[Rice Catalyst Issue 14]

USES OF HYDROXYAPATITE IN
DENTAL ENAMEL BIOFABRICATION
BY WILEY LIOU
ABSTRACT
Hydroxyapatite (HAp) is the primary
component of tooth enamel and is
important in the prevention of bacterial
and acidic decay. Over a lifetime, however,
the wearing of this outer tooth layer
may lead to recurring pain and disease,
making both prevention and restoration
of the teeth crucial for daily routines
like chewing and physical appearance.
Although HAp is a molecule already found
in bone tissue, scientists have explored
methods of artificially embedding HAp to
complement already existing enamel due
to its biocompatibility and rigidity. In this
review, multiple uses of HAp are explored,
as well as ways to artificially synthesize the
molecule. First, using HAp as an alternative
to fluoride in toothpastes may decrease
the toxicity of unintended ingestion. In
addition, immersing impacted teeth in a
HAp powder solution has demonstrated
evidence of rehardening, while a hydrogel
mat similarly facilitated the formation of
HAp crystals on the surface of sampled
teeth. Alternatively, creating a HAp sheet
to cover the surface of the tooth is shown
to be a less invasive procedure while
accomplishing the same task. Finally, three
3D printing techniques of HAp are explored.
While vat polymerization has to be
conducted at high heat, leading to possible
destructuring of HAp crystals, both inkjet
printing and extrusion printing can create
highly specific scaffolds at the expense
of high cost. Overall, mimicking tooth
enamel through the fabrication of HAp,
while tedious, can be a minimally invasive
alternative to crowns and bridges for dental
patients.
INTRODUCTION
The thin outer layer of a tooth, or the
dental enamel, is formed in a process
known as amelogenesis, in which cells
called ameloblasts are generated from
oral epithelium tissue. [1] As these cells
secrete amelogenin and enamelin, calcium
and phosphate ions crystallize on top of
the dentin surface, the structural layer of
teeth underneath the enamel. The result
is a hardened matrix composed of long,
parallel, and cylindrical rods around 3 μm
wide. [2] Dental enamel maintains this rigid
structure owing to the fact that it is 95% by
weight HAp, which stabilizes the enamel
structure through hexagonal phosphate
and calcium ion repetitions. (Figure 1). [1, 3]
Evolutionarily, HAp minerals in our enamel
have been used to protect against bacterial
decay of sensitive dentin, allowing the
safe consumption of sugars and acidic
substances. [4] However since enamel is
acellular, it does not naturally regenerate,
leaving it vulnerable to demineralization.
[4] When bacteria combines with ingested
food, it forms plaque, an acidic substance
that further decays the tooth. [5] Thus,
enamel restoration is necessary for
individuals who have deteriorated their
outer teeth layer and intend to prevent
tooth pain and gum disease.
As scientists consider candidates for the
synthetic fabrication of enamel, artificially
synthesized HAp is a particularly attractive
material due to its biomimetic capabilities.
[6] Because of its natural presence in
human teeth, biofabricated HAp is more
Figure 1: Molecular structure of hydroxyapatite Ca10(PO4)6·2(OH). The crystal structure of
hydroxyapatite allows for the formation of hydrogen bonds between the hydroxyl ions and
the phosphate groups, which stabilizes the compound. The presence of calcium ions in the
crystal lattice also gives hydroxyapatite a high degree of thermal and chemical stability. [3]
4 4 | C A T A L Y S T 2022-2023