Biomedical Engineering Reference
In-Depth Information
along with predictable degradation rates, makes PGA attractive for cartilage engineering experi-
ments [
352
-
354
]. Past studies have also shown that PGA promotes more proteoglycan synthesis
than other materials like collagen or poly-glycolide/lactide copolymers [
355
]. While the predictable
degradation profile of PGA is often seen as a positive trait, it can also cause problems for tissue engi-
neering applications that require scaffold integrity longer than a few months. For these applications,
other polymers have been investigated.
Another alpha polyester polymer used extensively in the medical field is poly-lactic acid (PLA),
which like PGA, has been approved by the FDA for implantation in humans. PLA generally degrades
slower than PGA with a total degradation time ranging from twelve months to over two years [
356
].
Again, the loss of mechanical properties and scaffold integrity occurs prior to this, which could
cause an engineered construct to fail prematurely. As with PGA, the degradation products of PLA
are resorbable, making it an attractive, biocompatible material for implantation. PLA exists in two
stereoisometric forms, giving rise to four different types of PLA: poly-D-lactide, poly-L-lactide,
poly-D,L-lactide, and poly-meso-lactide [
357
]. Applications of the PLA isomers range from drug
delivery to suture materials. However, the D and L monomers polymerize to form semicrystalline
structures that have been investigated as possible scaffolds for cartilage engineering. PLA scaffolds
are primarily made as non-woven meshes due to the previous success of this structure for neocartilage
formation. Studies have shown that chondrocytes might not have as great of an affinity for PLA
surfaces as PGA surfaces, but, over time, total cell numbers on the two materials are similar [
358
].
Due to its slower degradation rate, PLA scaffolds allow more time for matrix formation before
catastrophic loss of mechanical integrity. This is important for applications where the scaffold has
to bear loads for a significant period after implantation.
Poly-lactic-co-glycolic acid (PLGA) is a copolymer composed of PGA and PLA monomers.
The material properties of PLGA are dependent on the ratio of each monomer included in the
macromolecule. For example, a formulation with a large fraction of PLA will degrade slower than
one with a large fraction of PGA. Characterization of a 75/25 (PLA/PGA) copolymer of showed
a degradation time of 4-5 months, whereas a 50/50 copolymer degraded in only 1-2 months [
356
].
As with its base components, PLGA degrades into molecules that naturally resorb in the body.
General biocompatibility has been investigated in large and small animal models, as well as in clinical
trials [
346
,
359
]. PLGA has been used extensively as a suture material due to its high tensile strength
and controllable degradation rates. It can be fabricated in forms similar to PGA and PLA [
360
], with
the non-woven mesh being among the more preferred structures for recent cartilage engineering
studies.
Another popular synthetic polymer for cartilage engineering is poly-caprolactone (PCL).
This polymer possesses longer degradation times than PGA/PLA/PLGA and is generally stronger,
making it attractive for many orthopaedic applications [
351
]. PCL can be extruded into threads for
meshes/felts or formed into porous scaffolds through a salt-leaching process. As with other polyesters,
PCL degrades through hydrolytic scission, but this process can take one or two years to completely
degrade the material [
356
]. While this means the scaffold remains at the implant site, it can also help
