When grasping objects, humans actively adjust grip force in response to surface slipperiness and motion. Previous studies have showed that corrective actions occur after tactile afferents signal surface friction or slip events. However, the influence of the mechanical behavior of the skin on the development of contact forces is poorly understood. In this study, using contact kinematics derived from a natural reach-and-grasp task, we applied a glass surface onto restrained fingers via a robotic manipulator under low- and high-friction conditions. Contact forces were measured with a force sensor, and skin deformations were captured using a high-speed camera. As expected, the normal force remained unaffected by friction, however, interestingly the tangential force rose more slowly and peaked lower under low friction. This resulted in a higher normal-to-tangential force ratio, resembling friction-dependent scaling of grip-to-load force ratio observed in active grasping. The skin partially slipped throughout contact development, with the proportion of the slipped area first decreasing and then increasing. The time course of tangential force correlated with the extent of skin slip, both varying with friction. The findings demonstrate that skin mechanics potentially influences the grip stabilization during the initial phase of object handling, which doesn't involve feedback-driven grip force adjustments.

Abstract: A task as simple as holding a cup between your fingers generates complex motor commands to finely regulate the forces applied by muscles. These fine force adjustments ensure the stability and integrity of the object by preventing it from slipping out of grip during manipulation and by reacting to perturbations. To do so, our sensorimotor system constantly monitors tactile and proprioceptive information about the force object exerts on fingertips and the friction of the surfaces to determine the optimal grip force. While the literature describes the transient responses, humans can generate to react to perturbations in load force, it is yet to be determined if humans can also react to abrupt changes in friction while already holding an object. Only recently technology using imperceivable ultrasonic vibrations became available to modulate friction in real time to investigate this question. In this study, we used an object with an integrated friction modulation device suspended in a pulley system controlling the load. With this device, we explored the rapid adaptation of the sensorimotor system to changes in friction alone and in combination with changes in load. When load force and friction changed simultaneously, the grip force response was regulated based on the grip safety requirements. Participants increased their grip force in response to decrease in friction. However, they did not adjust their grip force when the friction increased, which is expected based on our biomechanical model of friction sensing mechanisms. (Figure presented.). Key points: Simple tasks like pouring water into a glass mobilize intricate interactions between fingertip sensory inputs and motor commands to account for the weight change and friction. It has been investigated how humans react to force perturbations when holding an object, but very little is known about how frictional changes are sensed and acted upon while holding an object, for example, due to sweating or condensation. We engineered a unique experimental object that utilizes imperceivable ultrasonic vibrations to change the frictional properties of the surface in a few milliseconds. This apparatus enabled us to study how human subjects react to change of friction when gripping or holding an object. We showed that humans adjust the strength of their grasp when forces in the direction of gravity either increase or decrease; however, frictional change evokes adjustments only when friction decreases.

To robustly handle objects, robots must perceive mechanical interactions through touch with sufficient richness. New tactile sensors leverage miniature cameras to provide dense measurements of these interactions, allowing for the extraction of material properties and frictional information. Among the plethora of solutions, retrographic sensing is popular for its ability to finely resolve the shape of the object being touched. These sensors use a reflective membrane, illuminated at a shallow angle by three RGB lights from which fine details of the surface can be recovered. However, these retrographic sensors are unable to detect the lateral displacement of the membrane and, therefore overlook frictional information, which is crucial for grasping and manipulation. Embedding and tracking opaque markers has been a makeshift solution, but these markers occlude the membrane and are difficult to manufacture. In this paper, we introduce ShadowTac, a tactile sensor that combines retrographic illumination with non-intrusive markers created by colored shadows. We patterned the retrographic surface with a dense array of submillimeter dimples, which are small enough not to obstruct the view yet cast shadows large enough to be visible to the camera. ShadowTac captures a dense image of both the normal displacement field with fine details and a precise lateral displacement field by tracking the markers. Additionally, our sensor is easy to manufacture, as the dimple pattern can simply be molded. We evaluated the measurement reliability of ShadowTac and its effectiveness in estimating the incipient slip of arbitrary objects. The dense measurement of both the normal and shear deformation that the sensor captures makes it ideal for tracking dynamic interactions between robotic fingertips and manipulated objects.

Minimally invasive endovascular procedures use catheters that are guided through blood vessels to perform interventions, resulting in an inevitable frictional interaction between the catheter and the vessel walls. While this friction enhances stability during the intervention, it poses a risk of damaging the inner layer of the blood vessel wall during navigation, leading to post-operative complications including infectious diseases and thrombus formation. To mitigate the risk of adverse complications, we propose a new concept of a variable-friction catheter capable of transitioning from low friction during navigation to high friction for increased stability while performing the intervention. This variable-friction catheter leverages ultrasonic lubrication to actively control the frictional forces experienced by the catheter during the procedure. In this paper, we demonstrate a proof-of-concept for a friction control module, a pivotal component of the proposed catheter design. Our experiments demonstrate that the prototype effectively reduce friction by up to 11% and 60%, on average, on soft and rigid surfaces, representing its potential performance on healthy and calcified tissue, respectively. This result underscores the feasibility of the design and its potential to improve the safety and efficacy of minimally invasive endovascular procedures.

The emergence of the field of soft robotics has led to an interest in suction cups as auxiliary structures on soft continuum arms to support the execution of manipulation tasks. This application poses demanding requirements on suction cups with respect to sensorization, adhesion under non-ideal contact conditions, and integration into fully soft systems. The octopus can serve as an important source of inspiration for addressing these challenges. This review aims to accelerate research in octopus-inspired suction cups by providing a detailed analysis of the octopus sucker, determining meaningful performance metrics for suction cups on the basis of this analysis, and evaluating the state-of-the-art in suction cups according to these performance metrics. In total, 47 records describing suction cups are found, classified according to the deployed actuation method, and evaluated on performance metrics reflecting the level of sensorization, adhesion, and integration. Despite significant advances in recent years, the octopus sucker outperforms all suction cups on all performance metrics. The realization of high resolution tactile sensing in suction cups and the integration of such sensorized suction cups in soft continuum structures are identified as two major hurdles toward the realization of octopus-inspired manipulation strategies in soft continuum robot arms.

Abstract: When manipulating objects, humans begin adjusting their grip force to friction within 100 ms of contact. During motor adaptation, subjects become aware of the slipperiness of touched surfaces. Previously, we have demonstrated that humans cannot perceive frictional differences when surfaces are brought in contact with an immobilised finger, but can do so when there is submillimeter lateral displacement or subjects actively make the contact movement. Similarly, in, we investigated how humans perceive friction in the absence of intentional exploratory sliding or rubbing movements, to mimic object manipulation interactions. We used a two-alternative forced-choice paradigm in which subjects had to reach and touch one surface followed by another, and then indicate which felt more slippery. Subjects correctly identified the more slippery surface in 87 ± 8% of cases (mean ± SD; n = 12). Biomechanical analysis of finger pad skin displacement patterns revealed the presence of tiny (<1 mm) localised slips, known to be sufficient to perceive frictional differences. We tested whether these skin movements arise as a result of natural hand reaching kinematics. The task was repeated with the introduction of a hand support, eliminating the hand reaching movement and minimising fingertip movement deviations from a straight path. As a result, our subjects’ performance significantly declined (66 ± 12% correct, mean ± SD; n = 12), suggesting that unrestricted reaching movement kinematics and factors such as physiological tremor, play a crucial role in enhancing or enabling friction perception upon initial contact. (Figure presented.). Key points: More slippery objects require a stronger grip to prevent them from slipping out of hands. Grip force adjustments to friction driven by tactile sensory signals are largely automatic and do not necessitate cognitive involvement; nevertheless, some associated awareness of grip surface slipperiness under such sensory conditions is present and helps to select a safe and appropriate movement plan. When gripping an object, tactile receptors provide frictional information without intentional rubbing or sliding fingers over the surface. However, we have discovered that submillimeter range lateral displacement might be required to enhance or enable friction sensing. The present study provides evidence that such small lateral movements causing localised partial slips arise and are an inherent part of natural reaching movement kinematics.

Pipelines, vital for fluid transport, pose an important yet challenging inspection task, particularly in small, flexible biological systems, that robots have yet to master. In this study, we explored the development of an innovative robot inspired by the ovipositor of parasitic wasps to navigate and inspect pipelines. The robot features a flexible locomotion system that adapts to different tube sizes and shapes through a mechanical inflation technique. The flexible locomotion system employs a reciprocating motion, in which groups of three sliders extend and retract in a cyclic fashion. In a proof-of-principle experiment, the robot locomotion efficiency demonstrated positive linear correlation (r = 0.6434) with the diameter ratio (ratio of robot diameter to tube diameter). The robot showcased a remarkable ability to traverse tubes of different sizes, shapes and payloads with an average of (70%) locomotion efficiency across all testing conditions, at varying diameter ratios (0.7 1.5). Furthermore, the mechanical inflation mechanism displayed substantial load-carrying capacity, producing considerable holding force of (13 N), equivalent to carrying a payload of (≈5.8 Kg) inclusive the robot weight. This soft robotic system shows promise for inspection and navigation within tubular confined spaces, particularly in scenarios requiring adaptability to different tube shapes, sizes, and load-carrying capacities. The design of this system serves as a foundation for a new class of pipeline inspection robots that exhibit versatility across various pipeline environments, potentially including biological systems.

To gently grasp objects, robots need to balance generating enough friction yet avoiding too much force that could damage the object. In practice, the force regulation is challenging to implement since it requires knowledge of the friction coefficient, which can vary from object to object and even from grasp to grasp. Tactile sensing offers a window in the contact mechanics and provides information about friction. Notably touch can detect the precursor of the object slipping away from the grasp. To find this information, tactile sensors measure the deformation field of an artificial skin in both the normal and tangential direction. However, current approaches only react to slip and therefore react too late to perturbations. The object slips, inducing a failure of the grasp and damage. In this study, we introduce a method that uses machine-learning to anticipate slip by computing the so-called safety margin of the grasp. This safety margin represents the extra lateral force that maintains the contact away from the frictional limit. To find this value, we use a high-density camera-based tactile sensor to measure the 3D deformation of the surface via the movement of 82 colored markers. We trained a Convolutional Neural Network (CNN) to estimate the safety margin from the tactile images. Because it gives a distance to slip, the safety margin is a powerful metric for regulating grasp forces. As a testament of this effectiveness, we show that a simple proportional controller can robustly grasp a wide variety of objects. The results show that this control method outperforms slip detection methods, by reducing regrasp reaction times while decreasing grasping forces to 1-3 N.

To be fully integrated into the activities of our daily lives, robots need to be capable of traversing unstructured environments and interacting safely with their surroundings. Soft robots are perfect candidates since they can adapt to their surroundings through passive material compliance, rather than relying on complex control. However, the same compliance hinders the generation of propelling forces, and current approaches face a trade-off between traveling speed, action range, and control complexity. We overcome this trade-off by developing a locomotion mechanism based on the synergistic interaction between symmetric vibrations, elasticity, and asymmetric morphology. We then realize a rapid soft locomotor using inexpensive off-the-shelf components and requiring only elementary actuation and control. A single robotic unit can travel at speeds up to 100 mm/s when tethered and 35 mm/s when untethered. We derive a model that predicts the speed of the robot as a function of several design parameters and physical properties, highlighting the role of geometric asymmetries in the resulting anisotropic motion. Moreover, these elementary units can be added together to create more complex behaviors. By adding 2 units in parallel, the assembly can locomote and be steered following nonholonomic constraints. Our approach opens the door to a new class of low-cost soft robots that can travel fast and far with elementary fabrication and control, and which can be combined to achieve complex functions without compromising their essential simplicity.

Shortly after touching an object, humans can tactually gauge the frictional resistance of a surface. The knowledge of surface friction is paramount to tactile perception and the motor control of grasp. While potent correlations between friction and participants' perceptual response have been found, the causal link between the friction of the surface, its evolution and its perceptual experience has yet to be demonstrated. Here, we leverage new experimental apparatus able to modify friction in real time, to show that participants can perceive sudden changes in friction when they are pressing on a surface. Surprisingly, only a reduction of the friction coefficient leads to a robust perception. High-speed imaging data indicate that the sensation is caused by a release of a latent elastic strain over a 20 ms timeframe after the activation of the friction-reduction device. This rapid change of frictional properties during initial contact is interpreted as a normal displacement of the surface, which paves the way for haptic surfaces that can produce illusions of interacting with mechanical buttons.

Wearable vibrotactile actuators are non-intrusive and inexpensive means to provide haptic feedback directly to the user's skin. Complex spatiotemporal stimuli can be achieved by combining multiple of these actuators, using the funneling illusion. This illusion can funnel the sensation to a particular position between the actuators, thereby creating virtual actuators. However, using the funneling illusion to create virtual actuation points is not robust and leads to sensations that are difficult to locate. We postulate that poor localization can be improved by considering the dispersion and attenuation of the wave propagation on the skin. We used the inverse filter technique to compute the delays and amplification of each frequency to correct the distortion and create sharp sensations that are easier to detect. We developed a wearable device stimulating the volar surface of the forearm composed of four independently controlled actuators. A psychophysical study involving twenty participants showed that the focused sensation improves confidence in the localization by 20% compared to the non-corrected funneling illusion. We anticipate our results to improve the control of wearable vibrotactile devices used for emotional touch or tactile communication.

Transverse vibrations can induce the non-linear compression of a thin film of air to levitate objects, via the squeeze-film effect. This phenomenon is well captured by the Reynolds' lubrication theory; however, the same theory fails to describe this levitation when the fluid is incompressible. In this case, the computation predicts no steady-state levitation, contradicting the documented experimental evidence. In this Letter, we uncover the main source of the time-averaged pressure asymmetry in the incompressible fluid thin film, leading the levitation phenomenon to exist. Furthermore, we reveal the physical law governing the steady-state levitation height, which we confirm experimentally.

The sensation of touching virtual texture and shape can be provided to a touchscreen user by varying the friction force. Despite the saliency of the sensation, this modulated frictional force is purely passive and strictly opposes finger movement. Therefore, it is only possible to create forces along the direction of movement and this technology cannot stimulate a static fingertip or provide forces that are orthogonal to the direction of movement. The lack of orthogonal force limits the guidance to a target in an arbitrary direction and there is a need for active lateral forces to give directional cues to the fingertip. Here, we introduce a surface haptic interface that uses ultrasonic traveling waves to create an active lateral force on bare fingertips. The device is built around a ring shape cavity where two degenerate resonant modes around 40 kHz are excited with 90$^{\circ }$ phase shift. The interface provides active forces up to 0.3 N to a static bare finger uniformly over a 140×30 mm$^{2}$ surface. We report the model and design of the acoustic cavity, force measurements, and an application to create a key-click sensation. This work demonstrates a promising method for uniformly producing large lateral forces on a touch surface.

A surface texture is perceived through both the sound and vibrations produced while being explored by our fingers. Because of their common origin, both modalities have a strong influence on each other, particularly at above 60 Hz for which vibrotactile perception and pitch perception share common neural processes. However, whether the sensation of rhythm is shared between audio and haptic perception is still an open question. In this study, we show striking similarities between the audio and haptic perception of rhythmic changes, and demonstrate the interaction of both modalities below 60 Hz. Using a new surface-haptic device to synthesize arbitrary audio-haptic textures, psychophysical experiments demonstrate that the perception threshold curves of audio and haptic rhythmic gradients are the same. Moreover, multimodal integration occurs when audio and haptic rhythmic gradients are congruent. We propose a multimodal model of rhythm perception to explain these observations. These findings suggest that audio and haptic signals are likely to be processed by common neural mechanisms also for the perception of rhythm. They provide a framework for audio-haptic stimulus generation that is beneficial for nonverbal communication or modern human-machine interfaces.