Why a Special Issue on Acoustic Liquid Handling?

Certain topics stir excitement in the hearts and minds of scientists. Synthesizing new genes with never before seen capabilities, discovering cures for Ebola and other devastating diseases, devising new breakthrough drug therapy for cancer sufferers, expanding the understanding of protein structure via crystallography—those are topics that thrill and motivate. By comparison, liquid handling may appear prosaic. Whether it is a hose, a pipette, an eye-dropper, piston- or peristaltic-driven systems, or an ink-jet printer, there is a “been there, done that” attitude. It seems as though everything that could be said about a liquid handler has already been said many times over.

So why, then, are we devoting an entire issue of the Journal of Laboratory Automation to one specific type of liquid handling—acoustic droplet ejection (ADE)?

Precisely because ADE, a relative newcomer ¹ to liquid handling, enables new, far-reaching science. Experiments that were impossible for practical reasons become simple and easy. The ability to use sound waves to move liquids in precise nanoliter increments has wide-sweeping effects on driving exciting advances in science, including those topics mentioned above. ADE fundamentally changes what can be done, and it has facilitated breakthroughs in new areas of innovative research.

ADE is not another evolutionary step in the lab. It is correctly described as truly revolutionary.^2,3 The importance of control and precision with increasingly lower transfer volumes has become elevated as detection technologies have significantly improved, making miniaturization to high-density formats not only possible but also desirable for both throughput and financial reasons.

Already broadly employed in drug discovery, ADE is now becoming an essential part of many labs. Synthetic biology? Cures for emerging diseases? New therapies for cancer? All of these have exploited the advantages of ADE.

Kanigowska et al. ⁴ use ADE to generate new genes at radically lower costs. Their process would have been financially unfeasible were it not for ADE. This reduction in expenses echoes the drug discovery efforts reported by Wingfield et al. ⁵ Rasmussen et al. ⁶ use assay-ready plates made via ADE to test drugs against Ebola and other BSL-4 pathogens and open up faster, simpler ways to collaborate to end these scourges. Advances in individualized medicine that identify exactly what drugs or combinations of drugs are the best anticancer therapy for any specific individual are covered by Blom et al. ⁷ and Kulesskiy et al. ⁸ The miniaturization and speed provided by ADE move individualized systems medicine from theory to reality. Ericson et al. ⁹ and Wu et al. ¹⁰ use ADE to expand protein crystallography via miniaturization and crystallization with multiple ligands.

Of great interest to attendees of SLAS2015, the SLAS International Conference and Exhibition in Washington, D.C., as well as to all people interested in faster high-throughput assays is the article describing the first ever acoustic interface to a mass spectrometer. In “Novel Acoustic Loading of a Mass Spectrometer: Toward Next-Generation High-Throughput MS Screening,” Sinclair et al. ¹¹ elaborate upon the 2015 SLAS Innovation Award–winning presentation on the coupling of mass spectrometry with ADE to analyze as many as three assays per second. In a related article, Chin et al. ¹² show how they combine ADE with matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS).

As mentioned, ADE is used extensively in drug discovery. In this issue, Naylor et al. ¹³ expand the use of ADE in compound management to show how it preserves the potency of peptides. Edwards et al. ¹⁴ describe ADE-enabled high-throughput epigenetic methyltransferase assays.

Acoustic droplet ejection is fundamentally different from earlier technologies. It uses focused sound to propel droplets of liquids upward from a source location to a destination. The roots of ADE go back a century,^15,16 when Wood and Loomis used acoustic energy to propel droplets of liquid from a vessel. With modern ADE, a low level of sound energy is focused at the meniscus, causing a mound to form on the surface of the liquid. When a small additional amount of energy is put into the liquid, droplets are propelled upward to an inverted target or microtiter plate with unparalleled precision and accuracy.

Most important for achieving high scientific impact, ADE is a completely touchless transfer method. Traditionally, the term noncontact transfer has meant that the device transferring the fluid did not have to touch the solid or liquid surface of the destination—the liquid was just shot directly into the well. However, that traditional method has significant contact between the sample being transferred and tips or nozzles used to aspirate and dispense it. The ADE touchless technology moves the liquid from its source position to its destination without intervention from any other device. This completely eliminates the potential of solute in the solution from being absorbed by the transfer instrument (e.g., disposable pipette tips) while simultaneously preventing leachates, which can profoundly affect assays,^17,18 from contaminating the transferred liquid.

Touchless transfer generates IC₅₀ values that more accurately represent the true potency for drugs than when those same drugs are transferred via pipette. The IC₅₀ values obtained with ADE also generate pharmacophores that are more predictive of future assay results and that match more closely to pharmacophores developed from X-ray crystallography experiments. ¹⁹

Sound waves can be precisely controlled by radiofrequency generators and transducers. In turn, the high precision of the acoustic energy signal ensures that the precision and accuracy of the transfer are unparalleled. The coefficient of variation (CV) of acoustic transfers has been reported to be significantly lower than the CV of traditional liquid handlers—sometimes by as much as an order of magnitude. ²⁰

No longer is ADE restricted to a limited range of fluids (nor does it require liquid-by-liquid individual calibrations). The development of dynamic fluid analysis, as outlined by Sackmann et al., ²¹ generates automatic monitoring and calibrations of the system on a well-by-well basis in milliseconds. Buffers, solvents, 50% glycerol, or detergent-based solutions are automatically transferred with the same high precision as was developed for DMSO.

Each well is audited for depth of fluid and fluid properties. The individual well assessment provides researchers with the ability to transfer from any source well to any destination location. Furthermore, the volume transferred to any specific location can be modified for each transfer. This makes the technique extremely useful for dose-response experiments, as Dawes et al. ²² show. The ability to send fluid from multiple wells to the same destination makes combinatorial screening much easier to set up, as reported by Chan et al., ²³ Cross et al., ²⁴ Salzer et al., ²⁵ and others.^26,27

The opportunity to transfer low-nanoliter to mid-microliter volumes of genomic solutions, cells, and enzymes provides researchers with a means to dramatically reduce reagent and sample usage while improving results. Cain-Hom et al. ²⁸ show that they can reduce genotyping assay volumes by 75% while improving results. Agrawal et al. ²⁹ use ADE to miniaturize quantitative reverse transcriptase (qRT)–PCR assays, while Nebane et al. ³⁰ show how ADE allows miniaturization of a high-throughput RNA interference assay. Other researchers have used the microvolume transfers to run experiments in 1536-well^31–37 and 3456-well ³⁸ formats.

This special issue shows that the new capabilities of ADE can substantially change the way that science is done by expanding possibilities and allowing novel workflows in research and clinical applications. We, the guest editors of this issue, are excited with the content and optimistic that it can inspire others to deliver the next generation of innovations in liquid handling for further advancement in life sciences.