We report on cavitation in confined microscopic environments which are commonly called microfluidic or lab-on-a-chip systems. The cavitation bubble is created by focusing a pulsed laser into these structures filled with a light-absorbing liquid. At the center of a 20 microm thick and 1 mm wide channel, pancake-shaped bubbles expand and collapse radially. The bubble dynamics compares with a two-dimensional Rayleigh model and a planar flow field during the bubble collapse is measured. When the bubble is created close to a wall a liquid jet is focused towards the wall, resembling the jetting phenomenon in axisymmetry. The jet flow creates two counter-rotating vortices which stir the liquid at high velocities. For more complex geometries, e.g., triangle- and square-shaped structures, the number of liquid jets recorded correlates with the number of boundaries close to the bubble.
We report here the sonoporation of HL60 (human promyelocytic leukemia) suspension cells in a microfluidic confinement using a single laser-induced cavitation bubble. Cavitation bubbles can induce membrane poration of cells located in their close vicinity. Membrane integrity of suspension cells placed in a microfluidic chamber is probed through either the calcein release out of calcein-loaded cells or the uptake of trypan blue. Cells that are located farther away than four times Rmax (maximum bubble radius) from the cavitation bubble center remain fully unaffected, while cells closer than 0.75 Rmax become porated with a probability of >75%. These results enable us to define a distance of 0.75 Rmax as a critical interaction distance of the cavitation bubble with HL60 suspension cells. These experiments suggest that flow-induced poration of suspension cells is applicable in lab-on-a-chip systems, and this might be an interesting alternative to electroporation.
When bubbles oscillate close to cells in suspension they can cause viable or permanent poration of the cell membrane. This method named as sonoporation offers potentials for novel therapeutic applications in medicine i.e., cell killing or actively induced drug uptake. Until now, the details of bubble-to-cell interactions are not clarified especially due to a lack of experimental methods. Here we show that sonoporation of cells in suspension is made possible and can be studied in details when using a microfluidic system together with laser-induced cavitation bubbles.
Cavitation – the growth and collapse of mostly empty bubbles — is commonly attributed to large scale or very rapid flows, e.g. at ship propellors or at fuel injection nozzles. Cavitation is very aggressive to materials and one reason is its ability to focus fluid flows to very small scales; the bubbles concentrate the energy from the fluid during their shrinkage. Only recently the attention from largely free cavitation bubbles has shifted towards the study of more confined bubbles [1–5]. Here we report on an experiment to exploit cavitation in microfluidic systems or so called lab-on-a-chip devices for flow handling and biological cell manipulation. In microfluidics generally due to the small scales low Reynolds number flows are observed. Yet, cavitation bubble-induced flows allow to reach a high Reynolds number regime also on these small scales. By exploiting this rarely studied flow regime new techniques for liquid and cell handling become feasible. Here, we will report first on the effect of a channel wall on the bubble dynamics and then present an application for cell handling and membrane poration.
In this talk we give an overview on the usage of single cavitation bubbles to pump, mix, and manipulate cells in microfluidics. The bubbles are generated with a laser pulse in optically transparent lab-on-a-chip devices. The bubble pulsations is inherently fast, thus although the characteristic dimensions are small high Reynolds numbers flow can be achieved. Experiments show that depending on the channel height 2-dimensional or 3-dimensional fluid flow is generated. Interestingly, there exists a regime which can be described with the Laplace equation, thus it is essentially a potential inviscid flow. We will present the current work (of others and our group) on cavitation assisted pumping using the jetting effect, mixing flows through the creation of vorticity, and the interaction of a bubble with suspension cells. The first results on the latter promise a fruitful future for biologic relevant applications in integrated lab-in-chip devices.
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