Abstract
Dynamic materials that can sense changes in their surroundings and functionally respond by altering many of their physical characteristics are primed to be integral components of future "smart" technologies. A fundamental reason for the adaptability of biological organisms is their innate ability to convert environmental or chemical cues into mechanical motion and reconfiguration on both the molecular and macroscale. However, design and engineering of robust chemomechanical behavior in artificial materials has proven a challenge. Such systems can be quite complex and often require intricate coordination between both chemical and mechanical inputs and outputs, as well as the combination of multiple materials working cooperatively to achieve the proper functionality. It is critical to not only understand the fundamental behaviors of existing dynamic chemomechanical systems but also apply that knowledge and explore new avenues for design of novel materials platforms that could provide a basis for future adaptive technologies. In this Account, we explore the chemomechanical behavior, properties, and applications of hybrid-material surfaces consisting of environmentally sensitive hydrogels integrated within arrays of high-aspect-ratio nano- or microstructures. This bio-inspired approach, in which the volume-changing hydrogel acts as the "muscle" that reversibly actuates the microstructured "bones", is highly tunable and customizable. Although straightforward in concept, the combination of just these two materials (structures and hydrogel) has given rise to a far more complex set of actuation mechanisms and behaviors. Variations in how the hydrogel is physically integrated within the structure array provide the basis for three fundamental mechanisms of actuation, each with its own set of responsive properties and chemomechanical behavior. Further control over how the chemical stimulus is applied to the surface, such as with microfluidics, allows for generation of more precise and varied patterns of actuation. We also discuss the possible applications of these hybrid surfaces for chemomechanical manipulation of reactions, including the generation of chemomechanical feedback loops. Comparing and contrasting these many approaches and techniques, we aim to put into perspective their highly tunable and diverse capabilities but also their future challenges and impacts.
Highlights
Design of synthetic systems that sense and respond to environmental stimuli and chemical signals on both a microscopic and macroscopic level has been a longstanding scientific pursuit.[1]
From the nanoscale to the micro/macroscale, chemical signals correlated with a structural response, reconfiguration, or movement lie at the heart of many biological processes
The interplay between controllable parameters such as the location and type of stimulus,[23,26,28] the surface chemistry,[28] geometry,[26] and mechanical properties of HAR structures, 23,25 and the patterning and topography of the hydrogel within the confinement of the structures[25,27] provide for a highly tunable and customizable chemo-mechanical hybrid system based on just a few individual material components.[31]. We explore these different modes of chemo-mechanical actuation, examining how various chemical inputs as well as the physical combination of gel and HAR structure lead to a diversity of responses, actuation patterns, and functions
Summary
Design of synthetic systems that sense and respond to environmental stimuli and chemical signals on both a microscopic and macroscopic level has been a longstanding scientific pursuit.[1]. From the nanoscale (motor-like action of ATPases,[2] conformational changes of proteins allowing allosteric regulation3) to the micro/macroscale (phototropism,[4] proton gradients that drive the beating of flagella,[5] the motion of bacteria directed by chemotaxis6), chemical signals correlated with a structural response, reconfiguration, or movement lie at the heart of many biological processes Such diversity in the materials systems and mechanisms provides for an large range of elegant applications of chemomechanical actuators.[7] The beating of cilia, hair-like cellular structures, in the respiratory tract helps to prevent biofilm formation,[8] while the opening and closing of pedicellaria, flower-like extensions on the surfaces of sea urchins, are used for body cleaning and food capture.[9] Cephalopods, such as octopus and squid, are artful masters of camouflage, making use of pigment-containing cells (chromatophores) that expand and contract to allow the animals to rapidly change color.[10] The widespread demonstrated importance and versatility of such naturally occurring chemo-mechanical systems, their extensive diversity in terms of scale, mechanism, function, and structure, serve as inspiration for the development of “smart” materials that are able to autonomously sense and adapt via self-regulated structural reconfiguration. We describe systems in which the gels are either surface attached[23,24,25,26] or tethered to the HAR structures,[27] responding to homogenous chemical stimuli,[23,24,25,26,27,28] gradient stimuli,[29] and/or combinatorial stimuli,[27] and with programmable actuation directions and bending angles.[27,29] We explore how integration of the hybrid surfaces within different fluidic environments enables compartmentalized and spatially patterned application of stimuli further expanding the controllability of the chemo-mechanical response.[26,28] we examine how such hybrid surfaces can be used to mediate chemo-mechano-chemical processes including feedback loops, laying the foundation for unique routes to the dynamic control of mechanically-mediated chemical reactions.[28,30]
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