Mario El Tahchi

Artificial muscular microfibers: hydrogel with high speed tunable electroactivity

The advancement in polymer muscle-like actuator technologies is of immense importance in the development of biologically inspired designs for tissue substitutes where the mechanical robustness is combined with fast physico-chemical responsiveness, biocompatibility and electroactivity. The multiformability (films, fibres, tubules,…) and the multifunctionality (response to temperature, pH, electromagnetic excitation,…) of hydrogels make them a great candidate for electro-bio-active artificial implantable muscular fibers and as soft actuators for gentle micro-scale manipulations. With an end objective to conceive and fabricate a system with instantaneous response to external stimuli, we present the results of an experimental method used to prepare long thin self-assembled hydrogel fibers with aspect ratio greater than 105 (length/diameter). We show for the first time that these fibers are electroactive and dynamically tunable under low applied electric field (as low as 500 V m−1) with high potential in instantaneous sensing of pH and ionic strength variations in their surrounding medium.

Culture medium pH influence on Gluconacetobacter physiology: Cellulose production rate and yield enhancement in presence of multiple carbon sources

Gluconacetobacter genera are valued for bacterial cellulose (BC) and acetic acid production. BC is produced at optimal yields in classical microbiological media that are expensive for a large scale of production. In addition, BC usage for industrial purposes is limited due to low conversion rate into cellulose and to long incubation duration. In this paper, Gluconacetobacter isolated from apple vinegar was kinetically studied to evaluate cellulose production in presence of different carbon sources. Acetic and citric acid effect on Gluconacetobacter metabolism is clarified. It was shown that Gluconacetobacter uses glucose as a primary carbon source for cells growth and products formation. Acetic acid employment as a co-carbon source in Hestrin Schramm medium showed an increase of 17% in BC yield with a moderate decrease in the crystallite size of the resulting polymer.

Electropolymers for Mechatronics and Artificial Muscles

This chapter covers the main features and material examples of the electric fieldactivated electropolymers used for electromechanical applications. This class of electropolymers is very attractive in performing the energy conversion between the electric and mechanical forms and hence can be utilized as both solid-state electromechanical actuators and motion sensors. These polymers are also attractive for artificial muscles and for energy-harvesting applications. As will be discussed, the electromechanical response in this class of polymers can be linear, as in typical piezoelectric polymers and electrets, or nonlinear, as the electrostrictive polymers and Maxwell stress-induced response. Most of the piezoelectric polymers under investigation and in commercial use are based on poled ferroelectric (FE) polymers, including poly(vinylidene fluoride) (PVDF) and related copolymers. This chapter will discuss in detail the properties of these FE polymers. In comparison with the electromechanical responses in inorganic materials, the electromechanical activity in these polymers is relatively low. To significantly improve the electromechanical properties in these electropolymers, new avenues or approaches have to be explored. From the basic material consideration, these approaches include the strain change accompanied with the molecular conformation change, due to the polar vector reorientation, from the morphology change due to the ordering degree change in the interfacial layer between crystalline and amorphous regions, and from the Maxwell stress effect in soft polymer elastomers. This chapter will discuss the recent advances based on those approaches, which have produced remarkable improvements in terms of the electric field-induced strain level, elastic energy density, and electromechanical conversion efficiency in the electropolymers.

Photovoltaic Properties of Polyaniline-Titania Composite for Hybrid Solar Cells Applications

Solar energy harvesting has been extensively studied in the last three decades to provide a green energy source. Hybrid photovoltaics (HPV) based on titania (TiO 2 ) are researched for their easiness of production and low cost. Nanostructured mesoporous titania films and conductive polymers were used recently to form hybrid solar cells [1]. TiO 2 , mainly an n-type semiconductor with a band gap of 4.2 eV, is employed in several applications from which paints form the highest world use of titania making it an attractive material to use in HPV industry. On the other side, our targeted conductive polymer is polyaniline (PANI), a hole conductor polymer, which is used in such HPV cells due to its high charge-carriers mobility, absorption coefficient in the visible range and environmental stability. PANI and nanocrystalline TiO 2 films fabricated using spin coating or layer by layer assembly techniques behave as a p-n heterojunction diode and can be used as solar cells [2-4]. Precursor solutions are prepared by polymerizing aniline-HCl inside an aqueous solution of titania. To study the effect of the precursor concentration on the PANI-TiO 2 composite, polymerization of aniline is held in diverse TiO 2 concentrations in water. Industrial grade TiO 2 powders with particle size ranging from 200 nm to several μm are used. PANI-TiO 2 precursor solutions are dip coated or slot dyed on various substrates such as PMMA, PET and PP, all with metal oxide conductive coatings. Bulk PANI-TiO 2 pellets are prepared for comparison. The electrical and photovoltaic properties of the obtained films and pellets are investigated to choose the optimum blend composition for HPV cell. Finally a theoretical study and an analytical model of the HPV cell are presented relating the size of TiO 2 and PANI particles and their respective geometrical distribution inside the blend to the transport characteristics of charge carriers and the overall efficiency of the HPV cell.[1] M. McGehee, MRS Bulletin, Vol. 34, No. 2, February 2009.[2] Z. Liu, W. Guo, D. Fu and W. Chen, Synthetic Metals, Vol. 156, pp. 414–416, 2006.[3] Z. Liu, J. Zhou, H. Xue, L. Shen, H. Zang and W. Chen, Synthetic Metals, Vol. 156, pp. 721–723, 2006.[4] X. Zhang, G. Yan, H. Ding and Y. Shan, Materials Chemistry and Physics, Vol. 102, pp. 249–254, 2007.

Synthesis and characterization of a low-cost aerogel structure using classical drying methods for thermal insulation

Purpose Lightweight insulators are of major importance for the industry, in particular for thermal insulation in buildings. The purpose of this paper is to present a low-cost method to obtain aerogels with high insulating properties. Design/methodology/approach Silica aerogel structures have been obtained by a low-cost sol-gel synthesis method. Surface modification, using dimethyldichlorosilane and trimethylchlorosilane, has been applied in order to get different thermal and optical properties. Findings The samples show very promising thermal properties as tested through an infrared camera observation. A 5-mm-thick piece of aerogel is able to withstand the temperature of a brazing flame and reduce the heat by at least 150 degrees on its opposite side. The samples have a vitreous like structure as observed by SEM, some surface cracking is observed but they stay within acceptable limits. Originality/value A very good thermal insulator is obtained with a very simple and low-cost method. The bulkiness of the supercritical drying is eliminated and the obtained structure has good properties.

A New Method to Increase the Magnetoelectric Voltage Coefficients of Metglas/PVDF Laminate Composites

The magnetic flux density inside a Metglas sheet is much higher than that of the applied external magnetic field due to its high magnetic permeability, which is known as the magnetic flux concentration effect. Magnetic flux concentration of Metglas as a function of its sheet aspect ratio (width/length) was investigated for Metglas/Polyvinylidene fluoride (PVDF) laminar composites. Both the simulations and experimental results suggest that the magnetic flux concentration effect is markedly enhanced when the aspect ratio of a Metglas sheet is reduced. Consequently the magnetostriction of Metglas and the magnetoelectric (ME) voltage coefficients of the laminar composites are enhanced. The ME voltage coefficient for a laminar composite with a 1 mm wide and 30 mm long Metglas sheet (25 μm thick) is 21.46 V/cm•Oe, which is much higher than those reported earlier in similar laminar composites without making use of the flux concentration effect. The results demonstrate an effective means to significantly enhance the sensitivity of the magnetostrictive/piezoelectric composites as weak magnetic field sensors.