Transcript Degradable polymers for tissue engineering

Introduction to
Biomedical Engineering
Apr 29th 2014
Introduction to
Tissue Engineering
Ming-Long Yeh
葉明龍
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1 Stem cells
2 Morphogenesis, generation of tissue in the embryo
3 Tissue homeostasis
4 Cellular signaling
5 The extracellular matrix as a biologic scaffold for tissue engineering
6 Natural polymers in tissue engineering applications
7 Degradable polymers for tissue engineering
8 Degradation of bioceramics
9 Biocompatibility
10 Cell source
11 Cell culture: harvest, selection, expansion, and differentiation
12 Cell nutrition
13 Cryobiology
14 Scaffold design and fabrication
15 Controlled release strategies in tissue engineering
16 Bioreactors for tissue engineering
17 Tissue engineering for skin transplantation
18 Tissue engineering of cartilage
19 Tissue engineering of bone
20 Tissue engineering of the nervous system
21 Tissue engineering of organ systems
22 Ethical issues in tissue engineering
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The central tissue engineering paradigm
4
Introduction
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Tissue Engineering
It generally involves the use of materials
and cells with the goal of trying to
understand tissue function and some day
enabling virtually any tissue or organ on
the body to be made de novo重新.
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Introduction
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A foundation for the understanding and
development of the cell-based systems needed
for tissue engineering.
One of these areas involves the cells
themselves; include
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stem cell biology and cell sources, and
various aspects of cell culture including harvesting,
selection, expansion, and differentiation.
Other important themes involving cells, such as
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cell nutrition and cryobiology and cellular signaling.
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Introduction
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There are a variety of very important materials issues in
tissue engineering.
Cells adhere to the extracellular matrix material in the
body; this matrix has an enormous affect on how the
cells behave.
However, to try to recreate extracellular matrixes, is a
difficult task and therefore various materials have been
explored to provide substrates for cell growth in vivo .
The physical and chemical properties of these materials
are important.
These include natural polymers, degradable polymers,
and bioceramics.
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Introduction
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It also examines the important issue of tissue
compatibility and biomaterials compatibility
which are critical if these tissues are to be safe
and integrate with the body.
In addition, scaffold design and fabrication are
discussed.
Ways of using controlled release from materials
is examined;
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controlled release of different factors (e.g. growth
factors to promote vascularization) can provide an
important means of controlling and improving tissue
function.
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Introduction
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Once the scaffold system and the cell
system are developed, they have to be put
together.
This is generally done through a bioreactor
where the cells and materials are
combined with the right type of media and
flow conditions inside the reactor.
By combining all of these entities, a new
tissue may be created.
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Introduction
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Overall, this book provides a very useful guide for those
who wish to understand important issues such as
 cell biology,
 materials science, and
 bioreactor design with respect to tissue engineering,
providing specific examples of how tissue engineering is
accomplished.
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skin transplantation, cartilage, bone, nervous
system and organ systems
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Introduction
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In 1997, media all over the world were aroused
by a BBC documentary, Tomorrow’s World ,
showing what is now known as the Vacanti
mouse (Cao et al. , 1997).
Although the Vacanti experiment is truly
exemplary for the discipline of tissue
engineering, it is fair to say that the media
upheaval was not so much caused by the actual
experiment but even more by the spectacular
sight of a nude mouse that had apparently
grown a human ear on its back.
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For many, the Island of Dr Moreau *攔截人魔島 (Wells,
1896) had become reality and media hype was born.
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At first sight one would tend to say that the field has over
promised and has a history of not delivering on those
promises.
This statement would be too simple.
The eagerness of the media to report on the advances in
the field of tissue engineering is not so much caused by
publicity eager scientists; the actual cause is the
enormous demand in our society for technologies that
are able to repair, or even better regenerate, damaged
or worn out tissues and/or organs.
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Island of Dr Moreau
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The Mouse and the ear
Superhero Science- Limb Regeneration
Regenerative Medicine: Re-Growing Body Parts
Organ Printing
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Box 1 The Mouse and the ear: tissue-engineered
cartilage in the shape of a human ear
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A formal formulation of the discipline is traced back to
the paper of J. Vacanti and R. Langer in Science (1993).
Since then, the number of studies in tissue engineering
has grown rapidly.
A landmark study published in 1997 in Plastic and
Reconstructive Surgery by Y. Cao et al. (1997) attracted
the interest of a large audience; thanks also to a BBC
service on the subject.
In this paper it is shown how to successfully regenerate
the cartilagineous part of a 3-year-old child’s ear.
The work is useful also from an educational point of view,
as most of the ‘ingredients’ to perform a classic tissue
engineering experiment are present.
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The Mouse and the ear
Materials Science:
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First, a plaster mold of the ear of a 3-year-old
child was cast from an alginate impression of the
ear.
Then, a 100 μm thick non-woven mesh of
poly(glycolic acid) (PGA) was immersed in a 1%
weight/volume solution of poly(lactic acid) (PLA)
in methylene chloride for 2 seconds and
subsequently shaped into the plaster mold.
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The Mouse and the ear
Biology:
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calf chondrocytes (cartilage cells) were harvested from the
articular surfaces of a calf,
isolated by collagenase digestion, filtered, washed in cell
medium,
labeled with BrdU to control their viability, and
seeded onto the polymeric scaffolds (1.5 x108 cells in total).
The constructs were cultured in vitro for 1 week in an
incubator under physiological conditions (T 37°C; pco2
5%) and then implanted into a subcutaneous pocket on
the back of athymic無胸腺 mice.
Three groups of scaffolds were considered:
(I) scaffolds seeded with cells;
(II) scaffolds reinforced with an external stent and seeded with cells;
(III) externally stented scaffolds with no cells.
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The Mouse and the ear
Biochemistry:
 after 12 weeks, the constructs were explanted,
sectioned, and histologically stained with specific
markers for typical cartilage extra cellular matrix
components
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hematoxylin and eosin for general tissue formation;
alcian blue for glycosaminoglycan deposition;
Masson’s trichrome for collagen formation).
Immunohistochemistry was also performed to
confirm that present collagen was specific for
cartilage (type II).
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The Mouse and the ear
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The results showed extensive cartilage formation in the
scaffolds that were seeded with cells, while no cartilage
was present in the unseeded scaffolds.
Furthermore, scaffolds reinforced with an external stent
for the first 4 weeks of implantation maintained the
anatomical shape of the ear.
In contrast, the other scaffolds lost partially their integrity
and appeared of reduced size and distorted shape.
From these findings the scientists concluded that
cartilage formation is not mature enough in the first 4
weeks to counteract the contraction forces in the healing
process.
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The Mouse and the ear
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The Mouse and the ear
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This experiment was surely a success for
those years and definitely contributed to
boost the interest in the field.
If we consider the state of the art
nowadays, however, a number of
drawbacks still characterize this study,
some of which are mentioned here:
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The Mouse and the ear
1.
2.
Skin coverage is missing and is a critical
element of any ear reconstruction.
Bovine immature (young) chondrocytes were
used, while clinical application thereof is of
course highly unlikely and these cells are now
known to be partially unrepresentative for the
use of human cartilage cell sources.
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Such human sources tend to loose differentiation
capacity quite fast and are furthermore frequently
characterized by necrosis in the center of scaffolds
with a clinically relevant size.
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The Mouse and the ear
3.
An athymic or immunodeficient mouse model
is used here.
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4.
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Obviously, this is nothing more than a useful
screening model and large animal models are
required to test clinical relevance in the presence of
a functional immune system.
Scaffolds need to provide an adequate
mechanical stability to the construct at the time
of implantation
Implications on the growth rate of the artificial
ear compared to the growth rate of a 3-yearold child should be addressed before the final
implantation in the patient.
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Introduction
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The loss or failure of an organ or tissue is one of
the most frequent, devastating, and costly
problems in human health care. A new field,
tissue engineering, applies the principles of
biology and engineering to the development of
functional substitutes for damaged tissue. This
article discusses the foundations and challenges
of this interdisciplinary field and its attempts to
provide solutions to tissue creation and repair .
Langer and Vacanti (1993)
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If the need is indeed so high, and the field has so
extensively grown over the last decade, then why do we
still lack frequent clinical successes?
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There are two major reasons for this phenomenon.
 First, bringing a relatively simple medical device from
initial idea to a widespread clinical reality frequently
takes a minimum of 10 years.
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As the underlying technology for developing tissue
engineering products is less mature, and possibly more
complex, it is to be expected that clinical progress in this
field will be measured in decades rather than years.
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Second, tissue engineering is truly a
multidisciplinary field where acquired knowledge
from individual classical disciplines (e.g.
quantum physics, polymer chemistry, molecular
biology, anatomy) no longer suffices to make
substantial leaps.
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Individuals active in this field will have to acquire
multidisciplinary skills and be willing to look over the
borders of their home discipline.
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A clear example of that is the paragraph
on mass transport in the Cell Nutrition
chapter ( Figure I.1 ).
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Figure I.1 A glimpse at mass transport
theory.
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Figure I.2 Diagram sketching the layout of the book (groups biology,
technology and evaluation form basic to clinical practice.
Fundamental
Biology
Fundamental
Biomaterials
Applied
Biology
Engineering
Tissue
Engineering
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Stem Cells
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Stem Cells (Chapter 1) gives insight into
the different aspects of this cell and will
show that the stem cell is not a single
cell type but, in reality, encompasses
different categories of cells ranging from
the multipotent embryonic stem cell to the
apparently less potent adult stem cell.
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Two properties:
Stem cells
The ability to make identical copies of themselves
(self renewal) and
 the ability to form other cell types of the body
(differentiation)
These properties are also referred to as ‘stemness’.
Stem cells may potentially provide an unlimited supply of
cells that can form any of the hundreds of specialized
cells in the body.
It is because of these properties that stem cells are an
interesting cell source for tissue engineers.
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Stem cells
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Stem cells can be divided into two main groups:
 embryonic
and adult or somatic身體的stem
cells.
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Embryonic stem cells are responsible for
embryonic and fetal development and growth.
In the human body, adult stem cells are
responsible for growth, tissue maintenance and
regeneration and repair of diseased or damaged
tissue.
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Stem cells
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Stem cells
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Morphogenesis:
generation of tissue in the embryo
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The formation of tissues in the embryo.
Although most tissue engineers do not venture
into the realms of developmental biology and
research with a strong focus on developmental
biology is all too scarce in tissue engineering.
Obviously most tissue engineering constructs
will not be implanted into embryos but into
human beings after birth.
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Morphogenesis:
generation of tissue in the embryo
Thus important lessons for tissue engineering can be
learned from the formation of organs during embryogenesis.
(i) the origin of cells that contribute to the formation of a
particular organ,
(ii) the growth factors and their interrelationship in the
formation of an organ,
(iii) the mechanisms by which undifferentiated precursor cells
are induced to specialize into an organ specific cell type,
(iv) the subsequent steps in organ formation and
(v) the interaction between cells and their environment
consisting of both the extracellular matrix and neighboring
cells.
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Tissue Homeostasis
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Tissue Homeostasis
3.2 Tissues with no potential of regeneration
3.3 Tissues with slow regeneration time
3.4 Tissues with a high capacity of regeneration
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Although our body has the capacity to regenerate to a
large extent, there are tissues with no or limited capacity
of regeneration.
For example, the central part of our lens consists of
lens lamellas that are embryonic fossils that will not
have changed since they were developed during
embryonal life.
The cell populations of the photo receptor of the retina
and the auditory organ of Corti are not regenerated.
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Tissue Homeostasis
3.3 Tissues with slow regeneration time
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Our bones are constantly being renewed by an active
process which involves the breakdown and rebuilding of
new bone matrix by the osteoblasts.
This constant renewal, governed by the continuous
optimization of the load-bearing role of the bone by a
functionally adaptive remodeling activity (which is more
active in growing bone), is dominated by high magnitude,
high-rate strains presented in an unusual distribution.
Adaptation occurs at an organ level, involving changes in
the entire bone architecture and bone mass.
This process is continuously ongoing and results in a
total turnover time of 3 years for the whole bone
structure.
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Tissue Homeostasis
3.3 Tissues with slow regeneration time
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Cartilage has a similar turnover rate over time, although
the individual matrix components are renewed at various
rates.
The two major extracellular components in articular
cartilage, collagen type II and aggrecan, are relatively
longlived in the tissue
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non-enzymatic modifications by reducing sugars, thus ending in
accumulation of advanced glycation endproducts.
These accumulated endproducts reflect the half life of
the components that for collagen is estimated to be 100
years and for aggrecan to be 3,5 years.
The cellular turnover in cartilage is probably limited and
the localization of stem cells in articular cartilage is so far
undetermined.
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Tissue Homeostasis
3.4 Tissues with a high capacity of regeneration
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Human regeneration in intestine and epidermis of
the skin is characterized by a high turnover rate
where cells constantly proliferate and differentiate; a
process that is finalized by a discarding of the cells
into the intestine, or off the body.
The regeneration is maintained by a special resident
cell type, the stem cell situated at the basal lamina.
These cells persist throughout life their function is to
maintain homeostasis, and effect tissue
regeneration and repair ( Figure 3.3 ).
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Tissue Homeostasis
角質層 (stratum corneum)
顆粒層 Stratum Granulosum 為表皮層的一層
stratum spinosum 棘細胞層，棘狀層
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Cellular signaling
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Understanding how these cells interact is
recognized as pivotal for the success of
tissue engineering.
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Furthermore, in spite of all media attention
that befalls stem cells, in reality cells
represent only a small part of the dry
weight of living tissue.
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Cellular signaling
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Cellular signaling: the molecular mechanism
by which cells communicate
Tissue engineering, in analogy with
development and wound healing, is a dynamic
process in which the right cell type should be at
the right place at the right time in order to
establish a normally functioning tissue.
Communication between cells and the rest of the
body plays a crucial role in the coordination of
cell number, position and function.
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Cellular signaling
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Cellular signaling
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A central paradigm can be recognized in most events of
cellular signaling, which consists of three distinct steps (see
Figure 4.2 ):
1. Signal initiation: an extracellular ligand binds to a receptor
on the surface of the cell. Ligand binding changes activity
of the receptor, thus generating the signal.
2. Signal transduction: The activated receptor triggers a
signal transduction cascade in which intracellular proteins
are activated, ultimately leading to activation of a so-called
transcription factor in the cell’s nucleus.
3. Gene activation: The transcription factor binds to
regulatory sequences in target genes, resulting in gene
activation, protein synthesis and changed cellular
physiology.
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Figure 4.2 The paradigm of cell signaling. Although many variations on this
theme exist, many signaling events follow the paradigm of signal initiation by
an extracellular ligand, followed by signal transduction
into the nucleus where the change in gene expression underlies
physiological adaptation of the cell as depicted in Figure 4.1.
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Signal transduction
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The extracellular matrix as a biologic
scaffold for tissue engineering
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All, or at least most, cells interact with an
extracellular matrix, which, in contrast to the errant
opinion of some engineers and even biologists,
presents much more than mechanical support and
adds substantially to the biological interactions in
our body.
As tissue engineering typically combines scaffolds
with biologically active components as cells or
growth factors we felt that a chapter on the
biological equivalent: Extracellular Matrix (Chapter
5) could not be missed.
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Extracellular Matrix
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Scaffolds to support the constructive remodeling of
injured or missing tissues or organs can be
composed of synthetic or naturally occurring
materials.
Such scaffolds can be degradable or nondegradable, and these scaffolds can be engineered
to have specific mechanical and material properties
that closely approximate those of the tissue to be
replaced.
Ultimately, the scaffold should facilitate the
attachment, migration, proliferation, differentiation
and three-dimensional spatial organization of the
cell population required for structural and functional
replacement of the target organ or tissue.
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Extracellular matrix: Definition
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The extracellular matrix represents the
secreted product of resident cells within
each tissue and organ and is composed of
a mixture of structural and functional
proteins arranged in a unique, tissue
specific three-dimensional ultrastructure.
These molecules, mainly proteins, provide
the mechanical strength required for proper
function of each tissue and serve as a
conduit for information exchange (i.e.
signaling) between adjacent cells and
between cells and the ECM itself.
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Extracellular matrix: Definition
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The ECM is in a state of dynamic reciprocity
互惠(Bissell and Aggeler, 1987) and will change in
response to environmental cues such as
hypoxia and mechanical loading.
In the context of tissue engineering
applications therefore, use of the ECM as a
scaffold indeed provides structural support,
but perhaps more importantly provides a
favorable environment for constructive
remodeling of tissue and organs.
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Extracellular matrix: Composition
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The extracellular matrix represents a mixture of
structural and functional molecules organized in a
three dimensional architecture that is unique to each
tissue.
Most of these molecules are well-recognized and they
form a complex mixture of proteins,
glycosaminoglycans, glycoproteins and small
molecules arranged in a unique, tissue specific threedimensional architecture.
The logical division of the ECM into structural and
functional components is not possible because many
of these molecules have both structural and functional
roles in health and disease.
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Extracellular matrix: Composition
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For example, both collagen and fibronectin,
molecules that once were considered to exist
purely for their ‘structural’ properties, are now
known to have a variety of ‘functional’ moieties
with properties ranging from cell adhesion and
motility to promotion of or inhibition of
angiogenesis.
These ‘bimodal’ or multifunctional molecules
provide a hint of the diverse occult amino acid
sequences that exist within certain parent
molecules and which, in themselves, harbor
biologic activity.
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Extracellular matrix: summary
1. Scaffolds composed of extracellular matrix are useful in
many tissue engineering applications throughout the
body.
2 The ECM is composed of structural and functional
molecules arranged in a tissue specific three
dimensional structure.
3 The main components of the ECM are collagen,
fibronectin, laminin, and glycosaminoglycans.
4 Growth factors exist in the ECM in small quantities but
are potent modulators of cell behavior.
5 Following isolation from various tissues, the ECM
maintains structural and functional proteins that support
the host response to tissue injury.
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Extracellular matrix
6 These scaffolds promote constructive remodeling of host
tissue and elicit various biologic responses including
angiogenesis, chemotaxis, and antimicrobial properties.
7 The use of native xenogeneic scaffolds, devoid of cells
from the original tissue, results in host accommodation
as opposed to scaffold rejection. However, when the
ECM is modified by chemical or other cross-linking
methods, the biologic response is more consistent with
chronic inflammation, tissue destruction, fibrous
encapsulation, and scarring.
8 ECM scaffolds are currently used to treat a wide range of
tissue injuries, but their full potential will only be realized
following additional investigation and clinical application.
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Scaffolds
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At this point in the book a shift is made to the more
fundamental engineering aspects. Since not all scaffolds
and matrices are completely synthetic, a chapter on
Natural polymers (Chapter 6) seemed in place as well.
Most tissue engineers, actively involved in the design of
both synthetic and natural scaffolds or matrices, prefer to
have these degrade after implantation.
This is with good cause as the prolonged presence of
foreign material in the body may induce a variety of
unwanted effects such as implant-associated infection or
mutagenesis.
Two groups of authors discuss the aspects of implant
degradation and this is done in two related chapters:
Degradable polymers (Chapter 7) and Degradation of
Bioceramics (Chapter 8).
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Natural polymers
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Natural polymers are derived from renewable resources,
namely from plants, animals and microorganisms, and
are, therefore, widely distributed in nature.
These materials exhibit
 a large diversity of unique (and in most cases) rather
complex structures, and
 different physiological functions, and
 may offer a variety of potential applications in the field
of tissue engineering due to their various properties,
 such as pseudoplastic behavior, gelation ability,
water binding capacity, biodegradability, among
many others.
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Natural polymers
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In addition, they possess many functional
groups (amino, carboxylic and hydroxyl
groups) available for chemical (hydrolysis,
oxidation, reduction, esterification,
etherification, cross-linking reactions, etc.)
modification and/or conjugation with other
molecules, which allows an overwhelming
variety of products with tailorable chemistries
and properties to be obtained.
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Natural polymers
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Protein materials may offer an additional advantage as
they are able to interact favorably with cells through
specific recognition domains present in their structure.
On the other hand, the creation of hybrid materials – by
means of combining the advantages of different natural
polymers – may constitute a useful approach to
mimicking the natural environment of the extracellular
matrix and to obtaining scaffolding materials with
superior mechanical and biological properties.
Most popular natural polymers: chitosan, starch,
collagen, hyaluronic acid, elastin.
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Natural polymers
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An intrinsic characteristic of natural origin
polymers is their ability to be degraded by
naturally occurring enzymes, which may indicate
the greater propensity of these materials to be
metabolized by the physiological mechanisms.
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Natural polymers
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Another important aspect to consider when
using natural polymers, is that they can induce
an undesirable immune response due to the
presence of impurities and endotoxins
(depending on their source), and their properties
may differ from batch to batch during large-scale
isolation procedures due to the inability to
accurately control the processing techniques.
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Natural polymers
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Nevertheless, as knowledge about these natural
polymers increases, new approaches (including
methods for production, purification, controlling
material properties and enhancing material
biocompatibility) are likely to be developed for
designing better scaffolding materials to support
the development of more natural and functional
tissues.
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Natural polymers
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In summary, both natural and synthetic polymers
present important characteristics and, therefore,
one must recognize that the best biodegradable
polymer for biomedical applications might be
found by taking steps towards the development
of new biomaterials that combine the most
favorable properties of synthetic and natural
polymers.
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Natural polymers: Summary
1. A wide range of natural origin polymers have
frequent been used, and might in future be
potential useful in tissue engineering.
2 . Tissue engineering scaffolds comprised of
natural derived macromolecules have potential
advantages of biocompatibility, cell-controlled
degradability, and intrinsic cellular interaction.
3. However, they may exhibit batch variations and,
in many cases, exhibit a narrow and limited
range of mechanical properties. In many cases,
they can also be difficult to process by
conventional methods.
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Natural polymers
4 . In contrast, synthetic polymers can be prepared with
precisely controlled structures and functions. However,
many synthetic polymers do not degrade as desired in
physiological conditions, and the use of toxic chemicals in
their synthesis or processing may require extensive
purification steps. Many of them are also not suitable for
cell adhesion and proliferation.
5 . The combination of natural origin polymer with synthetic
polymers and the further development in emerging
methodologies such as recombinant protein technologies is
expected to lead to outstanding developments towards the
development of improved materials to be used in tissue
engineering application.
6 . No material alone will satisfy all design parameters in all
applications within the tissue engineering field, but a wide
range of materials can be tailored for discrete applications.
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Degradable polymers for
tissue engineering
A general definition of tissue engineering can be found in
The Williams Dictionary for Biomaterials ‘ Tissue
engineering is the persuasion of the body to heal itself
through the delivery to appropriate sites of molecular
signals, cells and supporting structures. ’ Although
very short, this definition addresses the three main
components of tissue engineering:
1. Molecular signals, such as growth factors, that stimulate
the proliferation and differentiation of cells in vitro and/or
in vivo, the infiltration by surrounding tissue,
2. Cells, to regenerate the lost or damaged tissue, and
3. Scaffolds and matrices, i.e. porous supporting structures
and/or gels that allow cell attachment and tissue
ingrowth.
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Degradable polymers for
tissue engineering
In addition to this, other important aspects regarding the
polymeric material that is to be used are:
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Biocompatibility is of utmost importance (see Chapter 9)
to prevent an adverse tissue reaction of the immune
system.
It should be possible to tune the rate of degradation
of the polymer.
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For instance, the material should not degrade too
quickly in load bearing areas, as it may cause a
significant decrease of the mechanical properties and
its consequent premature failure.
On the other hand, degradation should not be too
slow as it may prevent tissue regeneration.
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Degradable polymers for
tissue engineering
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The resulting degradation products should be
non-toxic and, therefore,
should not induce an inflammatory reaction.
They should dissolve in body fluids and, after
transportation via the lymphatic system, the
kidneys should be able to excrete them from
the body.
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Degradable polymers for
tissue engineering
1. Degradable polymers are suitable as scaffold material for
tissue engineering as interference with the development
and growth of new tissue and unwanted long-term
reactions are prevented.
2. Degradable polymers for use in tissue engineering
should be biocompatible, and the resulting degradation
products should be non-toxic. The time required to
complete degradation resorption depends on the
intended application and preferably matches the
formation of functional tissue.
3. Polymers are long-chain macromolecules formed by
covalent coupling of monomers. Copolymerization, i.e.
the preparation of polymers from two or more types of
monomers, is often employed to tune their properties. 84
Degradable polymers for
tissue engineering
4. For a polymeric material to be degradable in the
body it is necessary that the main chain of the
macromolecule contains labile degradable
bonds, such as ester, carbonate, or anhydride
bonds.
5. The most important degradable polymers are
polyesters. In the presence of water, chain
scission occurs by hydrolysis of ester bonds,
which lowers the molecular weight. The resulting
oligomers dissolve in the surrounding
environment or undergo further hydrolysis.
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Degradable polymers for
tissue engineering
6. The soluble degradation products are transported
from the implantation site via the lymphatic system
to the kidneys which excrete them from the body.
7. Mass loss of the polymeric device, due to removal
of the oligomers is referred to as erosion.
Distinction is made between surface and bulk
erosion processes.
8. Often, in vivo degradation is faster than in vitro
degradation. This is due to the tissue response
(inflammation, foreign body response) and the
presence of enzymes.
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Degradable polymers for
tissue engineering
9. Linear aliphatic polyesters such as poly(lactic acid)
(PLA), poly(glycolic acid) (PGA) and their
copolymers (PLGA) have been broadly used in
tissue engineering. These polyesters degrade via
hydrolysis of the main chain ester bonds. Their
degradation products (lactic- and glycolic acid) are
part of the Krebs metabolic cycle.
10. More advanced degradable polymers will have an
optimal balance between mechanical properties and
degradation rate. In addition, functional groups
attract cells and/or growth factors required for tissue
formation.
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Cell Sources
1. Adult stem cells are undifferentiated cells
present in differentiated tissues with a long term
self renewal capacity and a differentiation
potential toward the cells phenotypes of the
same tissues.
2. The main role of adult stem cells is to maintain
tissue homeostasis, and to replace cells lost due
to normal tissue turnover, injury or disease.
3. The first adult stem cells to be identified in the
1960s were the hematopoietic stem cells (HSC)
isolated from bone marrow.
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Cell Sources
4. A specific HSC niche exists in the bone marrow where
HSCs self-renew and are protected from entering a
differentiative or apoptotic fate. Exit of HSC from the
‘niche’ may determine their exposure to a
microenvironment promoting their differentiation into a
hematopoietic cell progeny.
5. Stratified分層的 epithelia, such as epidermis and corneal
epithelium, are organized in different cell layers.
Epithelial stem cells are located in the basal layer. Only
keratinocytes in the basal layer can divide. Basal
keratinocytes that become committed to terminal
differentiation, stop dividing, and migrate upward through
the different cell layers completing the differentiation
process.
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Cell Sources
6 . Although post-natal neural tissue had long been
thought to be unable to regenerate, two primary
locations of post-natal neurogenesis, fueled by postnatal neural stem cells, were identified: the
subventricular zone of the lateral ventricle wall (SVZ)
and the subgranular zone of the hippocampal
dentate gryrus (SGZ) in rodents, and the olfactory
bulb and hippocampus in humans.
7. In addition to HSC, a second population of stem
cells exists in the bone marrow. Originally identified
as bone marrow stromal stem cells, these cells were
later renamed mesenchymal stem cells (MSC).
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Cell Sources
8. When cultured in vitro or transplanted in vivo under
specific conditions MSC demonstrate a differentiation
potential toward several mesoderm derived cell lineages
such as osteogenic, adipogenic and chondrogenic.
9. Cells with properties similar to those exhibited by bone
marrow-derived MSC exist in other connective tissues,
such as adipose, skeletal muscle, etc.
10. Tissue specific stem cells may have the ability to cross
lineage specific boundaries, i.e. they are ‘plastic’. True
plasticity is defined as the ability of the progeny後代 of a
single cell to actually function as another cell type. Some
studies have challenged the concept of stem cell transdifferentiation potential by showing that a cell fusion was
responsible.
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Cell Sources
11. In terms of genetic disparity between donor and
recipient, grafts can be classified as autografts, isografts,
allografts or xenografts. Due to the presence of different
cell membrane antigens allogeneic or xenogeneic graft
are rejected by the body.
12. Stem cells such as MSC have a direct role in regulating
the immune response and can prevent/ reduce the
allogeneic or xenogeneic graft rejection. The
mechanisms by which stem cells control the immune
response are only partially known
An Isograft is a graft of tissue between two individuals who are genetically identical
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Cell Culture: Harvest, Selection,
Expansion and Differentiation
1. Autologous or allogenic cells can be utilized alone or in
combination with biomaterials to restore lost function of
tissue and organs.
2. Cells can be harvested from different locations of the
human body and, depending on the tissue to be restored,
processed for selection, expansion of cell number or reimplanted directly into the patient. In all cases, care has
to be taken to handle the harvested cells in a proper way
from the beginning.
3. When a special cell type is needed for implantation, cells
can be isolated from the initial harvest by targeting
special characteristics of the cells such as size, surface
markers, growth behavior and/or functional properties.
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94

Cells can be harvested
by using different
methods.
(A) Aspiration of bone
marrow from iliac crest
under local anaesthesia;
(B) Harvest of epithelial
cells from the bladder by
flushing with saline;
(C) Dermal core skin
biopsy.
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Empty full thickness defects in the trochlear groove (upper left);
tissue implant containing cells and covered with fibrin glue
(upper right); defect treated with implant and stapled with
resorbable PGA/PDS fixatives (lower left); and treated defect
96
after 12 months (lower right)
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98
Cell Culture: Harvest, Selection,
Expansion and Differentiation
4. The most common way of expanding the initial
cell number is by placing the cells in monolayer
cultures; although for some cell types such as
hematopoetic cells suspension cultures are
needed.
5. For clinical TE applications, the selection,
expansion and differentiation are made in
specially designated laboratories.
6. A potential problem with serial expansion in
monolayer is that the cells gradually change
their phenotype and that contaminating fast
growing cells could become too dominant.
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Cell Culture: Harvest, Selection,
Expansion and Differentiation
7. When the required cell number is achieved, the cells can
be differentiated by exposing them to specific feeder cell
lines, growth factors and/or to extra cellular matrix
proteins.
8. The cells can be combined with
synthetic scaffolds,
biomaterials,
forced into high density cultures or
exposed to physical forces to additionally stimulate
differentiation.
9. Most of the current clinical cell-based TE therapies
include expansion of the cell number in in vitro cultures
in special GMP laboratories. Future therapies will be
aimed at shortcutting this logistic and relatively
expensive procedures.
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Scaffold Design and Fabrication



Although some applications would involve direct injection
of cells into the tissues that one intends to regenerate,
most researchers active in the field of tissue engineering,
and working with cultured cells, strive for implanting
combinations of cells and scaffolds.
These so-called hybrid constructs will usually have three
dimensions and need to give access to both cells and
nutrients or have to allow an out flux of active ingredients
and waste products.
The chapter Scaffold Design and Fabrication (Chapter
14) presents the reader with the different ways to
manufacture scaffolds and explains the criteria these
scaffolds have to fulfill.
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Scaffold design and fabrication
102
103
Figure 14.3 a Scanning electron micrographs showing examples of scaffolds produced using
various processing techniques.
(A) liquid-solid phase separation and freeze-drying using 1,4-dioxane (PEGT/PBT),
(B) compression molding and porogen leaching (PEGT/PBT),
(C) nonwoven polylactic acid (PLA) mesh with 13 um fiber diameter,
(D) 3D fiber deposition (3DF) of PEGT/PBT scaffold with a 175- u m fiber diameter and 0.5104
mm fiber spacing.
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Scanning electron microscopy
images showing chondrocytes
attached to 3DF scaffolds
containing
(A)A homogeneous 1-mm fiber
spacing or
(B)an anisotropic pore-size gradient.
By generating pore-size gradients
within RP scaffolds. control over
the distribution of cells and
amount of ECM components,
such as GAG and collagen,
could be achieved.
(C) Polymer scaffolds fabricated with pore-size
gradients as a model for studying the zonal
organization within tissue-engineered
cartilage constructs. Tissue Eng, 11: 1297–
1311.
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107
108
109
110
111
112
113
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Dual spinnerets
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SFF: Scaffold Free Fabrication
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Bioreactors for Tissue
Engineering


Ideally the researcher working on a hybrid construct and
following the chapters, as they have been presented so
far, now faces one of the main obstacles of tissue
engineering: how to initiate cell growth and extracellular
matrix formation in three-dimensional constructs of
clinically relevant dimensions (usually centimeters
versus millimeters in many in vitro and animal studies).
These challenges, fundamentals and some solutions are
discussed in Bioreactors for Tissue Engineering
(Chapter 16).
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Bioreactors for Tissue Engineering
121
Cell seeding
122
Bioreactor with
dynamic compression
for
cartilage tissue engineering
123
Future automated bioreactor
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125




Although not complete or covering all of the subjects, for
which one could argue would be essential, the editors
feel that these first 16 chapters (as well as this chapter),
provide a solid basis for students who wish to acquire an
understanding for tissue engineering.
Much of the international research effort based on tissue
engineering is along the lines presented in these
chapters.
But it is also fair to say that some of the major
challenges are still to be found into making tissue
engineering a widespread clinical reality.
These clinical aspects are treated in the remaining six
chapters.
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17 Tissue engineering for skin transplantation
18 Tissue engineering of cartilage
19 Tissue engineering of bone
20 Tissue engineering of the nervous system
21 Tissue engineering of organ systems
22 Ethical issues in tissue engineering
127
Thanks!
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