OpenBionics Prosthetic Hand

OpenBionics Prosthetic Hand

A low-cost, 3D-printable, fully functional anthropomorphic prosthetic hand. The OpenBionics initiative (openbionics.org) publishes designs that cost under $200 and weigh under 300 g, aimed at closing the gap between commercial prosthetics (often tens of thousands of dollars) and what an individual or clinician can fabricate themselves.

How it works

The standout feature is a whiffletree-based differential mechanism. A single actuator drives all five fingers — but because of the differential, each finger stops independently when it contacts an object. That one trick unlocks 144 distinct grasp poses from a single motor, letting the hand conform to a wide range of objects (cylinders, pinches, key grips, hooks) without per-finger control.

A set of small mechanical locks lets the user selectively block specific fingers, which is how grasp selection is done on the physical device — no complex EMG classifier required at minimum.

What you get

• Fully printable 5-finger right hand (mirror files for left also available in upstream repo)
• HerkuleX DRS-0201 smart servo as the single actuator
• Wire-driven tendons with a differential bar mechanism
• Parametric CAD — scale the hand to fit different users using anthropometry tables
• Optional body harness + breakout board for integration with EMG or button control

Canonical printed parts

• palmDown, palmUp — two-part palm shell
• index, middle, ring, pinky, thumb — finger bodies (multi-phalanx each)
• baseHerkulex — mount for the smart servo
• mainBar, barIndexMiddle, barRingPinky — the whiffletree differential
• FlangePlate — wrist interface

The upstream repo contains ~280 STLs total covering left + right hands, the HDM variant, lockable mechanisms, and the wearable harness.

What you can do with it

• Teleop grasping demos — use the orobot cloud surface to command preset grasps (open, power grip, pinch, tripod).
• EMG or button-driven control — the upstream Arduino code reads muscle signals or switches and triggers grasps. This program wraps that with a cloud-side trigger surface.
• Research platform — anthropomorphic, under-actuated, fully open — a good baseline for grasp planning, tactile sensor integration, and rehabilitation engineering projects.
• Education — the whiffletree differential is a genuinely elegant piece of mechanical design; students can print one in a weekend and understand underactuated grasping hands-on.

Ethics note

This is a research-grade design published for open study and community iteration. For clinical use as a medical prosthesis, local regulatory review, professional fitting, and ongoing care are required — 3D-printable ≠ ready to wear.

Attribution & license

• Initiative: OpenBionics (openbionics.org)
• Upstream repo: https://github.com/OpenBionics/Prosthetic-Hands
• License: Creative Commons Attribution-ShareAlike 4.0 International (CC-BY-SA 4.0)
• Homepage: https://www.openbionics.org/

Links

• Upstream: https://github.com/OpenBionics/Prosthetic-Hands
• HerkuleX servo docs: see RobotShop product page (original dst-robot.com manual URL is no longer available)
• Project page: http://www.openbionics.org/

Build Guide

Assembly guide and parts list: OpenBionics Assembly Guide

Printing

This is a curated 24-part subset of the OpenBionics upstream (which has ~280 STLs across left/right hands, the HDM variant, locks, and harness). It covers one complete right hand plus the optional control/wear modules. Print in ABS/PLA, ~304 cm³. Most parts are print-one-each; the fingers are individual (each is a multi-phalanx body).

Core hand — print one each (14):

• palmDown, palmUp — the two-part palm shell
• index, middle, ring, pinky, thumb — the five finger bodies
• baseHerkulex — mount for the single HerkuleX DRS-0201 servo
• mainBar, barIndexMiddle, barRingPinky — the whiffletree differential bars (this is the mechanism that lets one motor drive all five fingers and conform to objects)
• BasePulleyInsidePalm, SupportPulleyPalmUpAssem — tendon-routing pulleys inside the palm
• FlangePlate — wrist interface

Optional module — selectable finger locks (4): dovetail_female, lockMechanismBracket, lockMechanismSupport, and the button hardware below let the user mechanically block individual fingers to pick a grasp. Print only if you want manual grasp selection.

Optional module — button control (3): buttonAxle, buttonBase, buttonFrame — the physical button assembly for triggering/locking grasps without EMG.

Optional module — wearable harness (4): HarnessEE, HarnessEntry, HarnessFemaleHole, HarnessMaleHole — the body-harness breakout for mounting the hand on a wearer. Print only for a worn build.

Left hand: This list is the right hand. The upstream repo contains full mirror files for a left hand (and an HDM variant) — add those from the OpenBionics repo if you need a left hand instead.

Not printed: 1× HerkuleX DRS-0201 smart servo (the single actuator), elastic/Dyneema tendon cord, ball bearings, springs, and M2/M3 fasteners per the BOM.