Despite technical and regulatory challenges, the prospects for tissue engineering
are good.
Tissue and organ failure, produced as a result of injury or other type
of damage, is a major health problem, accounting for about half of the total
annual expenditure in health care in the US1. Treatment options
include transplantation (human or xenotransplantation), surgical repair, artificial
prostheses, mechanical devices, and in a few cases, drug therapy. Ultimately,
however, major damage to a tissue or organ can neither be repaired nor long-term
recovery effected in a truly satisfactory way by these methods.
Tissue engineering is emerging as a significant potential alternative or
complementary solution, whereby tissue and organ failure is addressed by implanting
natural, synthetic, or semisynthetic tissue and organ mimics that are fully
functional from the start, or that grow into the required functionality. Initial
efforts have focused on skin equivalents for treating burns, but an increasing
number of tissue types are now being engineered, as well as biomaterials and
scaffolds used as delivery systems. A variety of approaches are used to coax
differentiated or undifferentiated cells, such as stem cells, into the desired
cell type. Notable results include tissue-engineered bone, blood vessels,
liver, muscle, and even nerve conduits. As a result of the medical and market
potential, there is significant academic and corporate interest in this technology.
Historical perspective An important component in the early development of tissue engineering was
the parallel development of artificial biomaterials. In the mid-1960s, artificial
skin for burn victims was being pursued as a symptomatic therapy2,
and later, synthetic fibers were being tried as artificial skin grafts for
burn treatment3. In the early 1970s, there were concerted efforts
to treat artificial surfaces to be used in implants in ways that would enable
them to avoid causing blood coagulation, by applying special heparin complex
coatings, for example4. Other efforts focused on the toxicology
profiles and biocompatibility of a variety of organic polymers considered
for implants or tissue engineering5, and the development of
novel gels as the basis for artificial skin6. In the late 1970s,
researchers experimented with collagen-based artificial skin for use in oral
mucosa injuries7.
In the 1980s, R&D in tissue engineering and biomaterials took off.
As part of this interest, several biomedical engineering departments were
established at major universities around the world. In 1981, a skin equivalent
consisting of a silicone cover over a sponge of porous collagen cross-linked
with chondroitin was used successfully to treat severe burns8.
In this decade, several products reached the market. Interpore's Pro-Osteon
coral-derived bone graft material was introduced in 1993. In 1996, Integra's
Artificial Skin was approved for as an in vivo, nonbiological tissue regeneration
product. Then, in 1998, the General and Plastic Surgery Devices Advisory Panel
to the US Food and Drug Administration recommended unconditional approval
of Apligraf (Graftskin) Human Skin Equivalent for the treatment of venous
leg ulcers. Apligraf, produced by Organogenesis, is the first manufactured
living human organ, specifically multilayered skin, to be recommended for
approval by an advisory panel to the FDA. Apligraf was approved for the treatment
of venous leg ulcers in Canada in 1997, and was launched there in August 1997
by Novartis Pharmaceuticals Canada (Dorval, Canada).
Current state Table 1 lists selected companies involved in tissue
engineering. Interestingly, the types of collaborations involving tissue engineering
companies can differ slightly from those done between traditional drug discovery
companies and big pharma. For example, LifeCell develops tissue grafts for
transplantation and the preservation of transfusable blood products. The company
has developed engineered porcine heart valves for replacement surgery in humans
in a 1993 partnership with Medtronic (Minneapolis, MN).
Table 1. Selected companies with tissue engineering programs.
Medtronic, a leading medical device company with a cardiac surgery business,
offers a complete line of mechanical and tissue prosthetic heart valves, and
the alliance with LifeCell gives it access to a significant tissue-engineered
product to complement its own lineup. This type of alliance illustrates an
alternate route to the big pharma path for commercializing tissue engineering
products—that is, through more specialized medical device companies.
Part of the interest and support for tissue engineering comes from the
armed forces, in that numerous battlefield-related medical applications exist
for tissue-engineered products and biomaterials. For example, Advanced Tissue
Sciences had part of its clinical trial for Dermagraft-TC in the treatment
of chemical burns funded by the US Army Institute of Chemical Defense. Dermagraft-TC
is an engineered human dermal tissue combined with a synthetic epidermal layer.
It covers and protects burns, helping to minimize infections and retain fluids
until a sufficient amount of the patient's own skin is available for
autologous grafting. The principal alternative is cadaver skin, but the problems
here include a limited supply, acute immunological rejection, and potential
pathogen transmission.
Another application of tissue-engineered products is in the toxicology
testing and in vitro markets, as alternatives to certain types of animal testing.
A good example of this approach was the acquisition in 1995 by Stratum Laboratories
(La Jolla, CA) of the In Vitro Laboratory Testing (IVLT) business of Advanced
Tissue Sciences. Stratum received license rights to manufacture and sell Skin2
in vitro laboratory testing kits, in addition to an option to extend rights
for up to six additional tissues. Skin2 is living human skin tissue used to
test skin care, household, chemical, and pharmaceutical products for a variety
of indications. The material is already approved by the US Department of Transportation
and Transport Canada for use in a corrosivity test, demonstrating significant
regulatory acceptance of this technology.
The use of progenitor-type cells as the starting point for developing differentiated
tissue material is the focus of considerable research. For example, it is
possible to take mesenchymal stem cells that reside in the adult bone marrow
and induce them to differentiate into chondrocytes by using specific tissue
culture media that include transforming growth factor 9.
Chondrocytes are constituents of cartilagenous tissue, and the possibility
of generating them in a controlled fashion creates possibilities for the development
of appropriate cartilage tissue for surgical procedures. Stem cells are also
used as a starting point for a multitude of other cell types used in tissue
engineering.
Wound repair is a key application for tissue engineering products. Although
most applications focus on the use of artificial skin to treat burns, different
disease conditions are also benefiting. For example, Advanced Tissue Sciences'
Dermagraft is a three-dimensional human neonatal dermal fibroblast culture
that has been grown on a biodegradable scaffold and cryopreserved. It has
been applied to foot ulcers that develop as a side effect of long-term diabetes.
In clinical trials, significant healing occurred with this material, especially
when the Dermagraft cells were alive and functioning properly10.
Finally, increasing effort is being focused on correlating the actual physics
of engineered cells that are to be used therapeutically. One recent report
describes how the pressure that is applied to chondrocytes transplanted into
articular cartilage defects is likely to inhibit the growth of these cells11. Thus, the actual mechanics of the environment into which bio-engineered
cells are used needs to be carefully considered to ensure that optimal benefit
is derived from these approaches.
Industry challenges Quality control of the materials used in various surgical applications
is a key challenge for the tissue engineering industry. For example, living
human cells are being used in scaffolds to repair structural tissue damage.
These materials need to be produced and cultured under good manufacturing
practice (GMP) conditions to meet FDA standards—especially the cells
that are grown ex vivo. As a result, the tissue engineering industry is striving
to create appropriate quality control standards and evaluate them.
A good example is that of autologous cultured chondrocytes used to repair
knee damage. Recently, Genzyme Tissue Repair reported on a quality control
program for this material that was based on evaluating 303 patients who had
received the material12. The program was based on the analysis
of several quantifiable parameters, including viability and freedom from pathogens;
results showed that the materials were, indeed, appropriate for their use,
and demonstrated one way to follow up with quality control monitoring of tissue-engineered
products.
Another challenge concerns acquiring a fundamental understanding of tissue
differentiation mechanisms that might be harnessed for the development of
tissue-engineered products. One product that the tissue engineering industry
is pursuing is "bone-on-demand," to be used in cases where new
bone formation is needed. An important component here is the bone morphogenetic
protein (BMP) complex, which is capable of inducing extraskeletal bone formation
at a concentration 1,000-fold lower than each of its constituents alone. Researchers
are trying to concentrate BMP complex locally, using appropriate implants
rather than the individual constituents, which could potentially lead to bone
formation13.
Finally, the industry is challenged to develop tissue-engineered products
for a number of surgery-related applications, such as vasculature. For example,
a recent report describes the generation of a tissue-engineered blood vessel
without any synthetic or exogenous materials. The vessels were produced by
wrapping a sheet of human vascular smooth muscle cells around a tubular support,
followed by wrapping a sheet of human fibroblasts around the first sheet.
After cell maturation, the tubular support was removed and endothelial cells
were seeded in the lumen of the putative blood vessel. The vessel showed all
the necessary key markers of activity, and also had a burst strength comparable
to human vessels. In short-term grafts in a dog model, this engineered vessel
demonstrated good handling and saturability characteristics14.
The future Tissue engineering has significant market potential and financial investment
continues apace. A 1997 survey15 of the field reported that
in that year alone, R&D expenditure directly linked to corporate tissue
engineering projects was about $0.5 billion, with a growth rate of about 22%
per year. This demonstrates the sustained interest in this area, driven in
part by positive results regarding specific products and processes in clinical
settings.
Technical advances in the various components of the industry will contribute
to market growth. One component is the availability of biomaterials that act
as scaffolds for tissue repair and reconstruction, or for the deposition of
engineered tissues and cells preceding implantation. An increasing amount
of R&D is directed toward addressing the properties of these scaffolds
with the goal of creating materials that have the desired functional profiles
for various applications.
For example, so-called blended-polymer scaffolds have an extended lifetime
in the body that is more suitable for orthopedic applications than nonblended
scaffolds16. Another study has shown how collagen-based scaffolds,
used to grow keratinocytes in artificial skin preparations, can be manipulated
by cross-linking collagen with glycosaminoglycan17. The result
was increased biological stability, which augmented the likelihood that the
keratinocytes would "take" and grow out of the scaffold. In yet
another application18, a sacchitin glycolipid-type membrane
prepared from the residue of a fungal fruiting body has been shown to have
significant promise in animal models of skin damage as a skin substitute that
facilitated wound healing and fibroblast growth. Development of new materials
of this type that enable different applications of tissue engineering is likely
to be the focus of considerable future research.
For the biological component of tissue engineering, rapid advances are
being made in identifying new cell types for use in tissue regeneration. For
example, undifferentiated stem cells are attracting intense interest because
of their capacity to be transformed into almost any cell type that may be
needed, and even fat cells can be directed to produce appropriate tissues
(see Table 1). In addition, promising artificial nerve
grafts or nerve guidance channels are being developed for nerve regeneration19.
In the future, efforts will likely increasingly focus on the development
of tissue-engineered products under consensus safety and efficacy standards,
including sourcing of cells and tissues, characterization and testing of the
materials, quality assurance and control, and preclinical and clinical evaluation20. The FDA has already provided some regulatory guidance concerning
specific materials, such as certain marketed artificial skin products; in
the next few years, these guidelines will likely be increasingly formalized
and structured, ensuring that tissue engineering products not only work but
are also safe.
Significant future developments will include the continued development
of artificial organs that use cells embedded into appropriate support structures.
A recent report describes the use of polyurethane foam as a good matrix into
which liver cells grew as spheroids, the system showing promise as an artificial
liver21.
The future will also see significant efforts to develop engineered vascular
grafts. An approach that will see increasing attention is that of taking a
scaffold that is a structurally intact xenogeneic vessel, such as a pig aorta,
removing all cells, and re-populating this with human autologous cells. A
recent report showed how this could be done in 2−3 weeks, opening up
the way for a good alternative to vascular engineering22.
The range of human tissue that can be engineered will also increase dramatically
in the future, so in addition to the traditional targets, such as skin and
liver, other tissues and organs will see their day. A great deal of excitement
in clinical circles is that of developing artificial human thyroid tissues
which are capable of producing T cells, and this will be a major area for
continued R&D23.
Finally, stem cells and their manipulation for therapeutic purposes will
continue to be a major area of development, because of the pluripotency of
these cells. For example, bone marrow stem cells contained in resorbable artificial
tubes have been shown to lead to effective healing of non-union defects in
rabbit radii24, and this opens up significant surgical alternatives
to organ and tissue damage.
Conclusions Tissue engineering is emerging as a vibrant industry with a huge potential
market. The biomaterials, scaffolds, artificial organs, and differentiating
cells that are combined to create a tissue engineering product address significant
medical needs, such as major tissue and organ damage or failure. The industry
faces numerous technical challenges, not the least of which is the establishment
of a consensus quality control program to ensure that tissue engineering products
work and are safe to use. Efforts to address these issues are underway, and
if past success is any indication, this technology is certainly one that will
have a major impact in future health care practice.
Reprinted from Nature Biotechnology 17, 508−510
(1999).
