NON LINEAR MATERIALS AND BIOMATERIALS

BIREFRINGENCE AND KERR EFFECT

The appearance of double refraction under the influence of an external agent is known as artificial double refraction or induced birefringence.

Optical Kerr Effect

Anisotropy induced in an isotropic medium under the influence of an electric field is known as Kerr effect.

A sealed glass cell known as Kerr cell filled with a liquid comprising of asymmetric molecules is used to study the Kerr effect.

Two plane electrodes are placed in parallel to each other.

When a voltage is applied to there electrodes, a uniform electric field is produced in the cell.

The Kerr cell is placed between a crossed polarizer system (Fig), When the electric field is applied, the molecules of the liquid tend to align along the field direction.

As the molecules are asymmetric, the alignment causes anisotropy and the liquid becomes double refracting.

The induced birefringence is proportional to the square of the applied electric field E and to the wavelength λ of incident light.

Fig. Kerr effect –Birefringence is induced in a liquid subjected to an electric field

The change in refractive influx is given by

∆μ= K λE²

Where K is known as the Kerr constant

EXPLAIN NON LINEAR PROPERTIES AND SECOND HARMONIC GENERATION

Basic Principle of Non Linear Properties

We know that a light wave is electromagnetic in nature ie., it consists of electric and magnetic fields.

When the light propagates through a material, it changes the properties of the medium, such as the refractive index.

It depends on the electric and magnetic fields associated with the light.

For example, we could not observe nonlinear effects with the ordinary light beam of low intensity, since the electric and magnetic fields associated with the light beams is very weak.

With the invention of laser, it is now possible to have electric fields which are strong enough to observe interesting non linear effects.

Thus if electric and magnetic fields are strong enough, the properties of the medium will be affected which in turn will affect the propagation of the light beam.

NON LINEAR PROPERTIES

Few of the nonlinear phenomena observed are

Second harmonic generation
Optical mixing
Optical phase conjugation
Soliton

SECOND HARMONIC GENERATION

In a linear medium, polarization P is directly proportional to the electric field E

P ∝E

P = ε_oχE

Where_o ε- Permittivity of free space

χ – electrical susceptibility

In nonlinear medium for higher fields

ie., higher intensities of light the non linear effects are observed.

In the above equation, 1^st term gives rise to dc field across the medium, the second term gives external polarization and is called first or fundamental harmonic polarisability.

The third term which oscillates at a frequency 2w is called second harmonic of polarization and other terms are referred as higher harmonic polarization.

Both first term (dc field) and third term (second harmonic of polarization) added together is called optical rectification.

The second harmonic generation is possible only the crystals lacking inversion symmetry. SHG crystals are quartz, potassium dihydrogen phosphate (KDP), Ammonium dihydrogen phosphate (ADP), Barium titante (BaTiO₃) and Lithium lodate (LiIO₃)

The observation of second harmonic generation by KDP is shown in figure.

Fig. Arrangement for observing second harmonic generation

When the fundamental radiation (1.064 m) from Nd: YAG laser is sent through SHG crystal like KDP, conversion takes place to double the frequency. i.e., half the wavelength (0.532 m) takes place.

BIOMATERIALS WITH THEIR PROPERTIES AND APPLICATIONS

The materials which are used for structural applications in the field of medicine are known as Biomaterials.

In the recent years, new biomaterials like nanobiomaterials are emerging up due to the requirements in the medical field for different applications.

CLASSIFICATION OF BIOMATERIALS

Based on the applications in the field of medicine, biomaterials are classified as

Metals and alloys biomaterials
Ceramics biomaterials.
Polymer biomaterials.
Composite biomaterials

Sometimes, a single material mentioned above cannot fulfill the complete requirements imposed for specific applications. In such case, combinations of more than one material are required.

Metals and Alloys

Metals and alloys are used as biomaterials due to their excellent electrical and thermal conductivity and mechanical properties.

TYPES OF BIOMATERIALS USING METALS AND ALLOYS

Cobalt based alloys
Titanium
Stainless steel
Protosal from cast alloy
Conducting metals such as Platinum

APPLICATIONS

The metals and alloys biomaterials are used in implant and orthopedic applications.

Stainless steel is the predominant implant alloy. This is mainly due to its ease of fabrication and desirable mechanical properties and corrosion resistant.
Proposal from cast alloy of Co –Cr –Mo is used to make stem and used for implant hip endoprosthesis.
The advanced version of protosal –10 from Co–Ni –Cr –Mo alloy is widely used in Hip joints, Ankle joints, Knee joints, leg lengthening spaceas.
ASTMF –136 (composition of Ti –6A1 –4V, EL1 alloy, forged) due to its high strength / weight ratio, high corrosion resistance and high bio compatibility, this alloy is used in dental applications for making screws, wires and artificial teeth.
Ni –Ti shape memory alloy is used in dental arch wires, micro surgical instruments, blood clot filters, guide wires etc.

CERAMICS

Ceramics are used as biomaterials due to their high mechanical strength and biocompatibility.

Types of Bio-Ceramic materials

Tricalcium phosphate
Metal oxides such as Al₂O₃and SiO₂
Apatite ceramics
Porous ceramics
Carbons and Alumina

Applications

Ceramic implants such as Al₂O₃and with some SiO₂ and alkali metals are used to make femoral head. This is made from powder metallurgical process.
Tricalcium phosphate is used in bone repairs.
Orthopedic uses of alumina consists of hip and knee joints, tibical plate, femur shaft, shoulders, radius, vectebra, leg lengthening spaces and ankle joint prosthesis. Porous alumina is also used in teeth roots.
Apatite ceramics are new bio active ceramics. They are regarded as synthetic bone, readily allows bone ingrowth, better than currently used alumina Al₂O_3.
Carbon has good biocompatibility with bone and other tissues. It has high strengths and an elastic molecules close to that of bone.
Carbon coatings find wide applications in heart valves, blood vessel grafts, percutaneous devices because of exceptional compatibility with soft tissues and blood.
Percutaneous carbon devices containing high density electrical connectors have been used for the chronic stimulation of the cochlea for artificial hearing and stimulation of the visual cortex to aid the blind.

Bio Polymers

Biopolymers are macromolecules (protein, nucleic acids and polysachacides) formed in nature during the growth cycles of all organisms.

Biopolymers find variety of applications as biomaterials.

The most prominent among them are collagens, muco-polysaccharides –chitin, collagens and its derivatives.

Collagnes which are major animal structural proteins are widely used in a variety of forms such as solution, gel, fibers, membranes, sponge and tubing for large number of biomedical applications including drug delivery system, vessels, valves corneal prosthesis, wound dressing, cartilage substitute and dental applications.

Biomaterials in Opthamology

Biomaterials find important applications in opthalmology.

They are used to improve and maintain vision.

Eye implants are used to restore functionality of cornea, lens, etc, when they are damaged or diseased.

The biomaterials include viscoelastic solutions intraocular lenses, contact lenses, eye shields, artificial tears, vitreous replacements, correction of corneal curvature.

Dental Materials

Polymers, composites, ceramic materials and metal alloys are four main groups of materials used for dental applications.

A large number of materials are tested for porous dental implants, which include stainless steel,Co –Cr –Mo alloy, PMMA,proplast and Daceon, velour coated metallic implants, porous calcium aluminate single crystal alumina, bioglass, vitreous and pyrolytic carbons.

The dental applications include impression materials, dentine base and ceorons, bridges, inlays and repair or cavities, artificial teeth, repair of alveolar bone, support for mandible .

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