MICRO-AND NANOSPHERES FOR TISSUE ENGINEERING - Micro and Nanotechnologies in Engineering Stem Cells and Tissues - page 207

interest for engineering of many tissues such as nervous 62 and osteochondral 63
tissues. In these applications, a gradient distribution of bioactive signals is estab-
lished to induce concentration-dependent cell responses. 64,65 To this end, Wang et al.
developed scaffolds containing reverse concentration gradients of two growth factors
(BMP-2 and insulin-like growth factor I (IGF-I)) through polymer scaffolds for
osteochondral reconstruction by introducing silk and PLGA microspheres as carriers
for each growth factor. 66 In that way, human mesenchymal stem cells (MSCs) were
stimulated to differentiate into osteoblasts and chondrocytes, respectively.
9.3.1.2 Delivery of Cells Besides delivery of therapeutic or biochemical com-
ponents, biodegradable and cytocompatible microspheres can also serve as cell
delivery vehicles to improve the biological performance of tissue engineering
constructs (Fig. 9.1) or to construct microscopic three-dimensional (3D) tissue
equivalents that mimic the native tissue structure. In contrast to conventional
hydrogel-based cell encapsulation approaches that normally lead to cell death
because of limited cell adhesion, migration, and communication, 67 the introduction
of microspheres as cell carriers into hydrogels not only provides cellular focal
adhesions (in case of arginine-glycine-aspartic acid (RGD)-containing polymers)
but also facilitates cells to overcome gel restriction and fully spread out into their
natural morphology. 67-70 Wang et al. proposed an injectable hydrogel scaffold based
on encapsulation of cell-laden gelatin microspheres into a continuous matrix of
agarose hydrogel, which exhibited strong potential for cell conveyance and regen-
eration of bone and other tissues. 67,69,70 Considering the above mentioned approach
as traditional scaffold-based “top-down” strategy to create cellularized constructs,
“bottom-up” tissue fabrication methods using cell-laden microspheres as building
blocks are potentially more powerful tools to construct 3D hybrid constructs
comprising both cells and biomaterials. 71,72 Matsunaga et al. recently developed
a method for rapid construction of macroscopic 3D constructs using a large number
of monodisperse cell-laden collagen microspheres with monodispersity to assemble
into uniform and shape-specific tissues. 71 Similarly, Pautot et al. proposed a colloidal
superstructure based on monodisperse silica microspheres for 3D neuronal network
formation. These microsphere-based bottom-up strategies showed many advantages,
including (1) a large surface area provided by microspheres for cell adhesion and
further functionalization; (2) abundant interparticle cavities, allowing for nutrient
exchange in vitro and in vivo; and (3) ease of manipulation and transportation of
colloidal microspheres. 71,73
9.3.2 Micro- and Nanospheres as Functional Components
to Modify Mechanical Properties of Scaffolds
9.3.2.1 Use of Micro- and Nanospheres as Porogens By embedding micro-
spheres into the continuous matrix of bulk materials, spheres can serve as porogen to
introduce porosity into otherwise dense biomaterials (Fig. 9.2). A typical example of
this strategy is the incorporation of microspheres into calcium phosphate cements
(CPCs), which exhibit slow degradation rates in vivo and consequently a lack of