To the Editor:

A paper published by Gregory Stephanopoulos and colleagues1 in your May 2014 issue described a microfluidic device for screening metabolic activity of cells. The authors put special emphasis on the fact that they had developed a fully integrated microfluidic platform allowing the encapsulation of cells into droplets that can be incubated, injected into a second device, fused and sorted. However, this approach was not introduced by the authors, but had originally been published by us2 in 2012. In our paper, we even concluded that our approach “should facilitate metabolic studies.” The general concept of combining different droplet manipulation modules into a single, fully integrated, cell screening device was patented even earlier3, with a priority date of July 13, 2007. However, neither our paper nor the patent was cited by Stephanopoulos and colleagues.

We also contend that many statements made by Stephanopoulos and colleagues are wrong. For example, they state, “Although others have demonstrated the use of microfluidic emulsion droplet technology to screen populations of cells, these previous applications have been limited to screening cells that have already completed their culturing process and that produce an analyte of interest physically connected to the cell by being either intracellular or membrane-bound.” In our approach2, hybridoma cells were screened for the functional activity of secreted, soluble antibodies. The culturing process was not completed at the time of encapsulation, and the analyte of interest was neither intracellular nor membrane-bound, but a secreted one, thus contradicting the aforementioned claims.

In addition, the authors1 go on to state, “In these examples, the initial formation of droplets involved the addition of only assay reagents to determine the activity of the enzyme. This is very different from the detection of extracellular metabolite production or consumption. In this scenario, the culture and assay steps cannot be performed simultaneously because the time scale for production or consumption is much longer than the assay time, which is on the order of seconds or minutes. Thus, a separate step is necessary to culture each individual cell in its own droplet.”

In our approach, the droplets did not contain assay reagents, but rather cultivation media. Exactly as Stephanopoulos and colleagues described in the Nature Biotechnology article1, we added the assay reagents later, subsequent to a cultivation step during which extracellular proteins were produced.

“Here, we have described a system to measure extracellular metabolite secretion or consumption, which uses a microfluidic droplet maker to encapsulate cells and growth medium, a syringe for multi-day microaerobic culturing of collected droplets, and a second microfluidic device containing coalescence, delay line, detection and sorting modules. In this second device, the reagents to detect the analyte of interest are added to the droplets collected in the syringe; the assay reaction occurs while the droplets are in the delay lines; and droplets are sorted and collected based on resulting fluorescence.” This is exactly the approach we first described in the paper by El Debs et al.2. It is noteworthy that we also obtained a fivefold higher throughput and an approximately twofold better enrichment than Stephanopoulos and colleagues did in their work1.

Our conclusion is that many aspects of the system described in their paper are not novel, and the authors should have cited all relevant previous work.