Biomedical Engineering Reference
In-Depth Information
hermodynamic luctuations (parts of the biomolecule bending to an adherence-
unfavorable position, collisions with other biomolecules in the solution, etc.) can
cause the biomolecule to leave the surface, a phenomenon known as
elution
. Certain
surfaces favor elution more than others, and surfaces (or surface coatings) that alto-
gether deter protein physisorption have attracted a lot of interest in BioMEMS, as we
will see. Elution can also be provoked by changing the solvent (e.g., water for ethanol),
as this disrupts all the hydrogen bonds and electrostatic interactions that were keep-
ing the molecules in place.
A biomolecule physisorbed onto a surface can be substituted or displaced by another
molecule that is attracted to the surface with a larger force.
As a second corollary, it follows that physisorbed coatings are unstable. Hence, for appli-
cations such as biosensors and implants, historically, many eforts have been directed toward
chemically anchoring biomolecules on surfaces to create surface coatings that are stable. But is
it worth the trouble?
2.1.1 Physisorption versus Chemisorption
Before we review the most common chemical immobilization strategies used in BioMEMS, we
briely point to frequent misconceptions about the stability of chemisorbed layers in biological
luids. he question that concerns us is the choice that the BioMEMS researcher must make
when a biomolecular coating for a surface is sought: Should the biomolecule under consider-
ation be physically adsorbed or chemically linked to the surface?
he advantages of physisorption are simple. As opposed to chemisorption:
Physisorption procedures require no expertise or special equipment.
Physisorption can be used in combination with most surfaces, in particular with the
ubiquitous, inexpensive glass slides or polystyrene petri dishes used as cell culture
substrates.
Physisorption does not
require
surface treatment (on most surfaces); however, in
some cases, it may be advantageous to treat the surface to enhance physisorption
(e.g., tissue-culture polystyrene petri dishes are treated with a plasma discharge that
is believed to introduce charged groups on the surface of polystyrene, which thus
becomes more adhesive to proteins and cells).
Reciprocally, those advantages can be listed as disadvantages of a chemisorption method:
Chemisorption requires a specialized substrate that allows for a chemical linkage
between the biomolecule and the surface; because the functional groups at the sur-
face of organic polymers are not always well characterized, these materials, otherwise
extremely appealing in BioMEMS for many other reasons, are not compatible with a
number of the chemistries commonly used to immobilize biomolecules (as we will
see).
Chemisorption oten requires the expertise of a chemist (not always available in bio-
logical laboratories). In modern laboratories with stringent safety standards, it also
mandates the presence of a chemical hood, proper exhaust in the building, and waste
disposal procedures. Implementation of safety training and precautions, even when it
is commonsensical, is bound to generate tedious paperwork and inspections (believe
me). So, even a chemist will tell you that physisorption is simpler than chemisorption.
Still, the notion that, in principle, chemisorption allows for a more stable anchorage between
the biomolecule and the surface has historically appealed to many groups in BioMEMS.
However, the degree and usefulness of this “stability” can be challenged:
