The assembly of scalable liquid compartments for binding assays in array formats constitutes a topic of fundamental importance in life sciences. This challenge can be addressed by mimicking the structure of cellular compartments with biological native conditions. Here, inkjet printing is employed to develop up to hundreds of picoliter aqueous droplet arrays stabilized by oil-confinement with mild surfactants (Tween-20). The aqueous environments constitute specialized compartments in which biomolecules may exploit their function and a wide range of molecular interactions can be quantitatively investigated. Raster Image Correlation Spectroscopy (RICS) is employed to monitor in each compartment a restricted range of dynamic intermolecular events demonstrated through protein-binding assays involving the biotin/streptavidin model system.

A simple, rapid, and highly controlled platform to prepare life‐inspired subcellular scale compartments by inkjet printing has been developed. These compartments consist of fL‐scale aqueous droplets (few µm in diameter) incorporating biologically relevant molecular entities with programmed composition and concentration. These droplets are ink‐jetted in nL mineral oil drop arrays allowing for lab‐on‐chip studies by fluorescence microscopy and fluorescence life time imaging. Once formed, fL‐droplets are stable for several hours, thus giving the possibility of readily analyze molecular reactions and their kinetics and to verify molecular behavior and intermolecular interactions. Here, this platform is exploited to unravel the behavior of different molecular probes and biomolecular systems (DNA hairpins, enzymatic cascades, protein‐ligand couples) within the compartments. The fL‐scale size induces the formation of molecularly crowded confined shell structures (hundreds of nanometers in thickness) at the droplet surface, allowing discovery of specific features (e.g., heterogeneity, responsivity to molecular triggers) that are mediated by the intermolecular interactions in these peculiar environments. The presented results indicate the possibility of using this platform for designing nature‐inspired confined reactors allowing for a deepened understanding of molecular confinement effects in living subcellular compartments.

This work presents the first reported imbibition mechanism of femtoliter (fL)-scale droplets produced by microchannel cantilever spotting (μCS) of DNA molecular inks into porous substrates (hydrophilic nylon). Differently from macroscopic or picoliter droplets, the downscaling to the fL-size leads to an imbibition process controlled by the subtle interplay of evaporation, spreading, viscosity, and capillarity, with gravitational forces being quasi-negligible. In particular, the minimization of droplet evaporation, surface tension, and viscosity allows for a reproducible droplet imbibition process. The dwell time on the nylon surface permits further tuning of the droplet lateral size, in accord with liquid ink diffusion mechanisms. The functionality of the printed DNA molecules is demonstrated at different imbibed oligonucleotide concentrations by hybridization with a fluorolabeled complementary sequence, resulting in a homogeneous coverage of DNA within the imbibed droplet. This study represents a first step toward the μCS-enabled fabrication of DNA-based biosensors and microarrays into porous substrates.

Inkjet printing is here employed for the first time as a method to produce femtoliter (fL) scale oil droplets dispersed in water. In particular, picoliter (pL) scale fluorinated oil (FC40) droplets are printed in presence of perfluoro-1-octanol (PFCO) surfactant at velocity higher than 5 m/s. Femtoliter scale oil droplets in water are spontaneously formed through a fragmentation process at the water/air interface by using minute amounts of non-ionic surfactant (down to 0.003% v/v of Tween 80). This fragmentation occurs by a Plateau-Rayleigh mechanism at moderately high Weber number (10 1 ). A microfluidic chip with integrated microelectrodes allows droplet characterization in terms of number and diameter distribution (peaked at about 3 microns) by means of electrical impedance measurements. These results show an unprecedented possibility to scale-up oil droplets down to the femtoliter scale which opens up several perspectives for a tailored oil-in-water emulsions fabrication for drug encapsulation, pharmaceutic preparations and cellular biology.

In article number 1900023, Valeria Vetri, Bruno Pignataro, and co‐workers describe a novel inkjet‐printing approach that allows to downscale up to femtoliter‐scale water droplets in oil drops on a chip, triggering the generation of sub‐cellular size artificial compartments with self‐selected submicrometer‐thin molecular and biomolecular shell structures. These systems are crowded, confined to the water surface, and show unique features (reduced water content, molecular proximity, spatial localization, and responsivity).