Fluoropolymers and nanomaterials, the invisible touchscreen hazards (2024)

Eric Arthur Johnson, an English engineer at the Royal Radar Establishment in Malvern, UK, first described his vision for developing capacitive touch screens in a 1965 paper entitled Touch display—A novel input/output device for computers [1]. Since then, touchscreen displays have transformed the way we interact with electronic devices and gradually gained a dominant presence on today’s consumer electronic devices. Indeed, touchscreens are used on most contemporary mobile computing devices, including smartphones, tablets, smartwatches, and laptop computers. Touch-enabled self-service kiosks are increasingly used in public settings such as schools, libraries, store check-outs, airport check-in's, and fast-food restaurants. They have become an indispensable part of our modern, technology-enabled lives [2]. Market analysis indicates that the current robust growth in the use of touchscreen displays will continue, with the global market for touchscreen displays expected to double its current size and reach over $166 billion USD by 2029 [3]. Touchscreens are very handy; however, how to keep them clean after hundreds of daily finger touches?

Screens that repel smudge

Touchscreen displays generally consist of a cover glass, touch sensors, and a display panel with backlight arranged from top to bottom [4]. One of the early challenges in developing touchscreens for daily use is how to keep their glass surfaces clean. Fingerprints, oil and grease, and smudges left on the surfaces compromise the clarity, appearance, and responsiveness of touchscreen devices. Therefore, the surfaces of touchscreens must be chemically treated to ensure that minimum amounts of fingerprints or other residues are left on the screen surface after finger contact, and that any smudges that do occur should be easily removable [5]. For that, anti-smudge coatings that are optically transparent and chemically inert have become the industry’s standard to create touch surfaces that repel oil, grease, and moisture, and they are easy to apply onto glass screens with fair durability under normal use [6, 7]. With the advancement of material science and processing, these goals have indeed been achieved with a plethora of commercial products offered for manufacturers and do-it-yourself enthusiasts.

Billion-dollar coating

The screen is a main component of many touch-enabled consumer electronic devices, and the application of anti-smudge coatings on the surface of the cover glass is an essential processing step. Apart from smartphones, tablets, and smartwatches, which account for the majority of consumer electronic devices equipped with touchscreens, they have also been incorporated into the design of laptop computer displays, printers, and some newer models of laptop touchpads [8, 9]. Driven by the strong demand for consumer electronics, especially mobile computing devices, the global anti-smudge coating market has already grown into a billion-dollar industry in its own right [10]. However, anti-smudge coatings do contain toxic compounds such as fluorochemicals and nanomaterials, as detailed below [11].

Per- and polyfluoroalkyl substances

The anti-fingerprint ability of electronic touchscreens is mainly gained by the use of fluorochemicals such as per- and polyfluoroalkyl substances (PFAS, Table 1). For example, perfluoropolyethers are commonly used because they form a low surface-energy lubricating layer, resulting from the enrichment of terminal –CF₃ groups at the surface [12,13,14], which exhibits hydrophobic and oleophobic properties [15,16,17]. In this vein, coating formulations containing perfluoropolyether–silane, introduced by Daikin, have captured a strong market share [15, 18, 19]. Other major manufacturers of anti-smudge coatings, such as Cytonix and Solvay, also used perfluoropolyether-based formulations [20,21,22].

These substances display toxicity. For instance, safety data of products listed in Table 1 indicate that they can cause irritation to skin and eyes with long-term harmful effects on aquatic organisms [23,24,25]. Moreover, the toxicological information of Daikin’s OPTOOL DSX and OPTOOL DAC-HP coatings shows that these coatings can irritate the skin of rabbits and be harmful to fish and aquatic organisms [23, 25]. Per- and polyfluoroalkyl substances (PFAS) have shown multi-organ toxicity, genotoxicity, reproductive toxicity, neurotoxicity, developmental toxicity, endocrine disruption, and are suspected carcinogens [26,27,28,29]. Recently, concerns have also been raised on fluoropolymers, i.e., a group of fluorinated macromolecules within the class of PFAS, over their impact on environmental and human health throughout their life cycles [30]. Perfluoropolyethers can decompose into fluorocarbon fragments and perfluoroalkoxy radicals, which are known to react with oxygen and moisture in air, leading to the formation of perfluorocarboxylic acids [31,32,33]. Overall, the presence of polyfluorinated substances on touchscreens is of concern regarding their possible transfer to fingers.

Liquid screen protectors, used for restoring touchscreen surfaces, contain nanomaterials such as silica nanoparticles (Table 2). When applied on a glass screen, the liquid screen protector penetrates into microscopic surface defects and bonds chemically with the glass substrate. These protectors have oleophobic properties, improving smudge resistance and making the surface smoother and easier to clean, restoring the snappy feel of touchscreen devices [11]. However, nanomaterials such as nanoceramics pose environmental and health risks [44]. Concerning nanosilica, animal studies have evidenced the adverse effects of silica nanoparticles on various organs, showing cardiovascular, skin, respiratory, neurological, liver, genetic, reproductive, and renal toxicities [45,46,47]. Further, rats exposed to silica nanoparticles showed behavioral changes, hepato-renal dysfunction, and hyperlipidemia [48]. Rashidian et al. [49] found that prolonged exposure to silica nanoparticles caused oxidative stress and reproductive issues in adult zebrafish.

Coating wear-off and dermal exposure

Dermal uptake of coating substances from touchscreen surfaces is a possible route of exposure to fluoropolymers and nanomaterials (Fig. 1). Organofluorine anti-smudge coatings, for instance, are typically applied as thin films on touchscreen surfaces, with thicknesses less than 10–15 nm [17, 34]. This very thin layer is prone to degradation and removal under the finger touch. As a consequence, these oleophobic coatings are by no means “permanent” and the industry has yet to establish a standard to ensure their long-term durability [14, 40, 42]. While an oleophobic coating should be able to sustain a smartphone’s typical 2-year life cycle, misuse, subpar quality, or poor conditions could cause it to wear out within weeks or months of use [12, 57].

Fig. 1

Touchscreens on consumer electronic devices and self-service kiosks profoundly changed how humans interact with technologies. These glass touchscreens are chemically treated with a thin layer of coatings on top. These coatings, referred to as anti-smudge coatings or anti-fingerprint coatings, are specially formulated to provide smooth, oleophobic, and easy-to-clean touch surfaces. At the core of these coatings, fluoropolymers, e.g., perfluoropolyethers or nanomaterials, e.g., silica nanoparticles, are the substances that render the screen surfaces smudge-proof by making them hydrophobic (water-repelling) and at the same time, oleophobic (oil-repelling). When users frequently touch these screens, fluoropolymers or nanomaterials may transfer to their fingers and enter the human bodies via dermal uptake and hand-to-mouth transfers during eating, cooking, or nonnutritive sucking by young children

Both fluoropolymer coatings and nanomaterial coatings tend to breakdown under repeated abrasion, causing screens to slowly lose their smudge resistance [12, 53]. The rate of wearing out is controlled by the nature of the object in contact, i.e., soft, hard, or sharp objects, by the contact frequency, e.g., light or heavy use, by the magnitude of forces applied during the contact, and by the micro-environment that the screen coating is exposed to, e.g., ultraviolet light, humidity, acids, oils, alcohols or other ingredients in food and drink, perfumes, or cleaning products. User habits, i.e., sliding forces and contact frequencies, are usually the main factors that determine how fast the coating thins and fails under normal use. In addition, contacts with hard objects such as keys, coins, buttons, or rough clothes on the screen accelerate the wear of the coating. During the mechanical wear process, the Si–C bond connecting perfluoropolyether and silane agent is the weakest and therefore prone to dissociation and potentially forming a new C–O bond, thus compromising the stability of the anti-fingerprint coating [58].

Moreover, using touchscreens in environments with the presence of dust, sand, grease, or moisture can cause the coating to wear out more easily. In the presence of oxygen, water molecules strongly influence the decomposition of perfluoropolyethers, where scission occurs both at the functional ends and the main chain via the C–O bond cleavage [59]. Furthermore, the high surface area of nanosized silica makes such coatings susceptible to contamination by trace amounts of water vapor and volatile organics such as plasticizers, silicone oil, and vacuum grease [60]. Also, exposure to UV light can induce the photodegradation of perfluoropolyethers and cause nanosized silica coating to generate significant amounts of free radicals, which accelerate the breakdown of hydrophobic groups [61, 62]. Finally, the use of solvent-based cleaning products also accelerates the wearing out of anti-fingerprint coatings [63].

“Zombie eaters” and young children

Around the world, screen use during eating is becoming very common. The use of smartphones, tablets, or other electronic devices during eating is particularly common among young people and those who eat alone. A survey of 2,000 adults in the USA found that up to 88% were so-called “zombie eaters” who gazed at or manipulated screens while eating, primarily for reading or sending emails, checking social media, or watching videos [64]. Indeed, screen-distracted eaters are frequently sighted in schools and workplaces, as well as at leisure venues such as food courts, fast-food restaurants, even in parks. In homes, electronic screens are also becoming the new “table staple” at dining tables.

When users come into contact with food after touching screen coatings, the risk of hand-to-mouth transfers of coating debris or associated chemical substances is significantly increased [65, 66]. Also, the use of electronic screens is increasing among young children, e.g., at ages 2–11 when digit sucking is common [67,68,69]. In a survey of nonnutritive sucking behaviors of children between 1 and 8 years of age, Bishara et al. [67] reported that 31% of the surveyed 1-year-old children engaged in digit sucking, with 12% of the surveyed 4- and 5-year-olds and 8% of the surveyed 6- and 7-year-olds showing the same behavior. Some adults also have finger-licking habits when cooking or eating food with sauces and condiments. For them, hand-to-mouth transfers of screen coating residues can easily occur.

Occupational hazards

The processing of organofluorine anti-smudge coatings has raised alerts on worker’s health. Ma et al. [70] reported an incident of acute poisoning in four workers at a Chinese factory applying anti-fingerprint coatings on mobile phone touchscreens, where the coating components and processing aids including perfluoro-1,3-dimethylcyclohexane, hexadecafluoroheptane, perfluoro-hexane, perfluoromethy lopentane, and perfluoro-2-methyl-2-pentene caused acute bronchitis, pulmonary edema, and/or myocarditis in those workers. To our knowledge, this study, which was published in the Chinese Journal of Industrial Hygiene and Occupational Diseases, was among the first reports of acute poisoning due to exposure to touchscreen coating components and processing aids in occupational settings.

60% of the world’s population owns smartphones

The public announcement of the first generation of Apple's iPhone, made by Steve Jobs in January 2007, was the defining moment that gives touchscreens the massive popularity we are seeing today. Now more than seventeen years have passed, global smartphone owners have reached a total user count of 4.9 billion, which represents 60.4% of the world’s population [71]. A 2019 survey among 1,600 young people in the USA showed that over two-thirds of the respondents (69%) owned their smartphones by the age of 12 [72]. During COVID-19, activities such as remote working, online classes, video conferencing, and media streaming grew substantially, which resulted in further increases in daily screen use among the general population [73]. This trend now continues well after the pandemic and will probably persist as the technologies continue to advance. Yet, knowledge gaps exist in the anti-smudge coatings of touchscreen surfaces with respect to their wear-off, dermal uptake, and hand-to-mouth transfer, e.g., by “zombie eaters” and young children, and the potential risks associated with these scenarios in daily activities.

Existing studies on contaminants from screen use have mainly looked at liquid crystal monomers leached from electronic screens [74,75,76]. In this research domain and on a different subject, Li et al. [65] studied the migration of persistent toxic substances from protective cases of mobile phones, noting the risk of hand-to-mouth transfer of these substances. Meanwhile, there is still a lack of data on human exposure to anti-smudge coating debris and potentially their transformational products via dermal uptake and hand-to-mouth transfer. Given the fact that fluoropolymers and nanomaterials are already under intense scrutiny for their adverse effects on environmental and human health, studies are needed to gauge the human exposure and assess the potential health effects resulting from this particular class of substances, especially among vulnerable groups such as young children, pregnant women, and those with occupational exposure. Further, toxicological data on touchscreen coatings used on consumer electronic devices must be provided to the regulators of chemical substances and the general public. Animal studies and cell cultures are needed to elucidate their effects on target organs under “normal use” scenarios with routine exposure or in particular environments.

Safer alternatives

Many have recognized that a world without PFAS is achievable, although the extent to which manufacturers are adopting alternatives remains unclear [77]. In the limited number of scholarly publications currently available, researchers have provided evidence from laboratory studies, proposing fluorine-free polyurethane coatings [78–80], non-fluorinated organosiloxane coatings [81, 82], and castor oil-based coatings [83] as alternatives for use as anti-smudge coatings on touchscreen cover glasses. Liu and co-workers recently reported a highly cross-linked, rigid oxazolidinone structure and a liquid-like molecular coating that demonstrated high hardness, flexibility, transparency, and dynamic omniphobic anti-fouling properties [84]. These studies offer new insights into alternative anti-smudge coatings for glass substrates. Given the widespread use of anti-smudge coatings on touchscreen devices, it is necessary to pursue safer alternatives while conducting more rigorous evaluations of current products regarding their human exposure levels, health risks, and environmental impacts through life-cycle analyses. Although this topic has not yet garnered widespread interest from the research community, the prominence of touchscreens as a primary type of user interfaces on electronic devices suggest that users will ultimately demand safe, durable, and effective anti-smudge coatings to maintain surface cleanliness and optical transparency with minimal risk.