Build a Helping Hand
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The human body is an impressive piece of machinery, and your hands are no exception. With some training, they can perform delicate and complex tasks like manipulating pens and tools to create art. At the same time, hands have the strength and durability to hold a person's own body weight up on steep rocks. Unfortunately, there is a rapidly growing demand for hand replacements. But fortunately, scientists have studied human anatomy and biology and created human-like hands used as artificial devices called prosthetics.These devices are especially needed in war-torn countries. Land mines (explosive devices concealed under the ground and designed to disable enemy targets ranging from combatants to tanks) are the main cause of devastating limb injuries, leading to an estimated 26,000 amputations per year. Prosthetics help those who have lost their hands to regain functionality, independence, and confidence. Some prosthetic designs mimic the human limb closely, while others use a more innovative, creative approach, like the "Cheetah Flex-Foot blades" used by long distance runners. Recent technological innovations and the dedicated work of biomedical engineers has resulted in remarkable progress, and while modern cutting-edge prosthetic limbs do not quite reach the functionality and/or visual appeal of their human counterparts, they are getting closer every day.
In this science project, you will design and create your own prosthetic hand model and put it to the test. Creating a prosthetic hand that performs all the tasks a human hand can handle is clearly a daunting task, so in this science project you will select only a couple of tasks to work on.
Before you start creating, you might want to do some background research on the anatomy of human hands. Study the musculoskeletal system. How do the sizes of the palm and different fingers compare? What is the role of the muscles, the ligaments, and the tendons in the movement of the finger? What gets pulled to make a finger move? What is the role of the bones or the skin? Next, study some robot hands. The robotics science project Grasping with Straws: Make a Robot Hand Using Drinking Straws or the elbow project referenced in the Bibliography can give you ideas on how to create prosthetic fingers or hands. As you look at these two references, pay attention to one major difference between them; in the case of the straw fingers, strings are only used to flex the finger. The elasticity of the straw is used to let the finger "spring" back to its original position. Rubber bands or springs are other ways to obtain this feature. In the elbow example, different strings are used to flex and extend the body parts, mimicking flexor muscles and extensor muscles. Can you think of some advantages and disadvantages of both ways, and decide which will work best for your prosthesis?
Once you have plenty of information, you will probably be eager to start creating. Before you do so, think of the qualities you would like your prosthetic hand to have. Should it be life-like? What is an acceptable weight and size? Should it be able to pinch something hard so it can hold a pencil or use a computer keyboard? Following the engineering design process, you will use these qualities to define the problem. Then you can create clear and precise design requirements, which will then be used to evaluate and test your prototype. Table 1 lists an example of how a problem translates into design requirements and test criteria.
|Desired Quality||Design Requirements||Test|
||The fingers need:
Now that you have a list of desired qualities for your prosthetic hand, including the design requirements, you can start brainstorming and create a first sketch of the design. Note that the anatomy of a prosthesis does not necessarily replicate the human anatomy, but you may still find that mirroring the anatomical structure of the human hand is a good place to start as you brainstorm for your project. Once your sketch is done, continue following the engineering design process to develop and build a prototype. Figure 1 shows some example designs of prosthetic hands created by Science Buddies staff using drinking straws, rubber washers, polymer clay, upholstery thread and rubber gloves.
Figure 1. Collage of prosthetic hands or fingers created by Science Buddies staff.
As another example, Figure 2 shows a hand made using wood. Many other materials could be used, such as irrigation tubing, rulers for fingers or to create an arm, or hinges from the hardware store or craft store can be handy to create joints. Try using a medical glove (or just its fingers) or skinny balloons (the kind used to make balloon animals) to mimic the skin and increase the hand's grip.
Figure 2. Prosthetic hand made by Science Buddies staff. The thumb has a separate joint from the other fingers so that it can move separately, and has a rubber band in the back to pull its spring back in place.
Once you have made your first model, take a good look. Do you like it? Does it inspire you to make it even better? This is your opportunity to learn. You can move on to the test and redesign step of the engineering design process. Put your hand to the test and record your results. State clearly what worked and what did not work so well, so you or other people can benefit from your findings!
Materials and Equipment
The materials you will need depend on the design choices you make for your prosthetic hand. Read the Abstract for a few ideas.
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