No abstract

Acoustic trapping of sub-micron particles can allow enrichment and purification of small-sized and low-abundance microorganisms. In this paper, we investigate the dependency of the ability to capture sub-micron particles on the particle concentration. Based on the findings, it is demonstrated that seed particles can be introduced to acoustic trapping, to enable capture of low-abundance sub-micron particles. Without using seed particles, continuous enrichment of 490 nm polystyrene particles is demonstrated in a rectangular capillary with a locally generated acoustic field at high particle concentrations, i.e. above 1% wt. Trapping of sub-micron particles at significantly lower concentrations was subsequently accomplished by seeding 10-12 micrometer-sized particles in the acoustic trap prior to the sub-micron particle capture. Furthermore, the new seeded-particle-aided acoustic trapping technique was employed for the continuous enrichment of bacteria (E. coli) with a capture efficiency of 95%. Finally, seed particle assisted acoustic trapping and enrichment is demonstrated for polymer-based particles down to 110 nm in diameter.

Non-contact trapping using acoustic standing waves has shown promising results in cell-based research lately. However, the devices demonstrated are normally fabricated using microfabrication or precision machining methods leading to a high unit cost. In e.g. clinical or forensic applications avoiding cross-contamination, carryover or infection is of outmost importance. In these applications disposable devices are key elements, thus making the cost per unit a critical factor. A solution is presented here where low-cost off-the-shelf glass capillaries are used as resonators for standing wave trapping. Single-mode as well as multi-node trapping is demonstrated with an excellent agreement between simulated and experimentally found operation frequencies. Single particle trapping is verified at 7.53 MHz with a trapping force on a 10 microm particle of up to 1.27 nN. The non-contact trapping is proved using confocal microscopy. Finally, an application is presented where the capillary is used as a pipette for aspirating, trapping and dispensing red blood cells.

This work proposes and demonstrates an acoustic trapping system where the trapping frequency is automatically determined and can be used to analyse changes in the acoustic trap. Critical for the functionality of this system is the use of a kerfed transducer that removes spurious resonances. This makes it possible to determine the optimal trapping frequency by analysing electrical impedance. It is demonstrated that the novel combination of a kerfed transducer and acoustic trapping in glass capillaries creates a high Q-value resonator. This narrows the frequency bandwidth but allows excellent performance, as confirmed by a ten-fold increase in the flow retention speed when compared to previously reported values. Importantly, the use of automatic frequency tracking allows the use of such a narrow bandwidth resonator without compromising system stability. As changes in temperature, buffer-properties, and the amount of captured particles will affect the properties of the acoustic resonator, corresponding changes in resonance frequency will occur. It is shown that such frequency changes can be accurately tracked using the setup. Therefore, monitoring the frequency over time adds a new feature to acoustic trapping, where experimental progress can be monitored and the amount of trapped material can be quantified.

Matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is currently changing the clinical routine for identification of microbial pathogens. One important application is the rapid identification of bacteria for the diagnosis of bloodstream infections (BSI). A novel approach based on acoustic trapping and an integrated selective enrichment target (ISET) microchip that improves the sample preparation step for this type of analysis is presented. The method is evaluated on clinically relevant samples in the form of Escherichia coli infected blood cultures. It is shown that noncontact acoustic trapping enables capture, enrichment, and washing of bacteria directly from the complex background of crude blood cultures. The technology replaces centrifugation-based separation with a faster and highly automated sample preparation method that minimizes manual handling of hazardous pathogens. The presented method includes a solid phase extraction step that was optimized for enrichment of the bacterial proteins and peptides that are used for bacterial identification. The acoustic trapping-based assay provided correct identification in 12 out 12 cases of E. coli positive blood cultures with an average score of 2.19 ± 0.09 compared to 1.98 ± 0.08 when using the standard assay. This new technology opens up the possibility to automate and speed up an important and widely used diagnostic assay for bloodstream infections.