In-vitro models of the systemic circulation have gained a lot of interest for fundamental understanding of cardiovascular dynamics and for applied hemodynamic research. In this study, we introduce a physiologically accurate in-vitro hydraulic setup that models the hemodynamics of the coupled atrioventricular-aortic system. This unique experimental simulator has three major components: 1) an arterial system consisting of a human-scale artificial aorta along with the main branches, 2) an artificial left ventricle (LV) sac connected to a programmable piston-in-cylinder pump for simulating cardiac contraction and relaxation, and 3) an artificial left atrium (LA). The setup is designed in such a way that the basal LV is directly connected to the aortic root via an aortic valve, and to the LA via an artificial mitral valve. As a result, two-way hemodynamic couplings can be achieved for studying the effects that the LV, aorta, and LA have on each other. The collected pressure and flow measurements from this setup demonstrate a remarkable correspondence to clinical hemodynamics. We also investigate the physiological relevancies of isolated effects on cardiovascular hemodynamics of various major global parameters found in the circulatory system, including LV contractility, LV preload, heart rate, aortic compliance, and peripheral resistance. Subsequent control over such parameters ultimately captures physiological hemodynamic effects of LV systolic dysfunction, preload (cardiac) diseases, and afterload (arterial) diseases. The detailed design and fabrication of the proposed setup is also provided.
Smart hydrogels are promising materials for shape-shifting structures regarding their large reversible deformation in response to external stimuli in the absence of mechanical loading. Actuators composed of responsive hydrogels have gained significant attention due to their low power consumption, bio-compatibility, fast response, and accessibility. Among these structures, bidirectional hydrogel-based actuators are more fascinating, especially when they have similar reversible bending in both directions. This paper introduces a new design concept of a hydrogel bilayer made of a poly (HEMA-co-DMAEMA) layer and a poly (HEMA-co-AA) hydrogel layer that swells at low and high pH, respectively. This structure is capable of bending in diverse directions while the pH of the aqueous bath alters. The main characteristic of this structure is having reversible bidirectional bending, which has similar behaviors in both directions, unlike previous hydrogel-elastomer bilayers. Then, we develop an analytical method to solve the swelling-induced bidirectional bending of a pH-sensitive hydrogel bilayer. On the other hand, the finite bending of bilayer structure is studied by the finite element method in several cases to demonstrate the validity and accuracy of the proposed analytical solution. Lastly, the impacts of material composition and geometrical factors are investigated to be used for bilayer actuator design and application.
Inspired by nature, active materials are used to developed complex 3D configurations considering their specific characteristics. One of the shape-shifting methods in smart structures is utilizing programmable materials in self-folding structures. Hydrogels due to their biocompatibility, controllable functionalities, large reversible deformations, and their sensitivity to environmental stimuli are vital candidates to be used in self-folding structures. To avoid the mechanical weakness of conventional hydrogels, in this paper polyampholyte tough hydrogel is inspected considering a transient electro-chemo-mechanical constitutive model combining a visco-hyperelastic model with Nernst-Planck and Poisson’s equations. After calibrating the material parameters and verifying the accuracy of the model and its implementation, we present two approaches in order to generate self-folding hydrogel-based structures: polymer structure with bilayer hinges and trilayer structure composed of a hydrogel film sandwiched between two elastomer layers. Next, diverse factors are examined in the self-folding of smart structures which conforms with experimental test data, including hydrogel swelling, structure thickness and stiffness, bilayer configuration and composition, the width of the bilayer as well as opening width in trilayer and layers thickness. Finally, several transient self-folding of 2D flat patterns which turn into 3D complex configurations are scrutinized such as the closure of box, pyramids, and flower-shaper gripper.
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