method of construction, nanofibers are typically made by a process called electrospinning. Electrospinning is a method by which a high voltage electric field is created, and the polymer used is pulled and stretched from one charged end to the oppositely charged end. This as a result creates a long, thin strand which is then spun directly onto a base layer which creates backing and support. The image on the right demonstrates this process in further detail.
Use of nanofibers in spinal injuries:
A specifically engineered material can be injected into the damaged spinal cord to help prevent scars and encourage damaged fibres to grow. This material, formed by engineers and microbiologists, contains molecules that self-assemble into nanofibers, which then act as a scaffold on which nerve fibres grow. Nerve cells, when damaged, scar and block nerve fibres so signals can no longer be carried to and from the brain across the central nervous system (CNS). Researchers are able to essentially inject a liquid directly into the spinal cord. The negatively charged molecules in the liquid start clumping together when they come in contact with positively charged particles like calcium and sodium ions in the body. These molecules then self-assemble into nanofibers which form a scaffold that can trap cells. On the surface of these nanofibers are biological molecules that stop scars from forming and encourage nerve fibres to grow.
In 2015, a directed research project where mice were injured by the spine and within 24 hours the liquid material was directly injected was conducted Results showed that the material significantly reduced the size of scars and stimulated the growth of nerve fibres through the scars considerably more than the control set of mice. Additionally, further data showed that the material promoted the growth of both nerve types: motor nerves (carry signals from brain to an effector (e.g., a muscle)) and sensory nerves (carry signals to the brain). In addition to this, the material boosted nerve stem cells (stem cells are cells that can differentiate into multiple different cell types) to become cells that create myelin. Myelin is an insulating layer around nerve fibres that allow them to conduct signals more effectively.
Another experiment showed that self-assembling biodegradable scaffolds made of nanofibers helped repair brain damage and return vision to mice who had been purposefully blinded. The fibres are made of peptides, which can be seen as the same as proteins. These peptides assemble themselves into fibres roughly 10 nanometres in diameter when they come in contact with salt levels found in the body in a process similar to the previous experiment. The peptides were then injected into the mice and formed networks of fibres that bridged severed brain tissues and allowed neural pathways to grow. Effectively demonstrating the success of the technology, the mice that were treated ended up with a wider range of movement and clearer sight than at the start of the experiment.
Use of nanofibers in lithium-ion batteries:
Lithium-ion batteries are a type of rechargeable battery that is charged and discharged as lithium ions move between the negative (anode) and positive(cathode) electrodes. Uses in the modern world include mobile phones, medical equipment, power tools, electric vehicles and more. The negative terminal is typically made-up carbon and the positive terminal is made up of lithium-cobalt oxide. During charging the positive electrode gives off some of its lithium ions and they travel across the electrolyte to the negative electrode in which they are stored. As this occurs, the battery then takes in and stores energy. When discharging, the ions simply move back to the positive electrode, releasing the energy that powers this battery. Lithium ions can be extremely useful batteries as they are extremely light and have a very high energy density as lithium is extremely reactive. Therefore, large amounts of energy can be stored in its bond. They also have no memory effect, so can be charged before they are completely discharged, unlike some batteries. Lastly, these batteries have electronic controllers that control the rate of charging and discharging to prevent overheating of the battery itself.
Scientists have found they can use carbon/ silicon nanofibers in a lithium-battery anode. By mixing silicon nanoparticles (roughly 30-50 nanometres in diameter) with several solutions and then heating to high temperatures in argon to form a carbon coating on the silicon particles, the nanoparticles change from yellow to black. Then using electrospinning, as stated before, nanofibers are formed from these carbon-coated silicon nanoparticles. Scientists then conducted two experiments where they used small discs of a mat of these particles or ground them into dust to test their electroconductivity in their use for lithium-ion batteries. Results showed that the carbon layer enhanced the electrical connection and bonding between Si particles and the ‘mat’ but having the particles in dust form proved to be more durable in electrical conductivity than the mat. The electrical conductivity of this material proves it to be an area of interest for use in lithium-ion batteries.