Optical properties of subdermal fat tissue

Rox Anderson et al.
Selective Photothermolysis of Lipid-Rich Tissues: A Free Electron Laser Study Wellman Centre of Photo medicine, Harvard Medical School, Boston. Massachusetts

Selective Photothermolysis is the principle of laser application technology for treatment of tissue that absorbs light more intensely than its surrounding tissue. Selective Photothermolysis is conditioned both by the type of light as well as the target. Each laser is characterised by its wave length. Each target tissue being treated has a preferential absorption for a certain wave length. The pulse width or duration of each delivery should be shorter than the thermal relaxation time of the tissue to assure that the heating is limited to the target without affecting the surrounding tissue.

This technique, normally associated to a cooling system to avoid heating of the skin surface, is successful in the treatment of vascular malformations, pigmented lesions, tattoos, pigmented hair, glaucoma, laryngeal lesions, etc.

In all of these applications, visible spectrum light or near-infrared laser or intense pulsed light is used to pump electronic photon absorption transitions in the target or chromophore (haemoglobin, oxyhaemoglobin, melanin, inks etc.).

Selective Photothermolysis has not yet been developed using vibrational absorption bands that exist in the infrared spectra emissions.

Far-infrared is considered a region of the electromagnetic spectrum with a molecular digital fingerprint; because the absorption of this delivery produces strong and narrow vibrations in the chemical bond structures.

While in theory Selective Photothermolysis could be performed using wavelengths great than 2,500 Nm, the reality is that these wavelengths are limited because strong absorption by water prevents photons from penetrating more than 0.1 mm into the tissue.

In contrast, the 900–2,800 nm region includes the most penetrating wavelengths, specifically around 1,200 Nm. Although in this spectral region, absorption is low because the photon energy is less than the electronic transition energy in most organic molecules and optical scattering is weak. In this band, there are weak vibrational absorption regions, consisting of overtones of the fundamental modes responsible for strong far-infrared absorption.

This study analyses the potential for Selective Photothermolysis of lipid-rich tissues by stimulating the vibrational absorption bands.

Lipids contain copious CH and CH2 bonds; and lipid-rich tissues usually have low water content compared with other tissues. Therefore, one part of the strategy is to use lipid vibrational bands that lie in between the dominant absorption bands of water.

** Most molecules synthesised by live organisms are optically active (with predominance of L-amino acids over the D-amino acids). The CH and CH2 bonds are potential vibrational absorption bands.

The rotational behaviour of macro-molecules is due to:

• The existence of asymmetric primary structures.

• The existence of a great of helicoidal biopolymers.

• The tertiary structures of the macro-molecules.

On this basis, molecular absorption is linked to circular dichroism. The behaviour of polarised light emitted on a type of optically active macro-molecule is different if the amino acid is D or L (dextrorotatory or levorotatory). Accordingly, a fingerprint is considered to exist in each molecular bond that emits on a different resonance and are recognizable.

The phenomenon of Dichroism within the field of optics has two acceptations that are related but different. A first phenomenon is the ability of some materials to fractionate a beam of polychromatic light into several monochromatic beams of differing wavelengths. It is not the same as the effect of diffraction and dispersion. The second phenomenon refers to the property of materials that on receiving the delivery of a beam of light in different polarisation planes, they display a level of absorption in different proportions after the reflection.

This study initially observed a spectra of human fat and water in the three bands for which absorption by fats exceeds that of water:

 915 Nm
1210 Nm
1720 Nm

It was also noted that heat capacity and thermal conductivity are lower in fat than in water, which tend to favour heating of fat.

Dr Serge Mordon. INSERM. Lille University Hospital, Lille. France. Mathematical Modelling of Laser Lipolysis.

In his study, Dr Serge Mordon quantifies precise laser energy to induce the effects of lipolysis.

Through the delivery of laser light at 1064 or 980 nm power: 6 W, and moving the optical fibre at 10 cm/sec, a surface temperature of 41 ºC is reached with the fibre at 8 mm below the surface in fat tissue.

Laser is transmitted to the adipocytes that absorb the power, expanding its volume and finally breaking; visible in histological analyses are: reversible cell damage (tumefaction), irreversible cell damage (lysis) and reduction in bleeding intensity compared with the aspiration product used in conventional techniques.

Volumetric reduction at 6 months can be defined by mathematical modelling, concluding that the reduction in fat tissue depends on the total amount of power applied. The study was performed based on a theoretical model of fat tissue and skin with an estimated dermal thickness of 2 mm, a hypodermal thickness of 20 mm and the optical fibre placed at 8 mm beneath the skin. Moving the fibre at a power of 6 W a total of 15 times at 10 cm per second without changing the plane and varying the horizontal orientation of the cannula 10º to newly move the optical fibre 15 times in the new orientation of the cannula until completely covering the 14 x 14 x 2.2 cm grid surface. With a total power deposit of 3,000 J and a total volumetric reduction of 5.2+/- 2.8 cm3.

So that to destroy 5 cm3 of fat tissue, an average total laser power of 3 kJ must be emitted.

Improvement is also observed in the skin retraction and concludes that it is sufficient to produce a temperature increase under the dermis of 48-50 ºC to induce the skin tightening component that is also sought after with this technique.

All of the information in this section is taken from scientific journals that are referenced at the start of each.