Microscale flow propulsion through bioinspired and magnetically actuated artificial cilia

Abstract

Recent advances in microscale flow propulsion through bioinspired artificial cilia provide a promising alternative for lab-on-a-chip applications. However, the ability of actuating artificial cilia to achieve a time-dependent local flow control with high accuracy together with the elegance of full integration into the biocompatible microfluidic platforms remains remote. Driven by this motive, the current work has constructed a series of artificial cilia inside a microchannel to facilitate the time-dependent flow propulsion through artificial cilia actuation with high-speed (>40 Hz) circular beating behavior. The generated flow was quantified using micro-particle image velocimetry and particle tracking with instantaneous net flow velocity of up to 10(1 ) μm/s. Induced flow patterns caused by the tilted conical motion of artificial cilia constitutes efficient fluid propulsion at microscale. This flow phenomenon was further measured and illustrated by examining the induced flow behavior across the depth of the microchannel to provide a global view of the underlying flow propulsion mechanism. The presented analytic paradigms and substantial flow evidence present novel insights into the area of flow manipulation at microscale.

FIG. 1.

Schematic of microchannel layout, fabrication process flow, and fabricated artificial cilia. (a) Dimensions of the microchannel and artificial cilia holes. (b) An acrylic substrate was first carved to define the geometry of the microchannel through micromilling and microdrilling (panels 1–3). Subsequently, the holes were filled with the mixture of polydimethylsiloxane (PDMS) solution and magnetic particles (panels 4 and 5). PDMS casting (panels 6 and 7) and microchannel enclosure through oxygen plasma surface treatment (panel 8). (c) Scanning electron microscope graph of the fabricated microchannel with artificial cilia protrusion from the microchannel bottom. Each artificial cilium is 50 μm in diameter.

FIG. 2.

Comparison of actual (solid line) and fitted (dashed line) circular trajectories of artificial cilia at five selected triggering frequencies. Fitted circle radius values at 10, 20, 30, 40, and 50 Hz are 31.8, 24.6, 18.4, 12.9, and 10.1 μm, respectively.

FIG. 3.

Generated time-dependent streamwise peak flow velocity at the rotational frequencies of 30 (a), 40 (b), and 50 Hz (c). Particle images were recorded at a frame rate of 200 Hz (first column). In the second column, the frame rate was identical to the corresponding selected rotational frequency for quantification of the induced net flow.

FIG. 4.

Calculated ensemble streamwise velocity distribution ((a) and (b)) and extracted velocity profiles (c) on five measurement planes across the depth of the microchannel. Data were collected at frame rates of 200 (a) and 40 Hz (b) with rotational frequency of artificial cilia at 40 Hz for three cycles of rotation. (c) Corresponding velocity profiles across the width of the microchannel (along the AB dashed line).

FIG. 5.

Time-reversible control was achieved through the change of rotational direction of artificial cilia. Induced maximum and minimum flow velocity values along the microchannel centerline (at a depth of 400 μm above the microchannel bottom) were plotted over each 1 s time window at the recording frame rates of 200 (a) and 40 Hz (b). Rotational speed was 40 Hz with clockwise rotation in the first 3 s and counterclockwise rotation in the other time frames.

FIG. 6.

Instantaneous particle tracking results with corresponding surrounding flow fields quantified through μPIV. (a) Illustration of induced particle trajectory (highlighted in red) with three phases of action under a cyclic rotational motion of an artificial cilium (dark circle). (b) Three phases of action of the particle's trajectory were identified showing the relationship between the axial position of particle and time. Time periods for phases I, II, and III are 0.05, 0.05, and 0.025 s, respectively, which also correspond to two, two, and one rotational cycles of the artificial cilium (with rotational frequency of 40 Hz), respectively. Axial positions of the particle over time were plotted (left panel) with surrounding axial velocity maps provided (right panel) in phases I (c), II (d), and III (e).