
5
time for the spontaneous bending of tissue rod is con-
trolled by the slower dissipative dynamics and the longer
time scale. Particularly for Hydra tissue rods, we can
estimate the magnitude of the two time scales as fol-
lows. Using ˜
Dr∼Eh3w,ξv∼η,w∼h,Ys∼Ehw,
and ξs=ξh2wwith ξbeing the interlayer friction coeffi-
cient [15], we obtain
τ∼ξv
˜
Drc4
0
∼η
Eh4c4
0
∼0.01 s, τs∼ξh
E∼1 min,(8)
where we have taken typical parameter values measured
in cell experiments [6,15]: h∼10 µm, c0∼1/100 µm−1,
η∼10−3Pa ·s, E∼1 kPa, and ξ∼1010 Pa ·s/m.
Therefore, the spontaneous bending of Hydra rods lies in
the limit of τs/τ 1 and is characterized by τs∼1 min in
consistent with the time duration of initial spontaneous
bending observed in Hydra regeneration experiments (see
Fig. 1(a)) [4].
The proposal here on the spontaneous bending of Hy-
dra tissue fragments driven by contractile supracellu-
lar actomyosin bundles should be generically present in
a broad range of cell aggregates and tissue fragments
during tissue regeneration or morphogenesis during em-
bryo development. Our mechanical model of Hydra frag-
ments can be easily extended to study the morphogen-
esis of other tissues and to include the couplings be-
tween the organization of cytoskeleton and the deforma-
tion/curvature of the tissue [25]. The effects of the per-
meation of water or other small molecules on the dynam-
ics of tissue bending can be explored similarly by model-
ing the tissue fragments as active-laminated gels [26,27].
Additional aspects of the cell biology, such as cell divi-
sion/apoptosis and cell morphological changes [18,19], or
active behaviors such as migration and oscillations, could
be incorporated as well [17,28].
The authors thank Samuel Safran, Kinneret Keren,
Zhihao Li, Yonit Maroudas-Sacks, Lital Shani-Zerbib,
and Anton Livshits for fruitful discussions. X.X. is
supported in part by National Natural Science Foun-
dation of China (NSFC, No. 12004082), by Guang-
dong Province Universities and Colleges Pearl River
Scholar Funded Scheme (2019), and by 2020 Li Ka
Shing Foundation Cross-Disciplinary Research Grant
(No. 2020LKSFG08A). J. Su and H. Wang contributed
equally to this work.
∗E-mail address: xu.xinpeng@gtiit.edu.cn
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