Biomedical Engineering Reference
In-Depth Information
The nature of the particle surface (i.e., size, surface area, hydrophobicity, charge
density, surface chemistry, and stability) will greatly affect its interaction with
surrounding biological moieties. The particle size is usually defined as the diameter
of a sphere that is equivalent in volume to the particle measured. Several methods
are used to determine size distributions: light scattering, differential mobility
analysis, time-of-flight mass Spectrometry (TOF-MS), microscopy, and others
[
80
]. Reducing particle size to the nano-level can modify the physicochemical
properties compared to the corresponding bulk material [
73
]. The size of
nanoparticles (NPs) determines the path they take in the body. It has been reported
that particles less than 30 nm in size are rapidly eliminated by renal excretion and
that larger particles are phagocytosed by macrophages. Nanoparticles of 30-150 nm
will go to the bone marrow, the heart, the stomach, and the kidneys and those of
150-300 nm will be found mainly in the liver and the spleen [
81
].
The surface area corresponds to the surface of the nanoparticle that is exposed to
the environment. This property is of great importance when we study toxicity of
NPs, because interactions with the biological organism occur at the interfacial area
of the material. The area-volume ratio establishes the number of possible reaction
sites on the particles. An increase in reactivity can be either advantageous
(increased ability of carrying drugs, increased uptake, etc.) or negative (toxicity,
induction of oxidative stress, etc.) [
73
]. The hydrophobicity of nanoparticles
controls the adsorption of plasma proteins on the surface of NPs and may also
play a role in the macrophage uptake. An augmentation in the hydrophobicity of the
particles facilitates binding to the cell membrane by forming hydrophobic
interactions. In fact, studies showed that the more hydrophobic the particles, the
larger the total amount of bound protein [
82
].
Among the physical characteristics of nanoparticles, the surface charge density
is an important one. It has major effects on the impact of the particle in the
organism. Indeed, the concentration of electric charge on a particle will cause or
inhibit some bindings and will change the dispersion of particles in the body,
considering a repelling force between like charges and an attractive force between
the opposite charges. Note that, although not attracted magnetically, nanoparticles
agglomerate. The presence of salts and electrolytes in biological solutions may
neutralize repulsion of surface charges on the nanoparticles, allowing particles to
agglomerate [
83
]. The surface charge density has a direct effect on the binding of
nanoparticles with cells. For example, macrophages present negatively charged
sialic acids on their surface [
83
]. Thus, positively charged nanoparticles will bind to
them. Consequently, these particles will be phagocytosed. Remarkably, the sign of
the charge (positive or negative) of the particle is not as influential as expected.
Indeed, studies show that charged particles, cations or anions, are more easily
absorbed by the cells than electrically neutral particles [
82
].
Surface chemistry also has a great influence on the interaction of the particle
with the biological environment. In order to stimulate or reduce the effects of
nanoparticles on the organism, it is possible to coat its surface with various
substances. To interact with specific biological targets, a coating acting as an
interface can be attached to the nanoparticle. Among all possible coatings, we
