Piezoelectric polymers may be key in the manufacturing of future generations of connected devices. Thanks to their capacity to generate electricity under mechanical stress, they could greatly reduce the energy consumption of intelligent sensors and make them autonomous. But in order to achieve this, researchers must be able to make these polymers compatible with classic production techniques in the plastics processing industry. All the possibilities are being considered, from 3D printing to bio-based materials.
Certain materials are able to generate an electric charge on their surface when deformed. This property, called piezoelectricity, has proven to be of particular interest in the field of connected devices. “Wearables”, which are intelligent devices worn on the body such as exercise trackers, are regularly subject to torsion, flexion and compression. By using piezoelectric materials in their design, there will be less need for batteries, reducing charging frequency and consequently increasing their autonomy. Piezoelectricity, although limited to ceramics since its discovery at the start of the 19th century, is gaining ground in the sector of polymers thanks to growing demand for flexible or transparent connected devices.
Nevertheless, these new plastics “will not compete with ceramics in their fields of application” warns Cédric Samuel, Materials Researcher at IMT Lille Douai (formerly Mines Douai and Télécom Lille). The coefficients that quantify the electricity produced by the piezoelectric effect are 15 times smaller for polymers than for ceramics: “30 picocoulombs per newton for the most efficient polymers, compared with 500 for ceramics” the scientist explained. But connected devices do not need high coefficients, since they only consume a small amount of energy. On the other hand, they require materials that are inexpensive to manufacture, a specification that would be met by piezoelectric polymers if researchers could make them compatible with classic production techniques in the plastics processing industry.
The researchers’ challenge – and it is a considerable one – lies in the processing and shaping of such materials. PVDF, which is currently the most efficient piezoelectric polymer, is far from easy to process. “Only a single type of PVDF crystal — the beta form — has piezoelectric properties,” Cédric Samuel explains. To obtain this form, PVDF must be deformed by more than 200% by stretching, at temperatures between 90 and 100°C. “This requires numerous processing and post-processing stages, which complicates the process and increases production cost” the researcher continued. Alternative options must be found in order to obtain a large-scale and inexpensive processing and shaping solution for piezoelectric PVDF.
Researchers are exploring various other possibilities. Working with the University of Mons (Belgium) through a co-supervised PHD thesis, IMT Lille Douai is concentrating more particularly on polymer blends combining PVDF with another plastic: PMMA. This provides two advantages. Not only is PMMA less expensive than PVDF, but the combination allows a piezoelectric form of PVDF to be obtained directly through extrusion. Scientists thereby skip several stages of processing. “The downside is that it leads to a lower piezoelectric coefficient,” Cédric Samuel points out, before adding, “but then again, applications for piezoelectric polymers do not necessarily need huge coefficients.”
Piezoelectric polymers through 3D printing
Although polymer blends are an option worth studying to improve processing of piezoelectric PVDF, they are not the only possible solution. Through the Piezofab project, which involves the two Carnot institutes of the IMT (M.I.N.E.S Carnot institute and Télécom & Société numérique Carnot institute) alongside IMT Atlantique (formerly Mines Nantes and Télécom Bretagne) and IMT Lille Douai, researchers are aiming to create sensors and electric generators from piezoelectric polymers through 3D printing. “We seriously believe we can succeed, because we have sufficient background on polymer-based additive manufacturing thanks notably to the expertise of Jérémie Soulestin on the subject,” declares Cédric Samuel confidently.
Researchers at IMT Lille-Douai will endeavor to test the feasibility of the process. To do so, they will work on a modified form of PVDF supplied by their partner PiezoTech, a company which is part of the Arkema chemicals group. This PVDF has the specificity of directly crystalizing in the piezoelectric from when manufactured using 3D printing. Although the cost of the modified polymer is greater than that of its standard form, the manufacturing process could allow a serious reduction of the quantities used.
This inter-Carnot project will lead researchers to study the relevance of piezoelectric polymers for connected devices. IMT Atlantique’s task will be to incorporate piezoelectric polymers into radio transmitters and characterize their properties during use. “One of their greatest strengths is the integration of systems for specific applications, such as monitoring individual exercise” the researcher explained, referring to work carried out by Christian Person.
Piezoelectric materials can also be bio-based!
In the two previously-mentioned options currently being studied by Cédric Samuel and his colleagues, the common factor is PVDF. However, “PVDF is an engineering polymer, which remains expensive compared to the commodity polymers” he underlines, “ideally, we would like to be able to use the commodity polymers of plastics processes, and preferably bio-based if possible” he continued. To achieve this, IMT Lille Douai is directing a cross-border European project called Bioharv which associates academic partners in France and Belgium. The Universities of Mons, Lille and Valenciennes as well as Centexbel, a scientific center specialized in the textiles industry, are working alongside the graduate school.
The researchers are most interested in two bio-based polymers, or bioplastics: Polyamide 11 and Polylactic Acid (PLA). The first has proven piezoelectric properties, although a lot weaker than those of PVDF. For the latter, it is a question of proving whether it can in fact generate electric charges. “Certain scientific articles lead us to suppose that Polylactic acid is a promising option, but there has not yet been a clear demonstration of its piezoelectricity” Cédric Samuel explained. In order to do so, the scientists must obtain PLA in its semi-crystalline form. “It’s a stumbling block, as PLA is currently not easy to crystallize” the researcher went on.
The Bioharv project is organized in several stages, gradually developing increasingly effective generations of piezoelectric polymers. It reflects a dual regional research dynamic focusing on both new textiles and the use of natural resources for designing the materials of tomorrow. The stakes are high because the petrochemical industry will not always be able to meet an increasing demand for polymers. Since PLA is produced using agricultural resources, connected devices in the future may be able to be made using corn or potatoes, rather than oil.