Chemically Controlled Bending of Compositionally Anisotropic Microcylinders

Soft materials that can undergo mechanical actuation in response to external stimuli, such as changes in temperature, light, pH value, or ionic strength, have attracted increasing attention because of their potential use as thinfilm actuators, smart sutures, and soft robots. These materials typically require specialty polymers, such as shape-memory polymers or use macroscopically layered films. In layered films, the anisotropic distribution of two polymers, or a polymer and a metal, is essential. This creates a mismatch in mechanical properties that gives rise to a defined bending. In principle, this concept is not limited to macroscopic multilayer films, but can be achieved with colloidal materials, as long as the required anisotropy can be realized and different parts of the colloidal object will respond differently to the external stimulus. In recent years, compositionally anisotropic microand nanoparticles have been devised using a range of different synthesis methods including microfluidic and lithographic techniques, particle replication in low surface energy templates, selective crosslinking of polybutadiene segments in terpolymers, lithographic patterning of microspheres, electrochemical and photochemical reduction, templating of porous membranes and nanotubes, surfactant aided growth, graft polymerization, and processes based on controlled surface nucleation. Alternatively, electrohydrodynamic co-jetting is a method to prepare particles and fibers with multiple compartments by transferring fluids through a set of capillaries that can process dissimilar materials. In the past, electrohydrodynamic co-jetting has resulted in particles with multiple compartments that contain different polymer blends, dyes, low-molecular weight additives, reactive molecules and even inorganic nanoparticles. If a reactive additive, such as a functionalized polymer, is added to one of the compartments, selective surface modification is possible and can result in spatially controlled immobilization of proteins or peptides. Because different compartments can be loaded with dissimilar materials, entirely new sets of functions can arise from unique synergistic effects, not just from the addition of the properties of the individual compartments. Herein, we report a new type of compositionally anisotropic microcylinders, where defined compartments within the same microcylinder undergo differential expansion due to the site-selective growth of a surface layer. The asymmetric expansion creates surface stresses resulting in significant and controllable bending of the microcylinders, which depends on the particle geometry and the architecture of the surface layers. Using finite element simulations, we verify the observed bending trends and derive a family of performance curves that predict a wide-range tunability of the actuation stroke based on the cylinder geometry and the amount of swelling. The microcylinders are fabricated based on electrohydrodynamic co-jetting followed by microsectioning. In brief, an electric field is applied to a compound droplet comprising two or more polymer solutions generated by laminar flow from a side-by-side arrangement of capillary needles. We have previously demonstrated the synthesis of particles and fibers from chloroform-based solutions of lactic acid polymers. In the case of fibers, high viscosities, combined with high solvent volatility and low charge-to-volume ratios, can result in an extremely linear and controlled jet migration without the bending and whipping instabilities commonly observed in charged jets. This situation enables the production of multicompartmental microfibers, which not only exhibit monodispersity with respect to diameter, but can also be aligned on rotating collectors. Such highly aligned fiber scaffolds can then be cut into monodisperse microcylinders. Importantly, particle diameters are controlled by altering the solution and process parameters during electrohydrodynamic co-jetting, while control over cylinder length is achieved by the microsectioning step. Spatioselective functionalization of one or more compartments of the cylinders has been achieved by incorporation of poly(lactide-co-propargyl glycolide) as an additive during fabrication of the microcylinders, and subsequent modification with biotin and streptavidin by click chemistry. As shown in the Supporting Information, Figure S1, we incorporated a poly[lactide-co[*] Dr. S. Saha, N. Clay, A. Donini, Prof. J. Lahann Department of Chemical Engineering University of Michigan, Ann Arbor, MI 48109 (USA) E-mail: lahann@umich.edu