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.

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. (Bao Lab)

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.

"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."

ORIGINAL: Stanford

By Tom Abate

One of the Most Important Tools in Science Now Fits Inside Your Phone

A spectrometer that fits in your mobile devices could let you scan yourself for skin cancer.

Illustration by Mary O'Reilly

We use them to spy on exoplanets, diagnose skin-cancer, and ID the makeup of unknown chemicals. They're on NASA spacecraft flying around Saturn's moons right now. Yes, right alongside the microscope, the optical spectrometer—an instrument that breaks down the light that something reflects or emits, telling you what its made of—is one of the most ubiquitous tools in all of science. Today, Jie Bao, a physicist at Tsinghua University in Beijing, China, has just discovered a fascinating way to make them smaller, lighter, and less expensive than we ever thought possible.

By using tiny amounts of strange, light-sensitive inks, Bao and his colleague Moungi Bawendi—a chemist at MIT—have designed a working spectrometer that's small enough to fit on your smartphone. Because of the tool's simple design and its need for only an incredibly small amount of the inks, Bao says, his spectrometer only requires a few dollars worth of materials to make. They report the research today in the journal Nature.

"THAT'S WHAT I'M REALLY HOPING FOR, SEEING THEM IN CELLPHONES IN THE VERY NEAR FUTURE."

"Of course we still have a lot of room for improvement. But performance-wise, even at this preliminary stage, our spectrometer works very close to what's currently being sold in the market," Bao says. "I think that's one of the most attractive results of our research: [This spectrometer] is already so close to a real product."

Printable Detectors

The way spectrometers work goes back to the 17th century, when Issac Newton showed that a prism could break up white light into distinct bars (technically wavelengths) of different colored light. Depending on the source of the light—say, a candle or the sun—that rainbow spectrum would change. Today, we know this happens because the atomic or molecular makeup of everything that either gives off or reflects light leaves an indelible fingerprint. And if you understand which materials leave which fingerprints, you can use light alone to find out what something is made of.

Bao says most modern spectrometers are made in more or less the same way. They diffract incoming light, then push it through a mechanically movable slit to see which exactly which wavelengths of light fit through which slits. This setup, because it involves complex moving pieces, is a total pain to shrink down in size. It's expensive, too, because accurate spectrometers require high-precision components and delicate alignment.

Jie Bao

But Bao's spectrometer works in a much simpler way. As if making micro-sized stained glass windows, Bao prints a tiny grid of 195 different-colored liquid inks directly onto a flat sensor. (That sensor, called a CCD sensor, is what your phone's camera uses to pick up light.) Each of the 195 windows is made of a material called colloidal quantum dots, and each "absorbs certain wavelengths of light, and lets others go," says Bao. When light hits each window and travels through, the underlying sensor records how the light changed. Later, a computer can compare the data from all of the windows and reconstruct what wavelengths made up the original light.

Cellphone Spectrometers

Right now, Bao's spectrometer is about the size of a quarter, and he says the underlying CCD sensors he uses can be bought online for less than a dollar a pop. Because he's using just a tiny drop of each of the colloidal quantum dot inks (which have only recently been developed) the cost all 195 drops is only on the order of a few dollars.

"THE PEOPLE WHO ARE PLANNING SPACE MISSIONS ARE WEIGHING EVERY GRAM."

Because spectrometers are so widely used in science, Bao sees a rainbow of possible uses for his new device. For one, he says, his spectrometers could be easily integrated into commercial smartwatches and phones, allowing everyday people to do things like self-identify skin cancer. "That's what I'm really hoping for, seeing them in cellphones in the very near future," he says.

And because spectrometers are so widely used on exploratory spacecraft, Bao sees an easier and far cheaper way to deck out the next generation of space explorers. "The people who are planning space missions are pretty much weighing every gram, and so this would be a very easy way to lose weight."

ORIGINAL: Popular Mechanics

By William Herkewitz

Jul 1, 2015