Applications In Soft Robotics Research Articles

E-Morph as a New Adaptive Actuator for Soft Robotics

The paper presents an electromagnetically-controlled shape morphing composite (e-morph) as a new variable stiffness changing actuator for soft robotic applications. The structure is composed of a granular core and an outer shell made of silicon, with channels filled with liquid metal. Under an electromagnetic force, the actuator can change its shape in a controlled way, while the jamming mechanism allows the granules used in the core to maintain the desired shape and change their stiffness at a rate higher than 6. This paper provides a detailed description of the concept, empirical and simulation results of the samples tested. Finally, suggestions are made for initial applications using this actuator.

Analytical Modeling of a Soft Pneu-Net Actuator Subjected to Planar Tip Contact

Soft actuators are compliant structures that are generally made of elastomers and generates large deformation. The behavior of these structures cannot be estimated accurately using infinitesimal strain theories. The objective of soft robotics applications is the controlled large deformation of these structures. In this article, we propose an analytical model for a pneu-net soft actuator. The model is based on the Euler–Bernoulli finite strain hyperelastic thin cantilever beam theory. The deformation of the air chambers is modeled using finite strain membrane theory. The analytical model is developed for two different states of the actuator: 1) free space; and 2) when the actuator was subjected to tip contact. The proposed theoretical model predicts the deformation and force characteristics of the actuator for the grasping state. The theoretical formulation of the developed model is different from previously developed infinitesimal strain models for the actuator, as it considers the axial stretch and forces applied to the actuator. In addition, it can be theoretically implemented on similar structured actuators for various applications. The theoretically calculated deformation and force characteristics of different actuators are compared with the finite element (FE) model and experimental characteristics. The results suggest that the proposed model can predict the actuator deformation and force characteristics as accurately as the FE model, but the computation time of the proposed model is less than 1% that of the FE model. The proposed model is further implemented on a three-finger gripper to predict the air pressure required for a stable grasp of different objects and is validated experimentally.

Kirigami-Inspired Programmable Soft Magnetoresponsive Actuators with Versatile Morphing Modes.

Untethered soft magnetoresponsive actuators (SMRAs), which can realize rapid shape transformation, have attracted widespread attention for their strategic applications in exploration, transportation, and minimally invasive medicine. It remains a challenge to fabricate SMRAs with complicated morphing modes (more than bending and folding), limiting their applications to simple shape-morphing and locomotion. Herein, a method integrating the ancient kirigami art and an advanced mechanical assembly method is proposed, which realizes 2D-to-3D and 3D-to-3D complicated shape-morphing and precise magnetization programming through cut-guided deformation. The kirigami-inspired SMRAs exhibit good robustness after actuating more than 10000 times. An integrated finite element analysis method is developed to quantitatively predict the shape transformation of SMRAs under magnetic actuation. By leveraging this method, a set of 3D curved responsive morphologies with programmed Gaussian curvature are fabricated (e.g., ellipsoid and saddle structures), specifically 3D multilayer structures and face-like shapes with different expressions, which are difficult to realize using previous approaches. Furthermore, a bionic-scaled soft crawling robot with significant obstacle surmounting ability is fabricated using the kirigami-inspired method. The ability of the method to achieve programmable SMRAs with versatile morphing modes may broaden its applications in soft robotics, color-switchable devices, and clinical treatments.

Insights into the viscohyperelastic response of soft magnetorheological elastomers: Competition of macrostructural versus microstructural players

Magnetorheological elastomers (MREs) are multifunctional composites that consist of an elastomeric matrix filled with magnetic particles. These materials respond to an external magnetic field by mechanically deforming and/or changing their magnetorheological properties. Such a multi-physical response has made them extraordinary candidates for a wide variety of applications in soft robotics and bioengineering. However, there are still some gaps of knowledge that prevent the optimal design and application of these MREs. In this regard, the effect of viscoelastic mechanisms remains elusive from a microstructural perspective. To the best of the authors’ knowledge, this work provides for the first time a numerical homogenization analysis for various magneto-active microstructures accounting for viscous deformation mechanisms. To this end, we propose an incremental variational formulation that incorporates viscoelasticity via internal variables, which is properly modified to deal with the continuity of Maxwell stresses. The proposed framework is applied to study the magneto-mechanical couplings in extremely soft MREs (stiffness < 10 kPa). Such a soft matrix promotes microstructural rearrangements while transmitting internal forces leading to macrostructural synergistic responses. The constitutive parameters are calibrated with experimental tests. The numerical results are accompanied with original magnetostriction tests considering different sample geometries and confined magneto-mechanical tests, reporting the macroscopic response. The results obtained in this work suggest that the effective magneto-mechanical response of the MRE is the outcome of a competition between macrostructural and local microstructural responses, where viscous mechanisms play a relevant role.

Harnessing fiber induced anisotropy in design and fabrication of soft actuator with simultaneous bending and twisting actuations

Most of the soft dielectric elastomer actuators (DEAs) are able to provide either planar or twisting deformations only. Development of a bio-inspired actuator with functionalities similar to the mammalian tongue or octopus tentacles is highly desirable for soft robotics applications. Here, design and fabrication of a multi-layered soft DEA capable of simultaneous bending and twisting actuation is reported through a unified computational and experimental approach. An anisotropic layer is introduced to couple the bending and twisting deformation in the conventional DEA. The fiber spacing and fiber orientation in the anisotropic layer is optimized with classical laminate theory (CLT) and finite element (FE) analysis. A multi-layered DEA with bending-twisting actuation is experimentally fabricated as a ‘proof of concept’. The computational and experimental results suggest that bend-twist coupling in multi-layered DEA can be suitably designed by varying the fiber spacing and orientation in the anisotropic layer. The twisting actuation of DEA enhances at the expense of bending deformation with an increase in the fiber angle. Finally, it is shown that a single anisotropic layer having 2 mm fiber spacing and 45° fiber orientation induces sufficiently large bending and twisting actuations in DEA. The findings of the present study will be a significant step ahead to develop bio-mimicking anisotropic actuators and grippers for soft robotics applications.

Development of an Innovative Soft Piezoresistive Biomaterial Based on the Interconnection of Elastomeric PDMS Networks and Electrically-Conductive PEDOT:PSS Sponges.

A deeply interconnected flexible transducer of polydimethylsiloxane (PDMS) and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) was obtained as a material for the application of soft robotics. Firstly, transducers were developed by crosslinking PEDOT:PSS with 3-glycidyloxypropryl-trimethoxysilane (GPTMS) (1, 2 and 3% v/v) and using freeze-drying to obtain porous sponges. The PEDOT:PSS sponges were morphologically characterized, showing porosities mainly between 200 and 600 µm2; such surface area dimensions tend to decrease with increasing degrees of crosslinking. A stability test confirmed a good endurance for up to 28 days for the higher concentrations of the crosslinker tested. Consecutively, the sponges were electromechanically characterized, showing a repeatable and linear resistance variation by the pressure triggers within the limits of their working range ( max = 80% for 1–2% v/v of GPTMS). The sponges containing 1% v/v of GPTMS were intertwined with a silicon elastomer to increase their elasticity and water stability. The flexible transducer obtained with this method exhibited moderately lower sensibility and repeatability than the PEDOT:PSS sponges, but the piezoresistive response remained stable under mechanical compression. Furthermore, the transducer displayed a linear behavior when stressed within the limits of its working range. Therefore, it is still valid for pressure sensing and contact detection applications. Lastly, the flexible transducer was submitted to preliminary biological tests that indicate a potential for safe, in vivo sensing applications.

Programmable thermochromic soft actuators with “two dimensional” bilayer architectures for soft robotics

Soft robots have unique advantages in the fields including human-machine interaction and bionics, yet there remain persistent challenges in developing multifunctional, programmable, and high-performance soft actuators. Here, we introduce a class of programmable thermochromic soft actuators (PTSAs) through liquid–vapor phase transitions, which can deform greatly through extreme volume change accompanying reversible liquid-vapor phase transition of embedded low boiling point fluid, and change several colors through reversible reactions of embedded thermochromic microcapsules. Using a "two dimensional" architecture, the initial shape and deformation type of the actuator with three different modes can be easily programmed. Respond to multiple stimuli, PTSAs have a variety of optional actuating methods including external heating, Joule heating, magnetic induction heating, and magnetic actuating. The suitable operating temperature and stretchability allows PTSAs to be used in wearable devices. Using PTSAs, a range of applications including color-shifting curly biomimetic maple leaves, color-shifting twisting biomimetic vines, zero-power holding and visually temperature sensing soft grippers, untethered magnetically actuated camouflage soft robots, and blood pressure sensing wristband are demonstrated, illustrating their broad prospects for applications in soft robotics, wearable devices, camouflage, and bionics.

Strain and temperature sensors made from fishing lines

Soft and flexible sensors are demanded in a wide variety of wearables and soft-robotic applications. We designed and developed novel integrated strain and temperature sensors using super-coiled transparent polymer fibres as an optical waveguide by exploiting changes in macro-bending optical losses induced by strain or heat. Polymers used in this research such as nylon and fluorocarbon are shaped as super-coils. The customized optical couplings were designed that can sustain tension induced by axial strain force or thermal stress (entropic spring effect). A customized method of transmission and measurement of the optical signal transferred through a super-coiled polymeric waveguide was designed to reduce attenuation losses. Transmitted optical signals were recorded in the range of −65 dBm to −45 dBm. Changes in the measured transmitted optical signal were recorded and compared for transparent mono-filament nylon fibre fishing line and fluorocarbon fibre fishing line. The trends of the measured parameters registered during the testing period showed a variation of ∼2%, good linearity and repeatability, however with minor hysteresis in the range of for nylon and the range of for fluorocarbon. Limited optical characteristics of these materials were compensated and substituted by high sensitivity introduced by a small macro-bend radius. These sensing devices combine high sensitivity with large comparative strain and a wide range of temperatures. Polymers used in this research exhibit the potential for use in SCP waveguide-based soft robotic sensors. Measured reaction times validate the potential of these sensors in measurements of moderate or slow speed processes. These sensing devices also combine a simple manufacturing process that uses abundant and cost-effective materials with a wide array of potential applications.

Fine liquid-core polymer fibers for microhydraulic applications: A versatile process design

Miniaturization is an essential requirement to advance areas where conventional mechatronic systems may struggle. Microhydraulic devices that combine resilience and compliance could thus revolutionize microrobot applications like locomotion and manipulation. Spurred by the deformability and structural stability provided by veins in insect wings, microscale liquid-core fibers were created, comprising of a polymeric sheath and a liquid core. A microfluidic co-extrusion spinneret was designed, assisted by computational fluid dynamics studies, to achieve such unique liquid-core fibers. Hydraulic pressure transfer tests were successfully applied on fine, up to 10 m long, oil-filled polyamide fibers. The results showed a pressure transfer with a fiber length-dependent delay of ∼ 20–100 s for fiber lengths of ∼ 1–10 m, and a viscoelastic behavior with relaxation times that behave linearly with fiber length. These findings enable the development of resilient and deformable microhydraulic systems within restricted available space, predestined for applications in soft robotics.

Investigation into a Lightweight Polymeric Porous Sponge with High Magnetic Field and Strain Sensitivity.

Recently, flexible sensors have gained significant attention due to their potential applications in soft robotics and biomimetic intelligent devices. However, the successful production of favorable flexible sensors integrated with high flexibility, sensitivity and excellent environment adaptability toward multiple external stimuli is still an enormous challenge. Herein, a lightweight polymeric porous sponge capable of detecting an external magnetic field and strain excitations is proposed by assembling a sodium alginate/chitosan (SA/CHI) porous sponge with micron carbonyl iron and nanoscale Fe3O4 magnetic particles (MPs). Based on the double network structure, the SA/CHI sponge possesses preferable mechanical strength and hydrophilicity, demonstrating its high flexibility and deformability. More importantly, the electrical response of the SA/CHI sponge sensors can display remarkable variation under external magnetic and mechanical stimuli due to their superior magnetic characteristics and electrical conductivity. Meanwhile, their sensing properties can maintain relatively stable recoverability and repeatability towards the periodic excitations and releases. Additionally, a potential mechanism is provided to investigate their stimuli-sensitive behavior. It is highly dependent on the microstructure variations in MPs and conductive multi-walled carbon nanotube (MWCNTs) networks. Due to its exceptional magnetic controllability and appropriate electrical sensitivity, the proposed sensor shows high potential in wearable multi-sensing electronics and intelligent transport devices.

4D printing of PLA/PCL-based bio-polyurethane via moderate cross-linking to adjust the microphase separation

Bio-polyurethanes with reversible thermal stimulation responsiveness and remarkable mechanical properties are desirable for 4D printed materials. Herein, polylactic acid (PLA)/polycaprolactone (PCL) based bio-polyurethane material (CBPU) showing excellent mechanical property (e.g. elongation at break reached 584%) and shape memory effect was designed and prepared. Moderate cross-linking of the bio-polyurethane using chitooligosaccharide (COS) successfully promoted the formation of a microphase-separated structure between the hard phase composed of isophorone diisocyanate (IPDI) and soft phase consisting of PLA and PCL. In addition, the CBPU changed autonomously between printed and temporary shapes at preset (−5 °C) and physiological (40 °C) temperatures, respectively, with shape recovery ratio >98%. Therefore, a combination of programmable printability, high toughness, and shape memory capability make CBPU very promising for a wide range of applications in bio-tissue engineering, soft robotics, and intelligent sensors.

Synergistic and counteractive effects of Bi-component plasticizers on structure and electric field-induced bending actuation behaviors of poly (vinyl chloride) (PVC) gels

Electroactive dielectric polymer gels (DPGs) have important applications in soft robots, wearable electronics, and biomedical devices etc. However, the relationship between structure and electromechanical properties of DPGs have not been fully understood so far. In this work, the effect of the bi-component plasticizers on the structure, mechanical properties, dielectric properties, and electric field induced dynamic bending actuation behaviors of plasticized poly (vinyl chloride) (PVC) gels with free electrodes were investigated using the dibutyl adipate (DBA) as major plasticizer and the branch structured bio-based triethyl 2-acetylcitrate (ATEC) or triethyl citrate (TEC) as minor second plasticizer (30 wt% of bi-component plasticizer). The PVC gel films bend towards the anode side when applied electric field under free electrodes measurement condition. The electric field induced capacitance change of PVC gels was important factor to influence the bending displacement, which were influenced by the interactions between plasticizers and PVC chains in PVC gels. The bending displacement of PVC/DBA/TEC and PVC/DBA/ATEC gels is about 80% and 140% of PVC/DBA at 7.2 kV (corresponding to 180 V/mm of nominal electric field), respectively. The synergistic and counteractive effects of bi-component plasticizers on the bending actuation of PVC gels are mainly attributed to the competition between plasticization effect of PVC and interactions of inter-plasticizers. Our results provide a new insight for the design of high performance electroactive dielectric polymer gel materials.

From carbon nanotubes to ultra-sensitive, extremely-stretchable and self-healable hydrogels

Bearing several merits including resemblance to tissues, tunable conductivity, and mechanical flexibility, conductive hydrogels have been widely applied in wearable sensors, implantable bioelectronics, artificial skins, etc. However, the fragile nature, lack of self-healing capability, and insufficient sensitivities at low strains of traditional conductive hydrogels cannot meet the increasing demands of flexible electronics. Here, carbon nanotubes (CNTs) were dispersed in water with the assistance of bis benzyl-modified polyethylene glycol (B-PEG181), which served as additional cross-linkers to prepare hydrogel by introducing acrylamide (AM) and N,N’-methylene bisacrylamide (MBAA). The synergy of physical entanglements and π-π interactions led to extreme stretchability (0∼132 mm/mm) and self-healable capability. In addition, the introduction of CNTs and lithium chloride endowed the hydrogel with ultra-high sensitivity (GF = 122.7, at 6.0∼8.0 mm/mm strain), superior to present sensors. Consequently, the obtained sensor could detect various human motions. It is envisioned that the remarkable performances empower the obtained gel promising applications in strain sensors, human-machine interfaces, and soft robots.

Strong Tough Poly Acrylic‐co‐acrylamide Hydrogels via a Synergistic Effect of Fiber and Metal‐Ligand Bonds as Flexible Strain Sensors

AbstractDeveloping conductive hydrogel‐incorporated strain sensors with high gauge factor (GF) and toughness for wearable applications is challenging. Herein, a facile strategy to fabricate strong, tough polyacrylamide‐co‐acrylic acid [P(AAm‐co‐AAc)] hydrogels via the synergy of fiber and metal‐ligand bonds is proposed. Through the secondary equilibrium approach, the pristine P(AAm‐co‐AAc) gel network is reconstructed with fiber and carboxyl–Zr4+ (Zirconium ion) coordination bonds intertwined over the entire gel matrix, generating a synergistic reinforcement in mechanical properties. The resultant hydrogels display a maximum tensile strength of 8.50 MPa and a fracture energy of 2.75 kJ m−2, which is 1–2 orders of magnitude greater than the original P(AAm‐co‐AAc) gels. It is also experimentally approved that by selecting different natural fibers, multivalent metal ions, and synthetic macromolecules containing carboxyl groups, the proposed approach is effective and can be generalized to fabricate strong, tough gels. Additionally, the electrical properties of obtained gel are evaluated by fabricating it into a stretchable strain sensor for object identification or human motion detection. The results reveal a high GF of 5.07 under a strain smaller 55%. These hydrogels are expected to enable numerous applications in soft robotics or wearable healthcare.

Semi‐Crystalline Rubber as a Light‐Responsive, Programmable, Resilient Robotic Material

AbstractPolymers with large and reversible light‐induced deformation offer a plethora of opportunities for the wireless control of small‐scale soft robots. However, their widespread adoption in real‐world applications is hindered, mainly due to their intrinsic softening upon illumination. Such limitation has detrimental effects on the achievable stress, durability, and precise positional control of the soft actuators after multiple cycles of use. Here, a synthetic rubber from a polybutadiene‐polyethylene copolymer is reported as a durable material for light‐controlled soft robots. The rubber can be programmed to exhibit various deformation modes controlled by visible‐to‐infrared light through a photothermal effect. Semi‐crystallinity of polyethylene within the rubbery network provides this material with a remarkable modulus at high temperatures (2.5 MPa at 100–140 °C), deformation repeatability (&gt;90%) and shape‐recovery (&gt;98%) after 100 actuation cycles subject to loads ranging from 10 to 10 000 times of its body weight (1.4 kPa–1.4 MPa). Soft robotic applications are demonstrated, such as thermally‐driven jumping and photo‐driven cargo transport carrying up to 1200 times its own weight. The results expand the portfolio of materials in designing remotely‐controlled, robust, and resilient soft robots working at small scales.

Design and Development of a Multi-Functional Bioinspired Soft Robotic Actuator via Additive Manufacturing.

The industrial revolution 4.0 has led to a burst in the development of robotic automation and platforms to increase productivity in the industrial and health domains. Hence, there is a necessity for the design and production of smart and multi-functional tools, which combine several cutting-edge technologies, including additive manufacturing and smart control systems. In the current article, a novel multi-functional biomimetic soft actuator with a pneumatic motion system was designed and fabricated by combining different additive manufacturing techniques. The developed actuator was bioinspired by the natural kinematics, namely the motion mechanism of worms, and was designed to imitate the movement of a human finger. Furthermore, due to its modular design and the ability to adapt the actuator’s external covers depending on the requested task, this actuator is suitable for a wide range of applications, from soft (i.e., fruit grasping) or industrial grippers to medical exoskeletons for patients with mobility difficulties and neurological disorders. In detail, the motion system operates with two pneumatic chambers bonded to each other and fabricated from silicone rubber compounds molded with additively manufactured dies made of polymers. Moreover, the pneumatic system offers multiple-degrees-of-freedom motion and it is capable of bending in the range of −180° to 180°. The overall pneumatic system is protected by external covers made of 3D printed components whose material could be changed from rigid polymer for industrial applications to thermoplastic elastomer for complete soft robotic applications. In addition, these 3D printed parts control the angular range of the actuator in order to avoid the reaching of extreme configurations. Finally, the bio-robotic actuator is electronically controlled by PID controllers and its real-time position is monitored by a one-axis soft flex sensor which is embedded in the actuator’s configuration.

Optical fiber specklegram sensor for multi-point curvature measurements.

We present a multi-point curvature sensor based on optical fiber specklegram measurements. Apart from the current approaches, the proposed system uses an ordinary multimode fiber excited with visible light as a reflection-type probe. Besides, this method discretizes the waveguide into segments connected by joints and assumes sequential bend events, simplifying the specklegram referencing for correlation analyses and avoiding laborious deep learning processing. Sensor characterization yielded a linear response with ∼1.3∘ resolution for single curvatures, whereas shape prediction experiments in the plane resulted in maximum errors of ∼3.5∘ and ∼5.4mm for angular and linear positioning, respectively. Furthermore, exploratory tests indicated errors <2.3∘ regarding probe curvatures in the space. This research introduces a feasible, straightforward alternative to the available shape sensors, enabling applications in medical probes and soft robotics.

A Highly Multi-Stable Meta-Structure via Anisotropy for Large and Reversible Shape Transformation.

Shape transformation offers the possibility of realizing devices whose 3D shape can be altered to adapt to different environments. Many applications would profit from reversible and actively controllable shape transformation together with a self‐locking capability. Solutions that combine such properties are rare. Here, a novel class of meta‐structures that can tackle this challenge is presented thanks to multi‐stability. Results demonstrate that the multi‐stability of the meta‐structure is strictly tied to the use of highly anisotropic materials. The design rules that enable large‐shape transformation, programmability, and self‐locking are derived, and it is proven that the shapes can be actively controlled and harnessed to realize inchworm‐inspired locomotion by strategically actuating the meta‐structure. This study provides routes toward novel shape adaptive lightweight structures where a metamaterial‐inspired assembly of anisotropic components leads to an unforeseen combination of properties, with potential applications in reconfigurable space structures, building facades, antennas, lenses, and soft robots.

Performance of flat-plate, flexible polymeric pulsating heat pipes at different bending angles

The heat transfer performance of a flat-plate, flexible polypropylene pulsating heat pipe (PHP) is evaluated experimentally when the device is bent at different angles, and for different layouts of the evaporator and the condenser. Flexible pulsating heat pipes are a passive thermal management technology with important prospective applications in foldable portable electronics, deployable systems (such as cube satellites), soft robotics, and many others. While a number flexible PHPs have been developed in the recent past, to date, there is insufficient information about the influence of the bending angle (conformation) and of the reciprocal positions of the heat sink and source (configuration) on their thermal performance. Several polypropylene PHP prototypes (250 mm ×100 mm ×1.5 mm) with a channel width of 5 mm and different loop geometries were fabricated using selective transmission laser welding technology. Ethanol and FC-72 were used as heat transfer fluids at a filling ratio of 50%. The PHP performance was evaluated through the equivalent thermal resistance of the device, calculated from the surface temperatures of both the evaporator and the condenser during an ascending/descending stepped heat input ramp applied to the evaporator. The experimental results show that conformation arrangements have no significant effect on the thermal performance, however it may affect the PHP start-up.

Paper-Based Piezoresistive Force Encoder for Soft Robotic Applications

This work demonstrates the design, implementation, and experimental results of a low-cost disposable flexible sensor system capable of both impact localization and measurement. The proposed flexible sensor structure utilizes a special series of Bristol paper as the main fabrication material, which is coated with electric paint graphite paste and silver paste. The implemented sensor system uses a planar absolute encoder-like sensing topology to locate the impact and has a low-cost and quick manufacturing process. The size of the structure is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${210}\times {18}.{56}$ </tex-math></inline-formula> mm with a thickness of approximately 340 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> . It has an electronic read-out consisting of three identical Wheatstone bridge circuits and instrumentation amplifiers for each bit. It can detect the external forces in the range of 0.6N to 12N with a spatial resolution of 2.4 cm and 0.55 cm in horizontal and vertical axes, respectively. The proposed sensor structure is tested in a series of experiments using a robotic setup consisted of a pantograph mechanism and a direct drive linear motor. The experiments illustrate the results with measurement sensitivity as small as 1N and proper fatigue resilience against repetitive loads.