Flexible Force Sensor Research Articles

Design and application of a new high-performance flexible six-axis force/torque sensor for massage therapy

Precise sensing of force magnitude and direction in massage therapy can significantly enhance treatment effectiveness. Despite advancements in flexible multidimensional force sensors, achieving comprehensive spatial force sensing with soft materials remains challenging. This study presents a novel flexible six-axis force/torque sensor, calibrated using a deep neural network. The calibrated sensor exhibits a maximum class I error of 0.603%F.S. and a maximum class II error of 0.751%F.S., indicating excellent measurement performance. Demonstrated in massage physiotherapy, the sensor effectively distinguishes between various techniques and accurately detects subtle force variations within the same technique. The present work introduces a novel strategy for designing flexible six-axis force/torque sensors, thereby laying the groundwork for advancements in intelligent robotics, human–computer interfaces, as well as in the fields of rehabilitation and medicine.

Optical Micro/Nanofiber Enabled Multiaxial Force Sensor for Tactile Visualization and Human-Machine Interface.

Tactile sensors with capability of multiaxial force perception play a vital role in robotics and human-machine interfaces. Flexible optical waveguide sensors have been an emerging paradigm in tactile sensing due to their high sensitivity, fast response, and antielectromagnetic interference. Herein, a flexible multiaxial force sensor enabled by U-shaped optical micro/nanofibers (MNFs) is reported. The MNF is embedded within an elastomer film topped with a dome-shaped protrusion. When the protrusion is subjected to vector forces, the embedded MNF undergoes anisotropic deformations, yielding time-resolved variations in light transmission. Detection of both normal and shear forces is achieved with sensitivities reaching 50.7dB N-1 (14% kPa-1) and 82.2dB N-1 (21% kPa-1), respectively. Notably, the structural asymmetry of the MNF induces asymmetrical optical modes, granting the sensor directional responses to four-directional shear forces. As proof-of-concept applications, tactile visualizations for texture and relief pattern recognition are realized with a spatial resolution of 160µm. Moreover, a dual U-shaped MNF configuration is demonstrated as a human-machine interface for cursor manipulation. This work represents a step towards advanced multiaxial tactile sensing.

An ultrasensitive flexible force sensor with nature-inspired minimalistic architecture to achieve a detection resolution and threshold of 1 mN for underwater applications

Highly sensitive flexible force sensors enable precise detection of underwater signals for monitoring biological activity, environmental conditions, and vehicle movement. Multilayer stack assembly and micro/nano structure array are often seen in most force/pressure sensors which are toughly hard to control the interlayer spacing and micro/nano structures alignment precisely, resulting in poor consistency and stability. Herein, we first reported a new force sensor with a nature-inspired minimalistic architecture, addressing such issues in an elegant and surprising approach by using a single-layer arched functional membrane with one microgroove. Inspired by the scorpions’ slit sensilla and mantis’ campaniform sensilla, a highly sensitive and waterproof flexible force sensor was fabricated. It is demonstrated that the force sensor has a sensitivity of 27.6 N−1, a high force resolution (1 mN), a fast response time of 70 ms, excellent stability over 5000 cycles and linearity (0.996), and a small force detection limit (≤ 1 mN), showing great potential in underwater environment sensing and motion monitoring of vehicles. This novel but minimalistic architecture provides a new direction in the development of sensors with advanced performance.

Cilia-Inspired Magnetic Flexible Shear Force Sensors for Tactile and Fluid Monitoring.

Recently, there has been a burgeoning interest in flexible shear force sensors capable of precisely detecting both magnitude and direction. Despite considerable efforts, the challenge of achieving accurate direction recognition persists, primarily due to the inherent structural characteristics and sensing mechanisms. Here, we present a shear force sensor constructed by a magnetically induced assembled Ni/PDMS composite membrane, which is magnetized and integrated with a three-axis Hall sensor, facilitating its ability to simultaneously monitor both shear force magnitude (0.7-87 mN) and direction (0-360°). The cilia-inspired shear force magnetic sensor (CISFMS) exhibits admirable attributes, including exceptional flexibility, high sensitivity (0.76 mN-1), an exceedingly low detection limit (1° and 0.7 mN), and remarkable durability (over 10,000 bending cycles). Further, our results demonstrate the capacity of the CISFMS in detecting tactile properties, fluid velocity, and direction, offering substantial potential for future developments in wearable electronics.

Design and FEA verification of high dimensional and multi-field smart soft material

Abstract We designed a sophisticated smart soft tissue material that simulates the properties of biological soft tissue and detects three-dimensional force. Inspired by electronic skin, this material uses capacitive pressure sensors and multi-layer sensor packaging. Finite element analysis and multi-field simulation were employed for structural design and verification. The study evaluated the theory and practicality of a flexible capacitive 3D force sensor for detecting pressure and shear force. Mechanical and electrical properties were confirmed through force and electric field simulations. The 3D force detection method proved feasible, achieving sensitivity levels of 10−4 kPa −1 in the X and Y directions, and 10−3 kPa −1 in the Z direction, demonstrating the effectiveness of our design.

Inductive Paper-Based Flexible Contact Force Sensor Utilizing Natural Micro-Nanostructures of Paper: Simplicity, Economy, and Eco-Friendliness.

Inductive contact force sensors, known for their high precision and anti-interference capabilities, hold significant potential applications in fields such as wearable and medical monitoring devices. Most of the current research on inductive contact force sensors employed novel nanomaterials as sensitive elements to enhance their sensitivity and other performance characteristics. However, sensors developed through such methods typically involve complex preparation processes, high costs, and difficulty in biodegradation, which limit their further development. This article introduces a new flexible inductive contact force sensor using paper as a sensitive element. Paper inherently possesses micro- and nanostructures on its surface and interior, enabling it to sensitively convert changes in contact force into changes in displacement, making it suitable for use as the sensor's sensitive element. Additionally, the advantages of paper also include its great flexibility, low cost, wide availability, and biodegradability. Performance testing on this flexible sensor showed good repeatability, hysteresis, sensitivity, and consistency. When used in experiments for monitoring human motion and respiration, this sensor also exhibited great detection performance. The proposed inductive paper-based flexible contact force sensor, with its simple structure, easy manufacturing process, cost-effectiveness, eco-friendliness, and good sensing performance, provides new insights into research for contact force sensors.

Flexible wide-range multidimensional force sensors inspired by bones embedded in muscle

AbstractFlexible sensors have been widely studied for use in motion monitoring, human‒machine interactions (HMIs), personalized medicine, and soft intelligent robots. However, their practical application is limited by their low output performance, narrow measuring range, and unidirectional force detection. Here, to achieve flexibility and high performance simultaneously, we developed a flexible wide-range multidimensional force sensor (FWMFS) similar to bones embedded in muscle structures. The adjustable magnetic field endows the FWMFS with multidimensional perception for detecting forces in different directions. The multilayer stacked coils significantly improved the output from the μV to the mV level while ensuring FWMFS miniaturization. The optimized FWMFS exhibited a high voltage sensitivity of 0.227 mV/N (0.5–8.4 N) and 0.047 mV/N (8.4–60 N) in response to normal forces ranging from 0.5 N to 60 N and could detect lateral forces ranging from 0.2–1.1 N and voltage sensitivities of 1.039 mV/N (0.2–0.5 N) and 0.194 mV/N (0.5–1.1 N). In terms of normal force measurements, the FWMFS can monitor finger pressure and sliding trajectories in response to finger taps, as well as measure plantar pressure for assessing human movement. The plantar pressure signals of five human movements collected by the FWMFS were analyzed using the k-nearest neighbors classification algorithm, which achieved a recognition accuracy of 92%. Additionally, an artificial intelligence biometric authentication system is being developed that classifies and recognizes user passwords. Based on the lateral force measurement ability of the FWMFS, the direction of ball movement can be distinguished, and communication systems such as Morse Code can be expanded. This research has significant potential in intelligent sensing and personalized spatial recognition.

Flexible Force Sensor Based on a PVA/AgNWs Nanocomposite and Cellulose Acetate.

Nanocomposites are materials of special interest for the development of flexible electronic, optical, and mechanical devices in applications such as transparent conductive electrodes and flexible electronic sensors. These materials take advantage of the electrical, chemical, and mechanical properties of a polymeric matrix, especially in force sensors, as well as the properties of a conductive filler such as silver nanowires (AgNWs). In this work, the fabrication of a force sensor using AgNWs synthesized via the polyol chemical technique is presented. The nanowires were deposited via drop-casting in polyvinyl alcohol (PVA) to form the active (electrode) and resistive (nanocomposite) sensor films, with both films separated by a cellulose acetate substrate. The dimensions of the resulting sensor are 35 mm × 40 mm × 0.1 mm. The sensor shows an applied force ranging from 0 to 3.92 N, with a sensitivity of 0.039 N. The sensor stand-off resistance, exceeding 50 MΩ, indicates a good ability to detect changes in applied force without an external force. Additionally, studies revealed a response time of 10 ms, stabilization of 9 s, and a degree of hysteresis of 1.9%. The voltage response of the sensor under flexion at an angle of 85° was measured, demonstrating its functionality over a prolonged period. The fabricated sensor can be used in applications that require measuring pressure on irregular surfaces or systems with limited space, such as for estimating movement in robot joints.

Flexible Tactile Sensors for 3D Force Detection.

As tactile force sensing has become increasingly significant in the field of machine haptics, achieving multidimensional force sensing remains a challenge. We propose a 3D flexible force sensor that consists of an axisymmetric hemispherical protrusion and four equally sized quarter-circle electrodes. By simulating the device using a force and electrical field model, it has been found that the magnitude and direction of the force can be expressed through the voltage relationship of the four electrodes when the magnitude of the shear force remains constant and its direction varies within 0-360°. The experimental results show that a resolution of 15° can be achieved in the range 0-90°. Additionally, we installed the sensor on a robotic hand, enabling it to perceive the magnitude and direction of touch and grasp actions. Based on this, the designed 3D flexible tactile force sensor provides valuable insights for multidimensional force detection and applications.

Implementing A Flexible Sensor to Identify Forces during Instrument-Assisted Soft Tissue Mobilization

Instrument-assisted soft tissue mobilization (IASTM) techniques use specialized hand-held instruments for applying controlled mechanical forces to the body with the goal of facilitating healing, improving range of motion, and reducing pain. Nevertheless, an optimal range of forces for achieving clinical outcomes has yet to be established. A barrier to advancing research on IASTM force optimization is the lack of commercially available instruments that quantify treatment forces. The aim of the current study was to assess the feasibility of attaching a flexible force sensor to a commercially available IASTM instrument to obtain valid force measurements. The validity of this novel approach was assessed by comparing data between the flexible force sensor and a force plate during a simulated treatment. Intraclass correlation coefficients, linear regression models, and Bland Altman plots all indicated excellent agreement between the force plate and flexible sensor when the instrument was used at 45°, 65°, and 90° treatment angles. Agreement between measures decreased when the instrument was held at 30°. Thus, commercially available instruments with attached sensors could make force measurement more accessible and feasible for a wider range of research settings, facilitating the advancement of IASTM research and ultimately informing clinical decision-making to improve patient care.

Reviews on Flexible Force Sensors Based on Fiber Assemblies with Mass Production Efficiency

AbstractFlexible force sensors show great potential applications in smart wearable devices based on their softness and ease of deformation. Textiles are extremely compatible with the human body, where mechanical forces generated by human movements and physiological activities create rich application scenarios for flexible force sensors. Currently, the bottleneck in the development of flexible sensors depends on the breakthrough of mass production efficiency, which has been a research focus in recent years to accelerate the application of smart textiles in various fields. This work reviews the research progress of flexible force sensors based on fiber assemblies with mass production efficiency, which is discussed in four aspects: sensing mechanisms, structure design, preparation methods, and application. Finally, the challenges faced by mass production of flexible force sensors are analyzed and summarized, and a few inspirations for their future development direction are provided.

Flexible Three-Dimensional Force Tactile Sensor Based on Velostat Piezoresistive Films.

The development of a high-performance, low-cost, and simply fabricated flexible three-dimensional (3D) force sensor is essential for the future development of electronic skins suitable for the detection of normal and shear forces for several human motions. In this study, a sandwich-structured flexible 3D force tactile sensor based on a polyethylene-carbon composite material (velostat) is presented. The sensor has a large measuring range, namely, 0-12 N in the direction of the normal force and 0-2.6 N in the direction of the shear force. For normal forces, the sensitivity is 0.775 N-1 at 0-1 N, 0.107 N-1 between 1 and 3 N, and 0.003 N-1 at 3 N and above. For shear forces, the measured sensitivity is 0.122 and 0.12 N-1 in x- and y-directions, respectively. Additionally, the sensor exhibits good repeatability and stability after 2500 cycles of loading and releasing. The response and recovery times of the sensor are as fast as 40 and 80 ms, respectively. Furthermore, we prepared a glove-like sensor array. When grasping the object using the tactile glove, the information about the force applied to the sensing unit can be transmitted through a wireless system in real-time and displayed on a personal computer (PC). The prepared flexible 3D force sensor shows broad application prospects in the field of smart wearable devices.

Flexible 3D Force Sensor Based on Polymer Nanocomposite for Soft Robotics and Medical Applications.

The three-dimensional (3D) force sensor has become essential in industrial and medical applications. The existing conventional 3D force sensors quantify the three-direction force components at a point of interest or extended contact area. However, they are typically made of rigid, complex structures and expensive materials, making them hard to implement in different soft or fixable industrial and medical applications. In this work, a new flexible 3D force sensor based on polymer nanocomposite (PNC) sensing elements was proposed and tested for its sensitivity to forces in the 3D space. Multi-walled carbon nanotube/polyvinylidene fluoride (MWCNT/PVDF) sensing element films were fabricated using the spray coating technique. The MWCNTs play an essential role in strain sensitivity in the sensing elements. They have been utilized for internal strain measurements of the fixable 3D force sensor's structure in response to 3D forces. The MWCNT/PVDF was selected for its high sensitivity and capability to measure high and low-frequency forces. Four sensing elements were distributed into a cross-beam structure configuration, the most typically used solid 3D force sensor. Then, the sensing elements were inserted between two silicone rubber layers to enhance the sensor's flexibility. The developed sensor was tested under different static and dynamic loading scenarios and exhibited excellent sensitivity and ability to distinguish between tension and compression force directions. The proposed sensor can be implemented in vast applications, including soft robotics and prostheses' internal forces of patients with limb amputations.

Highly Sensitive and Fast Responding Flexible Force Sensors Using ZnO/ZnMgO Coaxial Nanotubes on Graphene Layers for Breath Sensing.

The authors report the fabrication of highly sensitive, rapidly responding flexible force sensors using ZnO/ZnMgO coaxial nanotubes grown on graphene layers and their applications in sleep apnea monitoring. Flexible force sensors are fabricated by forming Schottky contacts to the nanotube array, followed by the mechanical release of the entire structure from the host substrate. The electrical characteristics of ZnO and ZnO/ZnMgO nanotube-based sensors are thoroughly investigated and compared. Importantly, in force sensor applications, the ZnO/ZnMgO coaxial structure results in significantly higher sensitivity and a faster response time when compared to the bare ZnO nanotube. The origin of the improved performance is thoroughly discussed. Furthermore, wireless breath sensing is demonstrated using the ZnO/ZnMgO pressure sensors with custom electronics, demonstrating the feasibility of the sensor technology for health monitoring and the potential diagnosis of sleep apnea.

Highly Sensitive Paper-Based Force Sensors with Natural Micro-Nanostructure Sensitive Element.

Flexible paper-based force sensors have garnered significant attention for their important potential applications in healthcare wearables, portable electronics, etc. However, most studies have only used paper as the flexible substrate for sensors, not fully exploiting the potential of paper's micro-nanostructure for sensing. This article proposes a novel approach where paper serves both as the sensitive element and the flexible substrate of force sensors. Under external mechanical forces, the micro-nanostructure of the conductive-treated paper will change, leading to significant changes in the related electrical output and thus enabling sensing. To demonstrate the feasibility and universality of this new method, the article takes paper-based capacitive pressure sensors and paper-based resistive strain sensors as examples, detailing their fabrication processes, constructing sensing principle models based on the micro-nanostructure of paper materials, and testing their main sensing performance. For the capacitive paper-based pressure sensor, it achieves a high sensitivity of 1.623 kPa-1, a fast response time of 240 ms, and a minimum pressure resolution of 4.1 Pa. As for the resistive paper-based strain sensor, it achieves a high sensitivity of 72 and a fast response time of 300 ms. The proposed new method offers advantages such as high sensitivity, simplicity in the fabrication process, environmental friendliness, and cost-effectiveness, providing new insights into the research of flexible force sensors.

Bionic octopus-like flexible three-dimensional force sensor for meticulous handwriting recognition in human-computer interactions

With the advantages of high flexibility, multifunctional integration, and wearing comfort, flexible tactile sensors hold great potential for application in the field of intelligent sensing system and human-computer interaction. However, the high sensitivity acquisition and the issue of decoupling of complex tactile signals, and improving compatibility of flexible tactile sensors in human-computer interfaces are still serious challenges in current research. In this study, drawing inspiration from the body structure of an octopus, we developed a flexible three-dimensional (3D) force sensor in which a CNT/Ecoflex conductive elastomer as the body of the bionic octopus-like sensor could capture the normal component of 3D spatial force, meanwhile a laser-induced graphene sensing film transferred to Ecoflex (LIG/Ecoflex) as the tactile wrists of the bionic octopus sensor could discern the magnitude and direction of the tangential component of the 3D spatial force. The bionic sensor offered excellent sensing and wearable characteristics, including fast and efficient response, cyclic stability, and flexible skin-fit performance. The unique structural design of the sensor enabled spatial force sensing capability and precise decoupling and output of 3D forces through the use of a multilayer perceptron (MLP) for mapping and calibration. The octopus-inspired flexible 3D force sensors were further attached to the fingertips of human fingers for successful extraction of handwriting minutia of wearer, showcasing their promising potential in areas such as smart wearables, handwriting recognition, and interactive human-computer interfaces.

Mineral Tanning‐Inspired Metal Ions Coordination Hydrogels with Outstanding Mechanical Strength and Toughness for Flexible Force Sensors

AbstractBy virtue of reversible energy dissipation and good ionic conductivity, metal ions have been extensively introduced to construct hydrogels. However, the traditional soaking method to prepare metal ions coordination hydrogels (MI‐HG) faces the challenge of diffusion and combination of metal ions, leading to weak mechanical properties. Inspired by the leather mineral tanning mechanism, a strategy that controls the accessibility of hydrogels by pickling and basifying processes thus significantly improving the strength and toughness of MI‐HG is proposed. To achieve this, polyvinyl alcohol/poly(acrylamide‐co‐acrylic acid) (PVA/PAM‐co‐PAA) hydrogels‐bearing ligands similar to those of leather collagen are fabricated. Zr4+, Cr3+, Al3+, and Fe3+ commonly used for mineral tanning are chosen as the metal source. The results demonstrate that the mechanical properties of the products (MI‐HGTanning) are significantly promoted, especially for Cr3+ and Zr4+. For example, the tensile strength of Zr‐HGTanning and toughness of Cr‐HGTanning reach 7.79 ± 0.41 Mpa and 28.01 ± 3.1 MJ m−3, approximately three and seven times that of their soaking samples. From macro to micro characterization and by theoretical simulation, the mineral tanning mechanism of metal ions first permeating and then effectively combining with carboxylates, thus improving the mechanical performance of MI‐HGTanning is deciphered, the products of which are promised applications in flexible force sensors.

Pressure-insensitive magnetic shear force sensor with pillar structure for versatile application

A flexible shear force sensor, exhibiting pressure-insensitivity, high sensitivity, and bidirectional measurement capabilities, is developed using a slender and flexible planar hall magnetoresistive (PHMR) sensing element combined with a pillar structure. The pillar structure, placed atop the PHMR sensing element, minimizes the influence of normal force on the sensor, enhancing its shear force sensing ability. This sensor features a 2.5 mm diameter pillar structure and measures bidirectional shear forces within a dynamic range of -0.1 N to 0.1 N, with a resolution of 0.4 mN and a sensitivity of 10 mV/N. The vertical force sensitivity is 500 times smaller than the shear force sensitivity, ensuring precise shear force measurements. This design simplifies signal monitoring, enabling accurate and precise measurements. Our sensor exhibits exceptional repeatability and nonhysteresis in shear forces, affirming its stability and reliability. These characteristics establish its suitability for measuring water and air flow velocities and various experimental applications, including braille reading. This shear force sensor, showcased in our study, demonstrates consistent performance across diverse environments, indicating its potential for applications requiring precise control and detection, such as robotic fingers, medical devices, and wearable monitoring systems.

Glycine/alginate-based piezoelectric film consisting of a single, monolithic β-glycine spherulite towards flexible and biodegradable force sensor.

Development of piezoelectric biomaterials with high piezoelectric performance, while possessing excellent flexibility, biocompatibility, and biodegradability still remains a great challenge. Herein, a flexible, biocompatible and biodegradable piezoelectric β-glycine-alginate-glycerol (Gly-Alg-Glycerol) film with excellent in vitro and in vivo sensing performance was developed. Remarkably, a single, monolithic β-glycine spherulite, instead of more commonly observed multiple spherulites, was formed in alginate matrix, thereby resulting in outstanding piezoelectric property, including high piezoelectric constant (7.2 pC/N) and high piezoelectric sensitivity (1.97 mV/kPa). The Gly-Alg-Glycerol film exhibited superior flexibility, enabling complex shape-shifting, e.g. origami pigeon, 40% tensile strain, and repeated bending and folding deformation without fracture. In vitro, the flexible Gly-Alg-Glycerol film sensor could detect subtle pulse signal, sound wave and recognize shear stress applied from different directions. In addition, we have demonstrated that the Gly-Alg-Glycerol film sensor sealed by polylactic acid and beeswax could serve as an in vivo sensor to monitor physiological pressure signals such as heartbeat, respiration and muscle movement. Finally, the Gly-Alg-Glycerol film possessed good biocompatibility, supporting the attachment and proliferation of rat mesenchymal stromal cells, and biodegradability, thereby showing great potential as biodegradable piezoelectric biomaterials for biomedical sensing applications.

A wireless fluorescent flexible force sensor based on aggregation-induced emission doped liquid crystal elastomers.

Flexible strain sensors have drawn a lot of interest in various applications including human mobility tracking, rehabilitation/personalized health monitoring, and human-machine interaction, but suffer from interference of electromagnetic (EM). To overcome the EM interference, flexible force sensors without sensitive electronic elements have been developed, with drawbacks of bulky modules that hinders their applications in remote measurement with power-free environment. Therefore, it is highly desirable to fabricate a compact wireless flexible force sensor but it is still a challenge. Here, we demonstrate a fluorescent flexible force sensor based on aggregation-induced emission (AIE) doped liquid crystal elastomer (LCE) experimentally. The proposed force sensor film can be used to measure force through the variation of fluorescent intensity induced by the extension or contraction of LCE film, which leads to reduce or increase of the aggregation degree of AIE molecules within. This compact wireless force sensor features lightweight, low-cost, high flexibility, passivity and anti-EM interference, which also enables the naked eye observation. The proposed sensor provides inspiration and a platform for a new concept of non-contact detection, showing application potential in human-friendly interactive electronics and remote-control integration platform.