The Third Thumb, a Prosthesis That Provides a New Kinesthetic Experience

In the first week of July 2017, a playful device massively shared on the web aroused curiosity, mainly from scientific communities and designers. Danielle Clode, a graduate of the Royal College of Art (RCA), created a 3D-printed prosthesis that is not intended to replace a limb but is added as a sixth digit, on one hand. Clode invented a new type of object halfway between a tool and a prosthesis. She plans to use it to learn more about the perception of prostheses by studying subjects who have all their limbs. Also, this digit will serve to explore another way to enhance human capabilities.

The digit was designed as part of her graduation project for the Product Design masters. Her creation began as a research project looking at how upper limb prostheses are attached to and controlled by the body.

Her invention, dubbed The Third Thumb, increases the user’s competency in gripping and manually manipulating things, or in artistic and physical performances. It notably improves and explores new artistic possibilities in music.

The Third Thumb received RCA’s Helen Hamlyn Design Award for Creativity. Installed next to the little finger, this artificial digit is connected to pressure sensors that allow the user to move it.

Simple Technology, Playful Experience, and Synesthesia

The prosthesis includes two motors mounted in a bracelet worn on the wrist of the hand equipped with the prosthesis. The motors operate a cable, which functions like the Bowden cable, integrated into the thumb structure to control the prosthesis articulations—mimicking the proximal, intermediate and distal phalanges. The cable also keeps the third thumb stable with two strips hugging the back of the hand and the part between the thumb and forefinger.

The motors are controlled by two pressure sensors attached to the users’ shoes, beneath the toes, and connected via Bluetooth. The device is based on the natural hand and foot coordination that we use spontaneously, for example, when driving a car, playing the piano, etc. The thumb and outer shells of the device’s various parts are printed in resin with a Formlabs SLA printer. Clode chose NinjaTek’s Ninjaflex polyurethane cables, printed in 3D and used in the field of robotics because of their resistance, flexibility and lightness.

The signals sent via Bluetooth translate the foot movements and activate different parts of the thumb. So, to make a gripping motion, you have to press your foot down. To extend the thumb, you simply lift the foot to release the pressure. The prosthesis then resumes the normal and relaxed position of the hand.

By manipulating this thumb, the bilateral coordination will try to adapt to the new limb, rendering the hands’ synchronized movements asymmetrical. Hand-eye coordination is also required, in an unusual way. Users will be able to develop a form of kinesthesia located in a part of the body, which, in proprioceptive awareness, is unrelated to a specific limb, as well as dexterity in a part of the hand that has never been experienced before. In fact, we believe that in addition to its potential in the field of intelligent prostheses, this technology can be used to develop memory and synaesthesia intelligence.