Biomedical Engineering Reference
In-Depth Information
13.3
Scaffolds
Three-dimensional scaffolds with a specific interconnecting porosity are of crucial
importance for the purpose of tissue generation and engineering. The mechanical
properties of the construct, its shape and its material properties are designed such as
to prevent an initial invasion of the surrounding tissues (if the scaffold is implanted
for tissue generation in vivo) and to integrate with the rest of the tissues at the
appropriate time. The internal surface of the scaffold provides the cells with the
necessary space to migrate, to attach, to survive and proliferate, and eventually to
differentiate into the needed cell phenotype. At the same time, the voids within the
scaffold, due to their initial porosity and as a result of a desired degradation process,
provide the space where new tissue formation and initial remodeling of such tissue
as well as vascularization within this tissue occur.
A wide variety of materials and designs has already been available both for exper-
imentation and in clinical use. We shall present here a brief summary of the key
issues that relate to the design of a scaffold for a particular application. A recent
review on the subject is available [
408
]. These issues are: material(s), porosity and
architecture, surface chemistry and topography, mechanical properties, degradation
kinetics, and fabrication techniques.
13.3.1
Materials
In a few cases, metals, such as titanium, tantalum and some alloys, have been used
as scaffolds (see Chaps.
6
,
7
and
8
). These metallic scaffolds (except magnesium
ones) and some ceramic ones do not degrade and remain permanently at the implant
site. However, while their stability may provide durability in certain settings, there is
an intrinsic disadvantage for their use. Their presence may stress shield the adjacent
newly produced tissue which in turn may result in tissue loss, mechanical failure at
the interface and, in case removal is deemed necessary due to infection for example,
the whole implant, scaffold and newly formed tissue may be lost.
Tissue-derived materials such as allograft bone, skin, intestinal submucosa,
xenogenic or allogenic heart valves, have also been used as scaffolds. All of them
have to become decellularized and treated in a way, that only the matrix remains,
thus creating an architecturally desirable porous structure for that particular sit-
uation. These scaffolds are essentially made of the
extracellular matrix
(ECM).
However, some decellularized scaffolds implanted in humans demonstrated a strong
inflammatory response and structural failure [
409
].
Natural biomaterials including collagen I, gelatine, silk protein (fibroin), fibrin,
alginate, hyaluronan, chitosan (chitin), and extracellular matrix models (Matrigel),
as well as plan-derived biopolymers, for example soy-based or starch-based, are
utilized.
Synthetic water-insoluble polymers or copolymers are also used. Poly(L-lactic
acid)(PLLA), poly(DL-lactic acid)(PDLLA), poly(lactic-co-glycolic acid)(PLGA),
