Native State Single-Cell Printing System and Analysis for Matrix Effects

Advances in Single-Cell Printing

Single-cell analysis is becoming an indispensable tool in modern biological and medical research. Single-cell isolation is the key step for single-cell analysis. Single-cell printing shows several distinct advantages among the single-cell isolation techniques, such as precise deposition, high encapsulation efficiency, and easy recovery. Therefore, recent developments in single-cell printing have attracted extensive attention. We review herein the recently developed bioprinting strategies with single-cell resolution, with a special focus on inkjet-like single-cell printing. First, we discuss the common cell printing strategies and introduce several typical and advanced printing strategies. Then, we introduce several typical applications based on single-cell printing, from single-cell array screening and mass spectrometry-based single-cell analysis to three-dimensional tissue formation. In the last part, we discuss the pros and cons of the single-cell strategies and provide a brief outlook for single-cell printing.

Development of a miniature protein mass spectrometer capable of analyzing native proteins

Current miniature mass spectrometers were usually designed for the detection of small and medium size molecules, including volatile (semi-volatile) compounds, drugs and lipids. In this study, a miniature protein mass spectrometer was developed in this work, which could serve as a biosensor for the rapid identification of proteins as well as their conformations. A linear ion trap with a field radius of 2.5 mm was designed to extend mass range of the instrument to over 6500 Th. Mass resolution and sensitivity of the instrument were also optimized for protein ions by increasing the buffer gas pressure and using a high-gain Faraday detector. It is then demonstrated that the mass spectra of native proteins, such as IgG1, could be acquired by coupling the instrument with a soft electrospray ionization source. As a proof-of-concept demonstration, results suggest that the current instrument could be used to identify target proteins and probe/distinguish their conformations in solutions.

Single Cell Metabolite Detection Using Inertial Microfluidics-Assisted Ion Mobility Mass Spectrometry

Single-cell metabolite measurement remains highly challenging due to difficulties related to single cell isolation, metabolite detection, and identification of low levels of metabolites. Here, as a first step of the technological development, we propose a novel strategy integrating spiral inertial microfluidics and ion mobility mass spectrometry (IM-MS) for single-cell metabolite detection and identification. Cells in methanol suspension are inertially focused into a single stream in the spiral microchannel. This stream of separated cells is delivered to the nanoelectrospray needle to be lysed and ionized and subsequently analyzed in real time by IM-MS. This analytical system enables six to eight single-cell metabolic fingerprints to be collected per minute, including gas-phase collisional cross section (CCS) measurements as an additional molecular descriptor, giving increased confidence in metabolite identification. As a proof of concept, the metabolic profiles of three types of cancer cells (U2OS, HepG2, and HepG2.215) were successfully screened, and 19 distinct lipids species were identified with CCS value filtering. Furthermore, principal component analysis (PCA) showed differentiation of the three cancer cell lines, mainly due to cellular surface phospholipids. Taken together, our technology platform offers a simple and efficient method for single-cell lipid profiling, with additional ion mobility separation of lipids significantly improving the confidence toward identification of metabolites.

Direct Infusion ICP-qMS of Lined-up Single-Cell Using an Oil-Free Passive Microfluidic System

When coupled online with mass spectrometry (MS), widely applied water-in-oil droplet-based microfluidics for single cell analysis met problems. For example, the oil phase rumpled the stability, efficiency, and accuracy of MS, the conventional interface between MS and the microfluidic chip suffered the low sample introduction efficiency, and the transportation rates sometimes unmatched the readout dwell times for transient signal acquisition. Considering cells are already "droplets" with hydrophilic surface and elastic hydrophobic membrane, we developed an oil-free passive microfluidic system (OFPMS) that consists of alternating straight-curved-straight microchannels and a direct infusion (dI) micronebulizer for inductively coupled plasma quadrupole-based mass spectrometry (ICP-qMS) of lined-up single-cell. OFPMS guarantees exact single cell isolation one by one just using a thermo-decomposable NH₄HCO₃ buffer, eliminating the use of any oil and incompatible polymer carriers. It is more flexible and facile to adapt to the dwell time of ICP-qMS owing to the adjustable throughput of 400 to 25000 cells/min and the controllable interval time of at least 20 ms between the lined-up adjacent single cells. Quantitative single-cell transportation and high detection efficiency of more than 70% was realized using OFPMS-dI-ICP-qMS exemplified here. Thus, cell-to-cell heterogeneity can be simply uncovered via the determination of metals in the individual cells.

Concentrating Single Cells in Picoliter Droplets for Phospholipid Profiling on a Microfluidic System

Cellular membranes are composed of a variety of lipids in different amounts and proportions, and alterations of them are usually closely related to various diseases. To reveal the intercellular heterogeneity of the lipid variation, an integrated microfluidic system is designed, which consists of droplet-based inkjet printing, dielectrophoretic electrodes, and de-emulsification interface to achieve on-line single-cell encapsulation, manipulation, and mass spectrometry (MS) detection. This integrated system effectively improves the single-cell encapsulation rate, and meanwhile reduces the matrix interference and continuous oil phase interference to the MS detection. Using this system, the heterogeneities between the normal and cancer cells are compared, and the heterogeneity of the same cells before and after the drug treatment changed obviously, indicating that this system can be used as a promising tool for studying the link between the alterations of lipid homeostasis and various diseases.