Biomedical Engineering Reference
In-Depth Information
TABLE 1.1
Scaffold Design Parameters for Bone Tissue Engineering [4]
Parameters
Requirements
Porosity
Maximum possible without compromising mechanical
properties
Pore size
200-400 µm
Pore structure
Interconnected
Mechanical properties of the cancellous bone
Tension and compression
Strength: 5-10 MPa
Modulus: 50-100 MPa
Mechanical properties of the cortical bone
Tension
Strength: 80-150 MPa
Modulus: 17-20 GPa
Compression
Strength: 130-220 MPa
Modulus: 17-20 GPa
Fracture toughness: 6-8 MPa
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Degradation properties
Degradation time
Must be tailored to match the application in patients
Degradation mechanism
Bulk dissolution in medium
Biocompatibility
No chronic infl ammation
Sterilizability
Sterilizable without altering material properties
deleterious
in vivo
due to lack of vascularization. Once the engineered tissue construct is placed in
the body, vascularization becomes a key issue for further remodeling in the
in vivo
environment.
Thus, angiogenesis is an essential step in the colonization of macroporous biomaterials during
osteointegration. Capillaries bring osteoprogenitor cells and the nutriments that are required for
their growth. They transport especially numerous angiogenic growth factors [8].
The main critical factors affecting bone formation are the pore size and pore interconnection
of the scaffold. Pore size is related to the
in vivo
bone tissue ingrowth, allowing migration and
proliferation of osteoblasts and mesenchymal cells, and matrix deposition in the empty spaces [9].
Pore interconnection provides the channel for cell distribution and migration allowing effi cient
in vivo
blood vessel formation. An incomplete pore interconnection could limit blood vessels
invasion. Small pore size could obstruct cell adhesion and bone ingrowth. Bone vascularization,
besides providing nutrients essential for tissue survival, plays also a crucial role in coordinating
the activity of bone cells and their migration for new bone formation [10].
Several studies have investigated the minimum pore size required to regenerate mineralized
bone. The minimum requirement for pore size is considered to be around 100 µm due to cell size,
migration requirements, and transport. However, pore sizes
300 µm are recommended due to
enhanced growth rate of a new bone and the formation of capillaries [3,4,11]. Pore size in the range
of 300-500 µm would promote vascularization and mass transportation of nutrients and waste
products, while the scaffold would maintain good mechanical integrity during
in vitro
culture and
in vivo
transplantation [12].
It is equally important to notice that tissue-engineering scaffolds should have enhanced biologi-
cal functions. Therefore, the incorporation of growth factors, such as bone growth factors (BGF)
and vascularization growth factors (VGF), or specifi c peptide sequences into the scaffolds or on
their surface is being considered as part of the integral design of scaffolds. Moreover, to improve
cell attachment and growth, the surface of scaffolds' struts needs to be pretreated (a process called
surface functionalization) [13-15]. The design of the surface properties of scaffolds is an important
step to achieve their successful
in vitro
and
in vivo
applications. A few approaches to surface modi-
fi cation of scaffolds are discussed below.
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