Development of a Minimally Invasive and Non-invasive Lipolysis Laser System for Effective Fat Reduction

In modern society, obesity does not mean being overweight, but it means a condition in which too much fat is accumulated in the body. Due to changes in eating habits and lifestyles, the obese population increasing by 20%. Methods for reducing fat for healthy life are being studied. 
In order to solve many complications caused by obesity, techniques for reducing and removing fat have been developed continuously. The method of lipolysis involves inserting a device into the body to suck up fat through a suction machine, destroying fat cells in the body using ultrasound equipment or a laser in vitro, and irradiating the laser directly to the adipose tissue through minimal invasion. There are non-invasive methods such as high frequency lipolysis, cool lipolysis, injection and drug ingestion.
 Laser lipolysis was first studied by Apfelberg in 1992. In 2002, Neira’s presentation on the effects of laser-assisted liposuction  led to increased interests in a laser-assisted lipolysis. After approval by the FDA in October 2006, the laser lipolysis is a commonly used method for removing superfluous fatty tissue and is being developed steadily. Laser lipolysis not only reduces fat and shortens recovery time, but also prevents bleeding by coagulating blood vessels, and it is effective for strengthening skin by inducing collagen production. Laser lipolysis generates a single wavelength using a diode laser or a Nd:YAG laser. When the laser beam meets the tissue, the beam is transmitted, scattered, reflected and absorbed. For biological effects, the laser beam must be absorbed into the tissue. The absorption rate of the laser beam for the tissue is determined by the natural frequency of molecules constituting the tissue and the frequency of the entering laser. In order to maximize the effectiveness of laser treatment, it is important to choose a wavelength that is appropriate for the treatment purpose. The wavelengths used for lipolysis are 924, 968 and 980 nm for the diode laser and 1064, 1319, 1320 and 1440 nm for the Nd:YAG laser. Fat reduction by the laser depends on the wavelength and energy of the laser. According to the theory of selective photothermolysis, tissue preferentially absorbs the energy of the laser based on the absorption coefficient of the fat and water of the wavelength. Figure 1 shows the absorption rate of water and fat with respect to the wavelength.

Figure 1
Comparison of the Laser Absorption Rate for Fat and Water.

Recently, we have been interested in the method of lipolysis by laser irradiation on skin without invasion. This is a way of reducing fat in the body by stimulating the adipose cell layer with the principle of hyperthermia treatment that the laser energy increases the temperature of the adipocyte to 42~47℃.  As shown in Figure 2, the 1064 nm wavelength region of the Nd:YAG laser has deep penetration depth compared to other wavelengths.

Figure 2
Depth of Skin Penetration by Wavelength.

Non-invasive lipolysis laser delivers the laser energy to the fat cells even when irradiated on the skin. 12 The mechanism of non-invasive laser lipolysis relies on temperature. 40~47℃ is reported as the death threshold of adipocytes, and the total decomposition of adipocytes happens at 50~65 ℃. 13-16 To protect the skin during laser irradiation, the hand-piece includes a cooling system. This cooling system does not have a therapeutic effect, unlike cryotherapy. The cooling system of the hand-piece allows the temperature of the skin to be 15 ℃ during treatment. 11 The disrupted adipocytes by the hyperthermia treatment are removed through the body’s natural mechanism, and inflammation induces macrophages to remove cellular debris. 12 In this paper, we performed three studies. First, we developed a minimally invasive laser system that reduces fat by direct irradiation on fat tissue. It had wavelengths of 1980 nm and 2300 nm which are good at the absorption of fat and water. Second, we developed a non-invasive laser system that reduces fat with hyperthermia treatment by raising the temperature of adipocytes with a 1060 nm penetration depth into the skin. Third, we confirmed the efficacy and safety of each system through animal experiments and confirmed the lipolysis effects when the two systems were combined.

Minimally Invasive Laser System 
Generally, DPSS 17 laser with the wavelength of 1064 nm uses Nd:YAG and Nd:YVO4 as a gain medium. To make a 1064 nm infrared laser source, either flash lamp excitation method or LD excitation method is used. For the excitation method of the flash lamp, dozens of watts of high power pulse lasers have been developed. However, Faraday rotators and polarizers are used to control the polarization, which limits the size of the laser system. We used a diode pumping method for a compact laser system. In this case, higher output is obtained when Nd:YVO4 is used as the gain medium rather than Nd:YAG.  Nd:YVO4 has an absorption rate of 808 nm that is five times higher than Nd:YAG, which is able to extends double the lifetime of the diode laser. As the single axis crystal with large double refraction, it is possible to obtain a linearly polarized beam without a polarization device by using Nd:YVO4 having the property of emitting a linearly polarized beam. The crystal of Nd:YVO4 is rectangular, with “a” and “c” directions orthogonal to each other. The laser rod usually orients the rod axis along an A-axis of the crystal. The Nd:YVO4 mount with cooling system can reduce the thermal lens effect generated when focusing the 808 nm excitation beam on the Nd:YVO4. As a result, a stable 1064 nm beam can be obtained. Sine pulsed lasers have the advantage of obtaining high peak power from low energy, AO Q-switch is used to create pulsed beams with repetition rates of tens of kHz.  In order to construct an intra cavity, an optical parametric oscillator composed of a mirror and a non-linear optical medium periodically polarized was placed inside the Nd:YVO4 laser resonator. In non-linear optical crystal used as a laser, rods are changed, the length of the crystal axis changes with temperature and the output wavelength changes due to the change in the length.

As shown in Figure 3, an oven with a heater and real-time temperature sensor was designed and made for temperature control of non-linear optical crystal for stable wavelength output. To select non-linear optical crystals to obtain wavelengths of 1980 nm and 2300 nm suitable for lipolysis, Bruner’s proposed “temperature dependent Sellmeier equation” for the refractive index of SLT was used. We calculated the temperature-dependent wavelengths of non-linear optical crystals such as CSP, PPLN, APMgLN, and PPSLT. As shown in Figure 4, we found that wavelengths of 1980 nm and 2300 nm were generated at 110℃ of PPSLT.

Figure 3
The Oven for Temperature Control of Non-Linear Optical Crystal, (a) Design, (b) Production and Application.

Figure 4
Variation of Wavelength of Non-Linear Optical Crystal With Temperature.

Three wavelengths, namely 532 nm wavelength, 2300 nm wavelength and 1980 nm wavelength, can be obtained at 110℃ of PPSLT. Among the wavelengths, mid-infrared wavelengths (1980 nm and 2300 nm), which are effective for lipolysis, are delivered to the fiber through dichroic filters and focusing lenses. The dichroic filter consists of three filters and a rotating motor to choose between 1980 nm and 2300 nm wavelengths respectively, or two combined wavelengths. The focusing lenses were designed and made to minimize the coupling loss when focused on fiber with 0.22 NA and 400 μm core size. Figure 5 shows the measurement results of the wavelength and power of the mid-infrared laser system for lipolysis. The temperature of non-linear optic crystal obtained wavelengths of 1980 nm and 2300 nm at 175℃. The 1980 nm and 2300 nm wavelengths are good absorption of water and fat, and the maximum power delivered to the fiber is 1.67 W. We have developed a minimally invasive mid-infrared lipolysis laser system that includes a laser head, temperature controller, power supply and cooling-system.

Figure 5
Output Wavelength and Power Measurement Results of the Lipolysis Laser System.

Non-invasive Laser System 
In the non-invasive laser, externally radiated laser energy is delivered to and absorbed by the adipocytes. 16 A source used a 1060 nm wavelength semiconductor diode laser with the deepest penetration into the skin. The hand-piece consists of a 1060 nm LD, optical system for the top-flat beam, cooling system and sapphire window. The rod shape LD has 40° and 80° divergence angle on the horizontal and vertical axis, respectively. The Gaussian-type beam has higher center beam intensity, and the beam intensity decreases toward the edges. The optical system is designed to increase lipolysis efficiency by irradiating a wider area with even beam intensity. In order to make the beam uniform, a cylindrical lens and a hand-piece coated with metal for diffuse reflection were used.