Division of Biomedical Engineering

Particles can be trapped and retained against flow by means of acoustic radiation forces. A common way of implementing an acoustic trap is to use a small piezoelectric transducer to generating a local acoustic standing wave field in a micro channel. Microparticles that enter the transducer region will be influenced by the primary l/2 standing wave field and focused in the pressure node plane (Fig 1a). At the same time the particles are also influenced by the lateral gradient field that force particles together in the pressure nodal plane (Fig 1 b-d). This mode of operation enables efficient enrichment of particles and cells and once trapped buffer conditions can be altered e.g. offering in situ labelling and washing.

Acoustic trapping works well under standard acoustophoresis conditions, i.e. when the radiation force on the particles is significantly larger than Stokes drag forces induced by acoustic streaming or perfusion flow. Consequently, acoustic trapping systems operated in the low MHz range display poor ability to trap submicron particles. However, if larger particles, so called seed-particles, 5-10 um, are pre-loaded in the trapping region, submicron particles will be attracted to the seed-particles by the inter particle forces that arise due to scattered sound interaction between the seed particles and the submicron particles, see figure below.

By preloading the trapping unit with seed-particles and aspirate e.g. a dilute bacteria suspension enrichment of pathogens can be accomplished in a short time period with minimal losses. More recently it has also been demonstrated that submicron bioparticles such as extracellular vesicles (EV) can be enriched and isolated and washed from its background matrix, enabling studies of biomarkers specifically linked to the EV population. The figure below shows a schematic of seed-particle mediated trapping of submicron particles. Most importantly, EV isolation can be accomplished from very small sample volumes (< 50 ul), enabling access to e.g. blood plasma or CSF biobanks.