Stretchable Tactile Sensor Research Articles

Flexible tactile sensing of magnetic hydrogel composites based on electrical impedance tomography

Flexible and stretchable tactile sensors show promising applications in robotics, prosthetics, and wearable electronics. This paper demonstrates an electrical impedance tomography (EIT) based highly flexible, touch-sensitive pressure sensor using magnetic hydrogels. EIT technology has been explored for continuous distributed strain sensing and tactile imaging. Magnetic hydrogel composites are synthesized by simultaneous polymerization and crosslinking of acrylamide with magnetic carbonyl iron particles (CIPs). The effects of CIPs percentage on morphology, sensitivity, and electrical/mechanical properties is investigated. EIT accurately discerns pressure quantity, shape, and spatial distribution. Lower CIPs volume fractions (3%) exhibit maximum sensitivity to strain fields. Mechanical testing show the hydrogels have excellent fatigue resistance, ideal for repeated compression loading. The results demonstrate the potential of EIT with magnetic hydrogel composites for tactile sensing and imaging, with applications in soft robotics, prosthetics, and wearable electronics. The tunable sensitivity and multifunctionality make these soft composites promising materials for flexible electronics.

Development of Kirigami‐Patterned Stretchable Tactile Sensor Array with Soft Hinges for Highly Sensitive Force Detection

AbstractFlexible and stretchable tactile sensors are attracted in the fields of soft robotics, wearable electronics, and healthcare monitoring. The sensing performance of tactile sensors is commonly affected by external deformations like stretching, bending, and twisting, thus they may fail to function on deformable object surfaces. This paper presents a stretchable tactile sensor array using kirigami‐patterned structural design and soft hinges to reduce the influences of deformation. The kirigami pattern of sensor array is parametrically studied to achieve the required expansion patterns. Laser engraving is employed to modify the micropillars on the force‐sensitive rubber surface to increase the sensitivity. Characterization tests show that the sensor array has high sensitivity (≈1.49 × 10−1 kPa−1) for force sensing, and the stretching and bending deformation have almost negligible effects on sensing performance. Under 40% stretching or 180° bending conditions, the measured resistance changes (ΔR/R0) is ≈0.03 and 0.06, respectively. To demonstrate the capability of developed sensor array, it is mounted on an expandable balloon surface for force detection. The recorded signals changed less than 1.5% during expanding process while rapidly rose under applied force, which indicated that the sensor array has the potential to effectively function on complex and deforming surfaces.

Simple and Effective Patterning Method of Liquid‐Metal‐Infused Sponge Electrode for Fabricating 3D Stretchable Electronics

AbstractStretchable electrodes have emerged as a promising technology for soft robotics and wearable devices. One fabrication method that guarantees sustained elasticity and high conductivity is directly filling or printing an elastic substrate with liquid metal (LM). However, it is difficult to create patterns with complex or 3D shapes, such as hole patterning, integration electrodes, and flexible breadboards. Therefore, a simple and effective patterning method for fabricating 3D stretchable electronics using LM electrodes is proposed in this study. The LM‐infused sponge electrode is fabricated by loading LM onto a polydimethylsiloxane (PDMS) sponge, which is fabricated using the salt‐leaching method in PDMS patterned by replica molding. The LM‐infused sponge electrode exhibits a minimal change in terms of resistivity under strain, bending, and twisting (∆R < 0.01 Ω cm−1). This enables various devices to be driven by the production of flexible breadboards, which makes it possible to integrate the proposed model into commercial electronics. Finally, a stretchable tactile sensor (self‐capacitive and self‐powered) is fabricated by integrating stretchable electrodes using only the LM‐infused sponges. In the future, stretchable electrodes fabricated in this manner can be used in various applications, such as soft robots and wearable devices.

An Artificial Intelligence‐Motivated Skin‐Like Optical Fiber Tactile Sensor

Soft and stretchable tactile sensors have received extensive attention for their potential applications in wearables, human–robot interaction, and intelligent robots. Herein, inspired by the functions of skin somatosensory signal generation and processing, an artificial intelligence‐motivated skin‐like optical fiber tactile (SOFT) sensor is proposed. It features multifunctional touch interaction capabilities including tactile amplitude and position and tensile strain. Four fiber Bragg gratings (FBGs) are embedded in a skin‐like three‐layer laminate structure of the SOFT sensor, forming a flexible tactile sensing array with a stretchability larger than 20%. Fusing the two‐level cascaded neural network, the position and magnitude of the contact force can be distinguished simultaneously. The recognition accuracy for contact position is up to 92.41% and the error is less than 4.2% within the force range of 0–3.5 N. Several SOFT sensor‐based interactive applications including pressure password interface and music playback are achieved by combining the artificial intelligence spatiotemporal dynamic logic analysis. Furthermore, the sensor is also capable of complex scenes involving tension and tactile sensing, such as dexterous hand perception and human–robot interaction control. This work provides novel insights into artificial intelligence‐based integrated skin that shows broad promise in intelligent prosthetics and bionic robotic.

Plasticized PVC‐Gel Single Layer‐Based Stretchable Triboelectric Nanogenerator for Harvesting Mechanical Energy and Tactile Sensing (Adv. Sci. 22/2022)

Stretchable Triboelectric Nanogenerators In article number 2201070, Jin Woo Bae, Ju-Hyuck Lee, and co-workers report a plasticized poly(vinyl chloride) (PVC)-gel single layer based stretchable triboelectric nanogenerator for mechanical energy harvesting and tactile sensing. The plasticized PVC-gel is transparent and stretchable, has highly tribo-negative property, and has improved electrical conductivity. It provides a novel approach of stretchable tactile sensor that can sense position and pressure without complicate electrode pattern.

Elongatable Gripper Fingers With Integrated Stretchable Tactile Sensors for Underactuated Grasping and Dexterous Manipulation

The ability to grasp a wider range of objects in size and shape directly relates to the performance of robotic grippers. Adapting to complex geometries of objects requires large degrees of freedom to allow complex configurations. However, complexity in controlling many individual joints leads to introduction of underactuated mechanisms, in which traditional finger designs composed of revolute joints allow only flexion/extension motions. In this article, we propose a length-adjustable linkage mechanism in the underactuated finger controlled by an antagonistic tendon pair. The resulting gripper can elongate the fingers for an increased task space or shorten them for a finer spatial resolution. For tactile sensing, hyperelastic soft sensors are used to stretch with finger elongation. Contact pressures measured by the soft sensors are used in force-feedback control for which either the joint angles or the link lengths are adjusted. Lastly, a multimodal control scheme that combines elongation and flexion modes is demonstrated with tasks of dexterous manipulation.

Active Perception in Non-Visual Recognition Environments by Stretchable Tentacle Sensor Arrays.

Smoke fog or other light-interference environments have intrinsic obstruction for visual recognition techniques to explore objects and surroundings. Alternatively, tactile perceptions, rather than visual observations, are commonly used by burrowing or deep-sea animals to communicate with environments. Bio-inspired by this natural wisdom, here, we demonstrate stretchable tentacle sensor arrays, which can recognize surrounding objects located in non-visual conditions such as smoke fog or dark environment. Each tentacle sensor is composed of two functional parts: a retractable tentacle with a magnetic top and an elastomer bottom containing copper coils. Different from traditionally passive tactile sensors, these tentacle sensors can actively stretch under the control of a syringe pump, yielding different electrical signals when in contact with the objects. Analyzing collected sensing signals of those tactile sensor arrays by the feature analysis model, complex morphological information of irregular objects in the smoke fog can be recognized. Our study reveals a fundamental connection between stretchable tactile sensors and feature analysis and demonstrates its practical potential for active perception in a non-visual recognition environment.

Flexible and Stretchable Fiber‐Shaped Triboelectric Nanogenerators for Biomechanical Monitoring and Human‐Interactive Sensing

AbstractWearable, flexible, and even stretchable tactile sensors, such as various types of electronic skin, have attracted extensive attention, which can adapt to complex and irregular surfaces, maximize the matching of wearable devices, and conformally apply onto human organs. However, it is a great challenge to simultaneously achieve breathability, permeability, and comfortability for their development. Herein, mitigating the problem by miniaturizing and integrating the sensors is tried. Highly flexible and stretchable coaxial structure fiber‐shaped triboelectric nanogenerators (F‐TENGs) with a diameter of 0.63 mm are created by orderly depositing conductive material of silver nanowires/carbon nanotubes and encapsulated polydimethylsiloxane onto the stretchable spandex fiber. As a self‐powered multifunctional sensor, the resulting composite fiber can convert mechanical stimuli into electrical signals without affecting the normal human body. Moreover, the F‐TENGs can be easily integrated into traditional textiles to form tactile sensor arrays. Through the tactile sensor arrays, the real‐time tactile trajectory and pressure distribution can be precisely mapped. This work may provide a new method to fabricate fiber‐based pressure sensors with high sensitivity and stretchability, which have great application prospects in personal healthcare monitoring and human–machine interactions.

Graphene–Polyurethane Coatings for Deformable Conductors and Electromagnetic Interference Shielding

AbstractElectrically conductive, polymeric materials that maintain their conductivity even when under significant mechanical deformation are needed for actuator electrodes, conformable electromagnetic shielding, stretchable tactile sensors, and flexible energy storage. The challenge for these materials is that the percolated, electrically conductive networks tend to separate even at low strains, leading to significant piezoresistance. Herein, deformable conductors are fabricated by spray‐coating a nitrile substrate with a graphene–elastomer solution. The electrical resistance of the coatings shows a decrease after thousands of bending cycles and a slight increase after repeated folding‐unfolding events. The deformable conductors double their electrical resistance at 12% strain and are washable without changing their electrical properties. The conductivity–strain behavior is modeled by considering the nanofiller separation upon deformation. To boost the conductivity at higher strains, the production process is adapted by stretching the nitrile substrate before spraying, after which it is released. This adaption meant that the electrical resistance doubles at 25% strain. The electrical resistance is found sufficiently low to give a 1.9 dB µm−1 shielding in the 8–12 GHz electromagnetic band. The physical and electrical properties, including the electro magnetic screening, of the flexible conductors, are found to deteriorate upon cycling but can be recovered through reheating the coating.

Stretchable tactile sensor with high sensitivity and dynamic stability based on vertically aligned urchin-shaped nanoparticles

Stretchable tactile sensor (STS) is promising for wearable electrical devices, human-machine interfaces, and electronic skin. However, developing a STS based on piezoresistive composite high-pressure sensitivity and dynamic stability remains challenging because stretching deformation destroys the original dispersed state of conductive fillers. This interference of stretching strain on the pressure sensing greatly reduces device performance. Here, we realize an STS based on a piezoresistive composite with different elastic modulus in its functional regions. The composite contains high elastic modulus region (59.1 MPa) of vertically aligned columns of urchin-shaped nanoparticles, and low elastic modulus region (2.4 MPa) of pure matrix. The sensor exhibits high-pressure sensitivity (12.05 kPa−1) owing to the increased conductive contact area between urchin-shaped nanoparticles in the high elastic modulus region. While stretching to 400% strain, the sensor exhibits excellent dynamic stability via strain accommodation in the low elastic modulus region. Our design to separate sensing from multiple stimulus by elastic modulus regulation is easy operative and universal. In addition, the sensor has a low hysteresis coefficient (5.25%), a good detection limit (22 mg), a low response/recovery time (<50 ms), and an excellent mechanical durability (cycled 10,000 times). Finally, we demonstrate the use of our STS for several important stretchable electronic applications to show the feasibility of our design.

Conformable core-shell fiber tactile sensor by continuous tubular deposition modeling with water-based sacrificial coaxial writing

Stretchable tactile sensors are popular in soft and conformable electronics, and innovations in their architectural design and manufacturing process are in demand for more advanced applications. In this work, a resistive-type core-shell tactile sensor was designed and fabricated via the coaxial ink writing of a continuous single core-shell fiber. Particularly, a facile tubular deposition modeling method was established, in which deionized (DI) water was selected as the core and first coextruded with the elastomeric shell and then scarified and replaced by injecting liquid metal (LM). To weave the continuous single core-shell fiber, tactile sensing nodes were formed easily and precisely to position-registered mechanical pressure stimuli. The resistive responses including sensitivity, coupling and reliability of the sensor architecture were examined, and the tracing mode was performed. Furthermore, the tactile sensor could be directly patterned on a 3D surface. The core-shell fiber-based tactile sensor design and its scalable manufacturing are significant for the advanced fabrication of 3D printed conformable electronics.

Ag nanowire-based transparent stretchable tactile sensor recognizing strain directions and pressure

In the last decade, extensive studies have been conducted to realize the functions of human skin based on stretchable electronics. An artificial skin, recognizing complex mechanical stimulation including pressure, strain and shear, and composed of transparent material, is an essential goal but has hardly been achieved. We fabricated a transparent integrated sensor system that can sense the strain direction and normal pressure of applied mechanical stimulation. Each sensor is composed of micropatterned Ag nanowire, forming a composite stretchable conductor with a block copolymer elastomer. The micropatterning and transfer process using thermoplastic elastomer facilitates the transparent conductor to show high transmittance with low sheet resistance at the same time. The designed transparent strain sensor responds linearly to strain, but does not respond to the orthogonal direction, which enables it to have strain-directionality. The applied mechanical signal, comprising normal force and directional strain, can be interpreted through the electrical signal observed from integrated sensors.

A Review of Printable Flexible and Stretchable Tactile Sensors.

Flexible and stretchable tactile sensors that are printable, nonplanar, and dynamically morphing are emerging to enable proprioceptive interactions with the unstructured surrounding environment. Owing to its varied range of applications in the field of wearable electronics, soft robotics, human-machine interaction, and biomedical devices, it is required of these sensors to be flexible and stretchable conforming to the arbitrary surfaces of their stiff counterparts. The challenges in maintaining the fundamental features of these sensors, such as flexibility, sensitivity, repeatability, linearity, and durability, are tackled by the progress in the fabrication techniques and customization of the material properties. This review is aimed at summarizing the recent progress of rapid prototyping of sensors, printable material preparation, required printing properties, flexible and stretchable mechanisms, and promising applications and highlights challenges and opportunities in this research paradigm.

3D Printable Soft Conductive Nanocomposites

3D printing is getting remarkable consideration in recent years in miscellaneous areas and being focused in many hi-tech future possibilities in engineering 1,2. 3D printable conductive polymer composites are gaining massive interest due to their impending application in the field of robotics, tactile, optical displays, microelectrochemical systems and in numerous other applications 3-5. Here in this work we prepared new types of 3D printable nanocomposites with tunable flexibility employing ionic liquid (IL) and micro-walled carbon nanotube (MWCNT) in polymer matrix. The developed soft nanocomposites demonstrated high conductivity and mechanical flexibility and diverse strain%. Thermal properties of the nanocomposites were studied by TGA and we found that thermal stability were very high for all nanocomposites regardless of the composition. Dynamic mechanical analysis (DMA) revealed information about their transition temperature for softening and by modifying MWCNT and polymeric content it is possible to tune this temperature according to desired requirements. The nanocomposites were printed by FDM 3D printer freely and on nonconductive printed base substrate according to the required shape. We also investigated their mechanical and conductive properties by tuning the composition of the constituents in the nanocomposites and discussed about the critical compositions and limitations for 3D printing techniques. These newly developed nanocomposites are expected to create pronounced impact in electromechanical fields to develop soft robotics, actuator and sensor devices. References Kalsoom, U., Nesterenko, P. N. & Paull, B. Recent developments in 3D printable composite materials. RSC Adv. 6, 60355–60371 (2016).Gross, B. C., Erkal, J. L., Lockwood, S. Y., Chen, C., Spence, D. M. Evaluation of 3D Printing and Its Potential Impact on Biotechnology and the Chemical Sciences. (2014).Wei, X., Li, D., Jiang, W., Gu, Z., Wang, X., Zang, Z., Sun, Z., 3D Printable Graphene Composite. Publ. Gr. 1–7 (2015). doi:10.1038/srep11181Guo, S., Qiu, K., Meng, F., Park, S. H. & Mcalpine, M. C. 3D Printed Stretchable Tactile Sensors. Mater. 29, 1701218 (2017).Muth, J. T., Vogt, D. M., Truby, R. L., Mengüç, Y.,Kolesky, D. B., Wood, R. J., Lewis, J. A., Embedded 3D Printing of Strain Sensors within Highly Stretchable Elastomers. Mater. 26, 6307–6312 (2014). Figure 1

Metal Deposition on a Self‐Generated Microfibril Network to Fabricate Stretchable Tactile Sensors Providing Analog Position Information

Stretchable conductors and sensors have attracted great attention for use in electronic skin and healthcare monitoring. Despite the development of many stretchable conductors, there are still very few studies that utilize the conventional methods making electrodes and circuits used in current industry. A method is proposed to fabricate a stretchable electrode pattern and a stretchable tactile sensor by simply depositing linear metal lines through a mask on a stretchable substrate. A method is developed of a self-generating microfibril network on the surface of stretchable block copolymer substrates. The formation mechanism of the microfibril network is studied with finite element method simulations. Metals (Au and Ag nanowires) are deposited directly on the substrate through a patterned mask. This study shows that strain-insensitive circuit and strain-sensitive sensor can be fabricated in a controlled way by adjusting the thickness of the deposited metal, which makes it easy to fabricate a tactile sensor by metal deposition. Also, by using the characteristic that the sensor has different sensitivity depending on the line pattern width, a novel sensor structure simultaneously providing analog-type position information and pressure value is proposed.

3D Printed Wearable Glucose Sensors

School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164 Wearable biosensor technology has attracted intensive research interests since it provides accurate monitoring of physiological samples for diagnosing, managing and treating disease and monitoring individual physical health without extracting blood sample. The most common detection technique depends on frequently correcting blood samples for measurement or continuous monitoring devices, which are expensive, painful and inconvenient. Thus, extensive efforts have been devoted to develop noninvasive sensors that rely on electrical and electrochemical techniques. Mechanical properties are important to wearable devices since they directly contact with human skin, which should need the Young’s modulus around 130–657 kPa [1, 2]. To meet this requirement, electrode materials should hold high viscosity at low-shear rates (103 to 105 Pa s) with shear thinning and viscoelastic behavior with high extensional relaxation times (>1 s), limiting their processing using common electrode fabrication technology. 3D printing technologies typically can process the materials with high concentration as well as high viscosity and shear-thinning behavior; utilize stretchable polymer composites with conductive compounds and have ability to construct complex, customized 3D geometries [3-4]. It can easily employ thin conductive films on stretchable substrates. In our study, our micro additive manufacturing of wearable glucose/lactase biosensor shows the advantage over conventional methods, including: (1) feasible design of objects with higher levels of integrate enzyme/electrode co-production (layer by layer), and (2) appropriate for modifications due to fine resolution and significant materials saving. This will enable us to achieve high conductivity and physiochemical stability of electrode on flexible substrates as well as thinner, higher aspect ratio structures with ideal three-dimensional stacking. We found that the sensitivity of our wearable sensor was much higher than that of traditional printed biosensor (~ 3 folds) due to the extreme fine and less defected surface area printed by DIW technique, which resulting in uniform surface carbon nanoparticle distribution. Besides, our wearable sensors show excellent reproducibility and long-time stability. Reference (1) Guo S Z, Qiu K, Meng F, et al. 3D Printed Stretchable Tactile Sensors[J]. Advanced Materials, 2017. (2) Muth J T, Vogt D M, Truby R L, et al. Embedded 3D printing of strain sensors within highly stretchable elastomers[J]. Advanced Materials, 2014, 26(36): 6307-6312. (3) Nesaei S, Rock M, Wang Y, et al. Additive Manufacturing With Conductive, Viscoelastic Polymer Composites: Direct-Ink-Writing of Electrolytic and Anodic Poly (Ethylene Oxide) Composites[J]. Journal of Manufacturing Science and Engineering, 2017, 139(11): 111004. (4) Truby R L, Lewis J A. Printing soft matter in three dimensions[J]. Nature, 2016, 540(7633): 371-378.

Development of a flexible and stretchable tactile sensor array with two different structures for robotic hand application

A flexible capacitance sensors array for robot hand application which could be used for objects distinction.

Unconstrained Measurement of Respiration Motions of Chest and Abdomen Using a Tactile Sensor Sheet in Supine Position on Bed

The management of health through daily monitoring of respiration is of major importance for early diagnosis to prevent respiratory and circulatory diseases. Such daily health monitoring is possible only if the monitoring system is physically and psychologically noninvasive. However, current unconstrained measurement methods cannot distinguish chest and abdominal movements in diagnosing sleep apnea. In this study, a flexible and stretchable tactile sensor sheet was developed to measure the static body pressure of a subject who lies on it and measure the pressure fluctuations induced by respiration or respiratory efforts. The results were compared with the measurements by band sensors that are widely used for measuring chest and abdominal movements in clinic. It was demonstrated that the sensor sheet can distinguish chest and abdominal movements in a supine position. The reasons why the pressure fluctuations measured by the sensor sheet are antiphase with the outputs of band sensors are discussed using a simple dynamic model.

Combined 3D printing technologies and material for fabrication of tactile sensors

Recently, 3D printing technology has taken the spotlight internationally with the recognition of the importance of the manufacturing industry. Currently, there are many mature 3D printing processes and materials. However, an absence of fabrication capability of smart structures such as sensors and actuators remains. In this research, we present a hybrid manufacturing process including directprint/cure (DPC) and projection-based stereolithography, along with printable materials for stretchable tactile sensors. The suggested DPC system consists of a robotically controlled micro-dispensing head, and a light curing module combined with projection stereolithography (PSL) retrofitted from a commercial projector. The materials developed in this research are based on a photocurable and stretchable liquid resin filled with multi-walled carbon nanotubes (MWNTs); this polymer/nanocomposite exhibits the piezoresistive property used in tactile sensing. We also used another hybrid process to develop a tactile sensor using a commercial machine to build the sensor body while a dispensing system was used to create the sensing elements. We have characterized the fabricated sensors with several experiments to detect the locations where forces are applied to the surfaces of the sensors. It is concluded that the suggested processes and materials are promising in developing accurate and reliable stretchable tactile sensors.

Flexible and stretchable fabric-based tactile sensor

We introduce a novel, fabric-based, flexible, and stretchable tactile sensor, which is capable of seamlessly covering natural shapes. As humans and robots have curved body parts that move with respect to each other, the practical usage of traditional rigid tactile sensor arrays is limited. Rather, a flexible tactile skin is required. Our design allows for several tactile cells to be embedded in a single sensor patch. It can have an arbitrary perimeter and can cover free-form surfaces. In this article we discuss the construction of the sensor and evaluate its performance. Our flexible tactile sensor remains operational on top of soft padding such as a gel cushion, enabling the construction of a human-like soft tactile skin. The sensor allows pressure measurements to be read from a subtle less than 1 kPa up to high pressures of more than 500 kPa, which easily covers the common range for everyday human manual interactions. Due to a layered construction, the sensor is very robust and can withstand normal forces multiple magnitudes higher than what could be achieved by a human without sustaining damage.As an exciting application for the sensor, we describe the construction of a wearable tactile dataglove with 54 tactile cells and embedded data acquisition electronics. We also discuss the necessary implementation details to maintain long term sensor performance in the presence of moisture.