We present a tactile-sensing method enabled by the mechanical compliance of soft robots: an externally attachable photoreflective module reads surface deformation of silicone skin to estimate contact force while: 1) locating the sensing module off the contact interface; 2) keeping the skin free of embedded transducers/wiring; and 3) enabling the rapid replacement and reconfiguration of the module. Locating the sensor off the contact interface reduces damage risk, and the nonembedded architecture preserves softness while simplifying the fabrication and maintenance through a plug-in and replaceable module. We first characterize the optical sensing element and the compliant skin and then determine the design of a prototype tactile sensor. Compression experiments validate the approach, exhibiting a monotonic force-output relationship consistent with theory, low hysteresis, high repeatability over repeated cycles, and small response lag over 0.1-10-mm/s indentation speeds. We further demonstrate integration on a soft robotic gripper, where the module reliably detects grasp events. Compared with liquid-filled or wire-embedded tactile skins, the proposed modular "add-on" architecture enhances durability, reduces wiring complexity, and supports easy redeployment across diverse robot geometries without disassembling the robot skin. Because the sensing principle reads skin-strain patterns, it also suggests extensions to other somatosensory cues-such as joint-angle or actuator-state estimation-from surface deformation. Overall, the leveraging surface compliance with an external optical module provides a practical and robust route to equip soft robots with force perception, while preserving structural flexibility and manufacturability, paving the way for robotic applications and safe human-robot collaboration.

Self-healing polymers, which can autonomously repair damage or be repaired through simple treatments, have garnered attention due to their flexibility in addressing damage and enabling long-term use in soft robotics. Many studies on the application of self-healing polymers to soft robotics have focused on realizing healable systems. Such a mechanism makes it possible to construct soft robots. Despite previous research on reconfigurable soft robots, the reconfiguration of actuators'critical components for determining actuation'has not been explored extensively. We propose a method for constructing reconfigurable soft pneumatic actuators using multi-material self-healing polymers, thus enhancing the adaptability and functionality of soft robotic systems. The actuator comprises stretchable and less-stretchable self-healing polymers, which allow for durable and versatile configurations. By adjusting the placement of the less-stretchable polymer on the actuator surface, we can design and alter the deformation, thus achieving various forms of actuation. Our research demonstrates the feasibility of actuators capable of bending, contracting, twisting, and elongating. We provide examples of their application in actuator fabrication, thus showcasing the potential of our study in inspiring the design and fabrication of soft robots utilizing self-healing polymers.

Soft pneumatic pouch actuators with simple structures and fabrication processes are being studied for application in soft robots. In addition, some research groups have tried to add sensors to the pouch actuators to estimate their states. In this study, we propose to embed a soft strain sensor made of liquid metal in the bladder of a pouch actuator made of stretchable styrene- ethylene-butadiene-styrene (SEBS) film, which can expand to a large extent under pressure to enable shape estimation. A method of processing the bladder of the pouch actuator to convert it into a sensor has yet to be proposed. We developed a fabrication process to form the small strain sensor in the SEBS film and evaluated the strain sensor's stretch characteristics. Finally, we confirmed that the deformation of a bending pouch actuator caused by pneumatic pressure is expressed in the electrical resistance of the embedded strain sensor.

We propose an optical tactile sensor using self-healing materials. The proposed tactile sensor consists of a structure that includes a diode, a phototransistor, and an optical waveguide made from self-healing materials. This design offers the advantage of being less susceptible to electromagnetic noise compared to traditional tactile sensors based on electrical detection principles. The sensor estimates the applied force by detecting changes in the total internal reflection caused by deformation due to contact force. In this study, we first established a fabrication method for the optical waveguide-based tactile sensor using self-healing materials. Subsequently, we measured the sensor output when a static load was applied to the fabricated tactile sensor and evaluated its characteristics. The results confirmed that the sensor output decreases in response to the applied load.

This study proposes a novel self-calibration method for eye tracking in a virtual reality (VR) headset. The proposed method is based on the assumptions that the user's viewpoint can freely move and that the points of regard (PoRs) from different viewpoints are distributed within a small area on an object surface during visual fixation. In the method, fixations are first detected from the time-series data of uncalibrated gaze directions using an extension of the I-VDT (velocity and dispersion threshold identification) algorithm to a three-dimensional (3D) scene. Then, the calibration parameters are optimized by minimizing the sum of a dispersion metrics of the PoRs. The proposed method can potentially identify the optimal calibration parameters representing the user-dependent offset from the optical axis to the visual axis without explicit user calibration, image processing, or marker-substitute objects. For the gaze data of 18 participants walking in two VR environments with many occlusions, the proposed method achieved an accuracy of 2.1$^\circ$, which was significantly lower than the average offset. Our method is the first self-calibration method with an average error lower than 3$^\circ$ in 3D environments. Further, the accuracy of the proposed method can be improved by up to 1.2$^\circ$ by refining the fixation detection or optimization algorithm.

In this letter, we investigated the estimation of cardiovascular signals, electrocardiogram (ECG) and photoplethysmogram (PPG) waveforms through skin vibrations as piezoelectric plethysmogram (PEPG) signals. ECG and PPG measurements are important for biomedical and security applications, and PEPG signals may be used to estimate ECG and PPG waveforms based on physiological activities in the cardiovascular system. We investigated the estimation procedure for ECG and PPG signal waveforms from PEPG signals using a frequency analysis. To evaluate estimation performance, we conducted an experiment using a dataset that included ECG, PPG, and PEPG signals. The results indicate that the ECG and PPG waveforms were successfully estimated from the PEPG signals, with errors of less than 15% and 10%, respectively.

In this paper, we propose a method to fabricate silicone soft pneumatic actuators with different motion characteristics from the same mold. In the proposed method, the motion characteristics are controlled by forming a pattern of hard silicone on the surface of an actuator base made of soft silicone.

Soft matter computers (SMCs) are promising control and computation mechanisms that do not use rigid materials. In such mechanisms, conductive droplets are used to switch the state of the voltage applied to connected loads, such as in an electric actuator. Without a complex structure, an SMC can only apply voltage in sequence starting from the first load. Therefore, soft robots with simple SMCs can only perform simple operations. In this study, to apply voltage to connected loads in any order and combination, the SMC design was extended by adjusting the spatial intervals between pairs of facing electrodes. The intervals between the pairs of facing electrodes for each load were designed uniquely. These pairs of facing electrodes are electrically connected by two conductive droplets at the same interval and a voltage is applied to the specified connected load. Based on the arrangement of conductive droplets, voltage can be applied to loads arranged in a pattern on a soft tube in any combination and order. An experimental evaluation was conducted to determine whether the proposed method could apply voltage to loads in a predefined pattern. Additionally, the applicability of the proposed method to soft robots was demonstrated by controlling soft actuators in any combination.

In our previous study, we proposed a method for judging whether the user is gazing at a semi-transparent virtual object or real objects behind it in augmented reality environments. This paper shows that the accuracy of our method can be improved by selecting the optimal thresholds for the fixation detection. Fourteen participants experienced a virtual reality environment in which there were a transparent subway map and buildings behind it in the distance of 2 m and 15 m away from each participant, respectively. As a result, the accuracy of our method has achieved 88.3 % and improved by 13.8 percentage points from the previous 74.5 %.

In this study, we propose a method to reconstruct photoplethysmogram (PPG) waveforms from other stealthily recorded physiological signals. The proposed method focuses on the frequency characteristics between two physiological signals and reconstructs the target PPG waveform using a regression model. We investigate the feasibility of the proposed method to reconstruct target PPG signals from respiratory (RSP) and PPG signals recorded at non-genuine measurement sites using the two datasets of physiological signals. The results indicate that the proposed method achieves similarities between the target PPG and reconstructed PPG signals with correlation coefficients more than 0.860.

In this study, we propose a countermeasure against a presentation attack on a photoplethysmogram (PPG)-based biometric authentication system. The countermeasure detects fake PPG signals by identifying PPG measurement sites on the body based on the difference between PPG waveforms recorded at genuine measurement and non-genuine sites without adding other sensing components. In an experiment, we computed the correlation coefficients as the similarity indices between PPG waveforms using two datasets, i.e., PPG signals recorded at multiple measurement sites on participants and mapped signals to generate fake signals for authentication. We then evaluated the proposed countermeasure using the feature values extracted from the PPG signals to identify the measurement sites on the body. The experimental results indicated that the identification of measurement sites as a countermeasure operated successfully for both PPG datasets, regardless of the presence or absence of waveform mapping, and exhibited an identification accuracy of more than 90 % regardless of the elapsed time.

Wearable haptic displays can provide haptic information while allowing for free body movement. Among these haptic displays, pneumatic haptic displays have the advantages of flexibility and lightweight; however, they require bulky air tubes and a heavy air compressor. To solve this problem, we propose a wearable haptic display that uses a liquid-to-gas phase change actuator and a Peltier device as a way to reduce the size of the entire system. A low-boiling-point liquid is encapsulated in the flexible bladder of the actuator, and the vaporization of the liquid, which induces the inflation of the actuator, is controlled by the external Peltier device. In this study, we implemented a pressure sensor to monitor pressure inside the liquid-to-gas phase change actuator. The pressure measurement will contribute to controlling the generated normal force. First, we characterized the pressure response concerning the design of the liquid-to-gas phase change actuator. Next, we evaluated the output normal force of the haptic display and confirmed that the maximum output force reached a few newtons, which is a similar level to the off-the-shelf wearable haptic display devices. Finally, a sensory evaluation revealed that the experimental participants perceived the haptic stimulus to their fingertips provided by the proposed haptic display in a few seconds. According to the obtained results, the proposed haptic display can be applied to applications such as human interfaces to provide force, allowing for a response time of a few seconds.

Abstract The use of dielectric elastomer actuators (DEA) in the development of compact and flexible tunable lens systems for microscopic imaging applications is appealing. However, there are still challenges with imaging system functionality and fabrication process easiness. In this paper, we proposed a multielectrode balloon-type DEA which is capable of tuning both the focus and optical axis of a lens system using 3-degrees-of-freedom movement of the DEA. We first investigated the DEA’s fundamental characteristics and found that effective microscale tangential and axial displacements were obtained up to approximately 10 Hz, and the displacements increased with applied voltage and air pressure. Assuming microscopic imaging application scenarios, we then conducted two-dimensional tracking and focus adjustment tests with a DEA-driven lens system. The result suggested that the position of the optical axis was successfully controlled using the captured image, and target images at different distances were successfully focused and defocused by the axial movement of the DEA.

In this study, we examined information leakage in photoplethysmogram (PPG)-based biometric authentication and assessed an attack against authentication based on the information leakage. Several approaches have been proposed to apply PPG to biometric authentication using a wearable device; however, there may be several attacks against PPG-based authentication. One of the attacks is a “presentation attack” (PA), which utilizes the information leakage originating from the various PPG measurement sites on a body. The PA records the victim’s PPG stealthily on non-genuine measurement sites and transmits it to the PPG sensor to break the authentication. We examined the information leakage and assessed the PA by evaluating feature values extracted from the PPG signals. We recorded the PPG signals of 12 participants on their fingertips and wrists. We compared the feature values extracted from the recorded PPG signals by computing the differences, correlation coefficients, and mutual information to examine the leakage of information required for the PPG-based authentication. We then assessed the feasibility of a PA based on existing PPG-based authentication algorithms and evaluated the contribution of each value to authentication and PA by computing the permutation importance of all feature values. The experimental results indicated that there might be information leakage and selection of feature values to reduce the feasibility of the PA up to 62.8 %.

Soft robots have been developed in recent years. In order for a soft robot to perform complex motions, it is needed to operate multiple actuators using multiple wires. Multiple wires increase the weight of the soft robot and limits the degree of freedom. We propose a system that uses a single channel to operate a specific actuator from a group of them by combining conductive droplet spacing and electrode spacing. In this system, two electrodes facing each other across the channel are placed at different intervals to connect the actuator to the power supply. While flowing insulating fluid in the channel, multiple conductive droplets are injected at various intervals. Regardless of the order in which the actuators are placed, only those actuators with electrodes that have the same intervals as conductive droplets will operate. This method can simplify the driving system and reduce the weight of soft robots.

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.

In recent years, soft robots have been attracting interest because of their potential for new application fields. Among soft actuators for soft robots, liquid-to-gas phase change actuators have the advantage of structural simplicity. However, the actuators require an external heater to vaporize a low boiling point liquid inside the actuator. The heater typically consists of a solid electrode that often reduces the durability and flexibility of the heater. In this study, we propose a small liquid-to-gas phase change actuator with an integrated liquid metal heater. A low boiling point liquid and liquid metal heater were encapsulated in a nylon-polyethylene bladder using wire molding. The proposed soft actuator has high flexibility and durability against bending because of the material used. We experimentally characterized actuators of different sizes with respect to the generated force, displacement, and time response. We also confirmed that the soft actuator could work even after bending 1000 times.

In recent years, soft tactile sensors comprising liquid metal and silicone rubber have been studied for tactile sensing of soft robots. In order to obtain detailed information about objects, the sensor array was configured by utilizing crossing of orthogonal microchannels filled with liquid metal as sensing elements in previous research. In this study, we propose the soft tactile sensor array with independent sensing elements. This sensor embeds narrow microchannels standing vertically in silicone rubber. Microchannels are filled with liquid metal and used as sensing elements. This structure realizes miniaturization and high-density placement of sensing elements while maintaining the independence of the sensing elements. We fabricated the sensor with 16 sensor elements of 0.4mm width embedded at 2.0mm intervals.

We propose a soft capacitive tactile sensor using the displacement of an air-water interface. This soft tactile sensor is composed of a soft contact part with a chamber and channel filled with water and a clip-shaped sensing part to detect the position of the interface. This tactile sensor changes the displacement of the air-water interface in the channel when a contact force is applied. The change in displacement is detected as a change in the capacitance at the sensing part. This sensor structure does not require stiff electrical elements to be embedded in the soft silicone rubber body, which improves the flexibility and durability of the sensor. Therefore, the proposed tactile sensor exhibits high flexibility and durability against bending and external stresses. Additionally, the proposed tactile sensor can detect not only press force but also pull force. This results in the detection of a press or pinch in a robot skin. The experimental results show that the proposed sensor can successfully detect both press and pull static forces. The sensitivity of the proposed sensor was 0.169 pF/N for an applied force in the press direction and -0.152 pF/N for a force in the pull direction based on the results of an evaluation of the static sensor characteristics. Furthermore, an eva-luation of the dynamic sensor characteristics revealed that the proposed tactile sensor has high repeat-ability and a quick response to an external force. Additionally, the hysteresis of the proposed tactile sensor is small, especially in the press direction. (c) 2021 Elsevier B.V. All rights reserved.