A Layer Jamming Actuator for Tunable Stiffness and Shape-Changing Devices

Variable stiffness performance analysis of layer jamming actuator based on bionic adhesive flaps

Soft actuators made of soft materials cannot generate precisely efficient output forces compared to rigid actuators. It is a promising strategy to equip soft actuators with variable stiffness modules of layer jamming mechanism, which could increase their stiffness as needed. Inspired by the gecko's the array of setae, bionic adhesive flaps with inclined micropillars are applied in layer jamming mechanism. In this paper, after the manufacturing process of the layer jamming actuator based on the bionic adhesive flaps is described, the equivalent stiffness models of the whole actuator are established in the unjammed and jammed states. And the shear adhesive force of a single micropillar is calculated based on the Kendall viscoelastic band model. The finite element simulation results of two bionic adhesive flaps show that the interlaminar shear stress and stiffness increase with the increase of pressure. The measurement of shear adhesive force show that the critical shear adhesive force of the bionic adhesive material is 3.2 times that of polyethylene terephthalate (PET) material, and exhibit the ability of anisotropic adhesion behavior. The variable stiffness performance of the layer jamming actuator based on bionic adhesive flaps is evaluated by three test methods, and the max stiffness reaches 8.027 N mm-1, which is 1.5 times higher than the stiffness of the layer jamming actuator based on the PET flaps. All results of simulation and experiment effectively verify the validity and superiority of applying the bionic adhesive flaps to the layer jamming mechanism to enhance the stiffness.

Scale-inspired programmable robotic structures with concurrent shape morphing and stiffness variation

Biological organisms often have remarkable multifunctionality through intricate structures, such as concurrent shape morphing and stiffness variation in the octopus. Soft robots, which are inspired by natural creatures, usually require the integration of separate modules to achieve these various functions. As a result, the whole structure is cumbersome, and the control system is complex, often involving multiple control loops to finish a required task. Here, inspired by the scales that cover creatures like pangolins and fish, we developed a robotic structure that can vary its stiffness and change shape simultaneously in a highly integrated, compact body. The scale-inspired layered structure (SAILS) was enabled by the inversely designed programmable surface patterns of the scales. After fabrication, SAILS was inherently soft and flexible. When sealed in an elastic envelope and subjected to negative confining pressure, it transitioned to its designated shape and concurrently became stiff. SAILS could be actuated at frequencies as high as 5 hertz and achieved an apparent bending modulus change of up to 53 times between its soft and stiff states. We further demonstrated both the versatility of SAILS by developing a soft robot that is amphibious and adaptive and tunable landing systems for drones with the capacity to accommodate different loads.

Learning Stiffness Tensors in Self-Activated Solids via a Local Rule

Mechanical metamaterials are often designed with particular properties for specific load-bearing functions. Alternatively, this study aims to create a class of active lattice metamaterials, dubbed self-activated solids, that can learn desired stiffness tensors from the elastic deformations they experienced, a crucial feature to improve the performance, efficiency, and functionality of materials. Artificial adaptive matters that combine sensory, control, and actuation elements can offer appealing solutions. However, challenges still remain: The designs will rely on accurate off-line and global computations, as well as intricate coordination among individual elements. Here, a simple online and local learning strategy is initiated based on contrastive Hebbian learning to gradually guide self-activated solids to possess sought-after stiffness tensors autonomously and reversibly. During learning, the bond stiffness of the active lattice varies depending only on its local strain. The numerical tests show that the self-activated solid can not only achieve the desired bulk, shear, and coupling moduli but also manifest uni-mode and bi-mode extremal materials by itself after experiencing the corresponding elastic deformations. Further, the self-activated solid can also achieve the desired time-varying moduli when exposed to temporally different loads. The design is applicable to any lattice geometries and is resistant to damage and instabilities. The material design approach and the physical learning strategy suggested can benefit the design of autonomous materials, physical learning machines, and adaptive robots.

Design and Development of a Continuum Robot with Switching-Stiffness

Continuum robots have the advantages of agility and adaptability. However, existing continuum robots have limitations of low stiffness and complex motion modes, and the existing variable stiffness methods cannot achieve a wide range of stiffness changes and fast switching stiffness simultaneously. A continuum robot structure, switching stiffness method, and motion principle are proposed in this article. The continuum robot is made up of three segments connected in series. Each segment comprises multiple spherical joints connected in series, and the joints can be locked by their respective airbag. A valve controls each airbag, quickly switching the segment between rigidity and flexibility. The motion of the segments is driven by three cables that run through the robot. The segment steers only when it is unlocked. When a segment becomes locked, it acts as a rigid body. As a result, by locking and unlocking each segment in sequence, the cables can alternately drive all the segments. The stiffness variation and movement of the continuum robot were tested. The segment's stiffness varies from 36.89 to 1300.95 N/m and the stiffness switching time is 0.25-0.48 s. The time-sharing control mode of segment stiffness and motion is validated by establishing a specific test platform and a mathematical model. The continuum robot's flexibility is demonstrated by controlling the fast bending of different segments sequentially.

Shape memory meta-laminar jamming actuators fabricated by 4D printing

Laminar jamming (LJ) technology is a hot topic because it allows for the transition from conventionally quick, precise, and high-force rigid robots to flexible, agile, and secure soft robots. This article introduces a novel conceptual design of meta-laminar jamming (MLJ) actuators with a polyurethane shape memory polymer (SMP)-based meta-structure fabricated by 4D printing (4DP). The sustainable MLJ actuators behave as soft/hard robots via hot and cold programming accompanied by negative air pressure. The advantage of MLJ actuators over conventional LJ actuators is that a continuous negative air pressure is not required to stimulate the actuator. SMP meta-structures with circle, rectangle, diamond, and auxetic shapes are 4D printed. Mechanical properties of the structures are evaluated through three-point bending and compression tests. Shape memory effects (SMEs) and shape recovery of meta-structures and MLJ actuators are investigated via hot air programming. MLJ actuators with auxetic meta-structure cores show a better performance in terms of contraction and bending with 100% shape recovery after stimulation. The sustainable MLJ actuators have the capabilities of shape recovery and shape locking with zero input power while holding 200 g weight. The actuator can easily lift and hold objects of varying weights and shapes without requiring any power input. This actuator has demonstrated its versatility in potential applications, such as functioning as an end-effector and a gripper device.

Soft robotics in wearable and implantable medical applications: Translational challenges and future outlooks

This work explores the recent research conducted towards the development of novel classes of devices in wearable and implantable medical applications allowed by the introduction of the soft robotics approach. In the medical field, the need for materials with mechanical properties similar to biological tissues is one of the first considerations that arises to improve comfort and safety in the physical interaction with the human body. Thus, soft robotic devices are expected to be able of accomplishing tasks no traditional rigid systems can do. In this paper, we describe future perspectives and possible routes to address scientific and clinical issues still hampering the accomplishment of ideal solutions in clinical practice.

Double-spiral: a bioinspired pre-programmable compliant joint with multiple degrees of freedom

Geometry and material are two key factors that determine the functionality of mechanical elements under a specific boundary condition. Optimum combinations of these factors fulfil desired mechanical behaviour. By exploring biological systems, we find widespread spiral-shaped mechanical elements with various combinations of geometries and material properties functioning under different boundary conditions and load cases. Although these spirals work towards a wide range of goals, some of them are used as nature's solution to compactify highly extensible prolonged structures. Characterizing the principles underlying the functionality of these structures, here we profited from the coiling-uncoiling behaviour and easy adjustability of logarithmic spirals to design a pre-programmable compliant joint. Using the finite-element method, we developed a simple model of the joint and investigated the influence of design variables on its geometry and mechanical behaviour. Our results show that the design variables give us a great possibility to tune the response of the joint and reach a high level of passive control on its behaviour. Using 3D printing and mechanical testing, we replicated the numerical simulations and illustrated the application of the joint in practice. The simplicity, pre-programmability and predictable response of our double-spiral design suggest that it provides an efficient solution for a wide range of engineering applications, such as articulated robotic systems and modular metamaterials.

Three-Dimensional Printable Ball Joints with Variable Stiffness for Robotic Applications Based on Soft Pneumatic Elastomer Actuators

This paper contributes to a new design of the three-dimensional printable robotic ball joints capable of creating the controllable stiffness linkage between two robot links through pneumatic actuation. The variable stiffness ball joint consists of a soft pneumatic elastomer actuator, a support platform, an inner ball and a socket. The ball joint structure, including the inner ball and the socket, is three-dimensionally printed using polyamide−12 (PA12) by selective laser sintering (SLS) technology as an integral mechanism without the requirement of assembly. The SLS technology can make the ball joint have the advantages of low weight, simple structure, easy to miniaturize and good MRI compatibility. The support platform is designed as a friction-based braking component to increase the stiffness of the ball joint while withstanding the external loads. The soft pneumatic elastomer actuator is responsible for providing the pushing force for the support platform, thereby modulating the frictional force between the inner ball, the socket and the support platform. The most remarkable feature of the proposed variable stiffness design is that the ball joint has ‘zero’ stiffness when no pressurized air is supplied. In the natural state, the inner ball can be freely rotated and twist inside the socket. The proposed ball joint can be quickly stiffened to lock the current position and orientation of the inner ball relative to the socket when the pressurized air is supplied to the soft pneumatic elastomer actuator. The relationship between the stiffness of the ball joint and the input air pressure is investigated in both rotating and twisting directions. The finite element analysis is conducted to optimize the design of the support platform. The stiffness tests are conducted, demonstrating that a significant stiffness enhancement, up to approximately 508.11 N·mm reaction torque in the rotational direction and 571.93 N·mm reaction torque in the twisting direction at the pressure of 400 kPa, can be obtained. Multiple ball joints can be easily assembled to form a variable stiffness structure, in which each ball joint has a relative position and an independent stiffness. Additionally, the degrees of freedom (DOF) of the ball joint can be readily restricted to build the single-DOF or two-DOFs variable stiffness joints for different robotic applications.

Redundancy Reduction for Sensor Deployment in Prosthetic Socket: A Case Study

The irregular pressure exerted by a prosthetic socket over the residual limb is one of the major factors that cause the discomfort of amputees using artificial limbs. By deploying the wearable sensors inside the socket, the interfacial pressure distribution can be studied to find the active regions and rectify the socket design. In this case study, a clustering-based analysis method is presented to evaluate the density and layout of these sensors, which aims to reduce the local redundancy of the sensor deployment. In particular, a Self-Organizing Map (SOM) and K-means algorithm are employed to find the clustering results of the sensor data, taking the pressure measurement of a predefined sensor placement as the input. Then, one suitable clustering result is selected to detect the layout redundancy from the input area. After that, the Pearson correlation coefficient (PCC) is used as a similarity metric to guide the removal of redundant sensors and generate a new sparser layout. The Jenson-Shannon Divergence (JSD) and the mean pressure are applied as posterior validation metrics that compare the pressure features before and after sensor removal. A case study of a clinical trial with two sensor strips is used to prove the utility of the clustering-based analysis method. The sensors on the posterior and medial regions are suggested to be reduced, and the main pressure features are kept. The proposed method can help sensor designers optimize sensor configurations for intra-socket measurements and thus assist the prosthetists in improving the socket fitting.

Friction Prediction and Validation of a Variable Stiffness Lower Limb Exosuit Based on Finite Element Analysis

The variable stiffness exosuit has great potential for human augmentation and medical applications. However, the model of the variable stiffness mechanism in exosuits is far from satisfactory for the accurate prediction and control of friction force. This paper presents a friction prediction model of a variable stiffness lower limb exosuit, verifies its prediction performance, and identifies its applicability. The friction force model was established by the Coulomb friction hypothesis. The equivalent coefficient, which is the core parameter of the model, was determined based on friction and squeezing force data obtained by tests and an ANSYS simulation. Experiments show that the prediction error of the proposed model can reach 15% with a proper structural dimension change constraint. The friction force control test showed that the achieved model can shorten the settling time of the step response by 26% and eliminate the steady-state error. Verifications indicate that the proposed method can provide guidance to the modeling of other friction/stiffness structures, especially friction-based wearable robot structure models and predictions.

Triple Stiffness: A Bioinspired Strategy to Combine Load-Bearing, Durability, and Impact-Resistance

Structures with variable stiffness have received increasing attention in the fields of robotics, aerospace, structural, and biomedical engineering. This is because they not only adapt to applied loads, but can also combine mutually exclusive properties. Here inspired by insect wings, the concept of "triple stiffness" is introduced and applied to engineering systems that exhibit three distinct deformability regimes. By implementing "flexible joints," "mechanical stoppers," and "buckling zones," structures are engineered to be not only load-bearing and durable, but also impact-resistant. To practice the performance of the design concept in real-life applications, the developed structures are integrated into 3D printed airplane wing models that withstood collisions without failure. The concept developed here opens new avenues for the development of structural elements that are load-bearing, durable, and impact-resistant at the same time.