An Ultrathin and Lightweight Soft Inflatable Actuator for Natural Tactile Sensory Feedback

An Ultrathin and Lightweight Soft Inflatable Actuator for Natural Tactile Sensory Feedback Hanna Scherer The BioRobotics Institute and Department of Excellence in Robotics & AI Scuola Superiore Sant’Anna Pisa, Italy [email protected] Francesco Iberite The BioRobotics Institute and Department of Excellence in Robotics & AI Scuola Superiore Sant’Anna Pisa, Italy [email protected] Giulia Caserta Rehab Technologies Lab Istituto Italiano di Tecnologia Genova, Italy [email protected] Nicolò Boccardo Rehab Technologies Lab Istituto Italiano di Tecnologia Genova, Italy [email protected] Jacopo Carpaneto The BioRobotics Institute and Department of Excellence in Robotics & AI Scuola Superiore Sant’Anna Pisa, Italy [email protected] Emanuele Gruppioni Centro Protesi INAIL National Institute for Insurance against Accidents at Work Vigorso di Budrio, Bologna, Italy [email protected] Silvestro Micera The BioRobotics Institute and Department of Excellence in Robotics & AI Scuola Superiore Sant’Anna Pisa, Italy [email protected] Tommaso Proietti The BioRobotics Institute and Department of Excellence in Robotics & AI Scuola Superiore Sant’Anna Pisa, Italy [email protected] This study introduces a soft inflatable actuator designed to provide rich, multimodal sensory feedback, including both pressure and vibration. The actuator, weighing less than 2 g and with a thickness of 0.4 mm, is a significant improvement over previous designs. Mechanical characterization demonstrates the actuator's capability to produce high-bandwidth vibrations up to 200 Hz and high forces of 28 Newtons at 60 kPa. Psychometric tests conducted with 14 able-bodied individuals and three transradial amputees show performance comparable to state-ofthe-art invasive and non-invasive solutions. Furthermore, the actuator successfully conveys different artificial roughness sensations to able-bodied individuals. In a classification task, amputees achieved an overall accuracy of 73.3%, with touch and pressure being the dominant among the elicited sensations. These results highlight the effectiveness, versatility, and lightweight nature of the proposed soft feedback actuator, showing its potential for integration into robotic systems for feedback restoration and augmentation. human-machine interfaces, multimodal feedback, sensory feedback, soft robotics, wearable haptics I. INTRODUCTION The human somatosensory system is fundamental for interacting with the environment, enabling crucial functions like proprioception and the perception of texture, pressure, and temperature [1]. The absence of sensory feedback significantly impairs user performance and engagement in a wide range of applications, including surgical robotics [2], teleoperation[3], and virtual reality [4]. This is particularly detrimental for individuals with sensory impairments such as amputees [5], stroke survivors, and patients with spinal cord injuries. While various haptic feedback systems have been developed to address this need, they often face challenges [6] related to size, comfort, and the ability to provide natural and intuitive sensations. To overcome these limitations, this work presents the design and validation of a lightweight and ultrathin soft pneumatic actuator (SPA) based on thermoplastic polyurethane (TPU). This actuator can produce a rich range of bioinspired, multimodal tactile stimuli, including both pressure and vibration, surpassing the performance of previous soft actuators. The proposed solution offers a simple, highly customizable, and cost-effective manufacturing process, enabling rapid prototyping and easy adaptation for applications in sensory restoration and augmentation. II. MATERIAL AND METHODS The soft actuator is fabricated from two ultrathin 0.2 mm heat-sealable TPU membranes. The desired actuator shapes are first cut using a laser cutter, along with a sheet of heatresistant paper that defines the internal air chamber. These layers are assembled and heat-sealed at 155°C for 100 seconds. The entire manufacturing process is completed in under 10 minutes, allowing for extensive customization of the actuator's geometry. For this research, actuators with diameters of 10 mm (for fingertips) and 25 mm (for the forearm and residual limb) were produced and evaluated. The actuator's mechanical properties were quantified through a series of characterization tests. An isometric force test measured the force transmitted as a function of inflation pressure, both in direct contact and with induced air gaps. A vibration test analyzed the system's frequency response from 1 to 200 Hz using an IMU to measure peak acceleration. To validate the actuator's performance in a human-in-the-loop context, a user study was conducted with 14 able-bodied participants and three individuals with transradial amputations. The study protocol, approved by the relevant ethics committees, included three distinct experiments: (1) a Just Noticeable Difference (JND) test using a two-alternative forced-choice method to determine pressure discrimination thresholds; (2) a perception-matching test where users compared virtual textures generated by the actuator to real 3Dprinted surfaces; and (3) an intensity classification test where amputees categorized stimuli into low, medium, and high 2025 I-RIM Conference October 17-19, Rome, Italy ISBN: 9788894580570 10.5281/zenodo.17629776 145 levels. Figure 1 (A) Detailed structure of the soft actuators, with the two TPU layers (1st and 3rd) and the heat-resistant layer (2nd). (B) Able-bodied mounting for the fingertip and forearm actuators. (C) Mounting of the soft actuators below a prosthetic socket. D) A direct encoding strategy devised for closed-loop scenarios. Figure adapted from [7] III. RESULTS Mechanical characterization confirmed the actuator's robust performance. The 25 mm TPU actuator generated up to 28 N of force at 60 kPa. Crucially, it demonstrated superior performance over a similar nylon-based actuator, especially when an air gap was present; with a 4 mm gap, the TPU design produced approximately 50% more force. The system also exhibited a wide and stable vibration bandwidth, showing a linear correlation between the setpoint frequency and the measured output up to 200 Hz. The peak acceleration consistently remained above the human Vibration Perception Threshold. The user study yielded compelling results regarding the actuator's effectiveness in both delivering force and texture information. The JND analysis resulted in Weber fractions (k) of 20% for the forearm, 19% for the fingertip in able-bodied users, and 22% for the amputee group, indicating a comparable sensitivity between prosthesis users and ablebodied individuals. In the perception-matching task, participants distinguished between the virtual textures— smooth, 1 mm spacing, and 4 mm spacing—with accuracies of 60%, 70%, and 60%, respectively. In the intensity classification task, the three amputees achieved an overall accuracy of 73.3%, substantially exceeding the 33.3% chance level. Analysis of the elicited sensations in amputees revealed that Pressure (25%) and Touch (20%) were the most dominant, suggesting a naturalistic feedback modality. Participants rated the device highly for comfort and pleasantness and indicated a willingness to use it for extended periods. Leveraging these encouraging results, a direct encoding of the sensory information was developed to preserve the richness of the evoked sensations just presented, intended to be used in a closed-loop scenario. The direct encoding implements a piecewise linear function translating the sensor values to actuator pressures. The piecewise function is defined by three regions: an initial constant value, a linear region and a final constant plateau. The two constant values are fundamental to filter out below threshold readings (the first) and extreme ones (the last plateau). IV. CONCLUSIONS This work extends a previous publication from the same authors [7] presenting an encoding strategy leveraging the promising results of the previous experiments that successfully validated the design and fabrication of an ultrathin, lightweight, soft inflatable actuator for delivering naturalistic and multimodal tactile feedback. The use of TPU offers significant advantages in force transmission, durability, and manufacturing simplicity over existing technologies [6], [8], [9], [10]. The actuator’s performance, demonstrated through comprehensive mechanical testing and user studies with both able-bodied individuals and amputees, establishes it as a powerful and versatile tool for human-machine interfaces and sensory restoration for prosthesis users[11]. Its minimal form factor (0.4 mm thickness) and high force-to-pressure ratio make it an ideal candidate for integration into prosthetics, virtual reality systems, and teleoperation interfaces where space and weight are critical constraints. Future research will be directed towards integrating this actuator into a closed-loop controlled prosthetic hand to perform functional tasks such as object size and compliance discrimination. ACKNOWLEDGMENT This work was supported by the NEXTGENERATIONEU and Ministero dell'Università e della Ricerca THE (IECS00000017)— Tuscany Health Ecosystem (DN. 1553 11.10.2022) and project MNESYS (PE0000006)—A Multiscale Integrated Approach to the Study of the Nervous System in Health and Disease (DN. 1553 11.10.2022), and by the Italian National Institute for Insurance against Accidents at Work (INAIL Centro Protesi, Vigorso di Budrio, Bologna, Italy) within the PR23-PAS-P2 Biointernect project. REFERENCES [1] E. P. Gardner and J. Martin, “Coding of Sensory Information,” Psychology, 2010. [2] A. Gerald and S. Russo, “Soft sensing and haptics for medical procedures,” Nat Rev Mater, vol. 9, no. 2, pp. 86–88, Jan. 2024, doi: 10.1038/s41578-024-00653-6. [3] R. V. Patel, S. F. Atashzar, and M. Tavakoli, “Haptic Feedback and ForceBased Teleoperation in Surgical Robotics,” Proceedings of the IEEE, vol. 110, no. 7, pp. 1012–1027, Jul. 2022, doi: 10.1109/JPROC.2022.3180052. [4] X. 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