Abstract
We present the development of a microfluidic AFM (atomic force microscope) cantilever-based platform to enable the local dispensing and aspiration of liquid with volumes in the pico-to-femtoliter range. The platform consists of a basic AFM measurement system, microfluidic AFM chip, fluidic interface, automated substrate alignment, external pressure control system and controlled climate near the dispensing area. The microfluidic AFM chip has a hollow silicon dioxide (SiO2) cantilever connected to an on-chip fluid reservoir at one end and a slicon nitride (Si3N4) tip with an aperture on the other end. A 3D printed plastic fluidic interface glued over the on-chip reservoir was used to connect microfluidics and macrofluidics. The fluidics is connected to an external pressure control system ranging from −0.8 bar to 5 bar with 0.1 bar resolution. This pressure range allows dispensing and aspiration of liquids through the cantilever tip aperture. The controlled climate with a temperature control range between 25°C – 40°C and humidity up to 95% near the dispensing area keeps the droplets for sufficiently long time before they evaporate. An array of droplets can be programmed to be dispensed automatically and access them again with a position accuracy of 1 micron. Experiments were performed with two types of cantilevers with different geometrical configurations. A minimum flow rate control of 50 fL/s was obtained and also frequency shift was monitored as the cantilever was filled with liquid. This platform will be used for various chemical and biological applications.
Highlights
The ability to controllably infuse or withdraw different types of reagents with nanometer precision unlocks the door for new processes that can lead to novel opportunities like cell surgery
In order to overcome these challenges and get closer towards the envisioned goals, we have developed a microcantilever Atomic Force Microscopy (AFM)-based platform to enable the dispensing and aspiration of liquids with volumes in the femtoliter range under controlled environment
The various parts in this system constitute a basic AFM measurement system, a microfluidic chip, an automated substrate alignment, a humidity chamber and a microfluidic interface connected to an external pressure control system
Summary
The ability to controllably infuse or withdraw different types of reagents with nanometer precision unlocks the door for new processes that can lead to novel opportunities like cell surgery. In attempts to manipulate liquid in the smallest possible regime, scientists have started to van Oorschot et al EPJ Techniques and Instrumentation (2015) 2:4 develop cantilever-based techniques (i.e. DPN [3], NADIS [4], NanoFountain Probe [5] and FluidFM [6]) which have been proved to surpass the picoliter barrier and handle volumes as small as few zeptoliters [7] These devices, compatible with Atomic Force Microscopy (AFM), combine the versatility of microfluidics with the high precision provided by the cantilever [6]. After filling the reservoir with liquid, a positive or negative pressure relative to the ambient is applied in the reservoir to infuse or withdraw liquids through an aperture located at the apex of the tip Constant volume systems such as a syringe pump used for applying pressure differences suffer from the effect of shock waves produced by the stepper motors and have a slow response at low flow rates. The results shown here represent an improvement over our earlier work on thermal pumping [8], evaporation based pumping and aspiration [11] and syringe-pump based pipetting [12]
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