Magnetic Material Engineering. Chapter 6: Applications in Medical and Biology Magnetic Material...

Magnetic Material Engineering

Chapter 6:

Applications in Medical and Biology



Chapter 6: Applications in Medical and Biology

Magnetic nanoparticles




Materials: Fe, Co, Ni, Gd

Spins of unfilled d-bands spontaneously align parallel inside a domain below a critical temperature TC (Curie) Laws:

B = H +4H

M = H

: Susceptibility




Hard Magnets: HC and Mrs have high values



Superparamagnetism - a size effect




Superparamagnetism:

• high saturation magnetisation MS

• no remanence MR = 0



Superparamagnetism

The Néel relaxation in the absence of magnetic field

Normally, any ferromagnetic or ferrimagnetic material undergoes a transition to a paramagnetic state above its Curie temperature. Superparamagnetism is different from this standard transition since it occurs below the Curie temperature of the material.

Superparamagnetism occurs in nanoparticles which are single domain. This is possible when their diameter is below 3–50 nm, depending on the materials. In this condition, it is considered that the magnetization of the nanoparticles is a single giant magnetic moment, sum of all the individual magnetic moments carried by the atoms of the nanoparticle.



Superparamagnetism

Magnetic anisotropy is the direction dependence of a material's magnetic properties.

In the absence of an applied magnetic field, a magnetically isotropic material has no preferential direction for its magnetic moment while a magnetically anisotropic material will align its moment with one of the easy axes.

Magnetic anisotropy is a prerequisite for hysteresis in ferromagnets: without it, a ferromagnet is superparamagnetic.



Magnetic anisotropy

• Are the magnetic properties same in all directions? No• It depends on the crystallographic direction in which the magnetic dipoles are aligned– Crystal anisotropy (Spin Orbit Coupling)– Shape anisotropy– Stress anisotropy– Externally induced anisotropy– Exchange anisotropy

• E = KVsin2θ (simplest form)– K the effective uniaxial anisotropy energy per unit volume– V particle volume – θ angle between moments and easy axis



Magnetic anisotropy



Superparamagnetism

Because of the nanoparticle’s magnetic anisotropy, the magnetic moment has usually only two stable orientations antiparallel to each other, separated by an energy barrier.

The stable orientations define the nanoparticle’s so called “easy axis”.

At finite temperature, there is a finite probability for the magnetization to flip and reverse its direction. The mean time between two flips is called the Néel relaxation time τN and is given by the following Néel-Arrhenius equation:



Superparamagnetism

• N is thus the average length of time that it takes for the nanoparticle’s magnetization to randomly flip as a result of thermal fluctuations. • 0 is a length of time, characteristic of the material; its typical value is 10−9 –10−10 second. • K is the nanoparticle’s magnetic anisotropy energy density and V its volume.• KV is therefore the energy barrier associated with the magnetization moving from its initial easy axis direction, through a “hard plane”, to the other easy axis direction. • kB is the Boltzmann constant andT is the temperature.



Magnetic NanoparticlesMagnetic Nanoparticles

Cancer TreatmentsCancer Treatments

MR

IM

RI

Dru

g D

elivery

Dru

g D

elivery

Manipulation of Cell membranesManipulation of Cell membranes

Cell

Cell

Separation

Separation



Nanomagnetic Particle



Magnetic Cell Separation / Cell Labeling

Diagram of Immunomagnetically labeled Cell



Magnetic Cell Separation / Cell Labeling



Capture of bacteria by using Vancomycin-conjugated magnetic Capture of bacteria by using Vancomycin-conjugated magnetic nanoparticlenanoparticle

Control experiment



Magnetic Drug Delivery



MRI

The principles of MRI rely on the spinning of specific nuclei present in biological tissues.

In nuclei that have an even mass number, i.e. # protons = # neutrons, half spin in one direction and half spin in the other. Nucleus has no net spin.

However, in nucleus with odd mass #, spin directions are not equal and opposite, so the nucleus has net spin or angular momentum.

These are know as MR active nuclei.



MRI

MR active nuclei are characterized by their tendency to align their axis of rotation to an applied magnetic field.

This occurs because they have angular momentum or spin and, as they contain positively charged protons, they posses electrical charge.

The laws of electromagnetic induction refer to three individual forces – motion, magnetism and charge – and state that if two of these are present, then the third is automatically induced.

MR active nuclei that have a net charge and are spinning (motion), automatically acquire a magnetic moment and can align with external magnetic field.

The strength of the total magnetic moment is specific to every nucleus and determines the sensitivity to magnetic resonance.



MRI - The hydrogen nucleus

The hydrogen nucleus is the MR active nucleus used in clinical MRI. It contains a single proton (atomic and mass number 1).

It is used because it is most abundant in the human body and its solitary proton gives it a relatively large magnetic moment.

Both of these characteristics enable utilization of the maximum amount of available magnetization in the body.



The hydrogen nucleus as a magnet

The laws of electromagnetism state that a magnetic field is created when a charged particle moves.

The hydrogen atom contains one positively charged proton that spins.

Therefore it has a magnetic field induced around it, and acts as a small magnet.

It has a north and south pole.

Each of which is represented by a magnetic moment.



Alignment

Quantum theory describes properties of electromagnetic radiation in terms of discrete quantities called quanta.

Applying quantum theory to MRI, hydrogen nuclei posses energy in two discrete quantities term low and high.

Low energy align their magnetic moments parallel to external field (spin-up).High energy align anti-parallel (spin-down).



The MR signal

When a patient is placed within and MR scanner, the protons in the patients tissues (primarily protons contained in water molecules) align themselves along the direction of the magnetic field.

A radiofrequency electromagnetic pulse is then applied, which deflects the protons off their axis along the magnetic field.

As the protons realign themselves with the magnetic field, a signal is produced.

This signal is detected by an antenna, and with the help of computer analysis, is converted into an image.



Image formation

The process by which the protons realign themselves with the magnetic field is referred to as relaxation.

Different tissues undergo different rates of relaxation, and these differences create the contrast between different structures, and the contrast between normal and abnormal tissue, seen on MRI scans.



MRI of a Female Rat

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Magnetic Material Engineering. Chapter 6: Applications in Medical and Biology Magnetic Material...

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