## 1 Introduction
[24] developed the \"PneuGlove,\" a pneumatic glove that can provide independent extension assistance to each digit while still allowing full arm movement. It was proved in combination with a virtual environment where different objects were presented with the aim of performing grasp-and-release tasks in a given time. The same principle has been used and improved in recent years in the device developed by [PERSON] [25], where finger flexion and extension are obtained thanks to three fabric layers and two air-tight bladders placed between each fabric pocket. Examples of endpoint devices are PHANToM by Geomagic and the Novint Falcon [22]. Different studies used the Novint Falcon in combination with other devices to build up a virtual reality environment for patients with diseases of mobility in the upper extremities [26; 27]. However, these technologies are bulky and expensive, so they are not the best choice for a low-cost platform for SCI patients. Tactile stimulation has been exploited with greater emphasis on vibrotactile or electro-tactile stimulation [21]. [PERSON] et al. [8] combined CyberTouch, a system composed of a glove with a vibro-tactile stimulator on each finger and the palm, with a VR environment for therapeutic training of reaching movements after spinal cord injury. Nonetheless, wearing a glove can sometimes be difficult and that is why researchers have shifted the focus to thimbles, smaller devices that are easier to wear and more \"realistic\" in the feedback that they can provide. [PERSON] and [PERSON] [28] developed the FinGAR (Finger Glove for Augmented Reality), a tactile thimble device that combines electrical and mechanical stimulation to selectively stimulate skin sensory mechanoreceptors. [PERSON] et al. [29] presented a wearable 3-DOF (degrees of freedom) fingertip device for interaction with virtual and remote environments. It consists of two platforms connected by three wires controlled by three small electrical motors and one force sensor placed in contact with the fingertip at the center of the platform for closed-loop control. Based on the same principles, the 3 RRS (Revolute-Revolute-Spherical) wearable fingertip device [30] is composed of two parallel platforms that are instead connected by three articulated legs, actuated by three motors in order to make and break contact with the skin and move or re-angle the mobile platform toward the fingertip. Recently, [31] developed a soft, flexible artificial skin made of silicone and electrodes. The system allows the artificial skin to adapt itself to the user and provide haptic feedback in the form of pressure and vibration. Strain sensors continuously measure the skin's deformation so that the haptic feedback can be adjusted in real time thanks to soft pneumatic actuators that form a membrane layer that can be inflated by pumping air into it. All these devices can be used for upper limb rehabilitation in spinal cord injury patients if used alongside computer-displayed virtual reality environments. The aim of this paper is to develop a set of serious games based on UL functional tasks that allow neurorehabilitation in SCI patients. Haptic feedback should be provided to the user by means of a non-detachable device to prevent the patient from becoming fatigued simply by holding it [26]. However, issues such as donning and doffing haptic gloves should be overcome. Therefore, a low-cost haptic thimble, such as the one presented in [32], should be built to simplify the interaction with the patient and the serious games. The haptic device must be thin enough so that a hand tracking system can extract finger parameters. Two haptic feedback effects are selected for their importance in serious game interaction: vibration and pressure. Vibration produces molecular mechanisms that mediate signal transduction at the tissue level [33]. The current general consensus about vibration is that the Pacinian corpuscles sense vibration with a frequency range from 40 to 800 Hz and a peak from 200 to 300 Hz.