Colloidal Robotics: Programming Structure and Function in Colloidal-Scale Material Through Emergence, Design and Logic

Abstract

Advancements in self-assembly and top-down fabrication approaches have enabled tailoring of colloidal materials, macromolecules and polymers, and both organic and inorganic nanoparticles to build advanced functional materials. Miniature sized robots made using such materials can have huge impacts in biomedical applications such as minimally invasive surgery, tissue engineering, targeted therapy, diagnostics and single-cell manipulation. This dissertation addresses building such robotic systems that are programmable at the elemental level and are tunable at the macroscopic level. Using coarse-grained particle simulations, analytical modeling, and mechanical design, I have developed three systems to this end that correspond to programming approaches for swarm intelligence, morphological control, and mechanical computing respectively. The first two systems use colloids possessing propulsion, a.k.a. active particles, that harness environmental energy into a propulsion force and can be developed using a wide variety of materials. The first system consists of particles that trigger propulsion only when in contact with other particles. An ensemble of such particles can be tuned externally to form and switch among crystals, gels and clusters as emergent behavior. Further, these systems possess enhanced transport dynamics, which is also tunable. In the second system, the active particles are connected end-to-end in a loop. When actuated, the loops fold into programmed shapes while the internal space is available to accommodate additional components such as sensors, controller, chemicals, and communication devices. The shape and motion information is encoded in the arrangement of active particles along the loop. Besides relevance of these systems in understanding the fundamental physics of non-equilibrium systems, they can be used to develop smart materials that can sense, actuate, compute and communicate. Physical experiments using kilobots—centimeter sized robots—are performed to demonstrate the scale invariance and feasibility of the design. The third system is inspired from the development of materials that respond to external stimuli by expanding or contracting, thereby providing a transduction route that integrates sensing and actuation powered directly by the stimuli. Our work motivates building colloidal scale robots using these stimuli-responsive materials. For maximum control using global triggers, computation ability needs to be incorporated within such robots. The challenge is to design an architecture that is compact, material agnostic, stable under stochastic forces, and employs stimuli-responsive materials. The third system resolves these challenges through an architecture that computes combinatorial logic using mechanical gates. It uses linear actuation—-expansion and contraction—-as input-output signals with the additional benefits of logic circuitry being physically flexible.