Acoustic trapping with seed-particles for submicron particle enrichment

Abstract

Acoustic trapping in disposable borosilicate capillaries utilize ultrasonic forces to capture/retain micro-particles or cells against fluid flow in a microfluidic-channel. A miniaturized ultrasonic transducer is used to locally excite a 4-MHz cross-sectional resonance in the capillary, creating an acoustic field gradient for retention of cells in non-contact mode. Due to competition between fluidic drag from induced acoustic streaming and primary radiation force the smallest particle size addressable with the trapping system is limited. Here, the typical transition occurs at single-micron particle diameters. However, trapping of single- or sub-micron biological species has highly relevant applications such as enrichment or purification of bacteria or viruses. This work investigates the influence of in-trap particle concentration on the trapping, and it is found that elevated concentrations allow capture of submicron particles. By preloading the acoustic trap with micron-sized seed-particles capture of submicron particles even at low concentrations is enabled. Using this technique, we demonstrate single event capture of bacteria as well as capture of 100nm particles. To provide analytical readout for identification/analysis of the trapped particles the acoustic trap is interfaced with a MALDI-MS instrument. Here, the acoustic trapping capillary is operated in aspirate/dispense mode allowing easy and flexible handling of small sample volumes.