Hydroxyapatite-Biodegradable Polymer Nanocomposite Microspheres Toward Injectable Cell Scaffold - Biomimetics: Advancing Nanobiomaterials and Tissue Engineering - page 220

the application range is limited in some cases, because the functionalities
of the microspheres are suppressed by the coating of the microsphere
surface with these molecular-level emulsifi ers. Strategies for the synthe-
sis of HAp-biodegradable polyester hybrid microspheres without the use
of the molecular surfactant or polymeric stabilizer should be developed;
however, there are few reports on this research topic [18, 19].
In this chapter, we will describe the fabrication method of HAp-
biodegradable polymer nanocomposite microspheres developed in our
research group. This method is named the “Pickering emulsion method,”
in which no molecular-level emulsifi er is used and an emulsion stabi-
lized by solid HAp particles is utilized. Here, the HAp particles play a
role of an emulsifi er, which stabilizes the emulsion as well as the result-
ing microspheres, and they also form the shell of the fabricated micro-
spheres to improve cell adhesion properties. First, we will explain what
the Pickering emulsion is and then show examples of the fabrication of
HAp-biodegradable nanocomposite microspheres using the Pickering
emulsion method.

9.2 Pickering Emulsion

9.2.1 What is a Pickering Emulsion?

An emulsion stabilized by adsorption of fi ne solid particles onto an oil/
water interface is called a “Pickering emulsion” [20-25]. Fundamental
research has been carried out based on physical chemistry and colloid sci-
ence, and the development of applications in cosmetics, pharmaceuticals
and food industries is expected. Several factors can be identifi ed which can
control the emulsion stability and the emulsion types [oil-in-water (O/W)
type (oil droplets are dispersed in continuous water phase) or water-in-oil
(W/O) type (water droplets are dispersed in continuous oil phase)]:
type of emulsifi er, its concentration, volume ratio of water and oil, tem-
perature, how emulsifi cation energy is added, container wall properties,
and addition order of each component [26]. In particular, when the par-
ticles are used as the emulsifi er, the contact angle ( q , generally measured
through the water phase), which indicates the wettability of the particles
to oil or water, is an important factor in determining the stability and the
type of emulsion. When hydrophilic particles adsorb onto the oil/water
interface, more than 50% of the particle surface is exposed to the water
phase; thus, q is less than 90

in the case
of hydrophobic particles. When the volume ratio of the water phase and
the oil phase is 1:1, hydrophilic particles preferentially stabilize an O/W
emulsion, and hydrophobic particles preferentially stabilize a W/O emul-
sion (Figure 9.1). Thus, the wettability ( q ) of particles provides a similar

° . In contrast, q is greater than 90 °