Biomedical Engineering Reference
In-Depth Information
only, we take brief look at eukaryotic cells and their surrounding environment.
This is followed by a summary foray into neuron structure and function.
2.2 Substrate Surface Parameters: A Pre´ cis
Biological cells of any type will exhibit a plethora of 'exterior' functional
groups, originating from protein, lipid and saccharide moieties, together with
domains of polar and hydrophobic character orchestrated in a highly
dynamical fashion. Accordingly, with respect to their interaction with bare or
coated substrates, the existence of interfacial functional groups and their spatial
distribution, roughness or morphology, physical pattern and structure (pillars,
etc.), free energy, charge and elasticity are all expected to play pivotal and
concerted roles.
As would be expected surface scientists have employed a large number of
analysis techniques for the characterization of surfaces and many of these have
been employed in connection with biomaterials and sensor technology. Indeed
there is much common ground with respect to surface analysis of devices and
materials. Included among this armamentarium are X-ray photoelectron
spectroscopy, atomic force microscopy, scanning tunneling microscopy,
secondary ion mass spectrometry, electron microscopy, confocal fluorescence
microscopy and contact angle measurement.
1,2
The last of these is important in
terms of the study of the surface free energy of a bare or adlayer-treated
surface. This leads to an appraisal of the critical role of interracial
thermodynamics with regard to the cell-substrate interaction, at least at the
level of a first approximation. This implies that such an interaction can take
place if there is an overall reduction in interfacial fee energy, with the caveat
that surface morphology will undoubtedly play a key role.
There are a huge number of electronegative atoms such as such as O and N
present on the exterior 'boundary' of cells which originate from membrane
proteins and lipidic moieties and other biochemical sources. Accordingly, this
leads to the possibility for extensive intermolecular hydrogen bonding with a
polar or hydrophilic surface. This involves an essentially electrostatic inter-
action of some 2-10 kcal mol
1
where functional groups with the hydrogen
atom may act as a donor or acceptor for formation of the bond. Since non-
polar domains are also expected on the surface of a cell the hydrophobic effect,
about which much has been written, will also play a role. This effect is generally
considered to originate from entropic considerations associated with the
exclusion of water and disruption of water-based hydrogen bonds. For an
excellent look at the hydrogen and the water molecule please consult ref. 3.
Given the importance of interfacial free energy, hydrophilic/hydrophobic
modification has received great attention with the aim of enhancing cell
attachment, or indeed avoidance of cellular interactions for particular appli-
cations. A surface can be simply rendered hydrophilic by the introduction of
negatively charged groups such as the carboxyl (-COO
) entity. Neutral
hydrophilic surfaces can be obtained by hydroxyl or amide groups, and cationic
hydrophilic surfaces are made by introduction of different amino groups
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