PROSTHETIC HAND

Time Period: September 2017 - October 2017

Course: Designing for the Human Body

Skills Developed: 3D Printing, Design Thinking, Brainstorming, User Testing, CAD Modeling

Software Used: SolidWorks, Cura Type A

Project Background

The task of this project was to, as a team, find an existing design of a prosthetic hand for an amputee on Thingiverse and 3D print it. From there, we were to design tests to test this hand making observations and gathering data. Based on the results, we were to brainstorm and implement modifications to the design of the prosthetic we 3D printed.

3D Printing

The first task we faced as a team was 3D printing an existing design for a hand prosthetic. We focused on printing and experimenting with a simple design, in order to understand the constraints and obstacles associated with 3D printing as a manufacturing technique. Looking at the designs on Thingiverse, we noticed that prosthetics that enable controlled grabbing movement used strings or pulley systems to provide motion while concepts with screws or joint connections move with less precision. We wanted to first print a simple prototype with a single homogeneous material before working on an iteration of the prosthetic that required external hardware and additional assembly.
Our first print was a purely mechanical hand, requiring assembly of small 3D printed pegs working as hinges. After printing multiple versions, we realized the file did not include the appropriate parts, including the palm to connect the wrist to the fingers. Our second design simplified the assembly even more, with built-in hinges and joints, requiring only some sanding and post-manufacturing clean-up. This second hand had no possible method of attachment to the amputee’s remaining limb, and moved freely on hinges without any control. After difficulty with our initial focus on a fully 3D-printed model, we diverged and created our third, final version of the hand with nylon and elastic string to ensure controllable arm movement. This prototype was considered for amputees with a working wrist, and used the bending of the wrist to control the opening and closing motion of the fingers.

Testing and Results

In order to test various features and uses of the prosthetic hand, we performed user testing. I volunteered to be the test-subject for all the tests. We experimented with diverse tests, focusing on the practical and performance properties of the hand such as grip, finger and knuckle strength, and fatigue in order to understand how this design can be applied to diverse users in various situations.

Grip Test:

First, we performed a grip test to observe if this prosthetic hand could grip common objects of various shapes and sizes. However, we found that our prototype hand was actually unable to grip any item. We noticed that the fingers were extremely short and could not reach past an amputated limb. Furthermore, the material of the product did not produce much friction, so the surfaces of the fingers were too slippery to hold onto objects.

Finger and Knuckle Joint Test:

We performed a finger and knuckle joint test to see how much weight the finger and knuckle joints could endure when the user tried to hold an object. The results of these tests proved that the joints were fairly strong and were able to withstand over 6N of force and the current knuckle joint could withstand over 20N of force. Although this is much less force than normal human fingers and hands handle, we realized that the strength shown in this current prototype was above average and could be improved by changing the material.

Fatigue Test:

We tested fatigue of our prototyped fingers by bending a finger 1000 times to observe how its movement and positioning might change. We noticed that over time, the string that enabled the fingers to open and close became loose and the joint slipped. We also applied extra force to break a finger to identify the initial failure location, learn how the joints break, and observe whether the other fingers still function. Surprisingly, neither finger joint broke; rather, the small stop restricting backward movement of the finger broke.

Range of Motion Test:

We wanted to study the range of motion of the prosthetic attachment and the ease of controlling the position. To do this, we measured at what angles the user must bend his or her wrist to properly open and close the prosthetic and the sustained control at various angles, noting any discomfort the test-subject felt at the wrist due to additional strain. The user was able to sustain different angle positions; however, felt strained when holding positions that were within a range of 30-degrees from the natural resting position, due to the strong elastic forces of the hinge mechanism.

Comfort:

The user felt discomfort on the wrist, as the prosthetic began to dig into their skin after long or repeated use. Some edges dug into the test-subject's skin and there was no comfortable attachment method with this particular model.

Brainstorming

When analyzing the data collected from user testing, the team broke down the main issues with the current hand prosthetic into 5 categories: Attachment + Comfort, Size + Fit, Grip + Strength, User Control, and Modular + Adaptable. The key insights gained from user research were detailed under these main categories, and were then prioritized into focus areas for brainstorming. We individually developed concepts and regrouped to discuss how to converge our 20+ concepts into three main ideas for the redesign.

Our criteria for choosing ideas stemmed from our defined goals, including: ability to grab everyday items (specifically cups and doorknobs), fit multiple people, be comfortable for long term wear, and provide more fluid and natural maneuvers. Ideas were prioritized in comparison to our focused constraints on a weighted decision matrix.

Redesign

Our top 3 redesign ideas from the brainstorming session were implemented into the new prosthetic hand design. They include:

​

Grip Pads:

Our redesign includes the addition of a grip-like material to the fingers to aid the user in picking up and holding items for a longer duration of time. This was inspired by current gripped gloves and shoes with a simple layer of silicon gel pads and silicon adhesive dots, commonly used on a smaller scale on gloves and socks to create friction to prevent slipping. These maintain the flexibility and shape desired while providing a simple solution to add grip strength.

​

Adjustable Velcro Straps:

After exploring multiple options for attachment methods, we converged on the idea of using velcro which is less expensive than molding an attachment specific to each user and can be adjusted. Furthermore, the velcro will not bend with the user as he/she attempts to maneuver this prosthetic, bending the wrist. This velcro will be attached at three locations for maximum security and translation of the user's movement: on the upper palm, at the wrist, and at the lower forearm at the base of the prosthetic.

Anthropomorphic Form:

Our redesign affects the form of the hand: extending finger length and palm size, and modifying the shape of the components of the prosthetic. The extended finger length will allow the user to grab larger objects and get a better hold on smaller objects. The larger palm size will make the device more comfortable for the user’s palm to rest in without the sides of the device digging into the skin. Filleting edges and replacing the rectangular brackets over the hand with arched brackets will allow the product to have a more anthropomorphic shape.

Conclusion

From this project, I learned a lot about integrating design with the body. When designing tests and brainstorming changes to make to the design, it took a lot of thinking and observing actions and behaviors that we tend to take for granted.

​

This was the first class where I began with an existing design and tested and iterated on it to improve the product. It was a great learning experience to start with someone else's idea which I enjoyed because it is another opportunity to gain insight and learn from other's ideas. It is also the first time I conducted user testing on a project, while in the past my projects were not products that a large population could benefit from.

​

In future classes, I hope to use the design process and user testing skills and methods to develop better projects that fulfill user needs.