Laura
Restrepo-Pérez
a,
Lluís
Soler
ab,
Cynthia S.
Martínez-Cisneros
a,
Samuel
Sánchez
*ab and
Oliver G.
Schmidt
ac
aInstitute for Integrative Nanosciences, Leibniz Institute for Solid State and Materials Research Dresden, Helmholtzstraße 20, 01069 Dresden, Germany
bMax Planck Institute for Intelligent Systems, Heisenbergstr. 3, D-70569 Stuttgart, Germany. E-mail: sanchez@is.mpg.de
cMaterials Systems for Nanoelectronics, TU Chemnitz, 09107 Chemnitz, Germany
First published on 11th February 2014
We demonstrate that catalytic micromotors can be trapped in microfluidic chips containing chevron and heart-shaped structures. Despite the challenge presented by the reduced size of the traps, microfluidic chips with different trapping geometries can be fabricated via replica moulding. We prove that these microfluidic chips can capture micromotors without the need for any external mechanism to control their motion.
Previous studies have shown the possibility to control the speed and directionality of these micromotors using external mechanisms such as magnetic fields,23,24 light,25,26 ultrasound27,28 or temperature.29,30 However, to our knowledge, the trapping of micromotors using patterned structures to confine the space where they swim, without the use of external sources, has not been experimentally reported.
While methods for trapping objects at the macroscale are well established and have been used since ancient times (i.e. for fishing and hunting), trapping self-propelled objects at the microscale becomes challenging due to the strongly reduced size of the traps. In a recent publication, Löwen's group reported a theoretical model in which static chevron-shaped structures can be used to trap self-propelled rod-like particles.31 Additionally, ratchets of different geometries have been previously proposed to redirect the motion of motile entities such as bacteria and molecular motors.32,33
Here, we integrate these two types of structures – chevrons and ratchets – to demonstrate the trapping and confinement of micromotors due to the steric hindrance that these physical boundaries cause on their movement. For this purpose, we developed a series of microfluidic chips containing patterns of different geometries and allowed micromotors to swim freely in these chambers without the influence of any external mechanism.
We first fabricated chips containing chevron-shaped structures and studied the feasibility of using these systems for the trapping of micromotors. We found a direct relation between the angle of the chevron apex and the trapping efficiency of these structures.
We also studied the effect of ratchets on the motion of artificial micromotors and we could observe that the rectifying effect previously reported for biological motile entities is also observed for artificial micromotors. These two structures were finally combined to create a microfluidic chip containing a heart-shaped reservoir in which micromotors get trapped over time due to the two previously mentioned mechanisms. We present the use of micropatterned walls for the control of trajectories of self-propelled particles.
To verify the feasibility of trapping micromotors with chevron-shaped structures, we conducted a series of experiments in which micromotors were placed in microfluidic chambers containing chevrons of different angles and without the effect of any external force. Initially, we used microfluidic chips containing single chevrons of 40°, 116° and 140°, following some of the values calculated by Löwen and co-workers. Preliminary experiments verified the influence of the chevron angle on the trapping of micromotors (Fig. 1). For the smallest angle (40°), micromotors remained trapped at the apex of the chevron and a retention time of over 14 s was found for the presented example in Fig. 1a. Bigger angles (116° and 140°), on the other hand, allowed micromotors to rapidly escape from the apex and no trapping was observed, as illustrated in Fig. 1b and c (see video S1 in the ESI†).
For a more detailed study of the influence of the chevron angle on micromotor trapping, microfluidic chips containing an ensemble of chevrons of different angles were used (Fig. 2a inset). For angles of 40°, 64°, 80°, 116° and 140°, videos were recorded and a parameter named trapping efficiency was measured. The trapping efficiency was defined as the ratio of the number of micromotors trapped to the number of micromotors that enter the trapping area. The trapping area is defined as the area of the triangle created by the two sides of the trap, as illustrated in Fig. 2b. A micromotor is considered to be trapped when it remains confined in the trapping area for a minimum of 10 seconds and until the end of the video acquisition process (ca. 50 seconds). The trapping efficiencies found for these angles are presented in the graph shown in Fig. 2a. As observed with single chevrons, trapping of micromotors is reduced for bigger angles as compared to their smaller counterparts. For our system, the relation between the chevron angle and its trapping efficiency was fitted with a sigmoidal function (coefficient of determination (R2) = 0.99). Fig. 2b shows structures containing 64° and 116° angles. In the time-lapse, the violet arrows point at two micromotors approaching the two different angles. Here, we can observe how multiple micromotors are being trapped in the 64° angle while no trapping is observed for 116° (see video S2 in the ESI†).
Opposite to what was observed in the theoretical model proposed by Löwen and coworkers, giant aggregates of micromotors are not observed in our traps. In our case, individual micromotor trapping dominates over collective self-trapping and only small aggregates of a few motors were observed to jam at the chevron apex. This is in accordance with previous studies that report that micromotors swimming under our regular conditions (H2O2 concentration, 2–10% and 0.01–1% surfactant) do not form swarms.34 After investigating the effect of the chevron-shaped structures on micromotor trapping, we fabricated a chip that combined the trapping angle of higher efficiency (40°) with a ratchet structure. In this way, we created a microfluidic chip containing a main reservoir and a heart-shaped reservoir (Fig. 3a). The heart-shaped reservoir was intended to concentrate or trap micromotors using two different mechanisms: first, the 40° angle structure serves as a trapping chevron to avoid the return of the micromotors to the main reservoir and second, the ratchet decreases considerably the amount of motors escaping from the trapping chamber thanks to the rectification of their trajectories towards the right side of the chamber. Fig. 3 shows the trapping mechanism of these two structures (see video S3 in the ESI†).
To verify the working principle of our system, we quantified the amount of micromotors present in the heart-shaped reservoir over a certain period of time. As shown in Fig. 4, we found a gradual increase of the number of micromotors over time and a maximum of 25 micromotors were trapped in 140 s.
To investigate the effect of the size of the aperture on the heart-shaped reservoir, microfluidic chips were fabricated with three different sizes of the aperture between the main chamber and the heart-shaped chamber: 300, 100 and 50 μm. For each of these cases, we calculated the trapping efficiency of the chamber, which was defined as the ratio of the total number of micromotors that remain trapped in the heart-like chamber to the total number of micromotors that enter the chamber (i.e. motors that remain in the chamber and motors that escape). The results obtained for each case are depicted in Fig. 5. It is shown that the higher efficiency is found for the microchip containing the 50 μm aperture, which is expected because the space available for micromotors to escape is reduced.
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Fig. 5 Trapping efficiency of the heart-shaped chambers with different aperture sizes. The insets show the microchip structures used in each case. |
Footnote |
† Electronic supplementary information (ESI) available: Supporting videos (S1; S2 and S3). See DOI: 10.1039/c3lc51419f |
This journal is © The Royal Society of Chemistry 2014 |