The navigable and plasma-generating catheter can be placed in the intervertebral disc, but the posterior annulus becomes challenging to approach via this conventional manner [1,2]. The aforementioned device and procedure constituted an attempt to expand the indications yielded from minimally invasive spine intervention that has recently shown positive clinical results [2]. Effective therapies to reduce the social and economic costs to workers in the younger generation, who suffer from disc herniation, and the development of safer and less complicated procedures for older patients are necessary in this field.

Increasing the applied voltage under the same output (W) of the radiofrequency generator to increase the electron agglomeration of the electrode will increase plasma ions resulting in increased substantial tissue removal [3]. However, since the increase of the applied voltage affects the corrosion of the electrode, the removal has an efficiency limitation. In order to improve the generator performance, it is necessary to compare the tissue removal rate by applying a pulse width modulation (PWM) [4]. This is a repetition of on-off of the frequency of maintaining effective stimulation while maintaining total energy transfer, preventing overheating and maintaining efficient plasma generation when applied to human tissue.

Changing the size and design of the plasma-generating electrode, more so making realistic improvements, is challenging owing to the minimum size being required for the minimally invasive intervention procedure and technical limitations for securing durability and function [5]. Given that this technology is currently being used in the clinical setting, attempts to increase the efficiency of radiofrequency generators are of practical value. It is theoretically possible to increase the plasma generation efficiency by minimizing the heat generated at the electrode tip during the plasma discharge and raising the peak voltage.

It is hypothesized that the peak voltage output of the power supply under the same system conditions improves the removal efficiency of artificial tissue.

All animal experiments were approved by the Institutional Animal Care and Use Committee (approval #2014AN0163), and conducted in accordance with the relevant animal guidelines. The study was carried out in compliance with the ARRIVE guidelines. The experimental contents are as follows: experiment to increase the amount of plasma generated through PWM, and control output average power (output power × duty ratio) to 45W to compare 100% duty ratio and 15% plasma generation.

The experimental apparatus was configured to reproduce the target tissue removal and to analyze the current, voltage, and resistance (Fig. 1).

Figure 1. Scheme and picture of the experimental apparatus for measuring the removal rate. RF, radiofrequency; ESU, electrical surgical unit; SUS, stainless steel; NP, nucleus pulposus.

The experimental protocols are as follows. First, the plasma generated through PWM was identified. As carbon damage due to electrode damage and sudden discharge is associated with an increase in the amount of current, PWM has been applied as a way to reduce or control the current and increase the voltager_ms [6]. The plasma generated at the electrode tip was recorded and applied via reduction of the duty ratio through the test board: the duty ratios of 100% and 15% were used. The waveform of the measured current, voltage, and effective value were recorded. The air bubbles and plasma generated in electrodes were observed in saline. Second, the impedance was measured by setting a micro-environment that can result when tissue is removed. Depending on the nature of the tissue or the surface of contact with the tissue impedance fluctuations are very large, and these states may lead to overcurrent formation, which can in turn lead to electrode breakdown or explosion. Through this experiment, the impedance change according to the positional situation between the electrode and the target tissue can be predicted, which can be the basis of the overcurrent blocking. Third, the rate of tissue removal was measured. The actual tissue removal rate was analyzed under the same conditions and environment (porcine nucleus pulposus). The tissues were extracted and inserted into the glass tube to measure the weight change over time (six attempts each). An output of 70W was set, and the continuous generating method (duty ratio of 0%) and pulse generating method (duty ratio of 15%) were applied. To prevent carbonization, the electrode rotated once per second inside the tissue. Bubbles and liquefied tissue were removed every 5–100 s.

The equipment applied are listed in Table 1. The sample tissues used were of porcine intervertebral disc nucleus pulposus (porcine nucleus pulposus 2 g). These tissues were extracted from a single porcine, using the lumbar nucleus pulposus with disc level of 3/4, 4/5, and 5/6.

Table 1 . The equipment applied.

Radiofrequency generator (RFG)	Manufacturer: Stryker(Model: Multigen RF) Sizes: 12.5 in. [317.5 mm] width × 8 in. [203.2 mm] height × 15 in. [381 mm] depth Supplying power: 100–120 V, 50–60 Hz / 230 V, 50 Hz / 50 Wmaximum power into 100 Ω resistive load Measurable range: 0–2000 Ω, Accuracy: Below 100 ± 30 Ω; above 100±10 Ω, Operation range: 35 Ω (1800 Ω at discharge) Heat generation: RF procedure: Accuracy +4° –2°C from 37° to 95°C, Lesioning time: 0-999 sec; default adjustable at system and file level, Frequency: 1 MHz, accuracy ± 10%
Plasma generating electrode	Manufacturer: U&I Co.(Model: L’DISQ ULTRA) Axial diameter: 0.19mm, Material: Stainless steel (SUS), Melting point: 1,800K, Thermal diffusion coefficient: 0.5-22mm2/s
Test board	The voltage and current output from the RFG were measured and enabled for wave-type and control before delivery to the plasma generating electrode. The measured current and voltage were analyzed and monitored using an ESU analyzer and an oscilloscope. The test board was also manipulated to control the duty ratio of the current applied to the electrode. (manufacturer: BL meditech, Gyeonggi-do, Korea)
Electrosurgical Unit Analyzer (ESU)	Manufacturer: Fluke Biomedical(Model: QA-ES II), Size: 342mm width × 132mm height × 395mm length Characteristics: Automatic power distribution measurement, including power, current, peak-to-peak voltage closed load only), and crest factor, Measurable range: 10–5200 Ω, Accuracy: ±5% of reading
Oscilloscope (OSC)	Manufacturer: Berkeley Nucleonics Corp (Model: P2025) Bandwidth: 200 MHz, Sample rate: 1 GS/s, Recording length: 40 M, DC gain accuracy: ±3%, DC accuracy: Average ≥16: ±(3% reading + 0.05 div) for ΔV, Sizes: 340 mm width × 177 mm height × 90 mm length

The primary outcome was tissue removal rate of radiofrequency generator using PWM method. Repeated measured analysis of variance (ANOVA) was used to compare the tissue removal rate of the radiofrequency with and without the PWM method, and the significance level was set to P < 0.05.

The radiofrequency ablation using PWM increased the removal amount in proportion to time of the animal’s nucleus pulposus in an ideal environment. This showed an efficiency of approximately 44% under the conditions mentioned.

The duty ratios were set as follows: 100% (output power 45 W) vs. duty ratio 15% (output power 300 W) (Fig. 2). The voltager_ms value was increased when the amount of current increase was limited. Elevated peak voltage enabled efficient discharge of plasma.

Figure 2. Duty ratio 100% (output power 45 W, above) vs. Duty ratio 15% (output power 300 W, below).

Plasma generated in physiological saline was qualitatively analyzed (Fig. 3). When the duty ratio was reduced, bubble formation during plasma generation occurred relatively quickly. Even if the same average power is applied, increasing the breakdown voltage through PWM with a duty ratio is effective for achieving rapid plasma generation. This is probably related to the initial bubble generation rate and the temperature rise around the electrode tip.

Figure 3. Shooting images during discharge with two outputs.

The applied resistance through the electrosurgical unit (ESU) analyzer was set, and the impedance was compared by measuring the test board, which presented the accuracy evaluation of the impedance measured on the test board. The measurements reflected the applied resistance well, with 1%–10% deviation (Fig. 4).

Figure 4. Measured impedances (output, 100 W). ESU, electrical surgical unit. Natural time flow presented on X-axis, the intensity of impedance on left Y-axis, and the variability (standard deviation, %) of difference between solid and dotted lines on right Y-axis.

The tissue removal rate of radiofrequency generator using the PWM method was assessed under the same conditions and environment. Tissue removal efficiency via PWM was increased by approximately 44% (P < .001) (Fig. 5). Regardless of PWM application, the removal amount of nucleus pulposus for 1000 seconds increased significantly (P < .001). Raw data of these results are presented in Table 2.

Table 2 . Tissue removal efficacy via PWM.

Time (sec)	0	100	200	300	400	500	600	700	800	900	1000
Removal weight (accumulated, mg)
with PWM	0	28.5	52.4	84.3	121.2	157.3	189.4	236.8	278.1	319.4	361.2
without PWM	0	19.8	36.4	56.5	82.2	107.8	134.2	169.9	194.8	223.6	251.2
Efficacy (%)	0	43.9	44.0	49.2	47.4	45.9	41.1	39.4	42.8	42.8	43.8

Efficacy = {(removal weight with PWM) – (removal weight without PWM)} / (removal weight with PWM) * 100 (%)..

PWM, pulse width modulation..

Figure 5. Tissue removal efficiency analysis with and without PWM application.

The PWM signals are a method of generating analog signals using digital sources. The PWM signal consists of two main components: duty cycle and frequency. Duty cycle represents the time the signal is in the “on” state as a percentage of the total time it takes to complete a cycle. The frequency determines the speed at which PWM completes the cycle (i.e., 1,000 Hz is 1,000 cycles per second), and therefore the speed at which the frequency switches between high and low states. When the digital signal is turned off and on with a constant duty cycle at high speed, the output may appear to behave like a constant voltage analog signal when the device is powered [7,8].

For example, to generate a 3-V signal using a digital source that can be ‘on’ at 5 V or ‘off’ at 0 V, an output of 5 V 60% of the time can be generated using PWM with a 60% duty cycle. When the digital signal cycles are fast enough, the voltage seen at the output appears as the average voltage. In this case, the average voltage can be calculated as 5 V × 0.6 = 3 V. Selection of 80% and 20% duty cycles results in 4 V and 1 V outputs.

The PWM signals are used in a variety of control applications. They are mainly used in DC motor control, but they can also be used for controlling valves, pumps, hydraulics, and other mechanical parts. The frequency at which the PWM signal should be set depends on the response time and the application of the powered system.

The PWM signal control of this radiation frequency generator needed to find suitable stability parameters is based on several experiments, and the average voltage should be above the minimum plasma generation voltage for efficient tissue removal. The improvements are expected to maintain high voltages under the same output and also the durability of the electrode tip to provide more time for spreading of the heat in the off state.

Finally, the tissue removal rate was improved by 44% compared with the removal rate without PVM. Although it is possible to maintain high voltage stability, it is difficult to predict the quantitative generation of plasma due to various environmental factors necessary for plasma discharge, and the degree of tissue removal by plasma transfer is quite significant depending on the soft tissue state (temperature, moisture, contact surface, etc.). This may indicate a difference. The 44% improvement in the removal rate under controlled experimental conditions is highly significant. Improving performance without changing the size and function of electrodes inserted into the human body is very clinically and industrially significant. Simple modifications and attentions to processes, licensing, applied techniques, clinical techniques, etc. can be applied directly to the site. This may be a major technology that can be applied to other medical devices using DC power to improve performance.

This study was supported by the Technology Innovation Program (or Industrial Strategic Technology Development Program) (20003688, Development of cosmeceutical and medical device platform using biodegradable metal patch delivering microcurrent) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

No potential conflict of interest relevant to this article was reported.