Liquid Metal Strain Research Articles

Highly Adhesive and Stretchable Epidermal Electrode for Bimodal Recording Patch.

For numerous biological and human-machine applications, it is critical to have a stable electrophysiological interface to obtain reliable signals. To achieve this, epidermal electrodes should possess conductivity, stretchability, and adhesiveness. However, limited types of materials can simultaneously satisfy these requirements to provide satisfying recording performance. Here, we present a dry electromyography (EMG) electrode based on conductive polymers and tea polyphenol (CPT), which offers adhesiveness (0.51 N/cm), stretchability (157%), and low impedance (14 kΩ cm2 at 100 Hz). The adhesiveness of the electrode is attributed to the interaction between catechol groups and hydroxyls in the polymer blend. This adhesive electrode ensures stable EMG recording even in the presence of vibrations and provides signals with a high signal-to-noise ratio (>25 dB) for over 72 h. By integrating the CPT electrode with a liquid metal strain sensor, we have developed a bimodal rehabilitation monitoring patch (BRMP) for sports injuries. The patch utilizes Kinesio Tape as a substrate, which serves to accelerate rehabilitation. It also tackles the challenge of recording with knee braces by fitting snugly between the brace and the skin, due to its thin and stretchable design. CPT electrodes not only enable BRMP to assist clinicians in formulating effective rehabilitation plans and offer patients a more comfortable rehabilitation experience, but also hold promise for future applications in biological and human-machine interface domains.

A Flexible, Architected Soft Robotic Actuator for Motorized Extensional Motion

To advance the design space of electrically‐driven soft actuators, a flexible, architected soft robotic actuator is presented for motor‐driven extensional motion. The actuator comprises a 3D printed, cylindrical handed shearing auxetic (HSA) structure and a deformable, internal rubber bellows shaft. The actuator linearly extends upon applying torque from a servo motor; the rubber bellows shaft is stretchable but resistant to torsional deflection, allowing it to transmit torque from the servo motor to the other end of the HSA. The high flexibility of the HSA and rubber bellows shaft enable the actuator to adaptively extend even when bent. The actuator's two components and its performance are mechanically characterized. Actuation strains of 45% elongation and a maximum blocked pushing force of about 8 N are demonstrated. The actuator's capabilities are showcased in two separate demonstrations: a crawling robot and a sensorized artificial muscle that integrates a microfluidic, liquid metal strain sensor. The architected material design approach for a robust, motor‐driven soft actuator provides several unique features—including a compact form factor and ease of use—over other motorized soft robotic actuators based on HSA assemblies or cable tendon mechanisms.

Liquid Metal and Carbon Nanofiber-Based Strain Sensor for Monitoring Gesture, Voice, and Physiological Signals

Liquid Metal and Carbon Nanofiber-Based Strain Sensor for Monitoring Gesture, Voice, and Physiological Signals

An Electronic Skin Strain Sensor for Adaptive Angle Calculation

Electronic skin (e-skin) with excellent flexibility and comfortable wearability has attracted considerable attention in the past decade. Flexible strain sensors are one of the key elements of e-skins, which are always attached to the joints of humans or robots for motion monitoring. However, most strain sensors previously reported cannot fit different joints to measure the bending angles without calibration. This work aims to propose a flexible strain sensor with an adaptive model for automatically calculating the bending angle of different joints. The proposed sensor is a screen-printed liquid metal strain sensor with a size of 120 mm (L) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times20$ </tex-math></inline-formula> mm (W) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 400\mu $ </tex-math></inline-formula> m (H). The real-time changes in different curvatures as the sensor bend has been investigated. Based on the curvature-strain-bending angle relationship of different joints, we use the Gaussian function to model the bending angle solution for adaptive curvature. Experimental results show that the proposed sensor can be served as the electronic skin on the joint’s surface to measure the bending angle adaptively, which provides a promising path for adaptive motion monitoring in human movement, soft robotics, prosthetic devices, etc.

Structural Reinforcement Effect of a Flexible Strain Sensor Integrated with Pneumatic Balloon Actuators for Soft Microrobot Fingers.

Motion capture of a robot and tactile sensing for a robot require sensors. Strain sensors are used to detect bending deformation of the robot finger and to sense the force from an object. It is important to introduce sensors in effective combination with actuators without affecting the original performance of the robot. We are interested in the improvement of flexible strain sensors integrated into soft microrobot fingers using a pneumatic balloon actuator (PBA). A strain sensor using a microchannel filled with liquid metal was developed for soft PBAs by considering the compatibility of sensors and actuators. Inflatable deformation generated by PBAs, however, was found to affect sensor characteristics. This paper presents structural reinforcement of a liquid metal-based sensor to solve this problem. Parylene C film was deposited into a microchannel to reinforce its structure against the inflatable deformation caused by a PBA. Parylene C deposition into a microchannel suppressed the interference of inflatable deformation. The proposed method enables the effective combination of soft PBAs and a flexible liquid metal strain sensor for use in microrobot fingers.

Characterization of a tactile sensor using a small, embedded strain gauge

In recent years, tactile sensors comprising flexible materials have been studied for soft robotics. Several conventional tactile sensors are based on a microchannel filled with liquid metal, for flexibility. In this study, we proposed a soft tactile sensor that is vertically embedded with a liquid metal strain gauge in an elastomer using a narrow wire mold. Despite the narrow and small design, the strain gauge can detect an applied force. In addition, the design has the potential to be arrayed in a dense setting. In this study, we evaluated our proposed tactile sensor with a single strain gauge and confirmed its sensing capability.

Robust, Self-Healable Siloxane Elastomers Constructed by Multiple Dynamic Bonds for Stretchable Electronics and Microsystems

Self-healable siloxane elastomers have recently attracted much interest in research for applications in flexible electronic devices. However, it remains a challenge for self-healable siloxane elastomers to realize good mechanical and rapid high-efficiency self-healable properties at room temperature. Herein, a desirable siloxane polymer elastomer was designed by the synergy of multiple dynamic bonds. In this self-healable system, the disulfide bonds as sacrificial bonds and hydrogen bonds as weak bonds could endow elastomers with rapid self-healability and high stretchability. Whereas, the Fe-coordination bonds could tune the robustness of a siloxane elastomer. Due to the synergy of multiple dynamic bonds, the self-healable PDMS-SS-DOPA1-Fe2 elastomer achieved high stretchability of 1100%, tensile stress of 1.11 MPa, and high self-healable efficiency of 96% (within 3 h). Moreover, after healing for 3 min at room temperature, the damaged siloxane elastomers were able to obtain a breaking strain of 250% and tensile stress of 0.5 MPa. As a proof of concept, based on elastomer films integrated with liquid metal alloy EGaIn (eutectic gallium–indium), stretchable electrodes and strain sensors were subsequently developed, opening the avenue to potential applications in flexible electronics and microsystems.

Measuring Dynamic Shear Force and Vibration With a Bioinspired Tactile Sensor Skin

There is a need to measure dynamic tactile information to enhance robotic hand performance during haptic exploration and object manipulation. Here, we report on the dynamic characterization of a flexible, bioinspired, and resistive microfluidic shear force sensor skin. When the skin is subjected to shear force, one side of the skin experiences tension while the other side experiences compression, both of which are measured by liquid metal strain gauges embedded in polydimethylsiloxane. The sensor can measure dynamic shear forces during stepwise unloading, incipient slip, and controlled vibration tests. The sensor can quantify vibration up to 800 Hz with an average minimum displacement of 0.43 μm, which is equivalent to or better than human fingertips. We demonstrate the utility of the sensor skin by performing experiments with a robotic hand and arm. The shear sensing skin senses tactile events while a robotic arm performs several manipulation tasks including pick and place, drop, and handover. The skin is robust when used on a robot manipulator and shows promise in providing rich and dynamic tactile information that is beneficial to robotic and prosthetic applications.

Bioinspired flexible microfluidic shear force sensor skin

There is a need to gather rich, real-time tactile information to enhance robotic hand performance during haptic exploration and object manipulation. Measuring shear forces is useful for grasping and manipulating objects; however, there are limited effective shear sensing strategies that are compatible with existing end effectors. Here, we report a bioinspired and flexible, resistive microfluidic shear force sensor skin. The sensor skin is wrapped around a finger-shaped end effector and fixed at the location of the nail bed. When the skin is subjected to shear force, one side of the skin experiences tension while the other side experiences compression and bulges similar to a human fingerpad. The tension and compression are measured by liquid metal strain gauges, embedded in PDMS, that are strategically placed adjacent to the nail bed, away from regions of direct finger-object contact. We present the sensor design, a finite element analysis static mechanical characterization model, as well as static response experiments. The resistive shear sensing skin exhibits greater than 10-bit dynamic range (up to 5N) that is insensitive to the applied normal force. The resistive shear sensing skin is intrinsically flexible and immune to fatigue and other problems of solid-state sensors when subjected to repeated, large strains.

Multi-Element Strain Gauge Modules for Soft Sensory Skins

In this paper, we describe the fabrication and testing of a sensory module composed of resistive strain gauges in an elastomer substrate. Each module contains three resistive gauges, providing sufficient information to reconstruct the geometry of the module. The modules are fabricated from two bonded sheets of silicone elastomer. The sensing element is a resistive strain gauge based on room-temperature liquid gallium–indium alloy contained within microchannels in the substrate. We demonstrate the functionality of the module by mechanically stretching it over a template and measuring the change in resistance of the embedded liquid metal strain gauges. Starting with known strains, we calibrate the device and fit a quadratic model. With the model and the measured error distribution, we can predict the uncertainty in the reconstructed position of the corners of the triangular modules, which we refer to as nodes.

Stretching and Twisting Sensing With Liquid-Metal Strain Gauges Printed on Silicone Elastomers

This letter reports ultra-stretchable strain gauges based on a liquid metal (eutectic gallium-indium) and a platinum-catalyzed silicone elastomer (EcoFlex). A custom liquid metal printing setup was constructed and operated in the pressure controlled mode to offer high quality printing by eliminating undulation typically encountered in the flow rate control mode via a syringe pump. Printed liquid-metal strain gauges were thoroughly tested under cyclic uniaxial stretching and twisting. We achieved the stretchability of ~700%, which can cover any high strain from human motion. In case of moderate strain of 350%, our liquid-metal strain gauge could be operated more than 4500 cycles without showing any degradation.

풍력발전기 블레이드 변형 측정을 위한 액체금속 스트레인 게이지 개발

In this paper, the embedding type novel liquid metal strain gauge was developed for measuring the deformation of wind turbine blades. In general, the conventional methods for the SHM have many disadvantages such as frequency distortion in FBG sensors, the low gauge factor and mechanical failures in strain gauges and extremely sophisticated filtering in AE sensors. However, the liquid metal filled in a pre-confined micro channel shows dramatic characteristics such as high sensitivity, flexibility and robustnes! s to environment. To adopt such a high feasibility of the liquid metal in flexible sensor applications, the EGaIn was introduced to make flexible liquid metal strain gauges for the SHM. A micro channeled flexible film fabricated by the several MEMS processes and the PDMS replication was filled with EGaIn and wire-connected. Lots of experiments were conducted to investigate the performance of the developed strain gauges and verify the feasibility to the actual wind turbine blades health monitoring.

Tibiotalocalcaneal Arthrodesis: A Biomechanical Assessment of Stability

Combined ankle and subtalar (tibiotalocalcaneal) arthrodesis is a procedure that can be used to successfully treat disabling foot and ankle arthropathy and is a reasonable salvage alternative to amputation for the treatment of nonbraceable neuropathic, diabetic, degenerative, or rheumatoid joints. Although many methods of tibiotalocalcaneal (TTC) arthrodesis have been described in the literature, the most popular current methods involve the use of crossed cancellous bone screws, plates, or a locked retrograde intramedullary rod. Fusion in these patients can be difficult, with significant complications including infection, malunion, and nonunion. A persistent nonunion can lead to failure of the hardware and recurrent deformity. We biomechanically tested the stability and micromotion in four methods of TTC arthrodesis using liquid metal strain gauges and Instron (Norwood, MA) material testing systems. Anatomically identical synthetic bones with properties very similar to human bone were instrumented and tested. Four instrumentation techniques were tested: 1) three crossed 6.5-mm cancellous screws, 2) two crossed 6.5-mm cancellous screws, 3) locked retrograde intramedullary rod, and 4) locked retrograde intramedullary rod augmented with a single anteromedial bone staple. Six separate specimens for each technique were tested. The three crossed cancellous screw technique provided the greatest stability with respect to micromotion (p < 0.05). The addition of a tibiotalar staple to the locked intramedullary rod conferred stability nearly equal to that of the three crossed cancellous screw fixation (p < 0.05). The locked intramedullary rod group and the two crossed cancellous screw group allowed significant micromotion at the arthrodesis sites, which was a full order of magnitude higher (p < 0.05) than in the three crossed cancellous screw group and the staple augmented intramedullary rod group. Biomechanically, a staple augmented locked intramedullary rod for TTC arthrodesis confers excellent stability nearly equal to the three crossed cancellous screw technique for TTC arthrodesis.

Strain and force transducers used in human and veterinary tendon and ligament biomechanical studies

Biomechanical studies often aim at determining the contribution (in terms of load or strain) of a tendon or ligament in posture, gesture or locomotion. To this end, many transducers have been developed since 30 years. These devices implanted within or attached to the inside of the tendon or ligament must be compliant enough to measure in vivo the tissue load or strain without interfering with the movement of man or animals. They can be transducers with variation of electrical resistance (liquid metal strain gauge, buckle transducer, implantable force transducer and pressure transducer), variation of magnetic field (Hall effect transducer) and variation of light flow (optic fibre). Their use requires surgery in order to implant them and it is limited in time because of their invasive character and the development of fibrous healing reactions. Besides, the transducer dimensions and its position in the tendon can influence the transducer output signal. Moreover, the latter may not reflect the behaviour of the tendon as a whole but only locally. In addition, a calibration is required in order to convert the output signal into a strain or a force. In animals, this calibration is generally made by a post-mortem procedure on dissected anatomical specimens; in man, an indirect calibration procedure using inverse dynamic calculations is generally performed. However, the calibration conditions cannot reproduce exactly the in vivo conditions. So far, only invasive transducers have allowed to measure strain or force in tendons with all constraints and limits mentioned above.

Dynamic fatigue properties of the dental implant–abutment interface: Joint opening in wide-diameter versus standard-diameter hex-type implants

Statement of problem. The clinical long-term success of single-tooth implant restorations depends, in part, on a stable connection between the prosthetic restoration and the implant body. Purpose. The purpose of this experiment was to investigate the fatigue life of UCLA-style abutment screws in wide-diameter versus conventionally sized dental implant restorations. Material and methods. Five 3.75 × 15-mm and five 6.0 × 15-mm hexed dental implants were used. Ten frameworks were fabricated, 5 with a single UCLA-style, 3.75-mm hexed gold alloy cylinder, and 5 with a single UCLA-style, 6.0-mm hexed gold alloy cylinder. To simulate a common laboratory procedure, 2 abutment interfaces were relieved with a one-quarter round bur for both diameters. The 3.75-mm implant used a Gold-Tite central abutment screw torqued to 32 Ncm, and the 6.0-mm implant used a titanium central abutment screw torqued to 25 Ncm. Frameworks were dynamically loaded (~10 Hz) with a 120 ± 10-N, 4-mm off-axis force. Liquid metal strain gauges were used to measure joint opening. Measurements were made at intervals of 103, 104, 105, and 5×105 cycles. Gauge output data were converted to displacement with a conversion factor determined by calibration. Linear regression analysis then was performed. Results. Two observations were made in this study. Two of three 3.75-mm nonadjusted specimens and all three 6.0-mm nonadjusted specimens maintained joint closure (range of opening 0-20 μm) while measured under dynamic loading. The median joint opening at 5×105 cycles for 3.75-mm nonadjusted specimens was 14 ± 7 μm; for 6.0-mm specimens, it was 11 ± 10 μm. Both 3.75-mm adjusted specimens and 1 nonadjusted specimen failed to maintain joint closure (excess joint opening >50 μm). One of the 3.75-mm adjusted specimens had abutment screw fracture. One of two 6.0-mm adjusted specimens failed to maintain joint closure because of screw fracture. Conclusion. The dental implant–abutment interface of 3.75-mm and 6.0-mm externally hexed implants experienced similar joint opening after periods of dynamic loading. Laboratory adjustment of the interface significantly decreased the service life of the abutment screw joint. (J Prosthet Dent 2001;85:599-607.)

Segmental in vivo vertebral kinematics at the walk, trot and canter: a preliminary study.

Understanding the pathophysiology of equine back problems, for clinical evaluation, treatment or injury prevention, requires understanding of the normal 3-dimensional motion characteristics of the vertebral column. Recent studies have investigated regional vertebral kinematics; however, there are no reported measures of direct in vivo segmental vertebral kinematics in exercising horses. Relative movements between 2 adjacent vertebrae were recorded for 3 horses that were clinically sound and did not have a known history of a back problem. A transducer consisting of 2 fixtures and an array of liquid metal strain gauges (LMSGs) was used to measure 3-dimensional segmental vertebral motion. The transducer was attached directly to Steinmann pins implanted in the dorsal spinous processes of adjacent vertebrae in 3 vertebral regions: thoracic (T14 to T16), lumbar (L1 to L3) and lumbosacral (L6 to S2). Rotational displacements between adjacent vertebrae were calculated from the differential outputs of the LMSG array during walk, trot and canter on a treadmill. Peak magnitudes of dorsoventral flexion, lateral bending and axial rotation were recorded continuously for each stride. The largest motion of the 3 instrumented vertebral segments was at the lumbosacral junction. In general, the greatest magnitude of segmental vertebral motion occurred during the canter and the least during the trot. The dynamic and continuous measure of 3-dimensional in vivo segmental vertebral motion provides an important new perspective for evaluating vertebral motion and back problems in horses.

Micromotion and dynamic fatigue properties of the dental implant–abutment interface

Statement of problem. Clinical loading may result in micromotion and metal fatigue in apparently stable implant screw joints. This micromotion may contribute to tissue inflammation and prosthesis failure.Purpose. This study investigated dental implant screw joint micromotion and dynamic fatigue as a function of varied preload torque applied to abutment screws when tested under simulated clinical loading.Material and methods. Fifteen noble alloy single-tooth implant restorations, each containing a hexed UCLA-style gold cylinder, were randomly assigned to 3 preload groups (16, 32, and 48 N·cm). Each group consisted of 5 implants (each 3.75 × 15 mm) and 5 square gold alloy abutment screws. A mechanical testing machine applied a compressive cyclic sine wave load between 20 and 130 N at 6 Hz to a contact point on each implant crown. A liquid metal strain gauge recorded the micromotion of the screw joint interface after 100, 500, 1,000, 5,000, 10,000, 50,000, and 100,000 cycles. Baseline data at 0 N·cm were collected before the application of the specified preload torque.Results. The 16 N·cm group exhibited greater micromotion (P<.001) than both the 32 and 48 N·cm groups at all cycle intervals (2-way ANOVA, Tukey HSD). Micromotion of the implant-abutment interface remained constant (P=.99) for each of the preload groups through 105 cycles.Conclusion. Under the loading parameters of this study, no measurable fatigue of the implant–abutment interface occurred. However, dental implant screw joints tightened to lower preload values exhibited significantly greater micromotion at the implant–abutment interface. (J Prosthet Dent 2001;85:47-52.)

Dynamic Analysis of In Vivo Segmental Spinal Motion: An Instrumentation Strategy

SummarySummary A transducer for measuring threedimensional segmental spinal motion was designed to directly measure dynamic rotations (Rx, Ry and Rz) about three orthogonal axes using an array of liquid metal strain gauges (LMSGs). The configuration of the LMSG array results in differential length changes due to segmental spinal motion. In vitro calibration utilized transducer attachment to Steinmann pins implanted into the dorsal spinous processes of anatomical spinal segments. The response of the LMSGs approximated linearity (R2 ≥0.980) over the calibrated ranges of angular displacement (i.e., ± 5°). On average, artifactual mechanical noise of the LMSGs was &lt;3% of the signal recorded during locomotion. The minimum resolution of the transducer was 0.07 degrees of flexion-extension, 0.46 degrees of lateral bending, and 0.56 degrees of rotation. Average resistive force for all transducers was 0.31 ± 0.05 Nm at the neutral articular position (0°) and 0.51 ± 0.03 Nm at 5° of flexion. Clinically, the modest mechanical resistance of the transducers did not affect spinal mobility nor locomotion. In vivo application of the transducer was demonstrated at thoracolumbar and lumbosacral spinal segments in horses treadmill locomotion. The transducer was designed and tested on an equine model, but may be adapted for other quadrupeds. The dynamic and continuous measure of three-dimensional in vivo segmental spinal motion will provide an important new perspective for evaluating normal and altered spinal motion.A technique was developed for directly measuring threedimensional segmental spinal motion in the thoracolumbar and lumbosacral spinal segments in horses during treadmill locomotion. The dynamic and continuous measure of three-dimensional in vivo segmental spinal motion will provide an important new perspective for evaluating normal and altered spinal motion associated with back problems.

Effect of calcaneal osteotomy and lateral column lengthening on the plantar fascia: a biomechanical investigation.

Medial calcaneal displacement osteotomy or lateral column lengthening fusion has been advocated to augment tendon transfer in planovalgus foot deformity associated with chronic posterior tibial tendon insufficiency. It is hypothesized that plantar fascia tightening occurs with these procedures, helping to restore a more normal longitudinal arch. To investigate this further, nine fresh-frozen cadaver below-knee specimens were used. A flatfoot model was created by sectioning of the posterior tibial tendon, spring ligament, talonavicular capsule, and deltoid ligament. A liquid-metal strain gauge, calibrated to measure fractional changes in length, was sutured proximally to the origin and distally into the thickest portion of the medial band of the plantar fascia. Specimens were axially loaded to 400 N and plantar fascia strain was measured. Fractional length changes in the plantar fascia were then measured after a medial displacement calcaneal osteotomy and after a lateral column lengthening through the calcaneocuboid joint. Tightening of the plantar fascia did not occur with either medial calcaneal displacement or lateral column lengthening. The plantar fascia became significantly less taut with both medial displacement and lateral column lengthening. We found that lateral column lengthening produced significantly looser plantar fascia than did medial displacement of the calcaneal tuberosity.

Mechanical properties of the tendinous equine interosseus muscle are affected by in vivo transducer implantation

Liquid metal strain gauges (LMSGs) were implanted in the tendinous interosseous muscle, also called suspensory ligament (SL), in the forelimbs of 6 ponies in order to quantify in vivo strains and forces. Kinematics and ground reaction forces were recorded simultaneously with LMSG signals at the walk and the trot prior to implantation, and 3 and 4 days thereafter. The ponies were euthanised and tensile and failure tests were performed on the instrumented tendons and on the tendons of the contra lateral limb, which were instrumented post mortem. The origo–insertional (OI) strain of the SL was computed from pre- and post-operative kinematics, using a 2D geometrical model. The LMSG-recorded peak strain of the SL was 5.4±0.9% at the walk and 9.1±1.3% at the trot. Failure occurred at 15.4±2.1% (mean±S.D.). The LMSG strain was higher than the simultaneously recorded OI strain 0.5±0.7% strain at the walk and 2.2±1.1% strain at the trot. Post-operative OI strains were only slightly higher than pre-operative values. Failure strains of in vivo instrumented SLs were 2.0±1.2% strain higher, and failure forces were slightly lower, than those of the contra lateral SLs that were instrumented post mortem. SL strains appeared to be considerably higher than those found in earlier acute experiments. Differences between in vivo LMSG and OI strains, supported by lower failure strains comparing in vivo and post mortem instrumented SLs, revealed that local changes in tendon mechanical properties occurred within 3 to 4 days after transducer implantation. Therefore, measurements of normal physiological tendon strains should be performed as soon as possible after transducer implantation.