The work on plastic MEMS is largely inspired from the micro-cantilever silicon MEMS used as electronic-noses, with adequate surface functionalization on the cantilever beam so that target molecules can be adsorbed to the surface of the cantilever and modify its flexure (in a static mode, through added mass) or its resonant frequency (in a dynamic sensing mode).
As it is the case for many printed electronic applications using polymers, Ayela summarized the benefits: a combination of low-cost synthesis, the good processability of organic materials and a large spectrum of optical, mechanical, electrical and chemical properties that can be accurately tuned to attain a specific functionality. But a key motivation behind the use of polymers in MEMS devices is the materials' low values of Young’s modulus, making the devices extremely flexible, ensuring large deflections even under a low stimulus, Ayela highlighted.
In fact, although today's commercial MEMS applications solely rely on silicon or glass-based devices, abundant literature revolves around polymer MEMS sensors and actuators built on substrates as diverse as Polystyrene, Polypropylene, Polyimide, cyclic olefin copolymer resins or the SU-8 epoxy-based photoresist to name a few.
Cédric Ayela presenting the architecture of the piezoelectric OFET cantilevers.
Here, Ayela leveraged the well-known piezoelectric properties of copolymer poly[(vinylidenefluoride-co-trifluoroethylene, abbreviated P(VDF-TrFE) to design a piezoelectric OFET (Organic Field Effect Transistor) integrated into a micro-cantilever made of flexible Polyethylene Naphthalate (PEN). His results published in Nature's Scientific Reports under the title "Piezoelectric polymer gated OFET: Cutting-edge electro-mechanical transducer for organic MEMS-based sensors" revealed that extremely sensitive MEMS sensors could be made, detecting strains from very low beam deflections.
Designed to operate as a mechano-electrical transducer, the triangular-shaped piezoelectric OFET stacks a bottom aluminium gate electrode, P(VDF-TrFE) and poly(1-vinyl-1,2,4-triazole) (PVT) gate dielectric layers, an organic semiconductor (Pentacene gave the best results) and gold source-drain (S/D) electrodes at the top. First, a polarization step is required to induce piezoelectricity (creating polarization states in the material), this was done with successive high-voltage sweeps across the gate and the source. But then as the cantilever is deflected (by added molecular mass from a target molecule), the mechanical strain changes the distance between hydrogen and fluorine atoms in the P(VDF-TrFE) layer, leading to a depletion of positive charge in the p-type semiconductor. This directly affects the OFET's characteristics, hence realizing a highly sensitive mechano-electrical transducer.