Fig. 11. Structure of flexible double-ring.The first prototype (showed in Fig. 10), was built from rapid prototyped parts, by means of additive manufacturing in 3D printing (Ultimaker® 2+). Its weight is 450 g in accordance with the indications set out in [52], [62] (RobHand_VC2.mp4 is a short video clip showing in detail the RobHand exoskeleton). The linkage-rotate mechanism (intermediate piece and proximal and distal rods), the back support platform and the thumb module are built with Poly Lactic Acid (PLA). The double-rings, which are in direct contact with the human skin, are made of flexible material (Filaflex 82a), improving the ergonomics of the device. The prototype incorporates five Actuonix L12 linear actuators (DC motors with screw mechanism, 30 mm stroke, 23N output force, low-cost and lightweight). Referring to [63], and [64], the maximum force level of the human fingers was measured to be 50 N. Since the proposed hand exoskeleton is designed for the assistance of patients having a partially paralyzed hand, the required force level of the finger module is determined to be 10 N [64]. Thus, the selected linear actuators have sufficient capability to realize the required force.During this development phase, usability and ergonomics tests were carried out with the collaboration of the Rehabilitation Service of the University Clinical Hospital (Valladolid, Spain). These tests were performed with five post-stroke patients (four man and one woman with ages between 30 and 60 years) who suffered from paresis in one of their hands (two in the left hand and three in the right one), and all in the chronic phase of the pathology. Four of them suffered from spasticity in the fingers with different levels of intensity. Sixty sessions of use of the exoskeleton were carried out with an approximate duration of sixty minutes each. Over the course of 20 weeks the patients underwent 3 weekly sessions. Fig. 12 shows the exoskeleton placed on the hands of two of the patients during the tests carried out.