*Article* **Medical Applications of Diode Lasers: Pulsed versus Continuous Wave (cw) Regime**

**Michał Michalik <sup>1</sup> , Jacek Szyma ´nczyk <sup>2</sup> , Michał Stajnke <sup>3</sup> , Tomasz Ochrymiuk <sup>3</sup> and Adam Cenian 3,\***


**Abstract:** The paper deals with the medical application of diode-lasers. A short review of medical therapies is presented, taking into account the wavelength applied, continuous wave (cw) or pulsed regimes, and their therapeutic effects. Special attention was paid to the laryngological application of a pulsed diode laser with wavelength 810 nm, and dermatologic applications of a 975 nm laser working at cw and pulsed mode. The efficacy of the laser procedures and a comparison of the pulsed and cw regimes is presented and discussed.

**Keywords:** laser diodes; pulsed and continuous wave (cw) regimes; medical applications; dermatology; laryngology

#### **1. Introduction**

We are approaching the 60th anniversary of laser medical applications. Shortly after the invention ruby lasers (with wavelength 694.3 nm) in the 1960s, Goldman et al. [1] started using it as therapy for melanoma, a human skin disease [2]. Later, in the 1980s, more powerful lasers, such as CO<sup>2</sup> lasers, argon lasers, and Nd:YAG lasers, were applied in the field of surgery (including laparoscopic), ophthalmology, dermatology, oncology, etc. An important step forward was the implementation of selective photothermolysis in dermatology by Anderson and Parrish [3], which are based on pigment-specific, shortpulsed lasers, e.g., Q-switched lasers.

Diode lasers (DLs), which first appeared in 1962, are still the most energy efficient and cost effective lasers. Therefore, they have found more and more applications in the field of medical therapies. Initially, DLs were not so popular as they gave power only in the order of mW. Diode lasers were used mainly for photobiomodulation (PBM)—previously also known as biostimulation or low-level laser therapy, LLLT— procedures, as well as for photodynamic therapy, where the wavelength is more crucial than high power [4,5]. Although PBM therapy was implemented by Endre Mester et al. [6] in 1967, for several decades it was mistrusted by many medical laser specialists. Only recently, after recognition of the role of cytochrome c oxidase in the mitochondrial respiratory chain as a primary chromophore and the introduction of the concept of "retrograde mitochondrial signaling", have attitudes changed. The significance of PBM in cell culture studies, resistance to fungal infections, mitigation of the side-effects of cancer therapy, pain and inflammation therapies, wound healing, muscle performance, etc. has become clearer. For example, Kowalec et al. [7] studied the Ceralas D15 diode laser which delivers optical power at 980 nm for wound and ulcer healing applications. The treatment enhanced wound healing and improved patient satisfaction and wellbeing. The study [8] confirmed the photomodulation efficacy of low power DL radiation at 740 nm (previously proven to be effective in wound healing) for

**Citation:** Michalik, M.; Szyma ´nczyk, J.; Stajnke, M.; Ochrymiuk, T.; Cenian, A. Medical Applications of Diode Lasers: Pulsed versus Continuous Wave (cw) Regime. *Micromachines* **2021**, *12*, 710. https://doi.org/ 10.3390/mi12060710

Academic Editors: Congyi Wu and Yu Huang

Received: 5 May 2021 Accepted: 12 June 2021 Published: 17 June 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

therapy of dry eye disease. The radiation can improve corneal surface, alleviate inflammation through decreasing of the neutrophils levels, etc. Espey et al. [9] demonstrated that PBM using 665 nm pulsed (200 ns) radiation from a DL with fluence 4 and 6 J/cm<sup>2</sup> results in a significant increase in sperm motility and velocity within 120 min post-irradiation.

High power (above 5 W) DL applications for surgery have taken place in dermatology (see, e.g., [10]) and oral surgical procedures (e.g., [11]). Bass [10] demonstrated DLs ability to eliminate vascular lesions, in a therapy called photosclerosis or thermocoagulation, using a DL emitting 810 nm wavelength radiation and has achieved satisfactory effects without scarring. The applied laser fluence during the square wave pulse (5–15 ms) was 14–42 J/cm<sup>2</sup> . The pulse interval was 32 ms (~31 Hz). Lesions treated included telangiectasias, spider veins, capillary dermal malformation and a cutaneous venous malformation. Telangiectasias were most responsive, usually disappearing after one treatment. Later, similar effects were achieved using a 980 nm diode laser by Desiate et al. [12]. Saetti et al. [13], after performing 22 endoscopic DL (810 nm) treatments of congenital subglottic hemangiomas, concluded that it is the safest and most effective (95% efficacy) therapy. The same cw DL radiation was used by Ferri et al. [14] to successfully treat Tis and T1 glottic carcinomas; all patients were able to eat without aspiration, as soon as the second day. Mittnacht et al. [15] applied DL radiation with power up to 450 W and λ = 808 and 940 nm to lung tissue. The laser with wavelengths 810 nm (3.5 W cw, 200 µm fiber) was used to treat oral Pyogenic granuloma [16].

More recently, Lee et al. [17] studied the efficacy of laser tonsillectomy using a 1940-nm laser working with a fluence of 12 W. The mean time for the procedure was 22.6 min and a notable reduction in pain at one week postoperative was elicited. Kang et al. [18] applied DL radiation with wavelength 1940 nm for the treatment of nasal congestion due to hypertrophied nasal turbinates. As the absorption coefficient of 1940-nm radiation in tissue is very high, the laser ablates tissue more precisely with less thermal damage. This clinical feasibility trial included eight patients with inferior turbinate hypertrophy. A rather low laser power of 4.5 W was applied leading to good medical results. In order to increase cutting efficiency of 940 nm DL, Agrawal et al. [19] studied the effect of various external chromophores (beetroot extract, erythrosine dye, hibiscus extract) applied on animal tissues. Staining of tissues with 3% erythrosine dye improved the efficacy of a 940 nm diode laser, by introducing sharper, wider cuts and clean incision with minimal charring when compared to beetroot, hibiscus, and saline chromophore.

In addition, the efficacy of 532-nm DL was investigated by treating a 50-year-old Korean female with oral erythro-leucoplakia [20]. Two months after the DL treatment, using a power of 6 W and 25 ms pulse, the operated region was well-healed without any significant scar contracture. Diode lasers emitting at wavelength 808 nm and different fluencies (12–14 J/cm<sup>2</sup> ) were tested for hair removal efficacy [21]; 30 ms laser pulses at a fixed rate of 7 Hz were applied. No significant difference was observed for both applied fluencies including patient comfort. The treatments were tolerated well without anaesthesia. The feasibility of a diode laser emitting at 1470 nm for blood vessel sealing was studied by Im et al. [22]. It was found that a power of 20 W and irradiation time of 5–10 s are adequate for effective sealing of blood vessels, although the higher power is required to cut the vessels.

Diode lasers with a central wavelength in the range 980 ± 10 nm have not been widely used up until now in high power clinical therapies. Romanos et al. [11] examined the wound healing after the application of a diode laser (980 nm) in oral surgical procedures, such as removal of soft tissue tumors, frenectomies, excision of gingival hyperplasias, vestibuloplasties, hemangioma removal, and periimplant soft tissue surgery. Laser radiation was applied both in pulsed and cw regime, with and without contact to the tissue. The advantages of this procedure were good coagulation properties; lack of bleeding, pain, scar tissue formation or swelling; and good wound healing. A few other examples related to otolaryngology procedures are known: turbinate reduction, nasal polypectomy, ablation of an oral papilloma, and photocoagulation of nasal telangiectasias [23]. Schmedt et al. [24]

has studied endovenous laser treatment of saphenous veins using a diode-laser emitting light of wavelength 980 nm which was transported via a 600 µm bare tipped optical fibre. Telangiectasias were most responsive, usually disappearing after one treatment [12]. Reynaud et al. [25] applied the 980 nm laser in laser-assisted lipolysis and Weiss et al. [26] in laser-assisted liposuction. Tunçel et al. [27] used DL (4–9 W) cw radiation to treat early glottic cancer and a year later Karasu et al. [28] applied DL radiation (3–5 W cw) to vocal fold polyps.

A Ceralas D15 diode laser delivering up to 15 watts of optical power at 980 nm using a quartz fiber delivery system was used to treat benign laryngeal lesions at officebased (outpatient) surgery—see [29]. Laser radiation (at power 12 W superpulse mode) was applied to a lesion through the working channel (3.7 mm in diameter) of the video fiberoptic esophagoscope. Some treated lesions such as: vocal polyps, leucoplakia, laryngeal hair showed significant improvement, yet required repeated procedures. On the other hand, patients with contact granuloma, subglottic stenosis and tracheal lesions showed partial remission with laser surgery. Recently, Karkos et al. [30,31] demonstrated the efficacy of a new "Π" surgical technique (using 980 nm DL laser, 4–9 W) postoperatively to improve quality of voice and swallowing. It was proven that the 980 nm diode laser appears to be safe and "friendly". Excellent long-term decannulation rates together with no significant deterioration in voice quality was achieved. Prazmo et al. [ ˙ 32] confirmed a positive effect of the repeated 980 nm laser pulsed irradiation (100 Hz) on intracanal *Enterococcus faecalis* biofilm elimination.

The effects of 975 nm radiation of dermatologic DL (in pulsed and cw mode) developed in IMP PAN was studied using optical phantoms of skin [33] before its clinical application [34]. Further research comparing the interaction effects of radiation at 532, 975, and 1064 nm was performed and reported by Milanic et al. [35]. It was concluded that the risks of the epidermis or subcutaneous tissue overheating are significantly reduced.

The aim of this paper is to describe and analyse the medical application of diode lasers operating in pulsed and cw regimes, with a special focus on laryngological or dermatological therapies. The results related to the authors' experience in the field are presented and discussed, including first simulations of dermatologic treatment. The advantage of pulsed laser application is discussed and its limitations are analysed.

#### **2. Materials & Methods**

The medical therapies analysed here were performed for several hundred patients treated in a private clinic, the Medical Centre MML in Warsaw (in the field of laryngology) and the private dermatology practice of Dr J. Szyma ´nczyk, in cooperation with the Institute of Fluid-Flow Machinery PAS in Gda ´nsk.

The Institute developed a dermatologic diode laser emitting at 975 nm, working at cw or pulsed regime—pulse lengths 100 ns−300 ms, and laser output power 20 W [36], which was later implemented for therapies of neurofibroma and hemangiomas [34]. The second diode laser applied in MML Centre generated radiation with wavelength 810 nm and a pulse duration 4 s. In both cases, high efficacy of laser treatments was registered. Efficacy of procedure was defined as the ratio of the number of patients with positive effects of treatment therapy to the total number of procedures performed.

Besides medical treatments, the theoretical modelling and analyses of laser radiation interaction with neurofibroma blisters were performed. Therefore, the classic fluid-solid interaction problem is simulated and solved, in which the use of the monolithic method [37] is justified. First of all, non-trivial coupling of the thermal-FSI type [38] is considered, with the laser beam providing a heat stream to the tumor surface. There is an unstable flow of heat stream through various types of tissues to the tumor interior filled with fluid. This fluid heats up and there is a phase change, combined with a rapid increase in pressure, which results in a significant non-linear increase in tumor volume due to the hyperelastic properties of the skin. The tumor eventually explodes after some time, less than the time it takes to reach the pain threshold. The Arbitrary Lagrangian-Eulerian (ALE) description

gives a proper foundation for monolithic methods in which simultaneous solution for all unknowns of the coupled fluid/solid system [39] and all interaction effects between the dependent equations are included. The set of balance equations in the well-known ALE form [40,41] are solved

$$\frac{\partial}{\partial t} \left\{ \begin{array}{c} \rho \\ \rho \mathbf{v} \\ \rho e \end{array} \right\} + \operatorname{div} \left\{ \begin{array}{c} \rho \mathbf{v} \\ \rho \mathbf{v} \otimes \mathbf{v} \\ \rho e \mathbf{v} \end{array} \right\} = \operatorname{div} \left\{ \begin{array}{c} 0 \\ \mathbf{t} \\ \mathbf{t} \mathbf{v} + \mathbf{q} \end{array} \right\} + \left\{ \begin{array}{c} 0 \\ \rho \mathbf{b} \\ \rho \mathbf{b} \mathbf{v} \end{array} \right\},\tag{1}$$

where *ρ* is the density of the continuum particle, **v** is velocity of the continuum particle, *e* = *cvT* + <sup>1</sup> 2 **v** 2 is total energy, *c<sup>v</sup>* is specific heat at constant volume, *T* is temperature of the continuum particle, **t** is the Cauchy stress flux, **q** = λ·*T*∇ is the molecular heat flux defined by Fourier law (λ is thermal conductivity coefficient), and **b** is the earth acceleration. The Cauchy stress flux can be divided into an elastic part and a diffusive part:

$$\mathbf{t} = \mathbf{P} + \mathbf{\tau}^c \,. \tag{2}$$

where **P** is an elastic momentum flux which is reversible and τ *c* is a total diffusive momentum flux which describes irreversible phenomena. Below the first introductory results of simulations are presented and analyzed.

#### **3. Results**

Here, the results of diode laser treatments performed in MML Centre (laryngology) and a private dermatology practise are presented and discussed.

#### *3.1. Pulsed Diode Laser 810 nm (5 W Fluence and Pulse Duration 4 s) in Laryngology Applications in MML Centre*

(i) Laser-assisted somnoplasty using the palisade technique, a method of treatment for snoring and sleep apnea, is characterized by high efficacy, a short recovery period, and minimal risk of complications [42]. The method is implemented for palatoplasty, surgery of palatoglossal and palatopharyngeal arch, and uvuloplasty. During the procedure, the diode laser fibre is introduced into the soft palate (see Figure 1), which results in the formation of linear intra-parenchymal adhesions that stiffen the palate and shift it in the vertical plane. The therapy results in the prevention of tissue vibration during sleep, which, in turn, leads to increased sleep comfort and maximally widened airways. There are several advantages for application of this laser-assisted procedure, e.g., it enables a shorter surgery time (30–40 min), under local anaesthetic conditions. Shortly after the procedure, the patient can be discharged.

**Figure 1.** Introduction of laser fiber into a soft palate.

In years 2007–2020, 84 diode laser-assisted somnoplasty procedures using palisade technique were performed. Complete clinical response was observed in 77 cases, and a partial response was seen in seven cases. The efficacy of the therapy reached 92%.

(ii) Separation of adhesions in nasal septum is needed due to postoperative complications see Figure 2. The adhesions being postoperative (iatrogenic) cicatrix appear between nasal conches and septum and inhibit normal air flow. After laser assisted separation, instead of the usual tamponade, a gel dressing, which dissolves after a certain period, is applied as sufficient. The laser procedure is safer for the patient and gives better results. From 2007 to 2020, 51 laser-assisted separation procedures were performed. Complete clinical response was observed in 49 cases, and a partial response was seen in two cases. The efficacy of the therapy reached 97%.

**Figure 2.** Laser assisted separation of adhesions in nasal septum.

(iii) Laser assisted frenuloplasty, a surgery for a short frenulum and frenectomy of labial frenulum is a simple, sensitive and safe medical procedure (Figure 3a). It is preceded by a local anaesthesia. The diode laser assisted therapy is bloodless and painless due to the character of laser radiation tissue interaction (increased coagulation). During the period 2007–2020, 62 diode laser-assisted frenectomy procedures were performed. Complete clinical response was observed in 61 cases, and a partial response was seen in one case, giving a procedure efficacy of 98%.

**Figure 3.** Frenectomy of labial frenulum (**a**) and laser-assisted closure of tonsillar crypts (**b**).

(iv) Laser-assisted closure of tonsillar crypts after removal of debris (known as tonsil stones) resulting from bacterial and viral infections (see Figure 3b). After the debris removal a diode laser fiber is introduced, which enables shrinking and closing of crypts. This is an ambulatory (also known as office-based or Outpatient) procedure under local anaesthetic, and is painless and bloodless. During the period 2007–2020, 31 diode laser-assisted closures of tonsillar crypts were performed. Complete clinical response was observed in 29 cases, and a partial response was seen in two cases, giving an efficacy of 96%.

(v) Laser-assisted haemostasis (coagulation) results from interaction of 810 nm radiation of diode laser with the blood and lymphatic vessels—see Figure 4a. The process enables bloodless procedures and eliminates haemorrhaging both during and postoperatively. The process efficacy reaches 100%.

(**a**) (**b**)

(vi) Laser surgery of laryngopharynx and larynx (Figure 4b) enables sensitive and precise operation, removal of deteriorated tissues and protection of healthy ones. The separated tissue can be sent for histopathologic diagnostics. During the period 2007–2020, 54 laser surgery procedures were performed. Complete clinical response was observed in 50 cases, and a partial response was seen in 4 cases, resulting in an efficacy for the procedure of 93%.

(vii) Laser-assisted removal of cancerous changes/tissues (papilloma, polyps, haemangiomas, vocal nodules) enables precise operation and reaching narrow channels in nasal, sinus and other regions—see Figure 5. There is a low risk of thermal damage to tissue, so introduced wounds normally heal fast. The procedures are relatively fast and less invasive than standard ones. The laser haemostasis inhibits haemorrhage. During the period 2007–2020, 67 diode laser-assisted removals of cancerous changes were performed. Complete clinical response was observed in 64 cases, and a partial response was seen in three cases, giving an efficacy for the procedure of 95%.

(viii) The laser-assisted blepharoplasty (popular cosmetic eyelid surgery) is a medical/cosmetic procedure leading to correction of upper eyelid drooping (Figure 6a). It consists in removal of skin surplus from the upper eyelid. The procedure enables an increase of eyeshot (improved field of vision) and face rejuvenation. Its efficacy reaches 99%. During period 2007–2020, 97 diode laser-assisted blepharoplasty procedures were performed. Complete clinical response was observed in 96 cases, and a partial response was seen in one case.

**Figure 5.** Laser-assisted removal of cancerous tissue. **Figure 5.** Laser-assisted removal of cancerous tissue.

**Figure 6.** The laser-assisted (**a**) blepharoplasty and (**b**) nasolacrimal duct anastomosis 12 months after procedure.

(ix) Laser assisted dacryocystorhinostomy (DCR) was performed using a diode laser 810 nm, at power 8–10 W and pulses 0.5–1 s, in the case of patients with tear duct obstruction [43]. An elastic laser fiber 0.4 mm wide was introduced through the tear duct towards the lacrimal sac. The procedure was performed for 60 patients (44 women and 16 men) with average age 60.9 years. Positive effects were observed in the case of 96%, 75%, and 78%, after three, six, and 12 months, respectively (Figure 6b). In two cases the procedure was repeated and general efficacy increased to 81%. The intraoperative use of mitomycin C during the procedure of nasolacrimal duct anastomosis with diode laser increases its effectiveness [44].

Summing up, the utilisation of a 810 nm diode laser allows not only the removal of damaged tissue but it leads to haemostasis of blood vessels which in turn results in higher safety of therapies. This is of special importance when dealing with blood engorged tissues, where the risk of postoperative complications can be much higher.

#### *3.2. Pulsed and Continuous (cw) Operation Regime of Diode Laser 975 nm Implemented for Therapy of Dermal Neurofibroma*

In the case of patients affected by *dermal neurofibroma* disease, therapy proceeded at different levels of laser power in order to find the optimum conditions. Figure 7 presents

7 weeks after laser treatment.

the effects in the case, when laser radiation with cw power 10 W and 15 W pulsed regime (pulse 50 ms, 10 Hz) was applied to treat right side of the décolleté area. In the second case (see Figure 8), cw power 12 W was applied. The check after ~4–9 weeks have shown that the best therapeutic and cosmetic results have been achieved for cw power of 10 W. In the case of higher powers the healing period was longer as well as the cosmetic effect less desirable due to tendency to scarring.

(**d**) (**e**) (**f**) **Figure 7.** Laser therapy of dermal neurofibroma at right side of the décolleté area using DL radiation with wavelengths 975 nm: and cw power 10 W; (**a**) view before irradiation, (**b**) soon after irradiation (**c**) 7 weeks after laser treatment; and with pulsed power 15W (pulse 50 ms, 10 Hz) left side of the décolleté area (**d**) view before irradiation, (**e**) soon after irradiation (**f**)

**Figure 8.** Laser therapy of dermal neurofibroma at right side of the neck below ear region using DL radiation with wavelengths 975 nm and continues wave, power: 12 W; (**a**) view before irradiation, (**b**) soon after irradiation (55 s), (**c**) 7 weeks after laser treatment, (**d**) one year after therapy.

Application of lower radiation powers does not significantly improve the final therapeutic or cosmetic effect, i.e., by flattening of irradiated distortions or reduced tendency to scarring. It looks like the applied pulsed regime gives slightly better results (compare Figure 7c,f), the procedure is slightly less painful and better tolerated by patients. However, the procedure lasted longer. In the case of the patient presented in Figure 8, the effects one year after therapy may point to the need for therapy repetition.

Due to dolorability of the therapy using both diode (975 nm), Nd:YAG and Ho:YAG lasers the treatment was preceded by local anaesthesia with 1% of lignocaine. However, patients' reactions and tolerance of these laser therapies were variable. In the case of diode laser (975 nm) patients do not experience real pain or any tissue warming despite deep coagulation. The tissue coagulation proceeds fast and effectively. The reaction to Ho:YAG laser irradiation (2100 nm) was different. Patients despite local anaesthesia very often suffered unpleasant high temperature effects due to the laser irradiation and coagulation process. The treatment (Ho:YAG laser irradiation) of the skin, necessary to achieve the required result, lasts significantly longer than in the case of the laser diode.

In order to understand better the phenomena and mechanism of neurofibroma therapy, a theoretical modelling and analyses of laser radiation interaction with neurofibroma blister was performed. As mentioned in Section 2 the fluid contained in these cancerous blisters is heated by laser radiation and evaporates rapidly increasing pressure. The blister eventually explodes after some time, e.g., at least 3 s for blisters of 1.8 mm in diameter and more than 5 s for blisters 3 mm in diameter. These results correlate well with the results of introductory simulations based on the ALE model described above, e.g., the full evaporation of liquid in the neurofibroma blister occurred after 3 s of laser heating—see Figure 9.

**Figure 9.** The simulated gas phase change during the period of 3 s.

Figure 10 presents the relation between temporal evolution of pressure inside the cancerous blister and the volume of fluid which has not yet undergone phase change (evaporated). The maximum calculated pressure is 817 kPa, after 3 s of laser irradiation. At that moment, 96% of the liquid had already evaporated. At that moment, the pressure forces surpass the elastic forces and explosion results.

**Figure 10.** Relation between volume of fluid inside skin blister and pressure inside.

#### **4. Discussions**

λ λ Diode lasers with wavelengths of 810 ± 10 and 980 ± 10 nm are used in cw and pulsed modes. The radiation is transmitted to the operation field using optical fibre, which may or may not contact the tissue being treated. These DL's promote less bleeding, cleaner and more adequate operative field, significant reduction in post-operative oedema associated with direct reduction in pain, and improvement in tissue repair (see, e.g., [45]). Besides, Hanke et al. [46] studied soft-tissue cutting-efficiency for DL emitting in the wavelengths (λ) range 400 to 1500 nm. They found that the cutting depth for 2.5 W laser radiation moving at the speed 2 mm/s is equal 530, 330, 260, 230 µ for λ = 445, 810, 980, 1064 nm, respectively. Total interaction zones change accordingly. The 980 nm radiation is slightly better absorbed by water than the 810 nm one, which results in a smaller interaction zone. For example, Goel et al. [47] stated "The diode laser 980 nm is usually preferred for DCR surgery as it provides a better ablation and narrower tissue area involvement versus 810 nm that creates better coagulation than the vaporization". Table 1 presents various medical applications of the mentioned lasers.

λ τ **Table 1.** Medical application of pulsed and cw diode laser with wavelengths 810 and 980 nm (λ denotes wavelengths, τ pulse lengths).



**Table 1.** *Cont*.

Although in the paper we focus on diode laser application, in otolaryngology various lasers have been used, following the first (in the late 70s) implementation of an argon laser for inferior turbinate reduction. Lasers have been later successfully applied for a variety of nasal pathologies, such as epistaxis, inferior turbinate hypertrophy, nasal and paranasal tumors, skin lesions, and pathologies of the nasopharynx—see e.g., [53]. Although, Abiri et al. [54] pointed to the argon laser as the superior for some laryngology problems (caused, e.g., by hereditary haemorrhagic telangiectasia) other lasers such as Nd:YAG (second harmonic) and diode lasers also give good results. However, the application of CO<sup>2</sup> laser radiation is limited due to the complexity of nasal anatomy and lack of appropriate elastic fibres.

The CO2, Nd:YAG (second harmonic), argon and diode lasers were also successfully applied to oral cavity and oropharyngeal lesions, such as hypertrophic gingivitis, chronic tonsillitis, benign and malignant tumors, etc. [55]. These lasers provide better haemostasis, greater cutting precision, and reduced postoperative edema when compared to other standard methods of surgery.

The first laser assisted dacryocystorhinostomy (DCR) was implemented (by Massaro et al. [56]) in order to increase the diameter of tear duct (nasolacrimal duct) whilst avoiding bleeding. The argon laser was used in order to generate a tear duct (4–6 mm wide), which allow tears from the lacrimal sac of the eye to reach the nasal cavity. Later, the advantage of various elastic fibres allowed the application of other wavelengths, e.g., 2120 nm of Ho:YAG laser [57], 810 [43] or 980 nm [58] diode lasers.

Fluence is a key parameter which should be carefully adjusted in order to cause minimal damage to tissues adjacent to the incision site. Another issue is related to pulse operation mode. It was observed during neurofibroma treatment that pulsed mode (50 ms, 10 Hz) was perceived by the patient as less painful than the cw regime. However, it led to a longer operation time. Besides, using a higher pulse power for a shorter period of time results in less tissue damage than using lower power for a longer period of time. This is of special importance for selective photothermolises studied by Anderson and Parrish [3], but the most popular diode laser used in medical therapies does not offer such possibilities. Therefore, cw mode operation is usually favoured in various therapies due to the reduced operation time. The exceptions are presented in Table 1.

#### **5. Conclusions**

The results of radiation tissue interactions depend upon the tissue absorption coefficient, the wavelength of the laser, power density, operation mode (including pulse lengths and frequency), and interaction time. Although these data are presently better described in various papers they are still not always fully provided.

From Table 1, it is clear that in the case of soft tissue surgery the cw operation mode is preferred by the medical staff. This is because of the limitations of pulse power in the most common diode lasers and its effect on operation time. However, one should remember that pulsed operation mode may result in less damage in tissue adjacent to the incision site. The 980 nm DL radiation may in some cases provide a better ablation and narrower tissue affected zone in relation to 810 nm laser which in turn will be better for coagulation.

Diode lasers are becoming increasingly popular in medical applications due to their small size, robustness and compactness, cost-effectiveness, and ease of operation as well as high efficiency (reaching up to 70%). Moreover, the price of diode lasers is getting more and more competitive in relation to other systems. However, the significant drawback of this technology is the scarcity of diode lasers with short and high power pulses, important, e.g., in the case of selective photothermolises therapy [3]. Pulse powers up to 150 W are available [59].

**Author Contributions:** Conceptualization, M.M., T.O and A.C.; methodology, M.M., J.S. and T.O; software, M.S.; validation, M.M., T.O. and A.C.; formal analysis, M.M., J.S., T.O. and A.C.; investigation, M.M., J.S. and M.S.; resources, M.M., T.O. and A.C.; data curation, M.M. and M.S.; writing—original draft preparation, M.M., M.S., T.O. and A.C.; writing—review and editing, M.M., J.S. and A.C.; visualization, M.M, J.S. and M.S.; supervision, M.M., T.O. and A.C.; project administration, M.M. and A.C.; funding acquisition, M.M and A.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study as mentioned in Appendix A.

**Conflicts of Interest:** There are no conflicts of interest.

#### **Appendix A. Patient Statement of Informed Consent for Surgical Treatment**

As a patient of MML Medical Center, I hereby agree to undergo treatment, which will consist of:

(Name of procedure)

I have been informed of the requirements, processes and stages of surgical treatment, its purpose, expected results and potential risks that may occur as a result of this treatment. I accept the multi-disciplinary treatment plan, which involves surgical treatment. I have been informed of the costs of treatment and accept these.

I have been informed of the possibility of early and late post-surgery complications and the accompanying risks. I have understood the explanations and asked all questions that are of interest to me in regard to this medical procedure. Should a situation arise requiring it, I agree to a modification of the surgical procedure to the necessary extent, in accordance with the principles of medical knowledge.

I hereby give conscious consent to perform this treatment under local/general anaesthesia and declare that I have not concealed any crucial information regarding my overall health status. I have been informed about the possibilities of medical complications during the procedure which will be conducted.

I have been informed of and agree to allow the necessary photographic and radiological documentation in connection with the treatments.

I have been informed about the necessity of reporting to post-treatment follow-up control visits. I have been informed that smoking and poor oral hygiene and failure to follow post-treatment recommendations can significantly exacerbate potential posttreatment complications and negatively affect the treatments success.

I submit to the following restrictions associated with the medical procedure: Performing Doctor Legible Patient signature

#### **References**


**Jia Fu 1,2,\* , Ian Richardson <sup>1</sup> and Marcel Hermans <sup>1</sup>**

	- **\*** Correspondence: j.fu@tudelft.nl

**Abstract:** Oxide dispersion-strengthened (ODS) Eurofer steel was laser welded using a short pulse duration and a designed pattern to minimise local heat accumulation. With a laser power of 2500 W and a duration of more than 3 ms, a full penetration can be obtained in a 1 mm thick plate. Material loss was observed in the fusion zone due to metal vaporisation, which can be fully compensated by the use of filler material. The solidified fusion zone consists of an elongated dual phase microstructure with a bimodal grain size distribution. Nano-oxide particles were found to be dispersed in the steel. Electron backscattered diffraction (EBSD) analysis shows that the microstructure of the heat-treated joint is recovered with substantially unaltered grain size and lower misorientations in different regions. The experimental results indicate that joints with fine grains and dispersed nano-oxide particles can be achieved via pulsed laser beam welding using filler material and post heat treatment.


**Citation:** Fu, J.; Richardson, I.; Hermans, M. Microstructure Study of Pulsed Laser Beam Welded Oxide Dispersion-Strengthened (ODS) Eurofer Steel. *Micromachines* **2021**, *12*, 629. https://doi.org/10.3390/ mi12060629

Academic Editors: Congyi Wu and Yu Huang

Received: 29 April 2021 Accepted: 26 May 2021 Published: 28 May 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Keywords:** oxide dispersion strengthened steel; ODS Eurofer; laser welding; microstructure; EBSD

#### **1. Introduction**

Due to their good high-temperature strength, corrosion resistance and radiation resistance, oxide dispersion-strengthened (ODS) steels are promising candidates for structural materials employed in elevated-temperature and nuclear applications [1]. The favourable properties of ODS steels are mainly attributed to the fine grains and homogenously dispersed nanosized oxide particles in the steel matrix [2]. These fine and thermally stable dispersoids hinder the motion of dislocations and grain boundaries, acting as trapping sites for both point defects and helium atoms generated during irradiation, resulting in an increased resistance to irradiation damage.

Despite the promising behaviour of ODS steels for use in advanced nuclear systems, joining these materials remains one of the major technological challenges limiting their deployment [3]. Joining ODS steels by solid-state methods such as spark plasma sintering (SPS), hot isostatic pressing (HIP) and friction stir welding (FSW) has been proven to be feasible by several authors [4–6]. The degradation of featured microstructures and mechanical properties can be minimised since these techniques do not create a molten zone in the joint area [5]. However, the costs of SPS and HIP are relatively high due to long processing times (1–5 h) [7] and the application of FSW is limited due to geometrical restrictions and tool wear [8]. The welding of ODS steels by traditional, fusion-based welding techniques such as gas metal arc welding and tungsten inert gas welding is problematic. As soon as a molten zone is produced, the oxide particles rapidly agglomerate and float to the top of the molten weld pool, resulting in a significant loss of strength [9]. Laser beam welding [10–12] can potentially be employed for joining ODS steels due to its highly concentrated energy input, leading to the melting of a small amount of base material, and consequently, the formation of a small heat-affected zone (HAZ). The study of Lemmen et al. [12] showed that PM1000 had a good laser weldability with a wide range of welding parameters. However, yttrium oxide clustering was found in all conditions,

causing a reduction in strength in the weld. Similar results were obtained by Liang et al. [13] who indicated that the nanoprecipitates were larger in the weld metal than in the base material. In summary, a new laser welding method needs to be developed to address the issue of microstructure and mechanical behaviour degradation.

In this study, pulsed laser beam welding was successfully employed to join ODS Eurofer steel with only minor deterioration of microstructure when compared to the parent material. The welding parameters were investigated and optimised to improve the microstructure of the joint. The microstructural features were characterised by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD).

#### **2. Experimental Details**

#### *2.1. Materials*

The alloy studied was ODS Eurofer steel with a nominal composition of Fe–9Cr– 1.1W–0.4Mn–0.2V–0.12Ta–0.1C–0.3Y2O<sup>3</sup> (wt%), produced via powder metallurgy. The production process started with mechanical alloying, where the precursor powders were mixed in a Retsch planetary ball milling machine under an argon atmosphere for 30 h at 300 rpm. The resultant powders were subsequently consolidated by spark plasma sintering (SPS, FCT group, Frankenblick, Germany) at a pressure of 60 MPa and a sintering temperature of 1373 K with a heating rate of 100 K/min. After a holding time of 30 min, disks of 40 mm diameter and around 10 mm thickness were produced (Figure 1). These parameters were selected based on our previous study [14].

**Figure 1.** Powder metallurgy-produced ODS Eurofer.

#### *2.2. Methods*

A 6 kW Yb:YAG laser was used for the welding experiments. The focusing optic has a focal length of 223 mm and projects a laser spot with a diameter of 0.6 mm. Two work pieces (30 × 20 × 1 mm<sup>3</sup> ) machined from the SPS-prepared disk were welded by pulsed laser beam welding with a peak power of 2500 W and a pulse duration ranging from 2 ms to 5 ms. A shielding gas of argon was delivered to the work piece at a flow rate of 8 L/min. Instead of moving straight in one direction, the laser beam was moved following the sequence indicated in Figure 2 in order to minimise the heat accumulation in the material and, therefore, shorten the melt pool lifetime. The time interval between each point is around 30 s. The distance between the centres of adjacent spots was 0.5 mm to ensure a continuous weld. A post heat treatment was conducted to recover the microstructure and release the residual stress generated during welding by normalising at 1423 K for 1 h, air cooling to room temperature, and then, tempering at 973 K for 1 h, followed by air cooling to room temperature.

**Figure 2.** Schematic illustration of welding strategies.

The microstructure of the joint was characterised using a Keyence Digital Microscope VHX-5000 and a JEOL 6500F SEM equipped with an energy dispersive spectrometer (EDS) and EBSD. The nano-oxide particles in the material were investigated using a JEM-2200FS TEM. To reveal the microstructure by OM and SEM, the samples were etched in a solution of 5 g ferric chloride, 50 mL HCl and 100 mL distilled water for 20 s. Specimens for EBSD were mirror polished followed by a colloidal silica polishing step. TSL orientation imaging microscopy (OIM) software was used for data processing and analysis. EBSD maps of inverse pole figure (IPF), grain average image quality (GAIQ) and kernel average misorientation (KAM) were implemented and analysed in the study. The TEM specimens were prepared by electropolishing disks with a diameter of 3 mm in a twin-jet electropolisher using 4% perchloric acid and 96% ethanol as electrolyte.

#### **3. Results**

#### *3.1. Parameter Optimisation*

To study the effect of a pulsed laser beam on the microstructure of ODS Eurofer, a number of spots were created on a plate with varying parameters. A short melt pool lifetime would be beneficial for retaining the Y2O<sup>3</sup> particles in the fusion zone. Therefore, a laser power of 2500 W and short pulse durations between 2 ms and 4 ms were applied. Optical micrographs of the cross-section of the spots can be seen in Figure 3. Material loss was observed in all conditions due to metal evaporation during the welding process. The width of the top of the "V"-shaped fusion zone is around 0.7 mm, which is very close to the beam size, indicating a concentrated heat input. The heat-affected zone (HAZ) is small in all cases, with a width of approximately 0.06 mm. It can be seen that partial penetration is obtained with a pulse duration of 2 ms and 2.5 ms. Large pores can be observed in the bottom of the fusion zone, probably because gas was trapped in the melt pool due to a short escape time. Full penetration is realised with pulse durations of more than 2.5 ms. As expected, more severe material loss was observed with longer laser beam pulses. Large pores managed to escape in these open keyhole conditions, while microvoids were found in the weld pool.

**Figure 3.** Optical micrographs of spots on the plate, with a power of 2500 W and a pulse duration of (**a**) 2 ms, (**b**) 2.5 ms, (**c**) 3 ms, (**d**) 3.5 ms and (**e**) 4 ms.

#### *3.2. Microstructure Characterisation*

δ δ δ In order to compensate for the material loss in the fusion zone, two ODS Eurofer square bars with dimensions of 30 × 1 × 0.5 mm<sup>3</sup> were attached to the top and bottom surfaces of the work piece to act as a filler material. A duration of 5 ms was used to realise full penetration. The material was joined using the pattern indicated in Figure 2. An SEM image of the weld seam is shown in Figure 4a. It can be seen that the material loss in the specimen is fully compensated by the filler material. In Figure 4b, the microstructure has both martensite grains (dark regions) and ferrite grains (bright regions), with the presence of microvoids. Martensite is formed during the rapid cooling process (~104–10<sup>6</sup> K s–1 [15]), while δ-ferrite is formed during heating, as the peak temperature of laser welding is definitely higher than the austenite–δ-ferrite transformation temperature (around 1400 K). However, since the δ-ferrite–austenite transformation is a diffusion-controlled process, the rapid solidification does not offer sufficient time to complete the phase transformation [16]. Consequently, residual ferrite is observed in the microstructure.

Figure 4d shows an enlarged image of the microstructure in the fusion zone. A large number of nanoprecipitates can be observed in the steel matrix. As shown in the TEM images in Figure 5a,b, finely dispersed Y2O<sup>3</sup> nanoparticles are observed in the microstructure. The particle sizes vary between 1 and 30 nm and do not show a significant difference in distribution between the fusion zone and base material. Figure 5c shows a dark field image of Y2O<sup>3</sup> particles (indicated by the arrows) pinning the grain boundaries in the fusion zone, which is beneficial for enhancing the mechanical properties and extending the working temperature range. The martensite lath structure in the fusion zone is revealed in Figure 5d. From Figure 4d, it is also worth noting that the Y2O<sup>3</sup> particles are not homogeneously distributed in the steel matrix. It seems that the smaller grains have a higher number of Y2O<sup>3</sup> precipitates than the larger grains. This can be explained as follows: The distribution of Y2O<sup>3</sup> is not perfectly homogenous even after a long period of mechanical alloying. Since Y2O<sup>3</sup> nanoprecipitates have a strong effect on impeding grain growth through a Zenertype pinning [17], grains with a higher density of Y2O<sup>3</sup> are presumably more resistant to recovery and growth.

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**Figure 4.** Morphologies of the weld seam with filler material: (**a**) SEM morphology of the **Figure 4.** Morphologies of the weld seam with filler material: (**a**) SEM morphology of the weld seam, (**b**) fusion zone, (**c**) fusion line, (**d**) enlarged fusion zone and (**e**) enlarged HAZ.

The grain size of the HAZ is smaller compared to that of the base material and clearly smaller than that of the fusion zone (Figure 4c), probably due to martensite transformation. An enlarged image of the HAZ (Figure 4e) shows a large number of precipitates with a size ranging from 0.1 to 1 µm in the steel matrix. They are found to be more preferentially located at the grain boundaries, possibly due to a decrease in volume-free energy [17]. These precipitates are rich in Fe, Cr, W and C, which can be identified as M23C<sup>6</sup> carbides (M = Fe, Cr and W) based on their size and chemical composition.

Figure 6 shows inverse pole figure (IPF) maps of the base material as well as the centre of the fusion zone and HAZ (P = 2500 W, t = 5 ms, with filler material) obtained by EBSD. Ultrafine grains smaller than 250 nm could not be indexed, causing the non-indexed (black) areas in the figures. It can be observed that the maps exhibit grain sizes ranging from the nanometre scale to a few micrometres. The microstructure of the fusion zone consists of elongated structures, while that of HAZ shows equiaxed grains. None of the regions show preferential grain orientation.

**Figure 5.** TEM images of the Y2O<sup>3</sup> nanoparticles in the (**a**) base material, (**b**) fusion zone, (**c**) Y2O<sup>3</sup> pinning the grain boundaries and (**d**) martensite lath.

**Figure 6.** Inverse pole figure (IPF) maps of different regions: (**a**) base material, (**b**) fusion zone, (**c**) HAZ, (**d**) heat-treated base material, (**e**) heat-treated fusion zone and (**f**) heat-treated HAZ.

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The average grain sizes were measured and are shown in Table 1. The grain size distribution was determined under the assumption that the minimum misorientation characterising grain boundaries is 15◦ . It can be seen that in the as-joined condition, the average grain size of different regions decreases from the fusion zone to the base material to the HAZ, which agrees with the observation from SEM. In the heat-treated condition, the average grain size of the fusion zone is still the largest, and that of the HAZ remains the smallest, but with smaller differences compared to the as-joined condition. Additionally, compared to the as-joined condition, there is a decrease in the grain size of the fusion zone, which could be ascribed to a refinement effect due to martensite transformation. Grain growth occurs in the HAZ, presumably resulting from the release of high stored energy due to phase transformation and distortion during laser welding. In general, the grain sizes of the joint overall do not grow significantly after the heat treatment, probably because of the strong pinning effect of Y2O<sup>3</sup> on the motion of grain boundaries.

**Table 1.** Average grain size of different regions obtained from EBSD data in Figure 6.


From Table 1, it can also be noted that the standard deviations of all conditions are high. This is due to a bimodal grain size distribution of the material. Taking the fusion zone in the heat-treated joint (Table 1) as an example, the grain size distribution has two peaks at 1.42 and 7.20 µm, respectively, as can be seen in Figure 7. Similar bimodal grain size distributions have been widely reported for powder metallurgy-prepared ODS steels [18–20]. In martensitic–ferritic steels, which generally contain 0.1–0.2 wt% C and 9–11 wt% Cr [21], this phenomenon could be due to the dual phase nature of the material. To further differentiate martensite from ferrite in the studied material, a chart of grain average image quality is shown in Figure 8a. Image quality (IQ) describes the quality of diffraction property of the analysed Kikuchi patterns. This factor can be used as an estimation of dislocation density or stored energy [22]. It can be seen that if a threshold value of approximately 40 is chosen, phases with image quality less than 40 are highlighted and shown in Figure 8b. In this way, martensite is differentiated from ferrite in the microstructure, since martensite exhibits lower image quality than ferrite due to its highly distorted lattice [23]. It can be noted that the smaller martensite grains are mainly located between the larger ferrite grains. This finding supports the dual phase microstructure of the material, and consequently, a high standard deviation of the grain size distribution.

The kernel average misorientation (KAM), which represents the numerical misorientation average of a given point with its nearest neighbours, was used to characterise local misorientations. The maps were calculated using a maximum misorientation of 5◦ . Figure 9a shows the KAM map of the fusion zone in the heat-treated joint. It can be observed that the grains standing out from the rest due to a larger size show almost no misorientation, i.e., no local lattice distortion. In addition, misorientations between 0.5◦ and 2◦ are observed near the grain boundaries. These small grain regions with higher misorientations can be linked to the areas with lower image quality. As shown in Figure 9b, the KAM map combined with the IQ map reveals overlapping regions, where martensite grains (yellow) have a higher misorientation and ferrite grains (dark blue) generally have a lower misorientation. By comparing KAM of different regions in the joint, this indicates whether the microstructure is changed significantly after welding or the heat treatment. The fusion zone and HAZ have a higher KAM than the base material, indicating a more distorted microstructure and probably a higher martensite fraction due to an additional thermal cycle

(Figure 10). Meanwhile, in the heat-treated condition, the KAM is smaller than that of the as-joined condition and shows no significant difference in different regions, demonstrating that the microstructure is substantially recovered and homogenous after the normalising and tempering treatment, which would be beneficial for the mechanical properties.

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**Figure 7.** Grain size distribution of the fusion zone in the heat-treated joint (P = 2500 W, t = 5 ms).

**Figure 8.** Fusion zone in the heat-treated joint (P = 2500 W, t = 5 ms): (**a**) Image quality in percentage showing highlighted low image quality and (**b**) Image quality map showing highlighted phases corresponding to martensite.

**Figure 9.** (**a**) Kernel average misorientation (KAM) map of the fusion zone in the heat-treated joint (P = 2500 W, t = 5 ms) and (**b**) Kernel average misorientation (KAM) map combined with image quality (IQ) map shown in Figure 8b.

**Figure 10.** Distribution of kernel average misorientation (KAM) of different regions in the as-joined and heat-treated joints.

#### **4. Discussion**

With controlled welding conditions, including an optimised laser power and a short pulse duration, a fully penetrated and not overheated spot weld was obtained. The welding defects observed in the joint are mainly material loss and microvoids. The material loss is generally caused by metal evaporation and spattering. This could be improved by tuning the focal position and the divergence angle [24,25]. With a reduced power density around the keyhole aperture, the speed of the melt flow as well as the instability of the keyhole will be decreased; therefore, the evaporation and spattering phenomenon will be limited. As for the porosity defects, it has been found that porosity can be effectively inhibited at the optimum frequency and duty cycle of the employed laser pulse [26]. In addition, the keyhole stability and material evaporation also have a large effect on the generation of porosity [27]. The use of nitrogen instead of inert shielding gas was found to be effective to suppress the formation of porosity defects, due to the occurrence of the on-and-off cycle of nitrogen plasma prior to the initiation of keyhole instability [27]. All of these could be used to reduce the welding defects and thus, improve the mechanical performance of the weld in future works.

The melt pool lifetime during fusion welding is crucial for retaining the microstructure and mechanical properties of ODS steel joints. Upon welding, the oxide particles will quickly float to the top of the melt pool and, consequently, form a depleted area, leading to significantly reduced mechanical properties. With traditional "continuous" welding, the heat will accumulate in the weld. Even though techniques such as laser beam welding have a very high cooling rate, the melt pool lifetime is still too long for ODS steels. The temperature of the fusion zone usually stays above the melting point for a relatively long period of time. It is therefore difficult, if not impossible, to obtain an undisturbed microstructure, i.e., a microstructure with fine grains and dispersed nano-oxide particles. For instance, Lindau et al. [9] studied the joining of ODS Eurofer via electron beam welding, which also has a characteristic high power density. The results showed that the nanodispersoids in the fusion zone unsurprisingly agglomerated to larger particles, causing a weak weld seam. Conversely, with the distributed pulsed spot welding procedure proposed in this study, the melt pool lifetime of each spot is reduced to the order of milliseconds, which is favourable for the retention of the microstructure and mechanical properties of the joint. It is known that the strengthening mechanisms of ODS steels are mainly based on grain boundary strengthening and dispersion strengthening [28], as neither the grain size nor the nanoparticle distribution is changed significantly after joining, and the strength of the joint will not be unduly reduced.

#### **5. Conclusions**

ODS Eurofer was welded successfully using a pulsed laser beam welding technique with a distributed pulse pattern. The welding parameters were optimised based on their effect on the microstructure. The microstructure of the joint in the as-joined and heat-treated conditions was investigated in detail. The main conclusions are as follows.


**Author Contributions:** Conceptualization, I.R.; methodology, I.R.; software, J.F.; validation, J.F.; formal analysis, J.F.; investigation, J.F.; writing—original draft preparation, J.F.; writing—review and editing, I.R. and M.H.; visualization, J.F. and M.H.; supervision, M.H.; project administration, M.H.; funding acquisition, M.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was carried out under project number T16010f in the framework of the Partnership Program of the Materials innovation institute M2i (www.m2i.nl, accessed on 28 April 2021) and the Netherlands Organisation for Scientific Research (www.nwo.nl, accessed on 28 April 2021).

**Data Availability Statement:** The raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

