Dosing of femto liter volumes using hollow cantilever AFM
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Abstract
The subject discussed in this thesis is the dosing of droplets in the femto (10-15) liter range. In this study, droplets were dispensed using a hollow cantilever atomic force microscope (AFM). Dispensing small volumes finds its applications in for example in single cell manipulation, where a drug is delivered inside a single cell to study the reaction of the cell to the drug. The working principle of an AFM can be compared with a vinyl player: a small needle is scanning a surface, which can be used for imaging with atomic resolution. For controlled dispensing of small droplets the needle and the cantilever were made hollow and a commercially available AFM system was adapted for fluidic applications. The hollow cantilever AFM chip is connected by tubing to a syringe which acts as fluidic reservoir. Small droplets of water will evaporate almost immediately; to increase the “lifetime” of the droplet a climate controller was added to increase the humidity. Therefore the influence of the climate controller on the AFM system was studied. With the described system droplets in the femto liter range were dispensed on a surface, however controlled dispensing remained a challenge. Therefore this research investigates the parameters influencing the droplet size and how these parameters can be controlled to dispense a desired volume. Two different ways of dispensing are experimentally investigated: just by touching the substrate and using the surface forces to pull liquid from the cantilever onto the substrate and by applying pressure to the liquid reservoir. For dispensing by surface forces, parameters such as surface energy and contact time are investigated to deliver a desired volume. A trend was observed that with increasing contact time an increase in volume is dispensed. For dispensing with additional pressure, a correlation was found between the time of applied pressure and dispensed volume. Dispensing of droplets of around 0.4 ? with a variation of 0.05 ? has been shown.