Airborne Cell Analysis

Abstract

A miniaturized analysis system for the study of living cells and biochemical reactions in microdroplets was developed. The technique utilizes an in-house-developed piezoelectric flow-through droplet dispenser for precise reagent supply and an ultrasonic levitator for contactless sample handling. A few-cell study was performed with living primary adipocytes. Droplets (500 nL) containing 3-15 individual cells were acoustically levitated. The addition of beta-adrenergic agonists into the levitated droplet using the droplet dispenser stimulated adipocyte lipolysis, leading to free fatty acid release and a consequent pH decrease of the surrounding buffer. The addition of insulin antagonized lipolysis and hence also the decrease in pH. The changes in pH, i.e., the cell response in the droplet, were followed using a pH-dependent fluorophore continuously monitored by fluorescence imaging detection. An image analysis computer program was employed to calculate the droplet intensities. To counteract droplet evaporation, found to affect the fluorescence intensities, a separate dispenser was used to continually add water, thus keeping the droplet volume constant.

Airborne chemistry: acoustic levitation in chemical analysis

“…The use of levitated drops exhibits the same miniaturisation benefits as the chip approach, such as diversity of application and low reagent and sample consumption. In addition to the obvious advantage of levitation for preventing the chemical and thermal contamination that accompanies contact between drops and external objects, the use of levitated drops also has the added advantage of increased sensitivity of detection, since no walls disturb detection [12,15].…”

“…2) consisting of an acoustic levitator combined with flow-through pL dispensers [14,44,45] has been used in our laboratory for bioanalytical purposes. This airborne analytical system [15,39,40,41,42] is equipped with different and exchangeable remote detection systems. The levitated drops are normally in the 100 nL-2 µL volume range and additions to the levitated drop can be made in the pL-volume range.…”

“…Among the most serious drawbacks is the risk of analyte adsorption to walls and interfaces [12,13,14,15,16,17], a problem multiplied in small-dimensional analytic systems where volumes in the picolitre (pL) or nanolitre (nL) range are used. Another problem is optical interference at the walls of the sample container hampering detection [15], especially since such low amounts of analytes are being used. Therefore, future advances in these areas require the development of analytical techniques capable of circumventing the use of contacting walls and thereby avoiding these problems and yet able to monitor minute cellular changes induced by ligands, drugs, activators, or inhibitors.…”

The Quest for High-Speed and Low-Volume Bioanalysis

“…As the surface grows, the evaporation rate eventually matches the liquid supply rate. This self-controlled evaporation also has been used to control liquid volumes in acoustically levitated droplets for wall-less nanochemistry by continuous solvent supply via a piezoelectric microdispenser (21).…”

Pumping of mammalian cells with a nozzle-diffuser micropump

“…6 Santesson et al performed ultrasonic levitation of droplets containing a dozen cells which behaved similarly to non-exposed cells. 7 For many applications, the successful transport and handling of cells on a microfluidic chip 8 requires a safe and dedicated pump 9 that can be integrated with the chip or separated from the sample flow path.…”