Tissue engineering - Biomedical Engineering Desk Reference - page 650

In addition, another material including PEG, oligo
[poly(ethylene glycol) fumarate] (OPF), has been de-
veloped and characterized ( Jo et al. , 2001 ). Because of the
chemical structure of this oligomer, it can be used to form
biodegradable hydrogels with a high degree of swelling ( Jo
et al. , 2001 ; Temenoff et al., 2003). This material may find
uses in guided tissue regeneration applications because it
demonstrates relatively low general cell adhesion, but at
the same time, possesses an ability to be modified with
peptides that could encourage adhesion of specific cell
types ( Shin et al. , 2002 ; Temenoff et al., 2003).
Poly(3-caprolactone) (PCL) as well as blends and co-
polymers containing PCL have also been studied as scaf-
fold materials ( Suggs and Mikos, 1996 ). Polyphosphates
and polyphosphazenes have been processed into scaffolds
for bone tissue engineering ( Behravesh et al. ,1999;Renier
and Kohn, 1997 ). Pseudo-poly(amino acids), in which
amino acids are linked by both amide and nonamide bonds
(such as urethane, ester, iminocarbonate, and carbonate),
are amorphous and soluble in organic solvents and thus
processable into scaffolds. The most studied among these
polymers are tyrosine-derived polycarbonates and poly-
acrylates ( James and Kohn, 1996 ). By structural modifi-
cations of the backbone and pendant chains, polymer
families with systematically and gradually varied proper-
ties can be created.

Table 7.1.4-2 Scaffold materials and their applications a

Materials Applications

Poly( a -hydroxy esters)

Poly( L -lactic acid) (PLLA) Bone, cartilage, nerve

Poly(glycolic acid) (PGA) Cartilage, tendon,

urothelium, intestine,
liver, bone

Poly ( D , L -lactic-co-glycolic acid)
(PLGA)

Bone, cartilage,
urothelium, nerve,
RPE

PLLA-bonded PLGA fibers Smooth muscle

PLLA coated with collagen or
poly(vinyl alcohol) (PVA)

Liver

PLLA and poly(ethylene glycol)
(PEG) block copolymer

Bone

PLGA and PEG blends Soft tissue and tubular

tissue

Poly( L -lactic acid-co- 3 -
caprolactone) (PLLACL)

Meniscal tissue, nerve

Poly( D , L -lactic acid-co- 3 -caprolactone)
(PDLLACL)

Vascular graft

Polyurethane/poly( L -lactic acid) Small-caliber arteries

Applications of scaffolds

Poly(lysine-co-lactic acid) Bone, cartilage, nerve

Poly(propylene fumarate) (PPF) Bone

Tissue induction

Poly(propylene fumarate-co-ethylene
glycol) [P(PF-co-EG)]

Cardiovascular, bone

Tissue induction is the process by which ingrowth of
surrounding tissue into a porous scaffold is effected
( Fig. 7.1.4-1 A). The scaffold provides a substrate for the
migration and proliferation of the desired cell types. For
example, an osteoinductive material can be used to se-
lectively induce bone formation. This approach has been
employed to regenerate several other tissues including
skin and nerve.

PPF/ b -tricalcium phosphate
(PPF/ b -TCP)

Bone

Poly( 3 -caprolactone) Drug delivery

Polyhydroxyalkanoate (PHA) Cardiovascular

Polydioxanone Bone

Polyphosphates and
polyphosphazenes

Skeletal tissue, nerve

Cell transplantation

Pseudo-poly(amino acids) Bone

The concept is that cells obtained from patients can be
expanded in culture, seeded onto an appropriate polymer
scaffold, cultured, and then transplanted ( Bancroft and
Mikos, 2001 )( Fig. 7.1.4-1 B). The time at which trans-
plantation takes place varies with a specific application.
Usually the cells are allowed to attach to the scaffold,
proliferate, and differentiate before implantation. A scaf-
fold for bone cell transplantation should be osteocon-
ductive, meaning that it has the capacity to direct the
growth of osteoblasts in vitro and allow the integration
of the transplant with the host bone. This strategy is
the most widely used in tissue engineering and has been

Tyrosine-derived polyiminocarbonates
Tyrosine-derived polycarbonate
Tyrosine-derived polyacrylates

a

Adapted from Babensee et al. (1998) .

Mikos, 2003 ). Results indicate that this hydrogel sup-
ported the proliferation, osteogenic differentiation and
matrix production from seeded bone-marrow stromal
cells during 28 days of in vitro culture ( Behravesh and
Mikos, 2003 ).