A working prosthetic arm for a school project can be as simple as cardboard and string or as advanced as 3D-printed parts controlled by an Arduino. The approach you choose depends on your grade level, budget, and how much time you have. Here’s how to build one at three different complexity levels, from a single-afternoon build to a multi-week engineering project.
The Simple Version: Cardboard and String
This is the fastest option and works well for elementary through middle school projects. The total cost is under $5, and you can finish it in one to two hours. You’ll need cardboard, drinking straws, yarn or string, small beads, tape, and scissors.
Start by tracing your hand and forearm onto a piece of sturdy cardboard and cutting out the shape. Then cut your straws into one-inch pieces (half-inch if your hand is small). Tape the straw pieces along each finger of the cardboard cutout, leaving small gaps between them. Crease the cardboard at each gap by folding it back and forth. These creases act as finger joints.
Cut five pieces of yarn, each about 12 inches long, and tie a bead to one end of each. Thread each piece of yarn through the straw segments on one finger, starting at the fingertip and running down to the palm. The bead keeps the string from pulling through. When you pull a string at the base, the corresponding finger curls inward, just like a real tendon pulling a real finger. You can pull all five at once to make a gripping fist, or pull them individually.
This design directly mimics how your hand actually works. Your fingers don’t have muscles in them. Instead, muscles in your forearm pull on long tendons that run through sheaths, bending each finger at its joints. The yarn is your tendon, the straw pieces are the sheath, and the cardboard creases are the joints. That biological connection is worth explaining on your presentation board.
Making It Grip Real Objects
The basic cardboard version curls its fingers but struggles to pick anything up because cardboard is flat and slippery. A few upgrades fix this. Hot-glue small rubber pads (cut from a shelf liner or rubber band) to the inside of each finger segment. This adds friction. You can also layer two pieces of cardboard for each finger to make the structure stiffer, and add a thumb that opposes the fingers at an angle rather than sitting flat alongside them.
To test your hand’s grip for a science fair, try picking up objects of increasing weight: a cotton ball, a pencil, a ping pong ball, a small water bottle. Record which objects it can hold and for how long. You can also measure how much force your hand generates by hanging small weights from the fingertips and noting the maximum weight each finger supports before the string slips or the cardboard buckles. This kind of simple load testing shows you understand engineering evaluation, not just construction.
The Intermediate Version: 3D-Printed Parts
If you have access to a 3D printer (many school libraries and makerspaces have them), you can print a functional prosthetic hand using free designs from the e-NABLE community, a global volunteer network that creates open-source prosthetic designs. The printed plastic parts and basic hardware cost around $50 in materials, and the print itself takes roughly six to eight hours.
e-NABLE designs use the same tendon principle as the cardboard version, but with hinged plastic joints, proper finger curvature, and a wrist mechanism that closes the fingers when the wrist flexes. The parts snap or screw together, and you thread fishing line or cord through channels built into each finger. The result looks and functions much closer to a real prosthetic than cardboard does.
For a school project, printing an e-NABLE hand lets you discuss real-world impact. These designs have been fitted to thousands of people worldwide, and comparing their $50 material cost to the $42,000 price tag of a commercial motorized prosthetic hand makes for a compelling data point in any presentation.
The Advanced Version: Servo Motors and Arduino
For a high school robotics or engineering class, you can build a prosthetic arm that moves on its own using small servo motors and an Arduino Uno microcontroller. Each joint of the arm (shoulder, elbow, wrist, and individual fingers) gets its own servo motor, which receives instructions from the Arduino to rotate to a specific angle. A basic arm with a gripping hand needs at least five or six servos.
The frame can be 3D-printed, laser-cut from acrylic, or even built from popsicle sticks and hot glue for a lower-budget approach. Each servo mounts at a joint and connects to the next segment with a simple bracket or horn. You write a short program (called a sketch) on the Arduino that tells each servo what angle to move to and when. Libraries of pre-written code for servo control are built into the Arduino software, so you don’t need to start from scratch.
Budget for this level runs $40 to $80 for an Arduino starter kit with servos included. The programming is straightforward: the basic servo sweep example that comes with the Arduino software is only about 10 lines of code, and you modify it to control multiple servos in sequence to create a grabbing motion.
Controlling It With Muscle Signals
If you want to go further, a muscle sensor board lets you control the arm using electrical signals from your own muscles. When you flex your forearm, the sensor detects the tiny voltage your muscle produces and sends that signal to the Arduino, which then tells the servos to close the hand. This is the same basic principle behind real myoelectric prosthetics.
The sensor uses three adhesive electrode pads placed on your forearm: one in the middle of the muscle, one along the muscle’s length, and a reference electrode next to the muscle body. The sensor board runs on either 3.3 or 5 volts depending on your Arduino model. This addition costs roughly $40 to $60 for the sensor board and electrodes, and it transforms a robotics project into a biomedical engineering demonstration.
What Makes the Project Stand Out
Regardless of which version you build, the strongest school projects connect the engineering to the biology. Your presentation should explain why you made each design choice by referencing how real hands work. Strings act as tendons. Straw segments act as tendon sheaths. Cardboard creases or plastic hinges act as joints. Rubber grip pads act as skin friction. Servo motors act as forearm muscles. Every component has a biological parallel.
Include a testing section that shows you evaluated your design, not just built it. Pick three or four objects of different shapes and weights, and record whether your hand can grip each one, how long it holds on, and what fails first. If you built multiple versions or made improvements along the way, photograph each iteration and explain what you changed and why. That iterative process, building, testing, improving, is the core of engineering design, and teachers grade heavily on it.
For materials and sourcing: cardboard and string are free, 3D printing filament runs about $20 per kilogram (more than enough for a hand), Arduino starter kits with servos cost $40 to $80 on Amazon, and muscle sensor boards are available from electronics retailers like SparkFun. Most school projects can be completed in one to three weeks depending on complexity, with the cardboard version done in a single sitting.