Elodie Bouzbib

Lisheng Kuang

Paolo Robuffo Giordano

Anatole Lécuyer

Claudio Pacchierotti

Survey of Wearable Haptic Technologies for Navigation Guidance

Keywords: Wearable haptics, wearables, navigation guidance, survey, state-of-the-art, guidance strategies, wearable device

Wearable haptic technologies, known as wearable haptics, instantiate physical contacts with users, either to confirm actions or to communicate a surrounding information -over a worn-type interface. Two factors require to be addressed for their design: the body locus -a function of comfort and sensitivity, and the types of stimuli they provide. Wearable haptics have gained great popularity in the last decade, thanks to their effectiveness, ease of use, and variety of application scenarios. This paper provides a non-exhaustive review of the state of the art on wearable haptics for navigation guidance. We classify the existing literature through two dimensions: (1) the body part they stimulate and (2) the haptic stimulation they provide through their actuation technology. We then analyze the navigation guidance strategies they adopt for communicating with the wearer, to finally identify challenges and limitations reflected in their evaluation protocols. Compromises are to be drawn when eliciting guidance through wearable haptics, between acceptability, cognitive workload, usability and accuracy.

I. INTRODUCTION

H UMAN-ROBOT and human-machine interaction tech- nologies currently aim to provide new ways to interact and exchange information with machines or artificial digital environments. Yet, communicating information between humans and machines requires an effective mediating tool. In these regards, wearable haptic interfaces -haptic systems that users can literally "wear" -are perceived as a viable interaction medium [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives[END_REF], instantiating physical feedback to their users.

Wearable haptics enhance physical interactions, enabling users to feel remote and virtual worlds as well as perform a wide variety of exploration and manipulation tasks. They stimulate the human body (e.g. hands, arm, leg, head) to convey rich touch sensations, in an intuitive and understandable manner, delivered through small, lightweight, and often inexpensive packages [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives[END_REF]. These promising features justify the recent burst of available wearable devices communicating through haptics, which have successfully been employed in a wide range of scenarios, including robotic teleoperation [START_REF] Bimbo | Teleoperation in cluttered environments using wearable haptic feedback[END_REF], [START_REF] Ryu | Wearable haptic-based multi-modal teleoperation of field mobile manipulator for explosive ordnance disposal[END_REF], [START_REF] Pacchierotti | Cutaneous feedback of fingertip deformation and vibration for palpation in robotic surgery[END_REF], virtual and augmented reality [START_REF] Meli | Combining wearable finger haptics and augmented reality: User evaluation using an external camera and the microsoft hololens[END_REF], [START_REF] Gosselin | Design of a wearable haptic interface for precise finger interactions in large virtual environments[END_REF], motor rehabilitation and augmentation [START_REF] Bortone | Wearable haptics and immersive virtual reality rehabilitation training in children with neuromotor impairments[END_REF], [START_REF] Hussain | A wearable haptic ring for the control of extra robotic fingers[END_REF], surgical interventions [START_REF] Condino | Tactile augmented reality for arteries palpation in open surgery training[END_REF], [START_REF] Tinguy | Enhancing the stiffness perception of tangible objects in mixed reality using wearable haptics[END_REF], [START_REF] Meli | Sensory subtraction in robot-assisted surgery: fingertip skin deformation feedback to ensure safety and improve transparency in bimanual haptic interaction[END_REF], [START_REF] Abad | A Novel Untethered Hand Wearable with Fine-Grained Cutaneous Haptic Feedback[END_REF], assistive robotics [START_REF] Sorgini | Haptic-assistive technologies for audition and vision sensory disabilities[END_REF], and navigation.

Wearable haptics have been proven particularly useful and effective in providing navigation guidance information, mostly E. Bouzbib and A. Lécuyer are with Inria, CNRS, Univ Rennes, IRISA -Rennes, France. E-mail: {name.surname}@inria.fr L. Kuang, P. Robuffo Giordano, and C. Pacchierotti are with CNRS, Univ Rennes, Inria, IRISA -Rennes, France. Email: {name.surname}@irisa.fr thanks to their distributed and private rendering capabilities [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives[END_REF]. Haptic cues have been used to communicate turn-by-turn navigation [START_REF] Prasad | HaptiMoto: turn-by-turn haptic route guidance interface for motorcyclists[END_REF], guidance in industrial teleoperation [START_REF] Mo | A Lightweight Accessible Wearable Robotic Interface for Bimanual Haptic Manipulations[END_REF], [START_REF] Abi-Farraj | A haptic shared-control architecture for guided multi-target robotic grasping[END_REF], [START_REF] Pacchierotti | Cutaneous/tactile haptic feedback in robotic teleoperation: Motivation, survey, and perspectives[END_REF], rehabilitation, or assembly tasks [START_REF] Preechayasomboon | Chasm: A Screw Based Expressive Compact Haptic Actuator[END_REF], and support people with sensory disabilities [START_REF] Zhao | Enabling People with Visual Impairments to Navigate Virtual Reality with a Haptic and Auditory Cane Simulation[END_REF].

This paper provides a non-exhaustive review of the state of the art on wearable haptics for navigation guidance.We analyze the existing literature according to [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives[END_REF] the body part the wearable haptic devices and technologies stimulate, and (2) the type of haptic stimulation they provide. We then describe the navigation guidance strategies they adopt for communicating with the wearer, and finally discuss the challenges and limitations highlighted through wearable haptics for navigation guidance evaluation protocols.

II. DEFINITIONS, BACKGROUND AND SCOPE

In this section, we provide some definitions and background on haptics, wearables, navigation, and describe our scope at these fields' intersection.

A. Definitions and Background 1) Haptics -Perception and Experience:

We can consider our sense of touch (or haptic sense) as composed of two sensory sub-modalities, cutaneous and kinesthetic. On the one hand, cutaneous sensations are elicited by receptors and corpuscles embedded in the skin, which are present in different densities and types throughout our bodies. On a perception perspective, they enable us to identify local properties of the environment, such as the shape, temperature, and texture of an object we touch. Cutaneous perception for exploration and manipulation principally relies on measures of the location, intensity, direction, and timing of contact forces on the hands [START_REF] Birznieks | Encoding of direction of fingertip forces by human tactile afferents[END_REF], [START_REF] Johnson | The roles and functions of cutaneous mechanoreceptors[END_REF], [START_REF] Yousef | Tactile sensing for dexterous in-hand manipulation in robotics-a review[END_REF]. On the other hand, kinesthetic sensations originate from the muscles, joints, and skin. They enable us to identify the position and velocity of our limbs, as well as the forces they apply [START_REF] Birznieks | Encoding of direction of fingertip forces by human tactile afferents[END_REF], [START_REF] Johnson | The roles and functions of cutaneous mechanoreceptors[END_REF]. Kinesthesia is indeed strictly intertwined with proprioception. We can define the former as the cognizance of joint movement and the latter as the awareness of our own joints positions [START_REF] Lephart | The role of proprioception in the management and rehabilitation of athletic injuries[END_REF]. Haptics extends from the design of external stimulation simulating haptic features to the experience it provides on a user perspective. The Haptic Experience was defined in these regards [START_REF] Kim | Defining Haptic Experience: Foundations for Understanding, Communicating, and Evaluating HX[END_REF], discussing design parameters and usability requirements of a haptic stimulation as well as their potential impact on the experiential dimensions influencing the user experience.

With the growth of haptic technologies, many surveys in haptics are being drawn, from various perspectives. They have focused on haptic interaction techniques [START_REF] Talvas | A Survey on Bimanual Haptic Interaction[END_REF], haptic rendering technologies [START_REF] Wang | Haptic display for virtual reality: progress and challenges[END_REF], [START_REF] Mercado | Haptics On-Demand": A Survey on Encountered-Type Haptic Displays[END_REF], [START_REF] Rakkolainen | A Survey of Mid-Air Ultrasound Haptics and Its Applications[END_REF], or on specific applications such as medical/surgery training [START_REF] Rangarajan | Systematic Review of Virtual Haptics in Surgical Simulation: A Valid Educational Tool?[END_REF], [START_REF] Escobar-Castillejos | A Review of Simulators with Haptic Devices for Medical Training[END_REF], [START_REF] Coles | The Role of Haptics in Medical Training Simulators: A Survey of the State of the Art[END_REF], virtual or augmented realities [START_REF] Bermejo | A survey on haptic technologies for mobile augmented reality[END_REF], [START_REF] Wang | Multimodal Haptic Display for Virtual Reality: A Survey[END_REF], [START_REF] Bouzbib | Survey of Virtual Reality Interactions via Haptic Solutions[END_REF], telemanipulation [START_REF] Galambos | Vibrotactile Feedback for Haptics and Telemanipulation: Survey, Concept and Experiment[END_REF], industry training [START_REF] Xia | Haptics for Product Design and Manufacturing Simulation[END_REF], or a combination of them [START_REF] Varalakshmi | Haptics: State of the Art Survey[END_REF]. In this survey, we only focus on a class of haptic technology: wearable haptic technologies (see below).

2) Wearable Technologies: Wearables are a group of interfaces that can be worn onto the user's body. This class of devices includes flexible garments (e.g. gloves, t-shirts) and rigid accessories (e.g., watch, glasses, bracelet) [START_REF] Berglund | A survey of the historical scope and current trends of wearable technology applications[END_REF], [START_REF] Seneviratne | A Survey of Wearable Devices and Challenges[END_REF], [START_REF] Chen | Wearable Actuators: An Overview[END_REF]. They can be defined as "small electronic and mobile devices, or computers with wireless communications capability that are incorporated into gadgets, accessories, or clothes, which can be worn on the human body, or even invasive versions such as micro-chips or smart tattoos" [START_REF] Ometov | A Survey on Wearable Technology: History, State-of-the-Art and Current Challenges[END_REF]. They are mainly used for body-tracking, e.g., in sports applications [START_REF] Luczak | State-ofthe-art review of athletic wearable technology: What 113 strength and conditioning coaches and athletic trainers from the USA said about technology in sports[END_REF], rehabilitation/health monitoring [START_REF] Pantelopoulos | A survey on wearable biosensor systems for health monitoring[END_REF], [START_REF] Memon | Wearable technology for infant health monitoring: a survey[END_REF] or motion capture, and gained a global popularity as tools for the Internet of Things (IoT) [START_REF] Ometov | A Survey on Wearable Technology: History, State-of-the-Art and Current Challenges[END_REF]. A wearable therefore provides information about the users and/or its vicinity to an external display. In this survey, we focus on wearable haptics: the wearable device itself communicates an information to the users through haptic cues, which includes notifications about the users, their physical environment, or a remote/virtual one. They are defined by Pacchierotti et al. as haptic "interfaces capable of communicating with the human wearers during their interaction with the environment they share" [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives[END_REF].

Wearable haptic technologies have been presented and analyzed according to their field of application (e.g., sensory replacement [START_REF] Shull | Haptic wearables as sensory replacement, sensory augmentation and trainer -a review[END_REF], rehabilitation [START_REF] Wang | Interactive wearable systems for upper body rehabilitation: a systematic review[END_REF]), the technology they exploit (e.g., exoskeletons [START_REF] Fang | Wireality: Enabling Complex Tangible Geometries in Virtual Reality with Worn Multi-String Haptics[END_REF]), their form factor (e.g., gloves [START_REF] Perret | Touching Virtual Reality: a Review of Haptic Gloves[END_REF]), or the body part they stimulate (e.g., fingertip and hand [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives[END_REF]). In this survey, we provide a classification of wearable haptics technologies according to the body part(s) they stimulate and the applied stimulation. However, we only focus on a single application: navigation guidance.

3) Navigation: Navigation is defined as a "goal-directed travel through space" [START_REF] Montello | Spatial cognition[END_REF]. It refers to one's ability to displace in a remote, virtual, or physical environment. The literature has often focused on the means to provide users with navigation capabilities in different environments. For instance, users in a virtual environment can be teleported, walk in place, or walk naturally [START_REF] Laviola | 3D user interfaces: theory and practice, ser. Addison-Wesley usability and HCI series[END_REF]. Using these interaction techniques, users choose their arrival and displace themselves towards it. Nonetheless, in psychology, an entire field of research is devoted to navigation in itself, through spatial cognition, the knowledge of oneself in relation to our surroundings [START_REF] Devlin | 41 Environmental Perception: Wayfinding and Spatial Cognition[END_REF]. Taxonomies in this area classify navigation as either locomotion or way-finding [START_REF] Wiener | Taxonomy of human wayfinding tasks: A knowledge-based approach[END_REF]. While these terms often refer to the same task outside of the spatial cognition field, they are to be differentiated as they do not rely on the same cognition maps on a user perception perspective. To understand them, three spatial representations are available: (a) landmarks, unique configurations of perceptual events; (b) routes, as a sequence of landmarks; (c) survey or map, as sequences of routes [START_REF] Siegel | The development of spatial representations of large-scale environments[END_REF].

a) Locomotion: Locomotion refers to "the guidance of oneself through space in response to local sensorimotor information in the immediate surrounds" (i.e. obstacle avoidance, moving towards a target) [START_REF] Montello | Spatial cognition[END_REF]. It therefore only relies on (a) and (b), i.e. the local landmarks in the visible space around self. It does not require an entire mental model or cognitive map of the environment.

b) Way-Finding: Way-finding refers to "the planning and decision making that allows one to reach a destination not in the immediate sensory field" [START_REF] Montello | Spatial cognition[END_REF] (i.e. choosing the shortest route, orientating to non-local landmarks). Way-finding therefore relies on (a), (b) and (c); and requires memorizing the environment and creating a cognitive map of it. To reduce this cognitive load, way-finding can be aided (using signage, maps, navigation assistant) or directed, when the destination is specific [START_REF] Wiener | Taxonomy of human wayfinding tasks: A knowledge-based approach[END_REF].

c) Navigation Guidance: In this survey, the wording "navigation guidance" refers to the spatial cognition's locomotion: information provided to the users are in their direct vicinity. Navigation guidance "helps the user to find, locate, and reach a target without having to perform an exhaustive search within the space" [START_REF] Gacem | A design space of guidance techniques for large and dense physical environments[END_REF]. More specifically, techniques providing aided wayfinding giving information about the users' surroundings and requiring decision making (presence of stairs, doors, toilets etc. [START_REF] Paneels | What's around me? multi-actuator haptic feedback on the wrist[END_REF]); or unaided directed way-finding for target approximation or path finding (e.g. [START_REF] Hong | Evaluating Wrist-Based Haptic Feedback for Non-Visual Target Finding and Path Tracing on a 2D Surface[END_REF]) are not included in this paper. Similarly, papers on guidance outside of the navigation field (rehabilitation [START_REF] Kapur | Spatially distributed tactile feedback for kinesthetic motion guidance[END_REF], teleoperation/guided manipulation tasks [START_REF] Pacchierotti | The hring: A wearable haptic device to avoid occlusions in hand tracking[END_REF]) are also out of the scope of this paper.

This navigation guidance information can be provided through external tools (e.g., GPS) or be directly connected to the user. For instance, visually-impaired people are guided through their white cane, physically discovering their vicinity when moving along an unencumbered pathway. Guidance can also be provided without conveying explicit feedback to the user. Users navigating an environment can be subject to visual illusions and be physically redirected towards a destination without noticing a trajectory alteration [START_REF] Razzaque | Redirected walking[END_REF]. This survey addresses navigation guidance systems providing explicit feedback to the users.

B. Scope

This survey focuses on wearable haptics for navigation guidance applications, regardless of the type of information they provide to the wearer (e.g., trajectory tracking, obstacle avoidance). Technologies only providing visual feedback for guidance (such as non-vibrating glasses) are out of the scope of this paper, as to wearable haptics that could be employed for navigation but have never suggested to be used in such an application (e.g., a vibrotactile bracelet used for VR contact rendering), or systems whose purpose is to only render some environment properties (e.g., a haptic watch rendering the map of an environment for non-navigational purposes).

The contributions of this work are: (1) a classification of wearable haptics for navigation guidance through two dimensions: body area and stimulation type; (2) an overview of wearable haptics technologies for navigation guidance; (3) an analysis of the strategies to provide guidance information; (4) a summary of current challenges and limitations highlighted from the devices' evaluation protocols. A trade-off is often made between comfort and sensitivity. This continuum shows devices for navigation guidanceworn on various body parts according to their respective tactile sensitivity, or acuity. (A) Leg-worn device [START_REF] Khan | Wearable navigation assistance system for the blind and visually impaired[END_REF]; (B) Back wearable [START_REF] -H. Hamdan | Springlets: Expressive, Flexible and Silent On-Skin Tactile Interfaces[END_REF], (C) Armbands for wheelchair navigation aid [START_REF] Devigne | Power Wheelchair Navigation Assistance Using Wearable Vibrotactile Haptics[END_REF]; (D) Forehead headband [START_REF] Kajimoto | Forehead electro-tactile display for vision substitution[END_REF]; (E) Palm device integrated through a glove [START_REF] Günther | Tactileglove: Assistive spatial guidance in 3d space through vibrotactile navigation[END_REF]; (F) Finger rings integrated through a glove [START_REF] Mori | Understanding indoor orientation through wearable vibrotactile feedback[END_REF].

III. STIMULATED BODY PARTS

This section analyzes the body parts most often stimulated through wearable haptics for navigation guidance. We describe them according to two dimensions: comfort and sensitivity (see Figure 1) . Comfort relates to the "wearability" of the device (i.e. weight, size, shape, ergonomics) [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives[END_REF], while sensitivity relates to its acuity and related expected performance for communicating guidance during navigation.

Indeed, wearables have the benefit of being directly attached to the body in an often non-invasive, non-obtrusive, and convenient way. Wearable haptics usually rely on rigid controllers or PCBs that therefore need to be worn without compromising the users' comfort and freedom of movement [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives[END_REF]. For instance, a rigid link should not be placed on a body joint, as it would prevent or impair potential limb displacements and rotations. Prior design guidelines for wearable haptics indicate that absolute characteristics (e.g., the weight and size of a device) might be misleading in measuring its "wearability" [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives[END_REF]. Instead, such characteristics should be evaluated considering the part of the body where the interface is worn. As an example, a device too heavy for a fingertip might be perfectly comfortable if worn on the forearm.

In terms of sensitivity, and as per Penfield's homunculus [START_REF] Schott | Penfield's homunculus: a note on cerebral cartography[END_REF], a given artificial haptic stimulation is interpreted differently depending on its location, according to density of mechanoreceptors in the skin [START_REF] Vechev | Tactiles: Dual-mode low-power electromagnetic actuators for rendering continuous contact and spatial haptic patterns in vr[END_REF], [START_REF] Dunkelberger | A multi-sensory approach to present phonemes as language through a wearable haptic device[END_REF], [START_REF] Khurshid | Effects of grip-force, contact, and acceleration feedback on a teleoperated pick-and-place task[END_REF].

There are many ways to measure and describe sensitivity, according to the target stimulation and information to convey. In general, we define an absolute perceptual threshold as the smallest amount of stimulus energy necessary to produce a sensation, and a differential perceptual threshold, or Just Noticeable Difference (JND), as the smallest amount of stimulus change necessary to achieve some criterion level of performance in a discrimination task [START_REF] Gescheider | Psychophysics: the fundamentals[END_REF]. For example, the vibrotactile absolute perceptual threshold (or vibrotactile sensitivity) can be defined as the lowest vibratory frequency/amplitude for which a vibration is perceived. Similarly, kinesthetic sensitivity can be defined as the minimal physical stimulus eliciting a kinesthetic perception, and its acuity defines the accuracy of the estimation of a limb position after having been moved by an external person [START_REF] Pickett | Measuring kinaesthetic sensitivity in typically developing children[END_REF]. Other representative examples for our purposes are the "tactile sensitivity", also referred to as tactile acuity or two-point threshold, which has been defined as the distance threshold for which users are able to discriminate a two-point stimulus [START_REF] Weinstein | Intensive and Extensive Aspects of Tactile Sensitivity as a Function of Body Part, Sex, and Laterality[END_REF]; the "error of localization" (or "point localization" [START_REF] Weinstein | Intensive and Extensive Aspects of Tactile Sensitivity as a Function of Body Part, Sex, and Laterality[END_REF], [START_REF] Myles | The Tactile Modality: A Review of Tactile Sensitivity and Human Tactile Interfaces[END_REF]), which is defined as the separation between a reference point and what is identified by the user as the reference point [START_REF] Loomis | Sensitivity to shifts of a point stimulus: An instance of tactile hyperacuity[END_REF]; and the tactile directional sensitivity (or shift), which is defined as the threshold distance for which users are able to tell the direction of a movement [START_REF] Loomis | Sensitivity to shifts of a point stimulus: An instance of tactile hyperacuity[END_REF]. We note that the wording acuity in the common vocabulary usually refers to a resolution; though when used in haptics, it often refers to a spatial tactile sensitivity rather than other stimulation (kinesthetic, thermal etc.).

A. Head

The head is a human body part which assembles many receptors in charge of collecting important external information, including vision (eyes), smell (nose), hearing (ears), taste (mouth). It also includes lots of sensitive neurons distributed around the skin of the neck and cheeks. Most of the head parts (neck, ears, forehead, cheeks) also have a high sensitivity to thermal variations, which can be leveraged to provide further information to the user [START_REF] Nasser | Thermo-haptic earable display for the hearing and visually impaired[END_REF].

In this section, we gather systems stimulating the head, including the neck, ears, back of the head, forehead, mouth, and tongue.

Neck. We are already familiar with wearing headbands, necklaces, and earrings; yet, as most wearable haptic devices and technologies require to be in firm contact with the skin, neck devices are usually considered to be too uncomfortable or even dangerous [START_REF] Machida | Suitable body parts for vibration feedback in walking navigation systems[END_REF]. Sensitivity-wise, the area of stimulation can easily be determined from a single point stimulus [START_REF] Morrow | The neck as a potential site for vestibular tactile sensory substitution[END_REF]. In this respect, the neck was shown to be one of the most effective body part to detect vibratory frequencies (absolute threshold of 0.72 Hz) [START_REF] Bikah | Supracutaneous vibrotactile perception threshold at various non-glabrous body loci[END_REF] (see Figure 2.A); however, this refers to the back of the neck, as the front has a lesser tactile spatial acuity [START_REF] Morrow | The neck as a potential site for vestibular tactile sensory substitution[END_REF].

Ears. Most glasses-like haptic interfaces usually stimulate the user at the ears [START_REF] Szczerba | A wearable vibrotactile display for automotive route guidance: Evaluating usability, workload, performance and preference[END_REF], [START_REF] Nukarinen | Delivering directional haptic cues through eyeglasses and a seat[END_REF], [START_REF] Lee | Activearring: Spatiotemporal haptic cues on the ears[END_REF] and have the benefit to be able to also easily stimulate the user's vision (see Figure 2.B), which makes it promising for Augmented Reality applications. More generally, ears can be stimulated through earrings-like devices, which seem like a convenient location for wearable haptics [START_REF] -H. Hamdan | Springlets: Expressive, Flexible and Silent On-Skin Tactile Interfaces[END_REF], providing high sensitivity and comfort. One benefit is their symmetry around the users' head: an independent stimulation of two haptic-enabled earrings can easily indicate the direction to follow.

Back of the head. Many wearables are already available for covering the users' head, often attached to its back. In this regard, we see wearable haptic solutions for guidance with helmets [START_REF] Mann | Blind navigation with a wearable range camera and vibrotactile helmet[END_REF], [START_REF] Kerdegari | Head-mounted sensory augmentation device: comparing haptic and audio modality[END_REF], [START_REF] Bertram | Sensory augmentation with distal touch: the tactile helmet project[END_REF], [START_REF] Hoppe | Odin's Helmet: A Head-Worn Haptic Feedback Device to Simulate G-Forces on the Human Body in Virtual Reality[END_REF] (see Figure 2.C), wigs [START_REF] Tobita | Smartwig: wig-based wearable computing device for communication and entertainment[END_REF], hats [START_REF] Woźniak | Gapeau: Enhancing the Sense of Distance to Others with a Head-Mounted Sensor[END_REF], and caps [START_REF] Kaul | Haptichead: A spherical vibrotactile grid around the head for 3d guidance in virtual and augmented reality[END_REF], [START_REF]Haptichead: 3d guidance and target acquisition through a vibrotactile grid[END_REF], stimulating mostly the back of the head, in spite of its usually lesser sensitivity due to the presence of hair. This can be seen as a compromise, as the back of the head provides the largest head area and therefore can be stimulated to provide spatially more information.

Forehead. The forehead provides a convenient stimulation location as it guarantees an area of application spanning 360 • [START_REF] Spakov | Sequential and simultaneous tactile stimulation with multiple actuators on head, neck and back for gaze cuing[END_REF]. Moreover, it is rarely employed for other purposes. Headbands such as [START_REF] De Jesus Oliveira | Experiencing guidance in 3d spaces with a vibrotactile head-mounted display[END_REF], [START_REF] Dunbar | Augmenting human spatial navigation via sensory substitution[END_REF], [START_REF] Kuang | Wearable cutaneous device for applying position/location haptic feedback in navigation applications[END_REF], [START_REF] Kajimoto | Forehead electro-tactile display for vision substitution[END_REF] are therefore quite popular for wearable haptic guidance applications exploiting on the head. Sensitivity-wise, the forehead has a tactile acuity of 15 mm [START_REF] Weinstein | Intensive and Extensive Aspects of Tactile Sensitivity as a Function of Body Part, Sex, and Laterality[END_REF] and a 10 mm directional sensitivity (see Figure 1.d). It has been indicated as the best body locus for pressure sensitivity [START_REF] Weinstein | Intensive and Extensive Aspects of Tactile Sensitivity as a Function of Body Part, Sex, and Laterality[END_REF], [START_REF] Myles | The Tactile Modality: A Review of Tactile Sensitivity and Human Tactile Interfaces[END_REF] and as the second best area for detecting vibratory frequencies [START_REF] Bikah | Supracutaneous vibrotactile perception threshold at various non-glabrous body loci[END_REF].

Mouth. Within the head, the mouth, lips, and tongue are the most sensitive areas to thermal cues [START_REF] Luo | High-density thermal sensitivity maps of the human body[END_REF], yet mostly vibrations cues are currently explored. These areas also are the most sensitive parts of the head according to Penfield's homunculus [START_REF] Schott | Penfield's homunculus: a note on cerebral cartography[END_REF]. They have been exploited by Tang et al. [START_REF] Tang | An oral tactile interface for blind navigation[END_REF] for the delivery of guidance information to blind users. Yet, they are still not popular body areas to stimulate, as they are quite inaccessible and inconvenient: humans swallow their saliva, speak, or eat, and therefore cannot be often encumbered by a tongue device. Moreover, their respective areas are quite limited and do not allow to leverage their high sensitivity (e.g. the upper lip for instance displays a 5 mm tactile acuity [START_REF] Weinstein | Intensive and Extensive Aspects of Tactile Sensitivity as a Function of Body Part, Sex, and Laterality[END_REF]).

B. Torso

The torso, defined as the central part of the body, including the front and the back sides, is a large and convenient area for wearable feedback, as the stimulation can occur in a garment such as a jacket or a belt [START_REF] Buchmann | Directional interfaces for wearable augmented reality[END_REF]. Even though its sensitivity is quite limited compared to other high-sensitive body areas, Piateski and Jones [START_REF] Piateski | Vibrotactile pattern recognition on the arm and torso[END_REF] showed that the torso is capable of accommodating twice the information received at the fingertips thanks to its significantly larger area. Sensitivity-wise, the potential presence of fat makes it the worst body locus to detect vibratory frequencies (0.91 Hz) [START_REF] Bikah | Supracutaneous vibrotactile perception threshold at various non-glabrous body loci[END_REF]. It features a 30 mm tactile acuity and a 10 mm directional sensitivity for the front, as well as 40 mm tactile acuity (see Figure 1.b) and over 5 mm directional sensitivity for the back [START_REF] Loomis | Sensitivity to shifts of a point stimulus: An instance of tactile hyperacuity[END_REF], [START_REF] Norrsell | Human, tactile, directional sensibility and its peripheral origins[END_REF]. It also displays a 10 mm error of localization [START_REF] Loomis | Sensitivity to shifts of a point stimulus: An instance of tactile hyperacuity[END_REF], similar on the belly and the back -which is almost twice as much as the forehead one. Yet, the torso is the second most sensitive body locus to pressure (after the forehead/face) [START_REF] Myles | The Tactile Modality: A Review of Tactile Sensitivity and Human Tactile Interfaces[END_REF], [START_REF] Weinstein | Intensive and Extensive Aspects of Tactile Sensitivity as a Function of Body Part, Sex, and Laterality[END_REF], therefore a large class of pressure-based stimulation exploits this spacious body area.

C. Arm

The arm is a very popular and comfortable area for a wearable, that can be integrated in sleeves [START_REF] Das | Providing navigation assistance through forcehand: a wearable force-feedback glove[END_REF] or arm bands [START_REF] Tsai | Guideband: Intuitive 3d multilevel force guidance on a wristband in virtual reality[END_REF], [START_REF] Henze | Non-intrusive somatosensory navigation support for blind pedestrians[END_REF], [START_REF] Devigne | Power Wheelchair Navigation Assistance Using Wearable Vibrotactile Haptics[END_REF]. A large amount of skin is available for stimulation, in various directions (proximal-distal -through its length, e.g. with a sleeve, or lateral-medial -through its circumference, e.g. with an arm band). Arms are also often used for wearing exoskeletons able to provide kinesthetic feedback to the elbow and/or shoulder. In this respect, kinesthetic sensitivity is dependent on both the limb displacement and its velocity. In fact, a movement over the shoulder would have an impact over the entire arm (upper arm, forearm and hand), as a movement on the elbow would directly impact the forearm (and ultimately the wrist would directly impact the hand). For instance, with the hypothesis of a straight 65-cm-long arm, a 1 • rotation on the shoulder would cause a 23 mm displacement on a fingertip. The same rotation applied on the wrist joint would only move it by 5 mm, if we consider a hand of 15 cm. Second, regarding speed, the same displacement for a faster movement is detected more easily than a slower one. For instance, at a 10 -80 • /s velocity range, the kinesthetic sensitivity threshold is at 1 • ; for a 1.25 • /s velocity it goes up to 8 • [START_REF] Jones | Kinesthetic Sensing[END_REF].

In terms of tactile acuity, the global arm sensitivity falls in 35-40 mm (see Figure 1.c) [START_REF] Weinstein | Intensive and Extensive Aspects of Tactile Sensitivity as a Function of Body Part, Sex, and Laterality[END_REF]. A gap detection task was also performed on this body area, using a two-surface discrimination threshold -as opposed to two-point discrimination threshold (what we call tactile acuity here). For this task, the stimulus orientation was shown to have a significant impact on the threshold detection. Indeed, this threshold is above 20 mm for a proximal-distal stimulation (forearm: 23 mm, upper arm: 20.1 mm), and decreases up to approximately 10 mm for a lateral-medial one (forearm: 9.75 mm, upper arm: 12.21 mm) [START_REF] Gibson | Tactile spatial sensitivity and anisotropy[END_REF]. Frequency-wise, the vibratory threshold is 0.83 Hz [START_REF] Bikah | Supracutaneous vibrotactile perception threshold at various non-glabrous body loci[END_REF].

No data was found regarding the upper arm and forearm directional sensitivities. Yet, using the previous information, we can already assume that a wearable can be easily placed on an arm band to provide a lateral-medial stimulation. A kinesthetic stimulation can also be sent to the upper arm for the entire arm to be stimulated, as seen in [START_REF] Lopes | Impacto: Simulating Physical Impact by Combining Tactile Stimulation with Electrical Muscle Stimulation[END_REF].

Similarly to the ears, the symmetry of the arms (and ultimately of the hands, wrists, legs, and feet) make them great candidate for guidance applications: a direction can be communicated to the appropriate limb whenever required.

D. Hands and Wrist

The wrist, hands, and fingers are the most stimulated body areas when considering wearable haptic interfaces. They regroup bracelets, watches [START_REF] Bauer | Enhancing perception for the visually impaired with deep learning techniques and low-cost wearable sensors[END_REF], gloves [START_REF] Günther | Tactileglove: Assistive spatial guidance in 3d space through vibrotactile navigation[END_REF] (see Figure 1.e), rings [START_REF] Mori | Understanding indoor orientation through wearable vibrotactile feedback[END_REF] (see Figure 1.f), that however can potentially encumber and impair their use during important grasping and manipulation tasks.

Hands are our main mean of communicating with the environment: we most often explore and manipulate our surroundings using our hands. They are the most represented body areas in Penfield's homunculus: as opposed to the back being represented by the same brain part, each finger has a dedicated brain part, discriminating their respective signals.The wrist has registered a vibratory frequency threshold of 0.83 Hz [START_REF] Bikah | Supracutaneous vibrotactile perception threshold at various non-glabrous body loci[END_REF]. As opposed to the arm (see previous subsection), proximaldistal and lateral-medial stimuli are quite similar for gap detection, with ratios (proximal to lateral) around 1 [START_REF] Gibson | Tactile spatial sensitivity and anisotropy[END_REF]. The palm tactile acuity is around 10 mm and the fingers about 2 -3 mm, which make them the most sensitive areas of the entire body. The fingers also display a 0.1 mm directional sensitivity [START_REF] Loomis | Sensitivity to shifts of a point stimulus: An instance of tactile hyperacuity[END_REF], which is therefore more than 40 times higher than the one on the back and makes it a really promising locus for guidance applications. The high acuity in the fingers is also practical as their small areas would not allow for a stimulation otherwise.

E. Legs and Feet

The lower trunk (legs and feet) is a usually comfortable area for wearables. They can be designed as leggings/leg bands [START_REF] Me | Wearable haptic-feedback navigational assistance for people with dementia: Preliminary assessment[END_REF], [START_REF] Che Me | Haptic modality for assistive navigation system of elderly with alzheimer's disease[END_REF], [START_REF] Meli | Combining wearable finger haptics and augmented reality: User evaluation using an external camera and the microsoft hololens[END_REF], [START_REF] Khan | Wearable navigation assistance system for the blind and visually impaired[END_REF] (see Figure 1.a), ankle bracelets [START_REF] Brown | A novel, wearable, electronic visual aid to assist those with reduced peripheral vision[END_REF], [START_REF] Demircan | A pilot study on locomotion training via biomechanical models and a wearable haptic feedback system[END_REF], [START_REF] Stock | FEEDI -A Smart Wearable Foot-Band for Navigation and Guidance Using Haptic Feedback[END_REF] or shoes [START_REF] Schirmer | Shoe me the way: a shoe-based tactile interface for eyes-free urban navigation[END_REF], [START_REF] Xu | Design and evaluation of vibrating footwear for navigation assistance to visually impaired people[END_REF], [START_REF] Meier | Exploring vibrotactile feedback on the body and foot for the purpose of pedestrian navigation[END_REF], [START_REF] Velãzquez | A shoe-integrated tactile display for directional navigation[END_REF]. When a human moves in the environment, these limbs are the most concerned and are therefore less sensitive to external stimuli compared to other body loci, i.e., the movement itself might partially mask the external stimulation provided by a wearable device. We should also consider that the calf, thigh, and gluteal region have a reduced tactile sensitivity (42 -45 mm [START_REF] Weinstein | Intensive and Extensive Aspects of Tactile Sensitivity as a Function of Body Part, Sex, and Laterality[END_REF]) and are less sensitive to vibrations than other body areas [START_REF] Wilska | On the Vibrational Sensitivity in Different Regions of the Body Surface[END_REF].

Finally, the ankle and feet seem to have similar sensitivities as the wrist and hands, compared to their attached limb (respectively, legs and arms). The ankle and wrist absolute frequency detection thresholds are also similar (0.83 Hz). The tactile acuity of the feet is around 20 mm. It is the second best area for vibration sensitivity (after the hands) [START_REF] Wilska | On the Vibrational Sensitivity in Different Regions of the Body Surface[END_REF]. Similarly, as the fingers in the upper trunk, the toes' tactile acuity is the highest among the lower trunk [START_REF] Weber | The sense of touch[END_REF]. Finally, thighs, glutes, and calves are less sensitive to temperature changes than soles and feet.

The advantage of using wearable haptics on this part of the body, despite its lesser sensitivity, is its comfort, its large area, its global symmetry on the body, and the fact that it is rarely used for other activities or devices.

F. Summary

This section analysed the different body parts most often used for wearing haptic devices during navigation guidance, focusing on their comfort and sensitivity. Machida et al. [START_REF] Machida | Suitable body parts for vibration feedback in walking navigation systems[END_REF] proposed these two dimensions to evaluate which body parts are the most convenient for communicating information through a wearable haptic device. Results showed that, comfort-wise, the ears and wrists (e.g. earrings and watches) are the most appropriate locations to stimulate, while the neck and ankle (e.g. necklace, ankle bracelet) lack comfort from the user's perspective. Sensitivity-wise, signals are easier to perceive on the hands than on the chest or waist. It is also important to consider the function of each body part and how wearing a device can affect them. Indeed, some parts of the human body are usually covered and their skin is not so often exploited for touch (e.g. torso, legs), while others are meant to be left free and commonly employed in exploration and manipulation tasks (e.g. hands). For this reason, choosing the body part most suited for a certain interface and task should consider multiple aspects, going well beyond the mere tactile acuity, also considering its comfort and wearability with respect to its everyday use.

We summarize our quantitative analysis in Table I, as a function of the considered parameters and with respect to each other (+++: best, ---: worst). This table aims at offering a subjective qualitative overview of the compromises to consider when designing a wearable technology for each part of the body. While we reviewed them considering our target application of navigation guidance, this information is useful for most designs of wearable haptics. [START_REF] Luo | High-density thermal sensitivity maps of the human body[END_REF], [START_REF] Weinstein | Intensive and Extensive Aspects of Tactile Sensitivity as a Function of Body Part, Sex, and Laterality[END_REF], [START_REF] Wilska | On the Vibrational Sensitivity in Different Regions of the Body Surface[END_REF], [START_REF] Myles | The Tactile Modality: A Review of Tactile Sensitivity and Human Tactile Interfaces[END_REF], [START_REF] Weber | The sense of touch[END_REF], [START_REF] Bikah | Supracutaneous vibrotactile perception threshold at various non-glabrous body loci[END_REF].

IV. STIMULATION TYPES AND ACTUATION TECHNOLOGY

This section examines the different techniques for generating wearable haptic stimuli during navigation guidance. We categorize what stimuli are generated, how they are exploited to communicate navigation guidance information, and which wearable haptics systems leverage them.

A. Kinesthetic

Exploiting kinesthetic feedback to generate directional cues involve applying external forces and/or torques to a joint or limb (see Section II-A1). Yet, proprioceptive sensations can also be elicited by locally deforming the skin around the joints, as it replicates a joint deformation caused by a limb displacement [START_REF] Edin | Skin strain patterns provide kinaesthetic information to the human central nervous system[END_REF], [START_REF] Collins | Cutaneous receptors contribute to kinesthesia at the index finger, elbow, and knee[END_REF], [START_REF] Shikata | Inducing elbow joint flexion by shear deformation of arm skin[END_REF].

A kinesthetic stimulation relies on muscles and tendons configurations, and can take advantage of a "reconfiguration" to directly guide the users through their environment. For instance, lower-body exoskeletons can literally move the users' limbs to follow the target guidance trajectory [START_REF] López-Larraz | Brain-machine interfaces for rehabilitation in stroke: A review[END_REF]. A similar effect can be achieved by directly stimulating the muscles using Electro Muscle Stimulation (EMS). It consists of small electrodes attached to the users' skin, which generate an electric signal activating the users muscles to achieve a target motion [START_REF] Auda | Around the (Virtual) World: Infinite Walking in Virtual Reality Using Electrical Muscle Stimulation[END_REF], [START_REF] Pfeiffer | Cruise Control for Pedestrians: Controlling Walking Direction using Electrical Muscle Stimulation[END_REF]. In these two techniques, users have thus no say in whether to follow the target direction or not. Their limbs are moved by the system to induce the wanted motion. This physical redirection approach is of course only available when wearing such systems on the lower trunk, e.g., actuating the users legs and feet through an exoskeleton to make them walk towards a target. Kinesthetic feedback can also provide information to be interpreted. A promising approach consists of a haptic sleeve using Pneumatic Artificial Muscles (PAMs), to provide pronation, supination, and abduction guidance feedback [START_REF] Das | Providing navigation assistance through forcehand: a wearable force-feedback glove[END_REF]. The advantages of PAMs are their safety, light weight, and flexibility. Indeed, they can be easily integrated into a garment suitable for different limbs, generating large forces while providing an easily interpretable guidance information. A PAM, or any other general pneumatic interface attached to a garment (e.g., [START_REF] Raitor | WRAP: Wearable, restricted-aperture pneumatics for haptic guidance[END_REF]) can inflate on a side of a limb to indicate the direction to follow, though this inflation is tricky to fully achieve around joints such as the wrist. Reversed PAMs (rPAMs) are therefore often preferred on joints. They can be deflated to indicate the direction to follow [START_REF] Skorina | A soft robotic wearable wrist device for kinesthetic haptic feedback[END_REF] (see Figure 3.A).

B. Skin Stretch

Skin stretch feedback refers to the deformation of the skin caused by an end-effector moving over its surface. The movement causes no relative displacement within the contact area and hence creates shear and normal stimuli to the skin [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives[END_REF]. Skin stretch offers many possibilities but was also recognized as potentially painful for users [START_REF] Aqeel | Skin stroking haptic feedback glove for assisting blinds in navigation[END_REF]. We consider three types of skin stretch for wearable haptics, shear, pressure, and twist, which depend on the direction of the external force generated by the considered device. When the displacement of the stimulation is parallel to the contact surface, it generates shear force; when the displacement is normal to the skin, it generates pressure; when the applied force is twisted with respect to the normal direction of contact [START_REF] Bark | Rotational skin stretch feedback: A wearable haptic display for motion[END_REF], [START_REF] Stanley | Evaluation of tactile feedback methods for wrist rotation guidance[END_REF], it generates a twist.

A common approach to provide skin stretch through shear forces is through belt haptic devices. They consist of an elastic belt fitted around a circular limb (e.g., waist [START_REF] Kayhan | A wearable haptic guidance system based on skin stretch around the waist for visually-impaired runners[END_REF], see Figure 3.B) and actuated through one or more motors. The actuated belt moves around the limb delivering skin stretch sensations, which direction is exploited to guide users. The intensity of the stretch can be dynamically adjusted by controlling the motor position and speed, so as to map the appropriate target trajectory angle [START_REF] Pezent | Spatially separating haptic guidance from task dynamics through wearable devices[END_REF]. If two motors are available, the belt can provide skin stretch at different pressure skin levels, which can exploited for guidance in collision avoidance scenarios. In general, skin stretch or skin drag does not require large stretches to provide rich and understandable stimuli, which makes it a very promising approach for small parts of the body, e.g., the fingers [START_REF] Je | Tactoring: A skin-drag discrete display[END_REF]. For example, Gleeson et al. [START_REF] Gleeson | Design of a fingertip-mounted tactile display with tangential skin displacement feedback[END_REF] placed the users' fingertips on a solid plate and performed guidance through micro displacements of the fingertips skin using a shear tactor.

Pressure sensations induced through a skin stretch interface providing, e.g., tapping [START_REF] Jin | VT-ware: A wearable tactile device for upper extremity motion guidance[END_REF], are currently explored but still rarely used for navigation guidance applications. They can be instantiated through solenoid actuators, placed as an array in a wrist band, and generate discrete and binary skin hitting sensations, interpreted as "left" or "right" by the user [START_REF] Alayon | Design of wearable wrist haptic device for blind navigation using microsoft kinect for Xbox 360[END_REF]; or using shape memory alloys -SMAs, integrated in flexible ergonomic on-skin stickers [START_REF] -H. Hamdan | Springlets: Expressive, Flexible and Silent On-Skin Tactile Interfaces[END_REF]. Their contraction provides skin stretch, stretch-induced pressure and motion perceptionused for guidance.

Finally, twist sensations are not (yet) represented in wearable haptics for navigation guidance.

C. Skin Stroke

An alternative to the above skin stretch stimulation is providing light skin stroking, which can elicit "tactile apparent motion" effects. Discrete skin stroking signals can indeed be perceived as single smooth continuous signal across the skin [START_REF] Kirman | Tactile apparent movement: The effects of interstimulus onset interval and stimulus duration[END_REF], "as if a rabbit were hopping on it" [START_REF] Geldard | The cutaneous" rabbit": a perceptual illusion[END_REF]. These discrete strokes can therefore communicate a continuous direction to the navigating user. Skin stroking was instantiated in [START_REF] Aqeel | Skin stroking haptic feedback glove for assisting blinds in navigation[END_REF] using brushes moving (anti-)clockwise, and integrated in a wearable bracelet. The signals are apparently well interpreted for assistance guidance [START_REF] Aqeel | Skin stroking haptic feedback glove for assisting blinds in navigation[END_REF] (see Figure 3.C) and wayfinding [START_REF] Strasnick | Brushtouch: Exploring an alternative tactile method for wearable haptics[END_REF], on top of being comfortable and pleasant to receive.

D. Thermal

Thermal cues are not very popular in the literature for guidance applications. Indeed, they show many drawbacks for providing guidance information to the users. First, only a small range of temperatures (24 • C -40 • C) is considered as appropriate, safe, and comfortable to send to the users [START_REF] Peiris | Thermovr: Exploring integrated thermal haptic feedback with head mounted displays[END_REF] (see Figure 3.D). The thermal resolution of the skin also shows a lower resolution and a longer response time than most visual, audio, or mechanical sensations. Whenever stimulated through temperatures, the skin also requires to be reset back to a neutral state, which therefore induces some potential delays between signals and undermines the detection of new stimuli [START_REF] Tewell | Heat-nav: Using temperature changes as navigation cues[END_REF].

Thermal cues are often stimulated through thermo-electric devices (most often including Peltier modules), which temperature is a function of the electrical current. They can be integrated into headbands [START_REF] Peiris | Exploration of cuing methods for localization of spatial cues using thermal haptic feedback on the forehead[END_REF], wristbands [START_REF] Peiris | Thermalbracelet: Exploring thermal haptic feedback around the wrist[END_REF], armbands [START_REF] Tewell | Heat-nav: Using temperature changes as navigation cues[END_REF], or earmuff-like devices [START_REF] Nasser | Thermo-haptic earable display for the hearing and visually impaired[END_REF], [START_REF] Narumi | Characterizing the space by thermal feedback through a wearable device[END_REF] for guidance purposes. For example, Narumi et al. [START_REF] Narumi | Characterizing the space by thermal feedback through a wearable device[END_REF] designed an ear device providing location-dependent thermal information to move in an existing space, which was divided into several thermal fields. People were then guided through the space by distinguishing the different thermal areas. A literal heat-map can also be mapped to the available space, as a gradient of the users' distance to target [START_REF] Tewell | Heat-nav: Using temperature changes as navigation cues[END_REF]. A more direct approach consists in designing patterns of thermal modules to provide guidance information using three stimuli variables: location (where the stimulus starts), number (of consecutive spatio-temporal stimuli) and direction ((counter-)clockwise stimuli) [START_REF] Peiris | Thermalbracelet: Exploring thermal haptic feedback around the wrist[END_REF], [START_REF] Peiris | Thermovr: Exploring integrated thermal haptic feedback with head mounted displays[END_REF]. Thermal stimulation is thus subtle to discriminate and its main advantage is its simple integration in wearable haptics, e.g., Peltier elements can be easily embedded in most end-effectors with very little footprint.

While LEDs or other techniques could be used to generate heat in wearable devices, Peltier elements are currently the most popular, as they guarantee a good efficiency with a relatively low voltage, and they ensure no burning sensations when touching the skin.

E. Electric (electrotactile)

In Section IV-A, we already discussed Electro-Muscle Stimulation. Apart from contracting the users' muscles and tendons, electric signals can also be sent through the users' skin so as to directly stimulate the nerve endings and provide tactile sensations. These systems are called electrotactile interfaces and provide low-level current pulses to the skin [START_REF] Pamungkas | Overview Electrotactile Feedback for Enhancing Human Computer Interface[END_REF]. These interfaces rely on electrodes, which are often flexible and very thin, facilitating their integration in wearable haptics technologies. They can be used as collision avoidance notifiers, for instance to inform users of potential obstacles or landmarks [START_REF] Meers | A substitute vision system for providing 3D perception and GPS navigation via electro-tactile stimulation[END_REF], or directly send a direction to follow [START_REF] Tang | An oral tactile interface for two-way communication[END_REF] (see Figure 3.E).

F. Vibratory

Vibrotactile cues are the most often employed to provide guidance information, mostly thanks to their cost-effectiveness and simplicity of implementation.

The human skin is sensitive to vibrotactile stimulation, at different frequencies or amplitudes depending on body loci (see Section III), making vibrations easily noticeable and reliable perception-wise. This justifies why most wearable haptics devices use vibrations, no matter the body area or form factor. However, sustained and prolonged vibrations are known to be uncomfortable [START_REF] Devigne | Power Wheelchair Navigation Assistance Using Wearable Vibrotactile Haptics[END_REF], [START_REF] Van Erp | Guidelines for the use of vibro-tactile displays in human computer interaction[END_REF]. The most popular actuation technologies among them are Eccentric Mass Motors (ERM) [START_REF] Bloomfield | Collision awareness using vibrotactile arrays[END_REF], [START_REF] Cassinelli | Augmenting spatial awareness with haptic radar[END_REF] (see Figure 3.F), where the mass is subject to a rotational force, Linear Resonant Actuator (LRA) [START_REF] Wang | Enabling independent navigation for visually impaired people through a wearable vision-based feedback system[END_REF], [START_REF] Dunbar | Augmenting human spatial navigation via sensory substitution[END_REF], where the mass is moved up and down along a line with a spring, voice-coil actuators, where a permanent magnet is suspended in an electromagnetic coil to produce vibrations [START_REF] Culbertson | Waves: A wearable asymmetric vibration excitation system for presenting threedimensional translation and rotation cues[END_REF] and piezoelectric actuators, which consist in beams deforming to vibrate against the users' skin [START_REF] Sorgini | Encapsulation of Piezoelectric Transducers for Sensory Augmentation and Substitution with Wearable Haptic Devices[END_REF]. Using the same principle, electroactive polymers can also provide vibrations against the user's skin [START_REF] Koo | Development of Soft-Actuator-Based Wearable Tactile Display[END_REF]. Indeed, these materials deform as a function of the received voltage; at an adequate frequency, they therefore provide a vibratory feedback [START_REF] Chen | Wearable Actuators: An Overview[END_REF]. Their flexible materials (e.g., electro-active textiles, dielectric elastomers [START_REF] Chen | Wearable Actuators: An Overview[END_REF]) seem adapted for wearable applications, though these have not been developed for guidance applications yet.

G. Summary

This section described the different types of feedback for navigation guidance and the technologies enabling them.

Although each of them has its pros and cons vis-à-vis navigation guidance, we can see some interesting and inspiring patterns. For example, compact and easy-to-use solutions such as ERM vibrators are largely the most popular actuation technology for the considered application. This is due to multiple reasons. First, they are inexpensive and trivial to use, which makes them straightforward to employ, even for novice designers. Second, their associated stimuli is familiar (e.g. smartphones notifications), noticeable and understandable. Third, they can be easily integrated into garments, for instance exploiting the symmetry of the human body and providing intuitive guidance cues. However, it is known that sustained vibrations become uncomfortable very quickly. Also, vibrotactile actuators need to be spaced out to be recognizable, preventing their use for navigation guidance in smaller parts of the body. In this respect, skin stretch and skin stroke devices seem very promising, as they need little space to convey rich information, they are not as complicated and cumbersome as, e.g., kinesthetic exoskeletons, and they are faster to react and convey changes in the stimulation than, e.g., thermoelectric devices. However, to provide effective information, skin stretch and stroke devices need to be well fastened to the body, which can make them uncomfortable to wear in some areas, especially for prolonged times. This aspect is also relevant for electrotactile interfaces, where small changes in the position of the electrode with respect to the skin can lead to significant changes in the delivered sensation [START_REF] Kourtesis | Electrotactile feedback applications for hand and arm interactions: A systematic review, meta-analysis, and future directions[END_REF].

This limitation complicates the integration of skin stretch, skin stroke, and electrotactile interfaces in garments or wearable structures. Indeed, the adaptation and customisation of wearable interfaces targeting panels of different users is an open problem in the community [START_REF] Malvezzi | Design of personalized wearable haptic interfaces to account for fingertip size and shape[END_REF].

V. NAVIGATION GUIDANCE STRATEGIES

We distinguish two methods for navigation guidance through wearable haptics: either the user is literally physically guided by the interface, or she receives information about her surroundings (e.g. landmarks, routes [START_REF] Montello | Spatial cognition[END_REF], [START_REF] Siegel | The development of spatial representations of large-scale environments[END_REF]). In this latter case, stimulation can be either interpreted as a where notification (Section V-A), and a when direction (Sections V-B-V-C). As mentioned in [START_REF] Gacem | A design space of guidance techniques for large and dense physical environments[END_REF], "guidance systems generally have two characteristics: 1) they orient the user's body movement, perhaps iteratively, to reach the target location, and 2) they indicate the target location". This section discusses the different stimulation strategies providing guidance navigation information. As per the Haptic Experience [START_REF] Kim | Defining Haptic Experience: Foundations for Understanding, Communicating, and Evaluating HX[END_REF], we define the haptic signal intensity as the overall perceived strength of feedback and the density as its rate or frequency, i.e., the number of noticeable stimuli per a given time.

A. Directions

Guidance information provides directions orienting users, often coupled with some additional strategies described later in Sections V-B and V-C. When providing a direction information, we rely on the angle to the target trajectory [START_REF] Pezent | Spatially separating haptic guidance from task dynamics through wearable devices[END_REF]. To interpret the direction correctly, we can exploit the causality feature [START_REF] Kim | Defining Haptic Experience: Foundations for Understanding, Communicating, and Evaluating HX[END_REF] of the feedback, i.e., the user is required to relate the haptic feedback she receives from its source, identify the direction, and then follow it. Beyond causality, direction is also shown to be better interpreted using density than intensity [START_REF] Bosman | Gentleguide: An exploration of haptic output for indoors pedestrian guidance[END_REF], [START_REF] Je | Designing skin-dragging haptic motions for wearables[END_REF] (see Fig. 4). In all cases, it is important to ensure that the considered directional sensations are understandable and perceived as different by the user, considering the tactile acuity of each part of the body with respect to the chosen type of stimulation (see Secs. III and IV).

1) Binary direction: 1D Direction: A binary direction can be seen as a 1D direction: it communicates a direction, and provides a "sense" (left or right). A straightforward strategy for providing directional guidance is to leverage the symmetry within the body (e.g., one actuator per arm, leg, or hand). For instance, a wearable device on each wrist can be worn to indicate left and right directions, and a simultaneous actuation of both of them indicates a "stop" signal [START_REF] Bosman | Gentleguide: An exploration of haptic output for indoors pedestrian guidance[END_REF]. Similarly, an array of four actuators can be designed, with binary cues to interpret (e.g. left/right, forward/backward [START_REF] Mori | Understanding indoor orientation through wearable vibrotactile feedback[END_REF], [START_REF] Schirmer | Shoe me the way: a shoe-based tactile interface for eyes-free urban navigation[END_REF]), or absolute cardinal directions [START_REF] Strasnick | Brushtouch: Exploring an alternative tactile method for wearable haptics[END_REF].

2) 2D direction: A 2D direction navigation strategy provides more information than a binary direction one: it is more precise regarding the angles and trajectories to follow. It provides a more accurate angle information than a simple sense. A first method is to increase the number of stimulation points: it allows for less interpretation and more intuitiveness, e.g., it can directly communicate the direction to follow in the transverse plane of the human user. The number of actuators and their spatial Examples of haptic feedback parameters from providing strategies to convey navigation guidance, from [START_REF] Kim | Defining Haptic Experience: Foundations for Understanding, Communicating, and Evaluating HX[END_REF]. (A) Intensity: the stimulation intensity indicates the direction to follow, e.g., [START_REF] Henze | Non-intrusive somatosensory navigation support for blind pedestrians[END_REF]. (B) Density: the rate of stimuli per a given time indicates when to take a turn, e.g., [START_REF] Szczerba | A wearable vibrotactile display for automotive route guidance: Evaluating usability, workload, performance and preference[END_REF]. (C) Causality: the user needs to identify the source of the feedback to choose the correct guidance direction, e.g., [START_REF] Schirmer | Shoe me the way: a shoe-based tactile interface for eyes-free urban navigation[END_REF].

distance can be used to provide the directional information.

For instance, and as a function of the local sensitivity, discrete or continuous signals can be provided [START_REF] Kerdegari | Head-mounted sensory augmentation device: comparing haptic and audio modality[END_REF]. Changing the distance between actuators has been proven to significantly affect the interpreted information: a smaller one conveyed the user with the sensation of a continuous stimulation, exploiting the "apparent motion" illusion -also called "funneling" [START_REF] Kaul | Vibrotactile Funneling Illusion and Localization Performance on the Head[END_REF], which was considered better for understanding directions to follow [START_REF] Kerdegari | Head-mounted sensory augmentation device: comparing haptic and audio modality[END_REF]. The sequences of actuation provide different types of directional information: arrays of actuators can be updated regularly to keep the user in the right direction (e.g., every 0.3 s in [START_REF] Tang | An oral tactile interface for two-way communication[END_REF]). Sequences can also indicate all of the wrong directions of an intersection [START_REF] Marston | Nonvisual route following with guidance from a simple haptic or auditory display[END_REF]. These "directional interfaces" are defined as "aiding users to orient themselves to find a way-point during navigation tasks" [START_REF] Buchmann | Directional interfaces for wearable augmented reality[END_REF].

3) 3D direction: Finally, there exists techniques to provide 3-dimensional direction information, with a height or depth perception on top on a 2D angle. Spatial and dynamic haptic illusion can be obtained by varying the time and spatial sequence of the multi vibrations [START_REF] De Jesus Oliveira | Experiencing guidance in 3d spaces with a vibrotactile head-mounted display[END_REF]. As opposed to previous techniques, the directions are updated regularly to guide the user in "real-time". A good synchronisation between actuators can also provide 3D directions (translation, rotation). For instance, simultaneously stimulating the fingers in opposite directions can indicate hand supination, pronation or flexion, and can be then exploited for navigation guidance [START_REF] Culbertson | Waves: A wearable asymmetric vibration excitation system for presenting threedimensional translation and rotation cues[END_REF].

B. Turn-by-Turn (GPS-like)

Guiding a user with a turn-by-turn strategy consists in providing a stimulation when she is required to take a turn, similarly to GPS guiding systems. In this case, the navigation information to modulate is the delay or distance before turning, in addition to the direction to turn to (see Section V-A). The frontier between delay and distance is thin in navigation guidance applications: Kiss et al. [START_REF] Kiss | Navigation systems for motorcyclists: exploring wearable tactile feedback for route guidance in the real world[END_REF] provide patterns to "turn far" and "turn near" (distance information) to eventually communicate to "turn now" (delay information).

Whether they rely on thermal [START_REF] Peiris | Thermalbracelet: Exploring thermal haptic feedback around the wrist[END_REF] or vibratory cues [START_REF] Alarcon | Design of a wearable haptic navigation tool for cyclists[END_REF], [START_REF] Brock | Using wrist vibrations to guide hand movement and whole body navigation[END_REF], [START_REF] Bial | Enhancing outdoor navigation systems through vibrotactile feedback[END_REF], turn-by-turn information are represented as (a) a change of intensity in the stimulus [START_REF] Alarcon | Design of a wearable haptic navigation tool for cyclists[END_REF], [START_REF] Henze | Non-intrusive somatosensory navigation support for blind pedestrians[END_REF] or (b) a change of density [START_REF] Szczerba | A wearable vibrotactile display for automotive route guidance: Evaluating usability, workload, performance and preference[END_REF], [START_REF] Günther | Tactileglove: Assistive spatial guidance in 3d space through vibrotactile navigation[END_REF]. As an example of intensity change, a thermo-electric wearable device can correlate its temperature to the delay before making a turn [START_REF] Nasser | Thermo-haptic earable display for the hearing and visually impaired[END_REF], [START_REF] Tewell | Heat-nav: Using temperature changes as navigation cues[END_REF]. A density change is instantiated with symmetric vibrators in [START_REF] Szczerba | A wearable vibrotactile display for automotive route guidance: Evaluating usability, workload, performance and preference[END_REF], where the frequency varies from 1 to 3 stimuli per 50 ms. With a higher density, the skin can consider the stimuli as continuous, as above the discrimination threshold (see Section III).

C. Obstacle Avoidance

Differently from the previous strategy, a wearable haptic system exploiting an obstacle avoidance strategy rely on the distance to obstacles. The user is free to choose her trajectory while avoiding obstacles on her way. This approach is similar to using a white cane: it provides haptic feedback whenever a collision might occur [START_REF] Dakopoulos | Wearable obstacle avoidance electronic travel aids for blind: a survey[END_REF], [START_REF] He | Pneufetch: Supporting blind and visually impaired people to fetch nearby objects via light haptic cues[END_REF], [START_REF] Meers | A substitute vision system for providing 3D perception and GPS navigation via electro-tactile stimulation[END_REF]. Once again, the feedback can be altered in intensity or density -the closer the obstacle, the stronger or the more frequent the stimulus. This strategy is adapted for vibratory [START_REF] Mann | Blind navigation with a wearable range camera and vibrotactile helmet[END_REF], [START_REF] Ghiani | Vibrotactile feedback to aid blind users of mobile guides[END_REF], [START_REF] Gibson | Analysis of a wearable, multimodal information presentation device for obstacle avoidance[END_REF], [START_REF] Woźniak | Gapeau: Enhancing the Sense of Distance to Others with a Head-Mounted Sensor[END_REF] and skin stroke [START_REF] Gleeson | Perception of Direction for Applied Tangential Skin Displacement: Effects of Speed, Displacement, and Repetition[END_REF] stimuli. In the same regards, a continuous signal can provide a "stop" signal [START_REF] Bosman | Gentleguide: An exploration of haptic output for indoors pedestrian guidance[END_REF], [START_REF] Bhatlawande | Way-finding electronic bracelet for visually impaired people[END_REF], or indicate all the wrong directions to redirect the users and avoid a trajectory deviation [START_REF] Marston | Nonvisual route following with guidance from a simple haptic or auditory display[END_REF], [START_REF] Brock | Using wrist vibrations to guide hand movement and whole body navigation[END_REF]. In the same line of observations, this strategy can also be adapted for target attraction, therefore relying on the distance to target.

VI. CHALLENGES & LIMITATIONS

Providing guidance through haptic signals seems straightforward -a direction, delay, or obstacle is indicated and the user interprets it. Taking advantage of the variety of cues available for stimulation, this information can potentially be very rich. Cues can be intertwined and/or complex patterns can be drawn to provide specific information. Though, wearable haptics for navigation guidance still show limitations -which are usually expressed in the devices' evaluation protocols.

A. Conceptual User Expectations and Acceptability

Extensive interviews and surveys have been conducted to define (1) user expectations [START_REF] Montuwy | What Sensory Pedestrian Navigation Aids For The Future?: A Survey Study[END_REF], (2) their prospective acceptability [START_REF] Profita | The AT Effect: How Disability Affects the Perceived Social Acceptability of Head-Mounted Display Use[END_REF], as well as to evaluate (3) wearable devices and technology usability [START_REF] Montuwy | What Sensory Pedestrian Navigation Aids For The Future?: A Survey Study[END_REF], [START_REF] Woźniak | Gapeau: Enhancing the Sense of Distance to Others with a Head-Mounted Sensor[END_REF] and (4) their requirements [START_REF] Gustafson-Pearce | Tugs-the tactile user guidance system[END_REF] for navigation guidance.

Using conceptual prototypes over a 600-participant panel, a survey showed that user expectations for guidance were preferably using visual (over 60%) and cartographic information (over 50%) than haptics (under 10%) [START_REF] Montuwy | What Sensory Pedestrian Navigation Aids For The Future?: A Survey Study[END_REF]. Similarly, acceptability criteria included utility, ease of use, ease of learning, safety, pleasantness and self-esteem and also displayed significantly negative feedback compared to maps [START_REF] Montuwy | What Sensory Pedestrian Navigation Aids For The Future?: A Survey Study[END_REF].

Conceptual vignettes evaluating sensory preferences were also evaluated using the UMUX questionnaire [START_REF] Finstad | The Usability Metric for User Experience[END_REF] and wearable acceptability questionnaire from [START_REF] Profita | The AT Effect: How Disability Affects the Perceived Social Acceptability of Head-Mounted Display Use[END_REF] (see also Section VI-C). This acceptability is evaluated from an external point of view: the interviewed participants rate their perception of user wearing a head-mounted wearable interface [START_REF] Profita | The AT Effect: How Disability Affects the Perceived Social Acceptability of Head-Mounted Display Use[END_REF]. On a user perspective, they communicate whether the wearable device user seemed independent, needed help or the wearable device, looked cool or nerdy. On an interaction perspective, they rate whether the interaction made them feel uncomfortable, if the wearer seemed awkward, normal, rude, distracting, or if the interaction seemed appropriate. On a device perspective, they evaluate whether the wearable device seemed useful or unnecessary. Users once again preferred navigation guidance through auditory feedback than haptic [START_REF] Woźniak | Gapeau: Enhancing the Sense of Distance to Others with a Head-Mounted Sensor[END_REF].

Prior to designing a (wearable haptics)-device or technology, identifying user experience dimensions through interviews and experts opinions helps identifying requirements. In these regards, Gustafson et al. [START_REF] Gustafson-Pearce | Tugs-the tactile user guidance system[END_REF] defined that wearable haptic devices (1) should be hidden, (2) should not impede on senses working to full capacity, (3) should be easy to use, and (4) should leave the hands free. Another approach consists in identifying the device's principal functions and potential failures, in order to mitigate their risks and/or occurrences and thus optimize the user experience [START_REF] Bouzbib | Let's Meet and Work it Out": Understanding and Mitigating Encountered-Type of Haptic Devices Failure Modes in VR[END_REF].

User expectations are important to consider as they enable the early identification of requirements and provide guidelines regarding the interfaces design. Indeed, even though haptics might not always seem like a promising and/or accepted approach for navigation guidance -we are more used to follow visual and auditory navigation cues than hapticsexperimental evaluations highlight a great potential for this type of application (see next subsections).

B. Cognitive Load

On a user perspective, the signal provided by the wearable haptic devices and technologies during navigation is required to be intuitive and easy to interpret. This requirement is usually reflected through cognition load quantification, for instance using the NASA TLX questionnaire [START_REF] Hart | Development of NASA-TLX (Task Load Index): Results of Empirical and Theoretical Research[END_REF]. This questionnaire evaluates mental, physical and temporal demands, performance, effort and frustration [START_REF] Hart | Development of NASA-TLX (Task Load Index): Results of Empirical and Theoretical Research[END_REF]. Haptics for guidance are shown to be cognitively less demanding than vision [START_REF] Nukarinen | Delivering directional haptic cues through eyeglasses and a seat[END_REF], [START_REF] Kerdegari | Head-mounted sensory augmentation device: comparing haptic and audio modality[END_REF], auditory feedback, or both [START_REF] Woźniak | Gapeau: Enhancing the Sense of Distance to Others with a Head-Mounted Sensor[END_REF], [START_REF] Szczerba | A wearable vibrotactile display for automotive route guidance: Evaluating usability, workload, performance and preference[END_REF] and guidance through haptics is displayed as intuitive and effective [START_REF] Fani | Multi-cue haptic guidance through wearables for enhancing human ergonomics[END_REF].

This cognitive load is also expressed through time for completion, which is shown to be significantly faster with haptics cues compared to visual ones [START_REF] Nukarinen | Delivering directional haptic cues through eyeglasses and a seat[END_REF], but only to some extent. Indeed, the more signals to interpret, the more cognitive load -which needs to be addressed when giving guidance to a visually-impaired person [START_REF] Khusro | Haptic Feedback to Assist Blind People in Indoor Environment Using Vibration Patterns[END_REF]. A complex pattern providing direction using vibration location and angle using vibration duration such as in [START_REF] Gustafson-Pearce | Tugs-the tactile user guidance system[END_REF] might lead to a higher cognitive load and/or a less intuitive understanding. Similarly, actuating two similar wristbands at the same time can be found redundant and produce more cognitive load and less comprehension of the guidance [START_REF] Mori | Understanding indoor orientation through wearable vibrotactile feedback[END_REF].

C. Usability

Another challenge regards the usability of wearable haptic systems for navigation, which is often reflected in the devices' evaluations. The most common protocol for usability is the SUS -System Usability Scale questionnaire [START_REF] Brooke | Sus-a quick and dirty usability scale[END_REF], which was shrunk to four questions in the UMUX -Usability Metric for User Experience questionnaire [START_REF] Finstad | The Usability Metric for User Experience[END_REF].

Visual feedback for navigation guidance shows (non significant) better score using SUS than Haptic & Visual [START_REF] Szczerba | A wearable vibrotactile display for automotive route guidance: Evaluating usability, workload, performance and preference[END_REF]. Similarly, using the SUS scale, auditory feedback was better perceived than haptics for providing directions to blindfolded participants [START_REF] Brock | Using wrist vibrations to guide hand movement and whole body navigation[END_REF]. Haptics was then suggested to assist users to keep straight directions and reduce their deviation while navigating. Similarly as the previous subsection, this usability is actually implementation-dependent. As an example, UMUX scores evaluating potential conceptual feedback modalities in [START_REF] Woźniak | Gapeau: Enhancing the Sense of Distance to Others with a Head-Mounted Sensor[END_REF] showed that audio feedback using headphones was perceived as the most usable. Yet, in practice, haptic feedback through vibrations and auditory feedback did not show a significant difference using SUS, and were both significantly more usable than visual feedback.

We indeed expect usability to significantly vary depending on the type of task considered, haptic feedback provided, and experience of the user, making the impact of the above results limited to the considered implementations.

D. Resolution & Accuracy

Even though haptics globally seems to reduce cognitive load for navigation guidance applications and is likely to be perceived as usable and viable, a compromise regarding resolution and accuracy error is also drawn [START_REF] Kerdegari | Head-mounted sensory augmentation device: comparing haptic and audio modality[END_REF], [START_REF] Woźniak | Gapeau: Enhancing the Sense of Distance to Others with a Head-Mounted Sensor[END_REF]. Indeed, when visual and auditory channels are available and free from other stimuli, accuracy seems to be higher than for haptic cues [START_REF] Nukarinen | Delivering directional haptic cues through eyeglasses and a seat[END_REF], [START_REF] Brock | Using wrist vibrations to guide hand movement and whole body navigation[END_REF], [START_REF] Szczerba | A wearable vibrotactile display for automotive route guidance: Evaluating usability, workload, performance and preference[END_REF]. This might be justified as haptics need to be interpreted, and can often lead to more confusion in orientations, for instance. "Left and right" can be easily recognized (93% recognition rate for [START_REF] Culbertson | Waves: A wearable asymmetric vibration excitation system for presenting threedimensional translation and rotation cues[END_REF]), while "up/down" and "forward/backward" directions tend to be harder to interpret (70% recognition rate [START_REF] Culbertson | Waves: A wearable asymmetric vibration excitation system for presenting threedimensional translation and rotation cues[END_REF]). Absolute directions are even trickier to communicate and ultimately interpret [START_REF] Buchmann | Directional interfaces for wearable augmented reality[END_REF].

Some criticism needs to be added whilst analysing wearable haptics accuracy results for navigation guidance. In fact, some evaluations are conducted with 2-Alternative Forced Choice, which can also increase the devices' results compared to asking the absolute perceived directions. Similarly, complex patterns might be tested with people sitting at the laboratory [START_REF] Nukarinen | Delivering directional haptic cues through eyeglasses and a seat[END_REF]. We can potentially imagine a drop in the devices performances considering the added cognitive load in an "in-the-field" experience, where participants are to be guided in real-time [179]. Finally, when multiple senses are compared, we have to consider the experiential designs of each condition: haptics might work better for guidance than an augmented reality map containing too many information, but not with a regular map [START_REF] Montuwy | What Sensory Pedestrian Navigation Aids For The Future?: A Survey Study[END_REF]. The participants are also to be accounted for: wearable haptics for seeing, blindfolded or legally blind users might provide different results.

E. Discussion

Future deployment of wearable haptics for navigation guidance purposes will highly depend on the increase of its acceptability and usefulness. As an answer to a locomotion issue or physical disability, wearable haptics are indeed useful -though for global navigation guidance, it still is not perceived as the best modality to stimulate. Signals may seem too complicated to interpret -the same signal can for instance represent a "constraint", to force the user to follow its opposite direction as if a virtual wall was erected; or on the opposite, can represent the direction to follow. Even so, these directions are lacking referential. Indeed, spatial cognition heavily relies on referential to enable oneself's orientation and knowledge of a future direction to follow. For instance, a wearable haptic bracelet worn on the wrist, vibrating on its left, should indicate to turn left (or as mentioned before, turn right). Though, the wrist has many degrees of freedom, it could induce a change of orientation of the wearable device. In this configuration, will the user have to interpret the signal as an absolute direction or as related to the wrist relative position? We could consider only integrating wearable haptics for navigation guidance in body parts that could act like a compass, for instance on the torso or on the head. However, and as mentioned in our first dimension, comfort and sensitivity are to take into account. Moreover, sensitivity in the field -while navigating -has not been investigated yet: we might not perceive our phone vibrating in our pocket while walking or cycling. In the same line of observations, current resolutions are still low. Reallife navigation does not only require binary directions, but most often many intersections or roundabouts where directions are to be communicated in a precise way. Globally, many compromises are to be drawn in order to design accepted, useful, understandable, usable, accurate wearable haptics for navigation guidance. This enhancement in the deployment of wearable haptics for navigation guidance could be instantiated through the integration of more precepts from spatial cognition -to better answer to users requirement from their perspective.

VII. CONCLUSION

This survey provides an analysis of wearable haptic devices and technologies for navigation guidance according to two main dimensions: (1) their body area and (2) their associated stimulation and actuation technology. We then identify the main navigation guidance strategies using wearable haptic devices. Finally, we describe current challenges and potential limitations of wearable haptics in this context. These limitations are usually reflected through evaluation protocols: haptic cues seem to reduce cognitive load and are usable, but they are still not catering for future users expectations and increase error rates in guidance. We mainly reflect in this paper how compromises need to be drawn when designing wearable haptics technology, notably for navigation guidance. The devices are to optimise the wearability and sensitivity while understanding the requirements for specific users. Depending on the types of users and/or coupling with other multimodal interfaces, they need to communicate clear instructions without obstructing the cognitive workload or usability.

Fig. 1 .

 1 Fig.1. Body areas for wearable haptics. A trade-off is often made between comfort and sensitivity. This continuum shows devices for navigation guidanceworn on various body parts according to their respective tactile sensitivity, or acuity. (A) Leg-worn device[START_REF] Khan | Wearable navigation assistance system for the blind and visually impaired[END_REF]; (B) Back wearable[START_REF] -H. Hamdan | Springlets: Expressive, Flexible and Silent On-Skin Tactile Interfaces[END_REF], (C) Armbands for wheelchair navigation aid[START_REF] Devigne | Power Wheelchair Navigation Assistance Using Wearable Vibrotactile Haptics[END_REF]; (D) Forehead headband[START_REF] Kajimoto | Forehead electro-tactile display for vision substitution[END_REF]; (E) Palm device integrated through a glove[START_REF] Günther | Tactileglove: Assistive spatial guidance in 3d space through vibrotactile navigation[END_REF]; (F) Finger rings integrated through a glove[START_REF] Mori | Understanding indoor orientation through wearable vibrotactile feedback[END_REF].

Fig. 2 .

 2 Fig.2. Examples of wearable haptic systems for the head: (A) Neck collar[START_REF] Schaack | Haptic Collar: Vibrotactile Feedback around the Neck for Guidance Applications[END_REF], providing guidance through vibrotactile feedback; (B) Earring[START_REF] Lee | Activearring: Spatiotemporal haptic cues on the ears[END_REF], associated with AR glasses, communicating guidance information through vibrotactile feedback; (C) Head helmet (back of the head and forehead)[START_REF] Hoppe | Odin's Helmet: A Head-Worn Haptic Feedback Device to Simulate G-Forces on the Human Body in Virtual Reality[END_REF], providing guidance in VR through kinesthetic feedback.

Fig. 3 .

 3 Fig.3. Our second dimension classifies the stimulation types for wearable haptics. Seven types of haptic stimulation for navigation guidance exist in the literature. We also associate the stimulated body parts in this figure to illustrate their variety from our first dimension. (A) Kinesthetic feedback using reverse-Pneumatic Artificial Muscles on the wrist[START_REF] Skorina | A soft robotic wearable wrist device for kinesthetic haptic feedback[END_REF]; (B) Skin stretch waist-belt device guiding blinded users[START_REF] Kayhan | A wearable haptic guidance system based on skin stretch around the waist for visually-impaired runners[END_REF]; (C) Skin stroke device using brushes to wear on the arm[START_REF] Aqeel | Skin stroking haptic feedback glove for assisting blinds in navigation[END_REF]; (D) Thermal feedback provided within a virtual reality Head Mounted Display[START_REF] Peiris | Thermovr: Exploring integrated thermal haptic feedback with head mounted displays[END_REF]; (E) Electric signals sent to a mouth-piece worn on the tongue, to guide blind users[START_REF] Tang | An oral tactile interface for blind navigation[END_REF]; (F) Vibrations sent to the fingertips to provide guidance[START_REF] Mori | Understanding indoor orientation through wearable vibrotactile feedback[END_REF].

  Fig.4. Examples of haptic feedback parameters from providing strategies to convey navigation guidance, from[START_REF] Kim | Defining Haptic Experience: Foundations for Understanding, Communicating, and Evaluating HX[END_REF]. (A) Intensity: the stimulation intensity indicates the direction to follow, e.g.,[START_REF] Henze | Non-intrusive somatosensory navigation support for blind pedestrians[END_REF]. (B) Density: the rate of stimuli per a given time indicates when to take a turn, e.g.,[START_REF] Szczerba | A wearable vibrotactile display for automotive route guidance: Evaluating usability, workload, performance and preference[END_REF]. (C) Causality: the user needs to identify the source of the feedback to choose the correct guidance direction, e.g.,[START_REF] Schirmer | Shoe me the way: a shoe-based tactile interface for eyes-free urban navigation[END_REF].

[

  179] N. K. Dim and X. Ren, "Investigation of suitable body parts for wearable vibration feedback in walking navigation," International Journal of Human-Computer Studies, vol. 97, pp. 34-44, 2017. Elodie Bouzbib is a postdoctorate researcher at Inria Rennes, France. She obtained a PhD from Sorbonne Université in Computer Science/Human Computer Interaction in 2021, for which she received the prize of 2022 Best Multidisciplinary PhD thesis from AFIHM. Her main research areas are HCI, Virtual Reality, Haptics and Robotics. She mainly prototypes artefacts to stimulate haptics in VR. Lisheng Kuang is a PhD candidate at CNRS-IRISA in Rennes, France. He got a master degree of control engineering from Harbin Institute of Technology (Shenzhen), in 2017. He was a research associate at the Italian Institute of Technology, Genova, Italy in 2019. He visited the Extended Reality and Robotics (xR²) Lab at Aarhus University in 2022. His research focuses on the design, prototyping, testing, and control of wearable robots for applications in human mobility aids. Paolo Robuffo Giordano received the M.Sc. degree in computer science engineering and the Ph.D. degree in systems engineering, both from the University of Rome "La Sapienza," Rome, Italy, in 2001 and 2008, respectively. From 2008 to 2012, he was Senior Research Scientist with the Max Planck Institute for Biological Cybernetics, Tübingen, Germany. He is currently a Senior CNRS Researcher and Head of the Rainbow Group, IRISA and Inria, Rennes, France. Anatole Lécuyer is director of research and head of Hybrid team at Inria Rennes, France. His resarch interests are in virtual/augmented reality, haptic interaction, 3D user interfaces, and brain-computer interfaces. Anatole Lécuyer obtained the IEEE VGTC Technical Achievement Award in Virtual/Augmented Reality in 2019, and was inducted in the IEEE Virtual Reality Academy in 2022. Claudio Pacchierotti is a tenured researcher at CNRS-IRISA in Rennes, France, since 2016. He was previously a postdoctoral researcher at the Italian Institute Technology, Genova, Italy. Pacchierotti earned his PhD at the University of Siena in 2014. Pacchierotti received the 2014 EuroHaptics Best PhD Thesis Award and the 2022 CNRS Bronze Medal. He is Senior Chair of the IEEE Technical Committee on Haptics and Secretary of the Eurohaptics Society.

TABLE I SUBJECTIVE

 I QUALITATIVE OVERVIEW OF RELATIVE CHARACTERISTICS OF TARGET BODY PARTS (+++: BEST, ---: WORST, /: NOT FOUND OR NOT APPLICABLE). INFORMATION IS NOTABLY EXTRACTED FROM