microscale actuation. However, these material-based actuators are often slow or only expand or contract axially, preventing use in applications that require volumetric expansions. To address such limitations of existing actuating materials, actuators based on the liquid-to-gas phase change of solvent inclusions encapsulated in a hyperelastic matrix have been realized. Phasechange actuators are capable of inducing rapid volumetric expansion similar to fluidic actuators, without being tethered to an external fluid source. [29][30][31][32] Accepting the advantages of phasechange actuators, we now turn our attention to granular media. Granular assemblies, consisting of discrete grains, can be tuned to accomplish a variety of different responses. For example, favorable self-assembly of granular media can be achieved via modulating interparticle interactions and external stimuli, [33][34][35][36] and granular assemblies with different packing configurations have been shown to yield different bulk properties. [36][37][38][39] Furthermore, granular media can exhibit optimized changes in stiffness during jamming transitions, enabling tunable moduli. [16,[40][41][42][43][44] When unjammed, the bulk material is compliant, and when jammed, the bulk material can achieve the stiffness of the grains. Finally, when mixed into carrier fluids, granular media can impart thixotropic behavior on the carrier fluid due to dynamic jamming, [45][46][47][48] enabling 3D printing of previously unprintable materials. With all the advantageous properties granular media has to offer, the technology has seen application in soft robotics, [49][50][51][52] medical devices, [53][54][55] flexible airfoils, [56] and programmable aggregate architecture. [57] Here, we bring together the unique advantages of phasechange soft actuators and granular media to introduce soft granular actuators made of discrete, volumetrically expanding grains. A single active grain consists of multiple solvent cores encapsulated in a hyperelastic silicone shell (Ecoflex 00-30). At elevated temperatures, the encapsulated solvent vaporizes and increases the internal pressure of the hyperelastic shell, inducing volumetric expansion of the grain. The grains are independently capable of rapid, high-force microscopic actuation, and are also easily arranged into granular assemblies to form larger-scale bulk actuators. Furthermore, agglomerates of active grains can self-assemble from disordered arrangements to conform around objects and exhibit variable moduli. Finally, the use of grains suspended in a carrier solvent or resin enables compatibility with granular self-assembly and 3D printing techniques, offering the potential to print volumetrically expanding actuators into freeform patterns across scales.Recent work has demonstrated the potential of actuators consisting of bulk elastomers with phase-changing inclusions for generating high forces and large volumetric expansions. Simultaneously, granular assemblies have been shown to enable tunable properties via different packing...
The use of polydopamine nanomembranes as a conformal coating with reversible adhesion is demonstrated. Here, two approaches to achieve reversible adhesion in polydopamine nanomembranes are reported: 1) controlling the surface chemistry using pH and 2) altering the texture through mechanical strain. pH‐dependent reversible adhesion in an aqueous environment can achieve differential pull‐off force of ≈0 and 1.93 ± 0.39 mN. Strain‐dependent adhesion in dry environments can produce between 0.26 ± 0.04 and 1.48 ± 0.06 mN, a range of 5.7 times difference. A theoretical framework based on elliptical Johnson–Kendall–Roberts theory to describe the relation between adhesion and the sinusoidal microtexture geometry is proposed. This model is in good agreement with experimentally determined values for interfacial adhesion in dry environments.
Catechol motifs are of particular interest owing to their ability to form many types of reversible bonds. [15][16][17][18][19] Polymer networks with catechol-cation coordination bonds have relaxation times that are governed by the affinity of the cation with the catechol group. [20][21][22] Grindy et al. [23] measured the viscoelastic modulus for networks with kinetically distinct metal coordinate crosslinks and demonstrated that reversible coordination junctions dominate the viscoelasticity of bulk materials. Catechol motifs with two hydroxy groups can also form hydrogen bonds among themselves. [24] Comparatively, there have been few reports on the role of hydrogen bonds between catechols in crosslinked networks. [25] Furthermore, the role of catechol concentration on bulk mechanics of polymer networks has yet to be elucidated in networks composed of model polymers. Here, we describe the role of hydrogen-bonded catechols on the bulk mechanical properties of the resulting networks.We recently synthesized catecholbearing ABA triblock copolymers that can self-assemble into mechanically robust networks wherein physical crosslinks are stabilized by inter-chain hydrogen bonding. [26] The copoly mer is synthesized through three steps and is processed into a network as shown in Scheme 1. Poly(ethylene glycol)-bromide (PEG-Br) macroinitiators are first prepared and then extended with an active esterified methacrylic acid (N-hydroxysuccinimide ester; NHSMA) by atom transfer radical polymerization. A blocks are then conjugated with dopamine to produce poly(NHSMA) 60 -b-PEG 227 -b-poly(NHSMA) 60 -Cat (ABA-Cat) polymers with a targeted conjugation ratio (r cat ) defined as the fraction of NHSMA monomers functionalized with a pendant catechol. Solutions of ABA-Cat polymer in N,Ndimethylformamide (DMF) self-assemble into networks through solvent exchange with water. We posit the network contains physical crosslinks composed of catechol-rich A blocks bridged together by PEG-based B blocks for following reasons. First, A and B blocks are immiscible in water and will be phase separated. Our previous study on structure showed solvent exchange disrupts crystalized PEG block by differential scanning calorimetry [26] and hydrogen-bonded Soft materials that contain dynamic and reversible bonds exhibit unique properties including unusual extensibility, reversible elasticity, and self-healing capabilities, for example. Catechol motifs are of particular interest owing to their ability to form many kinds of reversible bonds; however, there are few reports on the role of hydrogen bonds between catechols. Here, physically crosslinked self-assembled networks composed of catechol-functionalized ABA triblock co-polymers are synthesized and characterized to elucidate the role of intermolecular bonding between catechol motifs on bulk mechanical properties. The Young's moduli of equilibrated networks range from 16 to 43 MPa. Furthermore, the concentration of intermolecular interaction is controlled indirectly by synthesizing polymers with prescri...
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