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new materials

Smaller and more efficient

Nanofibers will make micro organism filtering and controlled release of pharmaceutical compounds more efficient

EDUARDO CESARPolymer nanofiber mat is more resistant to rupturesEDUARDO CESAR

Sophisticated fibers able to retain viruses, bacteria and extremely fine solid particles of liquids or gases, capsules coated by bio-absorbing membranes that release the medical drug into the organism over a given period of time and substrates for the growth of biological organs and tissues are the latest materials being developed in various places around the world. Polymer nanofibers are the main raw material in these products. The production process of polymer nanofibers, also being perfected, is dominated by few research centers around the world. Brazil now has a center of this kind, thanks to the work of the team headed by chemical engineer Rosario Elida Suman Bretas, a professor at the Reology Lab of the Federal University of São Carlos/UFSCar’s Material Engineering Department.

The research team has developed and filed a patent for two nanofiber production processes, which is conducted by means of a process called electrospinning. This process uses an electrical charge to draw fibers from a liquid. The first patent request, related to the production of polyamide 66 nanofibers (or 66 nylon) was financed by French multinational company Rhodia, which participated in the research project and filed the related patent in France. The second process, related to the production of nanofibers from polyamide polymer 66 nanocompounds (PA66) with montmollironite clay (MMT), had the financial support of FAPESP and the related patent request was filed in Brazil. Polyamide 66 is a widely used polymer in the production of textiles, internal reinforcement of tires, sutures, and lines for fishing rods.

Polymer nanofibers resemble a plastic wire composed of polymers or polymer compounds, the thickness of which corresponds to nanometers (1 millimeter divided by 1 million). They are thousands of times thinner than a hair strand or an ordinary textile fiber. They are currently being used by a few companies around the world, among which are the US’s eSpin Technologies, South Korea’s Nanotechnics and Japan’s Kato Tech – to manufacture filters able to retain pollutants of nanometric sizes. One of the material’s main characteristics is the extensive surface area, which provides a contact surface with the external environment that is significantly bigger than the contact surface of fibers produced by traditional methods, whose microscopic dimensions can be seen with the naked eye. “A fiber’s specific surface area or volume is in inverse proportion to its diameter. This means that the nanofibers have a more extensive area for the same volume of fibers, which is very important for a number of applications”, explains Rosario Bretas. “All the processes linked to surface phenomena, such as filtration, for example, are increased through the creation of this enormous surface area”, add materials engineer Thomas Canova, research and development manager at Rhodia Poliamida.

Therefore, the larger the fiber area, the higher the quantity of pharmaceutical compounds released into the organism by the bio-absorbable membranes (which are absorbed in the form of capsules by the human body or by adhesive patches placed on the skin) over a specific period of time. The same happens with devices for the growth of cells in capillary veins and organs and for the filtration of particles or pollutants. In this case, the larger the area of the fiber, the higher the quantity of pores and the better  the retention in the particles.

EDUARDO CESARIn the lab, syringe with electrode drips nanofiber to form the matEDUARDO CESAR

Another important characteristic of polymer compound nanofibers is the possibility of manufacturing fibers with better properties than those of the conventional fibers. This is possible because the compounds are produced from a combination of one polymer with a nanometric-sized particle. These particles, in turn, are able to improve the mechanical properties of a product, such as elasticity and resistance to rupture, the capacity to act as a barrier to various gases and to increase the level of biodegradability. “This is similar to mixing fiberglass and nylon, to increase nylon’s resistance”, says Rosario. Montmollironite, a kind of clay that provides 66 nylon with higher mechanical resistance, was the particle added to the polymer created by the researcher. The challenge of improving the properties of the compound is to make sure that each nanoparticle is widely dispersed and distributed throughout the entire polymer.

Although polyamide 66 is not biodegradable, the group from UFSCar is already working on the development of nanofibers from biodegradable and bio-absorbable polymer nanocompounds, the matrix of which are polyprolactone, poly lactic acid and polyhydroxibutirate polymers, among others. Montmollironite clay and carbon nanotubes are used in all the cases. Carbon nanotubes are cylindrical particles formed from carbon allotropes. “The main objective of these new studies which began two years ago, is to produce bio-absorbable compound polymer structures to support cell growth in site (on the human skin or mucosa to release drugs or help cell growth) and electricity-conducting compounds”, says Rosario.

According to the professor from UFSCar, the partnership with Rhodia was crucial to the success of the research project. “The company provided the synthesized polyamide 66 especially for electrospinning, that is, with specific molecular weight and adequate chemical composition. This allowed the polymer solution to have the ideal viscosity, conductivity and surface tension for electrospinning”, she points out. According to the researcher, polymer fibers with micrometric diameters can be manufactured by traditional spinning methods (fusion and coagulation spinning, for example), but electrospinning is the only technique able to produce nanometric polymer fibers. This method, created more than 70 years ago, has already resulted in more than 30 patents in the United States alone.

An electrospinning system is basically comprised of four kinds of equipment: a spinneret, which can be a hypodermic syringe needle; a copper or other metal electrode, a high-voltage (up to 30 kilo volts) direct current power supply, and a grounded collector, such as a rotating drum, to collect the nanofibers. During the electrospinning process, the polymer solution – the polymer plus the solvent – is placed inside the spinneret. Because of the surface pressure, it remains inside, without flowing out. Then, the metal electrode is placed into the solution and connected to the power supply. Electric pressure is applied and when a specific electric field is reached, the polymer solution inside the syringe starts to flow forming a jet.

This outflow occurs because when the electric pressure is applied to the polymer solution, an electric charge is induced at the surface of the droplet on the tip of the spinneret. “The mutual repulsion of charges produces a force that is directly opposed to the surface pressure”, explains Rosario. As the intensity of the electric field increases, the surface of the droplet at the tip of the spinneret stretches and forms a cone. When the electric field reaches a critical value, in which the repulsive electric force is higher than the force of the surface pressure, a jet of the polymer solution is produced at the tip of this cone. As the jet moves through the air, the solvent of the polymer solution evaporates, forming a polymer nanofiber. This nanofiber is deposited under the collector in the form of an unwoven nanofiber mat.

According to Rosario, electrospinning is the only known technique for the manufacturing of polymer nanofibers. To produce metal nanofibers, manufacturers can resort to chemical electroplating. The use of relatively high electric currents, the low productivity of the process and the need to use solvents, some of which are toxic, are the main drawbacks of electrospinning in comparison to conventional spinning methods. “The solvents employed in the process have to be evaporated. This is why it is better not to use toxic solvents. In our research studies, we use water, acetone, dichloromethane, and formic acid, which are not highly toxic solvents”, says Rosario, who claims she does not know of any other Brazilian research team that has been able to develop polymer compound nanofibers of 66 polyamide with montmollironite. “In Brazil, a group from the University of São Paulo’s Chemistry Institute has been working on this technique for a long time, but they use other kinds of polymers”.

MÁRCIA BRANCIFORT / UFSCARElectronic microscopy of polymer nanocompound nanofibers with montmollironite clay MÁRCIA BRANCIFORT / UFSCAR

The research work conducted by the group from UFSCar has partnered with researchers from other universities in Brazil and abroad. Professors Rodrigo Lambert Oréfice and Alfredo Góes, from the Federal University of Minas Gerais/ UFMG, are working on bone cell growth on the compound structures developed by Rosario. Another partner is chemist Luc Averous, from the Polymer Engineering for High Technologies Lab at France’s University of Strasbourg. Averous developed a new electrospinning method about to be patented; he is a specialist in the synthesis of biodegradable and bio-absorbable polymers. The partnership agreement has a two-fold objective. The first objective is the use of new bioabsorbable polymers he synthesized in future research projects. The second objective is to conduct comparative studies of the pioneer electrospinning method developed by his group with the method developed by the researchers from UFSCar.

Another partnership was established with Canada’s University of Alberta. The objective of this partnership is the research work conducted by professor and chemical engineer Uttandaraman Sundararaj, who developed gold and silver nanofibers through an electroplating process on aluminum oxide. He used this material to manufacture nanocompounds and added polystyrene. The nanocompounds can be used as piezoelectric sensors (that generate an electric field from the action of a mechanical effort), electric discharge systems and electromagnetic interference shielding, among other applications. “Our aim is to make these nanocompounds from the compound nanofibers of a conductor polymer with carbon nanotubes. The electric tests will be conducted at the University of Alberta”, says Rosario. The partnerships with the universities in Minas Gerais, France and Canada have the support of FAPESP and are part of a theme project coordinated by Rosário. Professors Elias Hage Júnior and José Alexandrino de Sousa, from UFSCar, are the head researchers in this project.

In regard to the two patent requests that have already been filed, the nanofiber production project, which was started in 2003, has resulted in the publication of four scientific articles in Brazilian and foreign journals. Two papers were presented at the 41st International Symposium on Macromolecules – Macro2006, held in Rio de Janeiro in July 2006, and at the Annual Meeting of the Polymer Processing Society, held in Italy in June 2008. The research studies conducted at UFSCar also had the support of the Conselho Nacional de Desenvolvimento Científico e Tecnológico/CNPq agency, which financed three scholarship grants.

The Projects
1.
Nanostructured polymer systems: processing and properties (nº 06/61008-5); Modality Theme project; Coordinator Rosario Elida Suman Bretas – UFSCar; Investment R$ 1.182.988,99 and US$ 643.499,18 (FAPESP)
2. Obtaining nanofibers through electrospinning (nº 07/57359-0); Modality Intellectual Property Support Program; Coordinator Rosario Elida Suman Bretas – UFSCar; Investment R$ 6.000,00 (FAPESP)

Scientific article
Guerrini, L. M.; Branciforti, M. C.; Canova,T.; Bretas, R. E. S. Electrospinning and Characterization of Polyamide 66 Nanofibers with different Molecular Weights. Materials Research. v. 12, n.2. 2009.

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