Plastics are being widely used in medicine in prosthetic, diagnostic,
therapeutic and storage applications. These may be categorised into:
- long term implant applications within the body tissues such as in
vascular grafts (artificial blood vessels), hip prosthesis, etc.
- long term contact applications such as with (i) mucosal surfaces
(dentures, intrauterine devices, etc.) (ii) conjunctiva (contact lenses)
- short term applications within the body or in contact with mucosal
surfaces as in endotracheal tubes, urinary catheters, etc.
- applications in contact with skin as in burn dressings, splints etc.
- applications such as for syringes, blood transfusion equipment,
hemolysis equipment etc. which are used to contain or administer
substances to be introduced parenterally into the human body and which
have no tissue contact.
- applications as materials in semi-polymerised state which are used in
plastic or orthopedic surgery and dentistry where catalytic polymerisation
is promoted in situ e.g. dental filling materials.
Emerging applications of polymers
1. Drug release systems
Polymeric drug delivery systems have wide applications in medicine and in
agriculture. They are used to improve therapeutic efficacy and safety of drugs
by delivering them at a rate dictated by the needs of the body over the period
of treatment and to the site of action, which may reduce size and number of
doses, side effects and biological inactivation and/or elimination. Various
terms such as ‘sustained release’, ‘controlled release’, ‘prolonged
release’, ‘timed release’, ‘programmed release’, and ‘extended
release’ are used quite often to denote the drug release action. With
controlled release polymeric drug delivery, one must not only be concerned
with the effects of the active agents on the host but also with the effects of
the polymer on the host.
There are many advantages for controlled delivery of drugs compared to oral
ingestion or injection. Controlled release of drugs from a polymer matrix
provides targeted drug delivery, extension of duration of activity at equal
level of active agent, improved patient compliance, increased patient comfort,
delayed action, reduction of side effects etc.
Generally, drug release systems can be classified into
- Physical incorporation of active treatment into suitable polymer matrix
- Synthesis of pharmacologically active polymers.
To qualify as a potential drug carrier, a polymer must satisfy certain
criteria -
- Polymer must be soluble and easy to synthesize; it must have a finite
molecular weight and narrow distribution.
- It should provide drug attachment/release sites, or the possibility of
incorporation of drug-polymer linkages; both the sites and the linkages
must display controlled stability.
- The polymer must be directable to predetermined cell types, either by
its inherent physicochemical properties or by the incorporation of
specific residues.
- It should be compatible with the biological environment.
- It should be biodegradable or be eliminated from the organism after the
fulfillment of the function.
Many polymers have been used as ‘reservoirs’ for targeted drug
delivery, the most common being silicones, cellulose based polymers,
hydrogels, EVA copolymer, segmented polyurethanes etc. The release of active
agents through the polymer occurs through diffusion of the agent in response
to a decreasing concentration gradient away from the releasing device.
Diffusion of active agents upto 400 molecular weight may efficiently occur in
polymers such as silicones. The properties of the polymers influence the
diffusion of an active agent to a great extent. Of these, the glass transition
temperature is the most important. Other factors such as degree of
crystallinity, addition of diluents, plasticisers, or fillers will also affect
diffusion of molecules through a given polymer. Biodegradable polymers such as
polyglycolic acid, polylactic acid and their copolymers are used with great
success. In these cases, the polymer matrix degradation occurs in situ and
they do not require surgical removal. These have been found extensively useful
especially in administering insulin and bovine serum albumin.
Table 1a & Table
1b lists the nondegradable and biodegradable polymers being
used in drug delivery systems today.
2. Wound Healing Systems
Rapid healing is required in treatment of extensive burns, trauma, diabetic
ulcers etc. where tissue damage is high. Loss of natural skin is compensated
by covering the wound with a dressing which is expected to prevent the loss of
body fluids and proteins from the body system, retard bacterial infection and
improve the healing process by providing a support for the proliferating
cells.
Castor oil, absorbent cotton wool, soft paraffin impregnated gauze are some
of the materials used from ancient times and still in current use. However,
synthetic polymeric wound dressings such as silicones, polyurethanes,
polyvinyl chloride or polyethylene have made their appearance in the market
recently. They are generally thin layer films which have a synthetic adhesive
as a coating on the inner surface that adheres well to dry skin at the wound
margins but does not adhere to the wound site. An advantage is that
antibacterial agents such as iodine can be incorporated into the membranes.
Recently, natural polymers such as collagen, fibrin, fibronectin, alginate and
hyaluronic acid have been studied as dressings for dermal wounds.
Currently bilayer wound dressings are getting popular in the medical field.
The outer layer is designed for durability, elasticity and suturability to
wound edges and it acts as the epidermis. The inner layer acts as the dermis
with maximum adherence and elasticity. Generally these wound dressings are
made of a fabric mesh such as cotton gauze or dacron net or biological
polymers bonded to a synthetic polymer film backing such as polyurethanes or
silicones.
Table 2 lists the different types of wound dressings, commercial brand
names, manufacturer and the material used for preparation.
3. Dental Materials
Though polymer based products have been in use in dentistry for quite some
time, with the advent of Bisphenol A-Glycidyl methacrylate (BIS-GMA), more and
more innovative products are coming into the market. BIS-GMA based chemical
and light cured restorative materials, bonding agents, pit and fissure
sealants are widely used for dental filling applications and for tooth
build-up. Currently the dental market constitutes generally 35-40% of the
total medical device market. It is estimated that India loses about Rs.1360
crores every year through foreign exchange towards import of dental products.
Polymers play a major role in most areas of restorative dentistry. The most
widely used impression materials are elastomeric polymers. Removable dentures
are from acrylic resin and other polymer. Table 3 lists the dental areas where
polymers are currently being used.
4. Artificial Pancreas
Diabetes is caused by the loss of the endocrine component of the pancreas
which secretes insulin. As a result, blood glucose levels climbs to levels far
exceeding the normal range. Frequent injection of insulin allows gross control
of glucose levels. The controlled administration of insulin can be carried out
in various ways. Insulin can be slowly released from various polymeric systems
such as ethylene vinyl acetate (EVA). Besides their lack of feed back control,
polymeric matrix controlled release systems have a non-renewable finite
loading capacity.
5. Artificial Kidneys
The function of the artificial kidney is to maintain body substances at the
right concentration. This is mainly achieved by the use of a semipermeable
membrane which purifies the blood against artificial liquids in a process
known as hemodialysis or peritoneal dialysis. In peritoneal dialysis, silicone
elastomer or polyurethane elastomer is used generally as catheters to access
the peritoneal cavity. A polyester cuff surrounds the segment of each
catheter. In hemodialysis, the dialyser is normally made of several thousand
hollow polymer fibres mounted in a polyurethane potting. The fibres are
encased in a polycarbonte housing containing appropriate polymer connectors.
The dialysis tubing is generally of Polyvinyl chloride. The membranes used are
generally based on cellulose or cellulose derivatives.
Market Scenario
It is estimated that a global market of 130 billion US dollars (~ Rs.5600
billion) per annum exists for biomaterials in the world as per recent data
available. The projected global market for year 2000 is to the tune of 260
billion US dollars (~ Rs.11,200 billion). The market in India in 1996 was
estimated to be around 790 million US dollars (~ Rs.3400 crores) per annum.
The growth rate for health care technology market in US was about 7% whereas
in Asia it rose to 19% during 1997-98. In India, the growth rate is estimated
to be 15-20% per annum. Polymers dominate the field of biomaterials as all
disposable devices and a good percentage of implants are fabricated from
polymeric materials. It is estimated that nearly 4-5% of the total world
consumption of polymers goes into medical device production.
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