J. Serrado Nunes

Influence of the β−phase content and degree of crystallinity on the piezo- and ferroelectric properties of poly (vinylidene fluoride)

The ferroelectric switching behaviour and piezoelectric response of poly(vinylidene fluoride) (PVDF) prepared by drawing at stretching ratios from 1 to 5 and temperatures from 80 to 140 °C has been studied. Stretching ratio and temperature deeply influence the α (non-ferroelectric) to β (ferroelectric) phase transformation. The variations in the phase content are accompanied by changes in the degree of crystallinity and the microstructure, all of them influencing the macroscopic piezoelectric and ferroelectric response of the material. This work shows how the piezo- and ferroelectric behaviour of PVDF depends on the aforementioned parameters and, in particular, on the crystalline β-phase content. Coercive electric field, remnant polarization and saturation polarization increase with increasing ferroelectric β-phase content in the sample. In a similar way, samples with higher β-phase content show higher d33 piezoelectric coefficients.

Lab-on-a-Chip With β-Poly(Vinylidene Fluoride) Based Acoustic Microagitation

This paper reports a fully integrated disposable lab-on-a-chip with acoustic microagitation based on a piezoelectric ß-poly(vinylidene fluoride) (ß-PVDF) polymer. The device can be used for the measurement, by optical absorption spectroscopy, of biochemical parameters in physiological fluids. It comprises two dies: the fluidic die that contains the reaction chambers fabricated in SU-8 and the ß-PVDF polymer deposited underneath them; and the detection die that contains the photodetectors, its readout electronics, and the piezoelectric actuation electronics, all fabricated in a CMOS microelectronic process. The microagitation technique improves mixing and shortens reaction time. Further, it generates heating, which also improves the reaction time of the fluids. In this paper, the efficiency of the microagitation system is evaluated as a function of the amplitude and the frequency of the signal actuation. The relative contribution of the generated heating is also discussed. The system is tested for the measurement of the uric acid concentration in urine.

Piezoelectric and optical response of uniaxially stretched (VDF/TrFE) (75/25) copolymer films

The phase diagram of the poly(vinylidene fluoride-trifluorethylene) (P(VDF-TrFE)) copolymer system shows for VDF contents of 50...85 mol% a ferroelectric (FE)-paraelectric (PE) phase transition below melting temperature. Investigations on P(VDF-TrFE) 75/25 samples revealed a slight anisotropic behaviour, which leads to a strongly anisotropic stretching effect both on the phase transition and on the amount and nature of the FE phase in samples subjected to mechanical stretching along the main directions of the film. In this work, both the refractive index n1,2 and the piezoelectric coefficient d33 of mechanically stretched P(VDF-TrFE) have been measured for samples with different levels of permanent deformation. These parameters are found to reflect the anisotropy of the permanently deformed samples. The stretching effect is most pronounced (n1,2) or limited (d33) to the vicinity of the yielding point of the material. Above the yielding point, almost the piezoelectric d33 coefficient of the non-deformed sample is observed for samples with large permanent deformation.

Functionally graded electroactive Poly(vinylidene fluoride) polymers

The phase transformation from α to β Poly(vinylidene fluoride) (PVDF) through an stretching process at a given temperature and stretching ratio allows controlling the phase content and therefore the electroactive properties of the polymer. Inhomogeneous stretching allows tailoring these properties along a given sample and to obtain graded electroactive and optical properties within the same plate of material from the micro to the macroscopic scale, increasing the potential of PVDF for advance applications.

Wearable brain cap with contactless electroencephalogram measurement for brain-computer interface applications

Many brain computer interfaces (BCI) are based on feature selection from electroencephalogram (EEG) signals. To acquire these signals, a set of electrodes is used, most of the times attached to a brain cap. Several brain cap designs have been presented, as well many different electrodes. So far, all the brain caps are difficult and uncomfortable to wear. Moreover, the electrodes, wet or dry, are also difficult to place. Despite the huge potential that a BCI offers, for disabled and healthy people, new brain caps are required to overcome these two main problems. In this work, a wearable brain cap is presented with the ability to measure the required EEG signals without requiring any electrical contact with the head. The wearable cap is obtained using a flexible polymeric material, with new integrated contactless electrodes. The electrodes may be obtained using a new electroactive gel, which is used to read the EEG signals.

A lab-on-a-chip for clinical analysis with acoustic microagitation based on piezoelectric poly (Vinylidene Fluoride)

Portuguese Foundation for Science and Technology (grants PTDC/BIO/70017/2006 and PTDC/CTM/69362/2006).