Biomedical Engineering Reference
In-Depth Information
oxygen, nutrients, and other water-soluble meta-
bolites [
These highly swollen structures retain high
permeability for oxygen and other water-
soluble nutrients and metabolites. Their use as
injectable scaffolds for bone and tissue repair
is particularly interesting because, when
injected into an irregularly shaped defect, they
can readily wet all surfaces of the injured site
and create a low-density aqueous cavity that
contains all the components necessary for bone
and tissue regeneration. If the spreading of the
fl uid also promotes adhesion to the surfaces of
the defect cavity, the hydrogel is likely to protect
the defect surfaces from unwanted soft tissue
that contains undesirable cellular elements,
maintaining at the same time an osteoconduc-
tive and osteogenic-like environment within
the scaffold. Under these conditions, new tissue
can form at the old bone-tissue interface and
on the skeletal network of the scaffold. Regen-
erated trabecular bone formed under these
conditions seems to mesh cleanly with the orig-
inal bone structure, with no visible transition
between the old and the new bone, and a fi nal
structure that is close to that of the original
bone (Fig.
]. If properly designed, natural
and synthetic hydrogel scaffolds can function
biomimetically, exhibit biocompatibility, and
cause
78
,
92
minimal
infl ammatory
responses,
thrombosis, and tissue damage [
].
An ideally designed hydrogel scaffold will
behave like an extracellular matrix with an
aqueous “matrix” to encapsulate the osmoti-
cally active components and provide a mechan-
ical fi berlike network that supports extant
mechanical stresses. Hydrophilic and hydro-
philic/hydrophobic polymers absorb large
quantities of water. By modifying the hydro-
philic/hydrophobic ratio of the polymer, one
can control the concentration of the aqueous
phase over a wide range. The mechanical fi ber-
like network that determines the viscoelastic
behavior of the hydrogel is generated by either
a permanent, covalently cross-linked structure
(an irreversible hydrogel) or a nonpermanent,
hydrogen-bonded skeletal network (a revers-
ible hydrogel) [
34
,
35
,
78
]. Appropriate viscoelastic
characteristics can be developed at polymer
concentrations as low as a few weight percent.
92
7
.
3
) [
28
].
Figure 7.3.
Histological sections of
the untreated and treated defects at 4
weeks. (A) Untreated, (B) treated with
commercial gel, (C) treated with silk
fibroin hydrogel, (D) treated with silk
fibroin hydrogel with full recovery. Red
arrows indicate the interface between
old bone (OB) and new bone (NB).
(A) The formation of NB in the untreated
cavity was restricted to the edge of the
defect. (B) Newly formed bone was gen-
erated radially inward from the defect
surface, leaving a distinct interphase
between OB and NB. (C) Newly formed
bone was generated radially inward
from the defect surface, with no notice-
able interface between OB and NB.
(D) Defect cavity completely filled with
NB with no noticeable interface between
OB and NB. Unpublished figures from
Fini, Motta, Torricelli, Giavaresi, Aldini,
Giardino, and Migliaresi, based on work
described in Fini et al. [28].
