Tissue Engineering
A Focus on Scaffolds
David Zeng
Cluster 1: Biotechnology
COSMOS, UC Davis
General Information on Tissue
Engineering
„
Definition: Technology combining genetic engineering of cells with
chemical engineering to create artificial organs and tissues
„
These cells can come from many sources: Autologous cells which come
from the individual itself, Allogenic cells which come from the same
species, Xenogenic cells which come from other species, and Syngeneic
cells which come from genetically identical individuals (twins, etc)
(Figure 1) Using tissue engineering, a human ear
grew onto a mouse
So what cells can be used?
„
Cells used in tissue engineering have
to be able to divide innumerous
times, so these cells are usually stem
cells
„
Stem cells are undifferentiated cells
with the ability to divide in culture
and give rise to different forms of
specialized cells. Stem cells are
divided into "adult" and "embryonic"
stem cells, the first class being
multipotent and the latter mostly
pluripotent
„
Because stem cells have the ability to
form specialized cells, at the moment
they are the best candidate for tissue
engineering
(Figure 2) A diagram of Stem cells division
Controversy
„
There is a a large amount of controversy surrounding tissue engineering
„
The main argument is the ethicality of using stem cells. Only embryonic
stem cells are capable of differentiating into any type of cell, and are able
to do this infinite times. However, we cannot obtain embryonic stem
cells without destroying the embryo with current technology.
„
However, the benefits far outweigh the harms. If tissue engineering
succeeds, we can implant new livers, hearts, brain tissue, lungs, arms, and
much more. In addition, with more research we may be able to obtain
embryonic stem cells without destroying the embryo.
Methods – The Scaffold
„
Cells are often implanted into an artificial
structure that is able to maintain the structure for
tissue formation. This artificial structure is a
scaffold
„
Scaffolds must: Allow cell attachment and
migration, enable transportation of vital cell
nutrients, and be biodegradable
„
Stem cells are seeded into a scaffold. The
scaffold is then implanted into the correct
position. A stimulus carried by the scaffold
triggers the cells to divide. The scaffold provides
nutrients and structure while the cells divide. The
scaffold biodegrades as the cells start to form a
structure strong enough to support itself.
(Figure 3) A scaffold
Methods – The Scaffold (cont.)
„
There are 3 “generations” of scaffolds: the first that matched the physical
properties of the environment, the second that produced certain chemicals
to stimulate cell division. The 3rd generation, “smart biomaterials”
combined the first two.
„
Several “smart biomaterials” for tissue engineering are activated by cells
or genes. Incorporation of a signal peptide such as RGD (arginineglycine-aspartic acid) into the biomaterial has attempted to adjust cell
adhesion and induce cell migration.
Challenges Scaffolds Must Overcome
„
Just allowing cell division isn’t enough. Although cells can divide, and
create the structure, the body part must be able to connect to existing
blood vessels, etc.
„
Furthermore, scaffolds are many times rejected by the immune system
because it is recognized as a foreign object
„
Tissue implanted with a volume greater than 2 to 3 mm cannot obtain
sufficient survival because provision of nutrition, gas exchange, and
elimination of waste products are limited by the large diffusion distance.
Thus, organs are not possible to be implanted using scaffolds at this
point.
Challenges Scaffolds Must Overcome
(Cont.)
„
In some cases, the scaffold will cause the degeneration of surrounding
tissues.
„
Furthermore, the process retrieving of cells needed for tissue engineering
can also lead to tissue degeneration.
„
When the scaffold is broken down, it releases acid that changes the pH of
the surrounding environment. Even a slight change in pH will affect
cellular function.
„
Overcoming these obstacles will take us ever closer to successfully using
tissue engineering
(Figure 4) Diagram of how a conventional scaffold operates, and how it is
insufficient to support large scale growth.
What can be done to overcome these
obstacles? The most promising solution to these obstacles is the
„
SFF (Solid Freeform) Scaffold
(Figure 5) A hydrogel
scaffold formed by
Solid Freeform
technique
„
The SFF uses layer manufacturing in order to create a
3D object. A computer generated model is created using
CAD (computer-aided design); then physical model is
created.
„
The model is made rapidly and with MRI and CT scans,
is extremely accurate. Thus, pore size, pore distribution,
and artificial vascular systems can be produced precisely
and can be custom-made for each individual.
„
The artificial vascular system lies within the scaffold and
supplies the cell with oxygen and nutrients, and removes
waste metabolites throughout the entire scaffold
Solid Freeform Scaffolds
„
Many times Solid Freeform Scaffolds also make a mold using a negative
of the envisioned scaffold, and material is poured into the mold.
„
A mold is created using a phase-change ink jet printer (the printer
translates the CAD into a physical model). The mold posseses a series of
interconnected and branched shafts running across the walls of the mold
„
Collagen Type I is cast into the mold and frozen at 200 C. The mold is
then immersed into a solution of ethanol which dissolves the mold and
ice crystals. The ethanol is then removed with liquid carbon dioxide.
„
The benefit of a collagen scaffold is that the body doesn’t reject
commonly reject it, the process doesn’t denature the collagen due to high
temperature, and offers the possibility of incorporating biological
molecules into the scaffold.
Solid Freeform Scaffolds Combined
with Nanotechnology
„
With the addition of nanotechnology, the scaffolds can be improved
greatly. Nano-particles are added into the scaffolds, which greatly
improves the scaffold in a multitude of ways.
„
Calcium phosphate bioceramics (ceramics used in implants), such as
hydroxyapatite(HA) and tricalcium phosphate, are promising
materials for bone tissue engineering since these ceramics reflect the
chemistry and structure of the native mineral components of bone
tissue
„
In addition to bone binding ability of HA, mimicking the size scale
of HA in natural bone may enable this composite scaffold to serve as
a mechanically improved three-dimensional substrate for cell
attachment and migration.
„
Bioactive HA particles seeded into scaffolds cause a greatly
improved rate of osteogenesis
„
Figure 6 (A collagen scaffold)
Solid Freeform Scaffolds Combined with
Nanotechnology (Cont.)
„
A PLGA/HA (polylactic-co-glycolic acid/hydroxyapatite) nano-particle
composite scaffold improved mechanical properties and enhanced
osteogenic potential in vitro and in vivo.
„
This research also demonstrated that additional exposure of the bioactive
HA particles allowed direct contact with cells and stimulated their
proliferation and osteogenic differentiation
Solid Freeform Scaffolds Combined
with Nanotechnology
(Figure 7) Radiographs of rat femoral defects (a) prior to implantation, (b)
immediately post-implantation, (c) treated with HA/TCP loaded human
Mesenchymal stem cells at 12 weeks after implantation (d) treated with HA/TCP
alone at 12 weeks after implantation.
Success?
„
The greatest success in the field thus far has been in the field of skin
tissue engineering. Complete epidermal and dermal bi-layer tissue
mimetics have successfully made their way from the laboratory to
patient treatment.
„
FDA-approved products such as Dermagraft and Appligraf have found
their way into the clinical treatment of burn victims and other patients
afflicted with skin disorders that require skin graft treatment.
„
The skin substitutes usually consist of an ex vivo expanded population of
the patient's own skin cells seeded onto a layer of chemically engineered
collage.
„
Successful bone formation has been reported in reconstructed skull and
mandibular defects in sheep, and in iliac wing defects in goats.
„
However, even with SFF scaffolds we are still not capable of creating
organs for use. The scaffolds still do not allow enough nutrients and
cannot support large organs to grow.
Conclusion
„
More and more research is being devoted to tissue engineering.
When tissue engineering succeeds, we will have another weapon
in our arsenal against human disease and accident.
„
At the moment, the most promising way to tissue engineer is by
using scaffolds. The most likely to succeed scaffolds are the Solid
Freeform Scaffold and Reverse Solid Freeform Scaffold.
„
With every passing day, draw nearer to completing tissue
engineering.
Works Cited
„
Falanga, Vincent et al. "Autologous Bone Marrow-Derived Cultured
Mesenchymal Stem Cells Delivered in a Fibrin Spray Accelerate Healing in
Murine and HUman Cutaneous Wounds*" TISSUE ENGINEERING (2007).
17 July 2007
<http://www.liebertonline.com/doi/pdfplus/10.1089/ten.2006.0278>.
„
Conte, Michael. "The ideal arterial substitute: a search for the Holy Grail?"
THE FASEB JOURNAL (1998). 17 July
2007<http://www.fasebj.org/cgi/content/full/12/1/43?maxtoshow=&HITS=1
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tfulltext=and&searchid=1&FIRSTINDEX=0&sortspec=relevance&volume=
12&resourcetype=HWCIT>.
„
Vacanti, Joseph. "State-of-the-art tissue engineering: From tissue
engineering to organ building." (2004). 17 July
2007<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W
XC-4F297MV7&_user=4421&_coverDate=01%2F31%2F2005&_rdoc=1&_fmt=&_orig=
search&_sort=d&view=c&_acct=C000059598&_version=1&_urlVersion=0
&_userid=4421&md5=68157fd05147a2741926d30e0d851a99>.
Works Cited
„
Moss, Steven C. "In Situ Mineralization of Hydroxyapatite for a
Molecular Control of Mechanical Responses in Hydroxyapatite-Polymer
Composites for Bone Replacement." Advanced Biomaterials Characterization, Tissue Engineering and Complexity, Boston,
Massachusetts, November 26-29, 2001. Boston: Symposia, 2001.
„
Kimelman, Nadav et al. "Review: Gene- and Stem Cell-Based
Therapeutics for Bone Regeneration and Repair." TISSUE
ENGINEERING (2007). 17 July 2007
<http://www.liebertonline.com/doi/pdfplus/10.1089/ten.2007.0096>.
„
Walker, John M. Biopolymer Methods in Tissue Engineering. Totowa,
New Jersey: Humana Press, 2004.
Pictures Cited
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(Title Background) http://www.sgeier.net/fractals/flam3/fractals/DNA.jpg
(Figure 1)
http://images.google.com/imgres?imgurl=http://www.csa.com/discoveryguides/stemcell/images/p
luri.jpg&imgrefurl=http://www.csa.com/discoveryguides/stemcell/overview.php&h=603&w=550
&sz=63&hl=en&start=0&um=1&tbnid=cZmJ6OdHTnJSRM:&tbnh=135&tbnw=123&prev=/ima
ges%3Fq%3Dstem%2Bcells%26svnum%3D10%26um%3D1%26hl%3Den%26client%3Dfirefoxa%26rls%3Dorg.mozilla:en-US:official%26sa%3DN
(Figure 2) http://www.belligerati.net/archives/MouseEar.jpg
(Figure 3) http://www.washington.edu/newsroom/news/images/scaffold.jpg
(Figure 4) http://www.ecmjournal.org/journal/papers/vol005/pdf/v005a03.pdf
(Figure 5) http://ieeexplore.ieee.org/iel5/9794/30880/01431934.pdf
(Figure 6) http://www.emeraldinsight.com/fig/1560120407012.png
(Figure 7) http://www.biomed.metu.edu.tr/courses/term_papers/Bone-TissueEngineering_gencsoy_files/image010.jpg