Nonlinear analysis of a fiber-reinforced tubular conducting polymer-based soft actuator

This study presents the analytical modeling of a fiber-reinforced tubular conducting polymer (FTCP) actuator. The FTCP actuator is a low voltage-driven electroactive polymer arranged in an electrochemical cell. The electrochemical model is developed following an electrical circuit analogy that predicts the charge diffused inside the actuator for an applied voltage. An empirical relation is applied to couple the two internal phenomena, viz., diffusion of the ions and mechanical deformation. Further, the finite deformation theory is applied to predict the blocked force and free strain of the FTCP actuator. The developed model is consistent with existing experimental results for an applied voltage. In addition, the effect of various electrical and geometrical parameters on the performance of the actuator is addressed

Stereocilia Membrane Deformation: Implications for the Gating Spring and Mechanotransduction Channel

AbstractIn hair cells, although mechanotransduction channels have been localized to tips of shorter stereocilia of the mechanically sensitive hair bundle, little is known about how force is transmitted to the channel. Here, we use a biophysical model of the membrane-channel complex to analyze the nature of the gating spring compliance and channel arrangement. We use a triangulated surface model and Monte Carlo simulation to compute the deformation of the membrane under the action of tip link force. We show that depending on the gating spring stiffness, the compliant component of the gating spring arises from either the membrane alone or a combination of the membrane and a tether that connects the channel to the actin cytoskeleton. If a bundle is characterized by relatively soft gating springs, such as those of the bullfrog sacculus, the need for membrane reinforcement by channel tethering then depends on membrane parameters. With stiffer gating springs, such as those from rat outer hair cells, the channel must be tethered for all biophysically realistic parameters of the membrane. We compute the membrane forces (resultants), which depend on membrane tension, bending modulus, and curvature, and show that they can determine the fate of the channel

Modeling and parametric optimization of 3D tendon-sheath actuator system for upper limb soft exosuit

This paper presents an analysis of parametric characterization of a motor driven tendon-sheath actuator system for use in upper limb augmentation for applications such as rehabilitation, therapy, and industrial automation. The double tendon sheath system, which uses two sets of cables (agonist and antagonist side) guided through a sheath, is considered to produce smooth and natural-looking movements of the arm. The exoskeleton is equipped with a single motor capable of controlling both the flexion and extension motions. One of the key challenges in the implementation of a double tendon sheath system is the possibility of slack in the tendon, which can impact the overall performance of the system. To address this issue, a robust mathematical model is developed and a comprehensive parametric study is carried out to determine the most effective strategies for overcoming the problem of slack and improving the transmission. The study suggests that incorporating a series spring into the system's tendon leads to a universally applicable design, eliminating the need for individual customization. The results also show that the slack in the tendon can be effectively controlled by changing the pretension, spring constant, and size and geometry of spool mounted on the axle of motor

Modeling Electrically Active Viscoelastic Membranes

The membrane protein prestin is native to the cochlear outer hair cell that is crucial to the ear's amplification and frequency selectivity throughout the whole acoustic frequency range. The outer hair cell exhibits interrelated dimensional changes, force generation, and electric charge transfer. Cells transfected with prestin acquire unique active properties similar to those in the native cell that have also been useful in understanding the process. Here we propose a model describing the major electromechanical features of such active membranes. The model derived from thermodynamic principles is in the form of integral relationships between the history of voltage and membrane resultants as independent variables and the charge density and strains as dependent variables. The proposed model is applied to the analysis of an active force produced by the outer hair cell in response to a harmonic electric field. Our analysis reveals the mechanism of the outer hair cell active (isometric) force having an almost constant amplitude and phase up to 80 kHz. We found that the frequency-invariance of the force is a result of interplay between the electrical filtering associated with prestin and power law viscoelasticity of the surrounding membrane. Paradoxically, the membrane viscoelasticity boosts the force balancing the electrical filtering effect. We also consider various modes of electromechanical coupling in membrane with prestin associated with mechanical perturbations in the cell. We consider pressure or strains applied step-wise or at a constant rate and compute the time course of the resulting electric charge. The results obtained here are important for the analysis of electromechanical properties of membranes, cells, and biological materials as well as for a better understanding of the mechanism of hearing and the role of the protein prestin in this mechanism

Analytical Modeling of Twist Actuators for Pretwisted Anisotropic Strips

In this paper, a twist actuator model for a strip-like open section is developed by application of variational asymptotic method. Piezoelectric Fiber Composite (PFC) with InterDigitated Electrode (IDE) has been used as the actuating material. The actuation effect in the presence of non-linear extension-twist coupling has been studied. For an open type of section, direct twist actuation stems mainly through structural anisotropy (structural coupling) because inplane shear actuation does not affect the induced twist much. Analytical derivation is done for a simple example and the effects of non-linear coupling discussed

Strain-generated electric charge.

<p>Charge per unit area generated in cellular membranes with prestin as a result of constant-rate application of the longitudinal strain to the outer hair cell (with circumferential strain changing accordingly to keep the cell volume constant). A) Three powers entering the viscoelastic model of the cell membrane, , B) three times associated with prestin, , and C) three strains rates .</p

Outer hair cell's active force measurement in the microchamber experiment.

<p>The cell is partially inserted into a large pipette (microchamber), and voltage V_app is applied to the chamber environment. The voltage in the microchamber results in the membrane potentials, V_in, and V_ex, of the included and excluded parts of the cell. These potentials have opposite signs, and the two (included and excluded) parts of the electromotile cell become, respectively, shorter and thicker and longer and thinner. The excluded part pushes the AFM cantilever of a prescribed stiffness, and the generated active force, F, can be estimated based on the displacement of the cantilever.</p

Predictions of Kelvin-Voight models.

<p>Predictions of a Kelvin-Voight-like model of membrane viscoelasticity with three different viscosities, (color lines) vs. the experimental values of the normalized amplitude of the outer hair cell active force (circles) from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037667#pone.0037667-Frank1" target="_blank">[38]</a>.</p

Analytical description of prestin-associated charge.

<p>Approximation of the solution of the Fokker-Planck equation obtained in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037667#pone.0037667-Sun2" target="_blank">[30]</a> (circles) by an electrical filter function (color surface) for different DC-potentials, V_DC, and frequencies, ω.</p