Liquid dosing on the micro-scale

Abstract

The processes that take place within cells are complex. Single-cell analysis is a method that is employed to gain further understanding of the working mechanisms on a single-cellular level. The controlled transport of substances through the cell membrane is important for studying the behaviour and responses of single living cells. Femtopipettes are used as a means to accurately target and sample cells with high viability rates. Different actuation principles exist for femtopipettes, but pressure actuation is identified as the most versatile and straightforward method. However, the volume dosing resolution of pressure driven femtopipettes lags behind other actuation methods. Throughout literature, the minimum reported dose is identified as 100 femtolitre (1 fL= 10^-15 L), achieved by applying a pressure pulse to a femtopipette. In this work, two new concepts are proposed and researched with the goal of increasing the volume dosing resolution of pressure driven femtopipettes. A multi-scale 3D printing strategy was employed where functional femtopipettes were successfully printed using two-photon-polymerization (2PP). The first concept incorporates a physical barrier in the form of a flexible membrane into the femtopipette. It was expected that the volume actuated by the deformation of the membrane could be controlled and calibrated. The deformation of 2PP printed membranes was characterized and the achieved volume displacement was well in the desired range of 100 fL. However, tests with liquid dosing did not succeed in achieving this same range, and difficulties were experienced with reproducibility. The second concept exploited the phenomenon of capillarity by incorporating axisymmetrical phaseguides as a means to control the position of the liquid-air meniscus. Consecutive geometrical steps were created that allowed for discrete and robust control over the liquid portion within the femtopipette. Step sizes of 10 picolitre (1 pL= 10^-12 L), 200 fL and 60 fL were fabricated and successfully tested, breaching beyond the current state of the art. This concept thus demonstrates both the level of customization and the unprecedented volume dosing resolution that was achieved.