Compliant Materials Research Articles

Self-locking and stiffening deployable tubular structures

Deployable tubular structures, designed for functional expansion, serve a wide range of applications, from flexible pipes to stiff structural elements. These structures, which transform from compact states, are crucial for creating adaptive solutions across engineering and scientific fields. A significant barrier to advancing their performance is balancing expandability with stiffness. Using compliant materials, these structures achieve more flexible transformations than those possible with rigid mechanisms. However, they typically exhibit reduced stiffness when subjected to external pressures (e.g., tube wall loading). Here, we utilize origami-inspired techniques and internal stiffeners to meet conflicting performance requirements. A self-locking mechanism is proposed, which combines the folding behavior observed in curved-crease origami and elastic shell buckling. This mechanism employs simple shell components, including internal diaphragms that undergo pseudofolding in a confined boundary condition to enable a snap-through transition. We reveal that the deployed tube is self-locked through geometrical interference, creating a braced tubular arrangement. This arrangement gives a direction-dependent structural performance, ranging from elastic response to crushing, thereby offering the potential for programmable structures. We demonstrate that our approach can advance existing deployment mechanisms (e.g., coiled and inflatable systems) and create diverse structural designs (e.g., metamaterials, adaptive structures, cantilevers, and lightweight panels).Weanticipate our design to be a starting point to drive technological advancement in real-world deployable tubular structures.

Physical phenomena during nanoindentation deformation of amorphous glassy polymers

We identify for visco-elasto-plastic (VEP) glassy polymers, physical phenomena during Berkovich nanoindentation, a locally imposed deformation. Live visuals via in situ nanoindentation indicate mainly sink-in during loading, with pile-up after unloading. Scanning Probe Microscopy (SPM) indicates significant volume conserving upflow below the tip, for these high ν, compliant materials, with compliance correlated high geometric fractional contact, hcΔhmΔ∼0.86−0.95. We adapt the ideal conical indentation framework to VEP Berkovich nanoindentation, to calculate the contact area and visually depict the upflow and the displacement paths, in the material. The combination of SPM and P−h data, indicates a mixed comparison with uniaxial modulus and yield stress, with conventionally defined hardness, H<3σy, and nanoindentation modulus, EN>E. By rationally removing viscoelastic (VE) effects from the loading P−h data, we find instant, zero-time hardness HL0>3σy. We apply the power law model to only the recovery onset, to estimate pure elastic recovery. We then deconvolute the VEP nanoindentation into the conventional EP (elasto-plastic) and elastic contributions, isolating the VE component. Constraint-induced sink-in, pile-up and VE recovery of the highly yielded tip-apex region, mirror the converse constrained deformation effects, governing the trends in the conventional EP EN and HL0 estimates for glassy polymers.

Soft Gripping Fingers Made of Multi-Stacked Dielectric Elastomer Actuators with Backbone Strategy.

Soft grippers, a rapidly growing subfield of soft robotics, utilize compliant and flexible materials capable of conforming to various shapes. This feature enables them to exert gentle yet, if required, strong gripping forces. In this study, we elaborate on the material selection and fabrication process of gripping fingers based on the dielectric elastomer actuation technique. We study the effects of mixing the silicone elastomer with a silicone thinner on the performance of the actuators. Inspired by nature, where the motion of end-effectors such as soft limbs or fingers is, in many cases, directed by a stiff skeleton, we utilize backbones for translating the planar actuation into a bending motion. Thus, the finger does not need any rigid frame or pre-stretch, as in many other DEA approaches. The idea and function of the backbone strategy are demonstrated by finite element method simulations with COMSOL Multiphysics® 6.5. The paper describes the full methodology from material choice and characterization, design, and simulation to characterization to enable future developments based on our approach. Finally, we present the performance of these actuators in a gripper demonstrator setup. The developed actuators bend up to 68.3° against gravity, and the gripper fingers hold up to 10.3 g against gravity under an actuation voltage of 8 kV.

Plantar pressure reduction in the heel region through self-adjusting insoles with a heel cup in standard and individualized rocker shoes

BackgroundEffectiveness of therapeutic footwear in reducing peak pressure in persons with diabetes and loss of protective sensation to prevent diabetic foot ulcers varies due to manual production and possible changing foot structure. A previous two-way approach to address this issue, featuring individualized 3D-printed rocker midsoles and self-adjusting insoles, proved effective in the forefoot but less in the heel. To address this, new insoles incorporating a heel cup are developed. MethodsIn-shoe pressure was measured, while persons with diabetes and loss of protective sensation with high peak pressure (≥ 200 kPa) in the heel walked on a treadmill with control and individualized rocker shoe paired with control and new insole. FindingsGeneralized estimating equations revealed significant decrease in peak pressure in the proximal heel with the new insole alone and combined with rocker shoe compared to rocker shoe alone. For the distal heel, significant decrease in peak pressure is shown with the combination of new insole and rocker shoe compared to control shoe. For the forefoot and toes (excluding hallux) significant decrease in peak pressure is shown using the rocker shoe alone or combined with the heel cup compared to control shoe. InterpretationThe new insole paired with rocker shoe is effective in reducing peak pressure in the distal heel. To have similar (or more) success in proximal heel, one could replace the rocker midsole with more compliant materials. The rocker shoe used separately or combined with a heel cup effectively reduces the peak pressure in the forefoot and other toes.

Biodegradable and Flexible Wood-Gelatin Composites for Soft Actuating Systems.

Compliant materials are indispensable for many emerging soft robotics applications. Hence, concerns regarding sustainability and end-of-life options for these materials are growing, given that they are predominantly petroleum-based and non-recyclable. Despite efforts to explore alternative bio-derived soft materials like gelatin, they frequently fall short in delivering the mechanical performance required for soft actuating systems. To address this issue, we reinforced a compliant and transparent gelatin-glycerol matrix with structure-retained delignified wood, resulting in a flexible and entirely biobased composite (DW-flex). This DW-flex composite exhibits highly anisotropic mechanical behavior, possessing higher strength and stiffness in the fiber direction and high deformability perpendicular to it. Implementing a distinct anisotropy in otherwise isotropic soft materials unlocks new possibilities for more complex movement patterns. To demonstrate the capability and potential of DW-flex, we built and modeled a fin ray-inspired gripper finger, which deforms based on a twist-bending-coupled motion that is tailorable by adjusting the fiber direction. Moreover, we designed a demonstrator for a proof-of-concept suitable for gripping a soft object with a complex shape, i.e., a strawberry. We show that this composite is entirely biodegradable in soil, enabling more sustainable approaches for soft actuators in robotics applications.

Multifunctional Nanocomposite Yield-Stress Fluids for Printable and Stretchable Electronics.

Compliant materials are crucial for stretchable electronics. Stretchable solids and gels have limitations in deformability and durability, whereas active liquids struggle to create complex devices. This study presents multifunctional yield-stress fluids as printable ink materials to construct stretchable electronic devices. Ionic nanocomposites comprise silica nanoparticles and ion liquids, while electrical nanocomposites use the natural oxidation of liquid metals to produce gallium oxide nanoflake additives. These nanocomposite inks can be printed on an elastomer substrate and stay in a solid state for easy encapsulation. However, their transition into a liquid state during stretching allows ultrahigh deformability up to the fracture strain of the elastomer. The ionic inks produce strain sensors with high stretchability and temperature sensors with high sensitivity of 7% °C-1. Smart gloves are further created by integrating these sensors with printed electrical interconnects, demonstrating bimodal detection of temperatures and hand gestures. The nanocomposite yield-stress fluids combine the desirable qualities of solids and liquids for stretchable devices and systems.

CHARACTERIZATION OF 3D PRINTED CONDUCTIVE FLEXIBLE MATERIALS FOR SOFT ROBOTIC APPLICATIONS

Soft robots composed of compliant and flexible materials can safely interact with humans and adapt to unstructured environments. However, integrating sensors, actuators, and control circuits into soft structures remains challenging. Additive manufacturing shows promise for fabricating soft robots with embedded electronics using conductive flexible composites. Nevertheless, there is still a limited understanding of the electromechanical behavior of 3D-printed conductive structures when subjected to the types of strains relevant to soft robotics applications. Optimized design requires characterizing the interplay between a soft component's changing shape and electrical properties during deformation. This study investigates the application of 3D printing technology to fabricate various geometries using a conductive, flexible material for soft robotic applications. The primary objective is to understand and characterize the behavior of differently shaped 3D-printed conductive materials under various mechanical stresses. Two distinct test setups are designed for conducting bending and tensile tests on the produced materials. Diverse geometries are printed using the conductive flexible material with desirable mechanical and electrical properties to employ tensile and bending tests. The experiments reveal a direct correlation between shape change and electrical resistance of the 3D printed materials, providing valuable insights into their adaptability for soft robotics. According to numerical results, honeycomb profiles are found to be the most linear and stable profile type. This research not only contributes to the field of flexible conductive materials but also lays the foundation for integrating these materials into future engineering designs, potentially enabling the development of highly responsive and adaptable devices for various industries.

Synergizing Structural Stiffness Regulation with Compliance Contact Stiffness: Bioinspired Soft Stimuli‐Responsive Materials Design for Soft Machines

Stiffness regulation strategies endow soft machines with stronger functionality to cope with diverse application requirements, for example manipulating heavy items by improving structural stiffness. However, most programmable stiffness strategies usually struggle to preserve the inherent compliant interaction capabilities following an enhancement in structural stiffness. In this study, inspired by the musculocutaneous system, we propose a soft stimuli‐responsive material (SRM) by combining shape memory alloy into compliant materials. By characterizing the mechanical performance, the flexural modulus increases from 6.6 to 142.4 MPa under the action of active stimuli, crossing two orders of magnitude, while Young's modulus stays at 2.2 MPa during programming structural stiffness. This comparative result indicates that our SRMs can keep a lower contact stiffness for compliant interaction although structural stiffness increases. Then, we develop three diverse soft machines to show the application potential of this smart material, such as robotic grippers, wearable devices, and deployable mechanisms. By applying our materials, these machines possess stronger load‐bearing capabilities. Meanwhile, these demonstrations also illustrate the efficacy of this paradigm in regulating the structural stiffness of soft machines while maintaining their compliant interaction capabilities.

Soft Robot Design, Manufacturing, and Operation Challenges: A Review

Advancements in smart manufacturing have embraced the adoption of soft robots for improved productivity, flexibility, and automation as well as safety in smart factories. Hence, soft robotics is seeing a significant surge in popularity by garnering considerable attention from researchers and practitioners. Bionic soft robots, which are composed of compliant materials like silicones, offer compelling solutions to manipulating delicate objects, operating in unstructured environments, and facilitating safe human–robot interactions. However, despite their numerous advantages, there are some fundamental challenges to overcome, which particularly concern motion precision and stiffness compliance in performing physical tasks that involve external forces. In this regard, enhancing the operation performance of soft robots necessitates intricate, complex structural designs, compliant multifunctional materials, and proper manufacturing methods. The objective of this literature review is to chronicle a comprehensive overview of soft robot design, manufacturing, and operation challenges in conjunction with recent advancements and future research directions for addressing these technical challenges.

Stretchable and Sweat-Wicking Patch for Skin-Attached Colorimetric Analysis of Sweat Biomarkers.

Stretchable sweat sensors are promising technology that can acquire biomolecular insights for health and fitness monitoring by intimate integration with the body. However, current sensors often require microfabricated microfluidic channels to control sweat flow during lab-on-body analysis, which makes effective and affordable sweat sampling a significant practical challenge. Here, we present stretchable and sweat-wicking patches that utilize bioinspired smart wettable membranes for the on-demand manipulation of sweat flow. In a scalable process, the membrane is created by stacking hydrophobic elastomer nanofibers onto soft microfoams with predefined two-dimensional superhydrophobic and superhydrophilic patterns. The engineered heterogeneous wettability distribution allows these porous membranes to achieve enhanced extraction and selective collection of sweat in embedded assays. Despite the simplified architecture, the color reactions between sweat and chemical indicators are inhibited from directly contacting the skin to achieve a largely improved operation safety. The sensing patches can simultaneously quantify pH, urea, and calcium in sweat through digital colorimetric analysis with smartphone images. The construction with all compliant materials renders these patches soft and stretchy to achieve conformal attachment to the skin. Successfully analyzing sweat compositions after physical exercises illustrates the practical suitability of these skin-attachable sensors for health tracking and point-of-care diagnosis.

Digital Light Processing of Soft Robotic Gripper with High Toughness and Self‐Healing Capability Achieved by Deep Eutectic Solvents

AbstractInspired by nature's flexible and adaptable organisms, soft robotics are motorless robots made from highly compliant materials to work in confined environments and manipulate delicate objects. However, soft robots often suffer from early failure because of unexpected damage. At the same time, it is challenging to manufacture the geometrically complex structures of soft robots. This study introduces resins based on deep eutectic solvents (DES) to fabricate a pneumatically driven soft gripper using digital light processing (DLP). The resins consist of choline chloride (ChCl) as a hydrogen bond acceptor, glycerol (Gly), and acrylamide (AAm) as hydrogen bond donors. By utilizing the intense hydrogen bonding within DES, the resin can be rapidly cured by photopolymerization to form tough ionogels without chemical crosslinkers. The DES ionogels exhibit remarkable toughness and self‐healing performance compared to common hydrogels. Furthermore, the ionogels show not only efficient energy‐dissipating behavior but also achieve rapid self‐recovery. Finally, the DLP‐printed soft gripper from the DES‐based resin performs successful actuation and healing of macroscopic damages. This work presents a simple strategy to 3D print a soft robotic gripper with high toughness and self‐healing capability.

Schiff base flexible organic crystals toward multifunctional applications

AbstractThe emergence of flexible organic crystals changed the perception of molecular crystals that were regarded as brittle entities over a long period of time, and sparked a great interest in exploring mechanically compliant organic crystalline materials toward next‐generation smart materials during the past decade. Schiff base compounds are considered to be one of the most promising candidates for flexible organic crystals owing to their easy synthesis, high yield, stimuli responsiveness and good mechanical properties. This paper gives an overview of the recent development of Schiff base flexible organic crystals (including elastic organic crystals, plastic organic crystals, and flexible organic crystals integrating elasticity and plasticity) from serendipitous discovery to design strategies and versatile applications such as stimuli responses, optical waveguides, optoelectronic devices, biomimetic soft robots, and organic photonic integrated circuits. Notably, atomic force microscopy‐micromanipulation technique has been utilized to bring the multifunctional applications of flexible organic crystals from the macroscopic level to the microscopic world. Since understanding mechanical flexibility at the molecular level through crystal engineering can assist us to trace down the structural origin of mechanical properties, we focus on the packing structures of various Schiff base flexible organic crystals driven by non‐covalent intermolecular interactions and their close correlation with mechanical behaviors. We hope that the information given here will help in the design of novel flexible organic crystals combined with other unique properties, and promote further research into the area of mechanically compliant organic crystalline materials toward multifunctional applications.

Design Optimization of a Hybrid-Driven Soft Surgical Robot with Biomimetic Constraints.

The current study investigated the geometry optimization of a hybrid-driven (based on the combination of air pressure and tendon tension) soft robot for use in robot-assisted intra-bronchial intervention. Soft robots, made from compliant materials, have gained popularity for use in surgical interventions due to their dexterity and safety. The current study aimed to design a catheter-like soft robot with an improved performance by minimizing radial expansion during inflation and increasing the force exerted on targeted tissues through geometry optimization. To do so, a finite element analysis (FEA) was employed to optimize the soft robot's geometry, considering a multi-objective goal function that incorporated factors such as chamber pressures, tendon tensions, and the cross-sectional area. To accomplish this, a cylindrical soft robot with three air chambers, three tendons, and a central working channel was considered. Then, the dimensions of the soft robot, including the length of the air chambers, the diameter of the air chambers, and the offsets of the air chambers and tendon routes, were optimized to minimize the goal function in an in-plane bending scenario. To accurately simulate the behavior of the soft robot, Ecoflex 00-50 samples were tested based on ISO 7743, and a hyperplastic model was fitted on the compression test data. The FEA simulations were performed using the response surface optimization (RSO) module in ANSYS software, which iteratively explored the design space based on defined objectives and constraints. Using RSO, 45 points of experiments were generated based on the geometrical and loading constraints. During the simulations, tendon force was applied to the tip of the soft robot, while simultaneously, air pressure was applied inside the chamber. Following the optimization of the geometry, a prototype of the soft robot with the optimized values was fabricated and tested in a phantom model, mimicking simulated surgical conditions. The decreased actuation effort and radial expansion of the soft robot resulting from the optimization process have the potential to increase the performance of the manipulator. This advancement led to improved control over the soft robot while additionally minimizing unnecessary cross-sectional expansion. The study demonstrates the effectiveness of the optimization methodology for refining the soft robot's design and highlights its potential for enhancing surgical interventions.

Stretchable and Smart Wettable Sensing Patch with Guided Liquid Flow for Multiplexed in Situ Perspiration Analysis.

Stretchable sweat sensors have become a personalized wearable platform for continuous, noninvasive health monitoring through conformal integration with the human body. Typically, these devices are coupled with soft microfluidic systems to control sweat flow during advanced analysis processes. However, the implementation of these soft microfluidic devices is limited by their high fabrication costs and the need for skin adhesives to block natural perspiration. To overcome these limitations, a stretchable and smart wettable patch has been proposed for multiplexed in situ perspiration analysis. The patch includes a porous membrane in the form of a patterned microfoam and a nanofiber layer laminate, which extracts sweat selectively from the skin and directs its continuous flow across the device. The integrated electrochemical sensor array measures multiple biomarkers simultaneously such as pH, K+, and Na+. The soft sensing patch comprises compliant materials and structures that allow deformability of up to 50% strain, which enables a stable and seamless interface with the curvilinear human body. During continuous physical exercise, the device has demonstrated a special operating mode by actively accumulating sweat from the skin for multiplex electrochemical analysis of biomarker profiles. The smart wettable membrane provides an affordable solution to address the sampling challenges of in situ perspiration analysis.

Intrinsically stretchable light-emitting drawing displays

Stretchable displays that combine light-emitting capabilities with mechanical compliance are essential building blocks of next-generation wearable electronics. However, their widespread applications are currently limited by complex device architecture, limited pixel density, and immature fabrication processes. In this study, we present the device design and material developments of intrinsically stretchable light-emitting drawing displays that can show arbitrary hand-drawing features. The alternating-current electroluminescent display uses a simplified architecture comprising coplanar interdigitated liquid metal electrodes, an electroluminescent layer, and a dielectric encapsulation layer. Ink patterns on the device are coupled with the interdigitated electrodes under alternating voltage stimulations, generating localized electric fields for bright emissions. Various inks are prepared for painting, stamping, and stencil printing. Arbitrary luminous features on the devices can be either long-lasting or transient in characteristics. These skin-like devices are made entirely of compliant materials that can withstand bending, twisting, and stretching manipulations. Due to the excellent mechanical deformability, the drawing displays can be conformally laminated on the skin as body-integrated optoelectronic communication devices for graphic information.

Effects of individually optimized rocker midsoles and self-adjusting insoles on plantar pressure in persons with diabetes mellitus and loss of protective sensation

IntroductionRocker shoes and insoles reduce peak pressure (PP) in persons with diabetes (DM) and loss of protective sensation (LOPS). However, they are handmade, leading to inconsistent effectiveness. If foot structure changes over time, high PP-locations also change. To address this, individualized algorithm based 3D-printed rockers and self-adjusting pressure-reducing insoles are applied. MethodsPP across seven foot regions was analyzed in 21 persons with DM and LOPS. Regions with PP < 200 kPa were considered not at risk (RnoR); regions with PP ≥ 200 kPa at risk (RaR). The aim was to offload RaR, while remaining PP < 200 kPa in RnoR. ResultsIndividualized rockers and self-adjusting insoles combined successfully reduce PP < 200 kPa (on average 24 % - 48 %) in all feet with toes, central and lateral forefoot identified as RaR. Same intervention reduces PP in 68 % of the feet with medial forefoot identified as RaR. With the heel as RaR, no intervention reduces PP successfully in all feet. ConclusionsIndividualized 3D-printed rockers combined with self-adjusting insoles reduce PP (< 200 kPa) in toes, central and lateral forefoot, but not in heels. Alternative insoles with medial arch support, heel cup and compliant midsole materials might enhance success rate across entire foot.

Exploration of surface milling machining process of C/SiC composites based on quality improvement

C/SiC composites have the advantages of high strength, oxidation resistance and high temperature resistance, which are widely used in aerospace high temperature structures. However, the machining apparent quality has also become an important factor limiting its application. Therefore, this paper investigates the effects of cutting force, tool parameters and tool wear on the apparent quality of ultrasonic vibration milling of C/SiC composites. And the effect of tool selection and tool path on the milling surface roughness of C/SiC compliant materials is proved by experiments. In order to provide certain reference for the improvement of the surface milling quality of C/SiC composite materials.

Stiffness modelling and analysis of soft fluidic-driven robots using Lie theory

Soft robots have been investigated for various applications due to their inherently superior deformability and flexibility compared to rigid-link robots. However, these robots struggle to perform tasks that require on-demand stiffness, that is, exerting sufficient forces within allowable deflection. In addition, the soft and compliant materials also introduce large deformation and non-negligible nonlinearity, which makes the stiffness analysis and modelling of soft robots fundamentally challenging. This paper proposes a modelling framework to investigate the underlying stiffness and the equivalent compliance properties of soft robots under different configurations. Firstly, a modelling and analysis methodology is described based on Lie theory. Here, we derive two sets (the piecewise constant curvature and Cosserat rod model) of compliance models. Furthermore, the methodology can accommodate the nonlinear responses (e.g., bending angles) resulting from elongation of robots. Using this proposed methodology, the general Cartesian stiffness or compliance matrix can be derived and used for configuration-dependent stiffness analysis. The proposed framework is then instantiated and implemented on fluidic-driven soft continuum robots. The efficacy and modelling accuracy of the proposed methodology are validated using both simulations and experiments.

Octopus-Inspired Muscular Hydrostats Powered By Twisted and Coiled Artificial Muscles.

Traditional robots are characterized by rigid structures, which restrict their range of motion and their application in environments where complex movements and safe human-robot interactions are required. Soft robots inspired by nature and characterized by soft compliant materials have emerged as an exciting alternative in unstructured environments. However, the use of multicomponent actuators with low power/weight ratios has prevented the development of truly bioinspired soft robots. Octopodes' limbs contain layers of muscular hydrostats, which provide them with a nearly limitless range of motions. In this work, we propose octopus-inspired muscular hydrostats powered by an emerging class of artificial muscles called twisted and coiled artificial muscles (TCAMs). TCAMs are fabricated by twisting and coiling inexpensive fibers, can sustain stresses up to 60 MPa, and provide tensile strokes of nearly 50% with <0.2 V/cm of input voltage. These artificial muscles overcome the limitations of other actuators in terms of cost, power, and portability. We developed four different configurations of muscular hydrostats with TCAMs arranged in different orientations to reproduce the main motions of octopodes' arms: shortening, torsion, bending, and extension. We also assembled an untethered waterproof device with on-board control, sensing, actuation, and a power source for driving our hydrostats underwater. The proposed TCAM-powered muscular hydrostats will pave the way for the development of compliant bioinspired robots that can be used to explore the underwater world and perform complex tasks in harsh and dangerous environments.

Aircraft Wing Design for Extended Hybrid Laminar Flow Control

Laminar flow offers significant potential for increasing the energy-efficiency of future transport aircraft. The German Cluster of Excellence SE2A is developing a new approach for hybrid laminar flow control. The concept aims to maintain laminar flow up to 80% of the chord length by integrating suction panels at the rear part of the wing, which consist of a thin suction skin and a supporting core structure. This study examines effects of various suction panel configurations on wing mass and load transfer for an all-electric short-range aircraft. Suction panel material, as well as thickness and relative density of the suction panel core are modified in meaningful boundaries. Suction panels made from Ti6Al4V offer the most robust design resulting in a significant increase in wing mass. For the studied configurations, they represent up to 33.8% of the mass of the wingbox. In contrast, panels made from Nylon11CF or PU1000 do not significantly increase the wing mass. However, the use of these materials raises questions about their robustness under operational conditions. The results demonstrate that the choice of material strongly influences the load path within the wing structure. Ti6Al4V suction panels provide sufficient mechanical properties to significantly contribute to load transfer and buckling stiffness. Locally, the share of load transfer attributed to the suction panel exceeds 50%. In contrast, compliant materials such as Nylon11CF or PU1000 are inherently decoupled from load transfer. Unlike the thickness of the suction skin, the relative density of the core structure strongly affects the wrinkling stiffness. However, wrinkling failure did not appear critical for the examined suction panel configurations. In the present study, the mechanical properties of Ti6Al4V cannot fully be exploited. Therefore, compliant suction panels made from Nylon11CF are preferred in order to achieve a lightweight solution, provided that they meet operational requirements.