Biomimetic Muscle Research Articles

Liquid Crystal Network Soft Actuator with Large Deformations Driven by High‐Frequency and Low‐Voltage Electric Field

AbstractIn this paper, electrically responsive liquid crystal network (LCN) based soft actuators are presented that can achieve large deformations under an alternating current (AC) electric field. These LCN actuators are prepared by photopolymerization of liquid crystal acrylate monomers with polar end groups, which form polar side chains in the networks. Actuation is initiated by applying a high‐frequency and low‐voltage electric field. When this AC field is applied, the LC mesogens, especially the polar side chains, can oscillate, decreasing the molecular order, and creating free volume in the LCNs, which further results in anisotropic deformation. The formed shapes range from bending to bulging, depending on the programmed molecular alignment. The actuation proceeds fast within 5 s both for activation and for the relaxation to the initial state upon switching the power on and off. This LCN soft actuator exhibits fast response, low power consumption, and high precision, making it a promising candidate for applications in soft robotics, microfluidics, and biomimetic muscles.

Design and Fabrication of Fibrous Spindle‐Like Constructs Using a Melt Electrohydrodynamic Writing Process

AbstractAdvanced manufacturing of 3D‐structured materials enables the production of biomimetic muscle tissues. While models of muscle tissue exist, current approaches possess a limited ability to capture essential elements of the muscle tissue microarchitecture. Therefore, this paper aims to engineer the intrinsically complex muscle spindle‐like ellipsoid geometry using a polymer melt‐based electrohydrodynamic (EHD) printing system. EHD systems have conventionally reported fiber deposition in a layerwise fashion. However, without mitigation, the observed fiber sagging and residual charge phenomena for the melt electrowriting (MEW) process limit the ability to produce layered fibrous 3D constructs with in‐plane fiber alignment. However, in this work, fiber sagging and residual charge phenomena are leveraged as part of the design intent to deposit nonoverlapping suspended fibers between two stationary walls toward spindle‐like construct fabrication. Specifically, herein the structural and mechanical properties of the MEW‐enabled spindle‐like constructs are analyzed as a function of the process and design parameters that govern control over fiber sagging and residual charge. The results indicate that the collector speed and wall‐to‐wall distance are the key parameters for tuning the spindle morphology. Moreover, cycle number and fiber diameter are identified as effective parameters for tuning the spindle mechanical properties.

Fabrication of 3D Biomimetic Smooth Muscle Using Magnetic Induction and Bioprinting for Tissue Regeneration.

Smooth muscles play a vital role in peristalsis, tissue constriction, and relaxation but lack adequate self-repair capability for addressing extensive muscle defects. Engineering scaffolds have been broadly proposed to repair the muscle tissue. However, efforts to date have shown that those engineered scaffolds focus on cell alignment in 2-dimension (2D) and fail to direct muscle cells to align in 3D area, which is irresolvable to remodel the muscle architecture and restore the muscle functions like contraction and relaxation. Herein, we introduced an iron oxide (Fe3O4) filament-embedded gelatin (Gel)-silk fibroin composite hydrogel in which the oriented Fe3O4 self-assembled and functioned as micro/nanoscale geometric cues to induce cell alignment growth. The hydrogel scaffold can be designed to fabricate aligned or anisotropic muscle by combining embedded 3D bioprinting with magnetic induction to accommodate special architectures of muscular tissues in the body. Particularly, the bioprinted muscle-like matrices effectively promote the self-organization of smooth muscle cells (SMCs) and the directional differentiation of bone marrow mesenchymal stem cells (BMSCs) into SMCs. This biomimetic muscle accelerated tissue regeneration, enhancing intercellular connectivity within the muscular tissue, and the deposition of fibronectin and collagen I. This work provides a novel approach for constructing engineered biomimetic muscles, holding significant promise for clinical treatment of muscle-related diseases in the future.

Simultaneous Sensing and Actuating Capabilities of a Triple-Layer Biomimetic Muscle for Soft Robotics.

This work presents the fabrication and characterization of a triple-layered biomimetic muscle constituted by polypyrrole (PPy)-dodecylbenzenesulfonate (DBS)/adhesive tape/PPy-DBS demonstrating simultaneous sensing and actuation capabilities. The muscle was controlled by a neurobiologically inspired cortical neural network sending agonist and antagonist signals to the conducting polymeric layers. Experiments consisted of controlled voluntary movements of the free end of the muscle at angles of ±20°, ±30°, and ±40° while monitoring the muscle's potential response. Results show the muscle's potential varies linearly with applied current amplitude during actuation, enabling current sensing. A linear dependence between muscle potential and temperature enabled temperature sensing. Electrolyte concentration changes also induced exponential variations in the muscle's potential, allowing for concentration sensing. Additionally, the influence of the electric current density on the angular velocity, the electric charge density, and the desired angle was studied. Overall, the conducting polymer-based soft biomimetic muscle replicates properties of natural muscles, permitting simultaneous motion control, current, temperature, and concentration sensing. The integrated neural control system exhibits key features of biological motion regulation. This muscle actuator with its integrated sensing and control represents an advance for soft robotics, prosthetics, and biomedical devices requiring biomimetic multifunctionality.

A Self‐Renewing Biomimetic Skeletal Muscle Construct Engineered using Induced Myogenic Progenitor Cells

AbstractSkeletal muscle represents a highly organized tissue that primarily regenerates by myogenic stem cells. Mimicking an in vitro skeletal muscle differentiation program that contains self‐renewing muscle stem cells and aligned myotubes is considered challenging. This study presents the engineering of a biomimetic muscle construct that can self‐regenerate and produce aligned myotubes using induced myogenic progenitor cells (iMPCs), a heterogeneous culture consisting of skeletal muscle stem, progenitor, and differentiated cells. Utilizing electrospinning, polycaprolactone (PCL) substrates are fabricated to facilitate iMPC‐differentiation into aligned myotubes by controlling PCL fiber orientation. Newly‐conceived constructs contain organized multinucleated myotubes alongside self‐renewing stem cells, whose differentiation capacity is augmented by Matrigel supplementation. Furthermore, this work utilizes single‐cell RNA‐sequencing (scRNA‐seq) to demonstrate that iMPC‐derived constructs faithfully recapitulate a step‐wise myogenic differentiation program. Notably, when subjected to a damaging myonecrotic agent, self‐renewing stem cells rapidly differentiate into aligned myotubes within the constructs, akin to skeletal muscle repair in vivo. Finally, this study demonstrates that the iMPC derivation protocol can be adapted to engineer human myoblast‐derived muscle constructs containing aligned myotubes, showcasing potential for translational applicability. Taken together, this work reports a novel in vitro system that mirrors myogenic regeneration and skeletal muscle alignment for basic research and regenerative medicine.

3D-printed hierarchical arrangements of actuators mimicking biological muscular architectures

Being able to imitate the sophisticated muscular architectures that characterize the animal kingdom in biomimetic machines would allow them to perform articulated movements with the same naturalness. In soft robotics, multiple actuation technologies have been developed to mimic the contraction of a single natural muscle, but a few of them can be implemented in complex architectures capable of diversifying deformations and forces. In this work, we present three different biomimetic muscle architectures, i.e. fusiform, parallel, and bipennate, which are based on hierarchical arrangements of multiple pneumatic actuators. These biomimetic architectures are monolithic structures composed of thirty-six pneumatic actuators each, directly 3D printed through low-cost printers and commercial materials without any assembly phase. The considerable number of actuators involved enabled the adoption and consequent comparison of two regulation strategies: one based on input modulation, commonly adopted in pneumatic systems, and one based on fiber recruitment, mimicking the regulation behavior of natural muscles. The straightforward realization through additive manufacturing processes of muscle architectures regulated by fiber recruitment strategies facilitates the development of articulated muscular systems for biomimetics machines increasingly similar to the natural ones.

Stiffness-Observer-Based Adaptive Control of an Intrinsically Compliant Parallel Wrist Rehabilitation Robot

Disability from injuries and diseases is a global problem affecting a large population; however, due to a lack of therapists and labor-intensive procedures, only a few benefits from rehabilitation. Robots can assist therapists in treating many patients simultaneously, but the existing solutions need improvements in their mechanism, actuation, and control. This article presents a four-link parallel end-effector robot for wrist joint rehabilitation. The proposed robot employs biomimetic muscle actuators (BMA) that provide intrinsic compliance to the robotic system. A fuzzy-based model is developed to identify the nonlinear nature of BMAs. The stiffness-observer learns subject-specific stiffness, which is used to modify the robot reference trajectories. An adaptive controller uses the fuzzy model and stiffness-observer and simultaneously controls the four BMAs to provide three degrees of rotational freedom to the robot end-effector. The feasibility of the robot mechanism and the controller was evaluated through proof of concept experiments conducted with three unimpaired human subjects. It was found that the controller was able to guide the robot–human system on the commanded trajectories in the presence of parallel actuation of compliant and nonlinear BMAs. Furthermore, the controller was also able to modify the commanded trajectories in the higher stiffness regions of the wrist workspace.

Impedance Control of a Wrist Rehabilitation Robot Based on Autodidact Stiffness Learning

Dynamic control of an intrinsically compliant robot is paramount to ensuring safe and synergistic assistance to the patient. This paper presents an impedance controller for the rehabilitation of stroke patients with compromised wrist motor functions. The control design employs a Koopman operator-based autodidactic system identification model to predict the anatomical stiffness of the wrist joint during its various degrees of rotational motion. The proposed impedance controller, perceiving the level of the subjects’ participation from their joint stiffness, can modify the applied force. The end-effector robot has a parallel structure that uses four biomimetic muscle actuators as parallel links between the end-effector and the base platform. The controller performance is corroborated by testing the end-effector robot with three healthy subjects.

An Environmental Friendly Tapioca Starch-Alginate Cultured Scaffold as Biomimetic Muscle Tissue.

Natural porous scaffolds have been studied and developed for decades in biomedical science in order to support cells with a simulated extracellular matrix in natural tissue as an ideal environment. Such three-dimensional scaffolds provide many degrees of freedom to modulate cell activity, such as porosity, pore size, mechanical strength, biodegradability, and biocompatibility. In this study, a porous, three-dimensional material of alginate incorporating tapioca starch was fabricated. A particular freeze-gelation method was applied to homogenously mix starch in the alginate, and the concentration was controllable. This pure natural composite porous scaffold was characterized physically and biologically. The synergistic functions, including biocompatibility, biodegradability, cell adhesion, and cell proliferation, were also investigated. A myogenic differentiation model further verified that the composite porous scaffold provided a suitable environment, supporting the differentiation effect in the myogenic process. The positive results demonstrated that this novel material has the potential to serve as a biomedical or clean meat appliance.

Ultralarge Contraction Directed by Light‐Driven Unlocking of Prestored Strain Energy in Linear Liquid Crystal Polymer Fibers

AbstractAnisotropic 1D contraction motion of polymeric actuating materials has drawn growing interests in fields ranging from soft robotics to biomimetic muscles. Although light‐driven liquid crystal polymers (LCPs) represent promising candidates to realize contraction (<20%) triggered remotely and spatially, there remain multitudes of challenges to develop an LCP system possessing ultralarge contraction rate. Here, a novel strategy combining shape memory effect and photochemical phase transition is presented to realize light‐driven contraction as large as 81% in a newly designed linear liquid crystal copolymer, where the eutectic mesogens of azobenzene and phenyl benzoate self‐organize into the smectic B phase. Importantly, this highly ordered structure as the switching segment firmly locks the stress‐induced strain energy, which is rapidly released by reversible trans–cis photoisomerization that destroys the lamellar liquid crystal phase, therefore leading to such ultralarge contraction. Fibers serve as light‐driven building blocks to achieve precise origami, to mimic the recovery of a “broken” spider web and to screen objects in different sizes, laying new ground for advanced applications of light‐driven LCPs from biomimetic robots to human assists.

Biomimetic Supramolecular Fibers Exhibit Water-Induced Supercontraction.

Spider silk is a fascinating material, combining high strength and elasticity that outperforms most synthetic fibers. Another intriguing feature of spider silk is its ability to "supercontract," shrinking up to 50% when exposed to water. This is likely on account of the entropy-driven recoiling of secondary structured proteins when water penetrates the spider silk. In contrast, humidity-driven contraction in synthetic fibers is difficult to achieve. Here, inspired by the spider silk model, a supercontractile fiber (SCF), which contracts up to 50% of its original length at high humidity, comparable to spider silk, is reported. The fiber exhibits up to 300% uptake of water by volume, confirmed via environmental scanning electron microscopy. Interestingly, the SCF exhibits tunable mechanical properties by varying humidity, which is reflected by the prolonged failure strain and the reversible damping capacity. This smart supramolecular fiber material provides a new opportunity of fabricating biomimetic muscle for diverse applications.

Water-driven actuation of Ornithoctonus huwena spider silk fibers

Spider silk possesses remarkable mechanical properties and can lift weight effectively. Certain kinds of spider silk have unique response to liquid, especially water, because of their hydrophilic proteins, β-sheet characters, and surface structure. The Ornithoctonus huwena (O. huwena) spider is a unique species because it can be bred artificially and it spins silk whose diameter is in nanometer scale. In this work, we report the “shrink–stretch” behavior of the O. huwena spider silk fibers and show how they can be actuated by water to lift weight over long distance, at a fast speed, and with high efficiency. We further rationalize this behavior by analyzing the mechanical energy of the system. The lifting process is energy-efficient and environmentally friendly, allowing applications in actuators, biomimetic muscles, or hoisting devices.

Spider silk as a novel high performance biomimetic muscle driven by humidity

The abrupt halt of a bumble bee's flight when it impacts the almost invisible threads of an orb web provides an elegant example of the amazing strength and toughness of spider silk. Spiders depend upon these properties for survival, yet the impressive performance of silk is not limited solely to tensile mechanics. Here, we show that silk also exhibits powerful cyclic contractions, allowing it to act as a high performance mimic of biological muscles. These contractions are actuated by changes in humidity alone and repeatedly generate work 50 times greater than the equivalent mass of human muscle. Although we demonstrate that this response is general and occurs weakly in diverse hydrophilic materials, the high modulus of spider silk is such that it generates exceptional force. Furthermore, because this effect already operates at the level of single silk fibers, only 5 microm in diameter, it can easily be scaled across the entire size range at which biological muscles operate. By contrast, the most successful synthetic muscles developed so far are driven by electric voltage, such that they cannot scale easily across large ranges in cross-sectional areas. The potential applicability of silk muscles is further enhanced by our finding that silkworm fibers also exhibit cyclic contraction because they are already available in commercial quantities. The simplicity of using wet or dry air to drive the biomimetic silk muscle fibers and the incredible power generated by silk offer unique possibilities in designing lightweight and compact actuators for robots and micro-machines, new sensors, and green energy production.

How super is supercontraction? Persistent versus cyclic responses to humidity in spider dragline silk

Spider dragline silk has enormous potential for the development of biomimetic fibers that combine strength and elasticity in low density polymers. These applications necessitate understanding how silk reacts to different environmental conditions. For instance, spider dragline silk ;supercontracts' in high humidity. During supercontraction, unrestrained dragline silk contracts up to 50% of its original length and restrained fibers generate substantial stress. Here we characterize the response of dragline silk to changes in humidity before, during and after supercontraction. Our findings demonstrate that dragline silk exhibits two qualitatively different responses to humidity. First, silk undergoes a previously unknown cyclic relaxation-contraction response to wetting and drying. The direction and magnitude of this cyclic response is identical both before and after supercontraction. By contrast, supercontraction is a ;permanent' tensioning of restrained silk in response to high humidity. Here, water induces stress, rather than relaxation and the uptake of water molecules results in a permanent change in molecular composition of the silk, as demonstrated by thermogravimetric analysis (TGA). Even after drying, silk mass increased by approximately 1% after supercontraction. By contrast, the cyclic response to humidity involves a reversible uptake of water. Dried, post-supercontraction silk also differs mechanically from virgin silk. Post-supercontraction silk exhibits reduced stiffness and stress at yield, as well as changes in dynamic energy storage and dissipation. In addition to advancing understanding supercontraction, our findings open up new applications for synthetic silk analogs. For example, dragline silk emerges as a model for a biomimetic muscle, the contraction of which is precisely controlled by humidity alone.