Dr. Stephen Levin originated the concept of Biotensegrity more than 30 years ago. He originally trained as an Orthopedic and Spine Surgeon and was formerly Clinical Associate Professor at Michigan State University and Howard University. He studied General Systems Theory with noted biologist, Timothy Allen, and now, retired from clinical practice considers himself a ‘Systems Biologist’. He has been closely allied with others working in the field of Design Science, emphasizing the work of Buckminster Fuller and its applications. He has written numerous papers that contribute to the understanding of how biological structures function like tensegrity structures.
In our conversation today we get into, well, first and foremost what biotensegrity is, the many ways that we are not like skyscrapers, how the difference between the bicycle wheel and the wagon wheel can illustrate the concept of how we are “pre-stressed”, what viscoelasticity is, the scapula as a sesamoid bone (every bone as a sesamoid bone really...), some examples of the many tensegrity structures we can find in nature if we know what we’re looking for, what the ichosahedron has going for it and why we should care, and more!
Brooke: For those unacquainted can you give us a simple, kind of nutshell definition of what biotensegrity is?
Stephen: Tensegrity is a word derived from tension and integrity which is a Buckminster Fuller term to indicate a continuous tension network. It's actually more than that. It's the compression elements of the structure are meshed within the tension elements so that the compression elements, the rods, the skeleton, do not press on one another.
It was derived from Kenneth Snelson's sculpture actually [Needle Tower, in resources]. Snelson was student of Fuller but it was Snelson who really made the first structure. He describes it as a closed structural system composed of three or more compression struts within a network of tension tendons. He says the compression rods float within this structure and they press outwardly against the tension member so it's a self-contained unit, and it's pre-stressed tension and compression unit. Tensegrity as a word sort of had lost its meaning so that we put bio in front of it, which is a biology adding to it, and it's really more narrowly defined and more specific than using tensegrity, which everybody uses for everything else. You can get away with things in tensegrity you just can't get away with in biotensegrity because lifeforms have their laws that they have to stick to.
Brooke: The difference between the mechanics of a bicycle wheel as opposed to a wagon wheel- that's a nice illustration of how we are pre-stressed. Can you talk about that a little bit?
Stephen: Aside from the Snelson sculptures, the closest you get to everyday life as a tensegrity is a bicycle wheel where the hub is suspended in the middle of a tension network of spokes. All your spokes are always under tension. In a wagon wheel, each spoke bears the full weight of the wagon, of course divided by the number of wheels you got on the ground. Each spoke bears the full weight and you actually are vaulting from one compression pillar of the spoke to the other compression pillar of the spoke.
The bicycle wheel, it works the opposite. All the spokes are working all the time. When you set a bicycle wheel, you tighten all the spokes, you pre-tension them, and then it stays that way even when you ride on the bicycle. Your load is distributed through the tension elements of the spokes. All the spokes are pulling on the hub all the time. It's by the opposite pulling of the spokes that the hub stays in place. It's like if you were doing tug of war and you had an equal side and the rope wasn't going anyplace. They're not staying right in the middle because it's equally pulled on both sides. Just as the spokes in that bike wheel are pre-stressed, all the tissues in the body are pre-stressed. They are always under tension. So muscles are never programmed lax. There's always muscle tone present. All the fascia and connective tissue, in fact all collagen have intrinsic tension within them. Even under deepest anesthesia, when you cut muscle it retracts and pulls apart. There's always tone to muscle, and you can never say your muscle is completely off.
Brooke: I think that's a beautiful illustration. Speaking of visuals, you say that tensegrity structures are ubiquitous in nature if you know what to look for. Can you give some examples of what we might be able to notice if we do know what we're looking for?
Stephen: There are no true man made tensegrities because even the man-made tensegrity structure itself uses the linear materials, the regular materials people build with. Tensegrities in themselves are non-linear, and we'll probably talk about that later. They're ubiquitous in nature; it's just recognizing them. Most of the obvious ones look like the Buckminster Fuller geodesic domes, like the Disney Epcot Center. Those can all be built as tensegrities. My favorite one is the dandelion puffball because that was of course a large structure that I recognized as being consistent with a tensegrity. The concept has been around for a long time. Icosahedrons were described in the mid-1960s I think it was in the lymphocytes and red blood cells, pollen grains, when you get down to the little things, but if you start looking at bigger things, things like raspberries and similar fruits and berries, puffer fish. In fact, most round spiky thinks are pretty obvious tensegrities when you look at them. Tensegrities actually can be recognized more from the mechanics of the structure than its outward appearance.
Brooke: That makes sense. On the opposite side of the spectrum, bioengineers oftentimes will describe human bodies or say that they are like skyscrapers. What are some of the many ways that we are not like skyscrapers?
Stephen: Tensegrities are built up from smaller units. In biology, the subunit, the cell of the tensegrity structure is the icosahedron, which is polyhedron with 20 triangular faces, and triangles are the only structures that are inherently stable with flexible hinges. These structures can have any outward appearance, from spheres to towers with limbs sticking out. It doesn't make any difference. They're all self-contained entities. They don't require gravity to hold them together.
Skyscrapers and towers need gravity to hold them together. The bottom bricks are held in place by the bricks above them, one on top of the other. When you build a skyscraper, the base has to be bigger and stronger and stiffer than the top, and if you tilt the tower over it not only will fall over, but it will pull itself apart because of the intrinsic sheer forces that develop. If you build biotensegrities, they really join together like bubbles in a foam and they sort of share walls and structural continuity.
If you go back to towers, I lived in Washington, D.C. area, and the classic model might be the Washington Monument, which is 550 foot tall, it's 30 foot thick at the base, it's five foot thick at the summit, and it's built stone on stone on stone held together with rigid mortar. The Washington Monument was almost toppled in an earthquake a couple of years ago. It got shaken up and got cracked because it has no flexibility in it. Trees on the other hand are broader at the top. They have much more weight at the top than the bottom. They withstand big winds, and they're sort of built upside down from a Newtonian concept.
You stop and think about animals including ourselves- we have our small and light bones in our feet. We actually stand on two little sesamoid bones under the first metatarsal, a little thing at the fifth metatarsal, and the heel bone, the calcaneus, which is as soft as eggshells. So we stand on our calcanei and of course we often have dense heads that put a lot of load on these structures. We're built upside down. We don't make sense in a Newtonian concept. All biological structures also have flexible joints, and we are omnidirectional. We don't break apart when we're turned upside down and shake it.
Brooke: I like how you mention that we don't need gravity to hold us together. When we have people who go into space or even just diving in different pressures under water and things like that, that we don't come apart.
Stephen: It's one of the characteristics of the tensegrity structure that is independent of these outside forces. It holds itself by internal forces.
Brooke: It's interesting, and the foam, the soap bubbles is a really nice one too because that's something we can interact with pretty regularly.
Stephen: We essentially are foams.
Brooke: I love that image. We are foams. You had mentioned icosahedron, which is pretty important I think to the concept of tensegrity. Can we break that down a little bit more and why we should care about this structure?
Stephen: When I started doing this, I tried to find some structure that looked like a cell and that would build from a cell. The icosahedron is one of the platonic solids going way back. It's a fully triangulated structure. Again, only triangles are inherently stable, so if you're going to have flexible hinges, you have to be triangulated. It's omnidirectional so that you can turn in any direction. It has the largest volume for surface area, so it's energetically in the sense of using materials that are most economical. It can be close-packed to fill space or would fill spaces like cellular space filling. It joins together. When it does join together, it'll share structures. It's like sharing the faces in the bubble, as we pointed out. The individual icosahedrons can actually then function as a one unit structurally, but it also has the ability to function as the individual unit. They become independent and interdependent at the same time. It can have an external or internal skeleton. You can internalize the compression elements instead of keeping it in the outside shell, and that internal creation is a self-emerging property that comes from the structure itself. It also has mechanical properties that are non-linear, viscoelastic, which is the same as biologic materials, so why wouldn't you use it?
Brooke: Can you describe viscoelasticity a little bit more, since you just dipped into that a bit?
Stephen: Viscoelastic suggests the material property has some qualities of a liquid, the viscus part, which is liquid, and some qualities of so-called the solid part, the elastic part. Elastic just means that the form may never return to its previous shape. The rubber band elastic is not a really good elastic. When you apply the term viscoelastic to biologic material, it's really a misnomer. Biologic materials are not hard matter; they're called condensed soft matter, or just simply soft matter. As a class of behavior, the hard materials side has been described as viscoelastic, but it's really a breed of its own.
The best example I can give you of this is silly putty and the green slime that the kids play with. If you recognize silly putty, it's a polymer, it's a mixture of things. Sometimes it's rigid, sometimes it's soft. You can bounce it. You can do all sorts of different things with it, and it behaves differently the different ways you load it. It depends on the rate of loading, the surface area, the temperature. By temperature, I mean only a few degrees. We operate in a very few degree level. Steel, you need to really get it hot. Biologic things have very slight temperature changes can do different things. The silly putty and the biologic density can be malleable, brittle, elastic, all these things at the same time.
Soft matter physics is the science of gels, foams, emotions and some composite mixture like cornstarch in water will show these kind of behavior. The Oxford University Lab actually has on their website, "Biology is soft matter come alive," and that's a quote right off their website. If you think as foam as behaving as silly putty, as it probably does, we have all these properties built into the viscoelasticity and the characters of biologic tissues. All the structures in biology behave the same as icosahedrons do, which have this viscoelastic property, which really isn't that, it's soft tissue mechanics.
Brooke: You mentioned the cornstarch and water thing. We used to use that at the Rolf Institute just to get a tangible sense for viscoelasticity, and it's really fun if you mix up a tray of it because if you touch it hard and fast, like with really pointed fingers, it just firms up like a wall and kind of pushes you away. If you touch it slow and broad, your fingers will sink in and then getting your fingers out, if you pull real quick, it'll keep you in there. It's really cool to play with that quality with touch and to get a sense for how responsive it is.
Stephen: I frequently use the example when you pull on your lip or your earlobe, and you'll get that kind of sensation where you can pull, it'll first give easily and then it stiffens up as you pull on it. It's not a linear stiffening up where you get an incremental point by point. It actually gets structurally stiffer in many ways. The harder you pull, the stiffer it gets. The silly putty gets stiffer the harder you pull on it or the faster you pull on it.
If you look at things like fascia for instance, then you're right, just as you play with it that the fascia does that. If you attack a body and push on it hard, it's going to resist you. You'll never get into its deeper layers. If you just sort of lean on it and move into it slowly, you'll get into the deeper tissues of it, and that's very typical of these kind of things. If you think silly putty or these slimy gel things, you'll see that's exactly the way it works. You look at something like the heel pad for instance. You run on your heel pad, and every time you hit it hard it protects you. When you walk on it, it gives you a soft, gentler kind of response and a more of a shock absorber, and if you touch it you can just sort of sink your finger into it. It's the same material. It depends on the rate of loading it that makes the difference.
Brooke: You mentioned how delicate a structure our calcaneus bone is, that heel bone, and that this model of us like skyscrapers just doesn't hold up. If we were loading it that way with all the force of everything else piling down on top of it, they wouldn't last us very long.
Stephen: The calcaneus is really very soft bone. If you get it in surgery, you can actually just poke at it and it'll break just by poking at it. It's really quite soft. One of our problems is that when you take these things out of the body and let them dry out, they become stiff and hard, and you're not dealing with them under normal test conditions that the body uses. If you're testing things at room temperature, well the body doesn't operate at room temperature. It operates at body temperature. If you take a thing like petroleum jelly, you keep it at room temperature and it's a thick jelly and you put it on your skin, it just slides all over the place. You have to now think of how are these people testing these materials outside the body? If you're not testing it at room temperature and using body conditions, then you got completely different results.
Brooke: That makes perfect sense. You point out that in the traditional paradigms of Newtonian biomechanics, these are some examples you gave: “The forces needed for a grandfather to lift his three year old grandchild would crush his spine, or touching a fish at the end of a fly rod would tear the angler from limb to limb.” I think these are some good examples about how this bioengineering Newtonian model doesn't really hold up to how we actually use our bodies.
Stephen: Rather than go through the math with you- it's difficult without a blackboard- let me point out at the Newtonian biomechanics calculation of spinal loading and joint muscle loading are based on a 350 year old model that assumes biologic material is hard matter, and we just discussed that it is not. We now know it's more like silly putty. That muscles act as binaries because they're on and off. We've already talked that muscles are always on. There's always tone in muscles. The assumption is that the muscles act as agonists and antagonists, when they mostly act synergistically. That the muscular system is an open kinematic chain system, one we know that much of if not all of muscular skeletal mechanics is really closed kinematic chains. We move one thing and something moves at the other end of this chain. You also understand that muscles are internal forces and can't resist external forces without external help. They're using the calculation as if the internal muscles can resist gravitational forces, and they cannot. You can go through all this and I don't have to use calculations or any math. I can just talking about this and say, "Hey, they're using the wrong model. You got to start over again."
Brooke: Perfectly said. We dipped into this a teensy bit, but going back to it, biologic systems are pretty invested in using the least energy expenditure necessary. What are some of the ways we've developed to use the least energy needed?
Stephen: I'm old enough to been in the Army, and the Army maxim was, "Never stand when you can sit. Never sit when you can lie down. Never stay awake when you can be asleep," and that applies to biologic systems. A non-linear stress strain curve is initially flat. The disc is flat. It just sort of flows along and then starts getting stiffer and stronger. The biologic system always wants to operate at the least energy point at the low part of the curve because as you increase that strain and it gets stiffer, you need much more energy. It's interesting that in the laboratory, most of the testing is done on the steep part of the curve and they sort of ignore the bottom part because the math there is very difficult.
Brooke: That's interesting to know. Diving into one structure in particular here, the shoulder is one of the least successfully modeled joint complexes using Newtonian mechanics, and you have a great paper on this, which I'll put in the show notes. It's titled The Scapula is a Sesamoid Bone. I love that image. For those listening, can you briefly touch on what a sesamoid bone is and how the scapula functions as one?
Stephen: The sesamoid bones, are those bones considered outside the axial skeleton that don't contribute to direct support of the body. Of course they're thinking of the body that has a column of bones, and the most common one that everybody knows about is the patella, the kneecap that sort of floats outside the knee and is buried within the tendon of the quadriceps muscle. There are others. One you don't think about very often is the hyoid bones in your neck where your voice box is. There are bones there, and that's sort of sitting in space not supporting anything. Of course while there are supporting ones, but not thought of those, are the two little sesamoid bones underneath the first metatarsal. These things are about the size of small peas, and they crush easily. They are like peanut shells. They'll just crush very easily and they're sitting there, and you'd expect it to get all this force. They're sitting in tendons, and those tendons act like leaf springs on a car and actually keep you from striking the bottom there. If those little sesamoids got hit by the first metatarsal, they'd be crushed like hammering against enamel.
I looked at the scapula, and the scapula sits there and it floats inside the chest wall. There's no direct loading between them. It's buried in muscles. It fits the definition of a sesamoid. Of course, I take it a bit further and I say in the biotensegrity model all bones float because that's a definition of tensegrity, and therefore all bones are sesamoid bones.
Brooke: Right, yeah, because the scapula, we'll think about that continuity with the clavicle bone, for those of us who are anatomy folks. That doesn't mean that it's not floating just because it has that bone nearby.
Stephen: If you look at the clavicle, it's also floating up there. It's only hinged to the sternum by a little ligament there at its joint, and of course the chest wall is moving up and down 16 times a minute through these flexible ribs, so there's no way you can pass a load through the chest to the axial spine.
Brooke: Thank you for that description of the float of the scapula and the float of everything. You've been working in the biosentegrity field for a long time and you've contributed a lot of wisdom in this field. Is there anything that you're currently really fascinated by in your own work?
Stephen: Actually this year is my 40th year working in biotensegrity. My original concept was sort of a eureka moment 40 years ago when I was sitting outside the Natural History Museum in Washington, D.C. contemplating the skeletons of dinosaurs when I recognized Snelson's sculpture across the lawn. The museum there where his sculpture is is directly across from the Natural History Museum, and I put the two together and then started building from there. Since then, I have been working on this concept and trying to figure out how these two mesh together.
The most recent things I've been working on have been the soft matter physics and the closed kinematic chain mechanisms, which are how these structures move and how they behave under different forces, which completely gets you away from the hard matter physics that is the staple of the present day biomechanics.
Brooke: Wonderful. That was a really fortuitous day at the Natural History Museum to see those two things together.
Stephen: The Hershell Museum opened a year before and I had gone down to the museum and walked around and couldn't figure out how this structure stood there. I just left it at that. I was a year later, sitting across the mall and I said, "Oh my god, the two of them match." I went across and then figured out from there, and then it took me a long time to figure out how to build the tensegrity. I called Snelson and got hold of Buckminster Fuller people and I did all sorts of things to finally figure it all out and get down to the icosahedron and then work back up from there.
Brooke: That's wonderful. Those giant dinosaurs couldn't have really existed with this structural skyscraper model.
Stephen: Absolutely not. There's no way that they could have functioned in any way but a tensegrity concept. Even more than anything else, the Diplodocus had a tail that was over 100 bones long and was held up in the air. It didn't drag on the ground. It used to whip around. There's no way that that that could have functioned unless it was in a tensegrity structure. The muscles are adjacent to the bone. There's no lever that you can possibly make out of it, so it has to function as a tensegrity.
Brooke: My son went through a very intense dinosaur period for a long time, so I'm pretty well versed- Because the tails were so long on these skeletons they were finding, they just always assumed and created the skeletons with the tail dragging on the ground until they finally realized they'd never found a fossil with a tail drag.
Stephen: Exactly. That was my professor. I was trained by the head of the Paleontology group over there, and he used to say, "In the sands of time there are footprints but no tail tracks."
Brooke: Which means we have to think of these things totally differently than we have. Thank you so much for all of the amazing work that you have been doing and for talking with all of us today.
Stephen: It has been my delight. Thank you very much.
Let's play with viscoelasticity by getting in touch with our preschool selves. Either get yourself some Silly Putty, or make a cornstarch goop. How you ask? The recipe is right here:
- Grab a large-ish tub of some kind. Flat on the bottom is best (because then maybe you can even stand in it- jump on it and it's firm, stand still and you sink.)
- Add cornstarch
- Add water very slowly and stir. You will know when you have added enough water when it's consistency gets goopy but not too liquid.
Then- once you have either Silly Putty or the cornstarch goop- you can play with its viscoelastic qualities:
Pull the Silly Putty apart quickly and it will snap and get more brittle. Pull it apart slowly and it will get soft like warm taffy.
With the cornstarch- touch it firmly or quickly- slap it, poke it, etc and it will firm up like a wall. Touch it broadly and softly and it will soften and become more like a liquid. The same goes for getting your hand out of the goop: pull your hand out quickly and it will firm up and hold on to you, pull your hand out slowly and your hand will be released.
Now you know what working in fascia feels like! And you have a better sense for how our bodies operate in both a viscous and elastic state simultaneously. Have fun!
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