## 2 State of the Art  
When interacting with computers or machines, _natural interaction_ usually refers to the final goal of the user experience that aims at being smooth, easy, and evident, i.e., natural to the user. However, natural interaction is a subjective concept [4] that can be applied to a wide range of fields. This is mainly due to different users having different preferences, expectations, and background. Several studies have been conducted regarding the interaction with virtual content without hands, that is, without direct contact from the user. Some approaches [5; 6] have made use of users' gaze to show or hide information when using AR displays. It has also been studied whether the use of head gazing or eye pointing [7] is more effective when interacting. Results indicate that users tend to prefer the head approach because of its better stability. Given that this type of interaction is quite widespread, some frameworks such as EyeMRTK [8] have been appearing to facilitate developer tasks. The survey by [PERSON] et al. [9] collects the history and current situation of gaze-enabled interaction in the context of mobile handheld devices. Finally, a survey about interaction methods for head-mounted displays that emphasizes touch and touchless-based interaction can be found in Reference [10]. Regarding direct manipulation, users may apply actions over elements like \"touching\" or \"moving\". In the end, the better the interaction adapts to what the user expects, the more natural it will feel. Hence, different systems and devices may offer different natural ways to interact with them. In wearable AR context, the use of hand gestures has been widely explored [11; 12]. Thanks to the apparition of more powerful devices, nowadays, it is possible not only to detect gestures but also to perform image analysis techniques in real time and to track hand movements (e.g., Kinect and Leap Motion [13; 14]). The preference of users for certain gestures has already been explored in previous works [15]. Nevertheless, the interaction with the virtual content is not natural yet. This is mainly due to one key component that has not been systematically integrated into this interaction: tactile feedback. The concept of haptic feedback promises to tear down the boundaries between the real, physical world and the virtual one. The combination of hand gestures and haptic feedback is positioned as a middle ground between precision and comfort, and its advantages have already been explored in the literature [16]. On its most basics, haptic feedback consists of presenting synthetic tactile stimuli to the users. Since this type of feedback can be generated on demand, it can be used to enhance the perception of the AR content. Whereas before the user could only see a digital object, now they can touch it or perceive it. There are many different types of haptic devices available today. Two main groups can be distinguished depending on whether the feedback is based on the perception of forces (_kinesthetic_) or produced in the skin of the user (_cutaneous_, tactile if the perception is over the fingers). The first group is formed by kinesthetic devices. This kind of device tends to present low portability because they are grounded. Some examples of these devices are the one used in the PHANToM project [17]--which consists of a grounded pen attached to a device that generates haptic feedback--or the one proposed in Reference [18]--a grounded, graspable haptic device that kept track of the user's hand and generated the feedback, e.g., to simulate contact with digital elements. The second group is composed of cutaneous devices, and it includes devices such as wristbands [19; 20; 21], haptic rings [22; 23], and gloves [24; 25; 26] or mid-air tools. More in detail, cutaneous devices can be divided into the following groups:  
* _Basic cutaneous devices._ This kind of device is one of the most popular, since they are simple and provide great results.