Artificial Scaffolds and Mesenchymal Stem Cells for Hard Tissues - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 165

form a triple-helix containing three separate peptide strands twisted around one
another. Collagens show high-tensile strength that makes them an essential
structural factor providing mechanical strength to hard tissues. Collagens carry
ligands supporting cell attachment and thus influence cell migration but also dif-
ferentiation. In addition, these ligands (reactive functional groups such as hydroxy
or amino) along the collagen backbone also enable interaction with bioactive
molecules, i.e., growth factors, provided by drug releasing systems. Collagen
composite materials developed in particular for bone regeneration include various
combinations of fibrillar collagen with HA, polysaccharides, polyethylene glycol,
cellulose derivatives or sodium hyaluronates. Chitosan fibrous scaffolds obtained
by wet spinning were coated with different densities of type II collagen to evaluate
the effect of this coating on MSC adhesion and chondrogenesis. The cell attach-
ment and distribution after seeding correlated with the density of type II collagen.
Cell number, matrix production, and expression of genes specific for chondro-
genesis were improved after culture in collagen-coated chitosan constructs [ 97 ].
Mauney and colleagues performed in vitro and in vivo studies of differentially
demineralized bone scaffolds using biologically-derived collagenous materials
such as intestinal submucosa or demineralized bone matrix as substrate to facilitate
the growth and differentiation of cells [ 98 ]. Beside collagen, its denaturated
derivative gelatine is used to prepare scaffold composites. Thus osteogenic dif-
ferentiation of bone marrow-derived SCs could be demonstrated using mixed
gelatine and chitosan-oligosaccharide scaffolds [ 99 ].
Synthetic polymers. Tissue-derived materials carry the risks of immune rejec-
tion, blood coagulation or tissue hypertrophy, and thus synthetic polymers are a
very attractive alternative. Synthetic polymer scaffolds provide the opportunity to
tailor physical properties such as molecular weight, molecular weight distribution,
and correlated mechanical properties. Major challenges are the design of 3D
architectures with defined porosity and a tailor-made surface adapted to specific
requirements concerning cell adhesion.
Synthetic scaffold materials for bone tissue engineering mainly comprise
polyesters, the most common being poly(lactic acid) (PLA), poly(glycolic acid)
(PGA), and poly(caprolactone) (PCL). In addition to homo-polymers, a huge
variety of synthetic co-polymers has been studied in the last two decades, and
recently reviewed by Zippel and colleagues [ 100 ]. In general, the polymers
themselves are biocompatible, and many of them are bioresorbable. Further
requirements include injectability and biodegradability. For bone regeneration,
scaffolds have to possess appropriate mechanical stability. Biodegradation rate and
mechanical properties can be varied through variation of molecular weight and
molecular weight distribution. Depending on the polymer synthesis methods,
linear polymers, branched structures, and 3D networks can be prepared. One of the
remaining problems is the formation and accumulation of a certain amount of
degradation products in a short time period due to bulk degradation. Although the
degradation products (e.g., lactic and glycolic acids) are also present in normal
metabolic pathways, these amounts may result in local inflammation.