Developing an Open-Source Pediatric Hand Prosthesis (v4)

This open-source project provides a lightweight, low-cost 3D-printed hand prosthesis for children, enabling everyday tasks like catching balls, riding a bike, or holding objects. Developed at TU Berlin, the design builds on the Kinetic Hand by Mat Bowtell but improves comfort, durability, and adjustability—all for under €10 in materials. Our goal: A prosthesis that grows with the child and empowers them to explore the world independently. All design files and documentation are open-source and available on GitHub, allowing others to replicate, adapt, or further develop the solution.

Design Overview

Feature	Solution	Benefit
Custom Fit	Uniform scaling (no geometry modification)	Fits 9-year-olds out of the box
Materials	PETG (body), TPU (joints), Dyneema® (strings)	Durable, flexible, low-stretch
Grip Optimization	Latex/hot-glue grip surfaces	Better hold on smooth or irregular objects
Modularity	Compatible with e-NABLE parts	Easy repairs/replacements
Cost	<€10 (filament + hardware)	<1 % of commercial prostheses

Version history

Version	Status	Name	Key Features	Date
v1	Proof of Concept	First Working Model (Flexy-Hand 2 by Gyrobot)	Identified issues: too large size, limited adjustability, poor grip, visible cables Used nylon strings → observed loss of force due to elasticity Basis for further research and benchmarking	April 2025
v2	Working prototype	Fit & Geometry Adaptation	Scaled Kinetic Hand by MatBowtell model for individual fit Adjusted wrist socket and gauntlet geometry Tested various scaling strategies (uniform vs. proportional) Settled on PETG and TPU for main components and Dyneema® for cords	May 2025
v3	Prototype	Grip Optimization & Cable Routing	Added grip surface testing (TPU, latex, hot glue) Settled on uniform scaling to keep wire channels functional Initial user feedback integrated Tuned in prinitng settings for optimal joint strength	June 2025
v4	Functional and tested	Final Prototype	Custom string tensioning clamp for one-handed operation Latex grip surfaces for improved usability Joints printed from softer TPU	July 2025

• tested non-proportional scaling (but settled on uniform scaling)
• printed multiple test pieces and incorporated user feedback

• replaced stretchy Nylon tensors with Dyneema® for reduced backlash
• selected softer TPU for hinges to lower required actuation force

• 

  added grip elements from latex/ developed process for in-place hot glue injection moulding

• selected PETG for main components for increased chemical resistance and durability
• rejected PLA for low heat resistance (e.g. prosthesis left in a car in the summer)

• developed new tensioner for one-handed adjustment

• Cost: <10 € per piece
• Weight: ~260 g
• Print Time: ~8 hours
• Assembly Time: ~2 hours

Within days, the child could:

• Hold and drink from a water bottle (previously required two hands).
• Catch a ball during play (improved hand-eye coordination).
• Ride a bicycle while gripping the handlebars.

• Precision tasks (e.g., holding a pen) remain difficult.
• Full closure requires significant wrist force (focus of next iteration).
• Separate movement of fingers

• 3D Printer: FDM (PETG/TPU).
• Files: GitHub Repository (STL files, assembly guide).
• Time: ~8 hours print time, 2 hours assembly.
• Additional Materials: Dyneema Small Screws, Latex or hot glue
• Tools: needle-nose pliers, box cutter, small screwdriver

1. Measure the user’s hand (following the instructions provided in the PDF for the Kinetic Hand).
2. Print parts (PETG for rigidity, TPU for flexible components).
3. Assemble (follow the official guide in the PDF for the Kinetic Hand).
4. Adjust tension (use the new screw-based tensor for easy calibration).
5. Test & iterate (involve the user for feedback!).

For a more detailed documentation, please refer to our Quick Start Guide on GitHub.

• Test our design? Share feedback via GitHub Issues.
• Improve it? Fork the repo and submit pull requests!
• Need help? Join the e-NABLE Forum (we plan to post updates there soon).

We are not in yet in contact with e-NABLE, but we plan to share our findings with them directly.

Feature	Status	How You Can Help
Hot-glue grip pads	Prototyped	Test durability with daily use model new fingers for hot-glue inserts on fingertips and phalanxes
Heat-shrink joints	In development	Stress-test with cycling loads
Improved print settings	In development	create optimised printing profiles for faster and more reliable prints with greater strength

Yes! The Kinetic Hand by Mat Bowtell was originally designed for adults, and our design is still built for this purpose. Use uniform scaling to achieve a good fit.

We gratefully acknowledge the open-source foundation by:

• Kinetic Hand by MatBowtell
• Flexy-Hand by Gyrobot

This project was developed as part of the course “3D-Druck in der Mechanik” (en: “3D-printing in mechanics”) at TU Berlin, supervised by Prof. Dr. C. Völlmecke, Yating Ou (M.Sc.), and Narges Panjalipoursangari (M.Sc.).

Special thanks to our young user and his family, for their continuous feedback and further improvements to the design.