Joan Marc Cabot

Comparing Microfluidic Performance of Three-Dimensional (3D) Printing Platforms

Three-dimensional (3D) printing has emerged as a potential revolutionary technology for the fabrication of microfluidic devices. A direct experimental comparison of the three 3D printing technologies dominating microfluidics was conducted using a Y-junction microfluidic device, the design of which was optimized for each printer: fused deposition molding (FDM), Polyjet, and digital light processing stereolithography (DLP-SLA). Printer performance was evaluated in terms of feature size, accuracy, and suitability for mass manufacturing; laminar flow was studied to assess their suitability for microfluidics. FDM was suitable for microfabrication with minimum features of 321 ± 5 μm, and rough surfaces of 10.97 μm. Microfluidic devices >500 μm, rapid mixing (71% ± 12% after 5 mm, 100 μL/min) was observed, indicating a strength in fabricating micromixers. Polyjet fabricated channels with a minimum size of 205 ± 13 μm, and a surface roughness of 0.99 μm. Compared with FDM, mixing decreased (27% ± 10%), but Polyjet printing is more suited for microfluidic applications where flow splitting is not required, such as cell culture or droplet generators. DLP-SLA fabricated a minimum channel size of 154 ± 10 μm, and 94 ± 7 μm for positive structures such as soft lithography templates, with a roughness of 0.35 μm. These results, in addition to low mixing (8% ± 1%), showed suitability for microfabrication, and microfluidic applications requiring precise control of flow. Through further discussion of the capabilities (and limitations) of these printers, we intend to provide guidance toward the selection of the 3D printing technology most suitable for specific microfluidic applications.

Fast high-throughput method for the determination of acidity constants by capillary electrophoresis. II. Acidic internal standards

A fast method for the determination of acidity constants by CZE has been recently developed. This method is based on the use of an internal standard of pK(a) similar to that of the analyte. In this paper we establish the reference pK(a) values of a set of 24 monoprotic neutral acids of varied structure that we propose as internal standards. These compounds cover the most usual working pH range in CZE and facilitate the selection of adequate internal standards for a given determination. The reference pK(a) values of the acids have been established by the own internal standard method, i.e. from the mobility differences between different acids of similar pK(a) in the same pH buffers. The determined pK(a) values have been contrasted to the literature pK(a) values and confirmed by determination of the pK(a) values of some acids of the set by the classical CE method. Some systematic deviations of mobilities have been observed in NaOH buffer in reference to the other used buffers, overcoming the use of NaOH in the classical CE method. However, the deviations affect in a similar degree to the test compounds and internal standards allowing thus, the use of NaOH buffer in the internal standard method. This fact demonstrates the better performance of the internal standard method over the classical method to correct mobility deviations, which together with its fastness makes it an interesting method for the routine determination of accurate pK(a) values of new pharmaceutical drugs and drug precursors.

Novel Instrument for Automated pKa Determination by Internal Standard Capillary Electrophoresis

The internal standard capillary electrophoresis method (IS-CE) has been implemented in a novel sequential injection-capillary electrophoresis instrument for the high-throughput determination of acidity constants (pK(a)) regardless of aqueous solubility, number of pK(a) values, or structure. This instrument comprises a buffer creation system that automatically mixes within a few seconds four reagents for in situ creation of the separation electrolyte with a pH range of 2-13, ionic strength of 10-100 mM and organic solvent content from 0% to 40%. Combined with 1.2 kV/cm and a short effective length (15 cm to the UV detector) fast 20 s electrophoretic separations can be obtained. The low standard deviation of the replicates and the low variation compared to reference values show that this system can accurately determine acidity constants of drugs by IS-CE. A single pK(a) can be determined in 2 min and a set of 20 measurements in half an hour, allowing rapid, simple, and flexible determination of pK(a) values of pharmaceutical targets.

Determination of acidity constants by the capillary electrophoresis internal standard method. IV. Polyprotic compounds

The IS-CE method is developed for pK(a) determination of polyprotic compounds. In this method, the internal standard (IS) and the polyprotic test compound are injected into the capillary electrophoresis (CE) system in buffers with appropriate pH. The pH of the buffers is not externally measured, but determined inside the capillary from the mobilities of the internal standards. Then the pK(a) values of the polyprotic compounds are obtained by fitting its mobilities to the in situ pH values. The method is faster than the classical CE method (a diprotic compound can be done in less than 15 min), and also than other methods like potentiometric and spectrophotometric titrations. The method has been successfully applied to 20 polyprotic test compounds of different chemical nature, including compounds with extreme or very close pK(a) values.

Determination of acidity constants of sparingly soluble drugs in aqueous solution by the internal standard capillary electrophoresis method

A set of 33 drugs with different solubilities, ranging from soluble to very insoluble, has been chosen in order to evaluate the performance of the internal standard CE method to determine acidity constants of compounds with limited solubility. The set of drugs tested in this work has been chosen as a function of their intrinsic solubility. For the most insoluble compounds, several analytical conditions to overcome the insolubility in aqueous buffers have been tested. This paper assesses the compound solubility limits for the IS‐CE method in aqueous pKa determinations, and also compares the determined pKas with the results from the literature data obtained by other methods. It is proved that IS‐CE method determines acidity constants of sparingly soluble drugs in aqueous media (compounds with logS down to around –6), whereas other reference methods require the use of aqueous–organic solvent buffers and extrapolation procedures to obtain the aqueous pKa for the same compounds.

Internal Standard Capillary Electrophoresis as a High-Throughput Method for pKa Determination in Drug Discovery and Development

A novel high-throughput method for determining acidity constants (pKa) by capillary electrophoresis (CE) is developed. The method, based on the use of an internal standard (IS-CE), is implemented as a routine method for accurate experimental pKa determination of drugs undergoing physicochemical measurements in drug discovery laboratories. Just two electropherograms at 2 different pH values are needed to calculate an acidity constant. Several ISs can be used in the same buffer and run to enhance precision. With 3 ISs, for example, the pKa of the test compound (TC) can be obtained in triplicate in less than 3 min of electrophoresis. It has been demonstrated that the IS-CE method eliminates some systematic errors, maintaining, or even increasing the precision of the results compared with other methods. Furthermore, pH buffer instability during electrophoretic runs is not a problem in the IS-CE method. It is also proved that after 16 h of electroseparation using the same buffer vial, pH may change by around one unit; but the pKa calculated by the IS-CE method remains constant. Thus, IS-CE is a powerful high-throughput method for pKa determination in drug discovery and development.

Fibre-based electrofluidics on low cost versatile 3D printed platforms for solute delivery, separations and diagnostics; from small molecules to intact cells

A novel and effective fibre-based microfluidic methodology was developed to move and isolate charged solutes, biomolecules, and intact bacterial cells, based upon a novel multi-functional 3D printed supporting platform, with potential applications in the fields of microfluidics and biodiagnostics. Various on-fibre electrophoretic techniques are demonstrated to separate, pre-concentrate, move, split, or cut and collect the isolated zones of target solutes, including proteins and live bacterial cells. The use of knotting to link different fibre materials, and the unique ability of this approach to physically concentrate solutes in different locations are shown such that the concentrated solutes can be physically isolated and easily transferred to other fibres. Application of this novel fibre-based technique within a potential diagnostic platform for urinary tract infection is shown, together with the post-electrophoretic incubation of live bacterial cells, demonstrating the cell survival following on-fibre electrophoretic concentration.

Electrophoretic separations on paper: Past, present, and future-A review

Point-of-collection (POC) devices aim for a fast, on-site detection for medical and environmental purposes. In this area, microfluidic Paper-based Analytical Devices (μPADs) have recently gained popularity because these are potentially cheap and environmentally friendly to produce, and easy to use. From an analytical perspective, paper is well known for its use as a substrate for chromatography, but less known for its use in electrophoretic separations. With the recent interest in μPADs, most applications are based on rather simple assays with relatively few applications incorporating an analytical separation. The focus of this review is on paper-based electrophoresis, originating with the key developments in the 1940s and 1950s as well as the recent developments of electrophoretic μPADs, and concluding with a critical discussion of the opportunities and challenges for electrophoretic μPADS in the future.

Isotachophoretic Fluorescence in Situ Hybridization of Intact Bacterial Cells

A counter-pressure-assisted capillary isotachophoresis method in combination with a sieving matrix and ionic spacer was used to perform in-line fluorescence in situ hybridization (FISH) of bacterial cells. A high concentration of sieving matrix (1.8% w/v HEC) was introduced at one end of the capillary, and the bacterial cells were suspended in the spacer electrolyte for injection. Using a 2 min injection with 18 psi counter-pressure, 50% of the cells injected into the capillary were hybridized with the fluorescently labeled oligonucleotide, and the excess unhybridized probe was separated from the hybridized cell-probe complexes in a two-stage ITP method. With an LOD (6.0 × 104 cells/mL) comparable with the CE analysis of a sample processed using an off-line FISH protocol, the total analysis time was reduced from 2.5 h to 30 min. Provided the appropriate probe is selected, this approach can be used for specific detection of bacterial cells in aqueous samples.

Enhanced physicochemical properties of polydimethylsiloxane based microfluidic devices and thin films by incorporating synthetic micro-diamond

Synthetic micro-diamond-polydimethylsiloxane (PDMS) composite microfluidic chips and thin films were produced using indirect 3D printing and spin coating fabrication techniques. Microfluidic chips containing up to 60 wt% micro-diamond were successfully cast and bonded. Physicochemical properties, including the dispersion pattern, hydrophobicity, chemical structure, elasticity and thermal characteristics of both chip and films were investigated. Scanning electron microscopy indicated that the micro-diamond particles were embedded and interconnected within the bulk material of the cast microfluidic chip, whereas in the case of thin films their increased presence at the polymer surface resulted in a reduced hydrophobicity of the composite. The elastic modulus increased from 1.28 for a PDMS control, to 4.42 MPa for the 60 wt% composite, along with a three-fold increase in thermal conductivity, from 0.15 to 0.45 W m−1 K−1. Within the fluidic chips, micro-diamond incorporation enhanced heat dissipation by efficient transfer of heat from within the channels to the surrounding substrate. At a flow rate of 1000 μL/min, the gradient achieved for the 60 wt% composite chip equalled a 9.8 °C drop across a 3 cm long channel, more than twice that observed with the PDMS control chip.