The entrapment and drainage of cells from a bubble film was studied by Michaels et
al. (1995a) by microscopically looking at individual draining films. Results are presented
in Table 15.5. In a serum-free medium they found that liquid draining was fast, while
cells drained at medium rates, with some cells remaining entrapped in the film. In the
presence of 0.1% PVP and 0.1% PEG drainage and cell movement was slow, leaving
most cells entrapped in the film. With 3% serum both liquid drainage and cell drainage
occurred at medium rates, leading to initial cell entrapment. However, with time the cells
drained from the film. Finally, in the presence of PVA, Pluronic F68 and Methocel the
drainage of liquid and cells was fast. These results indicate that PVA, Pluronic, Methocel
and serum may offer protection by allowing cells to flow out of the danger zone before
rupture. However, the experiments with PVP and PEG show that even if cells cannot
drain from the film they are still protected. This indicates that other mechanisms like
strengthening of the cells and reduction of the hydrodynamic forces associated with
bubble burst may be more important in offering protection.
Several mechanisms may be responsible for cell death in the bubble film. During
thinning of the liquid film in the bubble cap cells are exposed to capillary forces for a
film thickness less then or equal to a cell diameter. Capillary pressures were estimated by
Cherry and Hulle (1992) to be 1.9 10^4 N·m−^2. Comparing this to the resistance of a cell to
deformation as calculated by Zhang et al. (1992a), which is in the order of 600 N·m−^2 ,
these forces can deform and crush a cell. Whether this actually occurs depends on the rate
of film drainage compared to the rate with which a cell can be deformed and on the film
thickness at breakage. Cherry and Hulle (1992) calculated that the thinning event is much
faster than the deformation of the cell and thus assumed that these forces are not
important for cell damage. On the other hand, Chalmers and Bavarian (1991) stated that
they have observed cell deformation in the film cap together with what seemed to be
leakage of cellular compounds from the cell.
In addition, draining of the film itself may impose stress upon the cells. Cell death due
to direct contact of the cells with air can be excluded, because the exposure times are of
the order of seconds at most, which is very short.
Finally, cell death may be due to events at bubble-film rupture i.e. in the rapidly
retracting rim. The velocity of this rim may be estimated from the equation of Culick:
(20)where ρ(N·m−^1 ) is the surface tension, ρ (kg.m−^3 ) is the density of the liquid, and h (m) is
the film thickness. The film thickness and surface tension are known to vary over a film.
Together with difficulties in estimating the film thickness, this makes the outcomes of
this equation questionable. At a film thickness of 10μm and a surface tension of 64
mN·m−^2 a velocity of 3.5 m·s−^1 can be calculated. This is sufficiently rapid for the film
outside the rim not to notice of the film rupture, which means that cells are suddenly
struck by the retracting rim. Chalmers and Bavarian (1991) calculated that acceleration
rates experienced by cells are very high. Cherry and Hulle (1992) calculated the energy
dissipation, (m^2 .s−^3 ), in the retracting rim from the difference in surface energy (2σA)
and kinetic energy of the rim using
Lethal effects of bubbles in animal-cell culture 475