Susana Oliveira Catarino

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.

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.

A numerical study on the heat transfer generated by a piezoelectric transducer in a microfluidic system

The present work describes the modelling of heat transfer produced by the acoustic streaming phenomenon, generated through a piezoelectric transducer in a microagitator. Besides the fluids mixing, this phenomenon also promotes the fluids heating. The numerical approach used in this work comprises three main groups of equations: the piezoelectric, the compressible Navier-Stokes, and the heat transfer equations. It was concluded that the heat transfer due to the acoustic wave propagation, without other external heat sources, is not sufficient to increase significantly the fluid temperature.

Heating of samples by acoustic microagitation for improving reaction of biological fluids

This article describes an acoustic microagitator for being used in biological fluids analysis. It is known that a piezoelectric transducer, with its vibration, can be used for mixing fluids. However, in this case, the piezoelectric transducer is also used to heat the samples, improving the reaction of fluids that benefit from that effect. The piezoelectric transducer is fabricated from a poly(vinylidene fluoride) polymer, in the beta phase (β-PVDF). This concept is demonstrated theoretically and by measuring the temperature profile in a regular 1 cm optical lightpath glass cuvette, using capillary thermocouples. This system can further be included in a lab-on-a-chip device, acting as a microreactor, for clinical diagnosis.

Effect of β-PVDF Piezoelectric Transducers’ Positioning on the Acoustic Streaming Flows

This paper reports the numerical and experimental analysis of the acoustic streaming effect in a fluidic domain. The actuation of a piezoelectric transducer generates acoustic waves that propagate to the fluids, generating pressure gradients that induce the flow. The number and positioning of the transducers affect the pressure gradients and, consequently, the resultant flow profile. Two actuation conditions were considered: (1) acoustic streaming generated by a 28 μm thick β-poly(vinylidene fluoride) (β-PVDF) piezoelectric transducer placed asymmetrically relative to the fluidic domain and (2) acoustic streaming generated by two 28 μm thick β-PVDF piezoelectric transducers placed perpendicularly to each other. The transducers were fixed to the lower left corner of a poly(methyl methacrylate) (PMMA)cuvette and were actuated with a 24 Vpp and 34.2 MHz sinusoidal voltage. The results show that the number of transducers and their positioning affects the shape and number of recirculation areas in the acoustic streaming flows. The obtained global flows show great potential for mixing and pumping, being an alternative to the previous geometries studied by the authors, namely, a single transducer placed symmetrically under a fluidic domain.

An overview of modeling and simulation for lab-on-a-chip applications

The research team has been focused, in the last years, on lab-on-a-chip devices for clinical applications, where the acoustic streaming technique is used for promoting the mixing, the pumping and the chemical reactions of fluids inside the microstructures. This paper describes an overview of the advances achieved by the group in the modeling and simulation areas. In particular, it is focused the modeling and simulation of the acoustic streaming generated by a piezoelectric transducer: poly(vinylidene fluoride) - PVDF. The modeling of the acoustic streaming phenomenon includes the study of the piezoelectric effect generated by the transducers, the compressible Navier-Stokes equations and the heat transfer equations. The numerical results validate the experimental results previously achieved by the team and may represent a potential breakthrough in microfluids mixing.

Biological microdevice with fluidic acoustic streaming for measuring uric acid in human saliva

The healthcare system requires new devices for a rapid monitoring of a patient in order to improve the diagnosis and treatment of various diseases. Accordingly, new biomedical devices are being developed. In this paper, a fully-integrated biological microdevice for uric acid analysis in human saliva is presented. It is based on optical spectrophotometric measurements and incorporates a mixture system based on acoustic streaming, that enhances the fluids reaction due to both heating and agitation generated by this effect. Acoustic streaming is provided by a piezoelectric β-PVDF film deposited underneath the microfluidic die of the device. Further, it incorporates the electronics for the detection, readout, data processing and signal actuation. Experimental results proved that acoustic streaming based on this piezoelectric polymer is advantageous and reduces in 55% the time required to obtain the analysis results.

Assessment of the Deformability and Velocity of Healthy and Artificially Impaired Red Blood Cells in Narrow Polydimethylsiloxane (PDMS) Microchannels

Malaria is one of the leading causes of death in underdeveloped regions. Thus, the development of rapid, efficient, and competitive diagnostic techniques is essential. This work reports a study of the deformability and velocity assessment of healthy and artificially impaired red blood cells (RBCs), with the purpose of potentially mimicking malaria effects, in narrow polydimethylsiloxane microchannels. To obtain impaired RBCs, their properties were modified by adding, to the RBCs, different concentrations of glucose, glutaraldehyde, or diamide, in order to increase the cells’ rigidity. The effects of the RBCs’ artificial stiffening were evaluated by combining image analysis techniques with microchannels with a contraction width of 8 µm, making it possible to measure the cells’ deformability and velocity of both healthy and modified RBCs. The results showed that healthy RBCs naturally deform when they cross the contractions and rapidly recover their original shape. In contrast, for the modified samples with high concentration of chemicals, the same did not occur. Additionally, for all the tested modification methods, the results have shown a decrease in the RBCs’ deformability and velocity as the cells’ rigidity increases, when compared to the behavior of healthy RBCs samples. These results show the ability of the image analysis tools combined with microchannel contractions to obtain crucial information on the pathological blood phenomena in microcirculation. Particularly, it was possible to measure the deformability of the RBCs and their velocity, resulting in a velocity/deformability relation in the microchannel. This correlation shows great potential to relate the RBCs’ behavior with the various stages of malaria, helping to establish the development of new diagnostic systems towards point-of-care devices.

Red blood cells deformability as a malaria biomarker

Half of the worlds population, particularly in economic underdevelopment areas, is at risk of contracting malaria. The disease evolution causes biochemical, optical and morphological changes in host red blood cells (RBCs), so the early detection of these changes allows to acknowledge the presence and status of malaria. The main objective of this work is to study the RBCs deformability as a malaria biomarker, by measuring the deformation index (DI) of the RBCs in polydimethylsiloxane microfluidic channels. It was performed a comparative study among healthy ovine RBCs and glutaraldehyde synthetically modified RBCs (that were modified to increase their stiffness and mimic the behavior of the malaria parasite in cells). It was verified that healthy RBCs naturally deform when they pass through contractions with dimensions close to their average dimensions and quickly recover to their original shape after exiting the contraction. In contrast, when glutaraldehyde modified RBCs were tested, it was observed the presence of occlusions in the 6 and 8 μm width constriction microchannels, for concentrations higher than 0.024 and 0.032%, respectively. Also, the almost instantaneous shape recovery capacity, observed in healthy RBCs, was affected by the rigidiflcation treatment, since the DI analyzed at the outlet of the microchannel constriction was higher for modified samples, meaning that the cells did not recover to its original shape. For the 10 μm width constrictions, no occlusion was observed and the RBCs DI was similar for both microchannel inlet and outlet. Overall, it can be concluded that the increase in RBC stiffness causes the decrease of the RBCs deformability and the cell shape recovering capacity.

A numerical and experimental study of acoustic micromixing in 3D microchannels for lab-on-a-chip devices

This paper reports a numerical and experimental study of acoustic streaming and micromixing in polydimethylsiloxane microchannels. The mixing between two fluids flowing in microchannels was evaluated through the following conditions: (1) using a 28 μm thick β-poly(vinylidene fluoride) (β-PVDF) as a piezoelectric transducer actuated with a 24 Vpp and 40 MHz sinusoidal voltage; (2) using different flow rates. The results suggest that the mixing length increases as the flow rate increases and that the acoustic streaming phenomenon leads to a reduction on the mixing length. The good qualitative agreement between numerical and experimental results is a valuable indicator to predict the mixing performance of microfluidic devices, for improving biological fluid analysis in diagnosis lab-on-a-chip devices.