A tensegrity requires at minimum three conditions to fit either Kenneth Snelson's or Buckminster Fuller's definition:
1. They are discontinuously connected, that is, they do not transfer compressive loads. In these domes it is the tension forces that travel along the outer edges of the struts that are continuous. Similarly, if anatomical structures operate as tensegrities, then in most orientations the bones do not pass a direct load across the joint– rather the tension members; ligaments, tendons, and fascia transfer loads and the bones float in this tension matrix.
2) All tensegrities are prestressed under tension; they are self–supporting and independent of gravity. But the weight of the structure also adds to the prestress. As you increase the weight load the tensegrity tightens and gets smaller. The heavier the structure is, the greater the tension, and the less the range of motion. This presents real design problems when trying to model living systems that have and use joints with multiple degrees of freedom.
My models for example can emulatebiologic movement because I use elastic tension nets that are taut enough to maintain the shape of the model yet have enough residual elasticity to move through a wide range of positions. When the size and weight of a model increases, so does the prestress. It is always surprising to discover how high the tension levels climb when building large tensegrity structures. In some of Snelson’s largest sculptures (50’–100’) the tensile cables carry thousands of pounds of force. To make human scale tensegrity models that articulate and are prestressed is not a trivial challenge.
3) Tensegrities are self–contained non–redundant whole systems. All components are dynamically linked such that forces are translated instantly everywhere; a change in one part is reflected throughout. These features distinguish tensegrities from all other tension structures, e.g. a radio mast or a sailboat’s mast is fixed at the base and needs that fixed point to keep it upright. The boat does not need the mast for it’s integrity but the reverse is not true. Every part in a tensegrity is reliant on the entire structure for its continued existence. In terms of living forms, a discontinuity in a structure marks the boundary or interface between separate tensegrities. Also, molecules within cells within tissues within organs within bodies, and bodies within environments are all synergistically linked tensegrities in a hierarchical cascade from the smallest wholes to the largest.
At the macro scale of human anatomy, I see tensegrity as a diagrammatic way to model in four dimensions forces acting on complex shapes in terms of stress vectors. I suggest that this analytic technique can explain the structure of individual bones as well as aggregates that make up the body. Tensegrity in this sense is an applied principle– a map of the nature of structure. If tensegrity is seen not so much a building system as it is a description of the most efficient way that all form is organized, in terms of most economical use of energy and material, then we would expect parsimonious nature to utilize this principle universally. The task is to determine exactly how this might have been achieved at each scale of the continuum.
In the last 25 years tensegrity has come to be associated with various inquiries into the nature of living structure. Donald E. Ingber MD PhD, Professor of Pathology at the Harvard Medical School, has done cellular research that has been widely reported in major scientific journals as well in Scientific American, The Architecture of Life, January 1998. He has found conclusive evidence that tensegrity provides the best explanation for the cytoskeleton of the cell, its movement and behavior. Stephen Levin M.D., an orthopedic surgeon who coined the term biotensegrity has applied the principle to macro scale anatomy in papers and numerous lectures around the world for over 25 years.
Permission to post by T. Flemons
Images copyright T. Flemons 2006
Ref: Tom Flemons, Intension Designs.
Tom Flemons of Intension Designs created tensegrity models and sculptures for decades. His most recent work focuses on human anatomical structures, and his studio offers these models for sale to clinicians and healthcare professionals.
Additional information, images and video clips of his work can also be seen on: TensegrityWiki.com