1. Introduction
Exoskeletons are robotic devices which aim to help both healthy and unhealthy people in performing activities or recover some of their natural movements. They are devices attached to humans whose kinematic chain maps onto the human limb anatomy to assist in certain tasks. They aim to effectively, efficiently, and seamlessly transfer power to the user’s assisted limbs or joints to assist in tasks of daily living/rehabilitation (medical exoskeletons) or in work related tasks (occupational exoskeletons). They could be fully attached to the user (wearable and portable) or external to the person (grounded), giving the possibility of performing tasks in clinics, at home, or at work
[1][2][3][1,2,3].
During the past few decades, researchers have been working towards the development of exoskeletons with improved capabilities and “intelligence” levels to solve problems related to aging, disabilities, overloading, and muscle strain, which may prevent people from conducting a normal life, leading to marginalization with serious repercussions on both their working and private lives.
These devices are expected to play an important role in human health in the field of rehabilitation, assistive robotics and human power augmentation since research by the World Health Organization (WHO) has shown that more than 1.7 billion people worldwide suffer from musculoskeletal disorders (MSD), and this number is set to grow
[4][5][6][4,5,6]. These problems and more specific work-related musculoskeletal disorders (WRMSD) have been known for a long time. Literature dating back to the 1700s shows that Bernardino Ramazzini, an Italian physician and scientist who dedicated himself to the systematic study of occupational diseases, the social defense of workers and the observation of working conditions, had already identified this problem and provided suggestions for preventing harm at work in
De Morbis Artificum Diatriba [Diseases of Workers] [7][8][7,8]. As still occurs today, he observed that the working environment and prolonged, violent, and irregular motions and prolonged postures are the cause of a variety of common ailments affecting joints, bones, muscles, tendons, and ligaments
[8][9][10][8,9,10]. These afflictions can be episodic or chronic in duration and can also progress from mild to severe and impair the quality of life of people, reducing mobility, dexterity and people’s ability to work and perform daily living activities (ADL).
As reported by the European Agency for Safety and Health at Work (EU-OSHA) and the Italian Workers’ Compensatory Authority (INAIL)
[6][11][6,11], the most common health problems affect low back pain (more than 50%) and upper limbs (more than 40%).
The number of projects addressing wearable robotics has increased since the 2000s, thanks to research programs such as “
Exoskeleton for human performance augmentation” set up by the Defense Advanced Research Projects Agency (DARPA). Exoskeleton application areas are related to different fields such as military, medical, and industrial
[12][13][14][15][16][12,13,14,15,16].
However, relatively few of these studies have focused on wrist injuries, although they form the fourth most common site for musculoskeletal pain in the upper limbs
[17]. Review articles on upper limbs
[1][13][18][19][20][21][22][23][24][1,13,18,19,20,21,22,23,24] have reflected a similar lack of focus on the wrist and mostly do not differentiate between the hand and wrist or even the whole upper arm
[25]. It should be noted that from a clinical perspective, wrist pain and hand pain are very different entities, both in terms of diagnosis and management
[25]. From the other side, once wrist functionalities are impaired, there is a very significant impact on the hand and fingers, especially gripping
[26].
To address these problems, many different devices have been designed over the years. The majority of them are externally grounded and used to perform rehabilitation processes in clinics or at home with the aim of restoring human hand/wrist functionalities and dexterity and muscular strength after an injury has occurred. The lack of portability limits their usefulness for unstructured environment, outdoor applications, at work, or in everyday life activities where the users should be able to move freely. Here there is the need for fully wearable devices in which the entire system (including actuators, electronics, and batteries) is attached to the human body. Devices of this format can provide assistance, augmentation or rehabilitation wherever needed and at any time, and this can be extended to prevention of injuries at work, where wearable and portable wrist exoskeletons are highly desired. However, only a few are available in the market, and these are mainly focused on the hand by improving the user’s grasping strength (e.g., Carbonhand
® and Ironhand
® by Bioservo Technologies). Therefore, there is a very real and current need for specific wearable and portable devices conceived to mitigate wrist-related problems
[25].
2. Purpose
The “purpose” criterion, refers to the target objective of the device and knowledge of the end users
[19]. This is the starting point from which the device design should flow and begins with identification of intended application domain (e.g., healthcare, manufacturing, construction, agriculture, logistics and transport, consumers, etc.). It is important at this early stage that users (companies, hospitals, or research institutes) are fully involved in the design specification to ensure that the designers have a clear focus on real needs and user requirements/expectations. This should be a fully interactive and iterative process that values and prioritizes the input from stakeholders and potential customers on the product feasibility, design proof of concept and choices, and users’ needs and expectations. In parallel with the sector definition and user requirements and expectation assessment, it is vital to identify the targeted condition that the device aims to help (e.g., recovery of motor deficit, assistance during ADL tasks, strength augmentation, lowering fatigue at work, injury prevention, etc.). This entails close study (again involving active interaction with the users) of all details of the tasks (e.g., lifting and transporting loads, maintaining awkward postures or holding a static position, hand screwing or use of tools such as screwdrivers, etc.). It is particularly important to emphasize the key role played by the stakeholders and users who can help to identify critical case studies.
3. Kinematic Compliance
In case of wearable devices, the kinematic compliance deals with the design characteristics a robot should have to comply with the movements, DoF and RoM of a human. Often when designing human–robot interfaces there is the tendency to represent human limbs as a serial chain of rigid links linked by joints that can broadly be imitated through knowledge of RoM and DoF. However, as noted when considering the anatomy of the wrist, it is clear that the kinematics of human joints is often much more complex than single DoF or even multiple DoF robot joints. They have more DoF than they need to move in a three dimensional space
[12]. So, to be kinematically compliant with the human limb to which exoskeletons are attached, it is necessary to choose a combination of joints and DoFs to achieve a desired task. In particular, every anatomical joint involves a combination of rotational and translational movements since the instantaneous center of rotation (ICR) of human joints is not fixed in a specific position but migrates. Therefore, it has to be taken into account by designing kinematic chains with rotational and translational joints, both active and passive, which can help to compensate for constrained displacements between the device and the human limb. Note that kinematic mismatch could hinder human movement and give rise to discomfort and undesired forces at the human–robot interface
[12]. Some tips for the design of exoskeletons, reported in
[12], are that the device should be easily worn, suitable for people of different sizes, comfortable, lightweight, should never have more than six DoFs between two consecutive attachments, and does not need to copy exactly the kinematic structure of the human limb.
From the human wrist kinematic perspective, the device should support the main wrist movements (flexion/extension and radial/ulnar deviation
[27][46]. One of the most interesting movements to be actuated would be the DTM since it is the most complete and natural for the human wrist, but few examples of this format exist in the literature
[28][39], maybe because the choice and positioning of appropriate joints and links could be enough to guarantee a wrist natural movement. However, sometimes this leads to complex and bulky devices.
The development of devices which provide wrist assistance along the DTM, for both health care and occupational applications, seems to be a more promising line of development since, in the former case, it would allow a more rapid recovery from injuries or surgery intervention as mobilization through the DTM plane may be considered more stable and controllable
[29][30][31][35,37,40], while for the latter, it would follow the movement of the wrist in a more natural through a less complex kinematic architecture. From the occupational perspective, the DTM plane has been investigated as one of the major sources of hand osteoarthritis (OA), CTS, and ulnar impaction syndrome. A recent analysis of wrist injuries by Park et al.
[32][28], which was conducted on Korean fishermen, reported that oyster shuckers perform tasks which have been classified as a basic risk factor for wrist and hand musculoskeletal disorders (MSD) due to the performance of hand-bearing tasks that use repetitive or improper postures while simultaneously applying power. That study noted that during oyster shucker’s work, the hand holding the knife and applying force moves repeatedly along the
dart throwing motion plane.
For RoM, this is strictly dependant on the type of activities to be performed. Even if the human wrist RoM is well known, not all its workspace would be covered during manual handling activities. For safety reasons, it is highly recommended that motions at, or near, the endpoints are avoided to prevent problems such as hyper-extension or hyper-flexion.
4. Dynamics (Forces and Torques)
When designing exoskeletons in general, it is important to take into account how much force you need to apply to assist the human limb when performing a certain task, in which directions and with which intensity. The same assumptions are also valuable for wrist exoskeletons and depend on the specific activity you should perform. Several researchers have studied this topic
[27][33][43,46].
For a healthy wrist, without external loads, the maximum exercisable torque in ADL is 6–10 Nm for pronation/supination, 8–14 Nm for flexion/extension
[34][35][33,47], and the average estimated for radial/ulnar deviation is 1.3 Nm
[36][48]. In general males exert larger torque than females. It has also been experimentally determined that healthy males exert naturally an average maximum grip force of 311 N (SD = 80), while females exert 192 N (SD = 47)
[35][47]. However, the maximum prehension force required in daily activities is almost 70 N
[37][38][49,50]. Note that the grip force almost changes linearly as the wrist angles change
[35][47].
For rehabilitation purposes, since patients often have impairments, muscular spasticity, bone weaknesses, etc., it is better to apply lower torque values to avoid worsening medical conditions. Therapists and researchers indicate that, to complete rehabilitation therapy successfully, the required torque for flexion/extension should be greater than 0.35 Nm (minimum requirement of wrist torque during ADL) but not exceeding 1.5 Nm
[28][39][40][41][39,51,52,53] while not exceeding 0.5 Nm in radial/ulnar deviation
[42][34].
In the occupational sector, the situation may change due to different workloads for different activities. In these cases higher torques are required to lower muscular fatigue. Juul-Kristensenet et al.
[43][29] reported that for manual deboning of poultry, the muscular activities of the arm measured using electromyographic (EMG) signals could be estimated to have a median level of 6.25 N and a peak level of 20.71 N at the wrist during cutting, with a maximum hand grip force of 107.91 N (almost 11 kg)
[43][29].
In general, 3 kg is considered the weight above which loads have the potential to cause a risk of injury for manual handling activities. Usually, a torque equal to or greater than 3 Nm provides adequate assistance at the wrist while lifting 3 kg of load
[44][54] placed in the palm 0.1 m far from the joint. However this supported should be increased with the load and distance. These values match Hope and McDaid
[45][55] and suggest that typical maximum torque values for each DoF in a healthy wrist are 20 Nm (for flexion/extension and radial/ulnar deviation) and 9 Nm (for pronation/supination) when maintaining the wrist in a static position, 9 Nm (for flexion/extension and radial/ulnar deviation), and 7 Nm (for pronation/supination) when moving the wrist at a constant speed of 90°/s, while the values are 0.21 Nm (for flexion/extension), 0.25 Nm (for radial/ulnar deviation), and 0.72 Nm (for prontation and supination) when performing ADL.
5. Rigidity
Referring to
[19], the rigidity criterion is mainly related to the stiffness of the device and, in particular, the stiffness of the materials of the parts which form the exoskeleton structure and provide the required torques and forces. This characteristic can sub-group robotic devices into rigid, soft, or hybrid.
A device can be considered rigid if, at the interface between the device and the user, it is mostly composed of hard and stiff materials.
A device is soft if its human–robot interfaces are composed of soft and compliant materials such as foam, rubber, or silicon. This has the advantage of not hindering the natural movement of joints and can solve the kinematics problems of joint axis mismatch between the robot and the human. Some exoskeletons are often made rigid to provide high transmission efficiency and controllability [46][47][48][49][50][51][57,58,59,60,61,62], while others adopt soft mobile parts to better adapt to humans [28][36][44][52][53][54][55][56][57][58][59][60][61][39,48,54,56,63,64,65,66,67,68,69,70,71].
Softness is closely associated with comfort and ergonomic devices since hard components, such as battery casings and actuation units, can be placed remotely from the affected joint. This relieves the user from bearing localized weights and encumbrance. However, controllability, reliability, and power transmission efficiency become more challenging.
In all cases, special emphasis has to be put on the parts directly attached to the human body because they need to be rigid to properly transfer the assistive torque or force. They have to be designed in such a way to mitigate medical issues due to long-term pressure points caused by shear forces and tight anchor sites on the body. Hence, hybrid/compliant devices are increasingly seen as a good compromise between totally rigid or soft [40][41][42][62][63][64][34,52,53,72,73,74].
6. Ergonomics
Wearable devices should optimize, where possible, ergonomics, making the device easy to wear, tailored, customizable, lightweight, and safe.
To achieve this, any ergonomic wearable robot must address human kinematics compliance. As stated before, this starts with a proper choice of the type and number of DoFs (both active and passive) and of their placement. In a recent development, to make parts that are well tailored to the users’ limbs, 3D scanner technologies have been considered a reliable and increasingly common approach
[44][65][54,75]. Scan data are useful since they provide information on the shape, size, and metrics of a real object. In fact, using these data, it is possible to directly make a CAD model of human limbs such as a hand, wrist, arm, head, trunk, lower limbs, etc. In addition, it is possible to trace their shapes and design mechanical parts or garments where forces and torques should be applied.
In any wearable device, being lightweight is a must. To distribute the weights and reduce the inertia of mechanical components, one solution is to design parts that wrap around the limb following the longitudinal axis (in direction of the limb) rather than being perpendicular to the axis.
An additional solution to reduce weight is in the placement of the actuation. Remote actuation moves the actuators from the actuated joints/limbs, locally lowering and better distributing the weights and pressure points on the human body; however, implementation is not always easy. Although the use of rigid parts provides a stable platform to accurately and repeatedly apply forces and torques, the use of soft materials, textiles, and foams is highly recommended, whenever possible, especially on human–robot interfaces or as a coating for rigid parts, as it increases both comfort and safety.
Since one of the major limitations in developing wearable technology is the acceptance of the wearers, a human-centered approach is vital. Acceptance is a condition of the human psyche with various different meanings depending on the context. Regarding technological advances, there are several different models
[66][76]. Although these are based on different hypotheses, they share the assumption that users’ acceptance is influenced by several factors which predict the intention to use new technology. Among those factors are perceived usefulness, effort expectancy (or perceived ease-of-use), performance expectancy, social influence, and facilitating conditions which determine the self-predicted future usage. These are further influenced by moderating variables such as gender, age, experience and voluntariness of use
[66][76]. To improve the quality of exoskeletons and accelerate their adoption in real scenarios, these acceptance factors must be considered from the earliest stages of the design. An example is proposed by
Collinder and
Ekstrand [67][77], where they investigated risks of hand injuries in automobile manufacturing plants with a predictive assessment of hand ergonomics in early stages of production, providing explanations for the increase in injuries among operators and recommendations that could improve the design process of specific tasks where hand ergonomics are compromised.
A better understanding of the factors that prevent the widespread use of some types of exoskeletons could help to speed up their adoption, reducing both industrial and societal skepticism
[66][76].
7. Safety
As exoskeletons are physically attached to the user, safety is a paramount consideration. Since exoskeletons have both hardware and software elements, it is vital to fully consider different levels of safety for the 1—mechanical; 2—electrical; and 3—software systems
[20]. Safety precautions in the mechanical design consist of physical stops to prevent segments from excessive excursions and from causing harm to the user (e.g., cover forbidden spaces of the robot, cover empty or open spaces at joints to avoid injuring human skin while moving or pinching fingers, avoid floating cables). The electrical system should be equipped with different emergency shutoff switches (e.g., emergency button) to stop and release motor commands when something goes wrong. The software should have multiple control routines which check sensor status, joint angles, and joint torques to prevent sudden unexpected motion and by limiting maximum torque and velocity. It is something overlooked that also the software architecture must cover specific characteristics which are prescribed in standards
[16][20][16,20].
Safety requirements are important commercialization issues possibly preventing certification. In this respect, it is important to more clearly define the purpose and application domain of since medical and non-medical exoskeletons have their own regulatory requirements. Assistive exoskeletons for workers and healthy people with or without reduced physical capabilities (e.g., elderly people) are considered
machines, and they must comply with EC Machine Directive 2006/42/EC and ISO 12100:2010
[16]. Exoskeletons used by disabled people who have lost some functionalities or for rehabilitation purposes are considered
medical devices, and they must comply with EC Medical Directive 93/42/EEC and IEC 60601
[16]. Furthermore, for every electrical/electronic apparatus (including exoskeletons), Electromagnetic Compatibility (EMC) Directive 2014/30/EU must be observed
[16].
Although countries may have their own national regulations, general market rules are usually driven by international standards such as ISO and IEC (or EN standards), and this provide manufacturers with a legal presumption of conformity almost everywhere. For wrist exoskeletons in a working environment, the most pertinent safety and ergonomic considerations are regulations for manual handling of loads activities (ISO 11228,
[68][78]) and the assessment of static working postures (ISO 11226).
ISO 11228-1 is related to lifting, lowering, and carrying of loads with a mass of 3 kg or more, and to moderate walking speed, i.e., 0.5 m/s to 1.0 m/s on a horizontal level surface. This is based on working periods of 8–12 h. It covers moderate and high loads since 3 kg is considered the weight above which loads are considered a risk of injury for manual handling activities; 15 kg and 20 kg are considered the limiting weights which should not be exceeded for women and men, respectively. For weights between 3 and 15 kg, these should be handled with some precautions and under particular conditions (e.g., height from ground, shape of the object, number of repetitions per minute, the speed of the movement, the posture, the use of force, the presence of regular breaks, etc.)
[68][78]. In particular, for the wrist, ISO 11228-1 states that the ideal condition for lifting loads consists in a firm grip with neutral wrist posture, avoiding extreme deviations from the neutral position
[68][78].
ISO 11228-3 addresses handling of low loads at high frequency. Low loads are considered to be below 3 kg. In this range, musculoskeletal problems are mainly caused by the repetitiveness and speed of the task rather than the weight. awkward postures are those that cause a wrist at flexion > 45°, extension > 45°, ulnar deviation > 20°, radial deviation > 15°
[69][79], as experimentally measured by Marras et al. in
[70][80]. Wrist flexion causes increased pressure in its deep structures (carpal tunnel), and this pressure increases if incorrect and forceful gripping force is exerted simultaneously. Hence, this risk addresses the question: “
is the force used lower than that needed to handle 200 g with the fingers (e.g., pinching) or 2 kg with the whole hand (e.g., gripping)?”
[69][79].
These regulations provide a guide for risk injury assessment, with advice on the design and redesign of the working activities and the workplace. It considers a wide range of pertinent variables and factors. In each of these standards, there are several methods of injury risk assessment. OCRA (occupational repetitive actions) and HAL (hand activity level) are two of the quantitative methods used for the risk assessment of the wrist. OCRA is the most common and uses the OCRA index to evaluate each risk factor (repetitiveness, posture, strength, recovery, etc.) and then relates this to the frequency of specific movements actually being performed. The risk is then assessed as absent, low, or critical
[68][69][78,79].
Before any machine can be placed on the market, manufacturers must compile a fully detailed technical report (TR) describing each aspect of the machine, with drawings, calculation notes, risk assessment, instructions, standards used, and EC declaration of conformity.