Controlling the thermal-fluid environment of single cells
As stated on the Energetic BioMEMS home page, our overall goal is to control the thermal and fluid environment of large numbers of cells on the single-cell level. The projects below represent steps along that path. This includes the encapsulation of single cells into their own isolated fluid environment so that external biochemical stimuli can be controlled, using confinement to predispose a particular non-equilibrium thermodynamic phase, adjusting the concentration of chemicals in drops after their formation, or even separating the fluid environment of different parts of the same cell.
Controlled encapsulation of single-cells
Encapsulation of cells within picolitre-size monodisperse drops provides new means to perform quantitative biological studies on a single-cell basis for large cell populations. Variability in the number of cells per drop due to stochastic cell loading is a major barrier to these techniques. We overcame this limitation by evenly spacing cells as they travel within a high aspect-ratio microchannel; cells enter the drop generator with the frequency of drop formation (bars: 100 μm). This method makes use of inertial cell and particle ordering, an active area of research which is described in more detail in subsequent project descriptions. Click the title and image above to see the related publication and one of the supplemental high speed videos.
Nucleation and solidification in monodisperse microdrops
The precise measurement of nucleation and non-equilibrium solidification are vital to fields as diverse as atmospheric science, food processing, cryopreservation and metallurgy. The emulsion technique, where the phase under study is partitioned into many droplets suspended within an immiscible continuous phase, is a powerful method for uncovering rates of nucleation and dynamics of phase changes as it isolates nucleation events to single droplets. However, averaging the behavior of many drops in a bulk emulsion leads to the loss of any drop-specific information, and drop polydispersity clouds the analysis. Here we adapted a microfluidic technique for trapping monodisperse drops in planar arrays to characterize solidification of highly supercooled aqueous solutions of glycerol (a commonly used cryoprotectant). This system enabled the specific dynamics of solidification to be observed for over a hundred drops in parallel. Note that the small scale of these droplets allows much higher rates of cooling and heating than would be possible in bulk systems. We are extending this work into the realms of cryopreservation and atmospheric science. Click the title and image above to see the related publication and one of the supplemental high speed videos.
Concentrating chemicals in droplets that are already formed
A roadblock to the vitrification of cells is the requirement of high concentrations of cryoprotectant (CPA) chemicals and the damage caused by prolonged exposure of cells to these high concentrations above the glass transition temperature. Soybean oil (much more so than for other oils) is capable of dissolving small amounts of water as temperature is increased, but it does not effectively dissolve glycerol. We exploited this phenomenon to concentrate glycerol in single water droplets dispersed in the soybean oil by warming droplets and allowing the water to escape into the continuous phase. As illustrated in the study on nucleation above, confinement to picoliter volume droplets also reduces nucleation frequency for a given CPA concentration, facilitating cryopreservation through vitrification.
Separating the fluid environment of different parts of the same cell
This study utilized microfluidic cell culture to control the fluid environment of a continuous array of neurons so that mitochondrial axonal transport could be studied with laser scanning confocal microscopy. The function of this two-layer device was to separate the fluid environment of the neural cell body from that of the distal end of the axon by several long and narrow cross-section channels. Impaired mitochondrial transport along microtubules is thought to play a role in neurodegenerative diseases.
Inertial microfluidics: a tool for high-throughput cell handling
The ability to control the encapsulation of single-cells into monodisperse picoliter droplets which was described above required that the cells become evenly spaced out in the channel which precedes the drop generator. This arises by inertial ordering of cells, first by focusing the cells to two streamlines and then as they find an even spacing some time later. The projects below are within this focus area and deal with focusing and ordering in straight and then in curving microchannels. This is an active area of practical and theoretical research.
Understanding the focusing of particles in confined (straight) channels
Nonlinearity in finite-Reynolds-number flow results in particle migration transverse to fluid streamlines, producing the well-known ‘‘tubular pinch effect’’ in cylindrical pipes. Here we investigated these nonlinear effects in highly confined systems where the particle size approaches the channel dimensions. Experimental and numerical results revealed distinctive dynamics, including complex scaling of lift forces with channel and particle geometry. These results have broad implications for confined particulate flows.
Separation and filtration of cells using asymmetric curving channels
This study utilized secondary (Dean) flows which result from channel curvature to modify the focusing of particles and cells which occurs in straight channels. This allowed particles to be separated based on size and deformability. Due to the very high throughput (~1 mL / min), this has many potential applications in medical diagnostics. Click the image above to see the related publication and supplemental high speed videos.
Inertial focusing and ordering of particles in spiral microchannels
Similar to the ordering observed in straight microchannels above and also to the focusing observed in curving channels, this study instead utilized a spiral geometry to allow high throughput separation of particles based on size. This resulted in ordering of large particles along the roof and floor (alternating) of the microchannel.