**1. Introduction**

Nowadays, it is difficult to imagine functioning without polymers. Polymers are used in various industries, such as construction [1], electrical engineering [2], and medicine [3]. In recent years, more and more researchers from around the world have become interested in the characteristics of nano- and microplastic. Submicron polymer particles are defined as molecules of plastic smaller than 5 mm in diameter [4]. They are separated into two types: (1) Primary plastics, which enter the environment in their micro- and nanoscopic state, and (2) secondary plastics, which are the result of continuous environmental impacts on plastic litter and yield progressively smaller polymer fragments. Polystyrene (PS) is one of the most commonly used polymers in the production of plastic materials, together with polyethylene and polypropylene [5]. Its use is found mainly in food packaging, which, due to COVID-19, became a very busy branch of industry. Synthetic polymers have become a serious concern for the life and health safety of living organisms, as it is becoming clearer just how omnipresent plastic is in the ecosystem. Human activity is the main cause of contamination of the environment with polymers. Additionally, plastic production is continually increasing. It is estimated to reach 33 billion tons in 2050 [6]. The consequences of polymer particle debris for wildlife are becoming better and better documented.

Factors affecting the breakdown of polymers into smaller structures include UV radiation [7] or the influence of microorganisms [8–10]. The presence of polymers has already

**Citation:** Zaj ˛ac, M.; Koty ´nska, J.; Worobiczuk, M.; Breczko, J.; Naumowicz, M. The Effect of Submicron Polystyrene on the Electrokinetic Potential of Cell Membranes of Red Blood Cells and Platelets. *Membranes* **2022**, *12*, 366. https://doi.org/10.3390/ membranes12040366

Academic Editor: Francisco Monroy

Received: 1 March 2022 Accepted: 22 March 2022 Published: 26 March 2022

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**Copyright:** © 2022 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/).

been evidenced in soil [11], freshwater [12], oceans [13], plants [14], animal organisms [15], and food products [16]. Mass production of plastic articles in society poses great risks, as these items should be properly disposed of or recycled. If these processes do not take place, polymers will certainly end up in the environment. Given plastic's ubiquitous nature and small dimensions, its ingestion and subsequent impact on living organisms is a growing cause for concern. At cellular and molecular levels, alterations of immunological responses, neurotoxic effects, and the onset of genotoxicity have been observed in water-dwelling organisms exposed to polystyrene particles [17,18].

Domestic pigs (*Sus domestica*) are particularly useful in biomedical research for several reasons. This species is characterized by high fertility and occurs in many genotypes and phenotypes, which is important for the selection of appropriate genetic material for medicinal studies [19–22]. The usage of biological material from pigs raises less opposition in laboratory tests than tests on dogs or cats. *Sus domestica* is very important in medical sciences such as surgery [23,24] and pharmacy [25,26] because of correlations between their bodies and humans. Pigs are not only comparable in size, but also have analogies to humans in the structure of their digestive, nervous, circulatory, and urinary systems. These similarities in the anatomy and physiology of *Sus domestica* and *Homo sapiens* are one reason biological material from pigs can be used for reliable modeling of cellular processes of human bodies.

Soil is also believed to be a pathway for micropolymers to enter living organisms. As it was found out, plastic particles can enter plants grown in heavily polluted areas [27]. This could lead to the consumption of polymers while eating contaminated vegetables by humans or animals. Other studies have shown that digesting processes do not break down plastic, and synthetic polymers can be a component of manure [28]. Scientists discovered polymers in the muscle tissue of marine animals, such as turtles [29] or fish [30]. It is indicated that plastic particles can also cross the blood–brain barrier. The consequence of this is the accumulation of polymers in the microglia cells, worsening their ability to multiply and leading to their death [31]. This information sheds a different light on the effects of polymers on human cells, including cell membranes.

Thus, food consumption can be a direct path of entry to the human body for micropolymers. Scientists believe that polymer molecules can move up the food chain thanks to their presence in the bloodstream of animals [32]. Thus, any type of meat could be a potential route for micro- and nanopolymers to enter the human system. This could have very serious consequences for health and wellness. Many tests have to be carried out to assess the toxicity and transport pathway of micropolymers from livestock, e.g., pigs, to the human body.

Experimental studies of the electrical properties of natural cell membranes can contribute to our knowledge of their properties, function, and structure. The zeta potential (otherwise known as the electrokinetic potential, *ζ*) of membranes is an extremely important electrical parameter that depends on the composition of the cytoplasmic membrane. This parameter defines the stability of a system and allows one to determine whether modifications of the natural system with polymers affect changes in the structure of the natural membrane. The zeta potential varies according to many parameters, such as temperature, pH, conductivity (ionic strength), and solvent (viscosity). Thus, small modifications to any of these parameters may have a significant influence on the zeta potential values [33]. One technique used to estimate zeta potential is electrophoretic light scattering (ELS), a technique based on dynamic light scattering (DLS), in which the shift in the frequency or oscillation phase of the laser beam depends on the mobility of particles/cells in an alternating electric field [34].

The purpose of the research presented in this article is to assess the influence of PS particles on the zeta potential of *Sus domestica* blood components—red blood cells (RBCs), also referred to as erythrocytes and platelets (thrombocytes). We chose polystyrene as it does not generate reactive oxygen species in the presence of cells, and membrane damage by oxidative stress is unlikely [35]. Two sizes of two types of polystyrene were

used: 100 nm plain polystyrene (PS-100) and 200 nm plain polystyrene (PS-200), as well as 100 nm polystyrene with amino groups (PS-NH2-100) and 200 nm polystyrene with amino groups (PS-NH2-200). In addition, different particle concentrations and exposure times to the polymer were applied. Based on the knowledge of the authors, such a thorough analysis has never been performed before.
