Adding Haptics to a Second Skin

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Researchers from the University of Illinois at Urbana-Champaign, Northwestern University and Dalian University of Technology have completed a fairly ambitious project to recreate the functionality of pacinian corpuscles and Meissner's corpuscle via silicon-embedded semiconductors.

In short, they have prototyped an electrotactile second skin to be worn over the first, which adds additional pressure sensation to the user's own sensory capabilities – it makes the skin much more sensitive to touch.

Pacinian corpuscles deal with sudden sharp changes in pressure such as the feeling of suddenly coming into contact with a surface, whilst Meissner's corpuscles deal with slower changes in pressure over time such as running your finger over a surface and feeling the grain. Between the two, they account for the entire senses of pressure and texture. Because this second skin does not transmit to any other of the sensory nerve endings, data about heat, cold, or pain does not make it through it, unless the second skin itself is torn away.

The researchers anticipate a likely use as surgical gloves, greatly increasing the sensitivity and precision at a surgeon's fingertips. However the material could wrap any part of the body to increase sensation there. Ideal perhaps for someone who has limited sensitivity due to peripheral nerve or spinal damage.

So far, the new skin device has shown to be more than capable of responding with precision to the normal stresses and strains associated with hand and finger movement, greatly increasing the likelihood of its use for extended periods without losing sensitivity or deteriorating in other ways.

The electronic circuitry inside this skin is made out of of patterns of gold conductive lines and ultrathin sheets of silicon, integrated onto a flexible polymer called polyimide. The sheet is then etched into an open mesh geometry and transferred to a thin sheet of silicone rubber moulded into the precise shape of the body part it intends to cover. If they are to be used as surgeon's gloves, they must be custom made to the surgeon's fingers. A one-size-fits-all approach would leave the semiconductors attached in the wrong places, and risk putting the highest sensory enhancement in undesirable areas of the finger – such as away from the fingertips themselves.

The skin works by measuring the stresses and strains applied to the natural skin under it, in much the same way as the two corpuscles it emulates do. This stretching alters the capacitance of microelectrodes buried inside the circuit. As additional force is applied, the spacing between layers decreases in the exact same manner as the multi-layered onion of the pacinian corpuscle functions. The layers collapse onto one another, and the intensity of the collapse directly increases the capacitance and the as a result the electrical signal produced, at that location only, as more and more pairs of microelectrodes are crammed into the same tiny space.
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The researchers experimented with having the electronics on the inside of the device, in contact with wearer’s skin, and also on the outside. They believe that because the device exploits materials and fabrication techniques adopted from the established semiconductor industry, the processes can be scaled for realistic use at reasonable cost.

The research team hope to be able to include a temperature-sensing capability in the near future, and are currently looking at the possibility of encasing a living heart in the material, to monitor in real-time, every aspect of its performance by the pressures it places on the skin. If this is successful, then the skin will be a viable option for robotic and prosthetic uses as well – without a second, natural skin and its sensors below it.