Artificial Skin for Robotic Limbs

Since the new Star Wars trailer has brought with it an air of Sci-Fi, I felt our next story should fit the mood. This led me to a recent article published in the journal “Science” in which a team of researchers created an incredible form of artificial skin. Currently, prosthetic limbs can restore an amputee’s ability to walk or grip objects, but can in no way restore a sense of touch. Such a sense is critical to the human experience, said coauthor Benjamin Tee, an electrical and biomedical engineer at the Agency for Science Technology and Research in Singapore. Restoring feeling in amputees and people with paralysis could allow them to carry out several activities that were previously hindered, such as cooking, contact sports etc.

A break down of the components in the Artificial Skin discussed here, and how the optical-neural interface was constructed. Credit: Science. Source:

Well, these researchers at Stanford University have taken us one step closer to this goal by creating an electronic skin that can detect and respond to changes in pressure. The team named this product the “Digital Tactile System”, or DiTact for short, and it consists of two main components shown in the image to the right. The upper layer consists of microscale resistive pressure sensors shaped like tiny upside-down pyramids. These structures are made from a carbon nanotube-elastomer composite capable of generating a direct current that changes amplitude based on the applied pressure. This is because the nanotube structures are capable of conducting electricity. When these structures are moved closer together, electrictity can flow through the sensor. The distance between them will vary with the applied pressure, and the greater the pressure the smaller the distance. This decrease in distance will allow for a greater flow of electricty between the structures, causing the amplitude of the current to increase.

But one problem still remains! The human brain cannot interpret this information, as it is usually received in pulsed signals, similar to Morse Code, with greater pressure increasing the frequency of these pulses. The signal therefore had to be converted into something the brain could actually recognise, which is where the second layer of the artificial skin comes into play. This layer consists of a flexible organic ring-oscillator circuit – a circuit that generates voltage spikes. The greater the amplitude of the current flowing through this circuit, the more frequent the voltage spikes. And viola! We now have a pulsed signal. But the team had to show that this could be recognised by a biological neuron, otherwise the signal would stop once it reached such a cell. To do this, they bioengineered some mouse neuron cells to be sensitive to specific frequencies of light, and translated the pressure signals from the artificial skin into light pulses. These pulses were then sent through an optical fiber to the sample of neurons, which were triggered on and off in response. This combination of optics and genetics is a field known, oddly enough, as “Optogenetics”, and it successfully proved that the artificial skin could generate a sensory output compatible with nerve cells. However, it is worth noting that this method was only used as an experimental proof of concept, and other methods of stimulating nerve cells are likely to be used in real prosthetic devices.

This work is “…just the beginning…” according to Zhenan Bao, the leader of the team, adding that they also hope to mimic other sensing functions of human skin, such as the ability to feel heat, or distinguish between rough and smooth surfaces, and integrate them into the platform, but this will take time. There are a total of six types of biological sensing mechanisms in the hand, and this experiment reports success in just one of them. Nevertheless, the work represents “an important advance in the development of skin-like materials that mimic the functionality of human skin on an unprecedented level” according to Ali Javey, who is also working on developing electronic skin at the University of California, Berkley. Adding that “It could have important implications in the development of smarter prosthetics”.

With thought-controlled robotic limbs already being very real, this research represents the next key step in producing completely functioning prosthetic limbs, that could one day be almost indistinguishable from the real thing! Imagine being able to regain all forms of sense and movement in a limb you had once thought lost forever. That would be HUGE, and could drastically improve the quality of life of many amputees. Unfortunately, such a mechanical marvel is still very much in the future, but this research is an important stepping stone, and I wouldn’t be surprised if we start hearing more about this technology in the years to come. My next question would be, once we have achieved all of this, will we start working on a prosthetic arm able to mimic the effects of using The Force? I think we should make and arm that can use The Force! I think we all (secretly) want that.

Sources not mentioned in text:


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