Supin Chen

Micro-chemical synthesis of molecular probes on an electronic microfluidic device

We have developed an all-electronic digital microfluidic device for microscale chemical synthesis in organic solvents, operated by electrowetting-on-dielectric (EWOD). As an example of the principles, we demonstrate the multistep synthesis of [18F]FDG, the most common radiotracer for positron emission tomography (PET), with high and reliable radio-fluorination efficiency of [18F]FTAG (88 ± 7%, n = 11) and quantitative hydrolysis to [18F]FDG (> 95%, n = 11). We furthermore show that batches of purified [18F]FDG can successfully be used for PET imaging in mice and that they pass typical quality control requirements for human use (including radiochemical purity, residual solvents, Kryptofix, chemical purity, and pH). We report statistical repeatability of the radiosynthesis rather than best-case results, demonstrating the robustness of the EWOD microfluidic platform. Exhibiting high compatibility with organic solvents and the ability to carry out sophisticated actuation and sensing of reaction droplets, EWOD is a unique platform for performing diverse microscale chemical syntheses in small volumes, including multistep processes with intermediate solvent-exchange steps.

On Chip Droplet Characterization: A Practical, High-Sensitivity Measurement of Droplet Impedance in Digital Microfluidics

We demonstrate a new approach to impedance measurement on digital microfluidics chips for the purpose of simple, sensitive, and accurate volume and liquid composition measurement. Adding only a single series resistor to existing AC droplet actuation circuits, the platform is simple to implement and has negligible effect on actuation voltage. To accurately measure the complex voltage across the resistor (and hence current through the device and droplet), the designed system is based on software-implemented lock-in amplification detection of the voltage drop across the resistor which filters out noise, enabling high-resolution and low-limit signal recovery. We observe picoliter sensitivity with linear correlation of voltage to volume extending to the microliter volumes that can be handled by digital microfluidic devices. Due to the minimal hardware, the system is robust and measurements are highly repeatable. The detection technique provides both phase and magnitude information of the real-time current flowing through the droplet for a full impedance measurement. The sensitivity and resolution of this platform enables it to distinguish between various liquids which, as demonstrated in this paper, could potentially be extended to quantify solute concentrations, liquid mixtures, and presence of analytes.

Accurate dispensing of volatile reagents on demand for chemical reactions in EWOD chips

Digital microfluidic chips provide a new platform for manipulating chemicals for multi-step chemical synthesis or assays at the microscale. The organic solvents and reagents needed for these applications are often volatile, sensitive to contamination, and wetting, i.e. have contact angles of <90° even on the highly hydrophobic surfaces (e.g., Teflon® or Cytop®) typically used on digital microfluidic chips. Furthermore, often the applications dictate that the processes are performed in a gas environment, not allowing the use of a filler liquid (e.g., oil). These properties pose challenges for delivering controlled volumes of liquid to the chip. An automated, simple, accurate and reliable method of delivering reagents from sealed, off-chip reservoirs is presented here. This platform overcomes the issues of evaporative losses of volatile solvents, cross-contamination, and flooding of the chip by combining a syringe pump, a simple on-chip liquid detector and a robust interface design. The impedance-based liquid detection requires only minimal added hardware to provide a feedback signal to ensure accurate volumes of volatile solvents are introduced to the chip, independent of time delays between dispensing operations. On-demand dispensing of multiple droplets of acetonitrile, a frequently used but difficult to handle solvent due to its wetting properties and volatility, was demonstrated and used to synthesize the positron emission tomography (PET) probe [(18)F]FDG reliably.

High yield and high specific activity synthesis of [18F]fallypride in a batch microfluidic reactor for micro-PET imaging

[(18)F]fallypride was synthesized in a batch microfluidic chip with a radiochemical yield of 65 ± 6% (n = 7) and an average specific activity of 730 GBq μmol(-1) (20 Ci μmol(-1)) (n = 4). Specific activity was ~2-fold higher than [(18)F]fallypride synthesized in a macroscale radiosynthesizer, despite starting with significantly less radioactivity, and thus safer conditions, in the microchip.

On-demand droplet loading for automated organic chemistry on digital microfluidics

Organic chemistry applications on digital microfluidic devices often involve reagents that are volatile or sensitive and must be introduced to the chip immediately before use. We present a new technique for automated, on-demand loading of ~1 μL droplets from large (~1 mL), sealed, off-chip reservoirs to a digital microfluidic chip in order to address this challenge. Unlike aqueous liquids which generally are non-wetting to the hydrophobic surface and must be actively drawn into the electrowetting-on-dielectric (EWOD) chip by electrode activation, organic liquids tend to be wetting and can spontaneously flood the chip, and hence require a retracting force for controlled liquid delivery. Using a combination of compressed inert gas and gravity to exert driving and retracting forces on the liquid, the simple loading technique enables precise loading of droplets of both wetting and non-wetting liquids in a reliable manner. A key feature from a practical point of view is that all of the wetted parts are inexpensive and potentially disposable, thus avoiding cross-contamination in chemical and biochemical applications. We provide a theoretical treatment of the underlying physics, discuss the effect of geometry and liquid properties on its performance, and show repeatable reagent loading using the technique. Its versatility is demonstrated with the loading of several aqueous and non-aqueous liquids on an EWOD digital microfluidic device.

Optimization of microfluidic PET tracer synthesis with Cerenkov imaging

Microfluidic technologies provide an attractive platform for the synthesis of radiolabeled compounds. Visualization of radioisotopes on chip is critical for synthesis optimization and technological development. With Cerenkov imaging, beta particle emitting isotopes can be localized with a sensitive CCD camera. In order for Cerenkov imaging to also serve as a quantitative tool, it is necessary to understand how material properties relevant to Cerenkov emission, namely, index of refraction and beta particle stopping power, affect Cerenkov light output. In this report, we investigate the fundamental physical characteristics of Cerenkov photon yield at different stages of [(18)F]FDG synthesis on the electrowetting on dielectric (EWOD) microfluidic platform. We also demonstrate how Cerenkov imaging has enabled synthesis optimization. Geant4, a Monte Carlo program applied extensively in high energy physics, is used to simulate Cerenkov photon yield from (18)F beta particles traversing materials of interest during [(18)F]FDG synthesis on chip. Our simulations show that the majority (approximately two-thirds) of the (18)F beta particle energy available to produce Cerenkov photons is deposited on the glass plates of the EWOD chip. This result suggests the possibility of using a single calibration factor to convert Cerenkov signal to radioactivity, independent of droplet composition. We validate our simulations with a controlled measurement examining varying ratios of [(18)O]H2O, dimethyl sulfoxide (DMSO), and acetonitrile (MeCN), and find a consistent calibration independent of solvent composition. However, the calibration factor may underestimate the radioactivity in actual synthesis due to discoloration of the droplet during certain steps of probe synthesis. In addition to the attractive quantitative potential of Cerenkov imaging, this imaging strategy provides indispensable qualitative data to guide synthesis optimization. We are able to use this imaging technique to optimize the mixing protocol as well as identify and correct for loss of radioactivity due to the migration of radioactive vapor outside of the EWOD heater, enabling an overall increase in the crude radiochemical yield from 50 ± 3% (n = 3) to 72 ± 13% (n = 5).

Milliliter-to-microliter platform for on-demand loading of aqueous and non-aqueous droplets to digital microfluidics

We describe a world-to-chip interface for automated, on-demand loading of ∼1µL droplets from a large (∼1mL demonstrated) off-chip reservoir. Utilizing a combination of compressed inert gas and gravity to exert forces on the liquid, the technique employed is simple and robust, requiring no carrier fluid. Another key feature from a practical point-of-view is that all of the wetted parts are inexpensive and potentially disposable, thus avoiding cross-contamination in chemical and biochemical applications. The proof-of-concept and versatility of the platform are demonstrated with the loading of several aqueous and non-aqueous liquids on an electrowetting-on-dielectric microfluidic device.

Synthesis of 18 F-labeled probes on EWOD platform for Positron Emission Tomography (PET) preclinical imaging

We demonstrate the synthesis of 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) on an electrowetting-on-dielectric (EWOD) platform using multifunctional electrodes (for heating, temperature sensing, and EWOD driving). A revised chemistry to incorporate advantages of microfluidic volumes is used to produce multiple doses of PET tracer for preclinical imaging. The success is confirmed by imaging mice with radiotracers produced on chip. This is an important step toward the development of an affordable, universal synthesizer for imaging tracers used in Positron Emission Tomography (PET).

On-chip product purification for complete microfluidic radiotracer synthesis

Solid phase extraction was incorporated into an electrowetting-on-dielectric chip for radiochemical purification of a positron emission tomography tracer that was radiolabeled on the same chip. The radiotracer droplet was mixed with alumina particles, and the alumina particles were filtered out from the droplet through a line of pillars, all by electrowetting droplet movement. Fluorination reaction and on-chip purification were analyzed with both Cerenkov imaging and off-chip radio-thin layer chromatography measurements. The measurements were compared to test the validity of the combined use of filtration on-chip and Cerenkov imaging as an alternative approach for monitoring reaction yield without the need to extract sample from the chip.

Mirrored anodized dielectric for reliable electrowetting

Anodized dielectrics in a mirrored arrangement are proposed and evaluated for the parallel-plate electrowetting-on-dielectric (EWOD) configuration. As valve metal oxides with current rectifying effects, anodized dielectrics previously could only be used for EWOD under a restricted range of voltages. However, in a mirrored configuration, one side of the anodized dielectric pair is expected to always be under the correct bias to restrict current. Both the mirrored and typical configurations were tested with current leakage measurements on anodized alumina samples under cycles of negative and positive bias and a range of electric fields. The mirrored configuration was effective in limiting current leakage over all voltages applied.