\section{Wearable Haptics for the Hand}
\label{wearable_haptics_hand}

To understand how wearable haptics have been used to render haptic properties of virtual objects and haptic augmentations to tangible objects, we first need to briefly describe how the hand senses and acts on its environment.


\subsection{The Haptic Sense}
\label{haptic_sense}

The haptic sense has specific characteristics that make it unique in regard to other senses.
%
It enables us to perceive a large diversity of properties in the surrounding objects, through to a complex combination of tactile and proprioceptive sensations produced by numerous sensory receptors distributed throughout the body.
%
But it also allows us to act on these objects, to come into contact with them, to grasp them, to manipulate them and to actively explore them.
%
This implies that the haptic perception is localised at the points of contact between the body and the environment, \ie we cannot haptically perceive an object without actively touching it.
%
These two mechanisms, perception and action, are therefore closely associated and both essential to form the haptic experience of interacting with the environment~\cite{lederman2009haptic}.

% passive and active touch
% many receptors : cutaneous and kinesthetic sensory modalities
% hand mechanics and anatomy
% exploratory procedures
% grasp types


\subsubsection{Haptic Perception}
\label{haptic_perception}

Perceiving the properties of an object involves numerous sensory receptors embedded in the skin, but also in the muscles and joints of the hand, and distributed across the body. They are divided into two main modalities: cutaneous and kinesthetic.

\paragraph{Cutaneous Sensitivity}

Cutaneous haptic receptors are specialised nerve endings implanted in the skin that respond differently to the various stimuli applied to the skin. \figref{blausen2014medical_skin} shows the location in the skin of the four main cutaneous receptors that respond to mechanical deformation of the skin.

Adaptation rate and receptor size are the two key characteristics that respectively determine the temporal and spatial resolution of these mechanoreceptors, as summarized in \tabref{cutaneous_receptors}.
%
The adaptation rate is the speed and duration of the response to a stimulus.
%
Meissner and Pacinian receptors, known as fast-adapting, respond rapidly to a stimulus but stop quickly even though the stimulus is still present, allowing the detection of high-frequency changes.
%
In contrast, Merkel and Ruffini receptors, known as slow-adapting, have a slower but continuous response to a static, prolonged stimulus.
%
The size of the receptor determines the area of skin that can be sensed by a single nerve ending.
%
Meissner and Merkel receptors have a small detection area and are sensitive to fine skin deformations, while Ruffini and Pacinian receptors have a larger detection area.

\fig[0.6]{blausen2014medical_skin}{Schema of cutaneous mechanoreceptors in a section of the skin~\cite{blausen2014medical}.}

The density of mechanoreceptors varies according to skin type and body region.
%
Glabrous skin, especially on the face, feet, hands, and more importantly, the fingers, is particularly rich in cutaneous receptors, giving these regions great tactile sensitivity.
%
The density of the Meissner and Merkel receptors, which are the most sensitive, is notably high in the fingertips~\cite{johansson2009coding}.
%
Conversely, hairy skin is less sensitive and does not contain Meissner receptors, but has additional receptors at the base of the hairs, as well as receptors known as C-tactile, which are involved in pleasantness and affective touch~\cite{ackerley2014touch}.

There are also two types of thermal receptors implanted in the skin, which respond to increases or decreases in skin temperature, respectively, providing sensations of warmth or cold~\cite{lederman2009haptic}.
%
Finally, free nerve endings (without specialized receptors) provide information about pain~\cite{mcglone2007discriminative}.

\begin{tab}{cutaneous_receptors}{Characteristics of the cutaneous mechanoreceptors.}[
        Adaptation rate is the speed and duration of the receptor's response to a stimulus. Receptive size is the area of skin detectable by a single receptor. Sensitivities are the stimuli detected by the receptor. Adapted from \textcite{mcglone2007discriminative} and \textcite{johansson2009coding}.
    ]
    \begin{tabularx}{\linewidth}{p{1.7cm} p{2cm} p{2cm} X}
        \toprule
        \textbf{Receptor} & \textbf{Adaptation Rate} & \textbf{Receptive Size} & \textbf{Sensitivities}                                                          \\
        \midrule
        Meissner          & Fast                     & Small                   & Discontinuities (\eg edges), medium-frequency vibration (\qtyrange{5}{50}{\Hz}) \\
        Merkel            & Slow                     & Small                   & Pressure, low-frequency vibration (\qtyrange{0}{5}{\Hz})                        \\
        Pacinian          & Fast                     & Large                   & High-frequency vibration (\qtyrange{40}{400}{\Hz})                              \\
        Ruffini           & Slow                     & Large                   & Skin stretch                                                                    \\
        \bottomrule
    \end{tabularx}
\end{tab}

\paragraph{Kinesthetic Sensitivity}

\paragraph{Hand Mechanics}


\subsubsection{Hand-Object Interactions}
\label{hand_object_interactions}

%Perceiving the properties of an object involves numerous sensory receptors embedded in the skin, but also in the muscles and joints of the hand.
%
%Therefore, it requires an active exploration of the object with the hand and the fingers that requires coordination between the movements and the sensations in return.

\paragraph{Exploratory Procedures}

\paragraph{Grasp Types}


\subsubsection{Object Properties}
\label{object_properties}

Toucher activement un objet implique donc des mécanismes simultanés de la main à la fois sensoriels et moteurs.
%
Ainsi, les nombreux mouvements exploratoires utilisés guident la recherche d'informations sensorielles et permettent de construire une perception haptique des propriétés de l'objet.
%
Une distinction est généralement faite entre les propriétés matérielles, qui sont la perception de la surface, et spatiales, qui est la perception de la structure et de la géométrie de l'objet~\cite{lederman2009haptic}.
%
Nous décrivons ici les principales propriétés haptiques perçues.

\paragraph{Texture}

\paragraph{Stiffness}

\paragraph{Temperature}

\paragraph{Weight}

\paragraph{Spatial Properties}


\subsection{Wearable Haptics}
\label{wearable_haptics}


\subsubsection{Wearable Haptic Devices}
\label{wearable_haptic_devices}

\paragraph{Mechanics and Wearability}

\paragraph{Moving Actuators}

\paragraph{Vibrotactile Actuators}

\paragraph{Kinesthetic Actuators}


\subsubsection{Wearable Haptic Renderings}
\label{wearable_haptic_renderings}

\paragraph{Contact}

\paragraph{Texture}

\paragraph{Stiffness}

\paragraph{Temperature}

\paragraph{Shape}


\subsubsection{Evaluating the Haptic Feedback}
\label{wearable_haptics_evaluation}

Les nombreuses interfaces et rendus des dispositifs haptiques portables développés tendent à reproduire des expérience perceptuelles similaires, voire réalistes, à celles des interactions haptiques avec des objets réels.
%
Les méthodes psychophysiques sont notamment utilisées pour étudier le sens du toucher avec des objets réels et sont ré-employées pour pour évaluer la perception par les utilisateurs des rendus haptiques virtuels.
%
\textcite{choi2013vibrotactile} présentent particulièrement bien les questions perceptuelles et les méthodes d'évaluations à considérer.


\subsection{Conclusion}
\label{wearable_haptics_conclusion}
