Smart hydrogels are soft materials which can be applied in sensors and actuators especially in microfluidics in which the fluid–structure interaction is important. In this work, first, the behavior of a one-way hydrogel micro-valve is investigated by considering the fluid–structure interaction effect for a specified geometry of the micro-valve. Second, both the fluid–structure interaction and non-fluid–structure interaction simulations are conducted to study the fluid flow effect on the operational parameters of the micro-valve. The obtained results show that the fluid–structure interaction effects are important and have a considerable influence on the micro-valve parameters especially on its closing temperature. Thereafter, a precise study on the micro-valve is executed by considering the micro-valve operational parameters such as inlet pressure, head size, crosslinking density, and breaking pressure at different temperatures. The results show the importance of considering the fluid–structure interaction effect in the design of these devices.
Smart structures such as temperature-sensitive actuators made of soft hydrogels can be useful for designing biomedical devices. In this study, free swelling and force generation behavior of a bilayer actuator based on thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) hydrogel and an elastomer under various loading profiles has been investigated. The thermo-sensitive hydrogel was synthesized by redox-initiated radical polymerization. We then prepared a bilayer actuator made of temperature-sensitive PNIPAM hydrogel, and examined its behavior under different loading conditions such as free swelling and force generation to study different aspects of the response of the hydrogel, including homogeneous free swelling and confined swelling using two approaches for (monolayer) hydrogel. Experiments on free swelling and force generation swelling of the bilayer actuator were also performed. After implementing an appropriate hydrogel constitutive model within a finite element framework, material parameters (of both the hydrogel and an elastomer) required in fabrication of the bilayer were calibrated. Using these calibrated material parameters, the same test conditions were simulated, and the numerical results were compared to those of experiments; a good agreement was observed between the experimental tests and numerical outcomes. The results show that the material model parameters are reliable, and the finite element method can be used for designing more complicated structures based on thermo-sensitive hydrogels.
In this paper, a new approach for stress-softening of an isotropic, incompressible, hyperelastic and rectangular beam that undergoes cyclic bending-unbending deformation, is presented. Employing an exponential softening function, damage response of the hyperelastic beam due to cyclic finite bending is investigated. The stress-softening phenomenon occurs in elastomeric materials when they deform for the first time. Under the same deformation, the stress required in reloading is smaller than the initial loading stage. This is known as the Mullins effect. To verify the accuracy of the proposed solution, finite element analysis of the same problem is carried out. In this study, a principal stretch-based strain energy function i.e., Ogden model and an invariant-based function such as a newly introduced Exp–Exp model are used for all bending, unbending and re-bending procedures. The proposed method needs a much shorter time compared to FEM simulations. Thus, in design and optimization of the structures under bending that requires a large number of analyses, the proposed semi-analytical solution can be considered as an efficient tool for studying the effects of different material and geometrical parameters.
The behavior of a temperature-sensitive micro-channel have been investigated in this study which mainly includes a functionally graded (FG) hydrogel as a sensor or an actuator. In order to achieve this goal, both fluid-structure interaction (FSI) and non-FSI simulations are conducted for hydrogel with homogeneous property distribution as well as FG hydrogels with different number of layers (2–16 layers). Moreover, this study investigates the FG hydrogel cross-linking density that obeys a general exponential form. In addition to all mentioned, the FG hydrogels are considered in both ascending and descending states with vertically and horizontally functionally graded property distributions (VFG and HFG hydrogels). Subsequently, the importance of the difference between the FG and homogenous hydrogels has been highlighted in the findings of the study. Besides, the FSI influence has a vital role in these structures especially once an FG material is utilized. According to the findings, the ascending and descending distributions of the hydrogel properties may significantly affect the micro-channel behavior, especially in horizontally graded type. This process can be done in a way that for descending distribution of HFG there exist no closing state for the micro-channel.
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