# A Low-Cost Soft Robotic Hand Exoskeleton for Use in Therapy of Limited Hand-Motor Function
## 2 Materials and Methods
### Mechanical Design  
The mechanical design of the glove was based on the resting position of the hand, having the motor system actuate the closing of the hand and a passive system to return the hand to the resting position. This qualification made it necessary to have an inelastic material on the underside of the hand coupled to a motor system with position monitoring to close the hand and an elastic material on top of the hand that will return the hand to the neutral position. The elastic and inelastic tendons were secured to the fingers using 3D-printed rings. The motor system and position feedback system were mounted to the bottom side of the arm on a 3D-printed, ergonomically formed support system. The passive retraction system was mounted to the topside of the hand. All the separate components were secured to a 2.5 mm neoprene glove using a cyanoacrylate adhesive to form a contiguous unit.  
A set of low-profile rings was designed to affix the artificial tendons to the fingers, as shown in Figure 4b. Sizes ranging from 15 to 35 mm in diameter were created with customization in mind, as the end user can choose sizes to fit their hand without any need to edit the 3D models. A support platform with ergonomic contouring was created to house the motor system, guide the palmar tendons from the actuation system to the fingers and keep the wrist straight, as seen in Figure 4a.  
The requirement of having a simple mechanical design was the most significant driver our mechanical design process, as the exoskeleton must be simple enough to be put together by any layperson. Our design incorporates a snap-fit structure consisting of the ergonomic wrist structure, motor driver holders and an enclosure; as shown in the blown-up mechanical model in Figure 5a. The  
Figure 4: (**a**) Wrist support structure detailing palmar tendon guides and ergonomic contours. (**b**) 2D profile detailing tendon attachment points for the ring system.  
Figure 3: Custom actuator mechanism detailing various components.  
components were designed for simplicity, to be 3D-printed on any available 3D printer by the end-user and put together without any extra hardware or tools.  
Furthermore, adaptability to a wide array of hand and finger sizes played a crucial role in the design process. The mechanics of the exoskeleton were designed in a modular fashion, with three sizes of the support platform, shown in Figure 4a and 20 sizes of rings, shown in Figure 4b, ranging from 15 to 35 mm to allow fitment for a wide range of hand and finger sizes. The artificial tendons providing the actuating and restoring force to the fingers are easily adapted to the user's kinematics. The finger stroke length, seen as \(\Delta\)L in Figure 2a, can be customized by simply trimming the monofilament line to the correct length prior to securing it to the rings with the fingers fully extended. Once secured, the software then runs a calibration procedure to measure the appropriate \(\Delta\)L for the user. The restorative force, provided by 1/8\"shock-card, can be customized in a similar manner.  
Figure 5b shows the fully assembled robotic hand exoskeleton connected via a wire harness to the microcontroller. The top enclosure was purposefully removed in this figure to show the simplicity of construction and wire routing. The entire electromechanical assembly fits neatly within our mechanical enclosure, which is flat on top allowing two separate modes of operation while the hand is resting on a flat surface: one with the palm facing upwards and the other with the palm facing down with the flat surface of our enclosure resting on a table-top allowing for full range of motion of the fingers during a physical therapy session.