Supplementary Information
Kinetic behaviour of the cells touching substrate: the
interfacial stiffness guides cell spreading
Jianjun Li, Dong Han*, and Ya-Pu Zhao*
7 Supplementary Figures
1 Supplementary Method
2 Supplementary Movies
Supplementary Figures
Here shows 3 evidences of the existence of surface layer on PDMS after UV
treatment.
1. The non-specific adhesion reduced after the UV treatment, as shown in Fig. S1.
The averaged work of adhesion between micro-sphere and PDMS (60:1) is 81.2±22.5
fJ. When it was exposed to UV, the generated surface layer (perhaps as well as
hydroxyl) caused reduction to 25.4±18.1 fJ.
Figure S1 Non-specific adhesion between micro-sphere and PDMS
2. The PDMS surface became more hydrophilic. The contact angle of water is
reduced from 115.9° to 79.6° on PDMS (60:1), and the surface energy of PDMS
increased based on Young equation. This indicated the existence of silica layer on
PDMS.
Figure S2 Contact angle of water on PDMS a) pre UV b) after UV
3. The indentation curves of PDMS (60:1) show obviously difference between pre
UV and after UV. The PDMS after UV treatment has a bigger slope which means
the surface is much stiffer. In another word, it also indicates the existence of the
oxidizing layer.
Figure S3 The indentation curves of PDMS
Figure S4 AFM averaged curve and optimized FE curve on PDMS (80:1). For PDMS
(80:1), the most optimized parameters of silica layer are 190 nm and 8 MPa, which is
almost the same to PDMS (60:1). To reduce the difficulty of modeling in FEM, we
regarded it as the same.
Figure S5 The rigidity range that cells show different spreading behaviors on
substrates of PAAm has an upper limit of 33 kPa. (a) Time lapse recording of cell
spreading on different substrates. The extensions of lamellipodia or filopodia on
substrate of 33 kPa and 100 kPa, which are different from those on less stiff substrates,
are nearly the same. (b) The projected areas of cells after 20 min seeding increase
with the elasticity. ** p < 0.01 vs. projected areas on soft substrate by one-way
ANOVA analysis . The projected areas of cells on substrates of 2.3 kPa, 33 kPa and
100 kPa show no significant difference. (c) Cells on stiffer substrate spread much
faster than those on soft substrate, while the scaling law factors on substrates of 33
kPa and 100 kPa show no significant difference. ** p < 0.01. Scale bar: 10 μm.
Figure S6 Inhomogeneous porous network of PAAm gels. Scale bar: (a) 100 μm, (b)
20 μm, (c) 2 μm.
Figure S7 Normalized contact radius as a function of time. This result is in consistent
with the typical data on PDMS in our experiment.
Supplementary Method
Scanning electron microscopy of PAAm hydrogels
PAAm gels were attached to round coverslips with 10 mm in diameter, and fixed
with 2.5% glutaraldehyde in 0.01 M PBS buffer overnight at room temperature. After
washing with PBS three times, the substrates were treated with 1% osmium
ferricyanide at 4C for 6-8 hours. After a further washing step with PBS, the samples
were dehydrated in an ascending series of ethanol solutions from 30 to 100%. The
coverslips were placed in a critical point dryer (CPD 030, Bal-Tec), where the ethanol
was replaced with liquid CO2, which was heated to 40C and changed its state to
gaseous phase. The coverslips with samples were glued to pin stub specimen holder
with colloidal silver before observation. Samples were viewed by environment
scanning electron microscopy (Quanta 200, FEI) in low vacuum mode at 6 kV. In
order to maintain the original appearance of the samples, no metallic conductive layer
was coated on the surface.
To further confirm the network of hydrogels, bulk hydrogel samples were
quench-frozen, by plunging into liquid nitrogen. Then the samples were treated in the
above way.
Supplementary Movies
Supplementary Movies S1: Cell spreading on different substrates of PAAm.
Supplementary Movies S2: Cell spreading on different substrates of PDMS.