Page 238 - Biodegradable Polyesters

216 9 Environment-Friendly Methods for Converting Biodegradable Polyesters

7
44885 Kidney
6 5 Waiting list 15120 Liver
Number of patients × 10 5 4 3 Organ donors 3588 Lung
4159 Heart
2261 Kidney-pancreas
872 Pancreas
210 Heartlung
158 PancreasIslets
2
123 Intestin
1
Deaths 69057 Total

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

Figure 9.1 The need for tissue substitutes.
28 000 original articles and 6000 review articles. The number of TERM articles
continues to rise with nearly 4000 original articles published in 2010, compared
to a mere 360 a decade earlier, that is, a 10-fold increase in 10 years [6]!
Cells are often implanted or “seeded” into an artificial structure capable of
supporting three-dimensional tissue formation. These structures, typically called
scaffolds,are oftencritical, both ex vivo as well as in vivo, to recapitulating the
in vivo milieu and allowing cells to influence their own microenvironments.
Scaffolds usually serve at least one of the following purposes: (i) allow cell
attachment and migration, (ii) deliver and retain cells and biochemical factors,
(iii) enable diffusion of vital cell nutrients and expressed products, and (iv) exert
certain mechanical and biological influences to modify the behavior of the cell
phase.
How do the scaffolds work? The principle is illustrated in Figure 9.2.
Certain criteria were considered necessary for an ideal matrix for cell trans-
plantation. The matrix should be biocompatible, not inducing a tissue response in
the host, and completely resorbable, leaving a totally natural tissue replacement
following degradation of the polymer. The matrix should be easily and reliably
reproducible into a variety of shapes and structures that retain their shape when
implanted. As a vehicle for cell delivery, the matrix should provide mechanical
support to maintain space for tissue to form [7].
The interaction of the surface of the matrix with cells should support differen-
tiated cell function and growth and in certain situations should induce ingrowth
of desirable cell types from surrounding tissue.
Polymers in the group of polyesters, specifically the family of poly(lactic acid)
(PLA), poly(glycolic acid) (PGA), and copolymers of lactic and glycolic acids
(PLGAs), as well as poly(β-hydroxybutyric acid) (PHB) most closely fulfilled the
criteria outlined above, including biocompatibility, processability, and controlled
degradation [8]. These polymers, many descendant of absorbable suture materials
developed a couple of decades ago, were approved for in vivo use by the Food