The Science of Skin in Fiction

This article on the science of skin is part of the Science in Sci-fi, Fact in Fantasy blog series. Each week, we tackle medical or technical aspects of science fiction or a historical / world-building topic of fantasy with input from an expert. Please join the mailing list to be notified every time new content is posted.

The Expert: Gideon P. Smith

Gideon P. Smith MD PhD MPH is a physician-scientist. He specializes in cutaneous manifestations of autoimmune connective tissue diseases. He has published over 100 first and senior-author papers in scientific journals including in top tier journals such as Science. In his spare time, he writes science fiction and epic fantasy and has also written for the Science Fiction and Fantasy Writers of America. You can find more about him at his webpage/blog: www.gideonpsmith.com or follow him on Twitter: @gideonpsmith.

Skin in Fiction: Misconceptions, Injuries, and Repair

While skin is rarely the star of the show, it is often part of the plot. From minor battle wounds to whole body burns, skin damage and its subsequent repair is seen commonly both in science fiction and fantasy. I will not argue about what magic can do to heal someone, but science and medicine have more rigorous constraints. In this article, we are going to review what skin is, how it, its injuries, and its repair, are often misrepresented in science fiction, and how we might go about getting it right. But first, In order to understand what is wrong in science fiction with the way skin and skin repair are portrayed, we need to understand some facts about skin. And we also need to bust some of the common myths that trip writers up.

Myth 1: Skin is just a covering

Fact 1: If you remove the word “just” this myth would be true. It is a physical barrier. Simplistically, it keeps the good things in (blood, moisture, organs) and the bad things out (bacteria, funguses, insects, many chemicals). The outermost layer is composed of a protein called keratin, which makes skin waterproof. However, this protein also allows skin to remain flexible and tough, resisting minor damage. When the inevitable minor scrapes and cuts do occur it is the deeper skin layers that are capable of self-repair. Skin also has other barrier functions. Pigment secreting cells within the skin, called melanocytes, affect the color of our skin and help protect us from environmental ultraviolet.

However, skin is also so much more than just a barrier. It actually has many regulatory purposes. Different types of cutaneous nerves allow us to detect temperature (hot and cold), pressure, and sensation (itch and pain). Increased density of nerves in areas like fingertips enables us to do very delicate fine detail work and sense minute variations in things we touch. The blood vessels in the skin contract or dilate depending on whether our autonomic nervous system senses we need to conserve or shed heat, and the sweat glands within the skin, and the skin musculature that allows shivering, also help thermoregulate. The skin also has a somewhat unique immune function with its ability to both tolerate the outside world and yet react to it when attacked by insects, allergens, etc. The skin therefore not only is a covering but has many other functions, including communicating and responding to other internal states and organ systems.

Myth 2: Skin is the same thin veneer of tissue in all areas of the body

Fact 2:

Different areas of the body actually have unique and very specific functions and the thickness or structure of the local skin reflects this. Skin on the soles of our feet is much thicker than other areas to allow us to weight bear on it and not tear or shear as we walk or run. The skin of the back is also incredibly thick maximizing its protective capacity given movement here is usually large scale, but that of the eyelids, which has to allow fine movements and the folding away of the layer, is incredibly thin.

Myth 3: Compositionally skin is the same everywhere

Fact 3:

The skin not only varies in thickness of its different layers but it also has many functional biologic units within it, called skin appendages, and the type and density of these vary with the purpose of the skin. Skin on the soles of our feet has many nerve endings to help with proprioception (balance). Axilla, hands, feet, and groin perspire more because they have more eccrine glands, which not only keeps us cool but also secretes pheromones. Hair follicle density is highest on the scalp, but the high density of sebaceous glands on the face allows the production of a better protective oil layer for this highly mobile and exposed area.

Myth 4: Skin Regeneration is from the wound base outwards

Fact 4: This is partially true, in that skin doesn’t heal by floating above the wound, but it misses the point that there are patterns and stages to tissue growth. Early tissue is often just a framework to build more permanent skin over. It is made from the deposition of cells attracted to the wound from surrounding tissue and the blood. This early granulation tissue is often completely replaced as healing progresses. Local biologic chemicals, called cytokines, incite local cells to divide and reproduce, and either stem cells or the full range of cell types are essential if the skin is to reproduce all functions. Repair of nerves takes time and sensation often returns slowly, if at all. The color of the replacement cells will be determined by pigment-producing cells but if these are damaged, often scars will be pink, or white. Even should pigment return often it will recur in a patchy pattern starting around old hair follicles because of the stem cells there. Hair follicles, sweat glands, and sebaceous glands rarely regrow into scars and so scarred skin often lacks many of the functions of naïve skin.

Examples of Problematic Skin Science Tropes in Science Fiction

So how does this relate to the misrepresentation of skin and its repair in science fiction?

Robots with skin

The first trope is robots with fake skin. In the Terminator series, we are told that the T-800 developed its skin by placing the metal skeleton in a tank of blood plasma. The first problem with this is that while skin needs blood to supply oxygen and nutrients to grow, it doesn’t actually grow from blood plasma. Even granulation tissue needs more than this. For complete skin it needs skin cells. Even if we assume there were some unmentioned skin or stem cells, it still leaves us with three quandaries:

1. If the blood-plasma is outside, rather than in the usual cicrculatory system of mammals, the skin will form inside out.
2. Skin is waterproof. If they do somehow get it to form with the keratin layer outside so the terminator looks human, the new skin’s keratin layer will rapidly isolate it from this plasma bath from which it supposedly grows.
3. Once the robot is out of the tank, how does the skin survive? In Terminator 2, we learn their skin has the ability to regrow, but how does it do this without nutrients and oxygen? It still needs an internal circulatory system supplying oxygen and nutrients, or a technical alternate solution, but there is no evidence of this on the terminators.

In Murderbot the SecUnits have organic parts including skin and hair, and in Network Effect, we learn that this skin regenerates, rather than just being externally repaired. We are told SecU have rudimentary digestive systems, so these could take in nutrients and feed the organic parts of its body, but it also states SecU don’t need to eat. If this is the case, they still need to absorb nutrients some other way, or be able to shift their skin into some form of stasis state. It’s still not clear about how the organic skin is oxygenated, but at least nothing is written that rules this out.

Spray on skin:

In V, the original series, the ‘visitors’ spray on human skin over their lizard bodies to disguise themselves as human. This is probably fine, as long as what they are spraying on is a synthetic veneer, just designed to give the appearance of human skin (If it’s organic, that’s not only gross, it shares all the same problems as the robots with skins). But this raises another myth I commonly see – that if you cover skin you somehow die from “skin asphyxiation”. Neither humans nor reptiles breathe through their skin. So you can neither survive by the air absorbed by the skin, which impacts a fraction of a mm of skin, nor can you suffocate by covering the skin. This is a myth, first promoted by James Bond in Goldfinger  but I have seen this myth repeated in science fiction. There’s no such phenomenon.

More realistically, in some science fiction, skin replacement is done by spraying live fresh skin cells onto the wound. Remembering that skin varies in thickness, as well as function it becomes obvious this is a pretty unlikely instant solution. After all, how can one spraycan hold ubiquitous skin, not to mention match in color the recipient’s existing skin? Also, unless these are harvested from the individual, why does their immune system not reject this transplant?

However, in a much more primitive form, this approach does have some scientific merit. We can indeed spray on skin cells to wounds and burns, but these are usually harvested from the patients uninvolved skin areas, and with a lot of time, oxygen, and support this can mature into fresh skin. The more superficial the wound, the better it will work, but it will still look like a scar, and often not be color matching and sometimes not fully functional e.g. loss of sensation. Healing time still remains a concern as even if it regains function this takes slow growth.

Skin Repair Devices

Devices, that stimulate skin growth are another popular trope. Whether the lasers in Logan’s Run, or the dermal regenerators of Star Trek these often seem to work instantly. In our current world we rejuvenate skin with lasers – most commonly these days fraxel lasers. However, the laser does not create new skin. What it does is wound the skin by burning tiny holes in it and this then stimulates the natural (slow) healing to grow in fresher skin.

Dermal regenerators, which presumably work by stimulating existing cell growth, have the limitation that they should only be able to stimulate the cells that are there. Thus, someone severely burned, with skin cells already dying from chemical insult, or with cancerous skin, will not be a candidate for this. Dying skin won’t reproduce and damaged, mutated skin, just makes more mutants and potentially cancers, not normal skin.

Ways to Get Skin Science Right in Fiction

Thinking about what we learned above, how can you get the science right, and what might be some cool things to include in your science fiction?

First, if you are giving your robot or alien an artificial outer skin, be clear: is it something just sprayed on to look like skin, or is it real skin? If it is the latter, remember, skin is living tissue. Living tissue needs nutrients and oxygen brought in, and toxins and waste taken out. Even if you don’t want to explain how that happens know that in order to survive the skin needs it, so don’t write your story in such a way that this very necessity is impossible.

If you are going to heal skin, remember that lasers don’t make skin. They burn it. If skin is going to regenerate rapidly after laser it is likely going to need some topical medicine to stimulate it afterwards. Regenerators only stimulate existing skin so are likely limited to small areas with surrounding healthy skin.  Don’t pop someone with 90% burns over their body in the regenerator and expect them to survive unless you are supplying some non-immunogenic skin cells for growth.

And whatever you are doing normal healing/skin production often takes time and occurs in stages. Even if you repair the physical barrier, it likely won’t function normally in terms of appearance, flexibility, thermoregulation, or sensation for quite some time, as not only do functional structures need to grow, they need to integrate properly for feedback with the internal nervous system.

But these very limitations build in some potentially interesting plot elements. Maybe your hero’s skin was replaced but, like a babies skin, it hasn’t yet developed thermoregulation. This is going to limit your hero. They can’t run or exert themselves without risking overheating. Unless you design some kind of cooling suit for them. Or if their hand is burned, even if their skin is replaced, they may not have the necessary sensory nerve regeneration to crack the safe, read braille, or whatever other task their skin allows them to do. Your main character shouldn’t jump up three seconds later fully recovered. The time to heal can also build in some plot uncertainty. Will they recover their skill, or not? Maybe they get back some limited sensation and we are on the edge of our seats to find out if its enough, or in time. Will the plan go ahead as originally designed or will this new limitation require them to come up with a different solution?

Finally, however, I will concede if you are absolutely set on miracle skin repair in your world, that’s fine. This is science fiction after all. Far be it from me to say it will never be possible. But then you need to think about the social science implications. Stop and think about the other ways such technology will impact your world. We look old because of lines and wrinkles, damaged blood vessels, brown discoloration, scars, and collagen loss in the skin. If you can fix this in a second and take it all away because skin replacement is now easy and instant, how can you tell people’s ages and why does anyone look old in your science fiction?

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