Biomedical Engineering Reference
In-Depth Information
all geodesic structures (37,39). Tensegrity networks have the property that they
are self-stabilizing in the sense that they yield equilibrium configurations with
all cables in tension only due to the internal interactions between its compo-
nents, i.e., without the need for external forces. Specifically, the tension mem-
bers pull against the resisting compression members and thereby create an
internal tensile stress or "prestress" (isometric tension) that stabilizes the entire
system (the same prestress may be generated by the compression members push-
ing out against a surrounding resistance network). Moreover, both multimodular
and hierarchical tensegrity networks can be created that are governed by the
same rules and that exhibit integrated system-wide behaviors when exposed to
external stress (39).
In the cellular tensegrity model, the whole cell is a prestressed tensegrity
structure; however, geodesic structures are also found in the cell at smaller size
scales (37,39). In the model, tensional forces are borne by cytoskeletal micro-
filaments and intermediate filaments, and these forces are balanced by intercon-
nected structural elements that resist compression. These latter elements include
microtubule struts within the cytoskeleton and cell surface adhesions to the sur-
rounding extracellular matrix. However, biological systems are dynamic and
highly complex in that individual filaments can have dual functions and hence
bear either tension or compression in different structural contexts or at different
size scales. The tensional prestress that stabilizes the whole cell is generated
actively by the actomyosin apparatus within contractile microfilaments. Addi-
tional passive contributions to this prestress come from cell distension through
adhesions to the ECM and other cells, osmotic forces acting on the cell mem-
brane, and forces exerted by filament polymerization. Intermediate filaments
that interconnect at many points along microtubules, microfilaments, and the
nuclear surface provide mechanical stiffness to the cell based on their material
properties and on their ability to act as suspensory cables that interconnect and
tensionally stiffen the entire cytoskeleton and nuclear lattice. In addition, the
internal cytoskeleton interconnects at the cell periphery with a highly elastic,
cortical cytoskeletal network directly beneath the plasma membrane. The entire
integrated cytoskeleton is then permeated by a viscous cytosol and enclosed by a
differentially permeable surface membrane.
Unlike the isotropic viscous cytoplasm that dominated past models of cell
mechanics, the tensegrity-stabilized cytoskeletal network optimizes structural
efficiency (strength/mass ratio) by relying on internal tension, rather than on
continuous compression when exposed to an external force. Tensegrity systems
also can easily change shape with minimal energy consumption, for example, as
compared to classical truss structures that require an excessive amount of energy
even for minor shape modification. Most importantly, as with all complex net-
works composed of multiple interacting components, the macroscopic properties
of tensegrity networks (e.g., their mechanical stability, ability to grow and rear-
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