Protein-based Biointerfaces to Control Stem Cell Differentiation - Biointerfaces: Where Material Meets Biology - page 12

properties. Surface properties are able to affect the overall performance of
materials, control the first phase of material-cells interaction and trigger
specific biological responses.
Generally, surface functionalization approaches can be classified into
three main categories (physical, chemical and biological). However, this
distinction is not always strict and multiple modifications can be employed.
The choice of the right route is determined by either the material chemistry
or the envisioned application and every choice presents advantages and
drawbacks. Here, we will describe biological functionalization methods
(physisorption and covalent bonding), overlooking physical (e.g., polishing,
grinding, laser treatment) and chemical (e.g., ion bombardment, acid/alka-
line treatment, sol-gel, small molecule grafting) approaches.

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1.2.1.1 Physisorption
Physical immobilization is the simplest bio-functionalization method and
consists just in dipping the material in a solution containing the target
biomolecules. Physisorption (i.e., physical adsorption) relies on electrostatic
interactions, van der Waals forces, hydrogen bonds and/or hydrophobic
interactions. It can be applied to most types of surfaces and, in general, does
not require any surface pre-treatment. Although it is considered a gentle
method (i.e., non-destructive), the biomolecule-surface randomly-oriented
interaction can lead to conformational changes and subsequent loss of
functionality. In addition, due to the presence of weak interactions the
binding stability of adsorbed molecules is dramatically affected by en-
vironmental conditions, such as pH,

.

ionic strength and biomolecule

concentration. 76
Recently, a few approaches have been developed to introduce biomole-
cules by physisorption in a more controlled way. For example, Messersmith's
group at Northwestern University introduced a fast, reproducible, versatile
method to coat many different organic and inorganic surfaces by exploiting
alkaline oxidative polymerization of dopamine. 77 Having a thickness ranging
from few nanometers (nm) to 4100 nm, the polydopamine (pD) coating does
not affect the pristine material properties. The coating can be exploited to
introduce biomolecules through a single step, 78 i.e., with the biomolecules
entrapped in the pD layer, or two separated steps, with the molecules at-
tached on top of the pD layer. 79
Another interesting physisorption-based approach for ceramic materials
employed as bone substitutes involves the use of short (10-30 amino acids)
modular peptides having two domains: (1) a domain able to bind strongly
the surface through electrostatic interactions; and (2) a domain that intro-
duces a biologically relevant function (e.g., cell adhesive peptides or growth
factor-derived moieties). The mechanism behind the surface functionaliza-
tion mimics the natural mechanism by which human bone extracellular
matrix proteins bind hydroxyapatite (HA) into the body. Different sequences
have indeed been identified as responsible of this interaction in osteonectin