Wearable robots are expected to work very closely, to interact and collaborate with people in an intelligent environment [1]. Traditionally, wearable robotic structures have been mainly used in substitution of lost limbs (e.g., prosthetic limbs) or for human limb rehabilitation (e.g., exoskeletons). However, the progress in miniaturization and efficiency of the technological components is allowing more light and compact solutions, enhancing user's safety and comfort, while opening new opportunities for wearable robot use [2]. Together with exoskeleton and prosthesis, a very promising research direction seems to be that of adding robotic limbs to human, rather than substituting or enhancing them [3]. This addition could let the humans augment their abilities and could give support in everyday tasks to impaired people. This paper investigates how to compensate the capabilities of the human hand, instead of developing additional robotic extra-arms, as discussed for instance in [4]. The idea of using an extra-finger to support the human hand in grasping functions was initially proposed in [5]. Then, independently both in [6-8], the authors proposed the use of extra fingers to support the human hand to grasp objects whose size does not fit a hand or in executing bimanual tasks with one hand. The main difference is that in [7, 8], the goal was to minimize the size and the weight of the unique extra limb, while in [6], two extra fingers were used so to hold objects. While in [9] the authors developed a control strategy to grasp and manipulate objects, in [10] the authors mainly focused on the use of extra fingers for post-stroke patients.
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