Micronscale mobile robots are much more challenging than mobile millirobots, since it is almost impossible to integrate on-board computation, power, actuation, sensing and communication on them for meaningful operation durations. Moreover, scaling laws make the surface area- and length-related, short- or long-range physical forces (e.g., drag, surface tension, adhesion, friction, van der Waals forces) become much more dominant than the volume-related physical forces at the microscale [ ]. Even, the robot motion becomes much more stochastic going down to a few micrometer length scales due to the Brownian motion and other stochastic effects. Therefore, we need to propose new design, fabrication, locomotion and control approaches and methods to create mobile microrobots for given target applications and tasks.
As the first approach, we propose biohybrid designs that integrate genetically engineered bacterium and alga type of microorganisms to the designed robot bodies to enable self-propulsion, taxis-based sensory locomotion control and active cargo transport and delivery type of functions adaptively in biological media. Biohybrid microrobots exploit the cell’s inherent capability of harnessing biochemical energy in the microenvironment or inside the cell to power mobility and sensing. We exploit the microorganisms that can achieve high propulsion speeds, thereby providing high thrust power to the microrobots. Integrated physical bodies enable remote steering and targeted cargo delivery for functional tasks. These microrobots are highly stochastic and can be fabricated at high volumes cost-effectively. We have shown that bacteria-powered red blood cells and synthetic particles loaded with cancer drugs can be steered towards a target location, and bacteria sensing (e.g., chemotaxis, aerotaxis, pH-taxis) can be used to accumulate them to the tumor cell regions. Then, triggered light or other stimulus is used to deliver the drug on-demand in the target location effectively (around 3-4 orders of magnitude more effective than the same-size passive drug particles). We have demonstrated such local on-demand effective drug delivery functions in vitro.
As the second approach, synthetic mobile microrobots are actuated by external magnetic fields, acoustic waves or light to swim or surface roll/crawl inside the complex fluids and tissues of the human body. Our group uniquely brings together new functional materials with design and engineering strategies to develop fully synthetic microrobots that can dynamically interact with the environment. Fabrication of microrobots presents unique challenges concerning design, fabrication process, and encoding operational capabilities. Conventional microfabrication techniques usually provide relatively simple geometric structures, such as tubes, spheres, and surfaces, with limited design flexibility and function. Therefore, we combine 3D microprinting with tailor-made polymeric and hydrogel materials and nanocomposites to realize multi-responsive and multifunctional 3D complex microrobots that could not be conceivable with the alternative microfabrication methods.
