Embedded droplet printing in yield-stress fluids

Abstract

Microfluidic tools and techniques for manipulating fluid droplets have become core to many scientific and technological fields. Despite the plethora of existing approaches to fluidic manipulation, non-Newtonian fluid phenomena are rarely taken advantage of. Here we introduce embedded droplet printing—a system and methods for the generation, trapping, and processing of fluid droplets within yield-stress fluids, materials that exhibit extreme shear thinning. This technique allows for the manipulation of droplets under conditions that are simply unattainable with conventional microfluidic methods, namely the elimination of exterior influences including convection and solid boundaries. Because of this, we believe embedded droplet printing approaches an ideal for the experimentation, processing, or observation of many samples in an “absolutely quiescent” state, while also removing some troublesome aspects of microfluidics including the use of surfactants and the complexity of device manufacturing. We characterize a model material system to understand the process of droplet generation inside yield-stress fluids and develop a nascent set of archetypal operations that can be performed with embedded droplet printing. With these principles and tools, we demonstrate the benefits and versatility of our method, applying it toward the diverse applications of pharmaceutical crystallization, microbatch chemical reactions, and biological assays.

The rheology of direct and suspended extrusion bioprinting

“…Following removal of the applied stress, the suspension media very quickly recover their solid-like properties in a “self-healing” manner, entrapping and supporting the deposited bioink prior to cross-linking. 117 …”

“…The earliest example of this approach was by the Lewis group in 2011, 118 and a number of methods have been developed since. These include chopped slurries, 8,119,120 fluid gels, 7,121–124 nanoclays, 125,126 microgels, 117,127,128 polymer networks with dynamic or reversible bonds 9,111 and viscous solutions. 129,130 The increased shape complexity that can be achieved using this technique has made it an effective technique to print vascular networks within a tissue construct.…”

“…This occurs when at small length scales, the capillary pressure at the interface of the suspension medium and bioink is higher than the yield stress of the suspension medium, as shown in toroid formation. 107,134,135 It was, however, favorably exploited to enable embedded droplet printing by Nelson et al 117 Interfacial tension is often negligible when both a bioink and SM are aqueous, which is common to maintain cytocompatibility of both systems, and so the yield stress and elastic behavior of the SM determine the achievable feature resolution. 125 Increasing the SM yield stress has been shown to enable the production of smaller printed features.…”

On-demand modulation of 3D-printed elastomers using programmable droplet inclusions

“…S1A, consisted of Dowsil SE 1700 and silicone oil; the former is a nonflowing elastomeric paste (µ = 500,000 cP) used for direct writing applications (9), and the latter is a liquid used as a thinning agent. As opposed to previous reports of embedding of individual droplets into another phase at discrete points (20)(21)(22), the flowing PDMS outer phase is sufficiently capable of continuously shearing the aqueous inner phase into monodisperse droplets. The net result is the facile generation of PDMS emulsion inks that can be 3D printed into various geometries, as shown in Fig.…”

Direct-writing microporous polymer architectures – print, capture and release

“…14 In this paper, we demonstrate a breakthrough approach of fluid-on-fluid templating that addresses all of these challenges and enables rapid fabrication of functional, structured polymer films making these applications feasible and scalable. Extrusion of fluids within the bulk or on the surface of an elastomer matrix has been pioneered [18][19][20] to show the ability to capture and control functional materials in macro-scale patterns. Inspired by the BF method, we use the highly controlled, tuneable, direct-write benefits of drop-on-demand (DoD) inkjet printing to controllably print micron-scale droplets to a fluid surface, as illustrated in Fig.…”