Compact microfluidic device for rapid concentration of PET tracers

Microfluidic synthesis of radiotracers: recent developments and commercialization prospects

Positron emission tomography (PET) is a powerful diagnostic tool that holds incredible potential for clinicians to track a wide variety of biological processes using specialized radiotracers. Currently, however, a single radiotracer accounts for over 95% of procedures, largely due to the cost of radiotracer synthesis. Microfluidic platforms provide a solution to this problem by enabling a dose-on-demand pipeline in which a single benchtop platform would synthesize a wide array of radiotracers. In this review, we will explore the field of microfluidic production of radiotracers from early research to current development. Furthermore, the benefits and drawbacks of different microfluidic reactor designs will be analyzed. Lastly, we will discuss the various engineering considerations that must be addressed to create a fully developed, commercially effective platform that can usher the field from research and development to commercialization.

Microfluidic Evaporation, Pervaporation, and Osmosis: From Passive Pumping to Solute Concentration

Evaporation, pervaporation, and forward osmosis are processes leading to a mass transfer of solvent across an interface: gas/liquid for evaporation and solid/liquid (membrane) for pervaporation and osmosis. This Review provides comprehensive insight into the use of these processes at the microfluidic scales for applications ranging from passive pumping to the screening of phase diagrams and micromaterials engineering. Indeed, for a fixed interface relative to the microfluidic chip, these processes passively induce flows driven only by gradients of chemical potential. As a consequence, these passive-transport phenomena lead to an accumulation of solutes that cannot cross the interface and thus concentrate solutions in the microfluidic chip up to high concentration regimes, possibly up to solidification. The purpose of this Review is to provide a unified description of these processes and associated microfluidic applications to highlight the differences and similarities between these three passive-transport phenomena.

Automatic radioisotope manipulation for small amount of nuclear medicine using an EWOD device with a dimple structure

Delicate animal experiments and microdose clinical trials using short-lived radioisotopes require rapid preparation with high accuracy and careful attention to safety within a limited timeframe. We have developed an open-style electrowetting on dielectric (EWOD) device containing dimple structures for the rapid preparation of radiolabelled reagents. The device was demonstrated by automatic preparation of a technetium diethylenetriamine pentaacetate (99mTc-DTPA) with high chelation efficiency (99.7 ± 0.13%). Additionally, we demonstrated the single-photon emission computed tomography/computed tomography imaging of mouse kidney using the 99mTc-DTPA prepared with the EWOD device. The obtained organ tomographic images were sufficient for the evaluation of mouse models for specific diseases. These results indicate that manual radiolabelling for a small amount of nuclear medicine can be replaced by a process using the proposed EWOD device as a human error reduction technique.

Microfluidics-Coupled Radioluminescence Microscopy for In Vitro Radiotracer Kinetic Studies

Integrated bioassay systems that combine microfluidics and radiation detectors can deliver medical radiopharmaceuticals to live cells with precise timing, while minimizing radiation dose and sample volume. However, the spatial resolution of many radiation imaging systems is limited to bulk cell populations. Here, we demonstrate microfluidics-coupled radioluminescence microscopy (μF-RLM), a new integrated system that can image radiotracer uptake in live adherent cells growing inside microincubators with spatial resolution better than 30 μm. Our method enables on-chip radionuclide imaging by incorporating an inorganic scintillator plate (CdWO4) into a microfluidic chip. We apply this approach to investigate the factors that influence the dynamic uptake of [18F]fluorodeoxyglucose (FDG) by cancer cells. In the first experiment, we measured the effect of flow on FDG uptake of cells and found that a continuous flow of the radiotracer led to fourfold higher uptake than static incubation, suggesting that convective replenishment enhances molecular radiotracer transport into cells. In the second set of experiments, we applied pharmacokinetic modeling to show that lactic acidosis inhibits FDG uptake by cancer cells in vitro and that this decrease is primarily due to downregulation of FDG transport into the cells. The other two rate constants, which represent FDG export and FDG metabolism, were relatively unaffected by lactic acidosis. Lactic acidosis is common in solid tumors because of the dysregulated metabolism and inefficient vasculature. In conclusion, μF-RLM is a simple and practical approach for integrating high-resolution radionuclide imaging within standard microfluidics devices, thus potentially opening venues for investigating the efficacy of radiopharmaceuticals in in vitro cancer models.

Novel volumetric method for highly repeatable injection in microchip electrophoresis

A novel injector for microchip electrophoresis (MCE) has been designed and evaluated that achieves very high repeatability of injection volume suitable for quantitative analysis. It eliminates the injection biases in electrokinetic injection and the dependence on pressure and sample properties in hydrodynamic injection. The microfluidic injector, made of poly(dimethylsiloxane) (PDMS), operates similarly to an HPLC injection valve. It contains a channel segment (chamber) with a well-defined volume that serves as an "injection loop". Using on-chip microvalves, the chamber can be connected to the sample source during the "loading" step, and to the CE separation channel during the "injection" step. Once the valves are opened in the second state, electrophoretic potential is applied to separate the sample. For evaluation and demonstration purposes, the microinjector was connected to a 75 μm ID capillary and UV absorbance detector. For single compounds, a relative standard deviation (RSD) of peak area as low as 1.04% (n = 11) was obtained, and for compound mixtures, RSD as low as 0.40% (n = 4) was observed. Using the same microchip, the performance of this new injection technique was compared to hydrodynamic injection and found to have improved repeatability and less dependence on sample viscosity. Furthermore, a non-radioactive version of the positron-emission tomography (PET) imaging probe, FLT, was successfully separated from its known 3 structurally-similar byproducts with baseline resolution, demonstrating the potential for rapid, quantitative analysis of impurities to ensure the safety of batches of short-lived radiotracers. Both the separation efficiency and injection repeatability were found to be substantially higher when using the novel volumetric injection approach compared to electrokinetic injection (performed in the same chip). This novel microinjector provides a straightforward way to improve the performance of hydrodynamic injection and enables extremely repeatable sample volume injection in MCE. It could be used in any MCE application where volume repeatability is needed, including the quantitation of impurities in pharmaceutical or radiopharmaceutical samples.

Automatic concentration and reformulation of PET tracers via microfluidic membrane distillation

Short-lived radiolabeled tracers for positron emission tomography (PET) must be rapidly synthesized, purified, and formulated into injectable solution just prior to imaging. Current radiosynthesizers are generally designed for clinical use, and the HPLC purification and SPE formulation processes often result in a final volume that is too large for preclinical and emerging in vitro applications. Conventional technologies and techniques for reducing this volume tend to be slow, resulting in radioactive decay of the product, and often require manual handling of the radioactive materials. We present a fully-automated microfluidic system based on sweeping gas membrane distillation to rapidly perform the concentration and formulation process. After detailed characterization of the system, we demonstrate fast and efficient concentration and formulation of several PET tracers, evaluate residual solvent content to establish the safety of the formulated tracers for injection, and show that the formulated tracer can be used for in vivo imaging.