Out of equilibrium actuation of microprinted hydrogel objects for ultrasmall locomotor systems

• Ungleichgewichtsanregung von mikrogedruckten Hydrogelobjekten für ultrakleine Lokomotorsysteme

Zhang, Hang; Möller, Martin (Thesis advisor); Simon, Ulrich (Thesis advisor)

Aachen (2017)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

Self-propelling microobjects or colloids are a topical research subject for soft matter microrobots as well as for devices that mix, sort and circulate fluids. However, an artificial microswimmer that propels itself by pure body deformation is rather difficult to realize, since the deformations have to be non-reciprocal (or time-irreversible) during cyclic actuations, which is also known as the Scallop Theorem proposed by E. M. Purcell. Additional requirements to implement such a morphing microswimmer are a source of energy, sufficiently fast actuation, and a control mechanism for the repetition as well as for directing the motion.In this dissertation, a novel class of artificial microswimmers is presented that exploit the principle of out-of-equilibrium actuation by photothermal heating. The system consists of a crosslinked thermoresponsive hydrogel poly(N-isopropylacrylamide) laden with gold nanorods. Very fast temperature jumps localized to the volume of the microgel can be achieved by laser irradiation of the gold nanorods, which convert light energy into heat with efficiency close to unity. Because the diffusion of the gel network cannot follow fast temperature changes in time, the volume change can be effectuated out of equilibrium. Under out-of-equilibrium conditions, non-reciprocal motions are expected to take place due to the deviation in the swelling and shrinking path. The control over temperature change and rate inside a hydrogel network by photothermal heating is demonstrated, and the heating processes were shown to be distinct from the case of gold nanorods suspended in solution. Ultrasmall hydrogel objects were prepared by the PRINT (Particle Replication in Non-wetting Templates) technique, known to be effective in controlling the composition, size and geometry in the microscopic range. The actuation kinetics by photothermal heating was investigated based on a disk-shaped microgel under stroboscopic irradiations, and the actuation efficiency was estimated. Anisometric microgels have also been prepared, which demonstrated bending deformation and enhanced actuation amplitude due to the out-of-equilibrium conditions. Furthermore, bilayer hydrogel ribbons were fabricated that underwent reversible shape transformations upon swelling/shrinking. Spiral, helical or tubular geometries can be achieved depending on the initial aspect ratios of the ribbon. The helical microgels showed a response time on the millisecond scale upon photothermal actuation. The switch of the helicity can be triggered by irradiations due to the inversion of the ribbon curvature. The inversion process was found to be highly dependent on the environmental temperature and ionic species in the medium, which can be potentially utilized in sensing applications. By fine tuning of the irradiation conditions, rotational motion was achieved on both helical and spiral microgels. Analysis of the kinematics revealed non-reciprocal deformation paths under stroboscopic irradiations. When exposed to physical confinements, the helical microgels also exhibited translational motions.The results in this dissertation point out the possibility to design the modes, sequences, and amplitudes of complex body deformations of small hydrogel objects in a precise and purposeful way. The self-propulsion of microgels by dynamic control of shapes through photothermal heating provides an exciting avenue for developing soft micro-robotics in biomedical or microfluidic applications.