Hydrogen-doped viscoplastic liquid metal microparticles for stretchable printed metal lines.

Conductive and stretchable electrodes that can be printed directly on a stretchable substrate have drawn extensive attention for wearable electronics and electronic skins. Printable inks that contain liquid metal are strong candidates for these applications, but the insulating oxide skin that forms around the liquid metal particles limits their conductivity. This study reveals that hydrogen doping introduced by ultrasonication in the presence of aliphatic polymers makes the oxide skin highly conductive and deformable. X-ray photoelectron spectroscopy and atom probe tomography confirmed the hydrogen doping, and first-principles calculations were used to rationalize the obtained conductivity. The printed circuit lines show a metallic conductivity (25,000 S cm-1), excellent electromechanical decoupling at a 500% uniaxial stretching, mechanical resistance to scratches and long-term stability in wide ranges of temperature and humidity. The self-passivation of the printed lines allows the direct printing of three-dimensional circuit lines and double-layer planar coils that are used as stretchable inductive strain sensors.

Stretchable reflective coating for soft optical waveguides and sensors.

Soft robots must embody mechanosensing capabilities to merge with and act in the environment. Stretchable waveguides are making a mark in soft mechanical sensing since they are built from pristine elastomers. Therefore, they are insensitive to electromagnetic fields and weakly affect the deformations of the robot. However, issues in light-shielding, signal decoupling, and core-cladding interfaces are still open challenges. In this work, titanium oxide particles (TiO2) are dispersed in silicone elastomers to develop a soft optical shield coating. Results show that the added particles do not harden the matrix and reduce light transmission. Almost full NIR shielding is achieved by adding 1.0 vol% of TiO2 in 150 µm thick films. These properties make the proposed shielding coating an excellent candidate for soft mechanosensing. An open-access tool is developed to design soft optical devices by programming light transmittance at desired wavelengths by tuning, both, TiO2 concentration and film thickness. Finally, two proof-of-concepts are demonstrated, a soft waveguide and a soft strain sensor, by integrating the developed material to shield a transparent PDMS resin and a semi-transparent Ecoflex00-10 matrix, respectively. The soft waveguide can stretch up to 40% with very low optical loss, while the optical strain sensor can detect strain up to 90%. In both cases, bending, folding, and indentation of the devices have a significantly low impact on light transmission. These results can pave the way to design new optical transmission devices and sensors that exploit light reflection and that allow for discriminating different types of mechanical stimuli in soft robots.

Mechanical Sensing for Lower Limb Soft Exoskeletons: Recent Progress and Challenges.

Soft exoskeletons hold promise for facilitating monitoring and assistance in case of light impairment and for prolonging independent living. In contrast to rigid material-based exoskeletons, they strongly demand for new approaches of soft sensing and actuation solutions. This chapter overviews soft exoskeletons in contrast to rigid exoskeletons and focuses on the recent advancements on the movement monitoring in lower limb soft exoskeletons. Compliant materials and soft tactile sensing approaches can be utilized to build smart sensorized garments for joint angle measurements (needed for both control and monitoring). However, currently there are still several open challenges derived from the needed close interaction between the human body and the soft exoskeleton itself, especially related to how sensing function and robustness are strongly affected by wearability, which will need to be overcome in the near future.

Soft Smart Garments for Lower Limb Joint Position Analysis.

Revealing human movement requires lightweight, flexible systems capable of detecting mechanical parameters (like strain and pressure) while being worn comfortably by the user, and not interfering with his/her activity. In this work we address such multifaceted challenge with the development of smart garments for lower limb motion detection, like a textile kneepad and anklet in which soft sensors and readout electronics are embedded for retrieving movement of the specific joint. Stretchable capacitive sensors with a three-electrode configuration are built combining conductive textiles and elastomeric layers, and distributed around knee and ankle. Results show an excellent behavior in the ~30% strain range, hence the correlation between sensors' responses and the optically tracked Euler angles is allowed for basic lower limb movements. Bending during knee flexion/extension is detected, and it is discriminated from any external contact by implementing in real time a low computational algorithm. The smart anklet is designed to address joint motion detection in and off the sagittal plane. Ankle dorsi/plantar flexion, adduction/abduction, and rotation are retrieved. Both knee and ankle smart garments show a high accuracy in movement detection, with a RMSE less than 4° in the worst case.

A soft, stretchable and conductive biointerface for cell mechanobiology.

In mechanobiology the study of cell response to mechanical stimuli is fundamental, and the involved processes (i.e., mechanotransduction) need to be investigated by interfacing (mechanically and electrically) with the cells in dynamic and non-invasive natural-like conditions. In this work, we present a novel soft, stretchable and conductive biointerface that allows both cell mechanical stimulation and dynamic impedance recording. The biointerface stretchability and conductivity, jointly to the biocompatibility and transparency needed to perform cell culture studies, were obtained by exploiting the formation of wrinkles on the surface of a 90 nm thick conductive layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) on a pre-stretched 130 µm thick poly(dimethylsiloxane) (PDMS) substrate. Cell adhesion and proliferation of SH-SY5Y human neuroblastoma cells were evaluated, and cell differentiation on the corrugated surface was assessed. We demonstrate how the biointerface remains conductive when applying uniaxial strain up to 10%, and when cell culturing is performed. Finally, a reduction of about 30% of the relative impedance variation signal was measured, with respect to the control, as a result of the mechanical stimulation of cells.

The T-Blep: A Soft Optical Sensor for Stiffness and Contact Force Measurement.

This paper presents the Tactile Blep (T-Blep), an optical soft sensor that can measure the stiffness and force of different materials. The sensor consists of an inflatable membrane with an optical elements inside. The T-Blep can switch between stiffness detection and force detection modes, by changing the pattern followed by internal pressure of the membrane. Simulations reveal that a 1 mm-thick membrane enables differentiation of extra-soft, soft, and rigid targets. Furthermore, the sensitivity and FSO of the force estimation can be adjusted by varying the internal pressure. Force detection experiments exhibit a sixfold increase in detectable force range as internal pressure varies from 10 kPa to 40 kPa, with a force peak of 5.43 N and sensitivity up to 331 mV/N. A piecewise force reconstruction method provides accurate results even in challenging conditions (R2>0.994). Stiffness detection experiments reveal distinguishable patterns of pressure and voltage during indentation, resulting in a classification accuracy of 97%.

Soft, transparent, electronic skin for distributed and multiple pressure sensing.

In this paper we present a new optical, flexible pressure sensor that can be applied as smart skin to a robot or to consumer electronic devices. We describe a mechano-optical transduction principle that can allow the encoding of information related to an externally applied mechanical stimulus, e.g., contact, pressure and shape of contact. The physical embodiment that we present in this work is an electronic skin consisting of eight infrared emitters and eight photo-detectors coupled together and embedded in a planar PDMS waveguide of 5.5 cm diameter. When a contact occurs on the sensing area, the optical signals reaching the peripheral detectors experience a loss because of the Frustrated Total Internal Reflection and deformation of the material. The light signal is converted to electrical signal through an electronic system and a reconstruction algorithm running on a computer reconstructs the pressure map. Pilot experiments are performed to validate the tactile sensing principle by applying external pressures up to 160 kPa. Moreover, the capabilities of the electronic skin to detect contact pressure at multiple subsequent positions, as well as its function on curved surfaces, are validated. A weight sensitivity of 0.193 gr(-1) was recorded, thus making the electronic skin suitable to detect pressures in the order of few grams.

Sensorized objects used to quantitatively study distal grasping in the African elephant.

Nature evolved many ways to grasp objects without using hands: elephants, octopuses, and monkeys use highly dexterous appendices. From a roboticist's perspective, the elephant trunk is a fascinating manipulator, which strategies can empower robots' interaction capabilities. However, quantifying prehensile forces in such large animals in a safe, ethical, and reproducible manner is challenging. We developed two sensorized objects to investigate the grasping of an adult African elephant with deliberately occluded vision. A cylinder and a handle provided a distributed force (80 and 6 taxels) and inertial measurements in real-time, resisting dirt and shocks. The animal curled the distal portion of the trunk to grasp the tools. Using force and contact area data of the cylinder revealed the animal's ability to finely modulate pressure. The handle data provided insights into the energy-efficient behavior of the animal, with no significant grasping force changes despite variations imposed on both weight (5-15 kg) and initial position of the object.

Jointless Bioinspired Soft Robotics by Harnessing Micro and Macroporosity.

Although natural continuum structures, such as the boneless elephant trunk, provide inspiration for new versatile grippers, highly deformable, jointless, and multidimensional actuation has still not been achieved. The challenging pivotal requisites are to avoid sudden changes in stiffness, combined with the capability of providing reliable large deformations in different directions. This research addresses these two challenges by harnessing porosity at two levels: material and design. Based on the extraordinary extensibility and compressibility of volumetrically tessellated structures with microporous elastic polymer walls, monolithic soft actuators are fabricated by 3D printing unique polymerizable emulsions. The resulting monolithic pneumatic actuators are printed in a single process and are capable of bidirectional movements with just one actuation source. The proposed approach is demonstrated by two proof-of-concepts: a three-fingered gripper, and the first ever soft continuum actuator that encodes biaxial motion and bidirectional bending. The results open up new design paradigms for continuum soft robots with bioinspired behavior based on reliable and robust multidimensional motions.

3D printing stretchable and compressible porous structures by polymerizable emulsions for soft robotics.

UV-curable 3D printing compositions for the fabrication of stretchable and flexible porous structures for soft robotics are presented. The stereolithography-based printing compositions are water-in-oil (W/O) emulsions in which water droplets are the pore-forming material, and the continuous phase is a stretchable polyurethane diacrylate (PUA). The porosity of the printed objects is controlled by the material's micro-porosity and by the macro-porosity obtained by a cellular design. The mechanical behavior can be tailored by the composition of the emulsion, providing both compliance and strength while utilizing a unique optimization methodology for fitting the ink to the 3D printer. This approach enables developing materials having superior mechanical properties, with the highest reported elongation-at-break for 3D printed porous structures, 450%. The emulsion-based printing compositions were utilized for fabricating a soft robotic gripper with unique actuation performance that could not be obtained with commonly used materials.

Roughness encoding in human and biomimetic artificial touch: spatiotemporal frequency modulation and structural anisotropy of fingerprints.

The influence of fingerprints and their curvature in tactile sensing performance is investigated by comparative analysis of different design parameters in a biomimetic artificial fingertip, having straight or curved fingerprints. The strength in the encoding of the principal spatial period of ridged tactile stimuli (gratings) is evaluated by indenting and sliding the surfaces at controlled normal contact force and tangential sliding velocity, as a function of fingertip rotation along the indentation axis. Curved fingerprints guaranteed higher directional isotropy than straight fingerprints in the encoding of the principal frequency resulting from the ratio between the sliding velocity and the spatial periodicity of the grating. In parallel, human microneurography experiments were performed and a selection of results is included in this work in order to support the significance of the biorobotic study with the artificial tactile system.

Sensing Deformation in Vacuum Driven Foam-Based Actuator via Inductive Method.

Perception in soft robotics is crucial to allow a safe interaction to effectively explore the environment. Despite the inherent capabilities of soft materials, embedding reliable sensing in soft actuators or robots could introduce constraints in the overall design (e.g., loss of deformability, undesired trajectories, etc.) or reduce their compliant characteristics. Consequently, an adequate stiffness for both sensor and actuator becomes a crucial design parameter. In particular, for sensing the deformation related to actuation motion, sensing and actuating strategies must work in full mechanical synergy. In this view, an inductive sensing solution is presented, exploiting open-cell foam and a copper (Cu) wire in an Inductive Foam Sensor (IFS). Due to entangled air cells high deformability is enabled upon vacuum pressure, and proprioceptive information is provided. The IFS is then successfully integrated into the earlier developed Ultralight Hybrid Pneumatic Artificial Muscle (UH-PAM), which encases an elastomeric bellow skin and plastic rings. Such sensorized UH-PAM (SUH-PAM) is capable of a high contraction ratio (54% upon -80 kPa), while the inductive sensing shows a high sensitivity of 0.01031/1% and a hysteresis of 5.35%, with an average error of 1.85%, respectively. In order to implement a robust feedback control system, an adaptable proportional sliding mode control is presented. As a result, the SUH-PAM motion can be controlled to the mm-scale, with an RMSE of 0.925 mm, and high robustness against disturbances is demonstrated.

Development of the Ultralight Hybrid Pneumatic Artificial Muscle: Modelling and optimization.

Pneumatic artificial muscles (PAMs) are one of the key technologies in soft robotics, and they enable actuation in mobile robots, in wearable devices and exoskeletons for assistive and rehabilitative purposes. While they recently showed relevant improvements, they still present quite low payload, limited bandwidth, and lack of repeatability, controllability and robustness. Vacuum-based actuation has been recently demonstrated as a very promising solution, and many challenges are still open, like generating at the same time a large contraction ratio, and a high blocking force with enhanced axial stiffness. In this paper, a novel Ultralight Hybrid PAM (UH-PAM), based on bellow-type elastomeric skin and vacuum actuation, is presented. In particular, open-cell foam is exploited as a structural backbone, together with plastic rings, all embedded in a thin skin. The design and optimization combine numerical, analytical, and experimental data. Both static and dynamic analysis are performed. The weight of the optimized actuator is only 20 g. Nevertheless, a contraction ratio up to 50% and a maximum payload of 3 kg can be achieved. From a dynamic point of view, a rise time of 0.5 s for the contraction phase is observed. Although hysteresis is significant when using the whole contraction span, it can be reduced (down to 11.5%) by tuning both the vacuum range and the operating frequency for cyclic movements. Finally, to demonstrate the potentiality of this soft actuation approach, a 3 DoFs Stewart platform is built. The feasibility of performing smooth movements by exploiting open-loop control is shown through simple and more complex handwriting figures projected on the XY plane.

3D Printing Materials for Soft Robotics.

Soft robotics is a growing field of research, focusing on constructing motor-less robots from highly compliant materials, some are similar to those found in living organisms. Soft robotics has a high potential for applications in various fields such as soft grippers, actuators, and biomedical devices. 3D printing of soft robotics presents a novel and promising approach to form objects with complex structures, directly from a digital design. Here, recent developments in the field of materials for 3D printing of soft robotics are summarized, including high-performance flexible and stretchable materials, hydrogels, self-healing materials, and shape memory polymers, as well as fabrication of all-printed robots (multi-material printing, embedded electronics, untethered and autonomous robotics). The current challenges in the fabrication of 3D printed soft robotics, including the materials available and printing abilities, are presented and the recent activities addressing these challenges are also surveyed.

Artificial Roughness Encoding with a Bio-inspired MEMS- based Tactile Sensor Array.

A compliant 2×2 tactile sensor array was developed and investigated for roughness encoding. State of the art cross shape 3D MEMS sensors were integrated with polymeric packaging providing in total 16 sensitive elements to external mechanical stimuli in an area of about 20 mm(2), similarly to the SA1 innervation density in humans. Experimental analysis of the bio-inspired tactile sensor array was performed by using ridged surfaces, with spatial periods from 2.6 mm to 4.1 mm, which were indented with regulated 1N normal force and stroked at constant sliding velocity from 15 mm/s to 48 mm/s. A repeatable and expected frequency shift of the sensor outputs depending on the applied stimulus and on its scanning velocity was observed between 3.66 Hz and 18.46 Hz with an overall maximum error of 1.7%. The tactile sensor could also perform contact imaging during static stimulus indentation. The experiments demonstrated the suitability of this approach for the design of a roughness encoding tactile sensor for an artificial fingerpad.

Editorial for the Special Issue on Tactile Sensing for Soft Robotics and Wearables.

Tactile feedback is needed for the interaction of humans with a worn device, and to enable robots to gather environmental cues and react to their surroundings [...].

Toward Perceptive Soft Robots: Progress and Challenges.

In the past few years, soft robotics has rapidly become an emerging research topic, opening new possibilities for addressing real-world tasks. Perception can enable robots to effectively explore the unknown world, and interact safely with humans and the environment. Among all extero- and proprioception modalities, the detection of mechanical cues is vital, as with living beings. A variety of soft sensing technologies are available today, but there is still a gap to effectively utilize them in soft robots for practical applications. Here, the developments in soft robots with mechanical sensing are summarized to provide a comprehensive understanding of the state of the art in this field. Promising sensing technologies for mechanically perceptive soft robots are described, categorized, and their pros and cons are discussed. Strategies for designing soft sensors and criteria to evaluate their performance are outlined from the perspective of soft robotic applications. Challenges and trends in developing multimodal sensors, stretchable conductive materials and electronic interfaces, modeling techniques, and data interpretation for soft robotic sensing are highlighted. The knowledge gap and promising solutions toward perceptive soft robots are discussed and analyzed to provide a perspective in this field.

Biomechanics in Soft Mechanical Sensing: From Natural Case Studies to the Artificial World.

Living beings use mechanical interaction with the environment to gather essential cues for implementing necessary movements and actions. This process is mediated by biomechanics, primarily of the sensory structures, meaning that, at first, mechanical stimuli are morphologically computed. In the present paper, we select and review cases of specialized sensory organs for mechanical sensing-from both the animal and plant kingdoms-that distribute their intelligence in both structure and materials. A focus is set on biomechanical aspects, such as morphology and material characteristics of the selected sensory organs, and on how their sensing function is affected by them in natural environments. In this route, examples of artificial sensors that implement these principles are provided, and/or ways in which they can be translated artificially are suggested. Following a biomimetic approach, our aim is to make a step towards creating a toolbox with general tailoring principles, based on mechanical aspects tuned repeatedly in nature, such as orientation, shape, distribution, materials, and micromechanics. These should be used for a future methodical design of novel soft sensing systems for soft robotics.

Integrated Simultaneous Detection of Tactile and Bending Cues for Soft Robotics.

Soft robots should move in an unstructured environment and explore it and, to do so, they should be able to measure and distinguish proprioceptive and exteroceptive stimuli. This can be done by embedding mechanosensing systems in the body of the robot. Here, we present a polydimethylsiloxane block sensorized with an electro-optical system and a resistive strain gauge made with the supersonic cluster beam implantation (SCBI) technique. We show how to integrate these sensing elements during the whole fabrication process of the soft body and we demonstrate that their presence does not change the mechanical properties of the bulk material. Exploiting the position of both sensing systems and a proper combination of the output signals, we present a strategy to measure simultaneously external pressure and positive/negative bending of the body. In particular, the optical system can reveal any mechanical stimulation (external from the soft block or due to its own deformation), while the resistive strain gauge is insensitive to the external pressure, but sensitive to the bending of the body. This solution, here applied to a simple block of soft material, could be extended to the whole body of a soft robot. This approach provides detection and discrimination of the two stimuli (pressure and bending), with low computational effort and without significant mechanical constraint.

Integration of a miniaturised triaxial force sensor in a minimally invasive surgical tool.

This paper reports preliminary results on design and fabrication of a cutting tool with an integrated triaxial force sensor to be applied in fetal surgery procedures. The outer diameter of the proposed device is 7.4 mm, but a scaled down design can be easily achieved. Linearity and hysteresis tests have been performed for both normal and tangential loadings. A linear transformation relating the sensor output to the external applied force is introduced and discussed. The typical working range for the conceived instrument is around 0.3 N, while 20 N and 1 N are, respectively, maximum normal and tangential forces for which the device robustness has been assessed.