What Engineers Can Learn From the Design of the Penis

The mechanics of the erection may have applications for robotics: An Object Lesson.

Mike Brown / Reuters

The erection is a miracle of evolutionary biology. So advantageous is the inflatable penis, in fact, that it’s evolved independently in several corners of the animal kingdom—including mammals, turtles, and archosaurs (a group that includes birds and crocodiles), to name a few examples. Erect, the penis is “a virtually puncture-proof balloon that can be re-inflated at will,” the journalist David M. Friedman wrote in A Mind of Its Own: A Cultural History of the Penis, “no matter how often it has gone flat in the past, and why.”

Diane Kelly, a research assistant professor in comparative biology at University of Massachusetts at Amherst, has been researching the penis for more than 20 years. Over Skype, she said she’s currently researching “the evolution and morphology of the phallus in amniotes,” a group that includes mammals (including humans), turtles, crocodilians, birds, snakes, and lizards.

In a 2012 TED talk, Kelly explained how she got started. “One day I started thinking about the mammalian penis. It’s really an odd sort of structure,” she said. “Before it can be used for internal fertilization, its mechanical function has to change in a really dramatic fashion,” from the flexible structure that allows urine to exit the body to a rigid one that can copulate.

The means by which the penis becomes erect are well known, stretching back to Leonardo da Vinci’s 15th-century anatomical drawings, and to the work of Regnier de Graaf, who “produced the most thorough investigation of the penis to date” in 1668, as Friedman writes. Soon after that, de Graaf’s student Fredrik Ruysch created wax anatomical models based on cadaver dissections. These models “showed the expanding and shrinking organ to be a marvel of hydraulic engineering,” Friedman writes, thereby helping to disprove the then-widespread theory that “air” or “wind” causes erections.

These men helped establish the modern understanding of erection physiology. Here are the basics: Chemicals in the brain signal the body to relax the corpora cavernosa, two chambers inside the penis. Blood flows in through vascular openings and becomes trapped as the body restricts outflowing veins. Meanwhile, a third chamber, the corpus spongiosum, does not inflate, keeping the urethra open so that semen can pass. Eventually, circumstances (orgasm, or maybe a sudden interruption) prompt the body to let blood flow out of the area.

In its transformed state, Kelly recognized that the penis has the hallmarks of a hydrostatic skeleton, a type of structure that maintains stiffness by means of fluid trapped in a chamber. She began to wonder why mammalian penises did not bend, while creatures made of hydrostatic skeletons, like earthworms, could coil and uncoil at will. She took on the question for her dissertation, hypothesizing that the answer lay in a 90-degree arrangement of fibers in the collagenous tissue under the skin.

Collagen, a protein abundant in the human body, is present in a layer of penile tissue called the tunica albuginea. The collagen fibers lay crimped in the flaccid penis; erection unfolds them to their full length. In her research, Kelly discovered that collagen’s molecular structure is crucial to erectile stiffness. Viewing tissue samples under a microscope, she found that, as she had suspected, they had fibers arranged at roughly 90 degrees: some running the length of the penis, and others the width. This was different from more bendable hydrostatic skeletons, whose fibers are angled like a helix.

“If the wall around the erectile tissue wasn’t there, if it wasn’t reinforced this way, the shape would change, but the inflated penis would not resist bending, and erection simply wouldn’t work,” she explained in her TED talk. (In a 2007 study in the Annals of the New York Academy of Sciences, she also wrote that “the tensile stresses … are twice as great around its circumference as the stresses along its length.” This means the fibers running width-wise expand much less than the ones running the long way, to prevent the erection from “bulg[ing] out in an aneurysm.”)

“It’s an observation with obvious medical applications in humans,” she said in her TED talk, “[and] also relevant in a broad sense, I think, to the design of prosthetics, soft robots, basically anything where change in shape and stiffness are important.”

But the insight is not Kelly’s alone. The concept of using the design of the penis for other purposes is part of an established field called biomimicry, the science of applying nature’s design lessons to human problems. Velcro, for example, was inspired by the hook design of burdock burrs, and engineers have borrowed the design of spider webs to create a glass coating that makes windows more visible to birds. The term “biomimicry” was coined by Janine M. Benyus, the author of 1997 book Biomimicry: Innovation Inspired by Nature Benyus is also a founder of Biomimicry 3.8 and the Biomimicry Institute, two closely related organizations that aim to promote the biomimetic philosophy. (The “3.8” stands for the 3.8 billion-year head start evolution has had over human innovation.)

Ask Nature, a repository of biomimetic ideas established by the Biomimicry Institute, includes a listing for the human penis as an example of how “hydraulic action creates structural rigidity.” But Kelly believes the penises of other species may also offer insights for engineers. “There are significant differences” between humans and other animals, she said. She offers one an example: “Crocodilians have this bizarre, always erect, very densely collagen-packed phallus [that] sits inside their cloaca”—a cavity that is present in amphibians, reptiles, birds, and some fish—“and they extrude it before copulation.” Most mammals also have a baculum, or penis bone.

Curious about biomimetic applications for her research, Kelly reached out to a Bay Area company called Otherlab. They’ve just launched Pneubotics, a unit that bills itself as specializing in “all-fluidic, membrane based robotics” with high safety, low weight, and low cost. “I emailed them wanting to talk about it,” Kelly said, “and they were saying that, yes, fiber orientation is very important to them, for controlling the final shape of their robots after inflation.”

Saul Griffith, a principal at Otherlab and an MIT-trained physicist who’s familiar with Kelly’s work, said that his team considers something called the braid lock angle, which “dictates the optimal angle between fibers in a pressurized vessel.” He didn’t call his work biomimicry—“It’s a very vague term very loosely applied by people who have traditionally been pseudo-scientists,” he told me, and “we try to distance ourselves from the term.” But “you do care a lot about that [angle],” Griffith said, “and it is what gives the penis its anti-buckling characteristics, and it is what makes some of our [robotic] things very strong per their weight.”

Kelly’s general insight is correct, in other words. In their fine details, male sex organs have physical similarities to non-sexual objects both living and inanimate—and might have applications beyond the reproductive and sexual functions for which we tend to value penises most.

This article appears courtesy of Object Lessons.