All the while Domhnall Gleeson's Caleb is supposedly conducting a Turing test on Ava, the humanoid robot of Alex Garland's movie Ex Machina, he never touches her once. Her inventor Nathan is taking rather too much for granted there. True, until Ava is fully clothed and bewigged there is no mistaking, from the filigree mesh covering her limbs and torso, that she is a manufactured being. But to pass as a real human, as she would need to when at the movie's end she is out in the world, Ava should also 'feel' human too. And it is hard to imagine how a mechanical body, no matter how exquisitely made, could ever fool us into thinking we are touching a human quickened and warmed by the pulse of life.

Yet today there is more reason than ever to face that challenge. A few people worldwide who have lost limbs have been fitted with prosthetic replacements that can be controlled by their mind, using signals from their nerve endings connected to electrodes1. The technology, developed at the Rehabilitation Institute of Chicago, makes use of a surgical technique called targeted reinnervation2. Right now recovery of limb function is obviously the priority, but at some point it may well be thought desirable for the limb not only to function as but to look and feel like a wholly real one, to both the user and others. Learning to live with prosthetic limbs carries a whole range of psychological issues concerning a sense of 'body ownership' and feelings of social stigma: for example, an ability to conceal prosthesis use typically aids the person's sense of social integration and reduces potential emotional problems3.

Cabibihan et al. now report that with a good choice of materials, an artificial hand can be made to feel indistinguishable from a human hand when it touches subjects4. Selection of the best material for 'synthetic skin' was made by participants who were offered a choice of four different soft polymeric materials, all of them used previously for prosthetic or robotic applications, heated electronically to different temperatures. The subjects showed a clear preference for two of the materials, silicones with trade names Ecoflex and Prochima, and the preferred average temperature was 28.4 °C, close to the measured average of the participants' own fingertips.

Credit: PHILIP BALL

Cabibihan et al. then made an artificial hand from the Ecoflex silicone moulded over a resin replica of a human hand skeleton made by 3D printing. Their indentation measurements showed that it had rather similar skin compliance to a real hand over much of the surface. Participants were tested to see if they could distinguish the feel of the optimally warmed artificial hand from that of a real hand when touched on the forearm while the hand was kept hidden from them. The differences were barely significant, especially in terms of judgements about whether the hand was 'certainly' prosthetic. It would be fair to say that this synthetic hand passed the Turing touch test.