If your lab is feeling overcrowded with reagents, equipment, and personnel, then maybe it’s time to get small. These days, you can get about as small as you want, with the help of microfluidics. Surprisingly, the ink-jet printer is responsible for the inception of microfluidics, the principles of which were extended, beginning in the 1970s and 1980s, to systems of biological analysis in which fluids could be moved under continuous flow into microchannels using external pumps or reservoirs. The two key features of these systems distinguishing them from standard laboratory wetware were the constant and controllable flow of reagents, and their small size, quantified broadly as maintaining channels of less than one millimeter in at least one dimension.
Research, development, and production in the following years sought to optimize channel and substrate materials, fluid movement, and delivery with the ultimate goal of hermetically sequestering entire biological or chemical workflows onto a single miniaturized device, generating an empirical ideal—a laboratory on a chip (LOC), or alternatively, a micro total analysis system (TAS). Today, they are regularly used with great success in biomedical research fields including genomic preparation and analysis, high-throughput screening platforms, and the study of biomimetic organoid systems in tissue culture-based disease modeling. Their successful application to synthetic chemistry is both emerging and accelerating.
The benefits of going small, realized
The intrinsic benefits of moving workflows onto LOCs are potentially staggering in terms of cost, efficiency, and accuracy. Miniaturization imparts the obvious advantage of drastically lessening reagent use, and, downstream of experimental end points, minimizing waste, spill, and clean-up issues to make laboratory work both tidier and more sustainable.
Continuous flow guided by external pumps or reservoirs—or more recently, digital, droplet-based devices that use electrical and other potentials to mix or separate solutions—allow for repeatable delivery of precise volumes, and a high degree of control over reaction times and conditions. These factors contribute to facilitating measurement, manipulation, and detection in real time, rather than solely at end points, and an ever-increasing ability to remain on-chip from beginning to end without having to exit the LOC at intermediate or terminal steps to re-scale or re-calibrate. Less obviously, a microchannel imparts laminar, rather than turbulent, flow to the fluid that occupies it. Laminar flow is rare in nature, and changes fluid dynamics in important ways: 1) it reduces the Reynolds number indicative of the ratio between inertial and viscous forces, reducing unpredictability and backflow of reagents, resulting in greater fidelity of mixing; and 2) it promotes mixing via molecular diffusion, which in conventional labware would be stochastic and highly time-intensive.
Extensions into chemical synthesis and the rise of microreactors
Microfluidic devices have additional embedded characteristics that suit them specifically to chemical synthesis and to realizing a novel function as microreactors, an expanding field of study and production that has followed on the heels of their widespread use in biomedical research. In many ways, these new developments are complementary, especially in generating molecular and medical probes, and in pharmaceutical synthesis. Most importantly, their high surface tension and low gravitational forces translate into highly efficient heat and mass transfer properties, often two orders of magnitude greater than those measured in standard batch reactors, meaning that: 1) heat is quickly dissipated; and 2) reaction equilibrium can be very quickly reached. Therefore, highly exothermic (read: potentially explosive) organic synthesis reactions can often be performed with substantial efficiency and yield in microreactors, when they are too dangerous to scale up to batch reactors. For example, diazonium chemistry can generate an array of pharmaceutically relevant hydroxyarenes and chloroarenes. However, diazonium salts are sensitive to light, heat, and shock. Resulting destabilization can lead to explosions, and safety measures including constant cryocooling are often required. Synthesis in microreactors can mitigate the dangers and requirements, while often increasing yield and eliminating unwanted reaction by-products.
Increased reaction efficiencies have substantial additional benefits. For radiolabeled tracer probes such as those used in positron emission tomography, production and use can potentially outpace their rapid decay. Moreover, the principles of efficiency can carry over into testing and optimizing reaction conditions by multiplexing them in channel or, more powerfully, droplet format. In this manner, investigators can develop strategies to alter concentrations, compound residence times, and switch solvents in real time to pull out important intermediate products while increasing efficiency for end products. For instance, in production of azoles by combining diketone and hydrazine, one research group tested more than 200 separate reaction conditions over a 27-hour span while only using six milliliters of solvent and less than 100 milligrams of reactants.
Moving between microreactors and batch reactors to reach production scales can be problematic because of the common and unpredictable need to re-engineer synthetic routes between laboratory/experimental, pilot, and commercial operations. However, strategies to scale out, rather than up, have seen preliminary success. In other words, rather than re-assess synthesis reactions based on scale, investigators can maintain microreactor-scale operations while adding many more equivalent microfluidic devices to reach the total desired output. In this manner, the advantages of the microreactors’ small size and physical properties can be faithfully maintained, while avoiding re-optimization entirely.
Caveats, limitations, and solutions
There are, however, limitations to LOC microreactors. In practice, chip material dictates what kind of synthesis reactions can take place. Glass, or glass combined with silicon, has long been a preferred standard material for its comparatively inert nature. However, modern LOCs are most often composed of polydimethylsiloxane (PDMS), which imparts a flexibility amenable to creating cross-flow channels and controlling flow or pressure with specialized Quake valves.
One major problem in synthesis reactions is that materials like PDMS are susceptible to swelling or dissolution by organic solvents. Additionally, reactions that use bulk gravitational or buoyancy forces, including distillation or phase separation, are often more suitable to batch reactors. Finally, multiphase reactions, especially with solid or precipitating reactants, can clog microfluidic channels.
To move toward automating synthetic reactions for drug discovery, investigators have begun troubleshooting these and other limitations. Of note, proof-of-principle studies have shown that electrowetting on digital microfluidic devices (EWOD) unshackles microreactors from external pressure requirements that could otherwise clog flow channels, and from immobile polar organic solvents by encapsulating them with ionic liquids and driving their movement around the LOC via potentials generated by drag forces. The development and improvement of various digital LOC devices employing a multitude of strategies and materials, including paper, constitutes perhaps the most dramatic step into the future to unlock the potential of microreactors.