Inexpensive and nonconventional fabrication of microfluidic devices in PMMA based on a soft-embossing protocol.

This study describes an inexpensive and nonconventional soft-embossing protocol to produce microfluidic devices in poly(methyl methacrylate) (PMMA). The desirable microfluidic structure was photo-patterned in a poly(vinyl acetate) (PVAc) film deposited on glass substrate to produce a low-relief master. Then, this template was used to generate a high-relief pattern in stiffened PDMS by increasing of curing agent /monomer ratio (1:5) followed by thermal aging in a laboratory oven (200°C for 24 h). The stiffened PDMS masters were used to replicate microfluidic devices in PMMA based on soft embossing at 220-230°C and thermal sealing at 140°C. Both embossing and sealing stages were performed by using binder clips. The proposed protocol has ensured the replication of microfluidic devices in PMMA with great fidelity (>94%). Examples of MCE devices, droplet generator devices and spot test array were successfully demonstrated. For testing MCE devices, a mixture containing inorganic cations was selected as model and the achieved analytical performance did not reveal significant difference from commercial PMMA devices. Water droplets were successfully generated in an oil phase at rate of ca. 60 droplets/min (fixing the continuous phase flow rate at 100 µL/h) with size of ca. 322 ± 6 µm. Glucose colorimetric assay was performed on spot test devices and good detectability level (5 µmol/L) was achieved. The obtained results for two artificial serum samples revealed good agreement with the certified concentrations. Based on the fabrication simplicity and great analytical performance, the proposed soft-embossing protocol may emerge as promising approach for manufacturing PMMA devices.

Separation of carbohydrates on electrophoresis microchips with controlled electrolysis.

This study reports the separation of fructose, galactose, glucose, lactose and sucrose on glass microchip electrophoresis (ME) devices using a microfluidic platform adapted with external reservoirs for controlling the electrolysis phenomenon. The connections between external reservoirs and microfluidic platform were performed by saline bridges created using silicone tubing filled with BGE. The separation conditions were optimized and the best results were achieved using a BGE containing 75 mmol/L NaOH and 15 mmol/L trisodium phosphate. Electrophoretic separations were monitored using a capacitively coupled contactless conductivity detection system. The controlled electrolysis has successfully allowed the application of a higher voltage on the separation channel promoting the baseline separation of five carbohydrates within 180 s with great run-to-run repeatability (RSD < 1%). The achieved efficiencies ranged from 45 000 ± 6000 to 70 000 ± 3000 plates/m demonstrating a performance better than ME devices without controlled electrolysis. The proposed system offered good linearity from 1 to 10 mmol/L and LODs between 150 and 740 µmol/L. The use of external tubes for controlling the electrolysis phenomenon on ME devices has solved common problems associated to run-to-run repeatability and analytical reliability required for routine and quantitative analysis.

Simple, rapid and, cost-effective fabrication of PDMS electrophoresis microchips using poly(vinyl acetate) as photoresist master.

This study describes a simple, rapid, and cost-effective fabrication of PDMS electrophoresis microchips using poly(vinyl acetate) (PVAc) emulsion as photoresist master. High-relief microfluidic structures were defined on poly(vinyl acetate) previously deposited on printed circuit boards surfaces without cleanroom facilities and sophisticated instrumentation. After a UV exposure, channels with heights ranging from 30 to 140 µm were obtained by controlling the emulsion mass deposited on the master surface. The developing stage was performed using water rather than the organic solvents that are applied for conventional masks. The surface morphology was characterized by optical imaging, profilometry, and SEM. Based on the achieved results, the proposed method offers suitable reproducibility for the prototyping of electrophoresis microchips in PDMS. The feasibility of the resulting PDMS electrophoresis chips was successfully demonstrated with the separation of major inorganic cations within 100 s using a contactless conductivity detection system. The separation efficiencies ranged from ca. 67 900 to 125 600 plates/m. Due to the satisfactory performance and simplified instrumentation, we believe this fabrication protocol presents potential to be implemented in any chemical, biochemical, or biological laboratory.

Hand drawing of pencil electrodes on paper platforms for contactless conductivity detection of inorganic cations in human tear samples using electrophoresis chips.

This paper describes for the first time the fabrication of pencil drawn electrodes (PDE) on paper platforms for capacitively coupled contactless conductivity detection (C(4) D) on electrophoresis microchips. PDE-C(4) D devices were attached on PMMA electrophoresis chips and used for detection of K(+) and Na(+) in human tear samples. PDE-C(4) D devices were produced on office paper and chromatographic paper platforms and their performance were thoroughly investigated using a model mixture containing K(+) , Na(+) , and Li(+) . In comparison with chromatographic paper, PDE-C(4) D fabricated on office paper has exhibited better performance due to its higher electrical conductivity. Furthermore, the detector response was similar to that recorded with electrodes prepared with copper adhesive tape. The fabrication of PDE-C(4) D on office paper has offered great advantages including extremely low cost (< $ 0.004 per unit), reduced fabrication time (< 5 min), and minimal instrumentation (pencil and paper). The proposed electrodes demonstrated excellent analytical performance with good reproducibility. For an inter-PDE comparison (n = 7), the RSD values for migration time, peak area, and separation efficiency were lower than 2.5, 10.5, and 14%, respectively. The LOD's achieved for K(+) , Na(+) , and Li(+) were 4.9, 6.8, and 9.0 µM, respectively. The clinical feasibility of the proposed approach was successfully demonstrated with the quantitative analysis of K(+) and Na(+) in tear samples. The concentration levels found for K(+) and Na(+) were, respectively, 20.8 ± 0.1 mM and 101.2 ± 0.1 mM for sample #1, and 20.4 ± 0.1 mM and 111.4 ± 0.1 mM for sample #2.

Environmentally Friendly Manufacturing of Flexible Graphite Electrodes for a Wearable Device Monitoring Zinc in Sweat.

Electrochemical sensors based on graphite and polymers have emerged as powerful analytical tools for bioanalytical applications. However, most of the fabrication processes are not environmentally friendly because they often involve the use of toxic reagents and generate waste. This study describes an alternative method to produce flexible electrodes in plastic substrates using graphite powder and thermal laminating sheets by solid-solid deposition through hot compression, without the use of hazardous chemical reagents. The electrodes developed through the proposed approach have successfully demonstrated flexibility, robustness, reproducibility (relative standard deviation around 6%), and versatility. The electrodes were thoroughly characterized by cyclic voltammetry, electrochemical impedance spectroscopy, Raman spectroscopy, and scanning electron microscopy. As a proof of concept, the electrode surfaces were modified with bismuth and used for zinc analysis in sweat. The modified electrodes presented linearity (R2 = 0.996) for a wide zinc concentration range (50-2000 ppb) and low detection limit (4.31 ppb). The proposed electrodes were tested using real sweat samples and the achieved zinc concentrations did not differ statistically from the data obtained by atomic absorption spectroscopy. To allow wearable applications, a 3D-printed device was fabricated, integrated with the proposed electrochemical system, and fixed at the abdomen by using an elastic tape to collect, store, and analyze the sweat sample. The matrix effect test was performed, spiking the real sample with different zinc levels, and the recovery values varied between 85 and 106%, thus demonstrating adequate accuracy and robustness of the flexible electrodes developed based on the proposed fabrication method.

High adhesion strength and hybrid irreversible/reversible full-PDMS microfluidic chips.

To the best of our knowledge, this paper outlines for the first time high adhesion and hybrid irreversible/reversible microfluidic devices fully composed of polydimethylsiloxane (PDMS). These chips were fabricated by the sandwich bonding (SWB), a method that was recently deployed by our group. SWB offers simple, fast, and low cost operation requiring only a laboratory oven. The devices showed burst pressures of up to 4.5 MPa. This value is more than tenfold the pressures withstood by the full-PDMS chips described in literature. In terms of the reversible behavior, the ability for disassembling the chip slides is crucial in research and development stages, especially when the device integrates high-cost components or harsh cleaning steps are needed. Following successive steps of detachment and bonding, the channels still withstood high pressures of approximately 1.8 MPa. Finally, the emulsification of corn oil 4.0% w/w to polyglycerol polyricinoleate with 10.0 µmol L-1 rhodamine B aqueous solution was realized to show the relevance in enhancing the flow rate in microfluidics. Such experiment was conducted at total flow rates of 0.8-160.0 µL min-1. The decrease in size and polydispersity of the droplets was observed at increasing flow rates. Monodisperse emulsions were achieved only at 160.0 µL min-1.

Hydrodynamic injection on electrophoresis microchips using an electronic micropipette.

Here we report for the first time the use of an electronic micropipette as hydrodynamic (HD) injector for microchip electrophoresis (ME) devices. The micropipette was directly coupled to a PDMS device, which had been fabricated in a simple cross format with two auxiliary channels for sample volume splitting. Sample flow during the injection procedure was controlled in automatic dispenser mode using a volume of 0.6µL. Channel width and device configuration were optimized and the best results were achieved using a simple cross layout containing two auxiliary channels with 300µm width for sample splitting. The performance of the HD injector was evaluated using a model mixture of high-mobility cationic species. The results obtained were compared to the data obtained via electrokinetic (EK) injection. Overall, the HD provided better analytical performance in terms of resolution and injection-to-injection repeatability. The relative standard deviation (RSD) values for peak intensities were lower than 5% (n=10) when the micropipette was employed. In comparison with EK injection, the use of the proposed HD injector revealed an unbiased profile for a mixture containing K+ and Li+(300 µmol L-1 each) over various buffer concentrations. For EK injection, the peak areas decreased from 2.92 ± 0.20-0.72 ± 0.14Vs for K+ and from 1.30 ± 0.10-0.38 ± 0.10Vs for Li+ when the running buffer increased from 20 to 50mmolL-1. For HD injection, the peak areas for K+ and Li+ exhibited average values of 2.48±0.07 and 2.10±0.06Vs, respectively. The limits of detection (LDs) for K+, Na+ and Li+ ranged from 18 to 23µmolL-1. HD injection through an electronic micropipette allows to automatically dispense a bias-free amount of sample inside microchannels with acceptable repeatability. The proposed approach also exhibited instrumental simplicity, portability and minimal microfabrication requirements.