Monodisperse hydrogel microspheres by forced droplet formation in aqueous two-phase systems

Abstract

This paper presents a method to form micron-sized droplets in an aqueous two-phase system (ATPS) and to subsequently polymerize the droplets to produce hydrogel beads. Owing to the low interfacial tension in ATPS, droplets do not easily form spontaneously. We enforce the formation of drops by perturbing an otherwise stable jet that forms at the junction where the two aqueous streams meet. This is done by actuating a piezo-electric bending disc integrated in our device. The influence of forcing amplitude and frequency on jet breakup is described and related to the size of monodisperse droplets with a diameter in the range between 30 and 60 μm. Rapid on-chip polymerization of derivatized dextran inside the droplets created monodisperse hydrogel particles. This work shows how droplet-based microfluidics can be used in all-aqueous, surfactant-free, organic-solvent-free biocompatible two-phase environment.

Fragmentation mechanisms of confined co-flowing capillary threads revealed by active flow focusing

“…In fact, the optocapillary forcing intrinsically pinches the thread and, if sufficiently strong, may force the breakup by itself due to the strong pushing flow induced around the neck, regardless of the RP instability [34]. This 'scissorscut' forcing mechanism thus contrasts with remote forcing approaches using pressure transducers [25,26], actuation channels [54], or previously-reported low-power laser heating [28], that modulate the pressure of an already unstable thread to excite a specific mode but do not exert any local action that could force the breakup of an initially stable thread.…”

“…This reversibility in time is a definite asset compared to passive, geometry-based droplet production devices subject to long-lasting transient stages [42]. In addition, optically-based approaches such as presented here may be displaced on demand, featuring by this way an extended versatility compared to alternatives based on integrated electrical [24] or micromechanical [25] devices, which set actuation in space.…”

“…The main approaches explored to this end relied on the geometrical properties of the channel, for example by focusing a stream of fluid to be dispersed into another immiscible fluid with a lateral constriction [13,14], or by abruptly releasing a thread confined vertically when reaching a thicker terrace [15][16][17]. Many active approaches were also prompted to actuate the detachment of individual droplets from a fluid reservoir [18][19][20][21][22][23], such as electric-field induced ejection of droplets from a Taylor cone [24], or a mechanical stimulation of a liquid thread by a transducer [25,26]. In the jetting regime, droplet actuation was also provoked by AC electric fields [27], or modulated laser heating of the interface [28].…”

All-aqueous multiphase microfluidics

“…While direct droplet formation is observed in the PEG/salt system 26 with relatively high interfacial tension, breakup of all-aqueous jets in the low interfacial tension systems is usually difficult and results in polydisperse droplets because of the large Capillary and Weber numbers. 34 To assist the breakup of a w/w jet into monodisperse drops, we discuss two recently developed methods to generate all-aqueous emulsion.…”

“…To prevent the coalescence of droplets, photocurable monomers such as PEGDA 15 or dextran-HEMA 34 have been added to the emulsion phase, and the fast photo-polymerization helps to solidify the emulsion within seconds. However, photo-polymerization typically generates toxic free radicals, potentially harming the encapsulated species, especially the living ones.…”