Design and Evaluation of an Applicator for Magnetopriming Treatments

Seed priming is a physiological seed enhancement method for overcoming poor and erratic seed germination in many crop and flowering plants. Magnetopriming is a pre-sowing seed treatment with magnetic field that appears as a promising method to improve seed performances. This paper presents a cost-efficient design and optimization of an exposure system for magnetopriming treatments. The proposed static magnetic field applicator is modelled and designed with the aid of commercial software. The prototype is realized and tested based on the best set of geometry parameters for optimum performance, in terms of strength and high homogeneity of the magnetic flux density in the Region of Interest. Both analytical and measurement results are found to be in good agreement with the simulated results. The system is low cost, environmentally friendly and easy to operate. It allows seed treatments at different strengths with high homogeneity within the samples. In this way, the treatments can be carried out following good practice requirements strongly recommended for a high quality bioelectromagnetic research to assure reliability and reproducibility of the experiments.


I. INTRODUCTION
T HE world population is expected to increase to over 10 billion by 2100 and this will lead to a growing food demand [1]. Increasing the land surface used for agriculture is not a viable strategy because it is ecologically unsustainable, as this would imply a further increase in deforestation with a consequent loss of biodiversity. In last decades, the Green Revolution has played a central and exclusive role in increasing world agricultural productivity by its proposed approaches as high-yielding varieties, agrochemicals, and intensified farming [2]. On the other hand, intensive agricultural practices have generated a great environmental impact, whose effects are difficult to mitigate. Awareness of the complexity of the interactions between human production activities and the environment requires the urgent development and application of agricultural practices completely based on sustainability.
In the context of a sustainable agriculture, seed quality is an important trait that can improve food crop and contribute to meet the growing food demand [3]. High seed quality means good seed germination, sustaining an efficient growth and high productivity of food crops [4]. Germination is the very critical initial process of the entire seed development program. It is strongly influenced by intrinsic properties of the seed (e.g., genetic characteristics, state of conservation, age of the seed) which can define its quality [5]. Moreover, seed germination is greatly compromised by environmental constrains (abiotic and biotics factors), frequently occurring due to extreme weather events as consequences of the ongoing climate change [6]. Similarly, a slow and not uniform germination makes the development of plants more sensitive to adverse environmental conditions and leads to a not optimal crop production [3].
Pre-sowing treatments to improve seed quality have been known for a long time and are referred to as "seed priming" techniques. Seed priming aims to improve germination efficiency, in terms of speed and uniformity, and makes the seedling development more resistant to environmental stress, with positive effects on crop production [7]. There are numerous seed priming techniques, and their effectiveness depends mainly on the plant species, but also on the variety, morphology, and age of the seeds [8]. The most used seed priming techniques can be classified into chemical, biological and physical treatments. The chemical and biological techniques, although effective, are particularly long, complex, and expensive. They also often require the use of chemical agents which also have an additional cost in terms of environmental impact.
Physical techniques for seed priming have the advantage of being faster in their application, compared to the chemical and biological ones. In recent years, among physical seed priming techniques, magnetopriming had a significant development. There is an increasing number of publications demonstrating that exposure of seeds from different plant species to a magnetic field (MF) before sowing improves germination efficiency, plant development and productivity under both normal and stressed environmental conditions [9], [10], [11], [12], [13]. Despite numerous studies on the effects of MF exposure on seed This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ germination, it has been underlined a fragmentation of the results. This is a well-known issue in bioelectromagnetic research, and several authors, actively working in this contest, recommended to apply high methodological quality [14], [15], [16], [17], [18]. Regardless of the applied electromagnetic field frequency, many data reported in the literature are not informative due to inadequate electromagnetic or biological experimental procedures, preventing both the evaluation of the experimental results and the reproduction of the experiments by other laboratories. Accurate dosimetry is one of the mandatory requirements for good laboratory practices. The use of magnetopriming to improve seed quality is very promising, especially due to its low cost of application, ease of use and low environmental impact. However, the large heterogeneity of seeds (morphology and size), endpoints examined and exposure conditions adopted make difficult to summarize and compare the results.
A deep understanding of the interactions between MF and seed responses could revolutionize crop production and increase their resistance to disease and stress conditions, as well as the water and nutrient use efficiency.
Although some hypotheses have been proposed about the mechanisms that explain how this physical event is converted into a cellular response, the exact mechanism is still largely unclear. Magnetopriming induced by static magnetic fields (SMF) is carried out applying MF generated by electromagnets [19], [20], [21], [22] or permanent magnets [11], [23], [24], [25]. The magnetic flux density (B) and the effective dose can vary depending on the species, biological process observed, abiotic/biotic stress conditions, etc. Typically, B is in the range 50 mT -200 mT while the duration can vary from few minutes up to 24/48 h.
In addition to the MF strength, the uniformity of the field in the Region of Interest (RoI) is an important parameter to be addressed to have reproducible data. However, this information is difficult to obtain in published studies, and when inhomogeneity is reported it can strongly vary from very low values (0.5%-1.5%) typically achieved with electromagnets [19], [26] to relative high values (25%-30%) when permanent magnets are employed [11], [23].
The homogenization of B field conditions is a well-known challenge. For instance, in the Magnetic Resonance (MR) apparatuses, the experimental B field homogenization, called B shimming, is mandatory for reliable investigations. Common concepts and specific solutions for its experimental optimization are summarized in the review [27]. Passive shimming with ferromagnetic substances provides an inexpensive means of efficiently producing very strong and homogeneous magnetic fields. It can be achieved with ferromagnetic substances, opportunely introduced into the scan. However, the creation and adjustment of such passive shim assemblies is cumbersome and not flexible, and its practical shortcomings have prevented it from becoming the common method of choice in MR service best practice.
Inspired by MR apparatuses, in this paper we present an SMF applicator for magnetopriming treatments whose design was driven by the following working hypotheses: possibility to confine the magnetic field lines generated by the magnets, in such a way to maximize the B-field level and uniformity in the sample area; possibility to host samples of different sizes at various B-field strength; device based on permanent magnets, in such a way to avoid confounding factors like heating that could arise in the case of electromagnets or radiofrequency applicators; low cost of realization with low footprint (for use in laboratories/greenhouses); avoiding interference with other electromagnetic field sources (low/high frequencies), as well as guarantee the operators safety.
The applicator was realized and tested. The device works at variable B-strength between 50 mT and 200 mT, i.e., in the range where numerous positive results have been reported in the literature [28], [29], [30]. A high uniformity (about 95%-99%) was achieved in the RoIs by adopting careful design approaches that will be illustrated in the paper. We envision that a reliable, robust, small size applicator easily transported to the application place without requiring specialized installation and power supply, can be extremely advantageous, improving in this way the beneficial effect on the environment of this ecofriendly technique. In our research activity this device will be used either for investigations about the mechanisms promoting this effect or for finding the conditions suitable for potentiate seeds germination of crop plants particularly important in South Italy.
The paper is organized as follows. We present in Section II the numerical approach followed to obtain the B-field distribution in the case of two magnets opportunely located. The good agreement with the results achieved with an analytical approach assured us the accuracy of the method and different scenarios are thus considered to show the improvement attained in the applicator design. We performed a parametric study, allowing the reader to adopt the best scenario depending on the target of the study. The experimental setup for results validation is then described, and in Section III the comparison between analytical, measurement and simulations data are given. Finally, conclusions are drawn in Section IV.

II. METHODS
A set of simulations has been performed to achieve the optimal exposure conditions, in terms of maximum B-field and B-field uniformity achieved. Numerical dosimetry was carried out to evaluate B-field distribution in the case of two magnets at distance Δ, and to determine the average, minimum and maximum B value (B av , B min , B max ) as well as homogeneity degree in a RoI where the sample is located. To this goal, the magnetostatic solver of CST Studio Suite platform has been employed on a laptop equipped with an AMD RYZEN 9 5900HX, NVIDIA GeForce RTX 3080 and 32GB RAM. The dimensions of the computing grid (hexahedral mesh) in the RoIs were Δx = 1.5 mm, Δy = 2.08 mm, Δz = 1.93 mm. The simulation time varied between few hours up to ten hours depending on the complexity of the scenario. The coefficient of variation (CV), defined as the standard deviation (SD) of B values over the mean B value (B av ), was adopted as an indicator of the B nonuniformity degree [31]: being N the voxel number and B i the B value calculated in the i-th voxel. Two permanent cylindrical magnets (radius R = 22.5 mm, height L = 30 mm, remanence magnetic flux density B r = 1.2905 T) at a distance Δ varying, with 5 mm step, between 60 mm, minimum deviation from routine workflow of biological procedures (e.g., avoiding laborious or poorly reproducible positioning of seed container inside the exposure apparatus), and 120 mm was considered. The isocenter, where the greatest magnetic field homogeneity is expected in its neighborhood, is in (0, 10, 0) coordinates and two different RoIs were evaluated: RoI 15 (15 mm × 15 mm × 15 mm) and RoI 30 (30 mm × 30 mm × 30 mm) allowing at least 50/100 seeds contemporary exposed depending on the seed sizes. Results obtained with different configurations are shown in the following, and B-field homogeneity will be reported in terms of CV and B-field value in the isocenter neighbor. The following scenario were compared: i) B field distribution given by the two magnets at different Δ. ii) B field distribution given by the two magnets at different Δ when shimming plates with different thickness (1 mm-9 mm, 1 mm step) are located near the magnets. iii) B field distribution given by the two magnets at different Δ when an iron chamber (409 mm × 328 mm × 218 mm, 10 mm wall's thickness) contain the exposure system. iv) B field distribution in the case of the two magnets at different Δ when the iron chamber contains both the exposure system and plates with different thickness located near the magnets ( Fig. 1(b)). Case i) is the usual configuration reported in literature. As validation test, the simulation results achieved in this case were compared with that determined by adopting the analytical expression [32] for the magnetic field of a cylinder of finite length. Case iii) describes the previous configuration but with a B field confining structure. Case ii) and iv) represent respectively the shimming procedure applied when only magnets or magnets with shielding are considered.
A device was realized ( Fig. 1(a)) and tested. Measurements were carried out at Δ = 70 mm and Δ = 120 mm by mapping the magnetic flux density levels in the area where the sample would lay by means of a Hall gauss-meter (F.W. Bell, model 4048 with T-4048-001 transverse probe, 2% accuracy).

III. RESULTS
Plots of CV percentage in the different scenarios, depending on either the distance between the two magnets or kind of slab, are reported in Fig. 2 for RoI 30 . CV values result greater in the RoI 30 , being the volume wider than that occupied by the RoI 15 , and an improvement is observed when the system is enclosed in the shielded chamber. In the case i) and ii) a 5 mm slab was necessary to reduce the inhomogeneity of about 60%-80% whatever is the distance Δ (Fig. 2(a)). A similar result is obtained when the iron chamber is added, but a 4 mm slab was sufficient with the advantage of both a higher B strength and a lower SD (scenarios iii) and iv) in Fig. 2(b)). Furthermore, the comparison of the results shown in Fig. 2(a) and (b) highlights that, when the iron chamber is added, an additional CV reduction is achieved with respect to the corresponding scenario without chamber due to a smaller SD. For example, in the case of a 4 mm slab, the insertion of the iron chamber entails a CV percentage decrease of 36% and 15% at Δ = 60 mm and Δ = 120 mm respectively.
The average B-field strengths in the same conditions are shown in Fig. 3 for RoI 30 . A very similar behavior was observed in RoI 15 . As expected, B av decreases either with the increase of Δ or, at a fixed Δ, with the increase of the plat thickness. However, at either 4 mm or 5 mm depending on the scenario, B av becomes quite constant when higher thicknesses are adopted (Fig. 3(b) and (d)).
For the sake of clarity, the B-field strength behavior in the isocenter neighbor of RoI 30 with either a 2 mm or a 4 mm slab is compared to the no plat condition at Δ = 70 mm (Fig. 4(a) and (b)) and Δ = 120 mm (Fig. 5(a) and (b)).
It appears that the different transversal sizes of the chamber do not affect the symmetry along the x-axis and the z-axis (case    iii) and iv)) and a minimum is obtained in the isocenter along the y-axis, increasing the intensity of the field as we get closer to the magnets and bringing about the "bell shaped" curves. The B-field homogeneity improvement thanks to the 4 mm slab is evident, being the curves flattened. In Figs. 4 and 5, the comparison of numerical data with analytical data (case i) and of numerical data with measurements (case iv) are shown for RoI 30 and cases Δ = 70 mm and Δ = 120 mm. Measurements were carried out around the isocenter (0,10,0) of the prototype with 4 mm slabs inserted. As expected, a high symmetry was found in the transverse plane (x, z axes) with the overlap of the curves and similar values along longitudinal y axis were achieved. The numerical results closely match the experimental once, with a difference comparable to the accuracy of the instrument. We also found good agreement between numerical and analytical results with a difference of only 5%.
To help the reader interested in realizing one of these configurations, the values of B max , B min and B av numerically calculated in the different scenarios and with different Δ and slab's thickness are reported in Tables I and II in supplementary Appendix. These data allow one to find the best solution for certain values of B-field and show how slabs thicker than 5 mm do not improve the efficiency of the system. It is important to note that a negligeable B-field value is guaranteed outside the shield box. The external B-Field is less than 0.5 mT that is the exposure limit suggested to prevent inadvertent harmful exposure of people or sensible operators (e.g., with implanted electronic medical devices and implants containing ferromagnetic materials), as well as injuries due to flying ferromagnetic objects [33], [34].

IV. CONCLUSION
The influence of the MF on seeds does not pose any threat to the environment and the increase of the efficiency of physiological processes is particularly interesting. The effect of this action is greater vigor and a higher level of plant yield. The current contradictory and inconsistent outcomes from studies on varying effects of MF on plants could be related to species and/or MF exposure time and intensity. The design, implementation and testing of a portable applicator of magnetic field for magnetopriming treatments were shown.
A very simple configuration has been proposed allowing to easily replicate the applicator towards its ready use in laboratories as well as in greenhouses by unskilled operators, safe for both workers and the public. Other shapes of slabs and solutions will be examined to further improve these encouraging results. Passive shimming is performed with shims of ferromagnetic material in permanent magnet of MR systems with field strengths up to 0.5 T. Such systems often have a Field of View, corresponding with the portion of volume with strong B homogeneity, equivalent to a sphere of 40 cm in diameter or over 30 000 cm 3 . Thus, we are particularly confident about the scalability of the method both in terms of B strength and "homogenized" volume.
The prototype was adopted to expose Capsicum annuum L. seeds and their germination was compared to that of unexposed seeds considered as a control. Preliminary results indicate that 1 h exposure to 50 mT induced a significant (50%) increase germination (P < 0.05) in the short observation period (5 days after sowing). More details are reported in [35]. This initial investigation confirms that the magnetopriming treatment is a promising and ecofriendly approach to facilitate seed germination, being able to enhance the speed germination in a plant species characterized by an excellent basic germination capacity (around 98%).