If you lose your hand in an accident, a prosthetic could cost well over US$10,000. But 3D printing holds out the promise of making simple replacement hands available for far less.

In a closet-sized room in Tokyo’s Akihabara electronics district, the founders of robotics startup Exiii pore over the designs for their latest prototype. It’s a basic mechanical hand and forearm made from materials that only cost about $200. It’s also open source.

The Exiii Hackberry, as it’s called, has a flexible wrist, partially motorized fingers and low-cost parts such as an Arduino controller and a digital camera battery, all housed in a white 3D-printed plastic shell.

During a demo, co-founder Genta Kondo strapped an infrared distance sensor to his inner forearm and plugged it in to the Hackberry. The sensor measures the distance to the skin, so when a muscle contracts, it detects a change and sends a signal to the Arduino board. The sensor can be attached on a user’s upper arm, for instance, so that flexing biceps would activate the hand.

After a few false starts, Kondo flexed his forearm by making a fist and the fingers on the Hackberry closed, mimicking the movement. Although the servomotors emit a fairly loud screech, the grip is delicate enough to hold a business card without crushing it.

Compared to the previous, fourth-generation prototype hand, the Hackberry is now more compact and functional, with the battery use time extended from two hours to a full day. The number of servomotors has been reduced from six to three, one for the thumb, one for the index finger and one for the other three fingers, to keep it as light and simple as possible. Its plastic can now be printed on a consumer-grade 3D printer instead of an industrial one.

In experiments, the Hackberry has helped two Japanese without hands, as seen in a promo video where they are doing everyday things like tying a shoelace, turning the pages of a magazine and zipping up a jacket.

“The strongest point about the Hackberry is that it can pinch tiny objects,” said Kondo, a 28-year-old former Sony roboticist who co-founded Exiii last year.

Kondo and colleagues have experimented with prototypes that used electrical muscle signals in the skin, relayed via smartphone, to control the finger motors. But they’re now focused on using the infrared muscle sensor. It’s buggy, often failing to work during the demo, but it’s cheap and simple to set up. Last month, Kondo and his colleagues put the CAD files for the hand online and hopes to attract volunteers to help refine such prosthetics.

Exiii is one of several startups that are designing affordable appendages for people who either lost a hand in an accident or were born without one. The hacker approach also means that unlike high-cost, mass-market prosthetics, the 3D-printed hands can be quickly repaired and have custom designs and features such as NFC modules in the fingertips, which could do everything from unlocking smart locks to authenticating mobile payments.

The company recently received a $200,000 grant from Google, made via prosthetics startup Mission Arm, to develop its idea. The search giant also funded the E-Nable project to create even simpler 3D-printed hands for kids who have a palm and some degree of wrist motion.

Exiii plans to use its funds to help make the Hackberry a lot more robust so it’s ready for mass adoption. The cost hasn’t been decided yet but the company will push it on a nonprofit basis.

“I could see these being made by people around the world, using 3D printers set up in each town,” Kondo said, adding that regulations for medical devices would have to be fully met for the project to be successful.