Put very simply, the skin is comprised of two layers of rubbery plastic skin with a flexible circuit printed on, courtesy of the folks at Xerox Parc. Sandwiched between the two is a run of carbon nanotubes, which conduct electricity when they're pushed closer together. The harder the compression, the more current passes between them, which is how the skin can understand differences in pressure. 
 
 That, however, isn't enough, since that data would still have to be transmitted somehow into the user's brain. In the end, the team opted to harness a field of science called optogenetics, which involves genetically-engineering cells so that they react to specific frequencies of light. By creating optogenetic neurons that are capable of sensing light patterns, the team proved that it's possible to make this technology work in a person. 
 
 Admittedly, we're still years away from an initial human test, and project leader Professor Zhenan Bao doesn't believe that optogenetics is a suitable solution. There are also more things that just pressure that human skin can sense, like temperature and friction, all of which would still need to be replicated. Even so, this breakthrough could prove to be enormous in the hopefully not-too-distant future. 
 
 [Image Credit: Stanford University Bao Lab] 

 Sensing the force digitally 
 
 Our skin provides us with a flexible waterproof barrier, but it also contains a sensor array that feels the world around us. This array provides feedback and helps us to avoid a hot object or increase the strength of our grip on an object that may be slipping away. Tee et al. describe an approach to simulate the mechanoreceptors of human skin, using pressure-sensitive foils and printed ring oscillators (see the Perspective by Anikeeva and Koppes). The sensor successfully converted pressure into a digital response in a pressure range comparable to that found in a human grip. 
 
 Science, this issue p. 313; see also p. 274 

 Stanford engineers create artificial skin that can send pressure sensation to brain cell 
 
 Stanford engineers have created a plastic skin-like material that can detect pressure and deliver a Morse code-like signal directly to a living brain cell. The work takes a big step toward adding a sense of touch to prosthetic limbs. 
 
 Bao Lab Stanford chemical engineering Professor Zhenan Bao and her team have created a skin-like material that can tell the difference between a soft touch and a firm handshake. The device on the "golden fingertip" is the skin-like sensor developed by Stanford engineers. 
 
 Stanford engineers have created a plastic "skin" that can detect how hard it is being pressed and generate an electric signal to deliver this sensory input directly to a living brain cell. 
 
 Zhenan Bao, a professor of chemical engineering at Stanford, has spent a decade trying to develop a material that mimics skin's ability to flex and heal, while also serving as the sensor net that sends touch, temperature and pain signals to the brain. Ultimately she wants to create a flexible electronic fabric embedded with sensors that could cover a prosthetic limb and replicate some of skin's sensory functions. 
 
 Bao's work, reported today in Science, takes another step toward her goal by replicating one aspect of touch, the sensory mechanism that enables us to distinguish the pressure difference between a limp handshake and a firm grip. See video here. 
 
 "This is the first time a flexible, skin-like material has been able to detect pressure and also transmit a signal to a component of the nervous system," said Bao, who led the 17-person research team responsible for the achievement. 
 
 Benjamin Tee, a recent doctoral graduate in electrical engineering; Alex Chortos, a doctoral candidate in materials science and engineering; and Andre Berndt, a postdoctoral scholar in bioengineering, were the lead authors on the Science paper. 
 
 Digitizing touch 
 
 
 
 The heart of the technique is a two-ply plastic construct: the top layer creates a sensing mechanism and the bottom layer acts as the circuit to transport electrical signals and translate them into biochemical stimuli compatible with nerve cells. The top layer in the new work featured a sensor that can detect pressure over the same range as human skin, from a light finger tap to a firm handshake. 
 
 Five years ago, Bao's team members first described how to use plastics and rubbers as pressure sensors by measuring the natural springiness of their molecular structures. They then increased this natural pressure sensitivity by indenting a waffle pattern into the thin plastic, which further compresses the plastic's molecular springs. 
 
 To exploit this pressure-sensing capability electronically, the team scattered billions of carbon nanotubes through the waffled plastic. Putting pressure on the plastic squeezes the nanotubes closer together and enables them to conduct electricity. 
 
 This allowed the plastic sensor to mimic human skin, which transmits pressure information to the brain as short pulses of electricity, similar to Morse code. Increasing pressure on the waffled nanotubes squeezes them even closer together, allowing more electricity to flow through the sensor, and those varied impulses are sent as short pulses to the sensing mechanism. Remove pressure, and the flow of pulses relaxes, indicating light touch. Remove all pressure and the pulses cease entirely. 
 
 The team then hooked this pressure-sensing mechanism to the second ply of their artificial skin, a flexible electronic circuit that could carry pulses of electricity to nerve cells. 
 
 
 
 Importing the signal 
 
 
 
 Bao's team has been developing flexible electronics that can bend without breaking. For this project, team members worked with researchers from PARC, a Xerox company, which has a technology that uses an inkjet printer to deposit flexible circuits onto plastic. Covering a large surface is important to making artificial skin practical, and the PARC collaboration offered that prospect. 
 
 Finally the team had to prove that the electronic signal could be recognized by a biological neuron. It did this by adapting a technique developed by Karl Deisseroth, a fellow professor of bioengineering at Stanford who pioneered a field that combines genetics and optics, called optogenetics. Researchers bioengineer cells to make them sensitive to specific frequencies of light, then use light pulses to switch cells, or the processes being carried on inside them, on and off. 
 
 For this experiment the team members engineered a line of neurons to simulate a portion of the human nervous system. They translated the electronic pressure signals from the artificial skin into light pulses, which activated the neurons, proving that the artificial skin could generate a sensory output compatible with nerve cells. 
 
 Optogenetics was only used as an experimental proof of concept, Bao said, and other methods of stimulating nerves are likely to be used in real prosthetic devices. Bao's team has already worked with Bianxiao Cui, an associate professor of chemistry at Stanford, to show that direct stimulation of neurons with electrical pulses is possible. 
 
 Bao's team envisions developing different sensors to replicate, for instance, the ability to distinguish corduroy versus silk, or a cold glass of water from a hot cup of coffee. This will take time. There are six types of biological sensing mechanisms in the human hand, and the experiment described in Science reports success in just one of them. 
 
 But the current two-ply approach means the team can add sensations as it develops new mechanisms. And the inkjet printing fabrication process suggests how a network of sensors could be deposited over a flexible layer and folded over a prosthetic hand. 
 
 "We have a lot of work to take this from experimental to practical applications," Bao said. "But after spending many years in this work, I now see a clear path where we can take our artificial skin." 
 
 Media Contact 
 
 Tom Abate, Associate Director of Communications, Stanford Engineering: (650) 736-2245, tabate@stanford.edu 

 Researchers have developed a thin artificial skin that they hope could be used to help amputees feel what they're touching with prosthetic hands or limbs. 
 
 It's an unusual combination of a stretchy, rubber-based plastic and carbon nanotubes, with electrodes applied using an inkjet printer. It translates the pressure of a touch into a digital signal, the researchers report in the journal Science. 
 
 "It's skinlike in its functions," said Zhenan Bao, a chemical engineering professor at Stanford University who led the study team. 
 
 Stretchable skin with flexible artificial mechanoreceptors. Bao Research Group, Stanford University 
 
 Bao and colleagues took the idea a step further and used a new approach called optogenetics to send that signal directly to brain cells — in this case, cells taken from mice. Optogenetics involves using light to control a cell, and the approach has been used to control mouse behavior with the actual flip of a light switch. 
 
 It's far too early to try something like that in people, Bao said. For one thing, it involves genetically engineering the mice. 
 
 But the thin skin could be connected to a more standard electrode, she said. What's new is the delicacy of the sensor. "Some people have made pressure sensors but they have not been able to generate an electrical signal that can communicate with the brain," she said. 
 
 Their approach adds to a growing field of research aimed at making artificial limbs not only work better, but feel more lifelike to their users. 
 
 Model robotic hand with artificial mechanoreceptors. Picasa / Bao Research Group, Stanford University 
 
 Last year, a team at Case Western University reported on a system they developed for clipping a prosthetic hand into surgically implanted electrodes on a patient's forearm. It allowed users to feel what the fingers were doing. 
 
 Other teams have tried weaving the electrodes in among the nerves of the arm to allow the prosthetic to transmit signals that approximate touch. 
 
 The next step would be wireless transmission of signals, to cut down on the trail of wires that patients would need to have implanted. 
 
 The end goal is to allow people to control prosthetic devices with thought alone, and to manipulate objects with better dexterity. And the feedback helps conquer the phantom limb pain that plagues some people after amputations. 
 
 Artificial mechanoreceptors mounted on the fingers of a model robotic hand. Bao Research Group, Stanford University 
 
 Polina Anikeeva and Ryan Koppes, bioelectronics experts at the Massachusetts Institute of Technology, said the optogenetics approach could aid in getting rid of the wires. 
 
 "By optogenetic decoupling of the sensory and motor pathways, future prostheses can be integrated with minimal invasiveness, avoiding the need for convoluted wiring and establishing direct feedback from an artificial limb to the brain," they wrote in a commentary in Science. 
 
 "They may enable prostheses that deliver a sense of touch."
– Pretty soon, you won't be able to hide your limp handshake from someone with a prosthetic arm. At least that's the hope of the team of researchers at Stanford University who developed artificial skin that can sense touch. Engadget describes the team's invention—published Thursday in Science—as "two layers of rubbery plastic skin with a flexible circuit printed on." Billions of electricity-conducting carbon nanotubes are scattered between the layers. The closer they're squeezed together, the more electricity they conduct. "This is the first time a flexible, skin-like material has been able to detect pressure and also transmit a signal to a component of the nervous system," team leader Zhenan Bao says in a Stanford press release. It's the second part of that process that's proved tricky. NBC News reports Bao and her team used something called optogenetics to show electric signals generated by the skin can get to the brain. Basically, they turned pressure signals into light signals then genetically engineered mouse cells to respond to them. But researchers are still a long way from testing that in a human, and Bao doesn't believe optogenetics is the long-term solution, according to Engadget. The team hopes to one day use their technology to make prosthetics that can feel like human skin, but they still have a ways to go. According to the press release, pressure is only one of six sensing mechanisms—such as friction and temperature—that help skin let you know whether you're, say, petting a cat or a cactus. (A young boy got a new bionic arm from Iron Man himself.)