Leader: Luigi Bruno (UNICAL); Other collaborator(s):
This task will cover the design, development and validation of innovative materials and actuators systems for exoskeletons and prosthetics with a particular focus on nanostructured composite materials and Carbon fiber-based Twisted and Coiled Artificial Muscles (TCAMs). After mechanical and functional characterization studies, which will define a selection framework to identify the proper material(s), structural components, artificial muscles, and actuators systems will be applied to support motor rehabilitation.
Brief description of the activities and of the intermediate results:
During the reference period (November 2023-March 2024), the drafting of the review article on actuators used in upper limb wearable robotics was completed. The article was subsequently submitted and is currently undergoing review. Analysis of the upper limb's condition identified Twisted and Coiled Artificial Muscles (TCAMs) as potential candidates for innovative rehabilitation systems. Consequently, an experimental setup was designed for the production, heat treatment, and characterization of these artificial muscles. Additionally, preliminary tests were conducted to assess the performance of TCAMs in terms of force and displacement capabilities. The results demonstrated how this actuation technology could be utilized for low-load joint manipulation, such as wrist and phalanges movement. Furthermore, the thermo-mechanical behavior of TCAMs was analytically simulated, considering the actuator's geometry akin to a spring and utilizing the second theorem of Castigliano (CST). The analytical model was then implemented in Matlab. The numerical results will be compared in a subsequent phase with experimental data to establish the accuracy of the model employed. TCAMs were employed in the development of a first prototype of an active fixation mechanism for wearable exoskeletons. The system consists of a series of pulleys that accommodate the actuator's contraction and thus ensure the anchoring point's grip.
Main policy, industrial and scientific implications:
The research has several significant policy, industry, and practice implications:
Overall, the research has the potential to drive innovation in both the healthcare and robotics industries, improve rehabilitation practices, and influence policy decisions related to the integration of advanced actuation technologies into medical devices and rehabilitation equipment.
Please see the next reporting period.
An experimental setup was created for the on-site production and characterization of artificial muscles. Consequently, an experimental campaign was initiated to extrapolate the performance of the TCAMs. The tests involved single-ply artificial muscles, made from nylon 6,6 precursor filaments with an external silver layer to allow electrothermal actuation. Specifically, to understand their thermo-electro-mechanical behavior, experimental investigations were carried out by varying some fundamental production parameters, such as rotation speed, applied load, and power supply current. The results were presented in the "Biomechanics 1" session of the 53rd AIAS conference held in Naples on September 4th-7th 2024. Further experimental investigations are currently underway to complete the previously initiated campaign and verify the integrability of the TCAMs in a physical prototype of a wearable exoskeleton.
The results obtained from the experimental campaign conducted during the previous trimester enabled the initiation of a new study aimed at implementing novel actuation technology within a custom-designed exoskeletal gripping device. This device is fabricated using 3D printing techniques and incorporates silicone components to enhance ergonomic performance during use, as well as TCAMs to enable actuation. In parallel with the experimentation on the exoskeletal gripping prototype, efforts are underway to develop a thermo-electro-mechanical model capable of reproducing and predicting the behaviour of TCAMs when actuated by electrical stimulation. Specifically, the model consists of two fundamental components: an electro-thermal component designed to determine the temperature increase induced in the muscles by an electrical input and a thermo-mechanical component aimed at converting the temperature increase into the resulting axial displacement. The analytical model is entirely based on purely physical, macro-, and micromechanical considerations, thereby avoiding any reliance on empirical assumptions. The results provided by the analytical model were directly compared with experimental findings, demonstrating a high degree of accuracy. This has facilitated the drafting of a scientific article focusing on the experimental trials conducted with TCAMs and their analytical modelling.
During this trimester, research activities have focused on analyzing the experimental results obtained with Twisted and Coiled Artificial Muscles (TCAMs). The experimental data have been compared with a previously developed analytical model, demonstrating a high level of accuracy. This has laid a solid foundation for writing a scientific paper on the experimental tests and the corresponding analytical modeling of TCAMs. The first draft of the paper is already complete, and we are currently conducting an internal review before submitting it to a dedicated journal.
Meanwhile, Finite Element Analysis (FEA) has been carried out with the implementation of a dedicated subroutine to accurately replicate the thermo-mechanical behavior of these types of actuators. Specifically, the subroutine integrates the analytical model, which has been validated through experimental data, and has already been used in a preliminary series of simulations that yielded promising results. The subroutine and FEA implementation are still ongoing, cause the primary objective is to replicate the behavior of each type of precursor fiber tested experimentally.
In parallel, a collaboration with the Department of Mechanical Engineering at the University of Iowa (USA) has been initiated. The main goal is to enhance the performance of TCAMs, particularly in terms of efficiency and actuation rate. As part of this collaboration, bi-stability and mechanical instabilities are being investigated to enable faster deployment of the muscle geometry, thereby improving efficiency and significantly increasing the actuation rate.
In collaboration with task 3.4, our development of the dynamic fixing system for wearable robotics was presented at the 3rd IFToMM for Sustainable Development Goals Conference (I4SDG 2025) in Villa San Giovanni, Italy (June 9–12, 2025). Our teams were honoured with the I4SDG 2025 Best Research Paper Award, sponsored by the journal Robotics (ISSN 22186581), in recognition of the paper's technical excellence and relevance to sustainable robotics. During the period from April through June, the project achieved significant advances across simulation, publication, and hardware development, closely aligned with our goals. In April, we made substantial progress in enhancing our Finite Element Method (FEM) simulation framework by constructing a robust subroutine capable of modeling the thermo-mechanical behavior of Twisted and Coiled Artificial Muscles (TCAMs). Almost concurrently, we solidified a research partnership with the Department of Mechanical Engineering at the University of Iowa (USA). This collaboration prioritized improving both responsiveness and efficiency of TCAMs, exploring bistable structural designs expected to yield notable gains in actuation speed and energy performance. Throughout May, the simulation subroutine reached completion and was integrated into the FEM environment, successfully demonstrating accurate reproduction of TCAM thermo-mechanical responses. Parallel efforts continued with the internal review of our manuscript, which documents both the experimental testing on artificial muscles and their analytical modelling. Within the Iowa collaboration, we intensified investigations into mechanical instabilities, such as snap-through mechanisms and bistability, to further enhance actuation speed. By June, the internal peer review of the manuscript had concluded, allowing us to prepare its submission to a high-impact scientific journal. Simultaneously, our joint research initiative with Iowa culminated in the conceptualization and prototyping of a novel pneumatic artificial muscle. This design incorporates snap-through bistability to attain rapid actuation, while also offering enhanced versatility and configurability.
During the period from July through September, research activities continued with significant progress and major milestones achieved across publication, experimental testing, and collaborative development.
In July, following an extensive internal review process, the manuscript entitled “Experimental Analysis and Physics-Based Analytical Model on Twisted and Coiled Artificial Muscles (TCAMs)” was submitted to the journal Advanced Engineering Materials (Wiley). Meanwhile, the collaboration with the Department of Mechanical Engineering at the University of Iowa (USA) continued to focus on improving the performance and efficiency of artificial muscles. One of the primary limitations of TCAMs is their low energy efficiency, primarily due to the high electrical power required for actuation. To address this issue, we investigated alternative approaches based on mechanical instability and bi-stability, intending to minimize the energy input required for activation. The underlying idea is to provide only a minimal energy contribution capable of triggering specific instability points in the actuator, thus inducing full actuation.
In this context, pneumatic actuators exploiting mechanical instability and bi-stability were developed and prototyped, achieving remarkable performance in terms of both contraction and output force. These devices also demonstrated exceptionally high efficiencies (above 50%), especially when compared with conventional soft actuators. Moreover, depending on the materials used, some prototypes exhibited strong versatility and self-locking capability: in particular, rigid materials allowed the actuator to maintain the “closed” (fully contracted) configuration without any external energy input.
In August, the prototyping phase of the pneumatic actuators reached completion. Multiple designs were fabricated using various 3D printing techniques, including FDM and SLA, with different materials to assess the effect of stiffness and elasticity on performance. Experimental testing began soon after, aimed at mechanically characterizing this new class of actuators. The next step involves comparing the experimental results with those of commercially available pneumatic actuators to quantitatively evaluate the benefits of integrating mechanical instability and bistability in terms of both efficiency and actuation performance.
In September, the research group participated in the 54th AIAS Conference (Italian Scientific Society for Machine Design and Mechanical Construction), held in Florence from September 3rd to 6th. The conference provided an excellent opportunity to present the findings discussed in the submitted paper “Experimental Analysis and Physics-Based Analytical Model on Twisted and Coiled Artificial Muscles (TCAMs)” through an oral presentation. Concurrently, the collaboration with the University of Iowa continued, with the experimental phase on bistable pneumatic actuators nearing completion. The promising results obtained so far are now forming the basis for a new scientific manuscript, currently under preparation, that will document the design, experimental characterization, and performance advantages of these novel bistable actuators.
Scientific publications
Dissemination Events