Selective Stiffening in Soft Actuators by Triggered Phase Transition of Hydrogel‐Filled Elastomers

Abstract

Nature has inspired a new generation of robots that not only imitate the behavior of natural systems but also share their adaptability to the environment and level of compliance due to the materials used to manufacture them, which are typically made of soft matter. In order to be adaptable and compliant, these robots need to be able to locally change the mechanical properties of their soft material‐based bodies according to external feedback. In this work, a soft actuator that embodies a highly controllable thermo‐responsive hydrogel and changes its stiffness on direct stimulation is proposed. At a critical temperature, this stimulation triggers the reversible transition of the hydrogel, which locally stiffens the elastomeric containment at the targeted location. By dividing the actuator into multiple sections, it is possible to control its macroscopic behavior as a function of the stiffened sections. These properties are evaluated by arranging three actuators into a gripper configuration used to grasp objects. The results clearly show that the approach can be used to develop soft actuators that can modify their mechanical properties on‐demand in order to conform to objects or to exert the required force.

Mechanics Underpinning Phase Separation of Hydrogels

“…According to the literature and the discussion of Figure S1 in the Supporting Information, when the parameter χ depends on the swelling ratio J, there might exist two stable equilibrium solutions. 25 As a conclusion, although phase coexistence can be predicted when the parameter χ depends on the swelling ratio J, for the case of free swelling, the free energy (9) with constant χ cannot predict equilibrium phase coexistence.…”

“…For the free swelling case, we use the swelling ratio J as a variable and obtain the equivalent forms for eqs 7 and 8. The free energy (2) expressed as a function of J is (9) When the hydrogel is in equilibrium, eq 7 is equivalent to the following equation (10) The free energy W is plotted as a function of J at different values of χ, and points a, b, and c correspond to the equilibrium solutions when μ w = 0 (Figure 2a).…”

“…A hydrogel is composed of a polymer network dispersed in a solvent, and its volume can significantly change through the transport of solvent molecules. In response to small changes of external stimuli, such as the temperature, solvent composition, electric field, and light, certain hydrogels can undergo a discontinuous volume transition from a swollen state to a shrunk state, which is called a volume phase transition. − Hydrogels capable of a volume phase transition have diverse applications, including sensors, actuators, soft robots, drug delivery, and so on. During the process of a volume phase transition, which is governed by the kinetics of solvent migration, phase separation occurs, and the swollen and shrunk states coexist.…”

Liquid Crystal-Based Organosilicone Elastomers with Supreme Mechanical Adaptability

“…Elastomers with supreme mechanical adaptability have been used and developed for several years [1][2][3], which have not only recoverable large deformation capacity, but also ability to compensate external work during continuous deformation [4,5]. For instance, for widely used connection devices, i.e., rubber ring, it is desirable to not only provide certain tensile strength for load-bearing but also show a stress plateau as along as possible for severe service condition.…”

Entropy‐Mediated Polymer–Cluster Interactions Enable Dramatic Thermal Stiffening Hydrogels for Mechanoadaptive Smart Fabrics

“…The mostly adopted strategy is based on phasetransition hydrogels by incorporating LCST (lower critical solution temperature)-type polymers such as poly(Nisopropylacrylamide). [12][13][14] To inhibit the unwanted macroscopic volume contraction of LCST-type polymers, hydrophilic blocks are often introduced as molecular scaffolds to retain the released water at the micro-and meso-scales. [15][16][17][18] However, such a design usually leads to reduced modulus change amplitudes (generally < 20 times) and the maximum moduli at the stiffened state are also low (< 1 MPa).…”