Multi-Parameter Analysis in Single-cell Electroporation Based on the Finite Element Model 1 2 3 Hongmei Liu, Chenguo Yao, Member, IEEE, Yan Mi, Chengxiang Li, Yajun Zhao, Yanpeng Lv 4 State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400030, China [email protected] 5 6 7 8 9 Abstract—Pulsed electric fields have recently been the focus considerable attention because of their potential application in 12 biomedicine. However, their practical clinical applications are 13 limited by poor understanding of the interaction mechanism 14 between pulsed electric fields and cells, particularly in different 15 effect of electroporation exposed in different parameters, and the 16 optimal pulsed parameter is still vague. A multi-shelled dielectric 17 model considering the inner membrane was established by the 18 finite element software COMSOL in this paper. Different shock 19 protocols (different pulsed duration/strength/frequency/polarity) 20 was exposed to the cell model respectively to simulate and 21 analyze the influence of pulse parameters on varying degrees of 22 electroporation by comparing the dynamic development of the 23 pore radius and electroporation region. Results showed that the 24 conventional pulses have better efficiency in electroporation than 25 high frequency pulses; The monopolar pulses with cumulative 26 effect have much better electroporation effect than the bipolar 27 pulses which have weakening effect when duration of single pulse 28 reduced to nanosecond; and electric field strength was the major 29 factor that induced electroporation, particularly in the 30 recoverable pore, but it had minimal effect on pore expansion. 31 However, pulse duration affects the non-recoverable pore, such 32 that the high-intensity wide pulse is more useful in the field of 33 irreversible electroporation. The high-intensity short pulse can 34 increase permeability and maintain cell viability. This work done 35 by this paper maybe can provide theoretical basis for further 36 research on complicated cell systems and promote the clinical 37 application of pulsed electric field. either be reversible (which is used in electrochemotherapy [2] or gene transfection [3]) or irreversible (which is used in other 54 applications for tissue ablation [4]). Despite significant interest 55 in this technique, but its practical clinical applications or 56 development are limited by the poor understanding of the 57 interaction mechanism between pulsed electric field and cells; 58 in particular, electroporation and its effect on parameter 59 selection are poorly understood. 10 52 11 of 53 Numerous studies have used experimental techniques to provide insight into cell electroporation, such as uptake of 62 fluorescent molecules[5], measuring cell impedance[6], and 63 imaging cell transmembrane voltage[7]. And also many 64 different pulse parameters were selected in experimental 65 studies to investigate the effects of pulsed electric fields. Boris 66 Rubinsky et al.[8] shown that there is killing effect on human 67 liver cancer cell (HepG2) which exposed in 10 pulse with the 68 amplitude of 1.5kV/cm, duration of 300us, repetition frequency 69 of 1Hz, and the killing effect increase with the number of 70 pulse. Rafael V Davalos et al.[9] selected 80 pulses with the 71 amplitude of 2500V/cm, duration of 100us, repetition 72 frequency of 0.3Hz to treat nude mice solid tumor and it also 73 get a good ablation effect. And Yao et al.[10] conducted 74 preclinical animal studies by exposing liver in 90 pulses with 75 1500V/cm, duration of 100us, repetition frequency of 1Hz. 76 Result showed that the ablation boundary was clear. And so on. 77 We can see that these parameters are different, different pulse 78 parameters have different killing effect, but we just know a 38 Keywords—Dielectric model; electroporation; electroporation 79 vague answers about how much parameters influence on 39 region; finite elements; non-recoverable; optimal parameter; 80 treatment. Though many pulse parameters(90 pulses with 40 pulsing protocols; radii; recoverable; 81 100us, 1kV/cm~3kV/cm for irreversible electroporation(IRE), 82 8 pulses with 100us, 1kV/cm for reversible electroporation) 83 were studied and proposed in clinical application. In fact, it 41 I. INTRODUCTION 42 Pulsed electric fields have recently been the focus of 84 were judged just depend on quantitative analysis of the 85 experiments or based on the ability to produce the pulse 43 considerable attention because of their potential application in 86 generator. Moreover, corresponding parameter should be 44 biomedicine because of their broad potential applications in 45 biomedicine; pulsed electric fields induce cancer cell 87 selected pointedly because of organization is inhomogeneity, 88 but now, The optimal pulsed parameter is still not clear, the 46 electroporation, making them promising technologies for tumor 89 root cause is that the mechanism between pulsed electric field 47 therapy[1]. Electroporation occurs when a biological cell is 90 and cells, its interpretive competence is poor for the 48 exposed to electric pulses, resulting in the increase in cell 91 relationship between parameters and elecctroporated effects. 49 membrane permeability and the appearance of hydrophilic 92 Nevertheless, for different tissue, the optimal pulsed parameter 50 pores. Depending on the field intensity and duration or number 93 is still to be further studied, a theoretical model should be 51 of pulses applied, permeabilization of the hydrophilic pore can 94 established to supplement experimental knowledge and analyze Project supported by Fund for Innovative Research Groups of China (51321063); 95 the effect electroporation on parameter selection. In this study, 96 a finite element model of cells under the pulsed electric field National Natural Science Foundation of China (51477022); Natural Science Foundation Project of CQ CSTC (cstc2014jcyjjq90001); Fundamental Research Funds for the Central Universities (106112015CDJZR158804). 60 61 1 TABLE 1 PARAMETERS OF THE CELL ELECTROPORATION MODEL E B P rc dmem A N θ θnp C O dne Fig. 1 Multi-shelled dielectric model of the spherical cell A5 A4 A3 A2 A1 A6 A7 α=15° O B1 C1 E Fig. 2 Schematic diagram of sampling points on the cell membrane boundary Symbol rc rn dmem dne λm λmem λc λne λnp λp ε0 εm εmem εc εne εnp α rp N0 VEP Vrest q n ω0 F R T r* D β γ σ0 σ′ Fmax rh rt Value 10 µm 5 µm 5 × 10−9 m 40 × 10−9 m 0.2 sm−1 3 × 10−7 sm−1 0.3 sm−1 6 × 10−3 sm−1 1.35 sm−1 0.22 sm−1 8.85 × 10−12 m−3 kg−1 s−4 A−2 80 ε0 8.57 ε0 154.4 ε0 28 ε0 52 ε0 1 × 109 m−2 s−1 0.8 × 109 m 1.5 × 109 m−2 170 mV −80 mV 2.46 0.15 2.65 9.65 × 10−4 Cmol−1 8,314 J K−1 mol−1 295 K 0.51 × 10−9 m 5 × 10−14 m−2 s−1 1.4 × 10−19 J 1.8 × 10−11 J m−1 1 × 10−6 J m−2 2 × 10−2 J m−2 0.70 × 10−9 N V−2 0.97 × 10−9 m 0.31 × 10−9 m Definition and source Cell radius [13] Nucleus radius Membrane thickness [14] Nuclear envelope thickness [14] Extracellular conductivity [15] Membrane conductivity [16] Cytoplasm conductivity [17] Nuclear envelope conductivity [18] Nucleoplasm conductivity [18] Conductivity of the solution filling the pore Dielectric permittivity of the vacuum Media permittivity [15] Cell membrane permittivity [19] Cytoplasm permittivity [20] Nuclear envelope permittivity [18] Nucleoplasm permittivity [18] Creation rate coefficient [13] Minimum radius of hydrophilic pores [11] Equilibrium pore density at Vm = 0 [11] Characteristic voltage of electroporation [13] Rest potential [11] Pore creation rate [11] Relative entrance length of pores [13] Energy barrier within pore [13] Faraday’s constant Gas constant Absolute temperature Minimum radius of hydrophilic pores [12] Diffusion coefficient for pore radius [12] Steric repulsion energy [12] Edge energy [12] Tension of the bilayer without pores [12] Tension of hydrocarbon–water interface [12] Maximum electric force for Vm = 1 V [12] Constant for advection velocity [12] Constant for advection velocity [12] 2 was established to investigate cell electroporation and the influence of pulse parameters on varying degrees of 5 electroporation. This work done by this paper maybe can 6 provide theoretical basis for further research on complicated 7 cell systems and promote the clinical application of pulsed 8 electric field. 3 20 4 21 voltage (∆Ψ) can be computed using electromagnetic field theory by solving the following equation: ( ) 0 r ( 22 ( )) 0, t i (t ) 0 (t ). 23 9 (1) (2) When electroporation happened, pores provide new conductive pathways for the transmembrane current density J, 26 the additional term JEP, as follows: 24 10 II. MODEL AND METHODS A. Model of Electroporation Here, we establish a multi-shelled dielectric model of the 13 cell and consider this cell with a radius of 10 µm exposed to an 14 electric field of strength E in an underground field with a 15 conductive medium. its electrical characteristics are described 16 by conductivity and permittivity based on the finite element , 17 and every part of the cell was regarded as isotropic, linear, and 18 homogeneous media. As shown in Fig.1. The electric potential 19 Ψ inside and outside the cell ,and the induced transmembrane 25 11 12 27 J (t ) m0 (i 0 ) d 0 mem (i 0 ) d t iEP (t ) N (t ), (3) where iEP is the current through a single pore, N is the density of the pore, all of them can be characterized by the 30 following equation: 28 29 31 iEP (t ) ( i 0 )P rP2 A , d (4) evm 1 , (5) nvm ) / (0 nvm ) (0e0 nvm nvm ) / (0 nvm ) A. Effects of Pulse Strength 42 Regardless of shock duration or shape, increasing the shock e (0 e 43 strength increases the degree of electroporation, as shown by 44 the effect of electroporation exposed to the pulsed electric 2 dN (t ) N (t ) q ( (t )/VEP )2 e( (t )/VEP ) (1 e ). 2 (6) 45 field. For the single cell under 500 V/cm, no electroporation dt N0 46 occurs; however, the effect of electroporation increases when 47 this value is exceeded. The increase in pore density of A1 is 3 where vm (i 0 )( F / RT ) , then, we can get: 48 quasi-exponential with pulse strength, as shown in Fig. 3. As 49 the strength of the pulsed electric field increases, a larger area ( i 0 ) 0 mem ( i 0 ) J (t ) m (t ) , 4 (7) 50 reaches the transmembrane potential threshold, resulting in the d d t 51 expansion of the pore region. And also, The electric field 52 intensity also affects the distribution of the recoverable and 2 5 where m (t ) m0 N (t )P rP A is the variable −8 53 non-recoverable pores. The maximum pore radius 4.18 × 10 6 conductivity of the electroporated membrane. 54 m occurs at 0.5 µs after the pulse with an electric field of 0.75 7 Also, pore radius should be considered, for a cell with a 55 kV/cm, which is located at 31.5°. When the electric field 56 intensity is 1 kV/cm, the maximum pore radius reaches 5.07 × 8 total number of K pores, the rate of change of their radii rj is −8 57 10 at the 40.68° position. With 5 kV/cm, the maximum value 9 determined by the following ordinary differential equation [12]: −8 58 is 6.4 × 10 m and the location is 66.89°. Therefore, the cell dr j 59 membrane exhibits electroporation from non-electroporation, Vm2 Fmax D U (rj , Vm , eff ) ( 60 particularly recoverable electroporation, as the electric field 10 dt kT 1 rh / (r rt ) 61 increases. Then, the cell membrane exhibits electroporation (8) r* 1 62 characterized by non-recoverable electroporation. The region 4 ( ) 4 2 2 eff r ) j 1, 2,3, , K . r r 63 of non-recoverable electroporation, the maximum shift to the 64 pole, and the value of the maximum pore radius increase with 11 All parameters are defined in Table 1. 65 the increase in the electric field. 0 nvm timulation protocols 13 The cell was exposed to different shock protocols. 14 Specifically, this study investigated the effects of the 15 following: 1) Shock duration, by keeping the shock strength 16 constant and varying its duration. The shock duration used in 17 this study was 5–15 µs, and pulses with different durations 18 were tested. 2) Shock strength, by employing the variable 19 aforementioned duration for several different shock strengths. 20 The cell was exposed to electric fields of 0.25–5.0 kV/cm. 3) 21 Shock frequency, by applying trains of high-frequency pulses 22 while keeping the total duration of the trains at 10 µs; the 23 frequency was 0.2 and 2 MHz. The monophasic and biphasic 24 pulse trains were tested. Monophasic pulses had equal on and 25 off phases, whereas biphasic pulses had equal positive and 26 negative phases and the same total duration as that of the 27 monophasic pulse. S III. RESULTS The calculations were conducted in COMSOL Multiphysics using the Electric Currents, Transient Analysis 32 application mode [21]. In this study, we built a multilayer 33 dielectric sphere model, and two opposing copper electrodes 34 were applied; one electrode was set as the pulsed signal, 35 whereas the other electrode was set to the ground to obtain the 36 electric field under a pulse signal. All outside boundaries were 37 considered insulated. To analyze the evolution of pore radii on 38 the surface of the cell membrane, seven sampling points were 39 selected, as shown in Fig. 2. The angle between two points was 40 15°. 30 31 83.08 40 82 82.85 41.66 76 20 74.7 74 10.32 73.33 72 10 N S L 23.62 67.6 5.90 0 68 80 78 30 66 67 84 1.5 68 66 3.45 0.751 70 3 5 E (kV) Fig. 3. Effect of electric field intensity, E. N is the maximum pore density, B is electroporation area, and L is the fitted curve. 69 28 29 41 N(×1015m-2) 12 B. vm S(0) A 1 B. Effects of Pulse Duration 71 Regardless of shock strength, increasing the shock duration 72 increases the degree of non-recoverable electroporation; this 73 phenomenon is proven by the effect of electroporation exposed 74 to the pulsed electric field. 70 There are three types of pores: non-recoverable pores, recoverable pore, and no-pores. And this study found that when 77 cell exposed in the pulse with strength of 1.5kV/cm, perforated 78 area on the cell membrane increase with the duration increase 79 when the duration of pulse under 3us( do not show the figure in 80 the paper), but when beyond 3us, minimal change was 81 observed in the total electroporation region and density, and 82 there still no non-recoverable pores appear until the width up to 83 9.5 µs, the degree of non-recoverable electroporation increases 84 with the duration. 75 76 1 2 Fig. 4 shows the temporal–spatial distribution of pore radii under different duration. When the single-cell was exposed in 25 RE RE NO 20 3 the pulse with strength of 1.5kV/cm, width of 5 µs, the pores 120 2us 3us 6us 9us 10us RE Non-RE RE 2us 8us 12us 16us 20us 30us NO 100 80 rj (nm) rj (nm) 15 10 40 20 5 0 0 60 20 40 60 80 100 arc (o) 120 140 160 0 0 180 20 40 60 80 100 arc (0) 120 (a) 1.5 kV/cm, 5us (b) 1.5 kV/cm, 15 µs. (c) 1.5 kV/cm, 5us (d) 1.5 kV/cm, 15 µs. 140 160 180 Fig. 4. Effects of pulse duration on the temporal–spatial distribution of pore radii. (a)(c) is for the pulses with the strength of 1.5kV/cm, duration of 15us at different time. (b)(d) is for the pulses with strength of 1.5kV/cm, duration of 5us at different time. 4 5 were created and evolution with time, but the pore radii decrease after pulse treatment (Fig. 4 (a)(c)), which reveals that 8 all the pores are recoverable. When the pulse duration is 15 µs 9 (as shown in the Fig. 4 (b)(d)), the region of non-recoverable 10 pores expands significantly compared with that under the 11 duration of 10 µs. Moreover, minimal change is observed in 12 the total electroporation region and density. We can guess that 13 duration is a first kind important parameters for the pores 14 evolution, especially the creation of non-recoverable pore. conventional pulse with 1,000 V/cm amplitude and 10 µs width. However, under the monopolar and bipolar pulses with 27 1,000 V/cm amplitude, the pulse width was still 10 µs. After 28 the first pulse, recoverable electroporation occurred. The pore 29 radii became recoverable during the interval time (5 µs) 30 between the first and second pulses. With the occurrence of the 31 second pulse, the pore radii increased along with the 32 transmembrane voltage. However, no non-recoverable 33 electroporation occurred even after two pulses. Thus, the 34 conventional pulse is better than the monopolar and bipolar 35 pulses when the total high-level time inner burst is fixed. 36 Meanwhile, the evolution of pore radii was associated with the 37 square of the transmembrane voltage; thus, the evolution of the 38 pore radii exposed to the monopolar pulse was the same as that 39 exposed to the bipolar pulse. And then, we set the pulse width 40 to nanoseconds (500 ns), repetition frequency to 1 MHz, and 41 duty cycle to 50%. As shown in Fig.5 (c)(d). electroporation 42 exhibited a cumulative effect on the monopolar pulse, whereas 43 a weakening effect was observed on the bipolar pulse; the next 44 pulse exhibited an opposite polarity, resulting in the rapid 45 decrease and subsequent increase in the transmembrane 6 25 7 26 C. Effects of Shock Frequency and Polarity 16 The on time of different pulse types was fixed to 10 µs to 17 observe the principle of energy equivalence. The condition of a 15 single pulse, multiple monopolar pulses, and bipolar pulses were simulated in this study. The simulation results showed 20 that the membrane electroporated more easily under the 21 traditional single pulse, leading to cell death. The calculations 22 presented in Fig. 5 (a)(b) show the evolution of pore radii 23 exposed to different pulses. This finding implies that non24 recoverable electroporation occurred upon exposure to the 18 19 A1 A2 A3 A4 A5 A6 E 70 60 4 So monopolar pulses were more efficient at electroporation than bipolar electric pulses. 60 1.0 50 0.8 50 rj (nm) 3 40 0.6 30 0.4 20 10 40 0.2 0 0 2 4 6 8 10 12 T (us) 14 16 1.0 E 0.8 30 0.6 20 0.4 10 0.2 0 0 0.0 20 18 A1 A2 A3 A4 A5 A6 2 4 6 8 10 12 14 16 E (kV/cm) voltage, but was insufficient to reach the transmembrane voltage threshold and for electroporation to occur (Fig. 15(b)). rj (nm) 2 E (kV/cm) 1 0.0 20 18 T (us) (a) (b) ec.dV E 1.2 ec.dV E 1.0 0.6 1.0 0.8 0.4 ec.dV (V) 0.6 0.0 0.0 E (kV/cm) 0.5 0.3 0.6 E (kV/cm) ec.dV (V) 0.9 0.3 0.2 -0.3 -0.5 0.0 0.0 -0.6 -1.0 0 1 2 T (us) 3 4 5 0 1 2 T (us) 3 4 5 (d) (c) Fig. 5 (a)(b) : Temporal–spatial distribution of pore radii in different pulse frequencies. (a) Conventional pulse with pulse width of 10 µs; (b) Monopolar pulse with pulse width of 5 µs; (c)(d): Temporal distribution of the transmembrane voltage at different polarities. (c) Monopolar pulse with 500 ns × 4; (d) Bipolar pulse with 500 ns × 4. 5 cannot enter the cell even through the largest pore when the electric field is 0.6 kV/cm. However, when the electric field 28 exceeded 0.75 kV/cm, 70 kDa molecules can transmit into the 29 cell, but no significant increase in macromolecular 30 transmittance was observed. All these findings reveal that, with 31 the increase in the electric field, the number of pores increased 32 significantly, whereas the number of large pores changed 33 slightly. This conclusion is consistent with the findings on the 34 relationship of the electroporation region with electric field 35 strength. As the electric field increases, the number of pores 36 increases considerably (Fig. 11). When the strength of the 37 electric field exceeded 0.75 kV/cm, the number of recoverable 38 pores significantly increased, whereas the non-recoverable 39 pores changed only slightly (Fig. 12). From our simulation, we 40 conclude that the strength of the electric field is the major 41 factor that induces electroporation but has minimal effect on 42 pore expansion. For several applications that require reversible 43 electroporation, such as drug delivery, the high-intensity short 44 pulse can be used to increase permeability and maintain cell 45 viability. 26 6 I. DISCUSSION The model presented in this study allows us to investigate the temporal and spatial distributions of cell electroporation. 9 We evaluate how well the results of the simulation match those 10 of the experiments. During the evaluation of this simulation, 11 we noted that this study is unable to find in the literature all of 12 the parameters required by the simulation for a single cell. The 13 parameters listed in Table 1, such as cell radius, intracellular 14 and extracellular conductivities, and membrane thickness, were 15 obtained from external sources, other theoretical models, or 16 experiments. Thus, differences between experimental and 17 simulation results are to be expected. 7 8 Similarly, the permeability of the molecule was used to assess electroporation in different electric field strengths 20 [22][23]. The results revealed that, when the pulse duration and 21 the treatment time are same, the permeability is closely related 22 to the electric field strength. Gabriel [24] reported that 23 fluorescence intensity increases with electric field strength with 24 the use of propidium iodide. Demiryurek revealed that 10 kDa 25 molecules can transmit to cells, whereas 70 kDa molecules 18 19 27 1 Although electroporation is associated with the electric field, pulse duration is also an important factor. Saulis and 3 Saulė [22] revealed that, with the increase in pulse duration, the 4 pore radii also increased. We obtained a similar conclusion in 5 our simulation. When the electric field is 1.5 kV/cm and the 6 pulse duration is low, all the pores on the cell membrane are 7 recoverable after electroporation. However, when the pulse 8 duration reaches a critical value, the non-recoverable pore 9 appears on the membrane and its region increases along with 10 pulse duration and maximum cell radius. The pore density and 11 the entire electroporation area did not change significantly. The 12 pulse duration is the main factor that affected the development 13 of the pore radii under the appropriate electric field intensity. 14 Therefore, the pulse duration should be longer in a reasonable 15 range in the field of irreversible electroporation. 59 2 60 For high-frequency pulses, the influence of frequency and polarity is still unclear. Several researchers agree with the 18 energy equivalence principle; therefore, the conventional single 19 pulses and high-frequency monopolar and bipolar pulses would 20 have similar results upon treatment with cell suspension 21 [26][27]. Bilska [25] established a numerical model. Neumann 22 [28] built a resistance–capacitance cell circuit model, which 23 revealed that the degree of electroporation decreased with the 24 increase in pulse frequency by theoretical analysis. Kotnik et 25 al. [29][30] revealed that the influence of high-frequency 26 bipolar pulse perforation was significantly weaker than that of 27 monopolar pulses from electrophoresis, electrolysis, and other 28 points of view. Based on previous studies, Zimmerman [31] 29 proposed a weakening effect by bipolar pulses through an 30 experiment on calcium ion absorption. Bennett L.Ibey, Olga 31 N.Pakhomova, et.al [32] investigated whether the same 32 advantage existed for bipolar nanosecond-pulsed electric field 33 as compared to monopolar pulse. It show that bipolar 34 nanosecond electric pulses are less efficient at 35 electropermeabilization and killing cells than monopolar 36 pulses. In this study, we proposed the cumulative effect of the 37 high-frequency monopolar pulses and the weakening effect of 38 the bipolar pulses from the change of the transmembrane. From 39 the developing trends of the pore radii, we can conclude that a 40 high frequency is not conducive for the development of the 41 pore radii or non-recoverable electroporation, which benefits 42 reversible electroporation. The low frequency or the wide pulse 43 duration is beneficial to irreversible electroporation. All these 44 findings are summarized in our simulation, which requires 45 further experimental verification. 72 16 17 46 II. CONCLUSION In this study, a finite element model of cells under the pulsed electric field was established to investigate cell 49 electroporation and the influence of pulse parameters on 50 varying degrees of electroporation. The strength of the electric 51 field was the major factor that induced electroporation, 52 particularly the recoverable pore, but minimal effect was 53 exhibited on pore expansion. Pulse duration was observed on 54 non-recoverable pores. Thus, the high-intensity wide pulse is 55 more useful in the field of irreversible electroporation, whereas 56 the high-intensity short pulse can increase permeability and 57 maintain cell viability because of the cumulative effect of 58 monopolar nanosecond electric pulses and the weakening 47 48 effect of bipolar nanosecond electric pulses. Monopolar pulses are more efficient in electroporation than bipolar electric 61 pulses. The results explained the effect of electroporation on the selectivity of pulse parameters. 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