Biomedical Engineering Reference
In-Depth Information
Although steady-state behavior is often assumed when evaluating the bulk
properties of nanoparticulate suspensions, the nano-bio-interface is exposed to an
inhomogeneous and dynamic environment. This is a direct result from the distribu-
tion and spatial localization of proteins, lipids, and glycosylated structures of the
nanoparticles' microenvironment. Moreover, the interface experiences constant
fluctuations as a result of cellular turnover and environmental variations, namely,
secreted cell products. Furthermore, the nature of the particle influences the binding
of protein's surface ligands, and alterations to free surface energy may induce
conformational changes or oxidative damages. The microenvironments of the
particle can also chance as these particles can be engulfed inside the cell.
Events occurring at the nanoscale are still governed by Van der Waals (VDW),
electrostatic, solvation, and depletion forces [
77
]. VDW forces are a consequence
of the quantum mechanical movements of the electrons. These fluctuations result in
a small nonetheless significant dipole in the nanoparticle, which induces a dipole
moment in the atoms of the neighboring particle, triggering an attractive force
between both particles. The electrostatic force in the system results from surface
charges that inexorably occur on the particles when they come in contact with
water. The ionic strength in most biological fluids is approximately 150 mM
[
77
]. Thus, the electrostatic forces are, in all likelihood, to be screened within a
few nanometers of the surface. Solvation becomes important when dealing with
inorganic and hydrophilic nanoparticles. This phenomenon occurs when water
molecules attach to the particles with enough energy to create steric layers on the
surface of the nano-entities. This renders interactions and adherence of two
particles extremely difficult. On the other hand, hydrophobic attraction can occur
if the affinity of two surfaces for water is lower than that between water molecules.
However, these known interactions can be complicated by nonrigid compliant cell
membranes that can deform when interacting with a nanoparticle, due to the
former's fluidity and thermodynamics. Moreover, the cell surface is nonuniformly
charged due to the presence of surface proteins and other structures. This surface
heterogeneity varies between 10 and 50 nm and thus greatly alters its interaction
with nanoparticles. More importantly, cell surfaces are not passive, inducing a time-
dependent dynamic interface [
76
].
1.4.1.1 Nanoparticle-Protein Interactions
Immediately after its introduction in a physiological environment, proteins such as
apolipoproteins, fibronectin, vitronectin, and others, adhere to the nanoparticle
(Fig.
1.1
). Protein adsorption to various materials has been widely studied and it
has been found that factors such as electrostatic interactions, hydrophobic
interactions, and specific chemical interactions between the protein and the adsor-
bent play important roles in the characteristic of the bound protein-nanoparticle. It
is argued that to understand and predict the cell-nanomaterial interaction, the
particle and its “corona” of more or less strongly associated proteins from blood
or other body fluids should be considered. It is important to understand how cells
