Biomedical Engineering Reference
In-Depth Information
(-NH
2
, -NHR or -NR
2
). As we shall see later, coating of hydrophilic surfaces
with polysaccharides (e.g. dextrans) has been recognized as advantageous for
neuronal cell adhesion because each monomer within the chain carries up to
three hydroxyl groups. As discussed later so are also the so-called biomimetic
peptides. A particularly prevalent example is the key tripeptide sequence RGD
(Arg-Gly-Asp), which is present in fibronectin.
Aside from the electrostatic hydrogen bond, there are potentially other
charge-based interactions that can occur between the exterior of a biological
cell and a substrate surface, especially when the latter is expected to be charged
as may be the case for electrode materials. These range from formal
charge-charge interactions to those involving induced dipoles. A summary
compendium of the energy of these forces and their distance dependence
is given in Table 2.1. In this respect an interesting and potentially
important interaction, which has not been widely discussed, is that of the
membrane surface dipole electrical potential with either surface formal charge
or dipoles.
4
This potential originates largely from certain lipid components of
the biological membrane (the lipid composition of some types of cell is shown
in Table 2.2). The dipoles of the headgroups of lipid molecules at the surface are
aligned within an approximate (highly dynamic) sheet resulting in the surface
potential, which is distinct from the conventional potentials outlined above.
Interaction of this potential with, for example, the Gouy-Chapman double
layer present on electrodes could constitute an important adhesive, or indeed,
repulsive force.
d
n
4
t
3
n
g
|
0
n
3
.
Table 2.1 Basic energy equations for electrostatic interactions.
Interaction Potential energy (U)
Charge-charge U
¼
q
1
(q
2
/r)
Charge-dipole U
¼
q m cos y/(Dr
2
)
Dipole-dipole U
¼
m
1
m
2
(cos y
12
3 cos y
1
cos y
2
)/4 pe
0
r
12
3
Charge-induced dipole U
¼
1/2 a q
2
/(r
4
D
2
)
Key: q is formal charge, r is distance, m is dipole moment, D is dielectric constant, e
0
is permittivity,
a is polarizability, and y is the angle between charge and/or dipole (center). Subscripts 1 and 2 label
each of two dipoles.
Table 2.2 Examples of lipid composition of eukaryotic plasma membranes.
Cell
PC
PE
PI
PS
PA
CL
Sph
Ch
CB
SL
Fibroblast
43.2
16.1
7.6
6.4
1.5
-
12.2
13.0
-
-
Schwann
44.0
34.0
9.6
2.8
-
-
29.6
0.6
-
-
Dorsal root ganglia
(neuronal cell)
28.9
16.5
5.8
3.1
-
-
2.8
15.4
3.1
1.6
Reticular nucleus 34.0 27.5 - - - 6.3 12.6 19.6 - -
Human T cell 43.0 32.9 - 7.4 - - 10.4 - - -
Key: PC, phosphatidylcholine; PE, phosphatidylethanolamine; PI, phosphatidylinositol; PS,
phosphatidylserine; PA, phosphatidic acid; CL, cardiolipin; Sph, sphingomyelin; Ch, cholesterol;
CB, cerebrosides; SL, sulfatide.
