Biomedical Engineering Reference
In-Depth Information
acellular and cellular organic structures is a matter of special importance. Thereby, the first step is
represented by the formation of the acquired pellicle which occurs almost instantaneously on all
solid substrates exposed to the oral fluids
[17]
. The adsorption process is driven by physicochemi-
cal interactions of the tooth surface with proteins and glycoproteins from the oral fluids; the pellicle
formation means a gain in entropy. Fast area-wide pellicle formation is provided by the adsorption
of micelle-like globular structures, as most salivary components are not released as single mole-
cules but as aggregates
[17]
. These globularly structured aggregates have a diameter of
100
200 nm and are quite similar to milk micelles which could be a starting point for the develop-
ment of new strategies in preventive dentistry based on organic nanostructures
[18]
.
The physiological pellicle layer fulfills several functions besides lubricating the tooth surfaces.
It offers protection against erosive mineral loss and modulates de- and remineralization
[19
23]
.
However, these effects seem to be limited to short exposure duration and fluids with moderate
acidic pH
[20]
. Longer exposure to beverages or acids at low pH inevitably leads to destruction of
the pellicle and continuous mineral loss as shown in vitro and in situ. Due to the tremendous
increase in the consumption of erosive beverages and the prevalence of eating disorders, there is a
strong demand for preparations improving the antierosive properties of the pellicle.
Furthermore, the pellicle contains a couple of specific and unspecific antibacterial and antimi-
crobial proteins, glycoproteins, and peptides (such as secretory immunoglobulin A or lactoferrin) as
well as, and enzymes (such as lysozyme or peroxidase)
[17,24]
. Despite these antibacterial proper-
ties, the pioneer bacteria have adapted to the protective properties of the pellicle layer
[25]
. Many
pellicle components serve as receptors for specific and irreversible bacterial adhesion after the ini-
tial unspecific approach of bacteria to the tooth surface mediated by hydrophobic interactions, van
der Waal's forces, and other physicochemical interactions
[15,17]
. Thereby, bacterial glycosyl
transferases are remarkable key molecules synthesizing water insoluble and soluble glucans
[25]
.
These enzymes are already present in the acquired pellicle in an active conformation within min-
utes and can be regarded as pioneer molecules for bacterial biofilm formation
[25,26]
. In this closer
sense, biofilms have been defined as a structured community of bacterial cells enclosed in a self-
produced polymeric matrix adherent to an inert or living surface
[27]
.
The initial process of bioadhesion is an essential target of recent approaches in dental research
based on nanotechnology in order to prevent bacterial adhesion, to ameliorate remineralization, and
to avoid demineralization
[9,14]
. Furthermore, the more mature three-dimensionally organized bac-
terial biofilm could be prone to modification by nanomaterials. This means removal of already
established biofilms as well as direct antibacterial effects on adherent bacteria which is still a
drawback of many conventional antibacterial preparations. They are often of high efficacy against
planktonic microorganisms, but fail to substantially affect the adherent bacteria organized in three-
dimensional biofilms and embedded in an extracellular matrix
[28,29]
. Bacteria living organized in
these biofilms are effectively shielded and protected against external attacks like treatment with
detergents or antibiotics
[28]
. Nanoscaled particles may yield size-specific interactions with the bio-
film, but they could also serve as drug-release carriers with high affinity to the bacterial surfaces or
the extracellular matrix, respectively.
Besides the biofilm, the substrate of caries, and erosions, the dental hard substances themselves
have moved into the focus of nanoscientists. Both enamel as well as dentin represent unique and
hierarchically organized biological nanostructures
32]
. The inorganic building units
are hydroxyapatite (HA) nanocrystallites; in dental enamel, their length amounts up to several
[9,30
