Biological materials

• The major difference between biological materials and biomaterials is viability
• Most biological materials are bathed within body fluids.
• This is common except for specialized surface layers such as skin, hair, nails, etc…

Proteins

• Proteins are polymers that are made from amino acids.
• There are 20 amino acids known.
• The structure is

Collagen

• One of the basic structural proteins is collagen.
• It has the general amino acids sequence of Glycine-Proline-Hydroxyproline-Glycine-X
• It is arranged in a form of triple alpha helix

Elastin

• Another structural protein that is found in a large amount in elastic tissue that performs a supportive function, i.e. ligaments, skin, etc…
• The chemical composition is somewhat different from that of collagen.

Polysaccharides

• Are polymers of simple sugars.

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• Exist in tissues with highly viscous material that interacts readily with proteins, resulting in proteoglycans.

Examples of polysaccharides

• Hyaluronic Acid is found in vitreous humor of eyes, synovial fluid, skin, umbilical cord and the aortic walls.
• Chondroitin: similar structure to hyaluronic acid and is found in the cornea of the eyes.
• Chondroitin sulfate: plays an important role in the physical behavior of connective tissues as lubricating agents between tissues, or between collagen and elastin microfibrils

Mineralized tissue ( Bone)

• Their function is load carrying.
• Teeth has a direct contact with ex-vivo structures
• While function of bone is carried out inside the body.

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Composition of cortical bone

Organization of bone

Mechanical properties of bone

• The strength of bone depends on the sign of load (compressive or tensile)
• The effect of drying of the bone effect the strength, bone in normal cases is wet, so drying will lower toughness, fracture strength and strain of failure
• Effect of mineral content.

Collagen rich tissues

• Function mostly in load bearing capacity.
• Skin, tendon, cartilage, etc…
• Special functions such as the transparency for the lens of the eye and shaping of the ear, tip of nose, etc..

properties

• Physical properties varies according to the amount and structural variations of collagen fibers.

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BIODEGRADABLE MATERIALS FOR�MEDICAL APPLICATIONS

INTRODUCTION TO BIOMATERIALS

• During the last two decades, significant advances have been made in the development of biocompatible and biodegradable materials for medical applications.

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• In the biomedical field, the goal is to develop and characterize artificial materials or, in other words, “spare parts” for use in the human body to MEASURE, RESTORE and IMPROVE physical functions and enhance survival and quality of life.

What’s a biomaterial?

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• 1980 - Passive and inert point of view

Any substance or drugs, of synthetic or natural origin, which can be used for any period alone or as part of a system and that increases or replaces any tissue, organ or function of the body.

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• 1990 – Active point of view

Non-living material used in a medical device and designed to interact with biological systems.

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Classification of biomaterials

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First generation: INERT

Do not trigger any reaction in the host: neither rejected nor recognition

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Second generation: BIOACTIVE

Ensure a more stable performance in a long time or for the period you want

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Third generation: BIODEGRADABLE

It can be chemically degraded or decomposed by natural effectors (weather, soil bacteria, plants, animals)

Mean features for medical applications

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BIOFUNCTIONALITY

Playing a specific function in physical and mechanical terms

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BIOCOMPATIBILITY

Concept that refers to a set of properties that a material must have to be used safely in a biological organism

What is a biocompatible material?

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1. Synthetic or natural material used in intimate contact with living tissue (it can be implanted, partially implanted or totally external).

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• Biocompatible materials are intended to interface with biological system to EVALUATE, TREAT, AUGMENT or REPLACE any tissue, organ or function of the body.

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A biocompatible device must be fabricated from materials that will not elicit an adverse biological response

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Biocompatible material features

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• Absence of carcinogenicity (the ability or tendency to produce cancer)
• Absence of immunogenicity (absence of a recognition of an external factor which could create rejection)
• Absence of teratogenicity (ability to cause birth defects)
• Absence of toxicity

Applications for Medical Devices

1)Total implanted device

2)Partially implanted device

3)Totally externals device

Some examples

BIODEGRADABLE MATERIALS

What’s a biodegradable implant?

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Once implanted, should maintain its mechanical

properties until it is no longer needed and then be

absorbed and excreted by the body, leaving no trace.

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Biodegradable implants are designed to overcome the

disadvantages of permanent metal-based devices.

BIODEGRADABLE MATERIALS

Problems caused by permanent implants

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• Physical irritations
• Chronic inflammatory local reactions
• Thrombogenicity and long term endothelial dysfunction (for cardiovascular applications)
• Inability to adapt to growth
• Not allowed or disadvantageous after surgery
• Stress shielding, corrosion, accumulation of metal in tissues (for internal fixation applications)
• Repeat surgery necessary

BIODEGRADABLE MATERIALS

Advantages of biodegradable implants

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• More physiological repair
• Possibility of tissue growth
• Less invasive repair
• Temporary support during tissue recovery
• Gradual dissolution or absorption by the body afterwards.

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Note: these implants may act a new biomedical tool satisfying requirement of compatibility and integration.

BIODEGRADABLE MATERIALS

More used materials

BIODEGRADABLE MATERIALS

Synthetic Polymers

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General criteria of selection for medical applications

• Mechanical properties and time of degradation must match application needs

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Ideal polymer:

• must be sufficiently strong until surrounding tissue has healed
• does not evoke inflammatory or toxic response
• to be metabolized in the body after fulfilling its purpose, leaving no trace
• to be easily processable into the final product form
• must demonstrates acceptable shelf life
• to be easily sterilized

BIODEGRADABLE MATERIALS

Synthetic Polymers

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Main advantages

• Good biocompatibility
• Possibility of changing in composition and in physical-mechanical properties
• Low coefficients of friction
• Easy processing and workability
• Ability to change surface chemically and physically
• Ability to immobilize cells or biomolecules within them or on the surface (Drug Eluting Stent)

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BIODEGRADABLE MATERIALS

Synthetic Polymers

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Main advantages

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• Presence of substances that may be issued in the body [ monomers (toxic), catalysts, additives ] after degradation
• Ease of water and biomolecules absorption from surrounding
• Low mechanical properties
• In some cases, difficult sterilization

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Note: the final properties of a device depends both intrinsic molecular structure of the polymer and chemical and mechanical processes which it is undergone.

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BIODEGRADABLE MATERIALS

Magnesium Alloys Based

Orthopedic devices

• Pins
• Rods
• Screws
• Tacks (chiodini)

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Cardiovascular applications

• Stents

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BIODEGRADABLE MATERIALS

Magnesium Alloys Based

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Main advantages

High biocompatibility (Mg is present into the body and then

recognized as a not foreign element)

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Alloy’s elements are dissolved into human body during the

Degradation process Not toxic risk

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Not visible by X-ray and not seen by CT or MRI Does not

cause any artifacts

BIODEGRADABLE MATERIALS

Magnesium Alloys Based

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Main disadvantages

• Too high corrosion rate (Es: Mg stents corrode quickly both in vivo than in vitro after ~ 1 month).
• Degradation occurs before the end of healing process

How to adjust this ??

By alloy and surface treatment

or

By mechanical pre-processing to affect biocorrosion

resistance

BIODEGRADABLE MATERIALS

Magnesium Alloys Based

Metal degradation

• Biodegradability expressed in terms of corrosion.
• Very slow process, "ideally" should not influence device mechanical properties until tissue healing is over
• Biocompatibility is reduced from ion accumulation released from metal
• Rate of corrosion and mechanisms vary with applied "shear-stress"

BIODEGRADABLE MATERIALS

Polymers VS Metals

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Considerations in the selection

• Strength
• Overall time and rate of degradation/corrosion (a very high degradation rate can be associated with inflammations)
• Biocompatibility
• Lack of toxicity

BIODEGRADABLE MATERIALS

Polymers VS Metals

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Orthopedic applications (screws, tacks… )

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• Metal alloys present greatest load bearing, with similar results to non biodegradable metals (stainless steel)
• Polymers present lower load bearing. Appropriate preprocessing may improve their mechanical characteristics

BIODEGRADABLE MATERIALS

Polymers VS Metals

Vascular applications (stents…)

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• Magnesium alloys degrade too fast in biological environment and they dissolve in the body, not permitting the correct vascular remodeling. Mg is an element that exists naturally into the body, then it is good tolerated.

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• Polymers degrade slower than magnesium alloys. Fundamental to care about degradation product concentration, which may be toxic.