V. F. Cardoso

Micro and nanofilms of poly(vinylidene fluoride) with controlled thickness, morphology and electroactive crystalline phase for sensor and actuator applications

Poly(vinylidene fluoride), PVDF, thin films have been processed by spin-coating with controlled thickness, morphology and crystalline phases. The influence of the polymer/solvent mass ratio of the solution, the rotational speed of the spin-coater and the temperature of crystallization of the films on the properties of the material has been investigated. It is shown that high-quality films with controlled thicknesses from 300 nm to 4.5 µm and with a controlled amount of electroactive crystalline phases can be obtained in a single deposition step, which allows tailoring the material characteristics for specific applications.

Optimized SU-8 Processing for Low-Cost Microstructures Fabrication without Cleanroom Facilities

The study and optimization of epoxy-based negative photoresist (SU-8) microstructures through a low-cost process and without the need for cleanroom facility is presented in this paper. It is demonstrated that the Ultraviolet Rays (UV) exposure equipment, commonly used in the Printed Circuit Board (PCB) industry, can replace the more expensive and less available equipment, as the Mask Aligner that has been used in the last 15 years for SU-8 patterning. Moreover, high transparency masks, printed in a photomask, are used, instead of expensive chromium masks. The fabrication of well-defined SU-8 microstructures with aspect ratios more than 20 is successfully demonstrated with those facilities. The viability of using the gray-scale technology in the photomasks for the fabrication of 3D microstructures is also reported. Moreover, SU-8 microstructures for different applications are shown throughout the paper.

Improving the optical and electroactive response of poly(vinylidene fluoride–trifluoroethylene) spin-coated films for sensor and actuator applications

Poly(vinylidene fluoride–trifluoroethylene), P(VDF–TrFE), thin-films have been processed by spin-coating with controlled thickness. The influence of the thermal annealing and poling conditions on the properties of the material has been investigated. It is shown that thermal annealing strongly influences the microstructure and ferroelectric phase transition of the copolymer but does not significantly affect the degree of crystallinity of the samples. By increasing the annealing temperature, the samples undergo a transition from a microporous to a microfibrillar microstructure, accompanied by a decrease in the gauche defect density within the molecular chains that increases the ferroelectric transition temperature and enthalpy, and also influences the optical transparency of the films, which can achieve transmittances larger that 95% in the visible spectral range. The piezoelectric response of the material can be maximized by increasing the poling temperature at the cost of a decrease in the optical transparency of the film, due to the microstructural changes induced by the electrical field and the temperature. An optical transmittance as high as 90% along the visible spectral range is nevertheless maintained, demonstrating the suitability of the material for electroactive applications where transparency is also a relevant issue.

Smart-Optical Detector CMOS Array for Biochemical Parameters Analysis in Physiological Fluids

This paper describes the implementation of a smart-optical detector array for detection and concentration measurement of biochemical parameters in physiological fluids. Its application is in the low-cost microchip size analytical laboratories that use colorimetric detection, by optical absorption, as the analytical technique. The microlaboratory structure is composed of a microplate cuvette array containing the physiological fluids into analysis and an optical detector array underneath, which quantifies the light absorbed by those fluids. The detectors, together with their analog-to-digital (A/D) conversion, are designed and fabricated using a standard CMOS process. The on-chip A/D conversion is performed, simultaneously, using a 1-b first-order sigma-delta converter for each optical detector. The output signal of the device is a bit stream containing information about the absorbed light, which allows simple microcontroller interfacing. The proposed architecture has the main advantage of performing the simultaneous measurement of the light absorbed by the fluids, which avoids the errors that can be introduced due to light fluctuations in uncontrolled environments. In addition, the architecture allows on-chip calibration during each measurement. This means that the device can be reliably used in environments with noncalibrated light sources, e.g., in a doctors office. The A/D conversion design described here represents significant improvements when compared with the existing designs. Moreover, the microlaboratory application holds great promise, by both improving benefits (quality of health services provided) and reducing costs (of physiological fluid analysis services).

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.

Electroactive Poly(Vinylidene Fluoride-Trifluorethylene) (PVDF-TrFE) Microporous Membranes for Lithium-Ion Battery Applications

Poly(vinylidene fluoride-trifluorethylene) porous membranes were prepared by solvent evaporation at room temperature. The morphology of the membranes reveals an interconnected porous structure with porosity ranging from 72% up to 80%. The conductivity of the membranes and the LiClO4 uptake increases with increasing porosity. After uptake, the increase of the electrical conductivity is larger for the less porous membranes and the thermal stability of the electrical conductivity is increased.

Advances in Magnetic Nanoparticles for Biomedical Applications

Magnetic nanoparticles (NPs) are emerging as an important class of biomedical functional nanomaterials in areas such as hyperthermia, drug release, tissue engineering, theranostic, and lab-on-a-chip, due to their exclusive chemical and physical properties. Although some works can be found reviewing the main application of magnetic NPs in the area of biomedical engineering, recent and intense progress on magnetic nanoparticle research, from synthesis to surface functionalization strategies, demands for a work that includes, summarizes, and debates current directions and ongoing advancements in this research field. Thus, the present work addresses the structure, synthesis, properties, and the incorporation of magnetic NPs in nanocomposites, highlighting the most relevant effects of the synthesis on the magnetic and structural properties of the magnetic NPs and how these effects limit their utilization in the biomedical area. Furthermore, this review next focuses on the application of magnetic NPs on the biomedical field. Finally, a discussion of the main challenges and an outlook of the future developments in the use of magnetic NPs for advanced biomedical applications are critically provided.

Electroactive poly(vinylidene fluoride)-based structures for advanced applications

Poly(vinylidene fluoride) (PVDF) and its copolymers are the polymers with the highest dielectric constants and electroactive responses, including piezoelectric, pyroelectric and ferroelectric effects. This semicrystalline polymer can crystallize in five different forms, each related to a different chain conformation. Of these different phases, the β phase is the one with the highest dipolar moment and the highest piezoelectric response; therefore, it is the most interesting for a diverse range of applications. Thus, a variety of processing methods have been developed to induce the formation of the polymer β phase. In addition, PVDF has the advantage of being easily processable, flexible and low-cost. In this protocol, we present a number of reproducible and effective methods to produce β-PVDF-based morphologies/structures in the form of dense films, porous films, 3D scaffolds, patterned structures, fibers and spheres. These structures can be fabricated by different processing techniques, including doctor blade, spin coating, printing technologies, non-solvent-induced phase separation (NIPS), temperature-induced phase separation (TIPS), solvent-casting particulate leaching, solvent-casting using a 3D nylon template, freeze extraction with a 3D poly(vinyl alcohol) (PVA) template, replica molding, and electrospinning or electrospray, with the fabrication method depending on the desired characteristics of the structure. The developed electroactive structures have shown potential to be used in a wide range of applications, including the formation of sensors and actuators, in biomedicine, for energy generation and storage, and as filtration membranes.

Tailoring microstructure and physical properties of poly(vinylidene fluoride–hexafluoropropylene) porous films

This paper presents a systematic study for the production of poly(vinylidene fluoride–hexafluoropropylene) [P(VDF–HFP)], porous films using solvent evaporation (SE) and non-solvent-induced phase separation (NIPS) techniques. Parameters such as volume fraction of the copolymer solution, film thickness, time exposure to air, non-solvent, and temperature of the coagulation bath were investigated on the morphology, crystallization, and mechanical properties of the samples. Films with different porous morphologies including homogeneous pore sizes, macrovoids, and spherulites were obtained depending on the processing conditions, which in turn affect the wettability and mechanical properties of the material. Knowing that the phase content of the films also depends on the processing conditions, this paper shows that P(VDF–HFP) films with tailored porous morphology, electroactive phase content, hydrophobicity, crystallinity, and mechanical properties can be achieved for a specific application using the adequate SE and NIPS techniques conditions.

Ultrasonic Transducer Based on β-PVDF for Fluidic Microagitation in a Lab-on-a-Chip Device

This paper describes a fully-integrated lab-on-a-chip device for testing and monitoring biochemical parameters in biological fluids. The major innovation of this microdevice is the application of an acoustic microagitation technique with automatic electronic control based on a β-PVDF piezoelectric polymer placed underneath the microfluidic structures. Experimental results regarding the influence of the thickness of the polymer on the reaction rate of biological fluids are presented. Moreover, the study of the transmittance curve of β-PVDF with transparent conductive electrodes is also presented. Transparent electrodes are a constraint once the polymer is incorporate underneath the reaction chamber due to the analytical measurement by spectrophotometry.