We could direct swelling and shrinkage to distinct bending and twisting modes of motion, e.g. a tweezer-like motion of an L-shaped hydrogel object, whereas the ends punch each other on the beat of IR-light, a helix that stretches and inverses upon raising temperature and a wedge-like hydrogel object that undergoes transformation from a spiral to a stretched wiggeling oscillator. Actuation by light caused a cylic non-equilibrium process in which the forward stroke and the backward stroke differed in their motion sequence fulfilling the conditions for self-directed motion at low Reynolds Numbers. Irradiation energy transformed to mechanical work and enabled propulsion while the swimming direction is independent of the orientation of an external field. In the case of wedge-shaped microgel objects, the two ends undergo different but coupled modes of motion and swimming could be directed by the energy input between spinning on the spot and fast forward locomotion at a rate of 500 µm/sec with the wider end ahead. As the volume change of the gel objects depends on the environmental temperature and osmotic pressure, i.e. in particular on salt concentration, such microswimmers will navigate by outside temperature gradients as well as by gradients in salt concentrations.
Furthermore, we addressed fully autonomous motion, which requires, that the swimmer itself will control switching between energy take-up and relaxation, i.e. ensure that shrinkage upon heating is followed in due time by expansion during cooling under continuous irradiation. The hydrogel-object must dispose of a feed-back mechanism that switches off the heating when a certain volume shrink-age is achieved and switches on the heating again after a certain expansion. However, in order to avoid taht the gel-object does quickly run into an equilibrium configuration, a kind of enhancement is needed that retards the volume change and variations in the absorption spectrum. Both aspects, a work per¬forming cycle and the prevention of equilibration require introducing hysteric bistability. This can be demonstrated by a volume/temperature diagram corresponding to a heat engine. The change in temperature builds up an internal stress, either osmotic or elastic. Temperature rise and cooling by variation of the plasmonic heating is fast because of the fast heat transfer and the rise in temperature in the first step is faster then the change of the volume when the water is pressed out by the collapse of the gel. In the second step, however, the mechanical response should be faster than the temperature change. As small volume changes transform to fast bending and introducing, a barrier to the shape deformation causes a fast snap-back motion when the stress is sufficient to overcome the barrier. During the rest of the cycle, temperature and deformation must reverse, first by fast cooling and than by fast expansion. The introduction of a snap-motion has been realized by the design of bilayer micogels. A rectangular ribbon posses two primary bending modes, transverse and longitudinal bending, which impede each other depending on the length to width to height ratio. Snapping can be observed as the released internal stress switches bending from one main direction to another. Other snapping hydrogel objects we looked at were a dome type structure and disc shaped microgels with an embossed line pattern. At the snap transition, we could demonstrate strong coupling to small thermal fluctuations that lead to large oscillation. This is shown by a gel ribbon that undergoes 4 m amplitude oscillating motion at T 0,1 °C. The physical reason is that at the snap transition, bending is coupled to very small changes of temperature. Visualization of the flow pattern around the osciallting tips of the circularly bent ribbon clearly demonstrates that the ribbon performs work on its surrounding. We consider this an indication for an increased efficiency as described for a “critical heat engine" (Campisi M., Fazio, R. Nature Commun. 2016, 7, 11895). So far, we had to realize that the observed motility of our microgel objects was susceptible to very small variations in their dimensions and internal structure. Fabrication by the PRINT technique (Nano Letters, 2010, 10, 1421-1428) did not allow to prepare large ensembles that behaved identical. This limited our search for an optimized self-oscillating microgel. Therefore we established a new microfluidic synthesis of worm like microgels within which nanorods were oriented along the main axis enabling polarized feed back (not yet published)
