Despite intense research, biofilm presence in food manufacturing, processing, and storage remains a significant challenge [1]. Population growth and its associated rise in food supply chains and the demand for food have only exacerbated this issue [2]. According to the World Health Organization (WHO), $110 billion USD are lost each year in productivity and medical expenses from the lack of sale or the consumption of unsafe food in low and middle-income countries. An estimated 600 million cases of illness and 420,000 deaths occur every year [3]. Although there are multiple potential causes, the bacterial contribution to these statistics is likely to be considerable. As part of broader investigations to eliminate biofilm, attempts implementing nanotechnology and nano-formulations have shown promising results indicating significant biofilm reduction in several reports [4], [5]. As suggested by Barros et al. [6] nanoparticle containing natural products and phytochemicals can be applied through encapsulation or synergistically together with potent plant-derived materials such as curcumin [7], [8] or cinnamaldehyde [9], [10]. However, most approaches are based on substance migration to the vicinity of the cell and therefore suffer from temporary potency and leaching [11], [12]. Free nanoparticles (NPs) are susceptible to external conditions such as water and abrasion unlike anchored mediums. Due to their mobility, these materials are subject to regulations limiting their applicability [13]. The release and unintended leaching of active materials such as volatile molecules (e.g., carvacrol, thymol, cinnamaldehyde, and peppermint oil) can impart an undesirable taste, smell, or toxicity on the product [14]. For these reasons, a passive mechanism approach such as superhydrophobicity constitute a better alternative for anti-biofilm systems.

Superhydrophobicity; i.e., Cassie-Baxter wettability, is characterized by low roll-off angles (< 5 °), and is often associated with self-cleaning property that is highly desirable for various applications [15], [16]. In many animal and plant species, superhydrophobicity derived from surface morphology is known to be a solution to the array of challenges imposed by their habitat [17]. Since early reports of the lotus leaf self-cleaning mechanism in 1977, [18] thousands of articles have described attempts to mimic this phenomenon. The vast majority aim to form nano and micro-scale roughness to achieve a Cassie-Baxter heterogeneous wetting state and avoid the Wenzel homogenous wetting state [19], [20], [21].

Evidence shows that superhydrophobic wetting properties significantly affect biofilm adherence and formation [22]. Although superhydrophobic mechanisms are well-understood, current methods still grapple with material compatibility, scale-up and consistency [23].

The current study addresses these drawbacks by utilizing a Pickering emulsion templating approach to facilitate superhydrophobicity using non-fluorinated silica. An emulsion is a mixture of two or more immiscible liquids created by liquid-liquid phase separation [24]. Unlike standard emulsions that are stabilized by surfactants, Pickering emulsions are stabilized by solid particles and form colloidal systems [25]. One of the key advantages of Pickering emulsions over standard emulsions is that they form a large surface area of particles at the interphase. Due to their strong anchoring and added steric repulsion, colloids can inhibit coalescence to a greater extent than other materials. The properties of the final emulsion are largely determined by the size [26], shape [27], and wetting characteristics of the particles [28]. Solid particles allow for easy modification with high precision. Pickering emulsions are used in many applications, ranging from highly tunable emulsions with stimulus responsiveness to very stable and robust systems [29].

Here, we developed an inverse Pickering emulsion of water in DMC stabilized by hydrophobic silica particles with PCL dissolved in the DMC's continuous phase. The aqueous dispersed phase consists of a DMC-saturated solution of water. DMC is a green [30] and relatively safe [31] solvent compared to solvents commonly used today. Thus, developing an inverse Pickering emulsion stabilized by fluorine-free silica NPs with a safer solvent as the continuous phase presents a significant breakthrough compared to our previous work [32] and represents a significant step toward the development of superhydrophobic coatings with anti-biofilm properties for food and medical applications.

Since the 1980 s, DMC can be produced by recyclable catalysis of CO2 and methanol, resulting in a very sustainable, benign process [33], [34]. Besides its low cost, the degradability rates of DMC are approximately 90% after only 28 days [30]. In addition, DMC has no mutagenic or irritating effects when absorbed by inhalation or contact and has only mild toxicity when ingested [31]. Currently, industries as varied as paints, adhesives and fuel additives benefit from transitioning to DMC-based applications. [35] Thus, DMC production rates are expected to experience an exponential increase due to extensive consumption in the near future. The DMC market is expected to grow from 847 million USD in 2021–1148 million USD by 2026 [36]. Interestingly, DMC is partially miscible in water (∼14% at 25 °C). In the current study, the dispersed phase of the inverse emulsion consists of a saturated aqueous solution of DMC. Although relatively rare, Pickering emulsions composed of partially miscible phases have been reported by Binks et al., Clegg et al., and others [37], [38]. For a coating to be applicable on an industrial scale, it has to be cost-effective, consistent, and must have little to no effect on the properties of the substrate. Superhydrophobic coatings based on Pickering emulsions can be made with various solvents, polymers, and particle types, which all display different template patterns. The combination of unique patterns in conjunction with specific materials can produce advanced properties such as self-replacing mechanisms [39]. The use of a Pickering emulsion as a superhydrophobic platform allows for the incorporation of water in significant volumes, which increases biocompatibility and scalability, and reduces costs and carbon footprint. In addition, adding water has a minimal effect on the evaporation rate of the coating which is critical to many industrial processes that depend on fast drying times. What sets our approach apart is the use of Pickering emulsion to create unique surface roughness, which contributes to bacterial biofilm inhibition. Below we present findings indicating that there was a 90% and a 95% reduction in the survival rates of S.aureus and E.coli, respectively. For comparison, more toxic materials with lower biocompatibility have reported similar biofilm reduction rates of ∼2 log using fluorinated silica [40] and perfluorodecanethiol-modified silver NPs [41]. Hence, the unique surface roughness obtained by the Pickering template makes it possible to avoid the utilization of toxic hydrophobic materials such as fluorinated polymers to achieve high superhydrophobic performances. Here, we show that using DMC and polysiloxane-modified silica can produce a green, food-grade superhydrophobic coating with high anti-biofilm abilities. The research concept is illustrated in Fig. 1.