At the first stage of the work, the attractive force of the manufactured EN was calculated according to the method given in the Materials and methods section. For the detailed description of experimental procedure and calculations see the Exp. Section (2.4.). As a result, the average value of the attractive force of the EN for three particles was found for the voltage range of 1–6 V with a step of 1 V, and graphs of the dependencies of the needle attraction force on the distance between the center of the needle and the particle at various voltages were plotted (Fig. 2a), along with the dependence of attracting force of the EN depending on the applied voltage (Fig. 2b).
It was found that with a decrease in the distance between the EN and the particle, as well as with an increase in the applied voltage, the attractive force of the EN increases. The results are listed in Table 1.
In the next series of experiments, using EN, we studied the effect of the magnetic field gradient on MNPs loaded into HeLa cancer cells. Cells in culture were loaded with nanoparticles according to the method described by S. Fedorenko (Fedorenko et al. 2019). As a result, 100 percent of the cells in culture were internalized with MNPs (Fig. 3), which is consistent with the results previously obtained with these nanoparticles on motor neurons (Fedorenko et al. 2019).
Cells internalized with nanoparticles were placed on the object stage of a confocal microscope, and the EN were brought to the cell along with nanoparticles using a micromanipulator under visual control (Fig. 4). The average distance from the EN to the cell was 200 µm. The distance was determined using the confocal microscope software. The lens was focused on the tip of the EN and, using a Z-drive, the cell with MNPs was focused. The Z-drive scale could be used to determine the distance from the needle to the cage. Then the magnetic field was turned on for 180 seconds and video images were recorded in the XYZt mode. The recording was carried out for a voltage range of 1–6 V, for which the force of attraction of the EN was previously calculated (see Fig. 2a). MNPs form conglomerates in cells; observations were made of conglomerates with an average radius of 1 µm. The movement of the MNPs in the cells was detected after the voltage was applied (Fig. 5).
After recording the video, the experimental data were processed and the displacement of the MNP conglomerate was analyzed depending on the magnitude of the applied force. The analysis was performed by shifting the peaks of MNP fluorescence intensity in cancer cells using the Leica SP5 TCS confocal laser microscope software in the absence and presence of a magnetic field generated by the EN at various voltages of 1–6 V with a step of 1 V (Fig. 5). A relationship was also noted between the applied voltage and the displacement of MNP in HeLa cancer cells: as the voltage increased from 1 to 6 V, the displacement itself increased from 1.01 ± 0.03 to 2.03 ± 0.05 µm, respectively. The measurement results are listed in Table 1. The magnetic field induction at the EN tip was assessed using a constructed device based on a Hall sensor (Blokhin et al. 2020), the residual magnetization was removed by applying an alternating magnetic field (Table 1).
Table 1
Relationship between the applied voltage on the coil and the magnetic field induction, displacement of MNP in HeLa cancer cells and the average value of the magnetic force.
Voltage, V | Current, mA | Needle magnetic field induction, mT | Displacement value, µm | The value of the average force (at a distance of 200 µm between the EN and the particle), pN | The value of the average force (at a distance of 20 µm between the EN and the particle), pN |
1 | 29 | 3.3 ± 0.33 | 1.01 ± 0.03 | 0.056 ± 0.002 | 9.75 ± 1.82 |
2 | 59 | 6.0 ± 0.58 | 1.13 ± 0.04 | 0.103 ± 0.017 | 11.47 ± 2.02 |
3 | 86 | 7.4 ± 0.55 | 1.23 ± 0.03 | 0.127 ± 0.019 | 14.55 ± 2.61 |
4 | 116 | 8.1 ± 0.28 | 1.41 ± 0.05 | 0.198 ± 0.033 | 16.59 ± 2.96 |
5 | 144 | 9.5 ± 0.29 | 1.7 ± 0.04 | 0.236 ± 0.05 | 18.3 ± 2.02 |
6 | 169 | 10.7 ± 0.33 | 2.03 ± 0.05 | 0.302 ± 0.027 | 37.85 ± 3.4 |
According to the method described in the Exp. Section (1.5), the average viscosity of the cytoplasm of HeLa cancer cells was calculated at a voltage of 6 V. The results are listed in Table 2.
Table 2
Average viscosity of the cytoplasm of HeLa cancer cells, displacement value of the MNP and the average value of the magnetic force at a voltage of 6 V on the coil.
Voltage, V | Displacement value (average), µm | The value of the average force (at a distance of 200 µm), pN | The average viscosity of the cytoplasm of Hela cancer cells, Pa·s |
\({U}\) | \({d}{x}\) | \({{F}}_{{E}{N}}\) | \({\mu }\) |
6 | 2.03 ± 0.05 | 0.302 ± 0.027 | 1.45 ± 0.04 |
As a result of calculations, the average viscosity of the cytoplasm of HeLa cancer cells was 1.45 ± 0.04 Pa·s.
The literature data describe a fairly wide range of viscosities of the cell cytoplasm, which can differ by orders of magnitude. It is known that the viscosity of the cell cytoplasm depends on many parameters, including the density of cell compartments, cytoskeleton, etc. (Xie and Minc 2020). It has been shown that the viscosity properties of the cytoplasm also change depending on the stage of cell development (Chen et al. 2014). In early studies performed using the method of spectrally resolved fluorescence measurements of a porphyrin-dimer-based molecular rotor, the obtained values of the viscosity of the cytoplasm of HeLa cells are in the range of 0.05–0.2 Pa·s (Kuimova et al. 2009; Kuimova et al. 2008). In the work performed on fibroblasts and MDCK cells using rotational magnetic spectroscopy, the obtained viscosity values are also in the range of 0.1-1 Pa·s (Chevry et al. 2013). It should be also noted that the cell viscosity was measured by different methods, which can also affect the results obtained. The work of J.-F. Berret et al. represents the local viscoelasticity of living cells measured by rotational magnetic spectroscopy. Using this method, it was found that the viscosity of the cytoplasm of HeLa cancer cells is in the range of 10–100 Pa·s (Berret 2016). A. Bausch with coauthors in his article determined the viscosity of macrophages, it turned out to be 210 ± 143 Pa·s (Bausch et al. 1999). However, Y. Chen et al. noted in his work noted that the viscosity of fibroblasts, as well as the viscosity of spermatocytes, was 0.26 ± 0.21 Pa·s and 0.04–0.36 Pa·s, respectively (Chen et al. 2014). These parameters were calculated from the Brownian motion of microparticles inside cells at different stages of cell division. These differences are associated with a variety of methods for measuring the viscosity and the state of the cellular cytoplasm. In the present study, we determined the dynamic viscosity of the cell cytoplasm using the classical Stokes formula. The viscosity determination is based on the calculation of the speed of the particles movement inside the cells cytoplasm under the action of a known force. An analysis of the velocity of movement of MNPs in a medium under the action of a known force makes it possible to calculate the viscosity of the medium in which movement occurs (Rodríguez-Rodríguez et al. 2020). In particular, this method can be used to evaluate the viscosity of the cell’s cytoplasm (Bausch et al. 1999). Currently, various methodological approaches are used to measure the viscosity of the cell cytoplasm, including passive microrheology which measures the viscoelasticity of the sample of interest by tracking and analyzing the thermal motion of one or more probes either embedded inside or attached to the sample. The use of such approaches has a number of limitations, since Brownian motion is highly dependent on temperature (Chen et al. 2014). Theoretically, passive microrheology is valid for the determination of cellular viscoelasticity but only if the cell can be regarded as an equilibrium system (Wilhelm 2008). There are also active microrheology methods that imply active influence on test particles in a microsystem and registration of particle responses to this active influence. Such methods include the method of rotational magnetic spectroscopy, but its implementation requires the synthesis of special microwires and it requires the imposition of a field of a certain frequency and complex measurements and calculations (Berret 2016). There are methods using magnetic nanoparticles and magnetic tweezers (Bausch et al. 1999) they are based on the fact that a force of known magnitude, generated by a magnetic field, is applied to magnetic particles internalized into cells. Then, the trajectory and speed of movement of the particles are recorded and these parameters are used to calculate the viscosity. It is well-known that the movement of magnetic nanoparticles was recorded using light microscopes and conventional video cameras (Bausch et al. 1999; Cenev et al. 2018; Seon et al. 2020). In the present study we used a similar approach, but unlike previous works, we used new fluorescent magnetic nanoparticles that can be used in fluorescence confocal microscopy. The use of the confocal microscopy technique using fluorescent magnetic nanoparticles makes it possible to conduct studies in three dimensions and with a higher resolution than in conventional optical microscopy. The presence of a fluorescent label in nanoparticles allows the use of microparticles of smaller diameter, which have a better penetrating ability into cells. The particles used in our studies penetrated 100 percent of the cells (Fedorenko et al. 2019). Due to the presence of the label ([Ru(dipy)3]2+), they can be easily detected in cells. Previous studies have used a ballistic method to load cells with microparticles, which requires the special equipment (Chen et al. 2014).