Tissue maintain the tissue integrity for required

Tissue
engineering is a part of regenerative medicine related with developing new
biological substitutes for the replacement of damaged or diseased tissues in
human. Tissue engineering uses the combination of the cells, material and
engineering technique to generate the structure as the required organ to be
replaced. The material (polymers or metals) is first engineered into the
required shape and then the cells are seeded with proper culture media so that
the cells grow into the shape of the material. The engineered material is known
as scaffold, over which the cells proliferates producing a tissue or organ of
shape of scaffold.

The field of
tissue engineering has dramatically advanced in last two decades. Different
advancement in scaffold design and cell material interaction have led to
successful regeneration of various (skin, cartilages, muscles) tissues and
organs in human. Tissue engineering has become an integral part of regenerative
medicine, wound healing and surgery. For successful generation of any tissue,
the scaffold material and its property have a deciding role. The search for
ideal scaffolding material and appropriate scaffolding structure to fulfill all
the required criteria has been a challenging job in tissue engineering. So, in
recent days the metals or ceramics are being replaced by polymers for scaffold
design. Polymeric scaffold are drawing attention in this field because of their
unique features such as high surface to volume ratio, high porosity with
smaller pore size, biodegradation and enough mechanical strength to maintain
the tissue integrity for required time frame. Additionally, they offer distinct
advantages of biocompatibility, versatility of chemical properties and the
biological responses which are significant in the application of tissue
engineering and organ replacement.

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Scaffold
material can be synthetic or biological, degradable or non-degradable depending
upon the use. The polymers can be manipulated into any required criteria by
varying the composition, structure, arrangement of constituents. Naturally
occurring polymers such as proteins (collagen, gelatin, fibrinogen, elastin and
keratin), polysaccharides (starch, chitin, chitosan) and polynucleotide (DNA,
RNA) are biodegradable and are extensively used clinically in tissue
engineering. These natural polymers have bioactive properties and show better
interactions with cells that allow the cells to proliferate, differentiate and
migrate. Also, these natural polymers can act as ligand for the cell adherence
and cell signaling. For example collagen has a binding site that acts as ligand
for macrophages, use of fibrinogen in scaffold or directly in the tissue
replacement system induces the aggregation of macrophages and monocytes around
it which prevents the infection around the implant site. A collagen scaffold
for dermal tissue engineering developed by prof. Ioannis Yannas showed better
cellular growth with enough mechanical stability. This collagen based skin
graft was approved by FDA and was used in treatment of burnt skin.

Synthetic
polymers are widely used in tissue engineering. Poly-ethylene glycol (PEG),
Poly-lactic acid (PLA), Poly-caprolactone (PCL) and many other polymers are
mostly used for cellular modeling and growth of cells in required shapes. PEG
is particularly used as a component of hydrogel due to its property to imbibe
water and protein repellant characteristics. Due to this feature the use of PEG
allows cellular growth in the required shapes. Poly-vinyl acid (PVA),
Poly-acrylic acid (PAA) and Poly-hydroxyethyl methacrylate (PHEMA) are used in
different tissue engineered scaffolds as they are biocompatible and allow
cellular growth. However these polymers are not in use as they are not
biodegradable and FDA has restricted the use of such non degradable polymers in
scaffolds.

Besides
scaffolds for tissue development the polymers are also used in wound healing
and dressing that provides a natural environment for the healing process.
Though new tissues are not introduced the polymers coated with drugs and cell
growth enhancer protects the external wounds from bacterial infection and
promotes the cell differentiation and proliferation. Wounds and burns in
epidermis, dermis or deep burns are treated with polymeric dressing. Methyl
cellulose derived wound coverage bandages developed by Stashak et. al proved
far better than traditional dressing system using paraffin gauze. This not only
provided external protection to the wound but also prevent water loss along
with adequate supply of oxygen to the wounded tissue. Some natural polymers such as polysaccharides (chitin, chitosan,
heparin, chondroitin), proteoglycans and proteins (collagen, gelatin, fibrin,
keratin, silk fibroin, eggshell membrane) are extensively used in wounds and
burns management. Other synthetic polymers like poly-glycolic acid, poly-lactic
acid (PLA), poly-acrylic acid (PAA), poly-caprolactone (PCL), polyvinyl alcohol
(PVA), poly-ethylene glycol(PEG) exhibit in vivo and in vitro wound healing
properties and enhance re-epithelialization.

The polymers are
also used in clinical medicine as a direct substitute of natural tissue where
tissue growth is delayed or not possible. In avascular structures such as
cartilage and teeth, cellular growth is not possible due to lack of blood
supply. So, any injury or damage to that tissue is treated directly using
polymeric material. Delayed cellular growth is sometimes enhanced using drug
coated polymers. Poly-methylmethacrylate (PMMA) used as bone cement, used for
treatment of fractured bone and cartilage provides mechanical support to bone
and cartilage. PMMA is generally coated with gentamicin which prevents
infection and also promotes cellular aggregation around the injured site. These
are also used as tooth fillers. Dentures or false teeth are made of PMMA which
are used as tissue replacement where growth of new teeth is not possible.

Hence it can be
concluded that polymers are one of the most important materials in medical
field. In tissue engineering polymers have wide range of application. Polymers
are widely used in scaffold designing, wound healing and dressing and also in
some clinical medical applications for the replacement of natural tissues. Both
natural and synthetic polymers proved to be the best material for cellular
in-growth as compared to metals and other materials. Also the variability in
their chemical and mechanical properties of different polymers makes them
useful in wide range of applications.