V-Soft Pro: Toward Natural Bionic Limbs via Soft Robotics and Variable Stiffness Actuation

V-Soft Pro: Toward Natural Bionic Limbs via Soft Robotics and Variable Stiffness Actuation Giuseppe Milazzo SoftBots Lab, Istituto Italiano di Tecnologia, Genova, Italy [email protected] Giorgio Grioli SoftBots Lab, Istituto Italiano di Tecnologia, Genova, Italy Universit` a di Pisa, Centro di Ricerca Enrico Piaggio, Pisa, Italy gior[email protected] Antonio Bicchi SoftBots Lab, Istituto Italiano di Tecnologia, Genova, Italy Universit` a di Pisa, Centro di Ricerca Enrico Piaggio, Pisa, Italy [email protected] Manuel Giuseppe Catalano SoftBots Lab, Istituto Italiano di Tecnologia, Genova, Italy [email protected] Abstract—We present a transhumeral prosthesis with Variable Stiffness Actuators (VSAs) to mimic the adjustable compliance of human joints. The modular design accommodates different users’ residual limb shapes and biological control signals. Our design draws on biological compliance for natural interaction with the environment, aiming to improve prosthesis embodiment and users’ social interactions. Moreover, embedded elastic elements enhance safety, enable more natural movement, and enhance versatility in daily tasks. Index Terms—Soft Robotics, Variable Stiffness Actuators, Transhumeral Prosthesis I. INTRODUCTION Limb loss profoundly affects not only physical ability but also emotional well-being and social participation. Modern prostheses help restore body image and some function, yet most remain limited to basic motor tasks. In contrast, human limbs dynamically adjust stiffness through muscle coactivation, enabling safe, precise, and adaptive interaction with people and unpredictable environments [1]–[3]. Rigid prosthetic joints lack this flexibility, increasing risk during collisions and limiting delicate manipulation. These shortcomings highlight the need for prosthetic devices that allow users to actively control impedance, improving versatility, safety, and task performance [3]. A solution is to incorporate variable impedance directly into the hardware design of prostheses. Accordingly, VSAs use redundant actuation and nonlinear elastic elements to modulate joint stiffness, mimicking the coactivation of antagonistic muscles and enabling more adaptive, human-like behavior. This paper presents V-Soft Pro (V-SP), a modular platform for a transhumeral prosthesis with user-controllable stiffness [4]. The platform consists of two elbow modules, three wrist modules, and two hand modules, which can be combined to This work was supported by the European Union’s ERC Synergy Grant Agreement Natural BionicS (810346), and ERC Proof of Concept Grant Agreement V-Soft Pro (101212751). The content of this publication is the sole responsibility of the authors. The European Commission or its services cannot be held responsible for any use that may be made of the information it contains. (a) (b) Fig. 1. Two configurations of the V-Soft Pro transhumeral prosthesis, tailored to different user needs. Panel (a) shows a configuration optimized for humanlike weight distribution in proximal transhumeral amputations. Panel (b) depicts a version adapted for users with longer residual limbs. create a transhumeral prosthesis tailored to the user’s limb morphology and available biosignals for control. Section II details the V-SP hardware design and the implementation of its submodules. Section III demonstrates the functionality of Variable Stiffness (VS) transhumeral prostheses. Section IV discusses potential applications and limitations of the system, and provides conclusions. II. HARDWARE The V-SP platform features a modular and user-centered design, outilined in Fig. 2, allowing users to select configurations tailored to their residual limb shape, number of independent control channels, and daily requirements. The system includes two elbow modules, three wrist options, and two hand designs, all fully compatible, resulting in 12 possible combinations to accommodate individual needs and preferences. The VS-Elbow [5] incorporates redundant actuation and a nonlinear elastic transmission, enabling independent control of stiffness and flexion/extension. Two layouts support different residual limb morphologies: the Agonist-Antagonist (AA) version is compact and fits within the forearm, suitable for proximal transhumeral amputations, although its weight distribution concentrated in the forearm do not provide optimal comfort for distal amputees. The Distributed2 (D2) design distributes 2025 I-RIM Conference October 17-19, Rome, Italy ISBN: 9788894580570 10.5281/zenodo.17629788 157 Pisa/IIT VS-Elbow AA Pisa/IIT VS-Elbow D2 Pisa/IIT VS-Wrist Ottobock Wrist Rotator Ottobock Quick Disconnect Pisa/IIT SoftHand 2 Pisa/IIT SoftHand Pro Elbow Modules Wrist Modules Hand Modules Fig. 2. The V-Soft Pro platform. The V-Soft Pro transhumeral prosthesis supports a modular design that can be customized to meet the diverse needs of end-users, including residual limb morphology, dexterity, comfort, and available control inputs. mass evenly between the upper arm and forearm, providing more natural weight balance and improved ergonomics for users with shorter residual limbs. The VS-Wrist is the most advanced wrist in the platform, offering three active degrees of freedom and user-controllable stiffness [6]. It restores full wrist motion while allowing continuous, decoupled stiffness adjustment through modulation of nonlinear spring preload. Its hybrid parallel-serial architecture achieves three kinematic DoFs with only four motors, maintaining a compact and lightweight design. For users with simpler needs or fewer control inputs, V-SP is compatible with commercial alternatives, including the Ottobock Wrist Rotator (1-DoF myoelectric rotation) and the Quick Disconnect Wrist (passive rotation actuated by the contralateral arm). The SoftHand Pro [7] is a soft, underactuated hand that uses a single motor to drive 19 degrees of freedom in a synergistic manner, adapting automatically to a variety of object shapes. Its intuitive mechanical design enables versatile grasping with minimal control effort. For users with advanced EMG control skills, the SoftHand 2 introduces an additional actuator, expanding the range of grasping, manipulation, and fine gestures [8]. III. EXPERIMENTAL DEMONSTRATION We present a functional demonstration of the V-SP platform, configured with the VS-Elbow AA, the Ottobock Wrist Rotator, and the SoftHand 2, providing four kinematic degrees of freedom and one additional DoF for controlling elbow stiffness. Each motor is governed by a dedicated PID controller operating at 200 Hz, modulating a 20 kHz PWM signal. The system is powered by a single 24 V commercial battery. Figure 3 illustrates the V-SP system dynamically adjusting elbow stiffness to suit different tasks. This capability highlights the platform’s versatility in unstructured environments. The prosthesis can reduce elbow stiffness to navigate obstacles and safely absorb external forces, enabling gentle interactions. Conversely, it can increase stiffness to resist disturbances and support tasks requiring precision. abc Fig. 3. Functional demonstration of the V-Soft Pro system (from the supplemental multimedia material). In the first scene, the prosthesis operates with low joint stiffness to navigate an unstructured environment, allowing gentle interactions with nearby objects. In the second scene, reduced elbow stiffness enables the system to comply with external forces. The third scene shows the V-SP system increasing stiffness to counter disturbances, supporting precision tasks. IV. CONCLUSION Current robotic prostheses lack the controllable compliance inherent to human limbs, resulting in unnatural interactions with objects and people, and limiting both versatility and social engagement. The V-Soft Pro platform addresses these challenges by integrating VSAs that replicate the adaptive behavior of human joints, providing continuous stiffness modulation and more natural limb dynamics. Its user-centered, modular design allows customization of components based on amputation level, daily requirements, and myoelectric control abilities, offering configurations that optimize comfort and usability for each individual. By combining adaptive mechanics with modular flexibility, V-Soft Pro has the potential to enhance the overall user experience, delivering bionic limbs that feel more natural and personalized. Future work will include clinical investigations and additional quantitative evaluations to assess system performance and impact. REFERENCES [1] R. Osu, N. Kamimura, H. Iwasaki, E. Nakano, C. M. Harris, Y. Wada, and M. Kawato, “Optimal impedance control for task achievement in the presence of signal-dependent noise,” Journal of Neurophysiology, vol. 92, no. 2, pp. 1199–1215, 2004. [2] D. Borzelli, B. Cesqui, D. J. Berger, E. Burdet, and A. d’Avella, “Muscle patterns underlying voluntary modulation of co-contraction,” PLoS One, vol. 13, no. 10, p. e0205911, 2018. [3] A. 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