In the original Terminator movie, the killer robot played by Arnold Schwarzenegger was covered in living human skin. In the context of the storyline, this was necessary for the terminator to travel back in time because only living matter could go through, no technology. As a movie device this accomplished two things. It allowed the robot to be portrayed throughout most of the film with a living actor (rather than expensive CG robot). Also, as a plot device it explained why the terminator and the protector sent to fight it did not bring back advanced weaponry with them.

Regardless of the purpose, the device introduced an intriguing concept for robotics – covering a humanoid robot in living skin to make it appear indistinguishable from a human. Living skin is a highly adapted external covering. It is an effective barrier to keep out unwanted invaders. It retains moisture very well, while being able to keep itself moist and pliable through sweat glands. It is also self-repairing to some extent. Skin also contains an array of sensors to detect temperature, vibration, touch, and pressure.

Not all robots need this kind of covering. Many do well with a hard covering, and rubber gripping surfaces serve fine. But there is an emerging field of soft robotics, which are better suited to interacting with humans and operating in spaces designed for humans. A soft covering for such robots is a critical component, with silicon being the current state-of-the-art for passive skin. However, development is under way for active, stretchable, smart artificial skin for soft robots. Fully soft robots also need soft electronics and actuators, and those are also being developed.

The idea of covering a robotic frame with living skin is a parallel research program, with its own advantages and disadvantages. Researchers from Japan have recently been able to cover a robotic finger with living skin, and present their results. They demonstrate the potential advantages, but also challenges, of this approach. The process involved covering the robotic finger with a dermis equivalent using – “collagen solution containing normal human dermal fibroblasts”. This dermal layer was held in place as it formed so that it essentially shrank around the finger, conforming to its shape. This is a huge advantage over having to custom shape a sheet of skin to the robot. Next an epidermal layer made of karatinocytes was added on top.

The results were encouraging, but a long way from functional robotic living skin. The process basically worked, creating dermal and epidermal layers that were able to remain in place and stretch appropriately when the robotic finger was flexed and straightened. It formed an effective moisture barrier. It also demonstrated good electrical impedance, which the authors used to estimate how good a barrier it would be to unwanted external material. The skin could also repair itself when a collagen patch was provided over a wound.

However, the skin was also very fragile compared to normal human skin, due to a low collagen content in the dermis. The authors speculate that they will have to use a collagen medium with far higher concentration. Also it’s possible that the collagen did not form itself into a strong network the way natural skin would. Therefore the developmental process may need to be tweaked. So while the skin basically worked, it is not ready for deployment. The study is more of a proof of concept.

The “skin equivalent”, as they call it, also needed to be maintained in a careful environment, kept moist and provided nutrients and waste removal. This is perhaps the key challenge to fitting living skin over a robotic frame – there is no living biological system to support the skin. The researchers plan on adding sweat glands to allow the skin to maintain its own moisture. But they will also need to design a self-sustaining system to support the living cells, providing nutrients, oxygen, and waste removal. Since skin is a relatively thin sheet, this can probably be provided through diffusion from a nutrient liquid. Without a functional life-sustaining system for the cells, the result will be like the terminator. When his skin was damaged, it just started to rot away and attract flies. Ideally skin equivalent for robots would last years, and not just fluff off and die after mere days.

Another challenge is integrating the moist living skin with robotic electronics. In this study the researchers operated the finger will pullies, without any electronics in the finger itself. This obviously won’t work for an entire robot. Moisture-tolerant components also need to be developed to full realize this approach.

It’s too early to tell if this living skin approach will work, and if so, how long it will take to become reasonably functional. It’s still early days. We also need to consider how a mature version of skin equivalent will compare to synthetic robotic skin. The question is – how human-like can synthetic skin become, and how durable can living skin equivalent become? Perhaps both approaches will work best for different applications. At this point I predict you are all thinking about sexbots, which is clearly going to be a popular application (if science fiction movies like West World and Cherry 2000 are any indication).

As an aside, I would like to vent one of my sci-fi movie pet peeves. Often humanoid robots that fully mimic humans are portrayed as having human-like (but usually synthetic) skin, but when it is pulled back it reveals underneath hard electronic components. This is to make it obvious that the creature is a robot, and to emphasize the contrast between the human-like outer appearance and the robotic innards. But this just does not make sense. They would not look and feel human unless underneath that skin there were soft human-like structures. The newer version of West World gets it right, with synthetic muscles under that synthetic skin. This is why endeavors to develop soft robots that can truly mimic humans are working on all soft components, not just the skin. This is a movie trope that annoys me every time, especially in more recent films that should know better and have the CG technology to portray soft robots more realistically.

When functioning soft robotic technology exists, this movie trope will additionally become dated and look all the more silly.