3D-Printed Multi-Stimuli-Responsive Mobile Micromachines

Abstract

Magnetically actuated and controlled mobile micromachines have the potential to be a key enabler for various wireless lab-on-a-chip manipulations and minimally invasive targeted therapies. However, their embodied, or physical, task execution capabilities that rely on magnetic programming and control alone can curtail their projected performance and functional diversity. Integration of stimuli-responsive materials with mobile magnetic micromachines can enhance their design toolbox, enabling independently controlled new functional capabilities to be defined. To this end, here, we show three-dimensional (3D) printed size-controllable hydrogel magnetic microscrews and microrollers that respond to changes in magnetic fields, temperature, pH, and divalent cations. We show two-way size-controllable microscrews that can reversibly swell and shrink with temperature, pH, and divalent cations for multiple cycles. We present the spatial adaptation of these microrollers for penetration through narrow channels and their potential for controlled occlusion of small capillaries (30 μm diameter). We further demonstrate one-way size-controllable microscrews that can swell with temperature up to 65% of their initial length. These hydrogel microscrews, once swollen, however, can only be degraded enzymatically for removal. Our results can inspire future applications of 3D- and 4D-printed multifunctional mobile microrobots for precisely targeted obstructive interventions (e.g., embolization) and lab- and organ-on-a-chip manipulations.

Keywords: 3D printing; 4D printing; micromachine; microrobot; stimuli-responsive materials.

Figure 1

Schematic illustration of 4D printing of size-controllable microrollers and microscrews using external stimuli. (a) Schematic illustration of the components of stimuli-responsive material-based photoresists and printing process using a direct laser writing (two-photon polymerization) system. (b) Schematic representation of the two-way size-controllable microrollers/microscrews mechanism when induced by multiple stimuli, such as temperature, pH, and Ca²⁺ ions.

Figure 2

Temperature responsiveness of two-way size-controllable spherical microrollers and double-helical microscrews. (a) Optical microscopy images of the 3D-printed pNIPAM-AAc microrollers (the unit of laser power (LP) is mW). (b) Optical microscopy images of shrinkage of the microrollers in response to change in temperature. (c) Plot showing the relationship between the laser power for printing the material and resulting shrinkage properties due to the change in the cross-linking density. Different laser powers ranging from 0 to 50 mW were exposed to microrollers composed of pNIPAM-AAc. (d) The repetition test of the pNIPAM-AAc microrollers shrinking/deshrinking cycles without any deterioration. (e) Optical microscopy images of three different microscrews printed in different sizes and representative swelling/deswelling of a 4 × 4 microscrews array in response to temperature change. (f) Repetition test of the pNIPAM-AAc microscrews during temperature-dependent shrinking/deshrinking cycles.

Figure 3

pH and ion responsiveness of two-way size-controllable microrollers and microscrews. (a) Optical images of swelling/deswelling microrollers and microscrews in response to pH value. (b) The repetition test of the pNIPAM-AAc microrollers swelling cycles in response to pH. (c) Optical images of multiresponsiveness of microscrews in response to temperature and Ca²⁺ ion concentration change. (d) Repetition test of the pNIPAM-AAc microscrews shrinking/swelling cycles in response to temperature and Ca²⁺ ion change.

Figure 4

Temperature responsiveness of one-way size-controllable double-helical microscrews. (a) Schematic representation of one-way size-control mechanism by temperature change. (b) Optical microscopy images of gelatin-based microscrews swelling in response to temperature change. (c) Swelling kinetics of gelatin-based microscrews in PBS. (d) Swelling kinetics of different sizes of gelatin microscrews in response to temperature. (e) Effect of the laser power on the cross-linking and swelling of gelatin-based microscrews.

Figure 5

Actuation and steering demonstrations of the microrollers and microscrews using a rotating magnetic field, and demonstration of their spatial adaptability by their temperature-dependent size control. (a) Schematic illustration of magnetic actuation and steering of the microrollers. (b) Controlled surface rolling trajectory snapshots (blue lines) of a microroller at a 10 mT rotating magnetic field at 0.8 and 1.5 Hz on a smooth glass substrate inside deionized water. (c) Mean motion speed of the microrollers as a function of magnetic rotation frequency. (d) Magnetic steering control snapshots of the double-helical microscrews at 10 mT rotating magnetic field at 0.8 Hz. (e) Mean speed of microscrews with different rotation frequencies. (f) Schematic diagram and optical images of the microroller showing the spatial adaptability (shrinkage) by temperature control to pass through a channel smaller than its initial diameter.