Science: Artificial “Skin” System Transmits the Pressure of Touch

The system might someday be applied to prosthetics to mimic human skin’s ability to feel sensation.
Zhenan Bao describes the steps in building artificial pressure-sensing "skin" than can communicate with mouse brain cells. | Bao Research Group, Stanford University

Researchers have created a sensory system that mimics the ability of human skin to feel pressure. Furthermore, they transmitted the digital signals from the system's sensors to the brain cells of mice. These new developments, reported in the 16 October issue of Science, could allow many people around the world living with prosthetics to one day feel sensation in their artificial limbs.

The system, consisting of printed, plastic circuits, might someday be placed on robotic fingertips. Digital signals transmitted by the system would increase as the fingertips came closer to an object, with the signal strength growing as the fingertips gripped the object tighter.

To simulate this human sensation of pressure, Zhenan Bao of Stanford University and her colleagues developed a number of key components that collectively allow the system to function. "Since this work requires several parts — sensors, readout circuits, and a modified brain slice [to read the sensor signals] — the biggest challenge was to have all the parts work well together," Bao explained.

As our fingers first touch an object, how we physically "feel" it depends on the invisible electrical force that the object exerts on our skin. Therefore the research team used a sensor with a specialized circuit that translates the pressure of an object's electrical field into digital signals. As the electrical force increases, the digital signals change to reflect this.

Artificial mechanoreceptors mounted on the fingers of a model robotic hand
The thin plastic pressure sensors, here mounted on the fingertips of a robotic hand, could provide prosthetic limbs with some feeling. | Bao Research Group, Stanford University

To allow the sensory system to feel the same range of pressure that human fingertips can, the team needed a highly sensitive sensor. They improved upon existing sensors by using carbon nanotubes in formations that are highly effective at detecting the electrical fields of inanimate objects.

The researchers then explored ways to transmit the digital signal from the artificial sensory system to the cortical neurons of mice. Optogenetics, the use of light to control neuron transmission, is an appealing option; however, conventional light-sensitive proteins used in optogenetics do not stimulate neurons long enough for the digital signals to be sensed.

This led the team to engineer new optogenetic proteins that are able to accommodate longer intervals of stimulation. They then incorporated these newly engineered optogenetic proteins in a section of mouse brain in a lab dish, which allowed the neurons to fire in response to the digital stimulation pulse.

Bao noted that the printed circuits of the new sensory system would make it easy to produce in large quantities. And, the team is interested in improving upon their existing model. "We would like to make the circuits with stretchable materials in the future, to truly mimic skin," Bao said. "Other sensations, like temperature sensing, would be very interesting to combine with touch sensing."

[Credit for teaser image: Bao Research Group, Stanford University]