Ahmed Imran
Biomaterials – Basics and Applications
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Biomaterials - Introduction
Biomaterials = Bio + materials
Are those materials which are used to interact with the biological systems in a bio-chemical manner.
Simple physical contact, like that of our clothes touching the outside of skin, will not make the cloth
material as a biomaterial. Examples are: artificial organs like, heart valves, pacemaker, blood vessels,
artificial joints, ligaments, tendons, artificial kidney, liver, etc.
Such materials, which interact with the biologic tissues, need special considerations for
biocompatibility so that they can fulfill the following requirements:
1. Not affect the body
2. Not get affected by the body
3. Perform their intended function inside the body
4. Keep a good level of performance over several years, which may be around 20 or even 30 years.
5. Perform a large number of cycles of the work, like the heart beats about 70 times in a minute.
Doing this job for several year or decades (decade = 10 year period) may not be easy to achieve.
Biodegradable materials disintegrate in the biologic environment. For example, biodegradable sutures
(threads for stitching soft tissues) help in holding the soft tissue ends together. As the healing
progresses, the suture material disintegrates. The materials can be designed such that the rate of
disintegration matches with the rate of healing.
Biomaterials may be of biologic origin or of synthetic origin. For examples materials may be taken
from another part of the body, or from another person or from another animal. Similarly, artificial
organs may be made with polymers, ceramics, glasses or composite materials (composite = more than
one material combined together).
The biomaterials may be used for temporary or for long-term permanent function. The materials are
selected according to function. Function depends on properties and properties depend on structure.
Therefore, it is important to understand the structure, properties and their relationships of materials.
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Biomaterials – Basics and Applications
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Material Types
Biologic Materials
Grafting is one method of surgical transplantation or implantation to replace a damaged part or
compensate for a defect. There are four general categories:
1) Autografts: A graft of which tissue was removed from one part of the body and transplanted to
another site in the same person. This type of transplant is usually not rejected by the body.
2) Allografts or homografts: Transplants made from one person to another in the same species.
This may be successful when special efforts are made to prevent rejection.
3) Xenografts or heterografts: A graft of tissue which the donor and recipient are of different
species.
4) Lab grown tissues are also used developed with tissue engineering techniques.
Engineering Materials
Polymers. Polymers consist of small repeating units, or isomers, strung together in long chains. The
flexible structure of polymers has enabled this group of materials to be useful in applications from
plastic garbage bags to rubber tires. Even DNA has found this structure useful, storing genetic
information in thousands upon thousands of repeating sequences of polymers. In many materials,
processing conditions can induce the polymer chains to link with each other along the length of the
chain to produce a wide variety of mechanical properties. This is known as cross-linking. Crosslinking can increase the density of materials to improve their strength and hardness; however, crosslinked materials often lose their flexibility and become more brittle. These parameters are easily
varied in order to suit current biomedical applications.
The wide use of polyethylene (PE) in medicine illustrates the versatility typical of a polymer.
Depending on the processing of the material, PE can be elastic and flexible, or hard and smooth. Lowdensity polyethylene serves as tubing in catheters, while ultra-high-molecular-weight (UHMW)
polyethylene is one of the major articulating surfaces used in total hip or knee replacements. The
smooth surface of UHMW creates extremely low friction with other materials and increases the
durability of the artificial joint exponentially.
Hydrogels. comprise another subdivision of this unique group of materials. These polymer structures
form a hydrophilic cross-linked network that swells in water until an equilibrium is reached between
the retractive force of the network and the thermodynamic swelling force. They can exist as
homopolymers, copolymers, or multipolymers that are ionically neutral, anionic, cationic or
ampholytic. In general, hydrogels exhibit good biocompatibility, low degradation, and processing
ease. Hydrogels can be produced with a wide range of swelling characteristics, which play a major
role in determining solute diffusion rates, surface properties, refractive indexes, and mechanical
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Biomaterials – Basics and Applications
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characteristics. Cellulose derivatives such as poly(vinyl alcohol) (PVA), poly(ethylene glycol) (PEG),
and poly(N-vinyl 2-pyrrolidone) (PNVP) swell to a higher degree than poly(hydroxyethyl
methacrylate) (PHEMA) and its derivatives.
The applications of hydrogels cover a range just as broad as their physical properties. The ability of
hydrogels to swell and dehydrate depending on composition and environment can be exploited to
facilitate the controlled release of drugs. Soft contact lenses also rely on their hydrogel content to
induce an equilibrium between the lens and the surface of the eye, at the same time allowing gas
exchange and visibility without distortion. These polymers show promise in blood contact
applications as well as wound-healing and artificial cartilage and skin.
Biodegradable Polymers. Natural and synthetic biodegradable polymers have emerged in the field as
reliable and effective materials for purposes that require only temporary stability in order to support
tissue ingrowth. Biodegradable polymers degrade when placed in the body while allowing functional
tissue to grow in its place. The mechanism of degradation for these types of polymers includes
hydrolytic instability, hydration, molecular backbone cleavage, loss of molecular weight, and
solubilization.
Typical applications of these polymers include their use in absorbable sutures. These sutures degrade
slowly into byproducts that the body can remove itself through natural functions such as phagocytosis,
thus affording time for the wounded tissue to complete its own healing process. The sutures eliminate
the need for a second operation to remove them. Biodegradable polymers show promise in many
tissue engineering applications due to their ability to degrade while encouraging functional tissue from
the body to take its place.
Biodegradable polymers can be natural or synthetic. Some natural polymers include collagen, which
already comprises about 30% of the protein in the body, chitosan, which is derived from a
polysaccharide called chitin found in crustacean exoskeletons, and polyhydroxyalkanoates (PHA),
which are secreted by certain species of microorganisms. Synthetic polymers include poly(glycolic
acid) (PGA), which has been used in absorbable sutures, poly(lactic acid) (PLA), copolymers of PGA
and PLA, and polydioxananone (PDS).
Ceramics. This category consists of inorganic, nonmetallic compounds that exhibit a variety of
combinations of ionic and covalent bonding. They have been widely used in biomedical applications
for load bearing implants and the dental industry. Most members of this group are characterized by a
high Young’s modulus, very low elasticity, and a hard, brittle surface. They can be used for load
bearing applications, such as total hip replacements. Alumina, for example, is often used as a pair in a
ball and socket hip implant. Porous ceramic materials exhibit much lower strengths but have been
found extremely useful as coatings for metallic implants. The coating aids in tissue fixation of the
implant by providing a porous surface for the surrounding tissue to grow into and mechanically
interlock. Certain ceramics are considered bioactive ceramics if they establish bonds with bone tissue.
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This bonding activity increases the interfacial strength of adhesion greatly. Hydroxyapatite is a
natural mineral component of bone tissue that can be added to implants to encourage bone ingrowth.
Metals. The closely packed crystal structure and metallic bonding in metals or metal alloys render
them valuable as load bearing implants as well as internal fixation devices in large part for orthopaedic
applications as well as dental implants. 316 L stainless steel, titanium alloys, and cobalt alloys when
processed suitably contribute high tensile, fatigue and yield strengths; low reactivity and good ductility
to the stems of hip implant devices. Their properties depend on the processing method and purity of
the metal, however, and the selection of the material must be made appropriate to its intended use.
One complication that can occur from the use of metals in orthopaedic applications is the phenomenon
of stress shielding. In some situations, such as hip implantation, the high strength of the metal in the
implant induces it to assume more than its share of responsibility for the load in that region. This
decreases the load born by the surrounding tissue and therefore shields it from experiencing stress.
Lack of stress causes bone density to decrease as bone tissue resorbs, eventually baring the location
and causing complications in the implant/tissue interface.
Composites. The individual strengths and weaknesses of polymers, ceramics, and metals benefit
different applications. The porosity and hardness of ceramics support tissue integration into the
tissue/implants interface, but these properties could hardly suit a ligament replacement. A composite
material or structure that incorporates the desired characteristics of each of these materials must often
be designed in order to meet the stringent demands of living tissue. For example, a porous coating of
hydroxyapatite (HA), the major component of bone mineral, over the stem of a metal hip implant will
interlock mechanically with the surrounding tissue and aid in implant fixation. The smooth metal stem
can serve the load bearing capacity but lacks the ability to interact with the surrounding tissue.
Conversely, the bioactive ceramic HA is too brittle to handle the different stresses experienced by the
metal stem, but is capable of actively recruiting a secure tissue/implant interface. Most composite
designs combine strength and flexibility by reinforcing a relatively flexible material with a harder,
stronger one. In some cases, one or more of these materials may be degradable in order to encourage
tissue integration.
More notes
Biomaterials – Basics and Applications
Introduction to the course
The course involves basics of materials in terms of:
 Structure (geometric arrangement of atoms / molecules in bulk material)
 Properties (Characteristic response of a material to external stimulation)
 Function
 Categories of materials:
o Biologic
o Non-biologic: (metals and alloys, polymers, ceramics and composites)
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Applications of materials may be for:
 Hard-tissue or soft-tissue replacement
 Temporary or permanent
 Short term or long term
 different organ systems in the body (like cardiovascular, orthopedic, dental, ophthalmic)
Current trends and biomaterials will be discussed and
Major issues associated with the use of materials will also be studies.
Biomaterials:
Any material of natural or of synthetic origin that comes in contact with tissue, blood or biological
fluids, and intended for use in prosthetic, diagnostic, therapeutic or storage application.
The purpose of using a biomaterial is to replace or support function (of tissues or organs) in the body.
Biocompatibility
Biocompatibility-the ability of a material to perform with an appropriate host response in a specific
application
It is the most important consideration for the selection of a biomaterial in a given application.
Biocompatibility requires the host (body) and the material must interact in an acceptable manner
without causing harm to each other.
Biodegradable materials slowly disintegrate (break up into smaller particles / molecules) in the
biologic environment and are removed from the body through physiological processes. The materials
as well as the products of disintegration must be biocompatible.
Some of the important requirements for a biomaterial:
 inert or specifically interactive
 biocompatible
 mechanically and chemically stable or biodegradable
 process-able (for manufacturability)
 non-thrombogenic (if blood-contacting)
 sterilize-able
Medical Device Regulations
 All medical devices need approval from a regulating authority like HAAD in the UAE or FDA
in the US.
 The properties and safety of materials must be carefully assessed with respect to the specific
application in question and its degree of patient contact.
 The final assessment must be performed on the finished product, under actual use conditions.
 The regulatory authorities approve the medical devices (not the materials).
Related websites:
Ahmed Imran
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Biomaterials – Basics and Applications
Society for Biomaterials (www.biomaterials.org)
Biomaterials Network (www.biomat.net)
Medical Device Information (www.devicelink.com)
Medical Materials Engineering reference (www.engineeringreference.com)
United States Patents and Trademarks Office (www.uspto.gov)
MEDLINE (www.ncbi.nih.gov)
Biomaterials Journals:
 Journal of Biomedical Materials Research
 Biomaterials
 Journal of Biomaterials Science. Polymer Edition
 Journal of Biomaterials Applications
 Journal of Materials Science: Materials in Medicine
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