An Acoustofluidic Picoinjector

Abstract

Droplet microfluidics has emerged as a valuable technology for a multitude of chemical and biomedical applications, offering the capability to create independent microenvironments for high-throughput assays. Central to numerous droplet microfluidic applications is the picoinjection of materials into individual droplets, yet existing picoinjection methods often exhibit high power requirements, lack biocompatibility, and/or suffer from limited controllability. Here, we present an acoustofluidic picoinjector that generates acoustic pressure at the droplet interface to enable on-demand, energy-efficient, and biocompatible injection at high precision. We validate our platform by performing acid-base titrations by iteratively injecting picoliter volume reagents into droplets to induce pH transitions detectable by color change in solution. Additionally, we demonstrate the versatility of the acoustofluidic picoinjector in the synthesis of metallic nanoparticles, yielding highly monodisperse and reproducible particle morphologies compared to conventional bulk-phase techniques. By facilitating controlled delivery of reagents or biological samples with unparalleled accuracy, acoustofluidic picoinjection broadens the utility of droplet microfluidics for a myriad of applications in chemical and biological research.

Keywords: Acoustofluidics; droplet microfluidics; nanoparticle synthesis; picoinjection.

Figure 1.. Schematic of the acoustofluidic picoinjector.

(A) Channel design of the device. Oil and dispersed phase liquid meet at a cross-junction and form droplets, which are then injected with some other fluid by the interdigital transducer. (B) Device image. (C) Schematic of acoustic pressure at the picoinjection site. Surface acoustic waves from the IDT generate acoustic pressure to pressurize the fluid.

Figure 2.. Mechanism and characterization of the acoustofluidic picoinjector.

(A) Time-step simulation analysis of the picoinjection inside the microfluidic channels. (B) Acoustic pressure output by the interdigital transducer, indicating the effects of acoustic radiation forces at various locations near the IDT. (C) Comparison of picoinjection when acoustics are turned off and on Scale bars: 100 μm. (D) Picoinjection volume control based on the injecting fluid flow rate as a droplet reaches the picoinjection orifice. The volume of reagent injected has a strong linear correlation with the injecting fluid flow rate.

Figure 3.. Microfluidic titration using the acoustofluidic picoinjector.

(A) Schematic for on-chip acid-base titrations via acoustofluidic picoinjection. (B) Relationship between injecting fluid flow rate and pH for acetic acid titrated with sodium hydroxide. Inset images show the color transitions of the pH indicator methyl red as titration ensues. (C-E) Images showing droplet color at various points during titration.

Figure 4.. Nanoparticle synthesis via acoustofluidic picoinjector.

(A) Device setup for nanoparticle synthesis, including the redox reaction to synthesize gold nanoparticles with citrate. (B) TEM image and (C) size distribution of gold nanoparticles synthesized using the acoustofluidic picoinjector. (D) TEM image and (E) size distribution of gold nanoparticles synthesized using an off-chip bulk synthesis method. (F) TEM image and (G) size distribution of silver nanoparticles synthesized using the acoustofluidic picoinjector. (H) TEM image and (I) size distribution of silver nanoparticles synthesized using an off-chip bulk synthesis method.