Biomedical Engineering Reference
In-Depth Information
of connective tissue, bone tissue engineering has emerged with the most clinical suc-
cess (3, 4, 4-7, 7-9). Currently, bone is the most transplanted tissue, second only to
blood transfusions, with approximately 500,000 cases occurring annually in the United
States (10).
Bone tissue engineering is aimed at developing implantable substitutes to replace the
use of autograft and allograft treatments. At present, autografts and allografts are most
commonly used for bone grafting. Autografts are ideal based on their high acceptance
rate within the body and ability to become integrated into the skeletal system by being
osteoinductive, osteoconductive, and having osteogenic properties (11). Osteoinductive
refers to the graft's ability to attract surrounding mesenchymal stem cells into the area
of repair that can then become a source of osteoblasts, while osteoconductive refers to
the facilitation of vascularization and the orientation of haversian canal systems (10).
Osteogenic potential implies that osteoprogenitor cells are present in the graft itself (10).
However, autografts have also been associated with multiple problems including donor
site morbidity (12-14), chronic pain, nerve damage, infection, fracture, pelvic instabil-
ity, hematoma, and tumor transplantation (15). Allografts negate these concerns but
have their own limitations such as carrying the risk of causing an immune response
in the host, transferring diseases to the host (16), storing and transplanting of the allo-
graft, and/or weakening of the allograft's biological and mechanical properties during
the storage and transplant process that would have made it an ideal replacement for bone
constructs (17). The limitations of autografts and allografts have led to the use of tissue
engineered constructs for bone grafts. Scaffolds are a key component for developing
a tissue engineered bone construct for implantation into a critical bone defect. Ideally,
a scaffold should have the following characteristics for successful implantation: (1) be
biocompatible and bioresorbable with a controlled degradation rate to match cell/tissue
growth in vivo ; (2) have mechanical properties capable of withstanding the mechani-
cal loads experienced in the physiological environment during cell matrix maturation;
(3) be three-dimensional and allow for adequate diffusion for cell growth, nutrient deliv-
ery, and waste removal; and (4) have suitable surface chemistry for cell attachment,
proliferation, and differentiation (18). In order to accommodate all of these qualities,
a diverse portfolio of materials, fabrication techniques, and modifications have been
implemented over the years to achieve successful skeletal integration for use in clinical
applications.
Recently, investigators have focused on the use of natural or renewable materials as
a scaffolding choice over synthetically derived options. The main driving force for the
use of naturally renewable materials is that these materials are highly biocompatible,
biodegradable, offer chemical functionality (which is desirable for cell processes such
as attachment, migration, and differentiation), and provide a cheap and replenishing
source of material. Thus, renewable scaffolding materials can be defined as materials
that can be obtained from natural resources including plant, fungal, animal, or bacterial
derivation. Typically, these materials are some form of secondary product and must
undergo some chemical treatment and sterilization process before end use. Two reviews
by Mano et al . and Malafaya et al . have addressed the overall status of these types of
materials in tissue engineering, hence, this chapter will specifically focus on their use in
bone tissue engineering applications with in vitro or in vivo examples (19, 20).
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