Biomedical Engineering Reference
In-Depth Information
Despite the gaps in the theory, material scientists cannot wait till everything
is crystal clear and they practice surface modification in prevention of protein
adsorption. Grafting oligomers or polymers on the surface is a feasible tech-
nique. Pluronics
R
is an example of a commercial coating. It consists of triblock
copolymers polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-
PPO-PEO). The density of the film (surface coverage) seems to be the predominant
factor for success.
Not any detail has been given about the thermodynamical contributions (Gibbs-
free energy, enthalpy and entropy changes) or acid-base properties at the interface.
This section is nothing but a glimpse of what is going on in the field and it could not
be more because a
handbook theory
is not yet at hand. In Sect.
11.9
, we were won-
dering why our elastomer coating was behaving much better than expected from its
hydrophobic character and why it is adsorbing a few percent of water. Study of the
hydrophobic interaction may be the lead to an understanding of this phenomenon.
Super-Hydrophilic Surfaces
D 180
ı
.
At the other end of the line, extreme hydrophilicity, are surfaces grafted by water-
soluble long polymer chains such as polyethylene glycol (PEG). Hydrophilic sur-
faces are subdivided in:
An extreme hydrophobic case is teflon-water with a contact angle of
Clear
because of sharp/rigid boundary: ionic for surfaces decorated with -COO
or nonionic with -OH decoration.
Super-hydrophilic
with a diffuse boundary, absence of a driving force for protein
adsorption (zero interfacial free energy).
Theoretically, those super-hydrophilic surfaces should be anti-fouling (no deposit of
cells or proteins). They can easily be produced (grafting, physical adsorption, etc.).
Sepharose is, for example, a hydrogel with low cross-linking degree and adsorbs
proteins only in marginal quantities...all appealing properties for scientists inter-
ested in blood compatible materials. But reality appeared to be less attractive. Blood
platelet adhesion decreases with the water content of the hydrogel but increases
steeply after passing through a minimum. So, blood compatibility is poor. Moreover,
another unexpected effect was happening: most testing is performed for relatively
short periods of time but time, as we stressed repeatedly in former chapters, is an
important parameter. And indeed, superhydrophilic surfaces produce thrombi after
an uncontrollable period of time.
Superhydrophilic surfaces are slippery and listed applications making use of this
property are guide wires, catheters, endoscopes or contact lenses, a potentially more
successful area because of restrained contact times. All these applications concern
contact with soft tissue.
Other candidate molecules for enhancing hydrophilicity are polysaccharides:
inositol, the natural polymer alginate, heparin (widely used in blood contacting
devices), hyaluronan (eye surgery) [
469
]. They all merit attention but the main
