Chain Assembly Kinetics from Magnetic Colloidal Spheres.

Magnetic colloidal chains are a microrobotic system with promising applications due to their versatility, biocompatibility, and ease of manipulation under magnetic fields. Their synthesis involves kinetic pathways that control chain quality, length, and flexibility, a process performed by first aligning superparamagnetic particles under a one-dimensional magnetic field and then chemically linking them using a four-armed maleimide-functionalized poly(ethylene glycol). Here, we systematically vary the concentration of the poly(ethylene glycol) linkers, the reaction temperature, and the magnetic field strength to study their impact on the physical properties of synthesized chains, including the chain length distribution, reaction temperature, and bending modulus. We find that this chain fabrication process resembles step-growth polymerization and can be accurately described by the Flory-Schulz model. Under optimized experimental conditions, we have successfully synthesized long flexible colloidal chains with a bending modulus, which is 4 orders of magnitude smaller than previous studies. Such flexible and long chains can be folded entirely into concentric rings and helices with multiple turns, demonstrating the potential for investigating the actuation, assembly, and folding behaviors of these colloidal polymer analogues.

Rolling of soft microbots with tunable traction.

Microbot (µbot)-based targeted drug delivery has attracted increasing attention due to its potential for avoiding side effects associated with systemic delivery. To date, most µbots are rigid. When rolling on surfaces, they exhibit substantial slip due to the liquid lubrication layer. Here, we introduce magnetically controlled soft rollers based on Pickering emulsions that, because of their intrinsic deformability, fundamentally change the nature of the lubrication layer and roll like deflated tires. With a large contact area between µbot and wall, soft µbots exhibit tractions higher than their rigid counterparts, results that we support with both theory and simulation. Upon changing the external field, surface particles can be reconfigured, strongly influencing both the translation speed and traction. These µbots can also be destabilized upon pH changes and used to deliver their contents to a desired location, overcoming the limitations of low translation efficiency and drug loading capacity associated with rigid structures.