Chapter 6
Active Gels
6.1 Cytoskeleton as a Continuum
What is a rational approach to the mind-boggling complexity we touched upon in
the preceding chapter, even restricting to mechanical aspects and forgetting for a
while the chemistry of proteins and nucleotides, which really run the entire show?
A tentative answer is the active gel theory. The term, first appearing in the paper
by Kruse et al (2004), predates the notion of active matter; earlier usage of this
term was mostly medical. However, mechanical modeling of a viscoelastic cytogel,
though lacking both the appellation “active” and a reference to orientational order,
goes twenty more years back (Oster and Odell, 1984; Dembo and Harlow, 1986).
In a later review, Prost et al (2015) summarized the way to construct an active gel
theory:
One has first to identify slow variables relevant for a macroscopic or mesoscopic description.
[...] Clearly, overall mass, solvent mass, energy and momentum are conserved and, on
timescales short compared to production and degradation rates, we can consider the number
of actin monomers and motors to be conserved. In the following we consider isothermal
systems; thus we can ignore energy conservation.
Clearly, energy is not conserved in living systems, nor in any non-equilibrium system
interacting with environment, and the authors realize it:
[...] these systems lack time reversal symmetry, because energy is constantly transduced.
Thus, we can be satisfied that energy conservation is ignored, although isothermicity
is not a sufficient reason. The authors’ outlook is far more optimistic than the opening
sentence of this chapter:
Fortunately, after a century of skilled experimental work, a certain simplicity has begun to
emerge.
Indeed, the physicist should not care for interconnections of the cytoskeletal maze
and an unceasing dance of actin monomers and molecular motors any more than for
the molecular chaos in common fluids:
113
L. Pismen, Active Matter Within and Around Us, The Frontiers Collection,
https://doi.org/10.1007/978-3-030-68421-1_6
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
Active Gels
6.1 Cytoskeleton as a Continuum
What is a rational approach to the mind-boggling complexity we touched upon in
the preceding chapter, even restricting to mechanical aspects and forgetting for a
while the chemistry of proteins and nucleotides, which really run the entire show?
A tentative answer is the active gel theory. The term, first appearing in the paper
by Kruse et al (2004), predates the notion of active matter; earlier usage of this
term was mostly medical. However, mechanical modeling of a viscoelastic cytogel,
though lacking both the appellation “active” and a reference to orientational order,
goes twenty more years back (Oster and Odell, 1984; Dembo and Harlow, 1986).
In a later review, Prost et al (2015) summarized the way to construct an active gel
theory:
One has first to identify slow variables relevant for a macroscopic or mesoscopic description.
[...] Clearly, overall mass, solvent mass, energy and momentum are conserved and, on
timescales short compared to production and degradation rates, we can consider the number
of actin monomers and motors to be conserved. In the following we consider isothermal
systems; thus we can ignore energy conservation.
Clearly, energy is not conserved in living systems, nor in any non-equilibrium system
interacting with environment, and the authors realize it:
[...] these systems lack time reversal symmetry, because energy is constantly transduced.
Thus, we can be satisfied that energy conservation is ignored, although isothermicity
is not a sufficient reason. The authors’ outlook is far more optimistic than the opening
sentence of this chapter:
Fortunately, after a century of skilled experimental work, a certain simplicity has begun to
emerge.
Indeed, the physicist should not care for interconnections of the cytoskeletal maze
and an unceasing dance of actin monomers and molecular motors any more than for
the molecular chaos in common fluids:
113
L. Pismen, Active Matter Within and Around Us, The Frontiers Collection,
https://doi.org/10.1007/978-3-030-68421-1_6
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021