IRG 2:  Spatialtemporal Control of Active Materials

Faculty Coordinators: A.R. Dinner & B. Tian

IRG2 represents an ambitious effort to understand, design, and synthesize materials containing distributed molecular elements that convert chemical energy into mechanical work. Drawing on the myriad ways that biological systems have evolved to construct materials with specific responses to applied stimuli, this IRG aspires to achieve control of active materials and ultimately to create novel molecular assemblies for robust tunable shape change. Success of thi IRG would result in the identification of minimal combinations of elements capable of programmable amorphous shape changes, autonomous movement and collective behavior, and such a material could be tailored to environments and situations beyond the reach of biological systems.

By harnessing behaviors found in living tissue in novel molecular contexts, these new materials could provide smart actuation beyond materials that can be made now, with chemical functionalities/reactivities, environmental compatibilities (e.g., temperatures, pH, etc.), and durabilities beyond biological systems. For example, a sheet of such material could dynamically sculpt its shape, size, and rigidity in response to external forces and self-heal to enable repeated dramatic deformation. Overcoming the design and fabrication challenges for such a material would be a step toward realizing self-propelled mesoscale robotics with diverse applications.

Progress in active materials is currently limited by the lack of knowledge of their underlying physical principles. The issues are exemplified by the actin cytoskeleton, a class of active polymer networks that controls cell morphogenic processes. These materials use chemical energy in the form of ATP/GTP hydrolysis, and the mechanochemical feedbacks that dynamically control their structure and response are not well understood. We seek to achieve control of active materials and ultimately to create novel molecular assemblies with robust tunable functionalities.

Achieving these goals requires an interdisciplinary effort on three fronts: (1) dynamically controlling material structure, (2) engineering assemblies consisting of natural and synthetic components (and hybrids of the two) that can transform shape, and (3) establishing environments that elicit competing materials behaviors. All three focus areas are important for fully realizing the potential of active materials, but progress on any one would have immediate rewards for fundamental materials science.

Summary:

The ability to control network assembly dynamically and spatially modulate contractility could enable the design and implementation of a primitive ‘artificial cell’ that is capable of recapitulating the physical behaviors of living systems. Such a material should be able to respond to a varied external chemical/physical environment to change its shape, self-propel or divide, for instance. Success would be defined by identifying minimal combinations of elements capable of programmable shape changes, autonomous movement and collective behavior, and such a material could be tailored to environments and situations beyond the reach of biological systems.

“Phase diagram” for contractility elucidated from coarse-grained simulations and in vitro experiments.
A rational design for silicon-based modulation materials.
Coacervates promote actin assembly at concentrations below the critical concentration, c*. Scale bar: 2 μm
(top) Network structures elicited by different actin binding proteins. (bottom) Scaling of the motor mean square displacement of motors with the lag time (left) and the measurement time (right).
(Left) When alpha-actinin is absent, fascin binds continuously along the entire length of two-filament bundles. (Right) When alpha-actinin is present, fascin and alpha-actinin form distinct domains.
Chiral active fluids of magnetic colloids spinning under the action of a rotating magnetic field exhibit surface-localized motion and unusual surface waves and instabilities.