Specifications of the device
Each tactile unit consisted of a coil with 0.1 mm thick copper wire, with an inner diameter of 3.5 mm, an outer diameter of 5.5 mm, and a height of 9.3 mm. The coils had a total of 750 windings per piece. The magnetic rod in the center of the coil had a diameter of 3 mm and was 8 mm long. The rods had a magnetic rating of grade N42. The coils were surrounded by an aluminum case to shield the magnetic field. The tactile units were positioned in a grid of solid 3D printed plastic material. This di-magnetic material reduced the interference of the coils and magnets on each other. Moreover, the tactile units were disposed in the grid with alternated supplies polarity, so to further reduce interferences. The center distance between each of the tactile points was 11.0 mm. This distance was set below the two-point discrimination functionalities of limbs (the lowest threshold is 21.5 mm at the medial lower arm ). At the same time, the distance was large enough to ensure limited interference.
The electronic circuit driving each point in the grid was designed to reduce power consumption. This circuit consisted of a transistor in common emitter configuration and a large capacitor placed in series between the coil and the transistor collector. When the transistor is activated, the charge of the capacitor would allow an immediate high current (0.150 A) to flow through the coil, generating enough field to move the rod. Then, as soon as the capacitor is fully charged the current would settle on a lower steady value (0.050 A) determined by a resistor placed in parallel to the capacitor. This circuit allowed a reduction of power consumption by 67% and a maximum impact force of each tactile point of 90 mN when activated with 5 V (Figure 3B).
Sliding and strain bands
The slide and pressure band used continuous servomotors and two spools to allow the band to wind around them. A knot was added on the slide band so to indicate a specific location on the slider axis. The servomotors had dimensions of 50.4 x 37.2 x 20 mm and weighed 40 g. The spool had a radius of 0.6 cm. The servomotors required a current of 0.2 A at 5.0 V when operated, producing a torque of 2.0 kg/cm. Having two servos at opposite sides resulted in a maximum force of 2.4 kg. The rope applied pressure over its whole surface of roughly 6 cm2 (20 cm in length and 0.3 cm in width). The resulting maximum force was about 3.9 N/cm2. This is far below the average pain pressure threshold in extremities, which varies between 100 and 200 N/cm2 depending on the location . This implies that in case of erroneous activation of both servos, the band could not do any harm to the user. This configuration resulted in a sliding speed between 0 and 0.05 m/s. The total length of the rope was decided as at least two times the circumference of the targeted limb to ensure that the knot could travel fully around the limb. The case around the servo and spool was designed to ensure that the rope was neatly winded on the spool without unraveling.
For the compression band, micro-sized position-controlled servomotors were used. The servomotors had dimensions of 2.5x2.5x1.5 cm and weighed less than 10 grams. The servomotors required a current of 0.35 A at 5.0 V when operated, producing a torque of 2.5 kg/cm.
Verification of the device
The technical requirements regarding device safety and comfort were verified by looking at the rationale of the above-described design. None of the aforementioned actuators can reach an activation force that can be considered hazardous for the user. The case was designed in such a way that the parts in contact with the skin were comfortable and not hazardous. The agile design of the case allowed for wide variability in sizes. Anyone could wear the device on an upper or lower limb. The device was within the ergonomic weight limitations of 2.2 Kg . The device was found easy to wear using only one hand, as it can be worn as a sleeve.
The technical requirements regarding device functionalities were verified by bench tests meant to put the whole assembly under intense stress and unveil issues within the electronic and mechanic parts. For this, all actuators were automatically and intensively tested over multiple 2-hours sessions. Such bench tests did not result in any major issue. The calibration of the servomotors suffered minor deviations after hours of continuous activations. This indicated that a recalibration might be needed after intense use.
The delay between software commands and actuator activations was measured to be 261.6 ms on average, slightly above the commonly used threshold of 250 ms for humanly perceivable delays. A delay above 250ms might reduce the user’s performance .
Validation with able-bodied volunteers
The results from 2PD and MFT are shown in Figure 4 presented as improvement between before and after the training (i.e., difference between day 1 and day 5). As expected, the training sessions had little to no effect on the tactile sensitivity of the control areas (i.e., untrained skin patches) for both 2-points and force discrimination (p=0.683 and p=0.193, respectively). Interestingly, the force discrimination slightly worsened. When it comes to the stimulated skin patches, a trend of improvement was found for both 2-point and force discriminations. However, this trend was found significant only for the force discrimination (p=0.05) and not significant for the 2-points discrimination (p=0.12). It is good to note that the improvements have high subject-dependent variability. Outliers present both below the lower inner fence and above the upper inner fence, indicating mild outliers on both sides.
The scores from the training tasks and games overall improved for all participants. All participants advanced to more difficult levels, and the majority settled on medium and hard levels. Usually, participants passed the easy level after one or two sessions. The medium level was used the most on average from day 2 to day 4. Seven participants reached the hard level at least twice during all sessions, and the other four at least once.