In the prosthetics field, one of the most important bottlenecks is still the human-machine interface, namely the socket. Indeed, several amputees still reject their prostheses or report a low satisfaction level, due to factors dealing with the socket-tissues interaction. The aim of this paper is to describe the main parameters (displacements, stress, volume fluctuations and temperature) that affect the stump-socket interface and reduce the comfort of limb prostheses. In the review, a classification of the different socket types proposed in the literature is reported, together with an analysis of advantages and disadvantages of the different solutions, from multiple viewpoints. A specific focus concerns the technological solutions available commercially and in the state-of-the-art to face an altered distribution of stresses on the residual limb tissues, volume fluctuations affecting the stump overtime, and temperature variations affecting the residual tissues within the socket. The open challenges in this research field are highlighted and the possible future routes are discussed, towards the ambitious objective of achieving an advanced socket able to self-adapt in real-time to the complex interplay of factors affecting the stump, during both static and dynamic tasks.

This work explores the recent research conducted towards the development of novel classes of devices in wearable and implantable medical applications allowed by the introduction of the soft robotics approach. In the medical field, the need for materials with mechanical properties similar to biological tissues is one of the first considerations that arises to improve comfort and safety in the physical interaction with the human body. Thus, soft robotic devices are expected to be able of accomplishing tasks no traditional rigid systems can do. In this paper, we describe future perspectives and possible routes to address scientific and clinical issues still hampering the accomplishment of ideal solutions in clinical practice. In the past, rigid robotic systems have been the main research focus in traditional robotics due to their ability to provide high output forces and perform fast and precise position control tasks. However, a world in which robotic systems should directly interact with humans must drastically switch its focus towards new human-centered requirements to guarantee a safe physical human-robot interaction (Trimmer 2014; Murphy 2022). In robotic systems, safety can be achieved through specific planning and control strategies, such as working on robot navigation, collision detection and avoidance, stiffness and force control. Otherwise, it can rely on specific mechanical design rules (Pervez and Ryu 2008). Focusing on this second approach, soft robotics opens up novel possibilities for bridging the gap between humans and machines (Kim et al., 2013; Wood and Walsh 2013; Laschi et al., 2016). Indeed, soft robotics aims to develop “systems that are capable of autonomous behavior, and that are primarily composed of materials with moduli in the range of that of soft biological materials” (Rus and Tolley 2015). Thus, the soft paradigm turns out especially intriguing for biorobotic applications that require close interactions with the human body (Cianchetti et al., 2018). Against this background, this work analyses the current challenges and future perspectives of soft robots in wearable and implantable medical applications. Starting from the state-of-the-art in these fields, novel possible scenarios are figured out based on the exploitation of the advantages of the soft robotic approach and with the awareness that “soft robots will be capable of performing feats no current machines can accomplish” (Trimmer 2013). In the last decades, the development of ever more sophisticated wearable robots, such as limb prostheses and exoskeletons, has made it possible to transcend several disabilities through technological innovation. To accomplish this task and even more, achieving a physical and cognitive symbiosis of the device with the wearer is the key aspect (Pons 2019). Focusing on the physical interaction of wearable robots, huge research efforts are still required in the design area since “we still do not understand how to attach devices to the body, mechanically” (Herr, 2014). An ideal physical Human-Machine Interface (pHMI) should enable an unobtrusive biomechanical coupling with the user’s body, resulting similar to a “second skin” (Herr 2009). It should allow for proper alignment with natural joints and safe force transmissions, avoiding kinematic incompatibilities and motion restrictions. Furthermore, it should be comfortable. Indeed, even the most brilliant and advanced system is useless if comfort is not guaranteed. Current wearable pHMIs are rigid and passive structures. Although a soft layer is usually interposed between rigid materials and natural tissues, the unusual normal and shear stresses applied on the skin often lead to lesions and pain, and alter blood circulation. Volume and shape changes of the body—caused by fluid movements or muscle contraction/relaxation—affect the interface fitting, thus causing relative movements and altering in turn the stress distribution on tissues. The artificial physical barrier between the skin and the environment prevents the physiological thermal transfer mechanisms, thus affecting the homeostasis of the body and increasing the sweating rate. These aspects are the cause of several discomforts, dermatological and vascular complications, and, in the worst cases, device abandonment (Herr 2009; Wood and Walsh 2013). Solving these issues is particularly urgent in the prosthetic field, especially in lower limb amputees due to the higher loads involved at the residual limb/prosthesis interface, namely the socket (Paternò et al., 2018). Osseointegration is a recent method that allows for connecting the prosthesis directly to the patient’s bones, thus restoring the load transmission through the skeleton and overcoming socket-related issues. Even though it is the most promising approach, it is invasive and not easily acceptable by patients. Furthermore, several challenges have still to be faced, especially in terms of infection risks due to its percutaneous nature (Overmann and Forsberg 2020). Thus, smart prosthetic sockets integrating sensing and actuation technologies able to optimize fitting and stress distribution on tissues in an automatic and closed-loop fashion could be an alternative successful solution (Sanders et al., 2019; Weathersby et al., 2021). Recently, in the exoskeletons field, great steps forward have been carried out thanks to the exploitation of the soft robotics paradigm. Different solutions based on textile anchors (Figure 1A) and soft actuators (Figure 1B) have been developed, enabling designs more similar to suits than to robots (Wood and Walsh 2013; Masia et al., 2018; Walsh 2018). These designs show great advantages, especially in terms of physical interaction, avoiding the need for precise alignments with the user’s natural joints and eliminating movement restrictions due to weight and encumbrance. However, rigid systems remain the only available solution in some specific rehabilitation domains, and there is still great room for improvement to achieve real biomimetic systems able to meet the users’ acceptance, particularly in terms of comfort (Pons 2019; Xiloyannis et al., 2021). FIGURE 1. (A) Soft robotic exosuit for assistive and rehabilitation applications in post-stroke patients (Awad et al., 2020) (republished from Koch and Font-Llagunes, 2021); (B) Wearable device for ankle–foot rehabilitation based on McKibben pneumatic artificial muscles (Park et al., 2014) (republished from Koch and Font-Llagunes, 2021); (C) Soft wrist-wearable robot with a motor-driven tensioning system to improve force transmission (Choi W. H. et al., 2019) (courtesy of Professor Kyu-Jin Cho, Seoul National University, South Korea); (D) Motor-driven adjustable prosthetic socket for transfemoral amputees (Paterno et al., 2021) (courtesy of Professor Arianna Menciassi, Scuola Superiore Sant’Anna, Italy); (E) Sensorized orthosis made by vacuum lamination of a stretchable sensor system (Bauwens et al., 2020) (courtesy of Professor Jan Vanfleteren, Ghent University, Belgium); (F) Variable stiffness and shape orthosis (Ibrahimi et al., 2021) (courtesy of Professor Arianna Menciassi, Scuola Superiore Sant’Anna, Italy); (G) Active textiles based on fluid-driven artificial muscle fibers (republished from Phan et al., 2022); (H) Triboelectric All-Textile Sensor Array (TATSA) for epidermal pressure measuring (Fan et al., 2020) (republished from Tseghai et al., 2020); (I) Stretchable and washable strain sensor for human motion monitoring (republished from Tolvanen et al., 2018); (J) E-textile suit for surface electromyography for level-of-effort estimation (republished from Ohiri et al., 2022). All figures are republished under the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/). In this context, the blending of soft robotics and material science has a special potential to enable wearable interfaces with new capabilities mainly based on the tunability of the mechanical properties of their constitutive materials and the morphological adaptability of their structures. The constant progress in CAD-CAM techniques offers an attractive pathway toward the development of bioinspired materials with even more complex architectures (Studart 2016). Heterogeneous multifunctional composite materials could be used at the pHMIs to obtain stiffness gradients and damping properties. These interfaces could be designed and developed according to the properties of the underlying tissues and limb shape (Sengeh et al., 2013; Petron et al., 2017; Paternò et al., 2020). They could be combined with actuation systems to compensate for major volume and shape changes, thus guaranteeing proper fitting, over time. Along this path, the active pHMI of the wrist-wearable robot proposed by Choi H. et al. (2019) offers a promising solution that combines a multi-layer soft structure with a motor-driven corset for efficient fitting and force transmission (Figure 1C). Similarly, motor-driven sockets combined with silicone socks, namely liners, have been proposed in the prosthetic domain (Figure 1D). Alternatively, active interfaces able to modulate their stiffness and damping properties in a wide range could be integrated to improve the interaction with the biological tissues (Kornbluh et al., 2004). They could allow for a stiffened interface of the device when biomechanical stability is really needed, otherwise for a soft, compliant, and comfortable interaction. Technologies with a pseudo-planar design, such as layer jamming based on vacuum (Choi H. et al., 2019; Paterno et al., 2022) or electrostatic forces, are highly promising in this sense (Kornbluh et al., 2004; Herr 2014; Wang et al., 2019). In addition, the integration of vacuum jamming technology with soft pneumatic actuators has already demonstrated the potential to enable tunable stiffness and shape-changing interfaces able to improve both comfort and fitting of wearable devices (Figure 1F) (Ibrahimi et al., 2021). Pneumatic systems can benefit from many energy sources compatible with wearable application requirements (Wehner et al., 2014). Moreover, the development of textile-based pneumatic energy harvesting system (Shveda et al., 2022) or the use of soft valves that can control airflow by acting as switches for automated functions (Rothemund et al., 2018; Kendre et al., 2022) have paved the way for overcoming the main challenges of current bulky and expensive commercial mechatronic components for pneumatic systems. In this scenario, active fluidic textiles based on pneumatic artificial muscle fibers can represent a successful soft actuation solution (Figure 1G) (Kurumaya et al., 2019; Zhu et al., 2019; Phan et al., 2022). Indeed, they could be combined in pseudo planar design with a great variety of smart fabric sensors and e-textile technologies (Figures 1H–J) to achieve a pHMI able to adapt to the physiological changes of the body, over time (Castano and Flatau 2014; Islam et al., 2020). The pHMI of wearable robots must be equipped with sensors and actuators also to allow for intention decoding and sensory feedback functionalities. Indeed, a closed human-centered control loop involving efferent motor outputs and afferent sensory inputs is fundamental to integrate the device with the user, synergically. Focusing on non-implantable bidirectional physical interfaces, some technologies can be integrated into the external rigid parts of wearable robots since direct contact with the user’s tissues is not required (e.g., sensor network based on IMUs) (Lotti et al., 2022). Since rigid parts usually replicate the user’s body shape and are made of thermoformed materials, stretchable electronics can be employed to obtain compact and light designs, as shown in Figure 1E (Bauwens et al., 2020). Otherwise, solutions based on soft and compliant materials, such as textiles or elastomers, have to be used if contact with tissues is needed (e.g., EMG sensors) (Figures 1H–J). The integration of several technologies in physical wearable interfaces can be stated as the main challenge. In this regard, the recent advances in manufacturing processes, material and computer science, miniaturized and stretchable electronics have been pushing forward the evolution of a novel class of materials that can integrate computing and communication components, sensors, and actuators in increasingly compact and tight designs (McEvoy and Correll 2015). These so-called “robotic materials” can change not only their stiffness but also their shape or other physical parameters, and they can do this in a fully programmable way. The robotic material concept is based on the exploitation of miniaturized technologies that can sense material properties, process them, and activate desired actions. All these technologies have to be integrated into a continuous structure created by the material itself. This approach is in its infancy but the great progress in textile engineering, soft robotics, and smart material science in the last decades has opened up several novel avenues to accomplish this goal (Sanchez et al., 2021). By using soft and comfortable materials as continuous structures in contact with the user’s body, the route of robotic materials is particularly intriguing for the development of wearable interfaces that are not just passive and rigid structures, but robotic skins able to sense, exchange information with the user, and self-adapt in real-time and in a closed-loop fashion to the full spectrum of the wearer’s needs. Less than 10 years ago, the biomedical field experienced an unprecedented development of high-fidelity physical organ simulators with various applications, such as the training of aspiring practitioners and surgeons, and the extensive testing of novel biomedical devices and medical instrumentations (Maglio et al., 2021). Undoubtedly, this recent growth of human body-parts simulators has been strongly encouraged and accompanied by the concurrent establishment of soft robotics principles and technologies. Firstly, the engineering of long-lasting materials with tunable mechanical properties, such as elastomers, allowed an accurate replication of not only the haptic perception of the organ but also of its natural dynamics in the body. To give just one example, in 2021, Conte et al. (2021) presented a dynamic and passive vocal folds (VFs) simulator made by three different silicone materials, to mimic the natural multilayer morphology of the VFs (see Figure 2A). The geometry replicates the main features of the natural counterpart and the materials have mechanical properties that approximate the ones of the natural tissues, enabling the simulation of the vibratory characteristics of both healthy and pathological VFs. FIGURE 2. (A) Synthetic vocal folds simulator (courtesy of Dr Martina Maselli, Scuola Superiore Sant’Anna, Italy); (B) Soft robotic sleeve, pneumatically actuated by McKibben artificial muscles (republished from Park et al. (2022)); (C) Soft robotic left ventricle prototype, pneumatically actuated by a dual-layer of flattened and cylindrical McKibben actuators (republished from Park et al. (2022)); (D) Soft robotic artificial ventilator: a close-up on a supradiaphragmatic McKibben actuator placed on the right hemidiaphragm of an in vivo porcine model (republished from Hu et al., 2021); (E) Soft Anal Band System (Agency for Medical Innovation, Austria), hydraulically actuated (republished from Goos et al., 2013). (F) Soft robotic artificial bladder, hydraulically actuated by bellows actuators (Casagrande et al., 2022) (courtesy of Ms Giada Casagrande, Scuola Superiore Sant’Anna, Italy). All figures are republished under the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/). When the target body part has a natural motility given by the contraction of muscle fibers, the simulation of the organ physiology requires constitutive materials that have similar mechanical properties, and that are active and stimuli-responsive too. In this scenario, the integration of soft actuating technologies resulted to be game-changing, and the natural motion of several human organs was reproduced (Gong et al., 2021). In the past decade, outstanding results were achieved using soft robotics in cardiovascular therapy (Wamala et al., 2017). A significant branch of studies started from the development of a soft and active material in which an elastomeric matrix embeds McKibben artificial muscles (Roche et al., 2014). By helically disposing the actuators in a 3D shape, the dominant muscle layer of the heart myocardium was represented. As a result, upon actuation, the motion of the structure accurately simulated the apical twist of a human left ventricle. Lately, this bioinspired actuation strategy was implemented in a soft robotic sleeve that conforms around the natural heart to provide cardiocirculatory support (see Figure 2B) (Roche et al., 2017). The adaptability demonstrated in the interaction with the natural heart both ex vivo and in vivo would not have been possible either with rigid components or with compliant materials only; the introduction of soft robotics technologies enabled the controllable and dynamic change of material properties in relation to physiological needs. For implanted devices, the surface interaction with biological fluids, especially the blood, is among the biggest challenges that need to be faced. In fact, the human body reacts to intimate contact with a foreign body by activating an inflammatory response, that leads to the formation of fibrotic tissue and/or the coagulation cascade, with consequent thromboembolic complication. For cardiocirculatory devices, if one possibility is to avoid direct contact with blood, the consequent design space limitation probably leaves no option to those patients that, being affected by end-stage heart failure, need a partial, or even a total, organ replacement (Cohrs et al., 2017). In 2022, Roche et al. presented a design framework for optimizing both the biomechanics and the hemodynamics of a soft robotic synthetic ventricle (Figure 2C) whose functionality is within the physiological range of a healthy left ventricle (Park et al., 2022). However, prior to the implementation of the ventricle design for in vivo applications, the performances need to be verified in a simulator of human circulation, and a comprehensive assessment of the thrombogenic risk associated with the ventricle implant must be performed. Additional considerations can be made on the possibility of adjusting the functionality of a soft robotic device in relation to the body’s needs, an aspect of paramount importance that requires the integration of sensor technologies and control units (Polygerinos et al., 2017). Very recently, exciting results on the synchronization with the native respiratory efforts were shown by an implantable ventilator that consists of two McKibben actuators placed on the diaphragm of the patient and anchored to the ribs (Figure 2D) (Hu et al., 2021). The intimate functional and biological integration of an implantable soft system with the human body is a fundamental requirement, whose fulfilment cannot forget the design of robot-to-body interfaces and the selection of suitable materials. While the research for the development of a bio-adhesive or a bio-integrating component will be crucial in the near future to guarantee a safe and stable mechanical coupling of a soft robotic implant with the natural tissues, several biocompatible elastomers, whether for short- or long-term implants, are readily available in the market (www.elkem.com). However, their poor availability, the higher costs of the materials, and the expensive...

Fluidic mechanisms have stimulated research and development in minimally invasive surgery instrumentations, because of their good performance in limited size and their force/torque generation with respect to other types of actuation systems. Fluidic solutions can be divided in two major classes: (i) elastic fluidic actuators and (ii) piston–cylinder actuators. Elastic fluidic actuators generate lower forces with smaller displacements; nevertheless, piston–cylinder solutions require seals, which can generate friction and require maintenance costs for a good reliability. The proposed solution is based on a hybrid soft–rigid actuation, which aims to overcome the limitations of both previous solutions while preserving the main advantages of the overall fluidic approach. This approach results in very compact, powerful, and low-cost actuators, which are highly customizable and adaptable to specific constraints, in medical applications but even beyond. This article proposes a novel design of hybrid soft–rigid actuators to be used as basic mechanical joints for enabling pitch and roll degrees of freedom for a miniature robotic arm. Forces up to 1.4 N and up to 2.77 N have been obtained for the above joints, respectively, and even better performance can be reached (up to 3 N) with further improvements, as demonstrated in this article.

This study constitutes the first attempt to systematically quantify residual limb volume fluctuations in transfemoral amputees. The study was carried out on 24 amputees to investigate variations due to prosthesis doffing, physical activity, and testing time. A proper experimental set-up was designed, including a 3D optical scanner to improve precision and acceptability by amputees. The first test session aimed at measuring residual limb volume at 7 time-points, with 10 min intervals, after prosthesis doffing. This allowed for evaluating the time required for volume stabilization after prosthesis removal, for each amputee. In subsequent sessions, 16 residual limb scans in a day for each amputee were captured to evaluate volume fluctuations due to prosthesis removal and physical activity, in two times per day (morning and afternoon). These measurements were repeated in three different days, a week apart from each other, for a total of 48 scans for each amputee. Volume fluctuations over time after prosthesis doffing showed a two-term decay exponential trend (R² = 0.97), with the highest variation in the initial 10 min and an average stabilization time of 30 min. A statistically significant increase in residual limb volume following both prosthesis removal and physical activity was verified. No differences were observed between measures collected in the morning and in the afternoon.Clinical Trials.gov ID: NCT04709367.

In bioinspired soft robotics, very few studies have focused on fluidic transmissions and there is an urgent need for translating fluidic concepts into realizable fluidic components to be applied in different fields. Nature has often offered an inspiring reference to design new efficient devices. Inspired by the working principle of a marine worm, the sipunculid species Phascolosoma stephensoni (Sipunculidae, Annelida), a soft linear fluidic actuator is here presented. The natural hydrostatic skeleton combined with muscle activity enables these organisms to protrude a part of their body to explore the surrounding. Looking at the hydrostatic skeleton and protrusion mechanism of sipunculids, our solution is based on a twofold fluidic component, exploiting the advantages of both pneumatic and hydraulic actuations and providing a novel fluidic transmission mechanism. The inflation of a soft pneumatic chamber is associated with the stretch of an inner hydraulic chamber due to the incompressibility of the liquid. Actuator stretch and forces have been characterized to determine system performance. In addition, an analytical model has been derived to relate the stretch ability to the inlet pressure. Three different sizes of prototypes were tested to evaluate the suitability of the proposed design for miniaturization. The proposed actuator features a strain equal to 40–50% of its initial length—depending on size—and output forces up to 18 N in the largest prototypes. The proposed bioinspired actuator expands the design of fluidic actuators and can pave the way for new approaches in soft robotics with potential application in the medical field.

Background: Among the different factors affecting socket comfort, the pressure applied on residual limb tissues is a crucial parameter for the success or failure of any prosthetic device. However, only a few incomplete data are available on people with transfemoral amputation, in this regard. This work aims at filling this gap in the literature. Methods: Ten people with transfemoral amputation wearing 3 different socket designs were recruited in this study: 2 ischial containment sockets featured by proximal trim lines that contain the ischial tuberosity and ramus and greater trochanter, 2 subischial sockets with proximal trim lines under the ischium level, and 6 quadrilateral sockets with proximal trim lines that contain the greater trochanter and create a horizontal seat for the ischial tuberosity. The pressure values at the anterior, lateral, posterior, and medial areas of the socket interface were recorded during 5 locomotion tasks (ie, horizontal, ascent, and descent walking, upstairs and downstairs) by using an F-Socket System (Tekscan Inc., Boston, MA). Gait segmentation was performed by exploiting plantar pressure, which was acquired by an additional sensor under the foot. Mean and standard deviation of minimum and maximum values were calculated for each interface area, locomotion task, and socket design. The mean pressure patterns during different locomotion tasks were reported, as well. Results: Considering all subjects irrespective of socket design, the mean pressure range resulted 45.3 (posterior)–106.7 (posterior) kPa in horizontal walking; 48.3 (posterior)–113.8 (posterior) kPa in ascent walking; 50.8 (posterior)–105.7 (posterior) kPa in descent walking; 47.9 (posterior)–102.9 (lateral) kPa during upstairs; and 41.8 (posterior)–84.5 (anterior) kPa during downstairs. Qualitative differences in socket designs have been found. Conclusions: These data allow for a comprehensive analysis of pressures acting at the tissue-socket interface in people with transfemoral amputation, thus offering essential information for the design of novel solutions or to improve existing ones, in this field.