Surface-enabled propulsion and control of colloidal microwheels

ARTICLE Received 3 Sep 2015 | Accepted 18 Nov 2015 | Published 4 Jan 2016 DOI: 10.1038/ncomms10225 OPEN Surface-enabled propulsion and control of colloidal microwheels T.O. Tasci1, P.S. Herson2,3, K.B. Neeves1,4 & D.W.M. Marr1 Propulsion at the microscale requires unique strategies such as the undulating or rotating filaments that microorganisms have evolved to swim. These features however can be difficult to artificially replicate and control, limiting the ability to actuate and direct engineered microdevices to targeted locations within practical timeframes. An alternative propulsion strategy to swimming is rolling. Here we report that low-strength magnetic fields can reversibly assemble wheel-shaped devices in situ from individual colloidal building blocks and also drive, rotate and direct them along surfaces at velocities faster than most other microscale propulsion schemes. By varying spin frequency and angle relative to the surface, we demonstrate that microwheels can be directed rapidly and precisely along user-defined paths. Such in situ assembly of readily modified colloidal devices capable of targeted movements provides a practical transport and delivery tool for microscale applications, especially those in complex or tortuous geometries. 1 Chemical and Biological Engineering Department, Colorado School of Mines, Golden, Colorado 80401, USA. 2 Department of Anesthesiology, University of Colorado, Denver, Colorado 80045, USA. 3 Department of Pharmacology, University of Colorado, Denver, Colorado 80045, USA. 4 Department of Pediatrics, University of Colorado, Denver, Colorado 80045, USA. Correspondence and requests for materials should be addressed to D.M. (email: ). NATURE COMMUNICATIONS | 7:10225 | DOI: 10.1038/ncomms10225 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms10225 A B0 (mT) t the microscale, fluid dynamics are unique because Superparamagnetic beads assemble into microwheels by isotropic viscous forces dominate over inertial forces, a condition interactions induced by the in-plane rotating magnetic field typically characterized by Reynolds numbers (Re) less (Fig. 1a,b) with wheel size controlled by local bead density. than unity. Because propulsion schemes that rely on inertial Spinning microwheels lying flat on a surface have no net motion. forces cannot be used, translation requires approaches adapted For translation to occur they must be inclined relative to the to overcome the inherent reversibility of low-Re flows by breaking surface; therefore, to propel microwheels, we introduce a normal symmetry1–3. For example, microorganisms use undulating or component to the magnetic field to orient the field rotation axis rotating flagella and cilia for motility. In the push for towards the surface plane (Supplementary Movies 1 and 2). With technological devices small enough to move through microscale addition of a field in the z direction, Bz ¼ Bz0 cos (oft  fz), both channels (10–100 mm) over macroscale distances (41 cm), such symmetric and asymmetric microwheels reorient off the surface as those found in human vasculature, there is appreciable effort in to a defined camber angle, yc (Fig. 1c,d), and begin to translate. developing equivalent artificial approaches. Propulsion schemes Apparent in this approach is the similarity of microwheels to based on catalytic methods4–6 or on cellular machinery rolling tires where friction with the road, combined with tire analogues7–9 have shown good progress; however, significant rotation, propels wheels forward. One difference between challenges remain. Though catalytic swimmers can reach high microwheels and tires is that the camber angle can vary from speeds10, they require available solute fuel for propulsion and lying flat, yc ¼ 90°, and spinning without translation to fully concentration gradients for direction11. While top–down fabrication of flagella has led to speeds comparable to some microorganisms, they cannot be reconfigured for applications in dynamic or varying environments. Colloidal assembly is a promising alternative. These methods provide bottom-up fabrication using simple colloidal building blocks as components of microstructures that are rapidly and reversibly assembled into a variety of sizes and shapes. Fabrication is initiated via specific12,13 or non-specific14 interactions or supplemented with applied electrical15, optical16 Translation c n Bx,By,Bz = 1,1,1 or magnetic fields17 to enable switching and direct control of size, structure and function18. Used with superparamagnetic colloids, magnetic fields are well-suited to assemble structures in situ that Assembly and rotation are easily manipulated and rapidly disassembled after use. Fields Bx,By,Bz = 1,1,0 Bz of only a few milli-Tesla (mT) create sufficient dipole strength to B y induce colloidal assembly. Static applied fields align particles into chains, while rotating fields create net isotropic interactions that By can lead to compact aggregates19. In our studies, we use B Bx x superparamagnetic colloids and balance magnetic and viscous forces with appropriate field strengths and rotational frequencies to create reversible close-packed assemblies that subsequently Btotal (mT) 90 spin due to their net dipole interacting with the dynamic applied 15 Bz0/B0 field20,21. While rotating magnetic fields can construct 80 2 1 microwheels and create a driving torque, the reversible nature of low-Re flows dictates that spinning symmetric objects 70 3 suspended in fluid do not translate. For net movement to 60 10 occur, symmetry, either in the device or in the surrounding 5 geometry, must be broken. In an approach particularly 50 appropriate for microenvironments where surface to volume c 40 ratios are high and surfaces are plentiful, one way to break the symmetry is with a nearby wall. 30 5 Here, we show that rotating magnetic fields can be used to Dimers 20 assemble and spin microwheels that, when canted relative to Trimers the surface, roll smoothly and quickly with a high degree of 7-mers 10 directional control. We demonstrate this propulsion mechanism 0 0 with microwheels composed of 1, 2, 3, 7 and even 19 0 10 20 30 40 50 60 70 80 90 paramagnetic colloidal particles with translation speeds Tan–1 (B0 / Bz0) 4100 mm s  1. These results demonstrate a rapid and reversible microdevice assembly and powering method that overcomes Figure 1 | Field-induced assembly and rotation. (a,b) With application of many of the limitations inherent in biomimetic artificial the rotating magnetic field B þ B in the surface plane, colloids assemble x y micropopulsion strategies. via isotropic interactions and ‘sit and spin’ (scale bar, 20 mm). (c) With addition of a normal variable-phase component (Bz), the field rotation axis is oriented towards the surface plane, wheels ‘stand up’ at a camber angle, Results yc, and roll along the surface. (d) yc measured during wheel translation as a Microwheel assembly and translation mechanism. We beg (...truncated)