Abstract
Complementary to the quickly advancing understanding of the swimming of microorganisms, we demonstrate rather simple design principles for systems that can mimic swimming by body shape deformation. For this purpose, we developed a microswimmer that could be actuated and controlled by fast temperature changes through pulsed infrared light irradiation. The construction of the microswimmer has the following features: (i) it is a bilayer ribbon with a length of 80 or 120 upmu m, consisting of a thermo-responsive hydrogel of poly-N-isopropylamide coated with a 2-nm layer of gold and equipped with homogeneously dispersed gold nanorods; (ii) the width of the ribbon is linearly tapered with a wider end of 5 upmu m and a tip of 0.5 upmu m; (iii) a thickness of only 1 and 2 upmu m that ensures a maximum variation of the cross section of the ribbon along its length from square to rectangular. These wedge-shaped ribbons form conical helices when the hydrogel is swollen in cold water and extend to a filament-like object when the temperature is raised above the volume phase transition of the hydrogel at 32,^{circ } hbox {C}. The two ends of these ribbons undergo different but coupled modes of motion upon fast temperature cycling through plasmonic heating of the gel-objects from inside. Proper choice of the IR-light pulse sequence caused the ribbons to move at a rate of 6 body length/s (500 upmu m/s) with the wider end ahead. Within the confinement of rectangular container of 30 upmu m height and 300 upmu m width, the different modes can be actuated in a way that the movement is directed by the energy input between spinning on the spot and fast forward locomotion.Graphic abstract
Highlights
Self-propelling microorganisms [1] such as Euglena, a single-cell flagellate eukaryote, move by large amplitude periodic shape deformations
Microorganisms cope with this constraint by deformation of their body in a hysteresis sequence [6], which is one of the challenges in the design of periodically actuated microscopic swimmers
E (2021) 44 :79 of micrometers long, one micrometer thick and had a width of 5 micrometers. They were prepared from a thermo-responsive poly(N -isopropylacrylamide) (PNIPAm) hydrogel whose solubility decreased with temperature [20], causing volumetric shrinkage
Summary
We have described preparation and motility of helical ribbons that were actuated by periodic pulse irradiation with near IR-light [16,17,18,19]. We note that ribbons are long narrow strips possessing three distinct material length scales (thickness, width, and length) which produce unique shapes unobtainable by wires or filaments. Our ribbons were typically a few tens
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