Contents
1. Introduction
2. Materials in Medicine, Their Classification, and
Definition of Terms
2.1. Biomaterials
2.2. Classification of Biomaterials Based on Material-
Tissue Interactions
2.3. Design Factors and General Requirements of
Biomaterials
2.4. Nanotechnology Effects on Biomaterials
Development
3. Biomaterials-based Tissue Engineering and
Regenerative Solutions to Musculoskeletal Problems
3.1. Tissue Engineering for Articular Cartilage
3.1.1. Scaffolds
3.1.2. Cell Source
3.1.3. Signaling Factors
4. Conclusions
References
Materials scientists and engineers have been able to
develop new materials and modify physical, chemical,
mechanical, electronic, magnetic and optical properties
of new or existing materials to meet the ever increasing
demands for more advanced and/or tailor-made materials
for specific applications. These developments in materials
science and materials technology have been central to
many technological advances in all areas of our modern
civilization including communication, transportation,
energy, construction, aerospace, defense, and health care
sectors. Researches on interrelation between structure,
processing, properties and service conditions of materials
intended to be utilized in biological applications have
also been increasing in number and funds to further
increase the quality of lives of human beings by providing
novel materials or modifying the properties of existing
ones with more functionality and biocompatibility.
Biomaterials have already being used in a range of
established medical applications, including implants to
replace diseased joints, surgical-repair materials such as
sutures and repair meshes, and tissue such as breast
implants. For these established products, continuing R&D;
will improve key requirements, such as more durable joint
implants. Further developments in biomaterials' design
and biocompatibility will enable production of novel
implant structures. Biomaterials having properties that
enhance drug delivery and provide technologies for
alternative delivery routes and release mechanisms make
a significant contribution in the fast-growing field of
drug-delivery systems (DDS). Finely tuned drug delivery is
becoming a reality with the support of biomaterials,
particularly for the growing range of protein therapeutics
emerging from research in genomics and proteomics.
The efforts are not limited to those, and new materials
that would help repair or regenerate natural organs have
also been under continuous development by research
teams consisting of materials scientists, chemists,
biotechnologists, engineers and clinicians. These studies
have led to the development a multi/interdisciplinary
field named as tissue engineering (TE).
For the successful tissue regeneration and repairing, it
is indispensable to provide cells with a local environment
which enables them to efficiently proliferate and
differentiate, resulting in cell-induced tissue regeneration.
Biomaterials play an important role in the creation of this
regeneration environment in terms of the cells scaffold of
artificial extracellular matrix (ECM) and the delivery
technology of bio-signaling molecules to enhance the
cells potential for tissue regeneration with, in some
instances, utilization of stem cell technology. In addition,
biomaterials give cells culture conditions suitable for their
in vitro proliferation and differentiation to obtain a large
number of cells with a high quality for cell transplantation
therapy. Cells can be genetically engineered to activate
the biological functions by using the non-viral carrier of
biomaterials1.
Articular cartilage, also known as hyaline cartilage, is
a tough, extremely smooth, elastic tissue that covers the
ends of bones in joints, enabling the bones to move
smoothly over one another. Since articular cartilage has
no direct blood supply, when it is damaged through
injury or a lifetime of use, it does not heal as effectively
as other tissues in the body. Instead, the damage tends to
spread, allowing the bones eventually to rub directly
against each other and resulting in pain and reduced
mobility. Significantly damaged cartilage may progress to
a more serious condition, such as osteoarthritis (OA). It is
estimated that as many as two million Americans damage
the cartilage in their knee each year, often through
sports, traumatic injuries and work injuries. Additionally,
an estimated 27 million Americans suffer from
osteoarthritis (OA), also known as “wear-and-tear”
arthritis, a chronic degenerative joint disease characterized
by the breakdown of the joint's articular cartilage2.
The knee is the most common joint affected by
osteoarthritis, affecting millions of people all over the
world. OA of the knee has a significant impact on daily
living and it is one of the five leading causes of disability
among elderly men and women. The risk for disability
from osteoarthritis of the knee is as great as that from
cardiovascular disease.
Conventional treatments of articular cartilage
(medication, autografting, and total knee replacement)
are in temporary in nature, hence put additional burden
on health care systems and economies in all countries.
The impetus behind the researches on the regenerative
solutions to musculoskeletal problems lies in the above
factors.
This review aims to provide a brief introduction to the
types of materials in medicine (biomaterials), and to
design factors and general requirements of biomaterials,
and attempts to sum up the recent advances in engineering
articular cartilage, one of the most challenging area of
study in biomaterials based tissue engineering as an
example to the research on regenerative solutions to
musculoskeletal problems.
2. Materials in Medicine, Their Classification,
and Definition of Terms
Biomaterials applications were as far back as ancient
Phoenicia where loose teeth were bound together with
gold wires for tying artificial ones to neighboring teeth.
Bone plates were successfully implemented to stabilize
bone fractures and to accelerate their healing in the early
1900's. While by the time of the 1950's to 60's, blood
vessel replacement were in clinical trials and artificial
heart valves and hip joints were in development3.
Engineering materials are classified in three main
groups: Metals, ceramics and polymers. A fourth group is
often added to this classification, composite materials
which are made by combining at least two of the three
main classes. The properties and characteristics of
materials play important roles in almost every modern
engineering design, providing problems as well as
opportunities for new invention, and setting limits for
many technological advances4. Medical technology is
no exception to this statement, and many engineering
materials in all four groups have found applications as
well in medicine and dentistry. Any material that is used to make reliable, economic and physiologically acceptable
devices to substitute a part or a function of the body can
be assessed as a “biomaterial”.
2.1. Biomaterials
In the scope of this review, a biomaterial is any “synthetic
material” that is used to replace or restore function to a
body tissue and is continuously or intermittently in contact
with body fluids or living tissue5.
This description excludes many materials used for
devices such as surgical and dental instruments although
they are exposed to body fluids; they are not used to
replace part of a living system or to function in intimate
contact with living tissue.
Materials used for external prostheses or devices; such as
hearing aids and artificial limbs, are also excluded from the
above definition of biomaterials as the skin acts as a barrier
with the external world and hence, they are not exposed to
body fluids although they are in contact with the skin.
These materials are referred as “artificial materials”.
Materials of biological origin are referred as “biological
or natural materials”, and wood, skin, artery, collagen
and bone are common examples for such materials.
Materials in medicine can then broadly be classified as: i)
Biological materials; and ii) Synthetic Biomaterials. Biological
materials can be further classified into soft and hard tissue
types. In accord to the above definitions, synthetic materials
are further classified into: a) Metallic; b) Polymeric; c) Ceramic;
and d) Composite biomaterials, Table 1 shows these classifications
and some examples for each class6,7.
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Table 1: Classification of materials in medicine and representative examples |
Biomaterials are placed within the interior of the
body as implied in its definition of exposure to body fluids. Thus, a biomaterial must be biocompatible,
nontoxic and noncarcinogenic, as well as they must have
adequate physical and mechanical properties to fulfill
their anticipated use. Many available engineering
materials are not qualified as biomaterials due to the
requirement of placement within the body interior.
A number of devices and materials such as tooth fillings,
needles, sutures, bone plates, catheters, etc. are used in the
treatment of disease or injury8,9 and Table 2 provides a
brief listing of synthetic materials used for implantation.
Trauma, degeneration and diseases often make
surgical repair or replacement necessary. When a person has a joint pain the main concern is the relief of pain and
return to a healthy and functional life style. This usually
requires replacement of skeletal parts that include knees,
hips, finger joints, elbows, vertebrae, teeth, and repair of
the mandible. The worldwide biomaterials market is
valued at close to $24x109. Orthopedic and dental
applications represent approximately 55% of the total
biomaterials market. Orthopedics products worldwide
exceeded $13 billion in 2000, an increase of 12 percent
over 1999 revenues. Expansion in these areas is expected
to continue due to number of factors, including the
ageing population, an increasing preference by younger
to middle aged candidates to undertake surgery,
improvements in the technology and life style, obesity, a
better understanding of body functionality, improved
aesthetics and need for better function3.
2.2. Classification of Biomaterials Based on
Material-Tissue Interactions
It is a fact that no foreign material placed within a
living body is completely compatible. The only substances
that conform completely are those manufactured by the
body itself (autogenous) and any other substance that is
recognized as foreign, initiates some type of reaction
(host-tissue response). Thus, when a synthetic material is
placed within the human body, tissue reacts towards the
implant in a variety of ways depending on the material
type. The mechanism of tissue interaction, if any, depends
on the tissue response to the implant surface. In general,
a biomaterial may be described in or classified in three
groups based on the tissues responses10. These are
bioinert, bioresorbable, and bioactive, which are well
covered in range of review papers11-13.
2.2.1. Bioinert Biomaterials
The term bioinert refers to any material that once
placed in the human body has minimal interaction with
its surrounding tissue. Examples of these include stainless
steel, titanium, alumina, partially stabilized zirconia
(PSZ), and ultra high molecular weight polyethylene
(UHMWPE). Generally a fibrous capsule might form
around bioinert implants hence its biofunctionality relies
on tissue integration through the implant.
2.2.2. Bioactive Biomaterials
Bioactive refers to a material, which upon being
placed within the human body interacts with the
surrounding bone and in some cases, even soft tissue. This
occurs through a time dependent kinetic modification of
the surface, triggered by their implantation within the
living bone. An ion exchange reaction between the
bioactive implant and surrounding body fluids results in
the formation of a biologically active carbonate apatite
(CHAp) layer on the implant that is chemically and
crystallographically equivalent to the mineral phase in bone. Prime examples of these materials are synthetic
hydroxyapatite [Ca10(PO4)6(OH)2, glass ceramic A-W and
bioglass.
2.2.3. Bioresorbab le Biomaterials
Bioresorbable refers to a material that upon placement
within the human body starts to dissolve (resorbed) and
slowly replaced by advancing tissue (such as bone).
Common examples of bioresorbable materials are
tricalcium phosphate [Ca3(PO4)2] and polylactic-
polyglycolic acid copolymers. Calcium oxide, calcium
carbonate and gypsum are other common materials that
have been utilized since 1970's.
2.3. Design Factors and General Requirements of
Biomaterials
Biomaterials are used in the treatment or management
of a disease, condition, or injury to improve human
health by restoring the function of natural living tissues
and organs in the body. Therefore, a sound understanding
of relationships among the properties, functions, and
structures of biological materials is essential. From this
point, biological materials, implant materials, and
interaction between these two in the body are the three
aspects of the study of biomaterials.
Even in the preliminary stages of this field, surgeons
and engineers identified materials and design problems
that resulted in premature loss of implant function
through mechanical failure, corrosion or inadequate
biocompatibility of the component. Key factors in a
biomaterial usage are its biocompatibility, biofunctionality,
and availability to a lesser extent.
Biomaterials must have special properties that can be
tailored to meet the needs of a particular application. For
example, a biomaterial must be biocompatible, noncarcinogenic,
corrosion-resistant, and has low toxicity and
wear9,14. However, depending on the application,
differing requirements may arise. Sometimes these
requirements can be completely opposite. In tissue
engineering of the bone, for instance, the polymeric
scaffold needs to be biodegradable so that as the cells
generate their own extracellular matrices, the polymeric
biomaterial will be completely replaced over time with
the patient's own tissue. In the case of mechanical heart
valves, on the other hand, we need materials that are
biostable, wear-resistant, and do not degrade with time.
Materials such as pyrolytic carbon leaflet and titanium
housing are used because they can last at least 20 years
or more.
Metals are generally very stiff and have high fracture
toughness. In sharp contrast to the metals are the
polymers, which have low stiffness and fracture toughness.
However the polymers have high elongation to failure.
The high stiffness of metals, on the other hand, can be a
disadvantage since this can give rise to “stress shielding” in bone fracture repair. Stress shielding is a phenomenon
where bone loss occurs when a stiffer material is placed
over the bone. Bone responds to stresses during the
healing process. Since the stress is practically shielded
from the bone, the density of the bone underneath the
stiffer material decreases.
United States Food and Drug Administration (FDA)
approval should be obtained for the materials to be used
in vivo. A new material is subjected to a series of
biocompatibility tests in order to be FDA approved.
Biocompatibility requirements in general include acute
systemic toxicity, cytotoxicity, hemolysis, intravenous
toxicity, mutagenicity, oral toxicity, pyrogenicity and
sensitization. There are stringent data and documentation
requirements for all tests. Good Manufacturing Practice
(GMP) should be adhered to and this requires production
should be carried out in completely isolated clean rooms,
which is a manufacturing cost increase factor yet an
indispensible requirement.
Mechanics and dynamics of tissues and the resultant
interactions between them are also an important area,
known as biomechanics. Many sophisticated analyses can
be made through the use of finite element modeling and
analysis (FEM and FEA). These approaches, for example,
help to design better prosthesis or customize them for a
particular application.
It is imperative that we should know the fundamentals
of materials before we can utilize them properly and
efficiently. Meanwhile, we also have to know some
fundamental properties and functions of tissues and
organs. The interactions between tissues and organs with
manmade materials have to be more fully elucidated.
Fundamentals-based scientific knowledge can be a great
help in exploring many avenues of biomaterials research
and development.
The study of the relationships between the structure
and physical properties of biological materials is as
important as that of biomaterials, but traditionally this
subject has not been treated fully in biologically oriented
disciplines. This is due to the fact that in these disciplines
workers are concerned with the biochemical aspects of
function rather than the physical properties of materials.
Research on developing new biomaterials is an
interdisciplinary effort, often involving collaboration
among materials scientists and engineers, biomedical
engineers, biotechnologists, pathologists, and clinicians
to solve clinical problems. The design or selection of a
specific biomaterial depends on the relative importance
of the various properties that are required for the
intended medical application. Physical properties that are
generally considered include hardness, tensile strength,
modulus, and elongation; fatigue strength, which is
determined by a material's response to cyclic loads or
strains; impact properties; resistance to abrasion and
wear; long-term dimensional stability, which is described by a material's viscoelastic properties; swelling in aqueous
media; and permeability to gases, water, and small
biomolecules. In addition, biomaterials are exposed to
human tissues and fluids, so that predicting the results of
possible interactions between host and material is an
important and unique consideration in using synthetic
materials in medicine. Two particularly important issues
in biocompatibility are thrombosis, which involves blood
coagulation and the adhesion of blood platelets to
biomaterial surfaces, and the fibrous-tissue encapsulation
of biomaterials that are implanted in soft tissues.
Poor selection of materials can lead to clinical problems.
One example of this situation was the choice of silicone
rubber as a poppet in an early heart valve design.
The silicone absorbed lipid from plasma and swelled sufficiently
to become trapped between the metal struts of
the valve. Another unfortunate choice as a biomaterial
was PTFE (Teflon), which is noted for its low coefficient of
friction and its chemical inertness but it has relatively poor
abrasion resistance. Thus, as an occluder in a heart valve or
as an acetabular cup in a hip-joint prosthesis, PTFE may
eventually wear to such an extent that the device would
fail. In addition, degradable polyesterurethane foam was
abandoned as a fixation patch for breast prostheses,
because it offered a distinct possibility for the release of
carcinogenic by-products as it degraded.
Besides their constituent polymer molecules, synthetic
biomaterials may contain several additives, such as
unreacted monomers and catalysts, inorganic fillers or
organic plasticizers, antioxidants and stabilizers, and
processing lubricants or mold-release agents on the
material's surface. In addition, several degradation
products may result from the processing, sterilization,
storage, and ultimately implantation of a device. Many
additives are beneficial; for example, the silica filler that
is indispensable in silicone rubber for good mechanical
performance or the antioxidants and stabilizers that
prevent premature oxidative degradation of
polyetherurethanes. Other additives, such as pigments,
can be eliminated from biomedical products. In order to
achieve a “medical-grade” biomaterial, the polymer may
need to be solvent-extracted before use, thereby
eliminating low-molecular-weight materials. Generally,
additives in polymers are regarded with extreme suspicion,
because it is often the additives rather than the constituent
polymer molecules that are the source of adverse
biocompatibility15.
2.4. Nanotechnology Effects on Biomaterials
Development
Nanotechnology is a rapidly evolving field that
involves material structures on a size scale around 100 nm
or less. New areas of biomaterials applications may
develop using nanoscale materials or devices. For
example, drug delivery methods have made use of a microsphere encapsulation technique. Nanotechnology
may help in the design of drugs with more precise
dosage, oriented to specific targets or with timed
interactions. Nanotechnology may also help to reduce
the size of diagnostic sensors and probes. Transplantation
of organs can restore some functions that cannot be
carried out by artificial materials, or that are better done
by a natural organ. For example, in the case of kidney
failure many patients can expect to derive benefit from
transplantation because an artificial kidney has many
disadvantages, including high cost, immobility of the
device, maintenance of the dialyzer, and illness due to
imperfect filtration. The functions of the liver cannot be
assumed by any artificial device or material. Liver
transplants have extended the lives of people with liver
failure. Organ transplants are widely performed, but their
success has been hindered due to social, ethical, and
immunological problems. Since artificial materials are
limited in the functions they can perform, and transplants
are limited by the availability of organs and problems of
immune compatibility, there is current interest in the
regeneration or regrowth of diseased or damaged tissue.
Tissue engineering refers to the growth of a new tissue
using living cells guided by the structure of a substrate
made of synthetic material. This substrate is called a
scaffold. The scaffold materials are important since they
must be compatible with the cells and guide their growth.
Most scaffold materials are biodegradable or resorbable
as the cells grow. Most scaffolds are made from natural or
synthetic polymers, but for hard tissues like bone and
teeth ceramic materials such as calcium phosphate
compounds can be utilized. The tissue is grown in vitro
and implanted in vivo. There have been some clinical
successes in repair of injuries to large areas of skin, or
small defects in cartilage. Following section is a discussion
on tissue engineering for finding solutions to
musculoskeletal health problems, an area of current
research activity.
3. Biomaterials-based Tissue Engineering and
Regenerative Solutions to Musculoskeletal
Problems
Loss of human tissues or organs is a devastating
problem for the individual patient. Despite technological
advances in biomaterials engineering the need for organ
and tissue replacement is on rise.
Current technology for organ and tissue replacement
has limitations. These include donor scarcity, adverse
immunological response from the host tissue,
biocompatibility, infection, pathogen transfer, and high
cost to patient. Then, there is the perennial deficiency of
synthetic material to provide the multifunctional
requirement of organ. For example, bone is not just a structural element but also a “factory to produce bone
marrow”. These limitations prompt scientists worldwide
to consider alternative technologies, amongst which
tissue engineering has been heralded as the promising
answer. This is considered as a paradigm shift from
“finding replacements/substitutes to human tissue” to
“trying to have the lost human tissue re-grow”.
The term “tissue engineering” has now come to
encompass a wide range of strategies employing cells,
synthetic and processed natural materials, tissues,
cytokines and genes for the regeneration of tissue in vivo
or the production of tissue in vitro. Cell therapies and
tissue transplant procedures are thus now often
considered under the rubric of tissue engineering16.
The aim of tissue engineering is to restore tissue and
organ functions with minimal host rejection. This arose
from the need to develop an alternative method of
treating patients suffering from tissue loss or organ
failure. TE has been heralded as the new wave to
revolutionize the healthcare-biotechnology industry. It is
a multidisciplinary field and involves the integration of
engineering methods and principles, basic life sciences,
and molecular cell biology towards the fundamental
understanding of structure-function relationships in
normal and pathological mammalian tissue and the
development of biological substitutes to restore, maintain,
or improve tissue function
The success of tissue engineering lies in five key
technologies. They are namely: a) Biomaterials; b) Cells; c)
Scaffolds; d) Bioreactors; and e) Medical imaging
technology. It may seem simple to produce a one-off,
tissue-engineered product in the laboratory, but it is a
completely different matter to produce hundreds of
products of consistent quality for clinical use.
Fundamentally, TE involves a scaffold which acts as a
temporary extracellular matrix for the cells to adhere to,
differentiate and grow. Breakthrough has been made in
the development of a platform technology which
integrates medical imaging, computational biomechanics,
biomaterials, and advanced manufacturing to produce
three-dimensional porous load bearing scaffolds for
tissue engineering of bone17. The technology makes
use of polycaprolactone (PCL) bioresorbable polymer and
Fused Deposition Modeling's (FDM) rapid prototyping
advanced manufacturing fabrication process to produce
the scaffolds without a mold18. Controlled three
dimensional architecture with interconnected pores
enables good cells entrapment and facilitates easy flow
path for nutrients and waste removal, and demonstrates
long-term cell viability. Patient-specific scaffolds can now
be made using this technology. This biomaterial processing
technology has paved the way for patient-specific tissue
engineering concepts not dreamed of a few years ago.
Scaffolds are temporary materials structured in such a
way as to guide cell growth in vitro or in vivo. Cell growth
in a scaffold (matrix) can be facilitated by other biological
entities such as growth factors. Scaffold materials, cells,
and soluble cell regulators have to work together to
achieve optimal regeneration of tissues and organs. So
called “TE triad” is shown in Figure 1.
In this triad, the cell is the main character to generate
tissue. Native, stem or allogenic cells produce ECM to
constitute tissue. Porous scaffolds of adequate structural
strength made from absorbable biomaterials provide the
habitation for the cells. Cell behavior is controlled by the
environmental factors including chemical, physical and
biological variables and substances in the culture system19.
Materials for the scaffolds used in TE includes a
variety of absorbable or nonresorbable synthetic and
natural materials that are used together with a variety of
cells and regulators for tissue regeneration.
Another approach to the tissue or organ replacement
is reproduction of the whole organ or body by cloning
from somatic cells rather than reproductive cells. This is
how “Dolly” the sheep was reproduced. The spare body
part concept may not be acceptable in humans due to
ethical as well as technical problems. Some application
examples20 of TE are given in Table 3.
3.1. Tissue Engineering for Articular Cartilage
One major research area of TE is the cartilage tissue of
joints. The degeneration of articular cartilage and
associated arthritis (osteoarthritis and rheumatoid
arthritis) is among the most prevalent chronic condition
in all over the world. In osteoarthritis, the cartilage
covering the joint gradually wears away, exposing the
bone which in turn makes joint motion difficult and
painful. The condition is overwhelmed by two major
drawbacks: First, the regenerative capacity of cartilage
tissue is limited due to the sparse population of
chondrocytes, reduced presence of progenitors and the
avascular nature of the tissue. Second, the repaired tissue
that is formed is a combination of hyaline and fibrocartilage
that has poor mechanical properties when
compared to native articular cartilage and also tend to
degrade over time. Therefore, one early clinical finding in
1743 by Hunter on articular cartilage that “once destroyed,
is not repaired” has not been much changed21.
Conventional treatment methods of osteoarthritis or
damaged cartilage tissue includes: drug therapy that
often involves anti-inflammatory drugs and analgesics
(first line of treatment); surgical intervention/manipulation
wherein autologous chondrocyte implantation (ACI),
subchondral drilling, mosiacplasty, and allografts are
introduced (second line of treatment); and finally total
knee replacement which involves the replacement of the
arthritic knee with an artificial knee (third line of
treatment). The first line of treatment provides only
symptomatic relief. The second line of treatment often
leads to the formation of fibrocartilage that is
mechanically inferior to articular cartilage. The third line
of treatment is a potential solution, however, it involves
a major surgery and the implants are very costly. Besides,
the life of a knee replacement implant is in the range of
10-15 years.
TE provides an opportunity to overcome the limitations
associated with conventional treatment methods of
cartilage tissue loss. As stated, TE triad includes a
scaffolding system, tissue specific or progenitor cells and
growth factors (cell signaling molecules). The scaffolding
system is central to TE strategy as they provide cells with
a surface for adherence and 3D growth. Scaffolds can also
be used as a reservoir for growth factors that can be
delivered locally for a specific duration at a suitable rate.
Cartilage tissue is made up of a small population of
cartilage cells (chondrocytes) and largely extra-cellular
matrix (ECM) that is in turn mainly made up of type II
collagen and glucoaminoglycans (GAGs). These ECM
components are fibrous in nature and have diameters in
nanometer scales. From biomimetic approach, studies are
concentrated to develop nanofibrous 3D scaffolds that
mimic the type II collagen and GAG fibrils22. The
nanofibrous scaffolds can be fabricated using the
electrospinning technique that involves the application of a high voltage field (up to 10 kV/cm) to a polymer at the
tip of a needle by virtue of its viscosity. The polymer
solution is then provided with a voltage potential that in
turn provides charge to the polymer solution. As the
potential gradually increased the charge density on the
polymer solution increases and eventually leads to
columbic repulsion. When the repulsive forces exceed the
viscous forces of the polymer, a jet ensues from the tip of
the needle that initially has a straight path and then
undergoes instabilities to traverse a spiral path with
increasing diameter. This trajectory of the jet allows for
continuous thinning of the polymer jet as well as
evaporation of the solvent from the jet, eventually
leading to the formation of charged dry nanofibers that
are collected on a grounded metallic collector. The fibers
obtained using the electrospinning technique can be
altered both in terms of morphology and diameter via
modification in the fabrication parameters. The
morphology can vary from elliptical bead containing
fibers to smooth fibers and the diameters can range from
10's to 1000's nanometers. In one approach it is proposed
to use the nanofibrous scaffolds as growth factor delivery
system, wherein the growth factors will be linked to the
scaffolding system covalently using a linker molecule. This
system when implanted into an arthritic knee will be
exposed to proteases (enzymes that selectively cleave
specific bonds) that will cleave the covalent bond that
connects the growth factor to the nanofibrous scaffold,
thereby leading to the release of the growth factor. It is
expected that the released growth factor will then
provide the necessary signaling to enable enhanced cell
proliferation and function. Some nanofibers systems can
also be applied to other applications such as filter media,
sensors, electrically conducting nanofibers, optical
applications, material reinforcement, protective clothing
and cosmetics23.
3.1.1. Scaffolds
Biomaterials are used for promoting cartilage repair
by providing scaffolds for cell attachment, growth and
differentiation and could act as vehicles for protein and
gene delivery to regenerate functional tissue. For
cartilage, biomaterials should have several properties to
support viable repair.
Numerous scaffolding materials have been used for
cell delivery in cartilage regeneration. The primary focus
has been on both natural and synthetic polymers, in a
variety of forms24. These include hydrogels, sponges,
and woven or non-woven fibrous meshes as shown in
Figure 2. Scaffold architecture plays a major role in
dictating cellular behavior; therefore, a scaffold
architecture that mimics the natural environment may
facilitate the growth of seeded chondrocytes. A layered
agarose scaffold with such depth-dependent nonhomogeneity has been designed for good in vitro
regeneration of cartilage from chondrocytes25.
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Figure 2: Examples of different scaffold architectures used in
the engineering of cartilage tissues24 |
Scaffolds provide a 3D environment that is desirable
for the production of cartilaginous tissue. Ideally the
scaffold should: 1) have directed and controlled
degradation, 2) promote cell viability, differentiation, and
ECM production, 3) allow for the diffusion of nutrients
and waste products, 4) adhere and integrate with the
surrounding native cartilage, 5) span and assume the size
of the defect, and 6) provide mechanical integrity
depending on the defect location24. Scaffold
degradation can occur hydrolytically or enzymatically,
and by controlling degradation temporally and spatially,
scaffolds can enhance and direct new tissue growth. For
example, scaffolds with degradable and non-degradable
units show improved ECM distribution compared to
completely non-degradable scaffolds26 However, a
balance must be found since slow degradation may
impede new cartilaginous ECM production, while fast
degradation may compromise structural support and
shape retention. For instance, Solchaga et al.27 showed
that scaffolds with slower degradation rates yielded
cartilage of greater thickness in an osteochondral defect
model, but cracks and fissures were evident on the
cartilage surface.
Natural materials that have been explored to produce
scaffolds for cartilage engineering to date include:
collagen, fibrin, hyaluronic acid (HA), alginate, agarose,
gelatin, chitosan, chondroitin sulfate, silk, and cellulose28-35. Advantages of natural materials are that many of
them are natural bodily constituents that provide a
natural adhesive surface for cells and carry the required
information for their activity. Furthermore, the
degradation products are physiological ones and therefore
non-toxic. The major disadvantages of natural polymers
include sourcing, processing and possible disease
transmission. In addition, natural polymers may be
inferior mechanically and subject to variable enzymatic
host degradation24,36.
Synthetic polymers are more controllable and predictable,
where chemical and physical properties of a polymer
can be modified to alter mechanical and degradation
characteristics (design flexibility), and eliminates disease
transmission. Synthetic polymers currently explored for cartilage repair include: polyα-hydroxy esters such as
polylactic acid (PLA) and polyglycolic acid37,38. These
polymers have been approved for clinical use in the USA
and are manufactured for routine hospital or surgical use.
They are readily made into scaffolds for tissue engineering,
in the form of foam or woven or nonwoven fiber
mesh. Products of these polymers have much better
mechanical strength than those of natural substances,
which makes it easier for them to be fixed to the recipient
site, and makes them more resistant to the friction of
joint motion39. Their copolymers allow adjustment of
the degradation rate of the scaffold. This is important
because the residence time of the implanted polymer
must be sufficient to serve its scaffold purpose, but not so
long as to impede tissue regeneration. Other polymers of
interest include poly(ethylene glycol)-terephthalate, poly
(butylene terephthalate), poly (ethylene glycol) fumarate,
poly (N-isopropylacrylamide), polyurethanes and carbon
fiber scaffolds40-43. Disadvantages of synthetic polymers
are those that, unless specifically incorporated, they
do not benefit from direct cell-scaffold interactions,
which can play a role in adhesion, cell signaling, directed
degradation, and matrix remodeling. In addition, degradation
byproducts may be toxic or elicit an inflammatory
response. Excellent reviews outlining the advantages and
disadvantages of different scaffold structures and scaffold
materials are available elsewhere19,24,44, to
which interested readers are directed.
3.1.2. Cell Source
The optimal cell source for cartilage tissue engineering
is still being identified. Chondrocytes, fibroblasts, stem
cells, and genetically modified cells have all been explored
for their potential as a viable cell source for cartilage
repair45,46. Chondrocytes are the most obvious choice
since they are found in native cartilage and have been
extensively studied to assess their role in producing,
maintaining, and remodeling the cartilage ECM.
Fibroblasts are easily obtained in high numbers and can
be directed toward a chondrogenic phenotype47,48.
More recent studies have focused on stem cells, which
have multi-lineage potential and can be isolated from a
plethora of tissues. These progenitor cells can be
expanded through several passages without loss of
differentiation potential. Additionally, all of these cells
can be modified genetically to induce or enhance
chondrogenesis. The goal is to find an ideal cell source
that can be easily isolated, is capable of expansion, and
can be cultured to express and synthesize cartilagespecific
molecules (e.g., type II collagen and aggrecan).
3.1.3. Signaling Factors
As the third component of the tissue engineering
triad, stimulating factors have been employed to induce, accelerate, and/or enhance cartilage formation. For
instance, growth factors and other additives may be
added to culture media in vitro or incorporated into
scaffolds for in vivo delivery to control cellular
differentiation and tissue formation. Regardless of the
nature of the cells, standard culture conditions require
the presence of serum, basically of bovine origin. The risk
of undesired pathogen transmission has been debated
when the cells are implanted to humans. Autologous
serum-supplemented culture medium has become the
state of the art for ACI, but serum-free culture is more
attractive49. The avascular condition of natural cartilage
does not suggest that serum is needed to support the
chondrocytes. One study has even indicated that serum
hinders the chondrogenic ability of chondrocytes50.
Serum-free culture is worthy of further development to
develop regenerated cartilage for clinical application.
In addition, gene therapy has emerged as another
method of local delivery, where cells can be engineered
to over-express bioactive molecules. An additional
approach is the introduction of mechanical signals
through loading regimes such as hydrostatic or dynamic
compression or through the use of bioreactors. Since
many types of cartilage depend on mechanical forces to
maintain healthy function, this approach has been used
to alter cellular differentiation and tissue production.
4. Conclusions
The impact of biomaterials on further enhancement
of human health seems to be substantial. The success in
regenerative medicine appears to lie in the developments
of biomaterials based tissue engineering. Cell source,
scaffolds, and signaling factors make up the tissue
engineering triad. One of the biggest challenges for
cartilage tissue engineering is cell source. Current studies
on alternatives to chondrocytes are increasing, and the
potential and limitations of fibroblasts and stem cells are
being investigated. Novel biomaterials are being
continuously developed and are leading to distinctive
interactions with cells through controlled biomaterial
chemistry, structure, and the addition of biological
molecules. However, sequences and concentration of
growth factors which are needed to optimize cartilage
regeneration are not well developed. The incorporation
of stimulatory factors such as bioactive molecules, gene
therapy, mechanical loading, and bioreactors are being
studied to enhanced cartilage production.
Much closer matching of synthetic scaffolds to the
natural extracellular matrices can be achieved by
development novel biomaterials that meet specific
mechanical property requirements and cell-specific
interactions via high-throughput, analytical technologies
combined with recent advances in genetic engineering.
A successful tissue engineered cartilage product
should make it through the regulatory processes.
Although there are a number of patented cartilage
products, and some companies have already applied for
FDA approval; most of the studies on biomaterials were
performed using mostly young adult or even fetal animal
cells, and not with cells from elderly osteoarthritis
patients. All of the approaches without consideration of
clinical translation and feasibility needs would be, to
some extent, futile activity. In due course, extensive
research will be needed to determine whether the results
can be extended to the human situation. Therefore,
professionals working in this multidisciplinary area have
assumed huge responsibility. Expectedly within five years
or so, the results and findings of research teams, consisting
of an appropriate combination of engineers, clinicians
and basic science researchers, will lead to the development
of novel biomaterials and devices that will help improve
the quality of human lives, if not yet for the creation of
“The Six Million Dollar Man”.
Conflict of Interest
No conflict of interest is declared by the author.
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