The first lasers, the ruby lasers, were invented by Maiman1,2 in 1960. Shortly after, the first medical application was reported and since then, within the continuous development of technology, which has seen the development of many medical lasers for use in a various range of medical conditions. Depending on the medium used, laser have several wavelengths and produces light with physical characteristics that are different from the light generated by sunlight or incandescent lamps. The two aspects are the basis of the huge diversity of lasers application in medicine. When a laser beam interacts with the skin surface, part of the incident energy is reflected and some is transmitted through the skin tissue. The transmitted energy is scattered or reflected by natural coloring such as collagen fibers in the skin, and part of it is absorbed by chromophores such as hemoglobin and melanin, resulting in photothermal reaction, photochemical reaction, photodestructive reaction, and fluorescence. Subsequently, reversible or irreversible changes such as plasma formation occur.3-5
Kalium Titanyl Phosphate (KTP) medical laser is a type of flashlamp Pumped Solid State (FPSS) laser that pumps Neodymium: Yttrium-Aluminum-Garnet (Nd:YAG) Rod into a flashlamp. When a KTP crystal is inserted inside the Nd:YAG laser resonator and the Nd:YAG laser light passes through this crystal, the wavelength is changed from 1064 nm to 532 nm green light. The KTP laser can produce a continuous output laser effect when the pulsed laser has a long pulse width and a high pulse repetition rate. KTP lasers with these characteristics are highly absorbed by the melanin, have low skin penetration, and effective for the treatment of epidermis, pigmented lesions, red tattoos, and vascular lesions.6-11
Diode-Pumped Solid-State (DPSS) laser technology and Nd:YAG Rod is a technology that uses a high-power semiconductor laser, unlike conventional FPSS lasers that use a discharge tube as the pumping light source. This offers many advantages such as improved laser tip output characteristics through the same change in pulse width and an increase in life. As a result, DPSS laser technology was an active research topic from the 1980s, and in the 1990s. High-power semiconductor lasers capable of pumping solid-state lasers have since been put into practical use and commercialized in various forms. In addition, since it operates in a cooling device in small size due to its high output efficiency and low heat generation compared to conventional lasers, it is smaller in size, easier to use, and less expensive to maintain.
The aim of this study is to visually and histologically observe the changes in the epidermis, dermis, and muscles after irradiation with 532 nm FPSS laser and DP-SS laser, and to evaluate the effect of different pulse widths have on the tissues. In addition, we compared the strengths and weaknesses of DPSS and FPSS lasers, to find out and suggest the more appropriate medical application among the two.
The experimental animals were 12-week-old healthy Dunkin Hartley guinea pigs weighing 250-300 g. The lasers used were FPSS lasers (Laserscope, San Jose), and DPSS lasers (gold mine, Daejeon). All of the experiment protocols on animals on this study were approved by the Institutional Animal Care guidelines of Dankook University of Medicine, Cheonan, Korea. The animals were anesthetized before each irradiation, using Zoletil (Virbac, France) and Rompun (Bayer, Korea) mixed at a ratio of 4:1. The mixture was given with one-fifth of the original dose as required by intramuscular injection.
The 532 nm FPSS and DPSS lasers were subjected to waveform analysis using an oscilloscope (Le Croy, New York) under the same frequency (25 KHz) and energy (2 watts) conditions.
The guinea pig muscle with skin was removed, 1 watt (63.7 J/cm2), 2 watt (127.3 J/cm2), 4 watt (254.6 J/cm2), 8 watt (509.3 J/cm2). Energy was applied with a beam diameter (spot size) of 2 mm for 1, 5, 10, 15, 20, and 25 seconds. Photoevaporation (vaporization) and photonecrosis on the guinea pig muscles were observed using a 10× magnified surgical microscope (Carl Zeiss, Germany) to assess the changes on tissue immediately after laser irradiation. Necrosis was observed and compared. The depth, width, and severity of photoevaporation and photonecrosis for varying exposure time were observed with a 10-fold magnification operating microscope and a microelectronic measuring transducer. Volumes were calculated and compared to analyze the effectiveness of laser with different light intensities and exposure times.
Laser effects on the skin tissue at 1 watt (63.7 J/cm2), 2 watt (127.3 J/cm2), 4 watt (254.6 J/cm2), 8 watt (509.3 J/cm2) energies. Four skin parts per animal were irradiated with a beam diameter of 2 mm for 2 seconds. All tissue specimens after laser irradiation were fixed in 10% formalin and seperated to a thickness of 5 μm after the embedding with paraffin for staining like hematoxylin and eosin staining. Changes in the epidermis and dermis were observed. The epidermal lesion size and the effects of light evaporation and photonecrosis were observed with a 40× microscope field of view.
A one-way ANOVA test by using SPSS was used to determine the size and depth of the lesion where the reaction between laser and the muscle tissue, epidermis, and dermis occurred.
Waveform analysis of 532 nm laser under the same frequency (25 kHz) and the same energy century (2 watts) conditions was performed. The results showed that the pulse widths of the FPSS laser and DPSS laser were 635 ns and 153 ns, respectively; the pulse width of the DPSS laser was about 4 times shorter than that of the FPSS laser. The pulse heights of the FPSS and DPSS lasers are 3.6 mV and 15 mV, respectively, and the DPSS laser peak light intensity (or peak energy density) was consistent with the theoretical predictions compared to the FPSS laser.
After irradiating the muscle tissue with a laser, the volume of photoevaporation and photonecrosis with different exposure times was measured with a surgical microscope, compared, and analyzed (Table 1). One-way ANOVA comparison results showed that the FPSS laser achieved significantly larger light evaporation and photonecrosis than the DPSS laser when irradiated for 1 second or longer (
Table 1 . Vaporization and Necrosis volume after irradiating 532 nm FPSS and DPSS laser on guinea pig muscle (mm<sup>3</sup>)
| 1S | 5S | 10S | 15S | 20S | 25S | |
|---|---|---|---|---|---|---|
| 2 watt (127.3 J/cm2) | ||||||
| DPSS-V | 0 | 0 | 0 | 1 | 1 | 2 |
| DPSS-N | 1 | 1 | 3 | 4 | 5 | 6 |
| FPSS-V | 0 | 4 | 6 | 7 | 10 | 11 |
| FPSS-N | 5 | 7 | 8 | 9 | 12 | 17 |
| 4 watt (254.6 J/cm2) | ||||||
| DPSS-V | 0 | 0 | 1 | 1 | 3 | 5 |
| DPSS-N | 2 | 2 | 4 | 6 | 7 | 8 |
| FPSS-V | 0 | 5 | 8 | 9 | 13 | 13 |
| FPSS-N | 6 | 7 | 9 | 10 | 13 | 16 |
| 8 watt (509.3 J/cm2) | ||||||
| DPSS-V | 0 | 0 | 2 | 2 | 3 | 4 |
| DPSS-N | 2 | 3 | 4 | 7 | 8 | 10 |
| FPSS-V | 0 | 7 | 9 | 10 | 15 | 18 |
| FPSS-N | 7 | 7 | 10 | 11 | 15 | 18 |
DPSS-V, DPSS laser induced photoevaporation of guinea pig muscle; DPSS-N, DPSS laser induced photonecrosis of guinea pig muscle; FPSS-V, FPSS laser induced photoevaporation of guinea pig muscle; FPSS-N, FPSS laser induced photonecrosis of guinea pig muscle.
When skin tissues were irradiated with the FPSS laser, most of the epidermis was removed by evaporation and the epidermal damage around the laser-irradiated site was relatively large. With a light intensity of 1 watt (63.7 J/cm2) and 2 watt (127.3 J/cm2), only partial light evaporation and photonecrosis were observed on the surface of the dermis while with 4 watt (254.6 J/cm2), In the 8 watt (509.3 J/cm2) light intensity, widespread photoevaporation and photonecrosis were observed from the surface to the bottom of the dermis. Histological examination revealed dermal vaporization of the epidermis and dermis, and showed necrosis with denatured nuclear proteins in the deep part of the evaporated dermis. It was observed that the changes were more with increased light intensity. On the other hand, the DPSS laser had smaller epidermal lesions than the FPSS laser with all light intensities, and epidermal necrosis was predominant at 1 watt (63.7 J/cm2) and 2 watt (127.3 J/cm2). At 4 watt (254.6 J/cm2) and 8 watt (509.3 J/cm2), only small parts of the epidermis were evaporated and little dermis necrosis was observed. In this case as well, the histological changes gradually increased with an increase in the light intensity (Figs. 1 and 2).
Figure 1. Histologic findings after irradiation of FPSS and DPSS laser {532 nm, 2 watt (127.3 J/cm2)}, (H & E stain, ×40) on the guinea pig skin. Both vaporization and necrosis width and depth of after FPSS irradiation is significantly more than those of DPSS. (A) FPSS laser irradiation. (B) DPSS laser irradiation. Bar, width and depth of vaporization and necrosis.
Figure 2. Histologic findings after irradiation of 532 nm FPSS and DPSS laser irradiation of FPSS and DPSS laser {532nm, 8 watt (509.3 J/cm2)}, (H & E stain, ×40) on the guinea pig skin. The depth of vaporization and necrosis after the FPSS irradiation is significantly more than that of the DPSS. (A) FPSS laser irradiation. (B) DPSS laser irradiation. Bar, width and depth of vaporization and necrosis
One-way ANOVA test showed that the size of the epidermal lesion was larger (
Figure 3. The vaporization width after the FPSS and the DPSS laser (532 nm) irradiation on the guinea pig skin. The width of vaporization of the DPSS laser-induced lesion is significantly narrower than that of the FPSS at each laser dosage. *statistically significant.
Figure 4. The necrosis width after the FPSS and the DPSS laser (532 nm) on the guinea pig skin. The width of necrosis of the DPSS laser-induced lesion is significantly narrower than that of the FPSS at each laser dosage. *statistically significant.
Figure 5. The vaporization depth after FPSS and the DPSS laser (532 nm) on the guinea pig skin. The depth of vaporization of DPSS laser-induced lesion is significantly shallower than that of the FPSS at each laser dosage. *statistically significant.
Figure 6. The necrosis depth after the FPSS and the DPSS laser (532 nm) on the guinea pig skin. The depth of necrosis of the of DPSS laser-induced lesion is significantly shallower than that of the FPSS at each laser dosage. *statistically significant.
Due to the nature of the laser light, it can concentrate the degenerative force on the target lesion by temporarily applying a strong electromagnetic field on the specific part and in a specific direction.12 The thermal effect of laser on the skin can be attributed to two points. The first is the expansion effect of the rapid vaporization of water in the tissue as a biological element, and the second is the thermal denaturation of the tissue proteins as a biochemical element.13 The effect of thermal energy is first displayed if the laser light is not absorbed by the tissue, and the degree of the light energy absorption is determined by the wavelength of the laser. The influence by the nature of the absorbing material is still under investigation. Goldman et al.6 reported that laser treatment works through thermal energy, so various methods have been deviced to change the laser from continuous form to pulsating form to minimize normal skin damage. For example, Janda et al.14 and others reported that the wavelengths of each laser, the Nd:YAG, Diode, and Argonion lasers are irradiated with continuous wavelengths, and while the coagulation ability is outstanding there is severe skin damage. However, when with pulsating CO2, Ho:YAG laser, irradiation the coagulation ability is small, but the fine excision ability and the ability to vaporize are good. The 532 nm FPSS laser used in this experiment has a wide pulse width, a fast display interval, and a consistent continuous wave that zooms on the skin and muscles like a continuous wave laser achieving significant photoevaporation and photonecrosis. In other words, the 532 nm FPSS laser that used a discharge tube as the pumping light source had a pulse width of 635 ns as described above, and when the DPSS system was used, the pulse width was 153 ns. Therefore, when comparing two lasers under the same average output conditions, the FPSS and DPSS lasers have the same energy output (2 watts x 1 second = 2 J), and the pulse repetition rate of the two types of lasers is 25. Since it is the same at KHz/sec, the energy of each pulse is also the same at 2 J/25000 = 80 μJ. Therefore, the peak light intensity at the highest peak of the pulse is 126 watts for the FPSS laser, but 523 watts for the DPSS laser. Consequently, in the case of DPSS laser, the exposure time is 4.15 times less than in FPS; when strong light energy is concentrated on the skin tissue for a short time, the heat conduction effect on the surrounding tissue is minimized, and there is less skin damage than in the case of FPSS laser. Although the results of the present study experiments do not provide accurate values, the overall results show that the DPSS laser causes less damage to the surrounding tissues than the FPSS laser.
Based on the present study findings, the DPSS laser causes less damage to the living tissue than the FPSS laser with the same exposure time. In addition, the use of glass such as fine excision, during DPSS laser treatment is recommended because it causes minimal damage to the skin and has less effect on the dermis. Moreover, DPSS lasers are more suitable for medical use due to their low maintenance cost.15
Since the DPSS laser uses a semiconductor laser as the pumping light source, the energy conversion efficiency is high because of the difference in the generation mechanism, and since the thermal cooling device is not large, the system is small, and the stability of the pumping light source is stable. Owing to these advantages, DPSS lasers are widely used in the industrial processing of laser materials. However, the magnitude of their effects on the epidermis and dermis during skin laser procedures is still unclear14 The results of the present study showed that the epidermis and dermis are less damaged during DPSS laser irradiation, and treatments that are more accurate than the micro excimer laser are recommended. DPSS laser has been demonstrated to be more effective and convenient for medical use; it significantly reduces side effects such as heat damage on the skin dermis that occur when using the currently available lasers. In addition, since it is possible to perform microscopic operations that were only possible with expensive vacuum equipment such as excimer lasers, its use in microsurgery is expected to increase in the near future.14-17 In addition, based on this research results, we can expect research and development of more advanced medical lasers that are more efficient and safe.
In this study, we evaluated the effects of laser waveform changes on tissues, through a comparison between the FPSS laser and the DPSS laser. DPSS laser showed less histological degeneration of the epidermis than the FPSS laser, with the same wavelength but different waveforms. The differences in the effects were clearer with increased energy. Since the DPSS laser has less damage on the skin, deep tissues can be finely excised. Owing to its numerous advantages the use of DPSS laser is expected to increase across a broad range of medical fields in the future.
No potential conflict of interest relevant to this article was reported.
Concept and design: SJL, PSC, SHW. Analysis and interpretation: YAK, GJJ, HRC. Data collection: YAK, GJJ, HRC. Writing the article: YAK. Critical revision of the article: SHW. Final approval of the article: JSK, SJL, PSC. Statistical analysis: YAK, JSK. Obtained funding: PSC. Overall responsibility: SHW.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2020R1I1A3072797).
Author ORCID Information
Yoon Ah Kim Gil Jae Jung Hae Ryung Choi Jong Soo Kim Sang Joon Lee Phil Sang Chung Seung Hoon Woo |
 |
