A simple paper sheet can improve the quality of life for millions of people by removing resistant viruses from water. The sheet, made of cellulose nanofibers, is called the mille-feuille filter because its unique layered internal architecture resembles that of the French puff pastry mille-feuille (thousand leaves).
“With a filter material directly from nature, and by using simple production methods, we believe that our filter paper can become the affordable global water filtration solution and help save lives,” says Albert Mihranyan, professor of nanotechnology at Uppsala University (Sweden), who led the study. “Our goal is to develop a filter paper that can remove even the toughest viruses from water as easily as brewing coffee.”
Globally, more than 748 million people lack access to safe drinking water and basic sanitation. Improving access to safe drinking water is among the United Nation’s Sustainable Development Goals. Water-borne infections are among the global causes for mortality, especially in children under age of five, and viruses are among the most notorious water-borne infectious microorganisms. They can be both extremely resistant to disinfection and difficult to remove by filtration due to their small size.
Today we heavily rely on chemical disinfectants, such as chlorine, which may produce toxic by-products depending on water quality. Filtration is a very effective, robust, energy-efficient, and inert method of producing drinking water because it physically removes the microorganisms from water rather than inactivates them. But the high cost of efficient filters limits their widespread use.
WHAT ARE NANOFIBERS?
According to the National Science Foundation (NSF), the term nano refers to particles, structures or devices having at least one dimension less than 100 nanometers. In the textile industry, the word nanofiber is used to identify fibers with diameters less than one micron (i.e., 1,000 nanometers).
The filtration industry, which comprises the industrial, consumer and defense segments, represents the oldest field of application for nanofibers. Nanofibers allow for the fabrication of filtration media capable of retaining contaminants as small as 200 nanometers, including viruses, bacteria, multivalent ions and ultrafine particulate.
COMPOSITION, EXTRACTION AND MANUFACTURING OF CELLULOSE NANOFIBERS
Cellulose can be extracted from several natural sources, including wood, sea weed, rice straw, potato and cassava tubers, sugar cane, sugar beets and sweet potatoes. Cellulose can also be obtained from organic compounds produced from certain types of bacteria, known as
bacterial cellulose,
according to Margareth Gagliardi, a BCC Research analyst.
Cellulose nanofibers, also called nanofibrils, represent one type of cellulose nanostructures, which are generally designated with the term nanocellulose. Nanocellulose also includes cellulose nanoparticles (a.k.a. nanocrystals or nanowhiskers) and bacterial nanocellulose.
Cellulose nanofibers are typically manufactured by micro- or nanofibrillation of wood pulp produced during traditional paper-making processes, explains Gagliardi. Electrospinning is another process gaining popularity for the fabrication of cellulose nanofibers.
She says cellulose nanofibers exhibit several unique characteristics, including high porosity, high
mechanical strength, low thermal expansion, light weight and optical transparency, that make them suitable for applications such as separators for lithium ion batteries, membranes for supercapacitors, functional filters, transparent substrates for flexible displays, and materials for immobilization of enzymes and vitamins.
SMALLER PORES IN PAPER TRAP DEADLY VIRUSES CHEAPLY AND EFFECTIVELY
The pores of cellulose, one of the most common filtering media used in daily life from tea-bags to vacuum cleaners, are too large to effectively remove smaller viruses. The filter paper developed by Mihgranyan’s team has pores petite enough to stop even the smallest viruses from getting through. The filter’s pastry-like layers come from hot-pressing cellulose nanosheets from Cladophora, a green algae.
“Safe drinking water is a problem not only in the low-income countries,” Mihranyan notes. “Massive viral outbreaks have also occurred in Europe in the past, including Sweden.” In 2008, a massive viral outbreak in a Swedish municipality infected almost 20% of the local population with Norovirus due to poor water.
A successful filter should not only remove viruses but also offer high flow, low fouling, and be durable, features which make advanced filters expensive to develop. The mille-feuille filter seems to realize the long-awaited shift to affordable advanced filtration solutions.
According to Gagliardi, nanofiber products for the mechanical/chemical/environmental sector are forecast to remain the largest segment in 2020, with global revenues of nearly $1.1 billion, corresponding to a 53.5% share of the total market. This segment is projected to grow at a 32.8% CAGR. Filtration media for water treatment and air purification will continue to represent the main driver for market growth.