THE ROLE OF ATOMIC FORCE MICROSCOPY IN ADVANCING DIATOM RESEARCH INTO THE NANOTECHNOLOGY ERA - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 408

EPS adhesion values were signiicantly less, they still may have been affected
by residual adhesives from the nearby raphe and/or high loading forces (25
nN) applied to polymer, which has the effect of increasing the contact area of
tip with the polymer. Measurements on the EPS girdle coating using loading
forces < 1 nN showed adhesion forces of only ≈200 pN corresponding to the
picking up and subsequent detachment of 50

- 200 nm individual polymer

chains with similar elastic properties. 15

19.4.2 From Microscale to Single-Molecule Adhesives: Pads,
Tethers, Strands and Nanofibres

To study diatom adhesive interactions with surfaces, researchers have
prepared live “bioprobes” by attaching an individual living cell to a tipless
cantilever. 23 Using these probes, forces measured against a mica substrate
and antifouling coating, Intersleek™, showed comparable cell adhesion
strength for on the two surfaces, indicating cells secreted

an adhesive consisting of both hydrophilic and hydrophobic motifs. To
more directly probe diatom adhesives involved in such interactions, ly
ishing measurements on

Navicula sp.

, whereby the tip was
hovered above the surface, enabled single adhesive strands protruding
from the non-driving raphe of living cells to be “caught” by the tip. 24 Their
subsequent detachment from the tip recorded forces of ≈150 pN. To
enable strong adhesion to the surface, the cells secreted a conglomerate
of these strands in the form of a single micro-sized tether that extended
for ≈40 μm and terminated in a holdfast-like attachment to the surface
( Fig. 19.5a ) . 23 When the AFM tip was brought into direct contact with the
raphe, these tethers recorded an adhesion force > 20 nN and, because of
their high extensibility, could remain attached to the tip even when the z-
height limit of the piezo had been reached ( Fig. 19.5b, ii ) . Force proiles for
these measurements revealed an irregular sawtooth pattern ( Fig. 19.5b, i ) ,
indicating the successive unbinding of domains (i.e. inter- and intra-bonds
within strands and tethers) when the raphe tether was placed under stress.
These unbinding domains had previously been explained as “sacriicial
bonds” which give way under force before the backbone of the adhesive
breaks, effectively increasing its lifetime. 25 Rises and falls in the force (i.e.
sawtooths) over long extension distances also greatly increased the area
under the curve, or energy required to break the adhesives. This imparted
extra fracture toughness into the adhesive material. 25 Similar sawtooth
patterns were observed on regions of a glass slide, presumably the location
of residual adhesive, where a chain-forming species,

C. australis and P. viridis

had
been mechanically removed. 26 The cell samples were cultured with another

Eunotia sudetica ,