Adding a Third Thumb

When you think of prosthetics most people either think of artificial limbs, helping those with a missing or damaged body part, or perhaps, theatrical foam prosthetics for stage and screen. What doesn’t immediately come to mind is a prosthetic to enhance your everyday abilities, a ‘third thumb.’

As part of her graduate work in product design at the Royal College of Art in London, Danielle Clode has created an additional prosthetic digit, a ‘third thumb.’ The 3D-printed sixth digit began as a research project into how upper-limb prosthetics can be attached and controlled by the body.

The prototype of the third thumb is 3D-printed in three main parts from different materials. The cover for the hand , and wrist cover for the motors, are both printed in a smooth rigid grey resin. However the main structure of the thumb itself is printed out of a flexible polyurethane thermoplastic—Ninjaflex flexible filament. These three structural parts are then connected via a Bowden cable system.

The motion of the thumb is created by wires attached to motors on the wrist. The motors pull cabling, creating tension against the flexible 3d printed material of the thumb itself, curling it in a natural manner. Control of the motors is via two pressure sensors fitted into the wearer’s shoes under the toes, and these sensors communicate with the wrist-mounted motors via a Bluetooth connection.

While toe control might seem like an odd idea, it’s a fairly common option for upper body prosthetics, and anyone that’s gone skiing—where direction is more-or-less down to subtle pressure with your toes inside the ski boots—can confirm how much control you can get over gross movements with very small amounts of pressure, and how natural it can become after a short time.

There is some amazing work going on right now towards open bionics to assist those with lost limbs. However apart from the odd set of mechatronic cat ears, or an artificial tail or two, this is one of the few projects I’ve seen setting out to enhance the basic model human. It’s certainly the first I’ve seen to take the idea seriously, and by far the most polished.

The prosthetic formed part of the school’s Graduate Exhibit at the beginning of the month, and was one of this year’s recipients of an RCA Helen Hamlyn Award.