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Introduction

Several technologies have been developed for proceduresresulting in fat reduction by lipolysis and aspiration.Proponents of laser-assisted lipolysis have presented sev-eral potential advantages of this technique over tradition-al liposuction. These advantages have been described inan earlier white paper entitled “Selective Laser InducedMelting”. In this paper, a device designed specifically tosafely "melt" the largest volume of fat possible isdescribed. A properly selected laser provides an excellenttool for this task if the right wavelength (or wavelengths)is chosen for the specific tissue to be treated. Selectivityof a specific light wavelength for an absorbing substance(also called a chromophore) is a measure of how welllight is absorbed by that chromophore in comparison tothe absorption of that light by other chromophores thatwill also be exposed to the light in the tissue. For exam-ple, water and lipids are important chromophores inhuman fat with lipids being the most abundant(75–85%).

"Melting" refers to the process of lipolysis realized ideal-ly by direct absorption of the light energy by the lipids.The absorption of this energy causes the lipids to heatsufficiently to thermally alter the dense lipid and proteinenvelope encapsulating the lipid droplet ultimately lead-ing to leakage from the adipocyte, a “lipid liberatingmechanism” induced by selective photothermolysis. Lipidliberation occurs either during the laser emission or as aresult of delayed thermally-induced changes to the perme-ability of the envelope.

924 nm for Efficiency

Light at 924 nm is highly selective for lipids in compari-son to water since this light is absorbed approximatelytwo times more in lipids than in water. To demonstratethe effect of several different laser wavelengths, a studywas performed comparing the temperature profiles inhuman fat (Figure 1). Each laser emitted 20 watts (W)of continuous wave output power through a 600 micron,straight end-cut fiber tip for 300 milliseconds (ms) and allother factors were held constant. The inner solid line ineach of the four images of Figure 1 represents the 64˚ Ctemperature contour indicating the outer edge of the zoneof coagulation around the fiber tip where immediate lipidliberation and tissue coagulation occur. The outer line

represents the 50˚ C temperature contour demarcating theouter edge of the zone of lipid liberation. For the sameenergy deposited by all four laser wavelengths, the 924nm light creates the largest zone of lipid liberation andcoagulation of proteins and fibrotic tissue. Figure 2shows the percent volume of coagulation and lipidliberation relative to the volume for the 924 nm laser.

Figure 1. Temperature profile in adipose tissue at the end of a 20W,300 ms pulse from a 600 micron fiber compared across differentwavelengths. The y-axis is in units of millimeters.

Figure 2. Percent volume of damaged fat for 924 nm, 980 nm, 1064nm and 1320 nm laser light at 300 ms pulse and 20W output power.

The volume of coagulated tissue using the 924 nm wave-length light is thirty-three (33) times greater as comparedto 980 nm, approximately six (6) times greater as com-pared to 1,064 nm and 2.7 times greater as comparedto 1,320 nm. When comparisons are made of the lipidliberating volumes, the 924 nm light is ten (10) timesmore efficient than 980 nm, four times more efficientthan 1064 nm and about 2.5 times more efficient than1,320 nm.

SlimLipo™ Laser WavelengthsJames J. Childs PhD, Mikhail Smirnov PhD, Alex Zelenchuk PhD, and Gregory Altshuler, PhD, DSc

924 nm for Safety

For treatment safety, it is important to prevent burning ofthe skin. Since 924 nm has the highest selectivity for fatversus water (and skin which is predominantly water),this wavelength is the safest when operating near thedermis. To demonstrate this, Figure 3a shows the sameconditions as in Figure 1 but with the fiber placed 2 mmbelow the dermis/hypodermis junction indicated by thesolid horizontal line. Note that for the 1320 nm wave-length, the dermis has been significantly damaged. Incontrast, no damage is observed in the dermis by theother lasers. The power of the light for all wavelengthswas then adjusted to achieve the same level of efficiencyof fat melting, i.e. equal volumes of damaged adiposetissue, as is achieved with the 924 nm laser light at 20W.In this case, all laser wavelengths except the 924 nmwavelength cause damage to the dermis (Figure 3b).Table 1 lists the required powers for each wavelength toproduce equal volumes of damaged adipose tissue. Inthis table, the percent damage shown is based on 1.5 mmthick dermis. As demonstrated, the 1320 nm laser causes100% damage since the 50˚ C contour line extends up to1.5 mm from the dermis/hypodermis junction. In con-trast, due to its high selectivity for fat, there is no damageto the skin with 924 nm light. Note also that, while thedermis is not damaged by the 1064 nm light at 20 Wpower and 2 mm distance, the 924 nm wavelength meltsfour times more volume of fat (Figure 2). The 924 nmlight is therefore the safest and most efficient wavelengthto use for melting fat.

Figure 3a. Temperature profile in adipose tissue near the dermis at theend of a 20W, 300 ms pulse from a 600 micron fiber compared acrossdifferent wavelength lasers.

Figure 3b. Temperature profiles in adipose tissue at the end of a 300ms pulse from a 600 micron fiber compared across different wave-length lasers. The power of each laser is adjusted for equal damagevolume in adipose tissue. Damage to the dermis is defined by thevolume within the 50˚C contour.

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Table 1. Comparison of percent dermal damage across the four wave-lengths. For equal comparison, each laser's power is adjusted to pro-vide the same damage volume in fat for a 300 ms laser pulse. Damageis defined as tissue coagulation plus cell death and extends to over 1.5mm into the dermis for the 1320 nm wavelength. This is considered torepresent 100% dermal damage.

Table 1 clearly shows that considerably more power isneeded at the other laser wavelengths to meet the fatmelting effects of 924 nm. However, by increasing thewattage at other wavelengths in order to liberate lipids asefficiently as 924 nm, there is increased risk of damagingthe dermis and other deep water-based structures whenusing these other wavelengths.

Finally to address hemostasis, it is important to note thatlight at either 924 nm or 975 nm wavelengths is readilyabsorbed in blood as compared with the other wave-lengths evaluated in this paper. Table 2 provides theabsorption coefficients of blood with 15 gm/dL of thedesignated hemoglobin at each wavelength.

Table 2. Absorption coefficients of blood at the four wavelengthsstudied in this paper.

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975 nm for Flexibility

Laser light at 975 nm is also an important wavelengththat was specifically chosen to complement the 924 nmlaser light when the SlimLipo™ laser is used, for example,in fat tissue with tumescence. This wavelength waschosen because it provides the highest selectivity forwater yet has a low enough absorption coefficient inwater to provide good penetration into the tissue andtherefore sufficient volume heating. Light at 975 nm ishighly selective for water versus lipids because it isabsorbed about ten times more in water than in lipids.By choosing two wavelengths, one with high selectivityfor fat and the other with selectivity for water, one hasbetter control to “dial in” treatment for a given tissuecomposition. For example, in the treatment of fat withlittle tumescence, 924 nm light may only be used. Intreating fat that contains significant tumescence or whendesiring to provide selective heating of the dermis for skintightening, the 975 nm light may be used to optimizeoverall absorption of the light in these structures withhigh water content. Figure 4 below demonstrates theincrease in the melting volume when 975 nm laser light isadded to the 924 nm light in adipose tissue with highwater content (hydrated fat with 30% water content onthe right side) as compared with normal fat (left side).This demonstrates that a system combining two wave-lengths, each selective to one of the two primarychromophores in fat, is optimized with greater flexibilityto treat a variety of tissue environments.

Figure 4. Temperature profile comparison of single and mixed wave-lengths of the SlimLipo™ tip in normal fat and in fat with 30% water(hydrated fat).

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