Spatial Control over Catalyst Positioning for Increased Micromotor Efficiency

Abstract

Motion is influenced by many different aspects of a micromotor's design, such as shape, roughness and the type of materials used. When designing a motor, asymmetry is the main requirement to take into account, either in shape or in catalyst distribution. It influences both speed and directionality since it dictates the location of propulsion force. Here, we combine asymmetry in shape and asymmetry in catalyst distribution to study the motion of soft micromotors. A microfluidic method is utilized to generate aqueous double emulsions, which upon UV-exposure form asymmetric microgels. Taking advantage of the flexibility of this method, we fabricated micromotors with homogeneous catalyst distribution throughout the microbead and micromotors with different degrees of catalyst localization within the active site. Spatial control over catalyst positioning is advantageous since less enzyme is needed for the same propulsion speed as the homogeneous system and it provides further confinement and compartmentalization of the catalyst. This proof-of-concept of our new design will make the use of enzymes as driving forces for motors more accessible, as well as providing a new route for compartmentalizing enzymes at interfaces without the need for catalyst-specific functionalization.

Keywords: aqueous phase separation; autonomous motion; microfluidics; micromotor; spatial localization.

Figure 1

Schematic representation of the experimental design. (A) Close-up of the ATPS-based microfluidic chip to generate droplet-in-droplet morphology. An aqueous-two-phase jet is formed at the first cross junction, which is emulsified at the second cross junction by a surfactant containing oil. (B) Asymmetric microgels are obtained after UV-polymerization of the droplets generated by the microfluidic chip; upon addition to hydrogen peroxide the catalyst, catalase, will decompose the fuel to water and propelling oxygen. (C) Two methods to position the catalyst, either homogeneously through incorporation in the gel or spatially through adding it to the polysaccharide, templating phase.

Figure 2

Confocal microscopy images of the different micromotor systems. Catalase was labelled with a fluorescent dye, Alexa 647, to analyse its position inside the motor. Bright field images are shown left, and the corresponding fluorescence images are on the right. The position of the motor is shown in dashed lines and the fluorescence intensity profile (bottom left) was obtained over the solid line going from the opening inside the motor. Spatial control over the catalyst was obtained by dissolving the enzyme in the polysaccharide phase prior to injection into the chip. As a control the catalyst was dissolved in the PEGDA gel phase, and a homogeneous distribution throughout the bead was observed. Scale bar is 20 µm.

Figure 3

(A) Bright field microscopy overlay of bubble propulsion of the three different polysaccharide systems with an interval of 0.5, 1, and 6 s for dextran 10 kDa, 70 kDa and Ficoll 400 kDa, respectively. (B) Typical trajectories of each motor system. Scale bar is 20 µm.

Figure 4

(A) The average speed over 10 s of the enzyme-localized systems compared to the previously obtained homogeneous systems [6] for all three polysaccharides at 4% hydrogen peroxide concentration. (B) The instantaneous speed of both systems was analysed over a time-course up to 60 s after addition at 4% hydrogen peroxide concentration.