If you shook hands with a robot today, it would feel no different to grabbing a cold hunk of metal.
But one day, it may not be much different from shaking hands with a fellow human.
A team of researchers in Japan say they have taken the first step to making this a reality by grafting "living human skin" onto a robotic finger.
The lab-grown coating consists of real human skin cells and can heal itself when "wounded" with the help of a collagen bandage.
And like the real deal, the coating is water-repellent and wrinkles up when the robot's finger curls.
The study was published today in the journal Matter.
Shoji Takeuchi, an engineer at The University of Tokyo, said the findings were a first step towards building robots that look like us.
"Our goal is to develop robots that are truly human-like," said Professor Takeuchi, who led the study.
Wait … why do we want human-like robots?
The idea of making robots as human as possible may sound like something straight out of a dystopian sci-fi flick, but it's a major goal for roboticists.
Giving robots human qualities isn't just about making them more relatable; it could also help them carry out tasks more safely and efficiently, said Nicole Robinson, who specialises in robot-human interaction at Monash University.
"It's important that robots are able to effectively interact with a human-built world," said Dr Robinson, who was not involved in the study.
For example, a robot with a human-like hand is more likely to do a good job of picking up an object that has been designed for people, such as a coffee cup, Dr Robinson said.
Another goal is to make robots safer for people working alongside them.
But fabricating realistic "skin" for robots has been a massive challenge for roboticists.
While soft materials like silicone are strong and flexible enough to accommodate a robot's movements, they cannot self-repair if they rip.
It's also tricky to get a flat sheet of silicone to fit a robot's jagged body like a glove. Plus, silicone skin isn't all that convincing up close, Professor Takeuchi said.
"You have to have the hands of an artisan who can cut and tailor with skills," he said.
"The silicone rubber covers that are commonly used today may look real from a distance or in photos or videos, but when you actually get up close, you realise that it is artificial."
Creating lab-grown human skin
To grow human-like skin from scratch, Professor Takeuchi and his team relied on two key components of the dermis (the middle layer of our skin): collagen and dermal fibroblasts.
Collagen is a protein that gives skin its structure and stretchiness, while dermal fibroblasts play a role in wound repair.
When the team dipped the robotic finger into a solution containing these two components, the liquid shrank and wrapped around the finger like cling wrap, forming a dermis-like layer.
"We were surprised by how well the skin tissue conformed to the robot's surface," Professor Takeuchi said.
Next, the researchers coated the finger with cells called keratinocytes, which make up about 90 per cent of the epidermis, the outer, water-repellent layer of our skin.
When the team put the whole skin-like coating to the test, they found it was elastic enough to stretch and wrinkle as the robotic finger moved.
Taking things further, the researchers tested how well the coating could repel water.
The researchers placed a tiny polystyrene foam bead in front of the finger to see whether it could flick it away.
Polystyrene beads are typically tricky for robots covered in wet coating to handle, as they tend to get stuck on the robot's surface.
But in this case, the finger successfully flicked the bead away, confirming its outer skin could repel water just like our own.
To compare, the team used another robotic finger that had a dermis-like layer, but no epidermis on top.
While the finger tried its best, the pesky bead ended up getting stuck on the tip after several flicking attempts.
But the ultimate test was figuring out whether the skin-like coating could heal itself.
Using a surgical knife, the team made a small cut over the robot's middle joint and then bandaged it with a collagen dressing, a common approach for treating real wounds.
After spending a week sitting in a culture dish, the collagen bandage had morphed into the skin and held up after several joint movements.
When can I get a skin-covered robot?
Juxi Leitner, a roboticist at Monash University, said the most exciting part of the findings were their potential applications in areas outside of robotics, from designing realistic prosthetics to treating burn victims.
"It's an important step in showing what can be done," said Dr Leitner, who was not involved in the study.
But as the skin-like coating doesn't contain all the components of human skin, such as a hypodermis — the deepest layer of the skin — and blood vessels, Dr Leitner is doubtful it will stand the test of time.
"Right now, it's not going to live very long if it doesn't have something that can provide it with a way of surviving," said Dr Leitner.
Professor Takeuchi and his team have already started figuring out how to build "circulatory systems" into robot skin.
But the ultimate dream is to create robot skin complete with sensory neurons, hair follicles, nails, and sweat glands.
While we probably won't be struggling to tell the difference between a robot and a human at the grocery store anytime soon, Dr Robinson agreed the findings open up possibilities.
"The use of a skin equivalent to cover hardware components is an important step forward to achieve closer human-robot collaboration," Dr Robinson said.
