Microtransformers: Controlled microscale navigation with flexible robots

Artificial microswimmers are a new technology with promising microfluidics and biomedical applications, such as directed cargo transport, microscale assembly, and targeted drug delivery. A fundamental barrier to realizing this potential is the ability to independently control the trajectories of multiple individuals within a large group. A promising navigation mechanism for “fuel-based” microswimmers – for example, autophoretic Janus particles – entails modulating the local environment to guide the swimmer, for instance by etching grooves in microchannels. However, such techniques are currently limited to bulk guidance. This Rapid Communication will argue that by manufacturing microswimmers from phoretic filaments of flexible shape-memory polymer, elastic transformations can modulate swimming behavior, allowing precision navigation of selected individuals within a group through complex environments.

Figure 1

(a) Many applications require independent control of microswimmers in a group. Left: Directing the leading helical microswimmer [7] to the right by varying the applied magnetic field causes the trailing swimmer to crash into the channel wall. Right: By selectively heating filaments at the desired location, microtransformers in a swarm could be independently directed to different targets. (b) Changing the shape of a microtransformer switches between pumping, translating, and rotating modes, allowing navigation via a series of runs, punctuated by on-the-spot reorientations.

Figure 2

Schematic surface solute concentration and flow streamlines of a flexible phoretic filament with catalytic end caps, semicoverage $s_c=0.2$, and uniform positive mobility, in (a) the pumping configuration, (b) the translating configuration, and (c) the rotating configuration.

Figure 3

(a) The tangential slip velocity of the pump with catalytic end caps and uniform, positive mobility, as a function of arclength $s$. The maximum speed occurs at the boundary between the catalytic and inert portions of the filament for all intermediate semicoverages $s_c$, in an analogous manner to elongated Janus particles [24]. Inset shows streamlines from four regularized stokeslets mimicking the flow for $s_c=0.2$ [Fig. 2]. (b) Flow decay as a function of distance from the filament center along the $x$ axis, showing a sign change in the direction of pumping as the cap semicoverage is increased. The logarithmic plot (inset) demonstrates stressletlike $1/r^2$ decay. (c) The swimming velocity of the translating configuration as a function of cap semicoverage $s_c$ for catalytic end caps and uniform, positive mobility. Swimming is bend-first. Inset shows the maximum azimuthal velocities as a function of arclength $s$ for $s_c=0.1$ (lightest), $0.2,0.3,0.4,$ and 0.5 (darkest). (d) The angular velocity of the same filament in the rotating configuration, as a function of $s_c$, showing clockwise motion with ends trailing.