Sequential Operation Droplet Array: An Automated Microfluidic Platform for Picoliter-Scale Liquid Handling, Analysis, and Screening

Abstract

This contribution describes a sequential operation droplet array (SODA) system, a fully automated droplet-based microfluidic system capable of performing picoliter-scale liquid manipulation, analysis, and screening. The SODA system was built using a tapered capillary-syringe pump module and a two-dimensional (2D) oil-covered droplet array installed on an x-y-z translation stage. With the system, we developed a novel picoliter-scale droplet depositing technique for forming a 2D picoliter-droplet array. On this basis, an automated droplet manipulation method with picoliter precision was established using the programmable combination of the capillary-based liquid aspirating-depositing and the moving of the oil-covered droplet array, the so-called "aspirating-depositing-moving" (ADM) method. Differing from the previously reported droplet systems based on microchips, microcapillaries, or digital microfluidics, this method can achieve complete and flexible droplet manipulations, including droplet assembling, generation, indexing, transferring, splitting, and fusion in the picoliter range, endowing the present system with ultralow sample/reagent consumptions and substantial versatility in analysis and screening for multiple different samples. To demonstrate its feasibility and versatility, we applied the SODA system in multiple experiments required in drug screening, including the screening of inhibitors for capases-1 from a chemical library, the measurement of IC50 values for the identified inhibitors, and the screening of the synergistic effect of multiple inhibitors. In the experiments, the consumptions of samples and reagents are only 60-180 pL for each droplet microreactor, which are commonly 3-5 orders of magnitude lower than those of conventional multiwell plate systems, and 1-2 orders of magnitude lower than other droplet-based microfluidic systems for multiple sample screening. The ability of the SODA system in performing complicated and multistep droplet manipulations was further demonstrated in the serial dilution of nanoliter-scale inhibitor droplets with concentrations spanning 6 orders of magnitude for IC50 profiling, which includes droplet generation, indexing, splitting, transferring, and fusion with picoliter precision.

Nanoliter-Scale Protein Crystallization and Screening with a Microfluidic Droplet Robot

“…Nanowell-array chips were fabricated using standard photolithography and wet chemical etching techniques from glass plates with chromium and photoresist coating (Shaoguang Microelectronics, Changsha, China) as described elsewhere28. After an etching time of 60 min at 40°C in an etching solution (HF/NH 4 F/HNO 3 , 1.0/0.5/0.75 M), a nanowell array with each well size of ca.…”

“…using a micropipette puller (P2000/F, Sutter instrument, Novato, USA). Both the microchip and capillary were treated with 1% octadecyltrichlorosilane (OTCS, Sigma, St. Louis, USA) in isooctane (v/v) to make their surfaces hydrophobic28.…”

“…It can achieve flexible and programmable liquid handling operations for various protein crystallization experiments in the scale ranging from several nanoliters to sub-nanoliters, including large-scale protein crystallization screening, study of protein crystallization behaviors, and determination of the phase diagrams for proteins. The droplet robot was built using commercially available modules and microdevices, and can be produced by simple microfabrication techniques and equipment, on the basis of the design strategy of sequential operation droplet array (SODA)28. A tapered capillary (20–30 μm tip size) connected to a commercial syringe pump with picoliter-scale precision was used for nanoliter-scale liquid metering, aspirating and dispensing282930.…”

“…The droplet robot was built using commercially available modules and microdevices, and can be produced by simple microfabrication techniques and equipment, on the basis of the design strategy of sequential operation droplet array (SODA)28. A tapered capillary (20–30 μm tip size) connected to a commercial syringe pump with picoliter-scale precision was used for nanoliter-scale liquid metering, aspirating and dispensing282930. A nanowell array chip (Figure 1a1) fabricated using standard photolithography and wet chemical etching techniques and covered with a layer of oil to reduce droplet evaporation, serves as containers of protein crystallization droplet reactors with a high density of 676 droplets/cm 2 (Figure 1a2).…”

Microfluidics for cell-based high throughput screening platforms—A review

“…Furthermore, it is regarded as an effective way to increase the efficiency of drug discovery as most drug combinations are carried out using existing drugs which have passed through the strict clinical and safety examinations [70]. Based on the sequential operation droplet array (SODA) technique [71], Du et al [72] achieved multi-step operations for drug combination screening involving cell culture, medium changing, schedule-dependent drug dosage and stimulation, and cell viability testing in an oil-covered nanoliter-scale droplet array system, by using multiple droplet manipulations including liquid metering, aspirating, depositing, mixing, and transferring, as shown in Fig. 15.…”