In a few years’ time we’ll be able to plug it in (Image: DARPA/JHUAPL/HDT Engineering Services)

Imagine a bionic arm that plugs directly into the nervous system, so that the brain can control its motion, and the owner can feel pressure and heat through their robotic hand. This prospect has come a step closer with the development of photonic sensors that could improve connections between nerves and prosthetic limbs.

Existing neural interfaces are electronic, using metal components that may be rejected by the body. Now Marc Christensen at Southern Methodist University in Dallas, Texas, and colleagues are building sensors to pick up nerve signals using light instead. They employ optical fibres and polymers that are less likely than metal to trigger an immune response, and which will not corrode.

The sensors are currently in the prototype stage and too big to put in the body, but smaller versions should work in biological tissue, according to the team.


Whisper light

The sensors are based on spherical shells of a polymer that changes shape in an electric field. The shells are coupled with an optical fibre, which sends a beam of light travelling around inside them.

The way that the light travels around the inside of the sphere is called a “whispering gallery mode”, named after the Whispering Gallery in St Paul’s Cathedral, London, where sound travels further than usual because it reflects along a concave wall.

The idea is that the electric field associated with a nerve impulse could affect the shape of the sphere, which will in turn change the resonance of the light on the inside of the shell; the nerve effectively becomes part of a photonic circuit. In theory, the change in resonance of the light travelling through the optical fibre could tell a robotic arm that the brain wants to move a finger, for instance.

Signals could be carried in the other direction by shining infrared light directly onto a nerve – this is known to stimulate nerves – guided by a reflector at the tip of the optical fibre.

To use working versions of the sensors, nerve connections would need to be mapped. For example, a patient could be asked to try to raise their missing arm, so that a surgeon could connect the relevant nerve to the prosthesis.

Bionic dog

The researchers plan to demonstrate a working prototype on a cat or dog within the next two years. Before then, the sensor will need to be shrunk from hundreds of micrometres to 50 micrometres. The project has $5.6 million of funding from the US military’s Defense Advanced Research Projects Agency (DARPA).

Christensen says one day the sensors and optical fibre could acts as “jumper cord” to restore movement and sensation to patients with spinal cord damage, by routing nerves in the brain to the legs, circumventing the damaged area.

Ravi Bellamkonda, a bioengineer at the Georgia Institute of Technology in Atlanta, is impressed. “I would be excited to have them succeed – it is important to develop robust interfaces to the nervous system,” he says.

But Mark Gasson of the University of Reading, UK, says the sensors may still be rejected by the body. “Certainly these are largely biocompatible materials. However, I doubt you can totally rule out some form of immune response,” he says.