A novel method was used to produce a self-assembling 3D filter
FOR SESHADRI Ramkumar the task was cut out - developing a higher efficiency next generation chemical warfare suit. The warfare suit included developing a lightweight facemask. Higher efficiency was the bottom line and Dr. Ramkumar, who is an Assistant Professor at The Institute of Environmental and Human Health at Texas Technology, turned to nanotechnology, his area of specialisation, for answers.
"Adsorption was one way to provide higher efficiency suits to protect personnel against agents used in chemical warfare," Dr. Ramkumar said. "So we turned to nanotechnology to produce material with this characteristic." The micron to submicron size of the deadly chemicals used in chemical warfare, such as nerve agents, meant that the protective suit made of conventional high efficient particulate air (HEPA) filters were no good. "The HEPA filter can filter out particles greater than ten micron size," he noted, "so we had to produce filters of nano size."
Larger surface area
The smaller size of the material used in nanotechnology meant that the surface area was larger and hence provided more area for adsorbing chemical compounds. Using natural nanometer size particles and sticking to standard substrates is one way of producing nano filters. But these have their own problems and limitations.
Though the size of nano filters provided a large surface area for adsorption, what he achieved was producing a 3D honeycomb like structure, which is essentially a mesh within a mesh. This mesh within a mesh structure greatly enhances the surface area and also reduces the pore size and makes them largely uniform. All these enhance the entrapment capability of the filter.Dr. Ramkumar used the well known electrostatic forces to produce electro spinning of the fibres. In this, a polymer is first extruded into thin strands. As the strands become dry the moment they come out, an electric charge of 10-20 kilovolt is given to charge them. This results in the strands splitting into nanofibres. The charged nanofibres are then attracted to a collector screen, which acts as a substrate. The substrate is grounded and helps to remove the charge immediately from the deposited fibres. Usually, a metal is used as a collector screen and this allows the fibres to get rid of the charge. "This resulted in irregular shaped nanofibre structure," he explained. "What we did was replacing a conducting substrate with a non-conducting substrate made of materials such as glass and cotton."
With the conductivity of these materials [glass and cotton] being less compared with metal, the team observed that the fibres that were charged and got deposited on a collector screen still retained some charge. The amount of residual charge was not constant. And the fibres, of say, a positive charge, repel other fibres that are similarly charged, thus forcing them to get deposited elsewhere on the collector screen.
The team was in for a great surprise when a cotton fabric substrate was used as a collector screen. A self-assembling 3D honeycomb pattern was formed; this was unlike the fibres that were aligned in a straight manner when the collector screen was a metallic mesh. The paper by him and his team, published recently in the Journal of Applied Polymer Science, notes that it is the first time that a self-assembling phenomenon was seen in polyurethane electrospun nanofibres. He, however, does admit that self assembling in an orderly manner is beyond their control and that the pore size and shape will never be uniform as the size is in the nanometre scale.
Some order achieved
"We were able to achieve some order and not just a fibre web in a highly chaotic nanofibre system," he said. The "influence of the electrical properties of the collector on the self-assembling process" is far from being understood, the paper states. Yet the basic understanding of the way to influence and form a self-assembling 3D honeycomb structure will go a long way to produce next generation warfare protective gear. The pore size in such a gear will be smaller and the surface area larger for successful adsorption of lethal chemical compounds.
Power of destruction
The next generation nerve agents will combine the chemical with a biotoxin. "So there is no use in just making a mask unless the mask is able to not just adsorb but also destroy the agents," he said. Breaking the double bond between oxygen and phosphorus of the lethal organo phosphorus compound , which is a commonly used agent, was possible by making the metal oxides catalytically degrade the lethal compounds to form metal complexes. "Metal complexes [thus formed] are inert material," he noted.