Stem Cell-Based Tissue Engineering for Bone Repair - Tissue Engineering: Computer Modeling, Biofabrication and Cell Behavior - page 15

Ta b l e 1 Mechanical properties of polymer-based scaffolds [ 37 , 39 , 40 ]

Polymer Tensile strength (MPa) Modulus (GPa)

PLLA 10-60 0.35-2.8

PLLA-poly(ethylene glycol) 7-36 0.15-0.44

poly(lactide-co-glycolide) 20-40 2.7

poly(1,5-dioxepan-2-one) 27-39 0.16

Several copolymers of relevance to bone regeneration are poly(1,5-dioxepan),
PLLA-poly(ethylene glycol), and poly(lactide-co-glycolide).

Poly(lactide-co-glycolide) has been shown to induce bone regeneration, but its
poor mechanical properties makes it an ineffective material for bone scaffolds [ 4 ].
The PLLA-poly(ethylene glycol) copolymer is still being investigated. Modulat-
ing both components causes changes in the crystallinity of the scaffold, and it is
believed that this can be optimized to produce a structure that is similar to the crys-
talline structure of bone extracellular matrix, while also being hydrophilic, allowing
the attachment of cells and drugs to the scaffold [ 4 , 36 ]. Finally, poly(1,5-dioxepan)
is a polymer that is normally soft, giving it good elastic and degradation proper-
ties, but when copolymerized with lactide, results in a rigid crystalline structure.
Copolymerization with lactide improves the mechanical properties of the structure
and changes the hydrophilic nature of the scaffold as well [ 4 , 37 , 38 ]. Experiments
using poly(1,5-dioxepan-2-one) in different copolymer ratios in in-vitro cell culture
have shown that this polymer could be used as a scaffold for bone tissue engineering
[ 4 ]. Table 1 shows the mechanical properties of these polymer based scaffolds.
Therefore, as opposed to using one polymer to make up a scaffold, copolymeriza-
tion allows the creation of multiple reactive sites, with the potential for modulating
the chemical and mechanical properties of the scaffold depending on what medical
application is desired.
Although copolymerization allows the controlled modification of scaffold prop-
erties, several adverse effects of using synthetic polymers have been demonstrated
after implantation. The most relevant of these effects is the interaction of the poly-
mers with the host's immune system. Implantation of the polymers described above
have caused immune reactions in some patients ranging from the release of large
numbers of white blood cells, to resorption of the original tissue [ 4 , 41 ]. Therefore,
greater understanding of the relationship between biocompatibility of a material be-
fore implantation, and its mechanical adaptation when placed inside the body needs
to be further investigated. Proper sterilization of the material without compromising
biomechanical stability, in addition to possessing properties that inhibits fibrotic tis-
sue formation and immune reaction will help to create a synthetic biomaterial that
can be used safely for degradable tissue engineering scaffolds in bone [ 4 , 41 ].