Integrating chemistry, fluid flow, and mechanics to drive spontaneous formation of three-dimensional (3D) patterns in anchored microstructures.

Enzymatic reactions in solution drive the convection of confined fluids throughout the enclosing chambers and thereby couple the processes of reaction and convection. In these systems, the energy released from the chemical reactions generates a force, which propels the fluids' spontaneous motion. Here, we use theoretical and computational modeling to determine how reaction-convection can be harnessed to tailor and control the dynamic behavior of soft matter immersed in solution. Our model system encompasses an array of surface-anchored, flexible posts in a millimeter-sized, fluid-filled chamber. Selected posts are coated with enzymes, which react with dissolved chemicals to produce buoyancy-driven fluid flows. We show that these chemically generated flows exert a force on both the coated (active) and passive posts and thus produce regular, self-organized patterns. Due to the specificity of enzymatic reactions, the posts display controllable kaleidoscopic behavior where one regular pattern is smoothly morphed into another with the addition of certain reactants. These spatiotemporal patterns also form "fingerprints" that distinctly characterize the system, reflecting the type of enzymes used, placement of the enzyme-coated posts, height of the chamber, and bending modulus of the elastic posts. The results reveal how reaction-convection provides concepts for designing soft matter that readily switches among multiple morphologies. This behavior enables microfluidic devices to be spontaneously reconfigured for specific applications without construction of new chambers and the fabrication of standalone sensors that operate without extraneous power sources.

Chemical pumps and flexible sheets spontaneously form self-regulating oscillators in solution.

The synchronization of self-oscillating systems is vital to various biological functions, from the coordinated contraction of heart muscle to the self-organization of slime molds. Through modeling, we design bioinspired materials systems that spontaneously form shape-changing self-oscillators, which communicate to synchronize both their temporal and spatial behavior. Here, catalytic reactions at the bottom of a fluid-filled chamber and on mobile, flexible sheets generate the energy to "pump" the surrounding fluid, which also transports the immersed sheets. The sheets exert a force on the fluid that modifies the flow, which in turn affects the shape and movement of the flexible sheets. This feedback enables a single coated (active) and even an uncoated (passive) sheet to undergo self-oscillation, displaying different oscillatory modes with increases in the catalytic reaction rate. Two sheets (active or passive) introduce excluded volume, steric interactions. This distinctive combination of the hydrodynamic, fluid-structure, and steric interactions causes the sheets to form coupled oscillators, whose motion is synchronized in time and space. We develop a heuristic model that rationalizes this behavior. These coupled self-oscillators exhibit rich and tunable phase dynamics, which depends on the sheets' initial placement, coverage by catalyst and relative size. Moreover, through variations in the reactant concentration, the system can switch between the different oscillatory modes. This breadth of dynamic behavior expands the functionality of the coupled oscillators, enabling soft robots to display a variety of self-sustained, self-regulating moves.

Self-Propelling Macroscale Sheets Powered by Enzyme Pumps.

Nanoscale enzymes anchored to surfaces act as chemical pumps by converting chemical energy released from enzymatic reactions into spontaneous fluid flow that propels entrained nano- and microparticles. Enzymatic pumps are biocompatible, highly selective, and display unique substrate specificity. Utilizing these pumps to trigger self-propelled motion on the macroscale has, however, constituted a significant challenge and thus prevented their adaptation in macroscopic fluidic devices and soft robotics. Using experiments and simulations, we herein show that enzymatic pumps can drive centimeter-scale polymer sheets along directed linear paths and rotational trajectories. In these studies, the sheets are confined to the air/water interface. With the addition of appropriate substrate, the asymmetric enzymatic coating on the sheets induces chemically driven, buoyancy flows that controllably propel the sheet's motion on the air/water interface. The directionality and speed of the motion can be tailored by changing the pattern of the enzymatic coating, type of enzyme, and nature and concentration of the substrate. This work highlights the utility of biocompatible enzymes for generating motion in macroscale fluidic devices and robotics and indicates their potential utility for in vivo applications.

Chemically Driven Multimodal Locomotion of Active, Flexible Sheets.

The inhibitor-promoter feedback loop is a vital component in regulatory pathways that controls functionality in living systems. In this loop, the production of chemical A at one site promotes the production of chemical B at another site, but B inhibits the production of A. In solution, differences in the volumes of the reactants and products of this reaction can generate buoyancy-driven fluid flows, which will deform neighboring soft material. To probe the intrinsic interrelationship among chemistry, hydrodynamics, and fluid-structure interactions, we model a bio-inspired system where a flexible sheet immersed in solution encompasses two spatially separated catalytic patches, which drive the A-B inhibitor-promotor reaction. The convective rolls of fluid generated above the patches can circulate inward or outward depending on the chemical environment. Within the regime displaying chemical oscillations, the dynamic fluid-structure interactions morph the shape of the sheet to periodically "fly", "crawl", or "swim" along the bottom of the confining chamber, revealing an intimate coupling between form and function in this system. The oscillations in the sheet's motion in turn affect the chemical oscillations in the solution. In the regime with non-oscillatory chemistry, the induced flow still morphs the shape of the sheet, but now, the fluid simply translates the sheet along the length of the chamber. The findings reveal the potential for enzymatic reactions in the body to generate hydrodynamic behavior that modifies the shape of neighboring soft tissue, which in turn modifies both the fluid dynamics and the enzymatic reaction. The findings indicate that this non-linear dynamic behavior can be playing a critical role in the functioning of regulatory pathways in living systems.

Self-Generated Convective Flows Enhance the Rates of Chemical Reactions.

In chemical solutions, the products of catalytic reactions can occupy different volumes compared to the reactants and thus give rise to local density variations in the fluid. These density variations generate solutal buoyancy forces, which are exerted on the fluid and thus "pump" the fluid to flow. Herein, we examine if the reaction-induced pumping accelerates the chemical reaction by transporting the reactants to the catalyst at a rate faster than passive diffusion. Using both simulations and experiments, we show a significant increase in reaction rate when reaction-generated convective flow is present. In effect, through a feedback loop, catalysts speed up reactions not only by lowering the energy barrier but also by increasing the collision frequency between the reactants and the catalyst.

Collaboration and competition between active sheets for self-propelled particles.

Biological species routinely collaborate for their mutual benefit or compete for available resources, thereby displaying dynamic behavior that is challenging to replicate in synthetic systems. Here we use computational modeling to design microscopic, chemically active sheets and self-propelled particles encompassing the appropriate synergistic interactions to exhibit bioinspired feeding, fleeing, and fighting. This design couples two different mechanisms for chemically generating motion in fluid-filled microchambers: solutal buoyancy and diffusiophoresis. Catalyst-coated sheets, which resemble crabs with four distinct claws, convert reactants in solution into products and thereby create local variations in the density and chemical composition of the fluid. Via the solutal buoyancy mechanism, the density variations generate fluid flows, which modify the shape and motility of the crabs. Concomitantly, the chemical variations propel the motion of the particles via diffusiophoresis, and thus, the crabs' and particles' motion becomes highly interconnected. For crabs with restricted lateral mobility, these two mechanisms can be modulated to either drive a crab to catch and appear to feed on all of the particles or enable the particles to flee from this sheet. Moreover, by adjusting the sheet's size and the catalytic coating, two crabs can compete and fight over the motile, diffusiophoretic particles. Alternatively, the crabs can temporally share resources by shuttling the particles back and forth between themselves. With completely mobile sheets, four crabs can collaborate to perform a function that one alone cannot accomplish. These findings provide design rules for creating chemically driven soft robotic sheets that significantly expand the functionality of microfluidic devices.

Resonant amplification of enzymatic chemical oscillations by oscillating flow.

Using theory and simulation, we analyzed the resonant amplification of chemical oscillations that occur due to externally imposed oscillatory fluid flows. The chemical reactions are promoted by two enzyme-coated patches located sequentially on the inner surface of a pipe that transports the enclosed chemical solution. In the case of diffusion-limited systems, the period of oscillations in chemical reaction networks is determined by the rate of the chemical transport, which is diffusive in nature and, therefore, can be effectively accelerated by the imposed fluid flows. We first identify the natural frequencies of the chemical oscillations in the unperturbed reaction-diffusion system and, then, use the frequencies as a forcing input to drive the system to resonance. We demonstrate that flow-induced resonance can be used to amplify the amplitude of the chemical oscillations and to synchronize their frequency to the external forcing. In particular, we show that even 10% perturbations in the flow velocities can double the amplitude of the resulting chemical oscillations. Particularly, effective control can be achieved for the two-step chemical reactions where during the first half-period, the fluid flow accelerates the chemical flux toward the second catalytic patch, while during the second half-period, the flow amplifies the flux to the first patch. The results can provide design rules for regulating the dynamics of coupled reaction-diffusion processes and can facilitate the development of chemical reaction networks that act as chemical clocks.

Controlling the Spatiotemporal Transport of Particles in Fluid-Filled Microchambers.

The development of microscale devices that autonomously perform multistep processes is vital to advancing the use of microfluidics in industrial applications. Such advances can potentially be achieved through the use of "chemical pumps" that transduce the energy from inherent catalytic reactions into fluid flow within microchambers, without the need for extraneous external equipment. Using computational modeling, we focus on arrangements of multiple chemical pumps that are formed by anchoring patches of different enzymes onto the floor of a fluid-filled chamber. With the addition of the appropriate reactants, only one of the enzymatic patches is activated and thereby generates fluid flow centered about that patch. These flows drive the self-assembly of microparticles in the solution and localize the particles onto the activated patches. By varying the spatial arrangement of the enzymatic patches, and the sequence in which the appropriate reactants are added to the solution, we realize spatiotemporal control over the fluid flow and the sequential transport of microparticles from one patch to another. The order in which the particles visit the different patches can be altered by varying the sequence in which the reactants are added to the solution. By harnessing catalytic cascade reactions, where the product of one reaction is the reactant for the next, we achieve directed transport between the patches with the addition of just one reactant, which initiates the catalytic cascade. Through these studies, we show how the trajectory of the particles' motion among different "stations" can be readily regulated through intrinsic catalytic reactions and thus, provide guidelines for creating fluidic devices that perform multistep reactions in an autonomous, self-sustained manner.

Light-Induced Dynamic Control of Particle Motion in Fluid-Filled Microchannels.

The design of remotely programmable microfluidic systems with controlled fluid flow and particle transport is a significant challenge. Herein, we describe a system that harnesses the intrinsic thermal response of a fluid to spontaneously pump solutions and regulate the transport of immersed microparticles. Irradiating a silver-coated channel with ultraviolet (UV) light generates local convective vortexes, which, in addition to the externally imposed flow, can be used to guide particles along specific trajectories or to arrest their motion. The method provides the distinct advantage that the flow and the associated convective patterns can be dynamically altered by relocating the source of UV light. Moreover, the flow can be initiated and terminated "on-demand" by turning the light on or off.

Using Chemical Pumps and Motors To Design Flows for Directed Particle Assembly.

Mechanical and electrical pumps are conventionally used to drive fluid flow in microfluidic devices; these pumps require external power supplies, thus limiting the portability of the devices. Harnessing catalytic reactions in solution allows pumping to be shifted into the chemical realm and alleviates the need for extraneous equipment. Chemical "pumps" involve surface-bound catalytic patches that decompose dissolved reagents into the products of the reaction. The catalytic reactions thereby produce chemical gradients that in turn generate pronounced flow fields. Such chemically-generated flows can be harnessed to transport particles in the solution and regulate their self-organization into complex structures within confined chambers. The challenge, however, is determining the reactions and conditions that will yield "programmable" flows, which permit control over the structure formation. In this Account, we review our modeling efforts to design chemical pumps (and "motors") to regulate the motion and assembly of microscopic particles in solution. In the first scenario, microcapsules release reagents in a microchamber with stationary catalytic patches and thereby act as "fuel" for the microcapsules' self-sustained motion. As the reagent is consumed, the capsules aggregate into "colonies" on the catalyst-covered sites. The shape of the assembled colonies can be tailored by patterning the distribution of the catalyst on the surface. Hence, these chemical pumps can be utilized to regulate the autonomous motion and targeted delivery of microcarriers in microfluidic devices. Notably, this fundamental physicochemical mechanism could have played a role in the self-organization of early biological cells (protocells). In the second example, the catalysts are localized on mobile, active particles, which are called "motors". Reactants dispersed in the solution are decomposed at the surface of the motors and produce a convective flow that transports both the active particles and nearby passive, non-coated particles. Depending on the numbers of active and passive particles and the structure of the self-organized cluster, these assemblies can translate or spin and thus act as self-assembled "conveyor belts" or gears in the microchamber. The latter examples involve the formation of two-dimensional structures. In the final scenario, we devise a mechanism for assembling three-dimensional towerlike structures using microcapsules in solution. Here, chemicals diffusing from a central patch on a surface generate a radially directed flow along the surface toward the center. This toroidal roll of fluid lifts the capsules above the patch and draws out the cluster into a tower, whose structure can be tailored by varying the attractive capsule-capsule and capsule-surface interaction strengths. Hence, our method of flow-directed assembly can permit the growth of reconfigurable 3D structures from simple subunits. Taken together, these findings facilitate the fabrication of stand-alone microfluidic devices that autonomously perform multistage chemical reactions and assays for portable biomedical applications and act as small-scale factories to autonomously build microscale components.

Self-Organization of Fluids in a Multienzymatic Pump System.

The nascent field of microscale flow chemistry focuses on harnessing flowing fluids to optimize chemical reactions in microchambers and establish new routes for chemical synthesis. With enzymes and other catalysts anchored to the surface of microchambers, the catalytic reactions can act as pumps and propel the fluids through the containers. Hence, the flows not only affect the catalytic reactions, but these reactions also affect the flows. Understanding this dynamic interplay is vital to enhancing the accuracy and utility of flow technology. Through experiments and simulation, we design a system of three different enzymes, immobilized in separate gels, on the surface of a microchamber; with the appropriate reactants in the solution, each enzyme-filled gel acts as a pump. The system also exploits a reaction cascade that controls the temporal interactions between two pumps. With three pumps in a triangular arrangement, the spatio-temporal interactions among the chemical reactions become highly coordinated and produce well-defined fluid streams, which transport chemicals and form a fluidic "circuit". The circuit layout and flow direction of each constituent stream can be controlled through the number and placement of the gels and the types of catalysts localized in the gels. These studies provide a new route for forming self-organizing and bifurcating fluids that can yield fundamental insight into nonequilibrium, dynamical systems. Because the flows and fluidic circuits are generated by internal chemical reactions, the fluids can autonomously transport cargo to specific locations in the device. Hence, the findings also provide guidelines to facilitate further automation of microfluidic devices.

Organization of Particle Islands through Light-Powered Fluid Pumping.

The field of active matter holds promise for applications in particle assembly, cargo and drug delivery, and sensing. In pursuit of these capabilities, researchers have produced a suite of nanomotors, fluid pumps, and particle assembly strategies. Although promising, there are many challenges, especially for mechanisms that rely on chemical propulsion. One way to circumvent these issues is by the use of external energy sources. Herein, we propose a method of using freely suspended nanoparticles to generate fluid pumping towards desired point sources. The pumping rates are dependent on particle concentration and light intensity, making it highly controllable. Using these directed flows, we further demonstrate the ability to reversibly construct and move colloidal crystals.

Convective Self-Sustained Motion in Mixtures of Chemically Active and Passive Particles.

We develop a model to describe the behavior of a system of active and passive particles in solution that can undergo spontaneous self-organization and self-sustained motion. The active particles are uniformly coated with a catalyst that decomposes the reagent in the surrounding fluid. The resulting variations in the fluid density give rise to a convective flow around the active particles. The generated fluid flow, in turn, drives the self-organization of both the active and passive particles into clusters that undergo self-sustained propulsion along the bottom wall of a microchamber. This propulsion continues until the reagents in the solution are consumed. Depending on the number of active and passive particles and the structure of the self-organized cluster, these assemblies can translate, spin, or remain stationary. We also illustrate a scenario in which the geometry of the container is harnessed to direct the motion of a self-organized, self-propelled cluster. The findings provide guidelines for creating autonomously moving active particles, or chemical "motors" that can transport passive cargo in microfluidic devices.

Self-Propelled Nanomotors Autonomously Seek and Repair Cracks.

Biological self-healing involves the autonomous localization of healing agents at the site of damage. Herein, we design and characterize a synthetic repair system where self-propelled nanomotors autonomously seek and localize at microscopic cracks and thus mimic salient features of biological wound healing. We demonstrate that these chemically powered catalytic nanomotors, composed of conductive Au/Pt spherical Janus particles, can autonomously detect and repair microscopic mechanical defects to restore the electrical conductivity of broken electronic pathways. This repair mechanism capitalizes on energetic wells and obstacles formed by surface cracks, which dramatically alter the nanomotor dynamics and trigger their localization at the defects. By developing models for self-propelled Janus nanomotors on a cracked surface, we simulate the systems' dynamics over a range of particle speeds and densities to verify the process by which the nanomotors autonomously localize and accumulate at the cracks. We take advantage of this localization to demonstrate that the nanomotors can form conductive "patches" to repair scratched electrodes and restore the conductive pathway. Such a nanomotor-based repair system represents an important step toward the realization of biomimetic nanosystems that can autonomously sense and respond to environmental changes, a development that potentially can be expanded to a wide range of applications, from self-healing electronics to targeted drug delivery.

Theory of electrocaloric effect in a shape-changing container: gas in a nanotube.

Driven by applied voltage or Ohmic heating, bistable nanotubes filled with gas can transform between expanded and collapsed configurations and by doing so convert energy between mechanical, electrical, and thermal forms. The electrocaloric response, a reversible change of temperature in response to applied voltage, combines the advantages of a working fluid with the lack of internal interfaces characteristic of robust solid-state thermoelectric devices. Such devices could be constructed from any conductive two-dimensional atomically thin material wrapped into an appropriate geometry.

Interlinking spatial dimensions and kinetic processes in dissipative materials to create synthetic systems with lifelike functionality.

Biological systems spontaneously convert energy input into the actions necessary to survive. Motivated by the efficacy of these processes, researchers aim to forge materials systems that exhibit the self-sustained and autonomous functionality found in nature. Success in this effort will require synthetic analogues of the following: a metabolism to generate energy, a vasculature to transport energy and materials, a nervous system to transmit 'commands', a musculoskeletal system to translate commands into physical action, regulatory networks to monitor the entire enterprise, and a mechanism to convert 'nutrients' into growing materials. Design rules must interconnect the material's structural and kinetic properties over ranges of length (that can vary from the nano- to mesoscale) and timescales to enable local energy dissipations to power global functionality. Moreover, by harnessing dynamic interactions intrinsic to the material, the system itself can perform the work needed for its own functionality. Here, we assess the advances and challenges in dissipative materials design and at the same time aim to spur developments in next-generation functional, 'living' materials.

Theory of carbomorph cycles.

We present a theory of a reversibly deforming sp2-carbon-based system controlled by competing strain, surface, and electrostatic energies, a carbomorph. For example, external forces (such as electrostatic, chemical, interfacial) could convert a bistable carbon nanotube between the collapsed and inflated states. Such a system could operate as a voltage-controlled constant-force spring, a charge-controlled harmonic spring, or an electromechanical engine or generator (with linear stroke up to few microns) driven across a propagating quasi-one-dimensional structural phase transition.

Cellulose microfibril twist, mechanics, and implication for cellulose biosynthesis.

All-atom molecular dynamics simulations with explicit water solvent were used to investigate the microstructure and conformational dynamics of cellulose Iß microfibrils as a function of microfibril length and cross-sectional size and shape. Cellulose microfibrils quickly develop a right-handed twist, which then remains stable over the entire 10 ns simulation time. The helical angle is independent of microfibril length and inversely proportional to its cross-sectional area, in accord with the expectations of continuum theory for an intrinsic chiral twist that is opposed by torsional shear. These calculations provide-to our knowledge-the first estimates of the shear modulus of a cellulose microfibril from MD simulations. The internal strains caused by this helical twist, propagated indefinitely along the microfibril axis, could be relaxed by periodic regions of amorphous structure along the axis of the cellulose microfibrils.

Engineering confined fluids to autonomously assemble hierarchical 3D structures.

The inherent coupling of chemical and mechanical behavior in fluid-filled microchambers enables the fluid to autonomously perform work, which in turn can direct the self-organization of objects immersed in the solution. Using theory and simulations, we show that the combination of diffusioosmotic and buoyancy mechanisms produce independently controlled, respective fluid flows: one generated by confining surfaces and the other in the bulk of the solution. With both flows present, the fluid can autonomously join 2D, disconnected pieces to a chemically active, "sticky" base and then fold the resulting layer into regular 3D shapes (e.g. pyramids, tetrahedrons, and cubes). Here, the fluid itself performs the work of construction and thus, this process does not require extensive external machinery. If several sticky bases are localized on the bottom surface, the process can be parallelized, with the fluid simultaneously forming multiple structures of the same or different geometries. Hence, this approach can facilitate the relatively low-cost, mass production of 3D micron to millimeter-sized structures. Formed in an aqueous solution, the assembled structures could be compatible with biological environments, and thus, potentially useful in medical and biochemical applications.

Light-Powered, Fuel-Free Oscillation, Migration, and Reversible Manipulation of Multiple Cargo Types by Micromotor Swarms.

Through experiments and simulations, we show that fuel-free photoactive TiO2 microparticles can form mobile, coherent swarms in the presence of UV light, which track the subsequent movement of an irradiated spot in a fluid-filled microchamber. Multiple concurrent propulsion mechanisms (electrolyte diffusioosmotic swarming, photocatalytic expansion, and photothermal migration) control the rich collective behavior of the swarms, which provide a strategy to reversely manipulate cargo. The active swarms can autonomously pick up groups of inert particles, sort them by size, and sequentially release the sorted particles at particular locations in the microchamber. Hence, these swarms overcome three obstacles, limiting the utility of self-propelled particles. Namely, they can (1) undergo directed, long-range migration without the addition of a chemical fuel, (2) perform diverse collective behavior not possible with a single active particle, and (3) repeatedly and controllably isolate and deliver specific components of a multiparticle "cargo". Since light sources are easily fabricated, transported, and controlled, the results can facilitate the development of portable devices, providing broader access to the diagnostic and manufacturing advances enabled by microfluidics.