3D-printed stimuli-responsive soft microrobots

Abstract

Untethered microrobots, i.e., mobile microrobots, with overall sizes less than 1 mm are receiving significant attention due to their great potential to conduct targeted and minimally invasive therapeutic delivery and medical treatment of diseases in the local region of the biological environment. However, there are many technical barriers in the integration of conventional on-board sensors, actuators, and batteries into micro-scale systems. To overcome these limitations, stimuli-responsive active materials with both sensing and actuation properties are integrated to mobile microrobots. Stimuli-responsive materials can morph their shape and size via swelling and deswelling mechanisms in response to external chemical, physical and other stimuli, such as heat, pH, light, magnetic field, and acoustic field, with no aid from complex wires, sensors, or batteries. While conventional fabrication methods with passive materials have led to the production of static structures, 3D printing with stimuli-responsive materials opens a new direction for 3D objects possessing volumetric transformation behavior, material property change, and shape morphing ability. In this dissertation, I integrate different stimuli-responsive materials and 3D microprinting to provide more multifunctional, versatile and complex microrobot designs based on bioinspired design principles, with a wide range of stimuli-responsive sensing and actuation properties to use them in potential biomedical applications inside the human body. In the first part, I introduce magnetically steerable 3D-printed microroller and microscrew robot designs with stimuli-responsive materials to develop volume-controllable microrobots for spatial adaptation. These wirelessly controlled microrobots possess the ability to function in response to multiple stimuli, including magnetic fields, temperature, pH, and cations, which can enhance the adaptability of the microbots to various unstructured environments. Second, octopus-inspired architectures with temperature-responsive materials to control the adhesion properties for medical purposes is proposed. Introduction of pNIPAM material leads temperature-responsive volume morphing behavior enabled controllable tissue adhesion by using externally applied magnetic fields. Furthermore, I demonstrate the capability of implementing a wide range of medical tasks repeatedly via showing the repetition of attachment and detachment processes using an external magnetic field. Finally, I present a multifunctional pollen-grain-inspired hydrogel robot by 3D direct laser printing in order to enhance the functional diversity of microrobots in biological environments. I also demonstrated multi-responsive hydrogel structures to decouple the stimulus inputs of magnetically actuated locomotion, temperature-responsive controllable attachment, and pH-responsive on-demand cargo release, respectively. The temperature-responsive outer crust shells made of pNIPAM enabled controlling the attachment of the microrobot by shrinking up to 49%, revealing the robot’s spikes. In addition, the inner pollen-grain-inspired structure with spikes made of PETA with FePt nanoparticles demonstrated an improved attachment performance and magnetically guided locomotion along biological surfaces. The inner sphere made of pNIPAM-AAc successfully released drugs by pH-induced swelling.