From a commercial perspective the term “nano” describes a diameter of the fibrous shape at anything below one micron – or 1,000 nanometers. However, the most coveted properties of these materials tend to manifest themselves at below 500 nanometers (.5 microns) depending on the material and the property. As one begins to operate at the molecular level there is a pronounced shift in the governing physics. For example gravity becomes markedly less significant while Van der Waals forces, which have previously been considered completely insignificant by scientists, become incredibly strong. The ability to manipulate physics and significant properties and attributes of materials has proven over time to be incredibly valuable.
High Surface Area
As the fiber diameter shrinks into the nanoscale, the surface area to volume ratio increases up to 1,000 times higher than a microfiber. This property generally tends to bestow the mat of thousands of fibers with enhanced properties over one fiber in the form of:
- Chemical Delivery – Efficiency and time of drug or cosmetic delivery to the skin
- Catalysis – Efficiency of chemical or photo catalysis of a material for a given volume
- Electron & Photon Transfer – Efficiency of electron or photon transfer or prevention thereof
- Slip Flow – Efficiency in filtration and particle separation through the generation of slip flow at the surface of the fiber
Due to the size of nanofibers, the tensile strength of an individual fiber is difficult to analyze; however, tensile strength has been shown to increase by up to 40% over the same weight of material in a bulk or larger format.
- Increase the strength of a given weight of material
- Use less materials and achieve the same level of strength
Reduced Crack Propagation
Achieving the same level of strength with less amount of material is enhanced by the fact that the probability of the failure of one fiber is much higher than the failure of thousands of fibers. Furthermore, upon the development of a structural flaw or crack, it is impossible to propagate to other discrete structural components.
Thermal conductivity testing has shown that decreasing fiber diameter into the nanoscale increases thermal resistance of popular insulative materials by almost 50%.
Many materials when processed into nanofiber dramatically improve electron transfer. Combined with the extremely thin nature of a nanofiber web, these properties have substantial benefits in energy storage, photocatalytics and sensors.
Core research and product development of nanofibers has gained significant prominence in recent years as evidenced by more than 350 related patent applications and almost 600 journal publications in 2009 (Exhibit 1).
The primary interests in nanofibers revolve around increased surface area and novel mechanical properties of materials in the nanoscale. As fiber diameter decreases the integrity of mechanical properties of that material increases by orders of magnitude. Similarly, the surface area of nanofibers is substantially greater than fibers on the micron level and higher. These factors have profound implications in a broad range of applications.
Nanofiber research is taking place across a wide spectrum of industries. Some of the more prominent applications include:
Filters are perhaps the most obvious use of nanofibers, where filter performance is based on producing the highest flow rate while trapping and retaining the finest particles without blocking the filter. Nanofibers have improved interception and inertial impaction efficiencies and result in slip flow at the fiber surface, resulting in better performance at a given pressure drop. Nanofibers are currently incorporated into commercial filters in air, liquid and automotive applications in both industrial and consumer markets by some of the largest filtration companies in the world.
As fiber diameters decrease, desired mechanical properties increase in integrity. Specifically, strength to weight ratios are improved dramatically, reducing the probability of fiber failure. In addition, the nano scale pores created by the fibrous mesh provide the potential for strong insulation properties.
Nonwoven Consumer Products
The ability to load vitamins and drugs in nanofibers and deposit them on the skin through the high surface area is being explored for cosmetic applications. Diapers, wipes and other products that will benefit from fibers that are biodegradable, biocompatible and absorbent are potential beneficiaries of nanofibers.
Nanofibers are being applied to photovoltaics and batteries due primarily to increasing surface area of certain absorbent and catalytic materials. Battery separators and even electrodes are being developed from nanofiber materials. Nanofibers generated from ceramics and metals are being developed for capacitors and photovoltaics.
Ceramic nanofibers bearing nanoparticles of several types of noble metals are being developed for catalytic applications in both automotive and industrial settings. The dramatic increase in surface area enables less of the expensive metals (Platinum, Palladium, Rhodium) to be used while achieving the same levels of performance, which leads to severe cost reduction.
Healthcare applications are many and varied, including:
- Tissue engineering: Fiber mesh is used to replicate the extracellular matrix. The primary advantage of nanofibers is associated with the porous nature of their assembly and increased surface area.
- Drug delivery: The ability for nanofibers to be made from biodegradable, non-biodegradable and hybrid materials enables nanofibers to be utilized for drug delivery for antibiotics. The high surface areas enable high drug loadings and transfer to specific sites.
- Wound care: The use of nanofiber materials for wound care enables the creation of complex layered dressings that can include multiple therapeutic benefits in a single product. In addition, nanofiber materials offer more surface exposure of active ingredients, providing the opportunity to enhance and increase efficacy of new or existing products.