[Rice Catalyst Issue 14]

Figure 3: A comparison of the three 3D printing techniques discussed. A. Vat Polymerization generates a scaffold by using a DLP projector to
harden a scaffold layer by layer, with a platform rising as the resin hardens. [20] B, C. Inkjet printing and extrusion both involve pushing out
a scaffold through a nozzle that is able to be specific in size and shape. [26]
of the non-HAp-immersed tooth (Figure
2). These results together display evidence
that the HAp powder had rehardening
properties, with calcium ions adsorbing
onto teeth, and phosphate ions acting as an
agglomeration tool. [14]
Yet another approach to covering enamel
is to mimic the gel-like environment that
surrounds the teeth during amelogenesis,
which involves the transport of calcium
and phosphate ions from ameloblasts into
the enamel. [16] Unlike past experiments
that were conducted in aqueous solutions,
Cao et. al created an agarose-based
calcium chloride hydrogel mat to provide
a one-way transport of ions into damaged
teeth. [17] The researchers treated 2 mmthick
teeth samples with phosphoric acid
for deterioration, then created a four-layer
model by covering the damaged teeth
slices with the calcium chloride hydrogel,
an ion-free agarose gel, and a phosphate
solution. Looking at SEM photographs,
HAp crystals were visible after just two
days of immersion for the treated teeth.
Over the next few days, these rods
organized themselves into one axis, and
after six days, the gel that had previously
been located between the crystals had
been effectively eliminated. Furthermore,
a nanohardness test demonstrated similar
hardness values for the control and
repaired teeth, with a hardness increase of
3.04 ± 0.75 gigapascals (GPa)(p=.05) after 6
days. [17] Although this gel can be cheaply
produced, a limitation of the matrix
formed is that it is inorganic, meaning
that it is dissimilar to real enamel; further
research can be done to explore how to
embed enamel proteins like amelogenin
and enamelin into the matrix without
sacrificing low-cost.
Although gel immersion may enhance
dental enamel, its clinical roles are limited
due to the difficulty of isolating individual
teeth from patients for the sole purpose of
enamel repair. Rather than immersing the
entire tooth, Hontsu and his team at Osaka
Dental University created a film of HAp that
demonstrated evidence of tooth hardening.
[18] To create this sheet, they deposited
a film of HAp on a soluble substrate via
pulsed laser deposition, after which the
substrate is dissolved and the remaining
flexible HAp sheet is collected. [18] To
observe its ability to bind to the dentin
and enamel layers of teeth, photographs
provided evidence of sealing owing to the
moisture content of the created layer.
[18] However, research has yet to be
done to demonstrate whether or not this
protective coating demonstrates hardening
capabilities. Overall, though, the techniques
of powders, hydrogels, and mats have all
provided possibilities for enamel repair
and solutions to ailments like dentine
hypersensitivity.
3D PRINTING
A more advanced method to produce
synthetic grafts is through 3D printing
biomimetic material, also known as
biomanufacturing. The advantages of
these 3D-printed scaffolds are that they
are replicable, have great biocompatibility,
and can be customized to individual
needs. Though there are numerous
ways to develop artificial HAp through
3D printing, the ones that are successful
must have certain traits. Notably, the
HAp must be porous enough to allow
for mass transport, strong enough to
demonstrate hardening capabilities in
enamel and prevent degradation in the
mouth microenvironment, and nontoxic
to ensure an absence of side effects. [19]
Three methods of 3D printing are described
in this discussion.
Vat polymerization is a method of using
UV light in Digital Light Processing (DLP) to
manufacture a HAp scaffold synthetically.
[20, 21] In this process, liquid resin inside
of a large tank hardens due to UV light
causing molecules to bond. By lifting a
platform and exposing resin to light, the
3D print is generated layer by layer [20]
However, after the fabrication stage, the
resin must be sintered, or burned, at a
high temperature. To prevent cracks from
forming, this must be done in a gradual
process to an optimal temperature of 1250
°C, with a high flexural and compressional
strength compared to fabrication at 1200°
C and 1300° C. [21] Although this method
can create a scaffold that is accurate
and specific for an individual tooth,
the sintering process at extremely high
temperatures leads to concerns about
4 6 | C A T A L Y S T 2022-2023