Measurements of hair temperature avalanche effect with alexandrite and Nd:YAG hair removal lasers.

BACKGROUND AND OBJECTIVES: In this study, we investigate the photothermal response of human hair using a pulsed laser source employed in the hair removal treatment. The purpose is to understand the dynamics behind the most common clinical practice to better define the salient features that may contribute to the efficiency of the process. STUDY DESIGN/MATERIALS AND METHODS: Temperature changes of hair samples (dark brown color) from a human scalp (skin type Fitpatrick II) were measured by a thermal camera following irradiation with single and multiple neodymium: yttrium-aluminum-garnet (Nd:YAG) (1064 nm) and alexandrite (755 nm) laser pulses. Particularly, the hair was treated with an individual laser pulse of a sufficiently high fluence, or with a series of lower fluence laser pulses. We investigated the temperature increase in a broad range of fluence and number of pulses. From the data analysis we extrapolated important parameters such as thermal gain and threshold fluence that can be used for determining optimal parameters for the hair removal procedure. Our experimental investigations and hypothesis were supported by a numerical simulation of the light-matter interaction in a skin-hair model, and by optical transmittance measurements of the irradiated hair. RESULTS: An enhancement of the temperature response of the irradiated hair, that deviates from the linear behavior, is observed when hair is subjected to an individual laser pulse of a sufficiently high fluence or to a series of lower fluence laser pulses. Here, we defined the nonlinear and rapid temperature built-up as an avalanche effect. We estimated the threshold fluence at which this process takes place to be at 10 and 2.5 J/cm2 for 1064 and 755 nm laser wavelengths, respectively. The thermal gain expressed by the degree of the deviation from the linear behavior can be higher than 2 when low laser fluence and multiple laser pulses are applied (n = 50). The comparison of the calculated gain for the two different laser wavelengths and the number of pulses reveals a much higher efficiency when low fluence and multiple pulses are delivered. The avalanche effect manifests when the hair temperature exceeds 45°C. The enhanced temperature increase during the subsequent delivery of laser pulses could be ascribed to the temperature-induced changes in the hair's structural properties. Simulations of the hair temperature under Nd:YAG and alexandrite irradiation indicate that the avalanche phenomenon observed in the hair suspended in air may apply also to the hair located within the skin matrix. Namely, for the same fluence, similar temperature increase was obtained also for the hair located within the skin. CONCLUSION: The observed "avalanche" effect may contribute to the reported clinical efficacy of laser hair removal and may at least partially explain the observed efficacy of the brushing hair removal procedures where laser fluence is usually low. The repeated irradiation during the brushing procedure may lead to an avalanche-like gradual increase of the hair's thermal response resulting in sufficiently high final hair temperatures as required for effective hair reduction.

Measurement of Simulated Debris Removal Rates in an Artificial Root Canal to Optimize Laser-Activated Irrigation Parameters.

BACKGROUND AND OBJECTIVES: To compare temporal rates of debris removal from an artificial root canal for three laser-assisted irrigation modalities single-pulse super short pulse (SSP), and two dual-pulse X-SWEEPS and AutoSWEEPS, and for two fiber-tip (FT) geometries flat and radial, and to evaluate the dependence of the debris flushing rate on the delay between the SWEEPS laser pulse pair. STUDY DESIGN/MATERIALS AND METHODS: Laser-assisted irrigation was performed with a pulsed Er:YAG laser operating in single-pulse SSP and dual-pulse SWEEPS laser modalities. The laser energy was delivered to the water-filled model access cavity through a FT with either a flat or radial ending. The X-SWEEPS modality delivered pairs of laser pulses separated by a fixed adjustable delay, while with the AutoSWEEPS modality the delay was automatically and repeatedly swept between 200 and 600 microseconds. The debris removal rate was determined with the use of a digital camera by measuring the rate at which a simulated debris was being flushed out of the artificial root canal. RESULTS: The simulated debris removal rate of the AutoSWEEPS modality is almost three times higher compared with that of the SSP modality. Further, the flat FT outperforms the radial FT by a factor of more than five in the case of SSP, and by more than 10 with AutoSWEEPS. The X-SWEEPS flushing rate exhibits strong dependence on the delay between the SWEEPS pulse pair, with the highest removal rate measured to be more than seven times higher in comparison with SSP. CONCLUSION: Dual-pulse laser irrigation modalities (AutoSWEEPS and X-SWEEPS) exhibit significantly higher simulated debris removal rates in comparison with the standard single-pulse SSP laser-assisted irrigation. As opposed to the previously reported dependence of pressure generation on FT geometry, the flat FT's simulated debris removal rate significantly outperforms the radial FT. © 2020 The Authors. Lasers in Surgery and Medicine published by Wiley Periodicals LLC.

Characteristics of Non-Ablative Resurfacing of Soft Tissues by Repetitive Er:YAG Laser Pulse Irradiation.

BACKGROUND AND OBJECTIVES: Recently, several minimally invasive gynecological, ENT and esthetic procedures have been introduced that are based on delivering "smooth" sequences of Er:YAG laser pulses to cutaneous or mucosal tissue at moderate cumulative fluences that are not only below the ablation threshold but typically also do not require local anesthesia.  To explain the observed clinical results using "smooth-resurfacing," it has been suggested that in addition to the direct heat injury to deeper-lying connective tissues, there is an additional mechanism based on indirect triggering of tissue regeneration through short-exposure, intense heat shocking of epithelia. The goal of this study is to improve understanding of the complex dynamics of the exposure of tissues to a series of short Er:YAG laser pulses, during which the thermal exposure times transition from extremely short to long durations. STUDY DESIGN/MATERIALS AND METHODS: A physical model of laser-tissue interaction was used to calculate the temperature evolution at the irradiated surface and deeper within the tissue, in combination with a chemical model of tissue response based on the recently introduced variable heat shock (VHS) model, which assumes that the tissue damage represents a combined effect of two limiting Arrhenius' processes, defining cell viability at extremely long and short exposure times. Superficial tissue temperature evolution was measured during smooth-resurfacing of cutaneous and mucosal tissue, and compared with the model. Two modalities of non-ablative resurfacing were explored: a standard "sub-resurfacing" modality with cumulative fluences near the ablation threshold, and the "smooth-resurfacing" modality with fluences below the patient's pain threshold. An exemplary skin tightening clinical situation was explored by measuring pain tolerance threshold fluences for treatments on abdominal skin with and without topical anesthesia. The obtained temperature data and pain thresholds were then used to study the influence of Er:YAG laser sequence parameters on the superficial (triggering) and deep (coagulative) tissue response. RESULTS: The simulations show that for the sub-resurfacing modality, the parameter range where no excessive damage to the tissue will occur is very narrow. On the other hand, using pain tolerance as an indicator, the smooth-resurfacing treatments can be performed more safely and without sacrificing the treatment efficacy. Two preferred smooth-resurfacing treatment modalities were identified. One involves using optimally long pulse sequence durations (≈1-3 seconds) with an optimal number of pulses (N ≈ 10-30), resulting in a maximal short-exposure superficial tissue response and moderate coagulation depths. And for deeper coagulation, without significant superficial heat shocking, very long pulse sequences (>5 seconds) with a large number of delivered pulses are to be used in combination with topical anesthesia. CONCLUSIONS: A comparison of the simulations with the established smooth-resurfacing clinical protocols in gynecology, ENT, and esthetics suggests that, through clinical experience, the clinical protocols have been optimized for the maximal superficial heat shock triggering effect. Further research is needed to gain a better understanding of the proposed role of heat shock triggering in the clinically observed regeneration of cutaneous, vaginal, and oral tissues following Er:YAG laser smooth-resurfacing. Lasers Surg. Med. © 2021 The Authors. Lasers in Surgery and Medicine published by Wiley Periodicals LLC.

Determination of Optimal Separation Times for Dual-Pulse SWEEPS Laser-Assisted Irrigation in Different Endodontic Access Cavities.

BACKGROUND AND OBJECTIVES: The purpose of this ex vivo study is to investigate whether it is possible to pre-determine and set the optimal separation times for the SWEEPS Er:YAG laser pulses pair during laser-assisted irrigation of endodontic root canals based on known lateral dimensions of the endodontic access cavities of different types of teeth. STUDY DESIGN/MATERIALS AND METHODS: As the optimal SWEEPS laser pulse pair separation for enhanced shockwave generation depends on the life-cycle of a single-pulse bubble, measurements of the oscillation time T B of the Er:YAG laser-generated bubble were made in 23 different endodontic access cavities of different types of teeth progressively widened in three different steps, into larger cavities, for a total of 69 cavities of different shapes and sizes. Different fiber-tip geometries (flat and radial), laser pulse energies (10 mJ and 20 mJ) and depth of fiber-tip insertion (2 mm and 4 mm) were also investigated. The obtained data were then analyzed using the reported relationship between the bubble oscillation time and the diameter of a cylindrically shaped cavity. RESULTS: A good fit to the relation analogue for ideal cylindrical cavities was found by taking the characteristic diameter of the access cavity to be represented by the cavity diameter either in the mesiodistal (D min ) or buccolingual (D max ) direction, or alternatively by the average of the two diameters (D ave ). The best fit was obtained for D min (R 2 = 0.73) followed in order by D ave (R 2 = 0.71) and D max (R 2 = 0.63). CONCLUSION: In spite of the endodontic cavities being non-cylindrical and of varied shape and size, the bubble oscillation time T B and the corresponding optimal SWEEPS separation time can be well predicted using a single characteristic dimension of the access cavity. This finding enables a simple and practical method for determining optimal conditions for shock wave generation and enhanced photodynamic streaming in differently shaped and sized root canals, leading to improved treatment efficacy and safety of root canal irrigation. Lasers Surg. Med. 2020. © 2020 The Authors. Lasers in Surgery and Medicine published by Wiley Periodicals LLC.

Characteristics of Bubble Oscillations During Laser-Activated Irrigation of Root Canals and Method of Improvement.

BACKGROUND AND OBJECTIVES: Laser-activated irrigation of dental root canals is being increasingly used as its efficacy has been shown to be superior compared with conventional techniques. The method is based on laser-initiated localized fluid evaporation and subsequent rapid bubble expansions and collapses, inducing microfluid flow throughout the entire volume of the cavity. The irrigation efficacy can be further improved if optimally delayed "SWEEPS" double laser pulses are delivered into the canal. This study aims to show that the irrigation efficacy, as measured by the induced pressure within the canal, is related to the double pulse delay, with the maximal pressure generated at an optimal delay. The second aim is to find a method of determining the optimal delay for different cavity dimensions and/or laser parameters. STUDY DESIGN/MATERIALS AND METHODS: Experiments were made in transparent models of root canals where Er:YAG laser (λ = 2.94 µm, pulse duration tp = 25 or 50 microseconds, and pulse energies up to EL = 40 mJ) was used with a combination of cylindrical and conical fiber-tip geometries (diameters 400 and 600 µm). High-speed photography (60,000 fps) and average pressure measurements inside the canal were used for process characterization. RESULTS: The results show that a pressure amplification of more than 1.5 times occurs if the laser pulse delay approximately coincides with the bubble oscillation time. Correlations between normalized oscillation time and canal diameter for a wide range of laser pulse energies (R2 = 0.96) and between the average pressure within the canal and the bubble oscillation periods (R2 = 0.90) were found. A relationship between the bubble oscillation time and the diameter of the treated cavity was found depending on the bubble oscillation time in an infinite fluid reservoir. CONCLUSIONS: The bubble oscillation time within a constrained volume can be determined based on the known oscillation time in infinite space, which offers a fast and simple solution for optimization of the laser parameters. These findings enable determination of optimal conditions for shock wave generation, and improvement of root canal irrigation at the same dose of laser energy input, leading to improved treatment efficacy and safety. Lasers Surg. Med. © 2020 The Authors. Lasers in Surgery and Medicine published by Wiley Periodicals, Inc.

Numerical Study of Hyper-Thermic Laser Lipolysis With 1,064 nm Nd:YAG Laser in Human Subjects.

BACKGROUND AND OBJECTIVES: The aim of this study was to develop a numerical model for hyperthermic laser lipolysis in human subjects to improve understanding of the procedure and find optimal therapeutic parameters. STUDY DESIGN/MATERIALS AND METHODS: A numerical model of hyperthermic laser lipolysis (HTLL) on human subjects was developed that is based on light and heat transport, including the effects of blood perfusion and forced air cooling. Tissue damage was evaluated using the Arrhenius model. Three irradiation scenarios were considered: single skin area irradiation without and with forced air cooling, and sequential heating of four adjacent skin areas in a cyclical manner. An evaluation of the numerical model was made by comparing the recorded skin surface temperature evolution during an experimental HTLL procedure performed on the abdomen of ten human volunteers using a 1,064 nm Nd:YAG laser irradiation. RESULTS: A good agreement was obtained between the simulated skin surface temperatures and that as measured during the HTLL procedure. The temperature difference between the simulations and experiments was in the range of 0.2-0.4°C. The model parameters, which were fitted to the experiment were the perfusion parameter (0.36-0.79 and 0.18-0.49 kg/m 3 ·s for dermis and subcutis) and the subcutaneous tissue absorption coefficient (0.17-0.21 cm -1 ). By using the developed HTLL model and the determined parameters, temperature depth distributions and the resulting thermal injury to adipocytes were simulated under different treatment conditions. Optimal ranges of the HTTL treatment parameters were determined for different skin types, damaging adipocytes while preserving skin cells. The target subcutaneous temperatures were in the range of 43-47°C, which has been found to lead to programmed adipocyte death. The optimal treatment parameters were further used to define a range of recommended protocols for safe and effective multiarea cycled HTLL treatment of large body surfaces. Specifically, for the set of chosen optimal treatment parameters (4-5 treatment cycles, 1.2 W/cm 2 radiant exposure, and 60-130 W/cm 2 forced air heat-transfer coefficient) the threshold surface temperature during irradiation was found to be in the range of 31-38°C, depending on the skin type and heat-transfer coefficient. CONCLUSIONS: The developed numerical model allows for the calculation of the temperature distribution and the resulting injury to adipocyte cells within deeper lying fatty tissues under different clinical treatment conditions. It is demonstrated that by measuring the temporal evolution of the skin surface temperature and by stopping the laser irradiation at predefined skin surface threshold temperatures, it may be possible to monitor and control the effects of the HTLL procedure deeper within the tissue. As such, the model provides a better insight into the HTLL, and may become a tool for defining the range of safe and effective HTLL treatment protocols for patients with different skin types. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

Amplification of pressure waves in laser-assisted endodontics with synchronized delivery of Er:YAG laser pulses.

When attempting to clean surfaces of dental root canals with laser-induced cavitation bubbles, the resulting cavitation oscillations are significantly prolonged due to friction on the cavity walls and other factors. Consequently, the collapses are less intense and the shock waves that are usually emitted following a bubble's collapse are diminished or not present at all. A new technique of synchronized laser-pulse delivery intended to enhance the emission of shock waves from collapsed bubbles in fluid-filled endodontic canals is reported. A laser beam deflection probe, a high-speed camera, and shadow photography were used to characterize the induced photoacoustic phenomena during synchronized delivery of Er:YAG laser pulses in a confined volume of water. A shock wave enhancing technique was employed which consists of delivering a second laser pulse at a delay with regard to the first cavitation bubble-forming laser pulse. Influence of the delay between the first and second laser pulses on the generation of pressure and shock waves during the first bubble's collapse was measured for different laser pulse energies and cavity volumes. Results show that the optimal delay between the two laser pulses is strongly correlated with the cavitation bubble's oscillation period. Under optimal synchronization conditions, the growth of the second cavitation bubble was observed to accelerate the collapse of the first cavitation bubble, leading to a violent collapse, during which shock waves are emitted. Additionally, shock waves created by the accelerated collapse of the primary cavitation bubble and as well of the accompanying smaller secondary bubbles near the cavity walls were observed. The reported phenomena may have applications in improved laser cleaning of surfaces during laser-assisted dental root canal treatments.

Amplification of high-intensity pressure waves and cavitation in water using a multi-pulsed laser excitation and black-TiOx optoacoustic lens.

A method for amplification of high-intensity pressure waves generated with a multi-pulsed Nd:YAG laser coupled with a black-TiOx optoacoustic lens in the water is presented and characterized. The investigation was focused on determining how the multi-pulsed laser excitation with delays between 50 µs and 400 µs influences the dynamics of the bubbles formed by a laser-induced breakdown on the upper surface of the lens, the acoustic cavitation in the focal region of the lens, and the high-intensity pressure waves generation. A needle hydrophone and a high-speed camera were used to analyze the spatial distribution and time-dependent development of the above-mentioned phenomena. Our results show how different delays (td ) of the laser pulses influence optoacoustic dynamics. When td is equal to or greater than the bubble oscillation time, acoustic cavitation cloud size increases 10-fold after the fourth laser pulse, while the pressure amplitude increases by more than 75%. A quasi-deterministic creation of cavitation due to consecutive transient pressure waves is also discussed. This is relevant for localized ablative laser therapy.

Measurement of Pressures Generated in Root Canal During Er:YAG Laser-Activated Irrigation.

Objective: To measure distribution of pressures along the depth of the root canal during erbium-doped yttrium aluminum garnet (Er:YAG) laser-activated irrigation (LAI) with different modalities and fiber tip (FT) geometries. Background: A new LAI modality based on the delivery of synchronized pairs of Er:YAG laser pulses to generate enhanced irrigant streaming and shock wave emission was recently introduced. However, the influence of FT geometry on efficacy and comparison with single pulse modality is not yet presented. Methods: Pressures within a simulated root canal were simultaneously measured at 5 depths during LAI. Seven FT geometries (conical and cylindrical) and two modalities [Super Short Pulse (SSP) and dual pulse AutoSWEEPS] were compared. Results: Under the same conditions, average pressures using SSP at 20 mJ of laser energy ranged from 111 Pa for a conical 600 µm FT to 225 Pa for a flat 400 µm FT. The measured pressures for the SSP and the AutoSWEEPS at 20 mJ laser energy were 223 and 308 Pa at the most coronal level and 119 and 126 Pa at the apical constriction, respectively. Measured pressures and irrigant penetration depths at different root canal levels were found to be linearly correlated (R2 = 0.82; p < 0.01). Conclusions: The generated pressures get progressively reduced from the coronal toward the apical third of the root canal. A strong dependence on the FT design and laser modality was observed. Within the limitations of the study, the AutoSWEEPS modality is more effective than standard SSP in generating pressures within the root canal, without increasing the risk of extrusion.

Evaluation of Apical Extrusion During Novel Er:YAG Laser-Activated Irrigation Modality.

Objective: To evaluate apical extrusion during a novel erbium-doped yttrium aluminum garnet (Er:YAG) laser-activated irrigation (LAI) modality. Background data: A novel double-pulse Er:YAG modality (AutoSWEEPS) was introduced recently, replacing a single laser pulse with two micropulses that are separated by a varying time delay (which is continuously "swept" between 300 and 600 µsec). Although the proposed method demonstrated increased efficacy, no data were yet available on extrusion. Methods: The extrusion was evaluated on simulated canals (n = 6) using particle imaging velocimetry. In the first two groups, the irrigation device was a syringe coupled to either a 30-G open-ended or side-vented needle, with flow rates of 1, 2, 5, and 15 mL/min. In the second two groups, irrigant activation was performed with an Er:YAG laser, using either a super-short pulse (SSP) or AutoSWEEPS modality. The pulse energies were 5, 10, 20, 30, and 40 mJ and the frequency was 10 Hz. Results: The measured extrusion was most prominent during the open-ended needle irrigation, followed by the vented needle irrigation. Compared with the conventional needle irrigation (CNI), all the studied LAI modalities resulted in ∼3-20 times less extrusion. The AutoSWEEPS modality induced the smallest extrusion rate, which was always <1.5 mm3/sec and was also independent of the laser energy. Conclusions: Within the limitations of the study, our results demonstrate that the SSP and AutoSWEEPS laser-assisted irrigation methods exhibited less extrusion in comparison with CNI methods.

Wavelength dependence of photon-induced photoacoustic streaming technique for root canal irrigation.

Laser-enhanced irrigation of complex root canals appears to be a very promising technique to improve the outcome of root canal treatment. This applies, in particular, if the technique can be effective at very low laser energies in irrigating not only the main canal but also the small lateral canals. This is important in order to avoid potential undesirable effects at higher laser energies such as temperature increase, dentin ablation, or extrusion of irrigating solution beyond the apical foramen. An improved understanding of the role of laser parameters, such as laser wavelength and pulse duration, in irrigation of lateral canals is therefore desired in order to optimize treatment efficacy. The influence of laser wavelength and pulse duration on cavitation phenomena was studied using shadow photography and a method of measuring fluid flow in lateral canals based on tracking of movements of small air bubbles naturally forming in liquid as a result of laser agitation. A simulated model of a root canal including a narrow lateral canal designed to represent typical root canal morphology was used for the water flow measurements. The following three laser wavelengths with relatively high absorption in water were studied: Er:YAG (2.94 µm), Er,Cr:YSGG (2.73 µm), and Nd:YAP (1.34 µm). Among the three wavelengths studied, the Er:YAG laser wavelength was found to be the most effective in formation of cavitation bubbles and in generating fluid motions within narrow lateral canals. A comparison between the shadow photography and fluid motion data indicates that it is the bubble's radius and not the bubble's volume that predominantly influences the fluid motion within lateral canals. Based on the results of our study, it appears that effective minimally invasive laser-assisted irrigation can be performed with low Er:YAG laser pulse energies below 10 mJ.