Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
FIgurE 18.1 Schematic showing the principle steps of tissue engineering using a biomaterial scaffold. (a) A
porous scaffold material is seeded with cells, (b) maintained in a bioreactor for cell proliferation and ECM produc-
tion, and (c) eventually the cell-scaffold construct can be implanted at the site requiring tissue replacement.
This requires suitable biomaterials and their development can be described by three generations of
materials (Rabkin and Schoen 2002, Schoen 2004). The first generation included industrial materials
that had properties compatible with the structure to be replaced as well as bio-inertness , that is, induced
a minimal response from the host tissue to the implant. This was followed by bioactive materials that
allowed for controlled interactions with the host tissue, for example, degradable materials for which the
interface between implant and host tissue is gradually eliminated. The materials of the third genera-
tion are designed for interactions on a molecular level, that is, functional materials. This development
has been a prerequisite for tissue engineering in which biomaterial scaffolds are implanted at the tissue
replacement site.
The scaffold forms a supporting structure to guide cell arrangement and initial tissue development.
It can either be seeded with cells for growth and differentiation in vitro prior to implantation, as shown
in Figure 18.1, or designed to attract endogenous cells in vivo . After cell seeding, the scaffold is main-
tained in a so-called bioreactor (cf. Figure 18.1), providing mechanical support for the scaffold and an
environment with nutritional supply for the cells. In the bioreactor, the cells are able to proliferate,
produce extracellular matrix (ECM), and eventually the construct can be implanted at the desired site.
After implantation, further cell proliferation, cell differentiation, ECM production, and gradual scaffold
degradation can take place in vivo .
18.1.2 Scaffold Properties and Synthesis
It is crucial but far from trivial to design a tissue engineering scaffold that has suitable structure and
provides an appropriate biological environment for the desired type of cells. To achieve this goal, several
parameters need to be considered:
• The material structure and morphology should be favorable for cell adhesion and ingrowth. Many
scaffolds consist of porous material structures or fiber networks, making a three-dimensional
arrangement of cells possible.
• Interconnectivity between pores in the scaffold structure is necessary for transport of nutrition,
oxygen, and waste products. In addition, it facilitates tissue vascularization, that is, the build-up
of a blood vessel structure.
• The mechanical properties of the scaffold should be similar to those of the tissue or organ intended
for regeneration.
• The chemical/biochemical properties of the scaffold should be favorable for cell adhesion, prolif-
eration, differentiation, and ECM production.
• The scaffold material should be biocompatible, that is, no inflammatory or immune response
should be induced after implantation.
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