whether a cell will adsorb to an air-bubble. Adsorption only occurs if the change in free
energy upon adsorption is negative. This change is given by
(10)where γcv (N·m−^1 ) is the interfacial tension between the cells and the vapour, γlv (N·m−^1 ) is
the interfacial tension between the liquid and the vapour, γCL (N·m−^1 ) is the interfacial
tension between the cells and the liquid. Values for γcv and γcl were calculated by
measuring the contact angle between a medium drop on a monolayer of cells. Assuming
that the monolayer surface was chemically homogeneous and perfectly smooth and the
system was in equilibrium, semi-empirical equations were used to calculate the values for
the different surface tensions. Thus, they found that the values of γcl were very small (0–
0.5 mN·m−^1 ) and do not contribute very much to the value of dFads, which is in
accordance with the fact that cells are readily suspended in media. Values of γcv showed
little variation and were usually in the range of 60–70 mN·m−^1. In the absence of
additives the value of γcv-γlv was of the order of −2 mN·m−^1 , indicating that adsorption can
occur. Addition of 0.1% Pluronic or 0.3% Methocel resulted in a reduction of γlv with
about 10 mN·m−^1. Because, complete wetting of the monolayer occurred in this case, the
contact angle was zero and γcv could not be determined. However, as large changes in γcv
are not expected, this made dFads positive. Consequently, adsorption will not occur,
which is in accordance with the complete wetting of the monolayer surface.
In reality the membrane of a cell contains proteins (about 50%) and different lipids
including glycosylated lipids and cholesterol. Thus, it is likely that the surface of a cell
will be mainly hydrophilic, which is in accordance with the low γcl. However, parts of the
cell surface will be hydrophobic, allowing for attachment to air bubbles. Furthermore, the
membrane is fluid, meaning that with increasing contact time between a cell and a
bubble, a redistribution of hydrophilic and hydrophobic parts may occur, with the
hydrophobic parts concentrating on the cell-air interface. If we assume that the
hydrophilic part is completely wetted by the liquid, the tension imposed on a cell is given
by γlv. In order to prevent disruption of the cell, this tension is balanced by the surface
tension of the cell γcv. If γlv is larger than γcv, the cell will be disrupted. However, this γcv is
not the value determined by Chattopadhyay et al. (1995b), since their value is some kind
of average value over the cell membrane. The value that should be filled in here should
be representative of the value with which the cell membrane is held together.
Michaels et al. (1995a) measured the induction time (τ) for cell adhesion being the
time cells should be in contact with a bubble in order to become attached. Measurements
were done for serum-free medium with or without additives. Results are shown in Table
15.5. Clearly a negative free-energy change and cell-bubble contact is on itself not
sufficient for attachment, but also the duration of contact is of importance. The increase
in contact time for the different additives corresponds well with the decrease in surface
tension, as would be expected from the results of Chattopadhyay et al. (1995b).
In conclusion, in the absence of surface-active compounds on the bubble surface cell-
bubble contact may lead to direct cell death. In the presence of shear-protective additives
cells may adsorb to the bubbles if they have been in contact with the bubble for a certain
Lethal effects of bubbles in animal-cell culture 465