Biomedical Engineering Reference
In-Depth Information
research efforts in many laboratories, no other degrad-
able polymers are currently used to any significant extent
in the formulation of degradable sutures.
A temporary barrier has its major medical use in
adhesion prevention. Adhesions are formed between two
tissue sections by clotting of blood in the extravascular
tissue space followed by inflammation and fibrosis. If this
natural healing process occurs between surfaces that
were not meant to bond together, the resulting adhesion
can cause pain, functional impairment, and problems
during subsequent surgery. Surgical adhesions are a sig-
nificant cause of morbidity and represent one of the most
significant complications of a wide range of surgical
procedures such as cardiac, spinal, and tendon surgery. A
temporary barrier could take the form of a thin poly-
meric film or a meshlike device that would be placed
between adhesion-prone tissues at the time of surgery. To
be useful, such as temporary barrier would have to pre-
vent the formation of scar tissue connecting adjacent
tissue sections, followed by the slow resorption of the
barrier material ( Hill et al. , 1993 ). This sort of barrier has
also been investigated for the sealing of breaches of the
lung tissue that cause air leakage.
Another important example of a temporary barrier is
in the field of skin reconstruction. Several products are
available that are generally referred to as ''artificial skin''
( Beele, 2002 ). The first such product consists of an ar-
tificial, degradable collagen/glycosaminoglycan matrix
that is placed on top of the skin lesion to stimulate the
regrowth of a functional dermis. Another product con-
sists of a degradable collagen matrix with preseeded
human fibroblasts. Again, the goal is to stimulate the
regrowth of a functional dermis. These products are used
in the treatment of burns and other deep skin lesions and
represent an important application for temporary barrier
type devices.
An implantable drug delivery device is by necessity
a temporary device, as the device will eventually run out
of drug or the need for the delivery of a specific drug is
eliminated once the disease is treated. The development
of implantable drug delivery systems is probably the
most widely investigated application of degradable
polymers ( Langer, 1990 ). One can expect that the future
acceptance of implantable drug delivery devices by
physicians and patients alike will depend on the avail-
ability of degradable systems that do not have to be
explanted surgically.
Since PLA and PGA have an extensive safety profile
based on their use as sutures, these polymers have been
very widely investigated in the formulation of implant-
able controlled release devices. Several implantable,
controlled release formulations based on copolymers of
lactic and glycolic acid have already become available.
However, a very wide range of other degradable polymers
have been investigated as well. Particularly noteworthy is
the use of a type of polyanhydride in the formulation of
an intracranial, implantable device for the administration
of BCNU (a chemotherapeutic agent) to patients suf-
fering from glioblastoma multiformae, a usually lethal
form of brain cancer (Chasin et al., 1990).
The term tissue engineering scaffold will be used in
this section to describe a degradable implant that is
designed to act as an artificial extracellular matrix (ECM)
by providing space for cells to grow into and to reorganize
into functional tissue ( James and Kohn, 1996 ).
It has become increasingly obvious that manmade
implantable prostheses do not function as well as the
native tissue or maintain the functionality of native tissue
over long periods of time. Therefore, tissue engineering
has emerged as an interdisciplinary field that utilizes
degradable polymers, among other substrates and bi-
ologics, to develop treatments that will allow the body to
heal itself without the need for permanently implanted,
artificial prosthetic devices. In the ideal case, a tissue
engineering scaffold is implanted to restore lost tissue
function, maintain tissue function, or enhance existing
tissue function ( Langer and Vacanti, 1993 ). These scaf-
folds can take the form of a feltlike material obtained
from knitted or woven fibers or from fiber meshes. Al-
ternatively, various processing techniques can be used to
obtain foams or sponges. For all tissue engineering scaf-
folds, pore interconnectivity is a key property, as cells
need to be able to migrate and grow throughout the
entire scaffold. Thus, industrial foaming techniques,
used for example in the fabrication of furniture cushions,
are not applicable to the fabrication of tissue engineering
scaffolds, as these industrial foams are designed contain
''closed pores,'' whereas tissue engineering scaffolds re-
quire an ''open pore'' structure. Tissue engineering scaf-
folds may be preseeded with cells in vitro prior to
implantation. Alternatively, tissue engineering scaffolds
may consist of a cell-free structure that is invaded and
''colonized'' by cells only after its implantation. In either
case, the tissue engineering scaffold must allow the for-
mation of functional tissue in vivo, followed by the safe
resorption of the scaffold material.
There has been some debate in the literature as to the
exact definition of the related term ''guided tissue re-
generation'' (GTR). GTR is a term traditionally used in
dentistry. This term sometimes implies that the scaffold
encourages the growth of specific types of tissue. For
example, in the treatment of periodontal disease, peri-
odontists use the term ''guided tissue regeneration''
when using implants that favor new bone growth in the
periodontal
pocket
over
soft-tissue
ingrowth
(scar
formation).
One of the major challenges in the design of tissue
engineering scaffolds is the need to adjust the rate of
scaffold
degradation
to
the
rate
of
tissue
healing.
Depending
upon
the
application
the
scaffold,
the
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