In the last decade, we have witnessed a drastic change in the form factor of audio and vision technologies, from heavy and grounded machines to lightweight devices that naturally fit our bodies. However, only recently, haptic systems have started to be designed with wearability in mind. The wearability of haptic systems enables novel forms of communication, cooperation, and integration between humans and machines. Wearable haptic interfaces are capable of communicating with the human wearers during their interaction with the environment they share, in a natural and yet private way. This paper presents a taxonomy and review of wearable haptic systems for the fingertip and the hand, focusing on those systems directly addressing wearability challenges. The paper also discusses the main technological and design challenges for the development of wearable haptic interfaces, and it reports on the future perspectives of the field. Finally, the paper includes two tables summarizing the characteristics and features of the most representative wearable haptic systems for the fingertip and the hand.

This paper presents a novel electromyography (EMG)-driven hand exoskeleton for bilateral rehabilitation of grasping in stroke. The developed hand exoskeleton was designed with two distinctive features: (a) kinematics with intrinsic adaptability to patient's hand size, and (b) free-palm and free-fingertip design, preserving the residual sensory perceptual capability of touch during assistance in grasping of real objects. In the envisaged bilateral training strategy, the patient's non paretic hand acted as guidance for the paretic hand in grasping tasks. Grasping force exerted by the non paretic hand was estimated in real-time from EMG signals, and then replicated as robotic assistance for the paretic hand by means of the hand-exoskeleton. Estimation of the grasping force through EMG allowed to perform rehabilitation exercises with any, non sensorized, graspable objects. This paper presents the system design, development, and experimental evaluation. Experiments were performed within a group of six healthy subjects and two chronic stroke patients, executing robotic-assisted grasping tasks. Results related to performance in estimation and modulation of the robotic assistance, and to the outcomes of the pilot rehabilitation sessions with stroke patients, positively support validity of the proposed approach for application in stroke rehabilitation.

A novel wearable haptic device for modulating contact forces at the fingertip is presented. Rendering of forces by skin deformation in three degrees of freedom (DoF), with contact-no contact capabilities, was implemented through rigid parallel kinematics. The novel asymmetrical three revolute-spherical-revolute (3-RSR) configuration allowed compact dimensions with minimum encumbrance of the hand workspace. The device was designed to render constant to low frequency deformation of the fingerpad in three DoF, combining light weight with relatively high output forces. A differential method for solving the non-trivial inverse kinematics is proposed and implemented in real time for controlling the device. The first experimental activity evaluated discrimination of different fingerpad stretch directions in a group of five subjects. The second experiment, enrolling 19 subjects, evaluated cutaneous feedback provided in a virtual pick-and-place manipulation task. Stiffness of the fingerpad plus device was measured and used to calibrate the physics of the virtual environment. The third experiment with 10 subjects evaluated interaction forces in a virtual lift-and-hold task. Although with different performance in the two manipulation experiments, overall results show that participants better controlled interaction forces when the cutaneous feedback was active, with significant differences between the visual and visuo-haptic experimental conditions.

The past decade has seen the emergence of rehabilitation treatments using virtual reality (VR) environments although translation into clinical practice has been limited so far. In this paper, an immersive VR rehabilitation training system endowed with wearable haptics is proposed for children with neuromotor impairments: it aims to enhance involvement and engagement of patients, to provide congruent multi-sensory afferent feedback during motor exercises and to benefit from the flexibility of VR in adapting exercises to the patient's need. An experimental rehabilitation session conducted with children with cerebral palsy (CP) and developmental dyspraxia (DD) has been performed to evaluate the usability of the system and proof of concept trial of the proposed approach. We compared CP/DD performance with both typically developing children and adult control group. Results show the system was compliant with different levels of motor skills and allowed patients to complete the experimental rehabilitation session, with performance varying according to the expected motor abilities of different groups. Moreover, a kinematic assessmentbased on the presented system has been designed. Obtained results reflected different motor abilities of patients and participants, suggesting suitability of the proposed kinematic assessment as a motor function outcome.

This work presents the Haptic Thimble, a novel wearable haptic device for surface exploration. The Haptic Thimble combines rendering of surface orientation with fast transient and wide frequency bandwidth tactile cues. Such features allow surface exploration with rich tactile feedback, including reactive contact - no contact transition, rendering of collisions, surface asperities and textures. Above capabilities were obtained through a novel serial kinematics wrapped around the finger, actuated by compact servo motor for orienting the last link, and by a custom voice coil for actuating the plate in contact with the fingerpad. Performance of the voice coil were measured at the bench in static and dynamic conditions, assessing the capability of reproducing generic, wide-bandwidth (0-300 Hz) tactile cues. Overall usability of the Haptic Thimble was explored within a virtual environment involving exploration of virtual surfaces. Finally, a perceptual experiment executed in a teleoperated environment with kinesthetic feedback, showed that the addition of tactile feedback, provided through the Haptic Thimble, significantly improved performance of an exploratory task

This paper presents a novel haptic device providing both kinesthetic and cutaneous cues informative of shape geometry at the contact point. The system is composed of a supporting kinesthetic haptic interface and an innovative fingertip haptic display that can instantaneously orient a small plate along the tangent plane at the contact point with a virtual shape and bring it in contact with the fingertip. We show how this local augmentation of displayed haptic information can improve human performance in shape exploration, by assessing perception thresholds in curvature discrimination. When kinesthetic feedback was enriched with cutaneous cues, we found a significantly lower threshold for curvature discrimination (1.51 Ϯ 0.2 m -1 vs. 2.62 Ϯ 0.61 m -1 , p Ͻ .05) for stimuli constituted of spheres with curvature ranging in the interval from 4 -6 m -1 . This confirms the importance in haptic perception of the stimulation of cutaneous mechanoreceptors at the fingertip.

Most current hand exoskeletons have been designed specifically for rehabilitation, assistive or haptic applications to simplify the design requirements. Clinical studies on post-stroke rehabilitation have shown that adapting assistive or haptic applications into physical therapy sessions significantly improves the motor learning and treatment process. The recent technology can lead to the creation of generic hand exoskeletons that are application-agnostic. In this paper, our motivation is to create guidelines and best practices for generic exoskeletons by reviewing the literature of current devices. First, we describe each application and briefly explain their design requirements, and then list the design selections to achieve these requirements. Then, we detail each selection by investigating the existing exoskeletons based on their design choices, and by highlighting their impact on application types. With the motivation of creating efficient generic exoskeletons in the future, we finally summarize the best practices in the literature.Index Terms-hand exoskeletons, rehabilitation hand exoskeletons, assistive hand exoskeletons, haptic hand exoskeletons

In this paper we propose a full upper limb exoskeleton for motor rehabilitation of reaching, grasping and releasing in post-stroke patients. The presented system takes into account the hand pre-shaping for object affordability and it is driven by patient's intentional control through a selfpaced asynchronous Motor Imagery based Brain Computer Interface (MI-BCI). The developed antropomorphic eight DoFs exoskeleton (two DoFs for the hand, two for the wrist and four for the arm) allows full support of the manipulation activity at the level of single upper limb joint. In this study, we show the feasibility of the proposed system through experimental rehabilitation sessions conducted with three chronic post-stroke patients. Results show the potential of the proposed system for being introduced in a rehabilitation protocol.