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On-chip unthethered helical microrobot for force sensing applications

Abstract : Microfluidic chips that could perform chemical and biological processes on a few centimeter square footprint have been an active area of research in the past decades. Among other advantages, this platform offers a closed and controllable environment that allows reproducible measurements and avoids external contamination. However, such closed environments prevent the use of tethered probes to measure or apply a specific force on an element inside the microfluidic chip. Therefore we propose to use a helical rotating microrobot inside a microfluidic chip to answer this need. The proposed microrobots are designed with 3D laser lithography, and have a helical shape of 5.5 µm in diameter and around 50 µm length. A thin ferromagnetic layer is deposited on these microrobots which allows us to propel and control them with a homogenous external rotating magnetic field.The first challenge is the stable integration of these microrobots inside microfluidic environments. Therefore, in this thesis we first proved that these microrobots can use their own mobility to integrate themselves selectively (one by one) inside a microfluidic chip through a microchannel connected to an open reservoir. For this, we have developed a 3D motion where the microrobot evolves in the fluid and two different 2D motions where it evolves on a surface. By switching easily from one motion to another, the microrobots can use the different advantages of each motion to get sufficient mobility required for this integration. We named our microrobot design Roll-To-Swimm (RTS) in reference to this characteristic.Then in order to use a microrobot as on-chip force sensor, a precise characterization of the force generated by the helical shape is necessary for each RTS. A characterization method is proposed, where the different environment parameters (parasite flow, temperature gradient and impact of near surfaces on the flow) are controlled precisely thanks to the microfluidic environment. The characterization shows that the force range of the RTS is between 10 and 45 piconewton with a maximum error of 38 %. We also conclude that the main component of this error (73 %) is due to the evolution of the RTS magnetization. Therefore the efforts to reduce this error should first focus on the magnetization property of the RTS. This error can also be temporarily reduced by characterizing the RTS just before its use in another experiment.Finally, we present three different proofs of concept to demonstrate that our characterization method brings helical microrobots closer to potential on-chip force sensing applications. Firstly, we show that it is possible to measure the diminution of the RTS force when it is pushing a micro spherical bead. This is essential toward applying force on an object with this RTS or to use beads as an interface between the RTS and the surface to measure friction forces. A microbead with 10 µm in diameter at the tip of the RTS reduces it propulsion of 6 %.Secondly, we use the RTS characterization to measure local flow speed. We demonstrate this feature by measuring flow profiles in fluid channels. We show the potential use for of microrobot control by proposing an automatic control of the RTS that adapts the motion to the measured flow. This control has been tested experimentally with different flow conditions. Thirdly, we use the characterization of the RTS to perform numerical simulations in order to find a control strategy in small microchannels. Indeed we demonstrate that for microchannels below 20 times the RTS diameter, the channel walls have an impact on the RTS motions. The model of this simulation has been validated by comparing this result with experimental data. Finally we propose a control scheme for maintaining the RTS centered in a curved microchannel by only using a 2D image feedback.
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Antoine Barbot. On-chip unthethered helical microrobot for force sensing applications. Automatic. Université Paris Saclay (COmUE), 2016. English. ⟨NNT : 2016SACLS425⟩. ⟨tel-01447622⟩

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