Recent Advances in Self-Powered Wearable Flexible Sensors for Human Gaits Analysis

The rapid progress of flexible electronics has met the growing need for detecting human movement information in exoskeleton auxiliary equipment. This study provides a review of recent advancements in the design and fabrication of flexible electronics used for human motion detection. Firstly, a comprehensive introduction is provided on various self-powered wearable flexible sensors employed in detecting human movement information. Subsequently, the algorithms utilized to provide feedback on human movement are presented, followed by a thorough discussion of their methods and effectiveness. Finally, the review concludes with perspectives on the current challenges and opportunities in implementing self-powered wearable flexible sensors in exoskeleton technology.

Evolution of Musculoskeletal Electronics

The musculoskeletal system, constituting the largest human physiological system, plays a critical role in providing structural support to the body, facilitating intricate movements, and safeguarding internal organs. By virtue of advancements in revolutionized materials and devices, particularly in the realms of motion capture, health monitoring, and postoperative rehabilitation, "musculoskeletal electronics" has actually emerged as an infancy area, but has not yet been explicitly proposed. In this review, the concept of musculoskeletal electronics is elucidated, and the evolution history, representative progress, and key strategies of the involved materials and state-of-the-art devices are summarized. Therefore, the fundamentals of musculoskeletal electronics and key functionality categories are introduced. Subsequently, recent advances in musculoskeletal electronics are presented from the perspectives of "in vitro" to "in vivo" signal detection, interactive modulation, and therapeutic interventions for healing and recovery. Additionally, nine strategy avenues for the development of advanced musculoskeletal electronic materials and devices are proposed. Finally, concise summaries and perspectives are proposed to highlight the directions that deserve focused attention in this booming field.

Actuation mechanism of a nanoscale drilling rig based on nested carbon nanotubes

With the increasing emphasis on health and the continuous improvement of medical standards, more and more micro/nano devices are being used in the medical field. However, the existing micro/nano devices cannot effectively solve various problems encountered in medical processes and achieve specific therapeutic effects. Based on this, this article designs a new type of nanoscale drilling rig. The nanoscale drilling rig is composed of double-layer nested carbon nanotubes with multiple electrodes, and is powered by an external power source, making it easy to perform long-term surgery in the human body. Through coding strategies, we can adjust the surface charge density and distribution of the nanoscale drilling rig, thereby controlling its periodical rotation and achieving precise medical treatment. In addition, in order to control the length of the nanoscale drill bit, meet the treatment needs of different parts of the human body, and reduce damage to the human body, we have designed a structure of ion electric double layers so that the drill bit can be fixed in different positions, reducing the risk of treatment to a certain extent. This drilling rig enriches the functions of micro/nano devices, which is beneficial for the development of the medical industry.

Highly Resilient Noncovalently Associated Hydrogel Adhesives for Wound Sealing Patch

The development of hydrogel adhesives with high mechanical resilience and toughness remains a challenging task. Hydrogels must exhibit high mechanical resilience to withstand the inevitable movement of the human body while simultaneously demonstrating strong wet tissue adhesion and appropriate toughness to hold and seal damaged tissues; However, tissue adhesion, toughness, and mechanical resilience are typically negatively correlated. Therefore, this paper proposes a highly resilient double-network (DN) hydrogel wound-sealing patch that exhibits a well-balanced combination of tissue adhesion, toughness, and mechanical resilience. The DN structure is formed by introducing covalently and non-covalently crosslinkable dopamine-modified crosslinkers and physically interactable linear poly(vinyl imidazole) (PVI). The resulting hydrogel adhesive exhibits high toughness and mechanical resilience due to the presence of a DN involving reversible physical intermolecular interactions such as hydrogen bonds, hydrophobic associations, cation-π interactions, π-π interactions, and chain entanglements. Moreover, the hydrogel adhesive achieves strong wet tissue adhesion through the polar hydroxyl groups of dopamine and the amine group of PVI. These mechanical attributes allow the proposed adhesive to effectively seal damaged tissues and promote wound healing by maintaining a moist environment.

Hybrid-Driven Origami Gripper with Variable Stiffness and Finger Length

Soft grippers due to their highly compliant material and self-adaptive structures attract more attention to safe and versatile grasping tasks compared to traditional rigid grippers. However, those flexible characteristics limit the strength and the manipulation capacity of soft grippers. In this paper, we introduce a hybrid-driven gripper design utilizing origami finger structures, to offer adjustable finger stiffness and variable grasping range. This gripper is actuated via pneumatic and cables, which allows the origami structure to be controlled precisely for contraction and extension, thus achieving different finger lengths and stiffness by adjusting the cable lengths and the input pressure. A kinematic model of the origami finger is further developed, enabling precise control of its bending angle for effective grasping of diverse objects and facilitating in-hand manipulation. Our proposed design method enriches the field of soft grippers, offering a simple yet effective approach to achieve safe, powerful, and highly adaptive grasping and in-hand manipulation capabilities.

Soft-Hard Janus Nanoparticles Triggered Hierarchical Conductors with Large Stretchability, High Sensitivity, and Superior Mechanical Properties

There is a long-standing conflict between the large stretchability and high sensitivity for strain sensors, a strategy of decoupling the mechanical/electrical module by constructing the hierarchical conductor has been developed in this study. The hierarchical conductor, consisting of a mechanically stretchable layer, a conductive network layer, and a strongly bonded interface, can be produced in a simple one-step process with the aid of soft-hard Janus nanoparticles (JNPs). The introduction of JNPs in the stretchable layer can evenly distribute stress and dissipate energy due to forming the rigid-flexible homogeneous networks. Specifically, JNPs can drive graphene nanosheets (GNS) to fold or curl, creating the unique JNPs-GNS building block that can further construct the conductive network. Due to its excellent deformability to hinder crack propagation, the flexible conductive network could be stretched continuously and the local conductive pathways could be reconstructed. Consequently, the hierarchical conductor could detect both subtle strain of 0-2% and large strain of up to 370%, with a gauge factor (GF) from 66.37 to 971.70, demonstrating outstanding stretchability and sensitivity. And it also owns large tensile strength (5.28 MPa) and high deformation stability. This hierarchical design will give graphene-based sensors a major boost in emerging applications.

Wearable Loop Sensor for Bilateral Knee Flexion Monitoring

We have previously reported wearable loop sensors that can accurately monitor knee flexion with unique merits over the state of the art. However, validation to date has been limited to single-leg configurations, discrete flexion angles, and in vitro (phantom-based) experiments. In this work, we take a major step forward to explore the bilateral monitoring of knee flexion angles, in a continuous manner, in vivo. The manuscript provides the theoretical framework of bilateral sensor operation and reports a detailed error analysis that has not been previously reported for wearable loop sensors. This includes the flatness of calibration curves that limits resolution at small angles (such as during walking) as well as the presence of motional electromotive force (EMF) noise at high angular velocities (such as during running). A novel fabrication method for flexible and mechanically robust loops is also introduced. Electromagnetic simulations and phantom-based experimental studies optimize the setup and evaluate feasibility. Proof-of-concept in vivo validation is then conducted for a human subject performing three activities (walking, brisk walking, and running), each lasting 30 s and repeated three times. The results demonstrate a promising root mean square error (RMSE) of less than 3° in most cases.

All Knitted and Integrated Soft Wearable of High Stretchability and Sensitivity for Continuous Monitoring of Human Joint Motion

E-textiles have recently gained significant traction in the development of soft wearables for healthcare applications. However, there have been limited works on wearable e-textiles with embedded stretchable circuits. Here, stretchable conductive knits with tuneable macroscopic electrical and mechanical properties are developed by varying the yarn combination and the arrangement of stitch types at the meso-scale. Highly extensible piezoresistive strain sensors are designed (>120% strain) with high sensitivity (gauge factor 8.47) and durability (>100,000 cycles), interconnects (>140% strain) and resistors (>250% strain), optimally arranged to form a highly stretchable sensing circuit. The wearable is knitted with a computer numerical control (CNC) knitting machine that offers a cost effective and scalable fabrication method with minimal post-processing. The real-time data from the wearable is transmitted wirelessly using a custom designed circuit board. In this work, an all knitted and fully integrated soft wearable is demonstrated for wireless and continuous real-time sensing of knee joint motion of multiple subjects performing various activities of daily living.

A soft, self-sensing tensile valve for perceptive soft robots

Soft inflatable robots are a promising paradigm for applications that benefit from their inherent safety and adaptability. However, for perception, complex connections of rigid electronics both in hardware and software remain the mainstay. Although recent efforts have created soft analogs of individual rigid components, the integration of sensing and control systems is challenging to achieve without compromising the complete softness, form factor, or capabilities. Here, we report a soft self-sensing tensile valve that integrates the functional capabilities of sensors and control valves to directly transform applied tensile strain into distinctive steady-state output pressure states using only a single, constant pressure source. By harnessing a unique mechanism, "helical pinching", we derive physical sharing of both sensing and control valve structures, achieving all-in-one integration in a compact form factor. We demonstrate programmability and applicability of our platform, illustrating a pathway towards fully soft, electronics-free, untethered, and autonomous robotic systems.

Bending Angle Sensor Based on Double-Layer Capacitance Suitable for Human Joint

Goal: To develop bending angle sensors based on double-layer capacitance for monitoring joint angles during cycling exercises. Methods: We develop a bending angle sensor based on double-layer capacitive and conducted three stretching, bending, and cycling tests to evaluate its validity. Results: We demonstrate that the bending angle sensor based on double-layer capacitance minimizes the change in the capacitance difference in the stretching test. The hysteresis and root mean square error (RMSE) compared with the optical motion capture show hysteresis: 8.0% RMSE and 3.1° in the bending test. Moreover, a cycling experiment for human joint angle measurements confirm the changes in accuracy. The RMSEs ranged from 4.7° to 7.0°, even when a human wears leggings fixed with the developed bending-angle sensor in the cycling test. Conclusion: The developed bending angle sensor provides a practical application of the quantitative and observational evaluation tool for knee joint angles.

Ultralow-Latency Textile Sensors for Wearable Interfaces with a Human-in-Loop Sensing Approach

The evolution of wearable technologies has led to the development of novel types of sensors customized for a wide range of applications. Wearable sensors need to possess a low form factor and be ergonomic, causing minimal impediment of the user's natural movement. Various principles have been explored to meet these requirements, ranging from optical, magnetic, resistive flex sensing to 3D printed sensors and liquid metals such as those using eutectic gallium-indium. However, manufacturing techniques for most current wearable sensors tend to be complex and difficult to scale. Challenges also exist in achieving high sensitivity with noise resistance and robustness to false detections, especially in capacitive sensors. In this research, a novel ultralow-latency soft tactile and pressure sensor developed using off-the-shelf e-textiles is proposed, which overcomes some of these limitations. The sensor does not use any specialized equipment or materials for manufacture. A human-in-loop (HIL) sensing technique is demonstrated, which provides high sensitivity, high sensing bandwidth, as well as ultralow latency, which makes it ideal as a wearable input device. In addition, the HIL method provides other advantages such as high noise rejection and resistance to accidental triggers that could be caused by other humans or environmental factors owing to its high signal to noise ratio. Finally, two applications-a wearable keyboard and gaming input device-were demonstrated using these sensors.

Screen-Printed Corrosion-Resistant and Long-Term Stable Stretchable Electronics Based on AgAu Microflake Conductors

High-throughput production methods such as screen printing can bring stretchable electronics out of the lab into the market. Most stretchable conductor inks for screen printing are based on silver nanoparticles or flakes due to their favorable performance-to-cost ratio, but silver is prone to tarnishing and corrosion, thereby limiting the stability of such conductors. Here, we report on a cost-efficient and scalable approach to resolve this issue by developing screen printable inks based on silver flakes chemically coated by a thin layer of gold. The printed stretchable AgAu conductors reach a conductivity of 8500 S cm^-1, remain conductive up to 250% strain, show excellent corrosion and tarnishing stability, and are used to demonstrate wearable LED and NFC circuits. The reported approach is attractive for smart clothing, as the long-term functionality of such devices is expected in a variety of environments.

Closing the Wearable Gap: Foot-ankle kinematic modeling via deep learning models based on a smart sock wearable

The development of wearable technology, which enables motion tracking analysis for human movement outside the laboratory, can improve awareness of personal health and performance. This study used a wearable smart sock prototype to track foot-ankle kinematics during gait movement. Multivariable linear regression and two deep learning models, including long short-term memory (LSTM) and convolutional neural networks, were trained to estimate the joint angles in sagittal and frontal planes measured by an optical motion capture system. Participant-specific models were established for ten healthy subjects walking on a treadmill. The prototype was tested at various walking speeds to assess its ability to track movements for multiple speeds and generalize models for estimating joint angles in sagittal and frontal planes. LSTM outperformed other models with lower mean absolute error (MAE), lower root mean squared error, and higher R-squared values. The average MAE score was less than 1.138° and 0.939° in sagittal and frontal planes, respectively, when training models for each speed and 2.15° and 1.14° when trained and evaluated for all speeds. These results indicate wearable smart socks to generalize foot-ankle kinematics over various walking speeds with relatively low error and could consequently be used to measure gait parameters without the need for a lab-constricted motion capture system.

A new 3D, microfluidic-oriented, multi-functional, and highly stretchable soft wearable sensor

Increasing demand for wearable devices has resulted in the development of soft sensors; however, an excellent soft sensor for measuring stretch, twist, and pressure simultaneously has not been proposed yet. This paper presents a novel, fully 3D, microfluidic-oriented, gel-based, and highly stretchable resistive soft sensor. The proposed sensor is multi-functional and could be used to measure stretch, twist, and pressure, which is the potential of using a fully 3D structure in the sensor. Unlike previous methods, in which almost all of them used EGaIn as the conductive material, in this case, we used a low-cost, safe (biocompatible), and ubiquitous conductive gel instead. To show the functionality of the proposed sensor, FEM simulations and a set of designed experiments were done, which show linear (99%), accurate (> 94.9%), and durable (tested for a whole of four hours) response of the proposed sensor. Then, the sensor was put through its paces on a female test subject's knee, elbow, and wrist to show the potential application of the sensor as a body motion sensor. Also, a fully 3D active foot insole was developed, fabricated, and evaluated to evaluate the pressure functionality of the sensor. The result shows good discrimination and pressure measurement for different foot sole areas. The proposed sensor has the potential to be used in real-world applications like rehabilitation, wearable devices, soft robotics, smart clothing, gait analysis, AR/VR, etc.

A Time-Division Position-Sensitive Detector Image System for High-Speed Multitarget Trajectory Tracking

High-speed trajectory tracking with real-time processing capability is particularly important in the fields of pilotless automobiles, guidance systems, robotics, and filmmaking. The conventional optical approach to high-speed trajectory tracking involves charge coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) image sensors, which suffer from trade-offs between resolution and framerates, complexity of the system, and enormous data-analysis processes. Here, a high-speed trajectory tracking system is designed by using a time-division position-sensitive detector (TD-PSD) based on a graphene-silicon Schottky heterojunction. Benefiting from the high-speed optoelectronic response and sub-micrometer positional accuracy of the TD-PSD, multitarget real-time trajectory tracking is realized, with a maximum image output framerate of up to 62 000 frames per second. Moreover, multichannel trajectory tracking and image-distortion correction functionalities are realized by TD-PSD systems through frequency-related image preprocessing, which significantly improves the capacity of real-time information processing and image quality in complicated light environments.

Thermophysiological aspects of wearable robotics: Challenges and opportunities

Technological advancements in the last two decades have enabled development of a variety of mechanically supporting wearable robots (i.e. exoskeletons) that are transitioning to practice in medical and industrial settings. The feedback from industry and recent controlled studies is highlighting thermal discomfort as a major reason for the disuse of the devices and a substantial barrier to their long-term adoption. Furthermore, a brief overview of the devices and their intended applications reveals that many of the potential users are likely to face thermal comfort issues because of either high exertion or medically related high heat sensitivity. The aim of this review is to discuss these emerging thermal challenges and opportunities surrounding wearable robots. This review discusses mechanisms, potential solutions, and a platform for systematically measuring heat transfer inhibition caused by wearing of an exoskeleton. Lastly, the potential for substantial metabolic rate reduction provided by exoskeletons to reduce worker thermal strain in warm-to-hot conditions is also considered.

Polyurethane-Based Stretchable Semiconductor Nanofilms with High Intrinsic Recovery Similar to Conventional Elastomers

Polymer semiconductors with large elastic recovery (ER) under high strain in thin film state are highly desirable for stretchable electronics. Here we report a type of stretchable semiconductor PU(DPP)x, by copolymerization of oligodiketopyrrolopyrrole-based conjugated block and hydrogenated polybutadiene flexible block via urethane linkage for intermolecular hydrogen bonding. By regulating block ratio, PU(DPP)35 with 35 wt % conjugated block exhibits high intrinsic ER > 80% under 175% strain (ε) in pseudo free-standing thin film state, comparable with commercial elastomers, and crack onset strain (COS) > 300% along with maximum hole mobility of 0.19 cm2 V-1 s-1 in organic thin film transistors to bring it to the best performing block copolymer-type stretchable semiconductors. Enhanced mobility is achieved using PU(DPP)35 as the binder for conjugated polymer PDPPT3. The 25 wt %-PDPPT3 blend displays mobility up to 1.28 cm2 V-1 s-1 along with COS ∼120%, and 10 wt %-PDPPT3 blend exhibits ER of 78% at ε = 150%, COS of ∼230%, modulus of 36.5 MPa, maximum mobility of 0.62 cm2 V-1 s-1 and no obvious degradation of mobility at ε = 150% after 100 cycles of strain. Moreover, the structural similarity enables the blend film uniform and stable microstructure against mechanical and thermal deformation. Notably, PU(DPP)35 and the blend are characterized by high mechanical performance similar to that of commercial elastomers in thin film state, and demonstrate their potential for high performance stretchable electronics.

Are Liquid Metals Bulk Conductors?

Stretchable electronics have potential in wide-reaching applications including wearables, personal health monitoring, and soft robotics. Many recent advances in stretchable electronics leverage liquid metals, particularly eutectic gallium-indium (EGaIn). A variety of EGaIn electromechanical behaviors have been reported, ranging from bulk conductor responses to effectively strain-insensitive responses. However, numerous measurement techniques have been used throughout the literature, making it difficult to directly compare the various proposed formulations. Here, the electromechanical responses of EGaIn found in the literature is reviewed and pure EGaIn is investigated using three electrical resistance measurement techniques: four point probe, two point probe, and Wheatstone bridge. The results indicate substantial differences in measured electromechanical behavior between the three methods, which can largely be accounted for by correcting for a fixed offset corresponding to the resistances of various parts of the measurement circuits. Yet, even accounting for several of these sources of experimental error, the average relative change in resistance of EGaIn is found to be lower than that predicted by the commonly used bulk conductor assumption, referred to as Pouillet's law. Building upon recent theories proposed in the literature, possible explanations for the discrepancies are discussed. Finally, suggestions are provided on experimental design to enable reproducible and interpretable research.

A stretchable sensor for force estimation in soft wearable robots

Soft wearable robots to assist human movements, such as exosuits, have rapidly gained attention thanks to their compliance, low weight and accessibility. However, force measurement in exosuits still rely on load cells and rigid sensors that are not wearable or unsuitable for applications outside the lab. Soft, stretchable and lightweight sensors that become invisible when integrated in an exosuit and perfectly conform to the human body represent a promising alternative. In this work, we developed a wearable sensing system based on a soft stretchable silicone-based strain gauge to measure the forces acting in the passive elastic elements of an exosuit. To measure sensor's accuracy, two unimpaired participants walked on a treadmill at speeds between 0.9 and 2.1 $\text{m}\text{s}^{-1}$. When comparing our solution to a state-of-the-art motion capture system, we found an average root mean square error in force estimation of 12.5% and a standard deviation of 7.4%. Furthermore, we showed the portability of our sensory system by monitoring the forces exerted by the wearable robot during outdoor walking. Our study shows the potential of using stretchable sensors to monitor walking patterns in studies outside the lab and to control human-robot interaction.

Durability Study of Thermal Transfer Printed Textile Electrodes for Wearable Electronic Applications

Textile-based electronics hold great promise because they can endow wearable devices with soft and comfortable characteristics. However, the inherent porosity and fluffiness of fabrics result in high surface roughness, which presents great challenges in the manufacture of high-performance fabric electrodes. In this work, we propose a thermal transfer printing method to address the above challenges, in which electrodes or circuits of silver flake/thermoplastic polyurethane (TPU) composites are prefabricated on a release film by coating and laser engraving and then laminated by hot-pressing to a variety of fabrics and textiles. This universal and scalable production technique enables fabric electrodes to be made without compromising the original wearability, washability, and stretchability of textiles. The prepared fabric electrodes exhibit high conductivity (5.48 × 10⁴ S/cm), high adhesion (≥1750 N/m), good abrasion/washing resistance, high patterning resolution (∼40 μm), and good electromechanical performance up to 50% strain. To demonstrate the potential applications, we developed textile-based radio frequency identification (RFID) tags for remote identification and a large-sized heater for wearable thermotherapy. More importantly, the solvent-free thermal transfer printing technology developed in this paper enables people to DIY interesting flexible electronics on clothes with daily tools, which can promote the commercial application of smart textile-based electronics.

Integrated stretchable pneumatic strain gauges for electronics-free soft robots

In soft robotics, actuators, logic and power systems can be entirely fluidic and electronics-free. However, sensors still typically rely on electric or optical principles. This adds complexity to fluidic soft robots because transducers are needed, and electrical materials have to be incorporated. Herein, we show a highly-stretchable pneumatic strain gauge based on a meandering microchannel in a soft elastomer material thus eliminating the need for an electrical signal in soft robots. Using such pneumatic sensors, we demonstrate an all-pneumatic gripper with integrated pneumatic strain gauges that is capable of autonomous closure and object recognition. The gauges can measure at least up to 300% engineering strains. The sensor exhibits a very stable signal over a 12-hour measurement period with no hysteresis. Using pneumatic sensors, all four major components of robots—actuators, logic, power, and sensors—can be pneumatic, enabling all-fluidic soft robots with proprioception and exteroception.

Anastasia Koivikko and colleagues report pneumatic strain gauges integrated into a soft robotic gripper which can recognize objects and close autonomously. The approach enables entirely fluidic robotic systems with no need for electrical or optical sensors.

Photopatternable Poly(dimethylsiloxane) (PDMS) for an Intrinsically Stretchable Organic Electrochemical Transistor

Patterning elastomers is an essential process for the application of elastomers to stretchable bioelectric devices. In general, replication of a mold and laser ablation are used for patterning elastomers. However, these methods are inefficient and time consuming due to complex patterning procedures and a heat-induced curing mechanism. In this work, we developed a photopatternable elastomer called thiol-ene cross-linked poly(dimethylsiloxane) (TC-PDMS). TC-PDMS showed high-resolution patternability (∼100 μm) through a direct patterning process. It also had high stretchability (∼140%) and low Young's modulus (∼2.9 MPa) similar to conventional PDMS. To demonstrate its practicability in stretchable bioelectric devices, TC-PDMS was applied to a passivation layer of an intrinsically stretchable organic electrochemical transistor (OECT), which showed a low leakage current (∼20 μA) and a high transconductance (0.432 mS) at high strain (60%). The stretchable OECT was able to record electrocardiographic (ECG) signals from human skin, and the measured ECG signals exhibited a high signal-to-noise ratio of 12.2 dB.

Configurable direction sensitivity of skin-mounted microfluidic strain sensor with auxetic metamaterial

Electromechanical coupling plays a key role in determining the performance of stretchable strain sensor. Current regulation of the electromechanical coupling in stretchable strain sensor is largely restricted by the intrinsic mechanical properties of the device. In this study, a microfluidic strain sensor based on the core-shell package design with the auxetic metamaterial (AM) is presented. By overriding the mechanical properties of the device, the AM in the package effectively tunes the deformation of the microfluidic channel with the applied strain and configures the directional strain sensitivity with a large modulation range. The gauge factor (GF) of the strain sensor in the radial direction of the channel can be gradually shifted from the intrinsically negative value to a positive one by adopting the AMs with different designs. By simply replacing the AM in the package, the microfluidic strain sensor with the core-shell package can be configurated as an omnidirectional or directional stretchable strain sensor. With the directional sensitivity brought by the rational AM design, the application of the AM-integrated strain sensor in the skin-mounted tactile detection is demonstrated with high tolerance to unintended wrist movements.

Soft Actuators Made of Discrete Grains

Recent work has demonstrated the potential of actuators consisting of bulk elastomers with phase-changing inclusions for generating high forces and large volumetric expansions. Simultaneously, granular assemblies have been shown to enable tunable properties via different packings, dynamic moduli via jamming, and compatibility with various printing methods via suspension in carrier fluids. Herein, granular actuators are introduced, which represent a new class of soft actuators made of discrete grains. The soft grains consist of a hyperelastic shell and multiple solvent cores. Upon heating, the encapsulated solvent cores undergo liquid-to-gas phase change, inducing rapid and strong volumetric expansion of the hyperelastic shell up to 700%. The grains can be used independently for micro-actuation, or in granular agglomerates for meso- and macroscale actuation, demonstrating the scalability of the granular actuators. Furthermore, the active grains can be suspended in a carrier resin or solvent to enable printable soft actuators via established granular material processing techniques. By combining the advantages of phase-change soft actuation and granularity, this work presents the opportunity to realize soft actuators with tunable bulk properties, compatibility with self-assembly techniques, and on-demand reconfigurability.

Intrinsically Stretchable Block Copolymer Based on PEDOT:PSS for Improved Performance in Bioelectronic Applications

The conductive polyelectrolyte complex poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is ubiquitous in research dealing with organic electronic devices (e.g., solar cells, wearable and implantable sensors, and electrochemical transistors). In many bioelectronic applications, the applicability of commercially available formulations of PEDOT:PSS (e.g., Clevios) is limited by its poor mechanical properties. Additives can be used to increase the compliance but pose a risk of leaching, which can result in device failure and increased toxicity (in biological settings). Thus, to increase the mechanical compliance of PEDOT:PSS without additives, we synthesized a library of intrinsically stretchable block copolymers. In particular, controlled radical polymerization using a reversible addition-fragmentation transfer process was used to generate block copolymers consisting of a block of PSS (of fixed length) appended to varying blocks of poly(poly(ethylene glycol) methyl ether acrylate) (PPEGMEA). These block copolymers (PSS₍₁₎-b-PPEGMEA_(x), where x ranges from 1 to 6) were used as scaffolds for oxidative polymerization of PEDOT. By increasing the lengths of the PPEGMEA segments on the PEDOT:[PSS₍₁₎-b-PPEGMEA_(1-6)] block copolymers, ("Block-1" to "Block-6"), or by blending these copolymers with PEDOT:PSS, the mechanical and electronic properties of the polymer can be tuned. Our results indicate that the polymer with the longest block of PPEGMEA, Block-6, had the highest fracture strain (75%) and lowest elastic modulus (9.7 MPa), though at the expense of conductivity (0.01 S cm^-1). However, blending Block-6 with PEDOT:PSS to compensate for the insulating nature of the PPEGMEA resulted in increased conductivity [2.14 S cm^-1 for Blend-6 (2:1)]. Finally, we showed that Block-6 outperforms a commercial formulation of PEDOT:PSS as a dry electrode for surface electromyography due to its favorable mechanical properties and better adhesion to skin.

Tactile Sensing for Minimally Invasive Surgery: Conventional Methods and Potential Emerging Tactile Technologies

As opposed to open surgery procedures, minimally invasive surgery (MIS) utilizes small skin incisions to insert a camera and surgical instruments. MIS has numerous advantages such as reduced postoperative pain, shorter hospital stay, faster recovery time, and reduced learning curve for surgical trainees. MIS comprises surgical approaches, including laparoscopic surgery, endoscopic surgery, and robotic-assisted surgery. Despite the advantages that MIS provides to patients and surgeons, it remains limited by the lost sense of touch due to the indirect contact with tissues under operation, especially in robotic-assisted surgery. Surgeons, without haptic feedback, could unintentionally apply excessive forces that may cause tissue damage. Therefore, incorporating tactile sensation into MIS tools has become an interesting research topic. Designing, fabricating, and integrating force sensors onto different locations on the surgical tools are currently under development by several companies and research groups. In this context, electrical force sensing modality, including piezoelectric, resistive, and capacitive sensors, is the most conventionally considered approach to measure the grasping force, manipulation force, torque, and tissue compliance. For instance, piezoelectric sensors exhibit high sensitivity and accuracy, but the drawbacks of thermal sensitivity and the inability to detect static loads constrain their adoption in MIS tools. Optical-based tactile sensing is another conventional approach that facilitates electrically passive force sensing compatible with magnetic resonance imaging. Estimations of applied loadings are calculated from the induced changes in the intensity, wavelength, or phase of light transmitted through optical fibers. Nonetheless, new emerging technologies are also evoking a high potential of contributions to the field of smart surgical tools. The recent development of flexible, highly sensitive tactile microfluidic-based sensors has become an emerging field in tactile sensing, which contributed to wearable electronics and smart-skin applications. Another emerging technology is imaging-based tactile sensing that achieved superior multi-axial force measurements by implementing image sensors with high pixel densities and frame rates to track visual changes on a sensing surface. This article aims to review the literature on MIS tactile sensing technologies in terms of working principles, design requirements, and specifications. Moreover, this work highlights and discusses the promising potential of a few emerging technologies towards establishing low-cost, high-performance MIS force sensing.

Undulatory Swimming Performance Explored With a Biorobotic Fish and Measured by Soft Sensors and Particle Image Velocimetry

Due to the difficulty of manipulating muscle activation in live, freely swimming fish, a thorough examination of the body kinematics, propulsive performance, and muscle activity patterns in fish during undulatory swimming motion has not been conducted. We propose to use soft robotic model animals as experimental platforms to address biomechanics questions and acquire understanding into subcarangiform fish swimming behavior. We extend previous research on a bio-inspired soft robotic fish equipped with two pneumatic actuators and soft strain sensors to investigate swimming performance in undulation frequencies between 0.3 and 0.7 Hz and flow rates ranging from 0 to 20c m s in a recirculating flow tank. We demonstrate the potential of eutectic gallium-indium (eGaIn) sensors to measure the lateral deflection of a robotic fish in real time, a controller that is able to keep a constant undulatory amplitude in varying flow conditions, as well as using Particle Image Velocimetry (PIV) to characterizing swimming performance across a range of flow speeds and give a qualitative measurement of thrust force exerted by the physical platform without the need of externally attached force sensors. A detailed wake structure was then analyzed with Dynamic Mode Decomposition (DMD) to highlight different wave modes present in the robot's swimming motion and provide insights into the efficiency of the robotic swimmer. In the future, we anticipate 3D-PIV with DMD serving as a global framework for comparing the performance of diverse bio-inspired swimming robots against a variety of swimming animals.

Skin-like hydrogel devices for wearable sensing, soft robotics and beyond

Skin-like electronics are developing rapidly to realize a variety of applications such as wearable sensing and soft robotics. Hydrogels, as soft biomaterials, have been studied intensively for skin-like electronic utilities due to their unique features such as softness, wetness, biocompatibility and ionic sensing capability. These features could potentially blur the gap between soft biological systems and hard artificial machines. However, the development of skin-like hydrogel devices is still in its infancy and faces challenges including limited functionality, low ambient stability, poor surface adhesion, and relatively high power consumption (as ionic sensors). This review aims to summarize current development of skin-inspired hydrogel devices to address these challenges. We first conduct an overview of hydrogels and existing strategies to increase their toughness and conductivity. Next, we describe current approaches to leverage hydrogel devices with advanced merits including anti-dehydration, anti-freezing, and adhesion. Thereafter, we highlight state-of-the-art skin-like hydrogel devices for applications including wearable electronics, soft robotics, and energy harvesting. Finally, we conclude and outline the future trends.

Emergence of flexible technology in developing advanced systems for post-stroke rehabilitation: a comprehensive review

OBJECTIVE

Stroke is one of the most common neural disorders, which causes physical disabilities and motor impairments among its survivors. Several technologies have been developed for providing stroke rehabilitation and to assist the survivors in performing their daily life activities. Currently, the use of flexible technology (FT) for stroke rehabilitation systems is on a rise that allows the development of more compact and lightweight wearable systems, which stroke survivors can easily use for long-term activities.

APPROACH

For stroke applications, FT mainly includes the "flexible/stretchable electronics", "e-textile (electronic textile)" and "soft robotics". Thus, a thorough literature review has been performed to report the practical implementation of FT for post-stroke application.

MAIN RESULTS

In this review, the highlights of the advancement of FT in stroke rehabilitation systems are dealt with. Such systems mainly involve the "biosignal acquisition unit", "rehabilitation devices" and "assistive systems". In terms of biosignals acquisition, electroencephalography (EEG) and electromyography (EMG) are comprehensively described. For rehabilitation/assistive systems, the application of functional electrical stimulation (FES) and robotics units (exoskeleton, orthosis, etc.) have been explained.

SIGNIFICANCE

This is the first review article that compiles the different studies regarding flexible technology based post-stroke systems. Furthermore, the technological advantages, limitations, and possible future implications are also discussed to help improve and advance the flexible systems for the betterment of the stroke community.

Gait Analysis Accuracy Difference with Different Dimensions of Flexible Capacitance Sensors

Stroke causes neurological pathologies, including gait pathologies, which are diagnosed by gait analysis. However, existing gait analysis devices are difficult to use in situ or are disrupted by external conditions. To overcome these drawbacks, a flexible capacitance sensor was developed in this study. To date, a performance comparison of flexible sensors with different dimensions has not been carried out. The aim of this study was to provide optimized sensor dimension information for gait analysis. To accomplish this, sensors with seven different dimensions were fabricated. The dimensions of the sensors were based on the average body size and movement range of 20- to 59-year-old adults. The sensors were characterized by 100 oscillations. The minimum hysteresis error was 8%. After that, four subjects were equipped with the sensor and walked on a treadmill at a speed of 3.6 km/h. All walking processes were filmed at 50 fps and analyzed in Kinovea. The RMS error was calculated using the same frame rate of the video and the sampling rate of the signal from the sensor. The smallest RMS error between the sensor data and the ankle angle was 3.13° using the 49 × 8 mm sensor. In this study, we confirm the dimensions of the sensor with the highest gait analysis accuracy; therefore, the results can be used to make decisions regarding sensor dimensions.

3D Printed Motor-Sensory Module Prototype for Facial Rehabilitation

This work demonstrates the first 3D printed wearable motor-sensory module prototype designed for facial rehabilitation, focusing on facial paralysis. The novelty of the work lies in the fast fabrication of the first fully soft working prototype, including feedback control, with a focus on the methodology for individual customization. Facial paralysis results from a variety of conditions, and more wearable and modular technologies are needed to address the complexity of facial movement rehabilitation. Smiling muscles are especially important for both expression and eating, and so this work focuses on this motion as an example of how the module can be applied to mimic and support needed muscle movement. A generalized actuator-sensor pair with a feedback control system is created to translate signals from smiling on the healthy side of the face (notably temporal and zygomatic branch) to actuation on the paralyzed side of the face for augmented physiotherapy. Fabric and a sensor fluid are integrated during the silicone printing process to create a multicomponent wearable that is ready to use with minimal postprocessing. The actuators' force and vertical contraction results under a 0.98 and 1.96 N load meet the 1-7 N requirements needed for smiling. It is a challenge to measure soft surface-based force and contraction ratio consistently; therefore, a novel modular surface is designed to simulate the interaction of skin and bone using 3D printed hard plastic (bone) and a silicone sheet (skin). The actuator is tested on top of four different repeatable and standardized surface morphologies, and results reveal that the actuator force application will vary based on topography and hardness of the facial surface. Demonstration of the complete system on the face while collecting sensor and pressure data serves as a proof-of-concept and motivates potential applications in rapid customization of highly specialized soft wearable orthotics, prosthetics, and rehabilitation devices. This unique actuator-sensor combination can have additional applications for wearables due to the (1) customizability, (2) closed-loop control, and (3) unique "grounding" test platform.

Printed and Laser-Activated Liquid Metal-Elastomer Conductors Enabled by Ethanol/PDMS/Liquid Metal Double Emulsions

Soft electronic systems require stretchable, printable conductors for applications in soft robotics, wearable technologies, and human-machine interfaces. Gallium-based room-temperature liquid metals (LMs) have emerged as promising candidates, and recent liquid metal-embedded elastomers (LMEEs) have demonstrated favorable properties such as stable conductivity during strain, cyclic durability, and patternability. Here, we present an ethanol/polydimethylsiloxane/liquid metal (EtOH/PDMS/LM) double emulsion ink that enables a fast, scalable method to print LM conductors with high conductivity (7.7 × 10⁵ S m^-1), small resistance change when strained, and consistent cyclic performance (over 10,000 cycles). EtOH, the carrier solvent, is leveraged for its low viscosity to print the ink onto silicone substrates. PDMS resides at the EtOH/LM interface and cures upon deposition and EtOH evaporation, consequently bonding the LM particles to each other and to the silicone substrate. The printed PDMS-LM composite can be subsequently activated by direct laser writing, forming high-resolution electrically conductive pathways. We demonstrate the utility of the double emulsion ink by creating intricate electrical interconnects for stretchable electronic circuits. This work combines the speed, consistency, and precision of laser-assisted manufacturing with the printability, high conductivity, strain insensitivity, and mechanical robustness of the PDMS-LM composite, unlocking high-yield, high-throughput, and high-density stretchable electronics.

Design of a Plantar Pressure Insole Measuring System Based on Modular Photoelectric Pressure Sensor Unit

Accurately perceiving and predicting the parameters related to human walking is very important for man-machine coupled cooperative control systems such as exoskeletons and power prostheses. Plantar pressure data is rich in human gait and posture information and is an essential source of reference information as the input of the exoskeleton control system. Therefore, the proper design of the pressure sensing insole and validation is a big challenge considering the requirements such as convenience, reliability, no interference and so on. In this research, we developed a low-cost modular sensing unit based on the principle of photoelectric sensing and designed a plantar pressure sensing insole to achieve the purpose of sensing human walking gait and posture information. On the one hand, the sensor unit is made of economy-friendly commercial flexible circuits and elastic silicone, and the mechanical and electrical characteristics of the modular sensor unit are evaluated by a self-developed pressure-related calibration system. The calibration results show that the modular sensor based on the photoelectric sensing principle has fast response and negligible hysteresis. On the other hand, we analyzed the area where the plantar pressure is densely distributed. One benefit of the modular sensing unit design is that it is rather convenient to fabricate different insole solutions, so we fabricated and compared several pressure-sensitive insole solutions in this preliminary study. During the dynamic locomotion experiments of wearing the pressure-sensing insole, the time series signal of each sensor unit was collected and analyzed. The results show that the pressure sensing insole based on the photoelectric effect can sense the distribution of the plantar pressure by capturing the deformation of the insole caused by the foot contact during locomotion, and provide reliable gait information for wearable applications.

Transparent Soft Actuators/Sensors and Camouflage Skins for Imperceptible Soft Robotics

The advent of soft robotics has led to great advancements in robots, wearables, and even manufacturing processes by employing entirely soft-bodied systems that interact safely with any random surfaces while providing great mechanical compliance. Moreover, recent developments in soft robotics involve advances in transparent soft actuators and sensors that have made it possible to construct robots that can function in a visually and mechanically unobstructed manner, assisting the operations of robots and creating more applications in various fields. In this aspect, imperceptible soft robotics that mainly consist of optically transparent imperceptible hardware components is expected to constitute a new research focus in the forthcoming era of soft robotics. Here, the recent progress regarding extended imperceptible soft robotics is provided, including imperceptible transparent soft robotics (transparent soft actuators/sensors) and imperceptible nontransparent camouflage skins. Their principles, materials selections, and working mechanisms are discussed so that key challenges and perspectives in imperceptible soft robotic systems can be explored.

Quasi-Passive Resistive Exosuit for Space Activities: Proof of Concept

The limits of space travel are continuously evolving, and this creates increasingly extreme challenges for the crew’s health that must be addressed by the scientific community. Long-term exposure to micro-gravity, during orbital flights, contributes to muscle strength degradation and increases bone density loss. In recent years, several exercise devices have been developed to counteract the negative health effects of zero-gravity on astronauts. However, the relatively large size of these devices, the need for a dedicated space and the exercise time-frame for each astronaut, does not make these devices the best choice for future long range exploration missions. This paper presents a quasi-passive exosuit to provide muscle training using a small, portable, proprioceptive device. The exosuit promotes continuous exercise, by resisting the user’s motion, during routine all-day activity. This study assesses the effectiveness of the resistive exosuit by evaluating its effects on muscular endurance during a terrestrial walking task. The experimental assessment on biceps femoris and vastus lateralis, shows a mean increase in muscular activation of about 97.8% during five repetitions of 3 min walking task at 3 km/h. The power frequency analysis shows an increase in muscular fatigue with a reduction of EMG median frequency of about 15.4% for the studied muscles.

Soft Inductive Coil Spring Strain Sensor Integrated with SMA Spring Bundle Actuator

This study proposes a soft inductive coil spring (SICS) strain sensor that can measure the strain of soft actuators. The SICS sensor, produced by transforming a shape memory alloy (SMA) wire with the same materials as that of an SMA spring bundle actuator (SSBA) into a coil spring shape, measures inductance changes according to length changes. This study also proposes a manufacturing method, output characteristics of the SICS sensor applicable to the SSBA among soft actuators, and the structure of the SICS sensor-integrated SSBA (SI-SSBA). In the SI-SSBA, the SMA spring bundle and SICS sensor have structures corresponding to the muscle fiber and spindle of the skeletal muscle, respectively. It is demonstrated that when a robotic arm with one degree of freedom is operated by attaching two SI-SSBAs in an antagonistic structure, the displacement of the SSBA can be measured using the proposed strain sensor. The output characteristics of the SICS sensor for the driving speed of the robotic arm were evaluated, and it was experimentally proven that the strain of the SSBA can be stably measured in water under a temperature change of 54 °C from 36 to 90 °C.

Honeycomb Jamming: An Enabling Technology of Variable Stiffness Reconfiguration

Jamming technologies are one of the promising approaches of variable stiffness mechanisms. However, there are problems limiting the broad application of jamming-based approaches such as a limited stiffening capacity and restricted stiffening position. This article presents a variable stiffness mechanism to achieve a rapid flexible to rigid state transition with biocompatibility, fail-safe design, and enhanced stiffening capacity. A novel strategy of reconfiguration of stiffening regions, which is entitled variable stiffness reconfiguration, is exploited to control not only the stiffnesses but also the positions and areas of the stiffening regions. At first, this article provides a new approach to the variable stiffness soft robotics community to enable both stiffness control and stiffening region adjustment. In this way, additional functions of the variable stiffness mechanisms including reproducing complex manipulator postures or customizing the soft gripper, through delivering functional units into or out of the devices, are demonstrated. Through reconfiguration, our design provides a generally applicable solution for a wide range of complex manipulator postures reproduced and objects grasped by reconfiguration of the stiffening regions. The variable stiffness mechanism is empirically evaluated with a comparison with other variable stiffness strategies in which the proposed solution shows greater stiffening capability, and an experimental search of optimal parameters of the honeycomb structure is presented. Finite element models, which have shown reasonable agreement with the empirical results, are constructed to model the stiffnesses, and an analytic model of the manipulator is derived to predict the posture.

Review of machine learning methods in soft robotics

Soft robots have been extensively researched due to their flexible, deformable, and adaptive characteristics. However, compared to rigid robots, soft robots have issues in modeling, calibration, and control in that the innate characteristics of the soft materials can cause complex behaviors due to non-linearity and hysteresis. To overcome these limitations, recent studies have applied various approaches based on machine learning. This paper presents existing machine learning techniques in the soft robotic fields and categorizes the implementation of machine learning approaches in different soft robotic applications, which include soft sensors, soft actuators, and applications such as soft wearable robots. An analysis of the trends of different machine learning approaches with respect to different types of soft robot applications is presented; in addition to the current limitations in the research field, followed by a summary of the existing machine learning methods for soft robots.

Liquid metal-based nanocomposite materials: fabrication technology and applications

Research on liquid metals has been steadily garnering more interest in recent times, especially in flexible electronics applications because of their properties like possessing high conductivity and being liquid state at room temperature. The unique properties afforded by such materials at low temperatures can compensate for the limitations of stretchable electronic devices, particularly robustness and their fluidic property, which can enhance the flexibility and deformation of these devices. Therefore, interest in liquid-metal nanoparticles and liquid metals with nanocomposites has enabled research into their fabrication technologies as well as utilisation in fields such as chemistry, polymer engineering, computational modelling, and nanotechnology. In particular, in flexible and stretchable electronic device applications, the research attention is focused on the fabrication methodologies of liquid-metal nanoparticles and liquid metals containing nanocomposites. This review attempts to summarise the available stretchable and flexible electronics applications that use liquid-metal nanoparticles as well as liquid metals with nanomaterial additives.

Functionalization of liquid metal nanoparticles via the RAFT process

The proper design and selection of RAFT agents allow the preparation of eutectic gallium–indium (EGaIn) based liquid metal nanoparticles with grafted polymers.

Synergizing microfluidics with soft robotics: A perspective on miniaturization and future directions

Soft robotics has gone through a decade of tremendous progress in advancing both fundamentals and technologies. It has also seen a wide range of applications such as surgery assistance, handling of delicate foods, and wearable assistive systems driven by its soft nature that is more human friendly than traditional hard robotics. The rapid growth of soft robotics introduces many challenges, which vary with applications. Common challenges include the availability of soft materials for realizing different functions and the precision and speed of control required for actuation. In the context of wearable systems, miniaturization appears to be an additional hurdle to be overcome in order to develop truly impactful systems with a high user acceptance. Microfluidics as a field of research has gone through more than two decades of intense and focused research resulting in many fundamental theories and practical tools that have the potentials to be applied synergistically to soft robotics toward miniaturization. This perspective aims to introduce the potential synergy between microfluidics and soft robotics as a research topic and suggest future directions that could leverage the advantages of the two fields.

Wearable sensors: At the frontier of personalised health monitoring, smart prosthetics and assistive technologies

Wearable sensors have evolved from body-worn fitness tracking devices to multifunctional, highly integrated, compact, and versatile sensors, which can be mounted onto the desired locations of our clothes or body to continuously monitor our body signals, and better interact and communicate with our surrounding environment or equipment. Here, we discuss the latest advances in textile-based and skin-like wearable sensors with a focus on three areas, including (i) personalised health monitoring to facilitate recording physiological signals, body motions, and analysis of body fluids, (ii) smart gloves and prosthetics to realise the sensation of touch and pain, and (iii) assistive technologies to enable disabled people to operate the surrounding motorised equipment using their active organs. We also discuss areas for future research in this emerging field.

Heterogeneous sensing in a multifunctional soft sensor for human-robot interfaces

Soft sensors have been playing a crucial role in detecting different types of physical stimuli to part or the entire body of a robot, analogous to mechanoreceptors or proprioceptors in biology. Most of the currently available soft sensors with compact form factors can detect only a single deformation mode at a time due to the limitation in combining multiple sensing mechanisms in a limited space. However, realizing multiple modalities in a soft sensor without increasing its original form factor is beneficial, because even a single input stimulus to a robot may induce a combination of multiple modes of deformation. Here, we report a multifunctional soft sensor capable of decoupling combined deformation modes of stretching, bending, and compression, as well as detecting individual deformation modes, in a compact form factor. The key enabling design feature of the proposed sensor is a combination of heterogeneous sensing mechanisms: optical, microfluidic, and piezoresistive sensing. We characterize the performance on both detection and decoupling of deformation modes, by implementing both a simple algorithm of threshold evaluation and a machine learning technique based on an artificial neural network. The proposed soft sensor is able to estimate eight different deformation modes with accuracies higher than 95%. We lastly demonstrate the potential of the proposed sensor as a method of human-robot interfaces with several application examples highlighting its multifunctionality.

A Simple Low-Cost Wearable Sensor for Long-Term Ambulatory Monitoring of Knee Joint Kinematics

OBJECTIVE

Accurate monitoring of joint kinematics in individuals with neuromuscular and musculoskeletal disorders within ambulatory settings could provide important information about changes in disease status and the effectiveness of rehabilitation programs and/or pharmacological treatments. This paper introduces a reliable, power efficient, and low-cost wearable system designed for the long-term monitoring of joint kinematics in ambulatory settings.

METHODS

Seventeen healthy subjects wore a retractable string sensor, fixed to two anchor points on the opposing segments of the knee joint, while walking at three different self-selected speeds. Joint angles were estimated from calibrated sensor values and their derivatives in a leave-one-subject-out cross validation manner using a random forest algorithm.

RESULTS

The proposed system estimated knee flexion/extension angles with a root mean square error (RMSE) of 5.0°±1.0° across the study subjects upon removal of a single outlier subject. The outlier was likely a result of sensor miscalibration.

CONCLUSION

The proposed wearable device can accurately estimate knee flexion/extension angles during locomotion at various walking speeds.

SIGNIFICANCE

We believe that our novel wearable technology has great potential to enable joint kinematic monitoring in ambulatory settings and thus provide clinicians with an opportunity to closely monitor joint recovery, develop optimal, personalized rehabilitation programs, and ultimately maximize therapeutic outcomes.

Additive manufacturing techniques for smart prosthetic liners

Elastomeric liners are commonly worn between the prosthetic socket and the limb. A number of improvements to the state of the art of liner technology are required to address outstanding problems. A liner that conforms to the residuum more accurately, may improve the skin health at the stump-socket interface. Previous work has shown that for effective thermal management of the socket environment, an active heat removal system is required, yet this is not available. Volume tracking of the stump could be used as a diagnostic tool for looking at the changes that occur across the day for all users, which depend on activity level, position, and the interaction forces of the prosthetic socket with the limb. We believe that it would be advantageous to embed these devices into a smart liner, which could be replaced and repaired more easily than the highly costly and labour-intensive custom-made socket. This paper presents the work to develop these capabilities in soft material technology, with: the development of a printable nanocomposite stretch sensor system; a low-cost digital method for casting bespoke prosthetic liners; a liner with an embedded stretch sensor for growth / volume tracking; a model liner with an embedded active cooling system.

Efficient Upper Limb Position Estimation Based on Angular Displacement Sensors for Wearable Devices

Motion tracking techniques have been extensively studied in recent years. However, capturing movements of the upper limbs is a challenging task. This document presents the estimation of arm orientation and elbow and wrist position using wearable flexible sensors (WFSs). A study was developed to obtain the highest range of motion (ROM) of the shoulder with as few sensors as possible, and a method for estimating arm length and a calibration procedure was proposed. Performance was verified by comparing measurement of the shoulder joint angles obtained from commercial two-axis soft angular displacement sensors (sADS) from Bend Labs and from the ground truth system (GTS) OptiTrack. The global root-mean-square error (RMSE) for the shoulder angle is 2.93 degrees and 37.5 mm for the position estimation of the wrist in cyclical movements; this measure of RMSE was improved to 13.6 mm by implementing a gesture classifier.