A guiding light: spectroscopy on digital microfluidic devices using in-plane optical fibre waveguides.

We present a novel method for in-plane digital microfluidic spectroscopy. In this technique, a custom manifold (.stl file available online as ESM) aligns optical fibres with a digital microfluidic device, allowing optical measurements to be made in the plane of the device. Because of the greater width vs thickness of a droplet on-device, the in-plane alignment of this technique allows it to outperform the sensitivity of vertical absorbance measurements on digital microfluidic (DMF) devices by ∼14×. The new system also has greater calibration sensitivity for thymol blue measurements than the popular NanoDrop system by ∼2.5×. The improvements in absorbance sensitivity result from increased path length, as well as from additional effects likely caused by liquid lensing, in which the presence of a water droplet between optical fibres increases fibre-to-fibre transmission of light by ∼2× through refraction and internal reflection. For interrogation of dilute samples, stretching of droplets using digital microfluidic electrodes and adjustment of fibre-to-fibre gap width allows absorbance path length to be changed on-demand. We anticipate this new digital microfluidic optical fibre absorbance and fluorescence measurement system will be useful for a wide variety of analytical applications involving microvolume samples with digital microfluidics.

A microfluidic technique for quantification of steroids in core needle biopsies.

Core needle biopsy (CNB) sampling is known to be inexpensive and minimally invasive relative to traditional tissue resectioning. But CNBs are often not used in analytical settings because of the tiny amount of sample and analyte. To address this challenge, we introduce an analytical method capable of multiplexed steroid quantification in CNB samples-those studied here ranged in mass from 2 to 8 mg. The new method uses digital microfluidics to extract steroids from CNB tissue samples (including a solid-phase extraction cleanup step) followed by analysis by high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS). The method has limits of detection of 3.6, 1.6, 5.8, and 8.5 fmol for estradiol, androstendione, testoterone, and progesterone, respectively. We propose that future generations of this method may be useful for regular quantification of steroids in core needle biopsy samples of breast tissue to inform dosage and timing of antihormone or hormone replacement therapies as part of a personalized medicine approach to treating a variety of hormone-sensitive disorders.

Attractive design: an elution solvent optimization platform for magnetic-bead-based fractionation using digital microfluidics and design of experiments.

There is great interest in the development of integrated tools allowing for miniaturized sample processing, including solid phase extraction (SPE). We introduce a new format for microfluidic SPE relying on C18-functionalized magnetic beads that can be manipulated in droplets in a digital microfluidic platform. This format provides the opportunity to tune the amount (and potentially the type) of stationary phase on-the-fly, and allows the removal of beads after the extraction (to enable other operations in same device-space), maintaining device reconfigurability. Using the new method, we employed a design of experiments (DOE) operation to enable automated on-chip optimization of elution solvent composition for reversed phase SPE of a model system. Further, conditions were selected to enable on-chip fractionation of multiple analytes. Finally, the method was demonstrated to be useful for online cleanup of extracts from dried blood spot (DBS) samples. We anticipate this combination of features will prove useful for separating a wide range of analytes, from small molecules to peptides, from complex matrices.

A digital microfluidic method for dried blood spot analysis.

Blood samples stored as dried blood spots (DBSs) are emerging as a useful sampling and storage vehicle for a wide range of applications. Unfortunately, the surging popularity of DBS samples has not yet been accompanied by an improvement in automated techniques for extraction and analysis. As a first step towards overcoming this challenge, we have developed a prototype microfluidic system for quantification of amino acids in dried blood spots, in which analytes are extracted, mixed with internal standards, derivatized, and reconstituted for analysis by (off-line and in-line) tandem mass spectrometry. The new method is fast, robust, precise, and most importantly, compatible with automation. We propose that the new method can potentially contribute to a new generation of analytical techniques for quantifying analytes in DBS samples for a wide range of applications.

A digital microfluidic method for in situ formation of porous polymer monoliths with application to solid-phase extraction.

We introduce the marriage of two technologies: digital microfluidics (DMF), a technique in which droplets are manipulated by application of electrostatic forces on an array of electrodes coated by an insulator, and porous polymer monoliths (PPMs), a class of materials that is popular for use for solid-phase extraction and chromatography. In this work, circular PPM discs were formed in situ by dispensing and manipulating droplets of monomer solutions to designated spots on a DMF device followed by UV-initiated polymerization. We used PPM discs formed in this manner to develop a digital microfluidic solid-phase extraction (DMF-SPE) method, in which PPM discs are activated and equilibrated, samples are loaded, PPM discs are washed, and the samples are eluted, all using microliter droplets of samples and reagents. The new method has extraction efficiency (93%) comparable to that of pipet-based ZipTips and is compatible with preparative sample extraction and recovery for on-chip desalting, removal of surfactants, and preconcentration. We anticipate that DMF-SPE may be useful for a wide range of applications requiring preparative sample cleanup and concentration.

Gradient elution in microchannel electrochromatography.

There is great interest in using microfluidic channels packed with a stationary phase for chemical separations of complex mixtures. A key advantage of such techniques is the use of electroosmotic flow (EOF), controlled simply by applying electrical potentials between reservoirs. A disadvantage for this technique, however, is a lack of compatibility with gradient elution separations. This limitation arises from the dependence of EOF velocity on run buffer content (including the concentration of organic modifier). Here, we introduce a method for implementing gradient elution in electrochromatography in which multiple run buffers are velocity-matched, such that the elution profile resembles that found in conventional HPLC. This method is driven entirely with EOF, meaning that pumps, valves, and pressure fittings are not required. The method was validated by application to separations of peptide standards and protein digests. These results suggest that microfluidic electrochromatography may be compatible with a wide range of applications that have previously been unexplored.

Soft lithography: masters on demand.

We report an ultra-rapid prototyping technique for forming microchannel networks for lab-on-a-chip applications, called masters on-demand. Channels are produced by replica molding on masters formed by laser printing on flexible copper printed circuit board (PCB) substrates. Masters of various designs and dimensions can be individually or mass produced in less than 10 minutes. Using this technique, we have fabricated channels as narrow as 100 microm with heights ranging between 9 microm and 70 microm. Multi-depth channel fabrication is also reported, using a two-step printing process. The functionality of devices formed in this manner is verified by performing in-channel electrophoretic separations and culture and analysis of primary mammalian cells.