Graça Minas

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.

Biomedical microfluidic devices by using low-cost fabrication techniques: A review

One of the most popular methods to fabricate biomedical microfluidic devices is by using a soft-lithography technique. However, the fabrication of the moulds to produce microfluidic devices, such as SU-8 moulds, usually requires a cleanroom environment that can be quite costly. Therefore, many efforts have been made to develop low-cost alternatives for the fabrication of microstructures, avoiding the use of cleanroom facilities. Recently, low-cost techniques without cleanroom facilities that feature aspect ratios more than 20, for fabricating those SU-8 moulds have been gaining popularity among biomedical research community. In those techniques, Ultraviolet (UV) exposure equipment, commonly used in the Printed Circuit Board (PCB) industry, replaces the more expensive and less available Mask Aligner that has been used in the last 15 years for SU-8 patterning. Alternatively, non-lithographic low-cost techniques, due to their ability for large-scale production, have increased the interest of the industrial and research community to develop simple, rapid and low-cost microfluidic structures. These alternative techniques include Print and Peel methods (PAP), laserjet, solid ink, cutting plotters or micromilling, that use equipment available in almost all laboratories and offices. An example is the xurography technique that uses a cutting plotter machine and adhesive vinyl films to generate the master moulds to fabricate microfluidic channels. In this review, we present a selection of the most recent lithographic and non-lithographic low-cost techniques to fabricate microfluidic structures, focused on the features and limitations of each technique. Only microfabrication methods that do not require the use of cleanrooms are considered. Additionally, potential applications of these microfluidic devices in biomedical engineering are presented with some illustrative examples.

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.

An array of highly selective Fabry-Perot optical channels for biological fluid analysis by optical absorption using a white light source for illumination

This paper describes a laboratory microsystem (Microlab) used to measure the concentration of biomolecules in biological fluids. Rather than just one measurement channel, it comprises 16 optical channels that enable the measurement of the concentration of 16 different biomolecules with the same device. An array of 16 optical filters based on Fabry–Perot thin-film optical resonators has been designed. Each optical channel is sensitive in a single wavelength with a FWHM less than 6 nm and with a peak intensity higher than 86%. The 16 optical channel array fabrication requires only four masks, used with different deposition time. The Microlab can easily be tuned during fabrication to analyse different biomolecules only by adjusting the deposition times without affecting the device layout. A commercially available passband optical filter with a passband wavelength in the range 450–650 nm is used. Th eM icrolab requires only a white light source for illumination due to the use of selective optical filters. The quantitative measurement of uric acid in urine is demonstrated. Such a device is extremely suitable for clinical diagnosis application in clinical laboratories and at a patient’s home because of its small size, low cost and portability.

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).

On-chip integrated CMOS optical detection microsystem for spectrophotometric analyses in biological microfluidic systems

An integrated optical detection microsystem, which includes photodetectors and a light-to-frequency converter for readout, is designed and fabricated in a standard CMOS process without extra masks. This detection microsystem is designed for use in biological microsystems for fluids analysis. The application is in the low-cost concentration measurement of biomolecules in biological fluids, by the optical absorption in a part of the visible spectrum defined by the specific molecule. Signals proportional to the intensity of the light transmitted through the biological fluid are available at the output in the form of bit streams, which allows simple computer interfacing. The quantitative measurement of uric acid in urine is successfully demonstrated. The photodiode responsivity is 224 mA/W at /spl lambda/= 495 nm (the wavelength at which the uric acid has its absorption maximum). The optical system sensitivity is 1 kHz/Wm/sup -2/ at /spl lambda/ =670 nm (using the TLS230 from Texas Instruments as reference).

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.

Dynamic Wet Etching of Silicon through Isopropanol Alcohol Evaporation

In this paper, Isopropanol (IPA) availability during the anisotropic etching of silicon in Potassium Hydroxide (KOH) solutions was investigated. Squares of 8 to 40 µm were patterned to (100) oriented silicon wafers through DWL (Direct Writing Laser) photolithography. The wet etching process was performed inside an open HDPE (High Density Polyethylene) flask with ultrasonic agitation. IPA volume and evaporation was studied in a dynamic etching process, and subsequent influence on the silicon etching was inspected. For the tested conditions, evaporation rates for water vapor and IPA were determined as approximately 0.0417 mL/min and 0.175 mL/min, respectively. Results demonstrate that IPA availability, and not concentration, plays an important role in the definition of the final structure. Transversal SEM (Scanning Electron Microscopy) analysis demonstrates a correlation between microloading effects (as a consequence of structure spacing) and the angle formed towards the (100) plane.

Narrow-band pass filter array for integrated opto-electronic spectroscopy detectors to assess esophageal tissue.

A strategy for spectroscopy tissue diagnosis using a small number of wavelengths is reported. The feasibility to accurately quantify tissue information using only 16 wavelengths is demonstrated with several wavelength reduction simulations of the existing esophageal data set. These results are an important step for the development of a miniaturized, robust and low-cost spectroscopy system. This system is based on a sub-millimeter high-selective filter array that offers prospects for a simplified miniature spectrographic detector for a future diagnostic tool to improve the diagnosis of dysplasia. Several thin-film optical filters are optimized and fabricated and its spectral performance is shown to be sufficient for the selection of specific wavelength bands.