Inspired by the remarkable surface wetting behavior of natural organisms, artificially designed superwettable systems have attracted significant attention from multidisciplinary scientists over the past two decades. Starch is an eco-friendly, nontoxic, and low-cost natural polymer that serves as an alternative to nonbiodegradable and/or bioincompatible synthetic polymers in these systems. This review explores the unique contributions of starch to superwettable systems from design principles to emerging applications. First, the fundamental theories and design principles underlying starch-involved superwettable systems are introduced. The specific design principles of these systems are comprehensively discussed from the aspects of intrinsic properties (e.g., hydrophilicity, film-forming properties, adhesiveness, and thermal decomposition), dimensionality (e.g., colloidal systems, zero-dimensional granules/particles, one-dimensional fibers, two-dimensional films/fibrous membranes/coatings, and three-dimensional fillers/porous materials/food textures), and biotransformation. It also provides an overview of their applications in functional biomaterials, oral delivery systems, emulsion polymerization, packaging technology, food taste modulation, and water treatment, with particular emphasis on intelligent systems. Each section summarizes recent advancements, highlighting the chemical and structural features. Finally, the review considers prospects for these superwettable systems, focusing on underutilized starch attributes and technical challenges.
Superwettable systems, such as lotus leaves (anti-contamination), cicada wings (antibacterial), and moth eyes (antireflection and antifogging), which serve as environmental and climatic condition adaptive mechanisms, have been used for thousands of years in various plants, insects, and animals [1]. Each instance of superwettability in nature results in well-defined chemical properties, structural profiles, and corresponding functionalities, offering valuable insights into the design of advanced functional systems. Therefore, the theory of superwettability has been extensively explored from a biomimetic perspective since the beginning of the 21st century, with related technologies and materials being incorporated into various emerging industrial applications [[2], [3], [4]]. For example, superhydrophobic windshields can be suitably installed on large vehicles to prevent fogging and icing; superhydrophobic coatings on cement and wood surfaces show promise for enhancing the longevity of roads and construction by mitigating water-related damage [5,6]; and superhydrophobic and superhydrophilic fabrics are sought after in the textile industry for water- and oil-resistant clothing, respectively. In the medical field, the adhesion of cells, proteins, and bacteria to biological surfaces is also considered an issue of superwettable interfaces. Thus, the fundamental theory, design principles, and diverse potential applications of these unique interface systems have become of great interest to researchers and industrialists across various fields.
Following the understanding of superwettability and its underlying principles, materials exhibiting superwettability have continued to be developed. Notable early contributions include the discovery of an anti-wetting soot/lycopodium mixture by Coghill and Anderson in 1907 [2], the identification of superhydrophilicity during silicon wafer pretreatment in 1959 [7], the development of superhydrophilic surfaces based on TiO2-coated glass slides under photoirradiation by Wang et al. in 1997 [8], and the demonstration of nanostructured materials composed of aligned carbon nanotube films exhibiting superamphiphobic properties (i.e., both superhydrophobic and superoleophobic) by Feng et al. in 2002 [9]. To date, superwettability has expanded into a broad spectrum of properties, including superhydrophilicity, superoleophilicity, superhydrophobicity, and superoleophobicity in air; superoleophilicity, superoleophobicity, superaerophilicity, and superaerophobicity under water; and superhydrophilicity, superaerophilicity, superhydrophobicity, and superhydroaerophobicity under oil [2]. These properties have been identified in various materials, including organic (polymers), inorganic (ceramics, glass, and carbon), and metallic materials [2,10]. However, as the global sustainable development strategy advances, concerns regarding the environmental impact of nonbiodegradable or toxic polymers on ecosystems and human health have become more pronounced [11].
Among alternative materials, natural biological macromolecules, such as polysaccharides and proteins, are the most abundant polymers [12,13]. Starch, the second most abundant polysaccharide after cellulose, is primarily sourced from crops, such as corn, wheat, rice, and potatoes [14]. It exists as semicrystalline microscopic granules with a crystallinity ranging from 15% to 45%. Starch consists of two main components: amylose (∼20%–25%) and amylopectin (∼75%–80%). Amylose has a linear structure (approximately 99% α-D-(1→4) glucosidic linkages) or slightly branched structure (approximately 1% α-D-(1→6) glucosidic linkages) and contains approximately 105–106 anhydrous glucose units (AGUs) per chain. Amylopectin, on the other hand, has a highly branched structure (approximately 5%–6% α-D-(1→6) glucosidic linkages) and approximately 106 AGUs per chain [15]. Although the chemical properties of starches from different botanical sources are largely similar, variations in their granular, crystalline, and molecular structures influence their properties and applications (Table 1) [[16], [17], [18], [19], [20], [21], [22]]. Starch is consumed primarily as a staple food that provides energy to humans, but it is also used in various nonfood industrial applications, including adhesives, sizing agents, chemical production, and other industrial products [23]. Starch stands out from other polysaccharides due to several characteristics: ① Compared with more expensive bioactive polysaccharides applicable to specific areas, such as antibacterial chitosan [24], antiparasitic pectin [25,26], and anti-aging hyaluronic acid [27], starch and cellulose are low-cost, nonbioactive polysaccharides with high yield and wide availability, making them suitable for many industrial applications (i.e., broad applicability); ② unlike cellulose, starch, with its combination of amylose and amylopectin, can be gelatinized in hot water and undergo gelation without the need for crosslinking agents (i.e., easy processability); and ③ starch is nontoxic, and the amylases that degrade starch are widely found in plants, animals, and microorganisms (i.e., favorable biocompatibility and biodegradability). These attributes make starch a promising substrate or auxiliary material in superwettable systems, in line with the principles of green chemistry. The chemical modifications, structural remodeling, and specific applications of superwettable systems are largely determined by the characteristics of the materials used. Compared with synthetic and other natural polymers, starch-based materials present distinct features [[28], [29], [30], [31], [32], [33]]: ① Starch-based materials can exist in dimensional forms ranging from zero-dimensional (0D) to three-dimensional (3D), offering diverse options for constructing substrates and surface micro/nanostructures; ② the hydrophilicity and water-soluble crystallinity (types A, B, C, and V) of starch contribute to its intrinsically high water affinity; ③ three reactive hydroxyl groups on each AGU provide anchoring sites for further chemical modifications; ④ the hydrolysis of starch by endo/exoamylases highlights its superior biotransformability and biocompatibility; and ⑤ the ability of starch to form gels and films and its high adhesiveness occur without the need for crosslinkers. These features position starch as a unique and promising material for the production of superwettable systems.
Driven by the increasing interest in superwettable systems, recent advances and perspectives on starch-involved superwettable systems are reviewed here, with a focus on their design principles and emerging applications (Table 2) [[34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72]]. First, the fundamental theories and design principles applicable to starch-involved superwettable systems are briefly introduced. Next, their specific design principles are discussed from the perspectives of intrinsic properties (e.g., hydrophilicity, film-forming ability, adhesiveness, and thermal decomposition), dimensionality (e.g., colloidal systems, 0D granules/particles, one-dimensional (1D) fibers, two-dimensional (2D) films/fibrous membranes/coatings, and 3D fillers/porous materials/food textures), and biotransformation. Finally, the applications of these systems in functional biomaterials, oral delivery systems, emulsion polymerization, packaging technology, food taste modulation, and water treatment are systematically summarized, with particular emphasis on intelligent systems. The review concludes by highlighting the unique contributions of starch to superwettable systems, addressing technical challenges, and offering perspectives on future developments.
2. Fundamental theory and design principle
Wettability is a fundamental interfacial property arising from the interaction between molecules of different phases. It is quantitatively evaluated through parameters such as the water contact angle (WCA)/oil contact angle (OCA), contact angle hysteresis (CAH), and sliding angle (SA) (Fig. 1). The surface energy and topography are the key factors governing wettability. Superwettability (contact angle (CA) > 150° or < 10°) represents an extreme case, characterized by a rough interface (i.e., micro- and/or nano-structuration) and specific hydrophilicity/hydrophobicity [73]. A minority of starch-involved superwettable systems are liquid–liquid interfaces, typically in the form of special colloidal systems, such as Pickering emulsions and micelles. The fundamental theory and design principles of these systems are closely linked to the specific form of the colloidal systems, and they will be detailed with illustrative examples in Section 4.1.1. In contrast, most starch-involved superwettable systems are classified as solid–liquid interfaces. According to Liu et al. [1] and recent studies [[74], [75], [76], [77]] on superwettability, several underlying theories and design principles associated with starch-involved superwettable systems are depicted along a time ruler (Fig. 1). The first theory of superwettability is inspired by an axisymmetric water droplet levitating over a hot liquid pool, where the formation of an insulating vapor layer results from the high mobility of the droplet (i.e., the Leidenfrost effect) [78]. Subsequently, Thomas Young, Wenzel, and Cassie and Baxter subsequently proposed wettability models for smooth (Eq. (1)), rough homogenous (Eq. (2)), and rough inhomogeneous surfaces (Eq. (3)) on the basis of surface energy (Eq. (4)) [75]:
where θCA is WCA or OCA and is the CA on rough surfaces; γla, γsl, and γsa refer to the surface tension (γ) at the liquid–air, solid–liquid, and solid–air interfaces, respectively; the roughness factor (r) is the ratio of the actual surface area to the horizontal plane area; A and f represent the surface area and area fraction, respectively; G is surface energy; n, T, and P represent the amount of substance, temperature, and pressure, respectively; and different materials on the surface are distinguished by the subscript i. In the real-life Cassie–Baxter model (Eq. (3)), the air–liquid–solid interface is the most considered, and some air is trapped between the surface protrusions and beneath the liquid given that the solid surface is wetted by liquid. Consequently, Eq. (3) can be given by Eq. (5):
in which, CA of liquid with free air (θa) is 180°; the area fractions of solid and air beneath the liquid drop are represented as fs and fa, respectively; and the sum of fs and fa is 1.
Moreover, CAH and SA can be calculated by models containing r, f, and γ, as shown in Eqs. (6), (7):
where θH and θSA refer to CAH and SA, respectively; m and w are the mass and width of the droplet, respectively; θadv and θrec are the advancing and receding CAs, respectively; and are the effective advancing and receding CAs, respectively; g is the acceleration due to gravity; and θe is the equilibrium CA. These models revealed that the level of surface energy determines hydrophilicity (CA < 90°) or hydrophobicity (CA > 90°), and high roughness parameters (i.e., f and r) result in great apparent wettability (CA > 150° or < 10°). Similarly, through the generalization of the wettability states in air to those under oil or water, these models are also applicable. In 1920, Langmuir reported that surface wettability could be altered by an absorbed monolayer of an organic compound, highlighting the importance of chemical modification. Following the early 2000s, surface roughness was redefined as fine dual-scale or hierarchical micro/nanostructures inspired by natural organism surfaces (e.g., lotus leaves). These findings provide a theoretical foundation for the design of superwettable systems based on surface energy and topography. Since then, various design principles have been proposed, with a focus surface chemical modification and the construction and robustness of micro/nanostructures for each type of superwettable system. Additionally, a broader range of materials have been adopted for fabricating artificial superwettable systems. The design of starch-involved superwettable systems follows these principles but requires comprehensive consideration of the chemical properties, macromolecular composition, and structural characteristics of starch to achieve successful integration.
3. Design from intrinsic properties of starch
As a conventional natural polymer, starch has various nonfood industrial applications owing to its intrinsic physicochemical properties, including gelatinization and thermal, rheological, and chemical properties [79]. A similar trend was observed in the superwettable systems (Table 2). While starch may not be the primary component in the following systems, it plays a critical role in constructing surface micro/nanostructures and regulating surface energy.
During gelatinization, starch granules absorb water and swell in the presence of heat, leading to the breakdown of internal hydrogen bonds and the release of starch chains, which increases viscosity [29,80]. The fully gelatinized starch transforms into a thick paste with high adhesiveness, serving as a bio-binder. In systems equipped with superwettable coatings, starch pastes are driven by surface tension to penetrate micro/nanoparticles or to settle between particle-based coatings and substrates. The solidified paste layers generally anchor or encapsulate these objects via intermolecular forces (e.g., hydrogen bonding and electrostatic attraction), facilitating the formation of durable surface micro/nanostructures (Fig. 2(a)). Starch binders are typically applied in two ways: ① blending with particles to form a coating, and ② applied as a surface sizing agent prior to coating. For example, Mater-Bi® (a commercial bioplastic containing thermoplastic corn starch) can be incorporated with fumed silica nanoparticles to create superhydrophobic (WCA = 176°, SA = 1°) and superoleophobic (OCA = 163°) coatings by spraying [34]. These nanoparticles spontaneously fuse together to form aggregates that anchor to the substrate by Mater-Bi® during chloroform evaporation, resulting in robust, sponge-like microstructures. Native cassava starch serves as an effective binding matrix for attaching kaolinite and CaCO3 particles to paperboard substrates using a rod coater, followed by electrospraying a second layer of polylactic acid to achieve superhydrophobicity. This formulation not only fills the porous paper surface but also enhances the surface roughness necessary for superhydrophobicity [35]. The favorable film-forming property of soluble starch allows nanocellulose and silica to adhere easily and stably to the nylon membrane via the dip-coating process (i.e., a simple sol–gel method). The resulting cellulose–starch–silica-coated nylon membrane exhibited excellent superhydrophilicity and underwater superoleophobicity (WCA = 0°, OCA = 159.5°) [36]. An alternative method involves depositing a thin layer of cassava starch paste on paper as a sizing agent (or a bio-binder) before treating with hexamethyldisilazane-treated silica nanoparticles using a rod wire coater. The nanoparticles are partially embedded in the starch layer, reinforcing the bonding strength between the nanoparticles and the paper substrate, resulting in superhydrophobic paper (WCA = 162°) with remarkable durability against mechanical stress (e.g., scratching and bending) and high atmospheric humidity [41]. Furthermore, gelatinized starch contributes to surface micro/nanostructures through self-deposition to form rough films. Yang et al. [38] demonstrated a simple strategy for constructing micro/nanostructures on cellulose surfaces via electrostatic layer-by-layer deposition of cationic starch and sodium alginate (Fig. 2(b)). Trichloromethylsilane further reduced the surface energy, resulting in an eco-friendly superhydrophobic paper (WCA > 155°). The surface roughness and hydrophobicity (WCA value) of the paper increased with the number of deposited bilayers of cationic starch/sodium alginate.
The thermal decomposition and amylose/amylopectin ratio of starch can also be utilized to regulate surface micro/nanostructures. Zhang et al. [39] first prepared polytetrafluoroethylene/potato starch composite fibrous membranes by centrifugal spinning and then obtained rough convex surface structures by heat treatment (370 °C for 420 min) to remove starch. Notably, the surface structure of the membranes could be controlled by adjusting the heat treatment time and amylose content (Fig. 2(b)). Rice grain-like structures, nanolike stripes, and micron-scale hemispherical protrusions appeared sequentially on the surface with increasing heating time, corresponding to the gelatinization, contraction, and decomposition of starch. Differences in the molecular chain structures of amylose and amylopectin influence their thermal properties, contributing to increased surface roughness (i.e., more pronounced hemispherical protrusions) with increasing amylose content. The hydrophobic carbon black formed during starch decomposition imparted low surface energy to the membranes. The thermal decomposition of starch successfully converted hydrophilic (WCA = 0°) polytetrafluoroethylene/starch fibrous membranes into superhydrophobic membranes (WCA = 160°, SA = 3.4°).
The numerous hydroxyl groups in starch not only impart high hydrophilicity but can also be chemically modified to introduce hydrophobicity. This chemical property has been effectively exploited by several researchers to regulate the surface energy of superwettable systems (Fig. 2(c)) [81]. For example, a hydrophilic sponge can be obtained by dissolving chitosan in a 1% acetic acid solution followed by freeze-drying. The surfaces of both sides (i.e., bottom and top) of the sponge, which were treated with stearic acid, became highly hydrophobic (WCA = 146° and 143°). When starch is added to the chitosan/starch composite sponge, the starch preferentially deposits on the bottom area, resulting in a gradient distribution of surface energy. The weaker interaction between polyhydroxy starch and hydrophobic stearic acid drives the latter to the top, whereas the introduction of starch reconstructs the bottom morphology into a rough crater-like structure. This creates an asymmetric superwettable system with a highly hydrophobic top surface (WCA = 136.2°) and a superhydrophilic bottom surface (WCA = 0°) [40]. Through chemical modifications, starch grafted with (heptadecafluoro-1,1,2,2-tetradecyl)trimethoxysilane and palmitoyl chloride can be uniformly distributed and tightly adhered to cellulose fibers in paper pulp via glutaraldehyde crosslinking, acting as a hydrophobic internal sizing agent in amphiphobic papers containing Ag@SiO2 (WCA = 154.8°, OCA ≈ 135°) to achieve low surface energy [37]. Additionally, combining chemical properties with adhesiveness can enable unconventional superwettable modes. Mater-Bi® acts as an adhesive, enabling the embedding of hydrophobic lycopodium particles on the surface of the substrate. The resulting materials achieve the threshold for superhydrophobicity (WCA ≈ 150°). However, the low particle content (< 50 wt%) limits the formation of continuous hierarchical roughness, leaving several regions free from particles. The hydrophilic Mater-Bi® in these areas restricts water droplet mobility (i.e., high SA), creating a “rose petal effect” [34]. While research on the intrinsic properties of starch in superwettable systems is still limited, its significant auxiliary role has been confirmed and warrants further exploration.
4. Design from dimensionality combined with chemical modification
As outlined in previous authoritative literature, one key design principle for superwettable systems involves the integration of interfacial materials of different dimensions [1]. This principle is also applicable to starch-based superwettable systems. Native starches typically exist as microscale granules, although some are at the nanoscale. Additionally, various starch micro/nanospheres, nanoparticles, and nanocrystals can be derived from native starch through physical and chemical treatments such as debranching–recrystallization, reverse precipitation, acid/alkali hydrolysis, and ball milling. Amphiphilic modified starch chains can self-assemble into micelles. These materials are classified as 0D starch materials (Fig. 3(a)). Like other synthetic and natural polymers, starch can be shaped into 1D micro/nanofibers using several advanced spinning techniques (Fig. 3(b)). Starch chains disperse in hot water, and the hydrogen bonding and chemically modifiable sites provided by the multihydroxy structure drive their reorganization into 2D films and 3D porous materials. Furthermore, 0D granules/particles can be spread onto 2D coatings or stacked into 3D fillers (Figs. 3(c) and (d)). The 1D fibers are generally intertwined into 2D membranes (Fig. 3(c)). The 0D and 1D materials are also popularly employed to decorate the surface of 2D and 3D materials, constructing hierarchical micro/nanostructures essential for superwettability. Notably, chemical modifications are critical for regulating surface energy because of the inherently hydrophilic nature of starch. In the following sections, we describe superwettable systems designed from starch materials with different dimensionalities and their associated chemical strategies.
4.1. 0D starch granules/particles
4.1.1. Colloidal systems
Superwettable colloidal systems based on 0D starch granules/particles generally exist as Pickering emulsions, which are fluid dispersions of one immiscible liquid phase in another. In these systems, incompletely wetted starch granules/particles accumulate and absorb at fluid interfaces to form viscoelastic films (i.e., physical barriers) via van der Waals forces and/or capillary action, which adjusts the steric hindrance or rheology of the interface and continuous phases [82]. The process involves the following steps: ① migration of starch granules/particles to the subinterface; ② dense adsorption at the interface; and ③ further stabilization of the system via rearrangement, conformational changes, or reorientation of granules/particles. This particle anchoring behavior is complex and mediated by various factors, such as size, the amylose/amylopectin ratio, wettability, morphology, concentration, and polydispersity, but the design can also be simplified from the perspective of surface chemical composition and micro/nanostructure. According to the literature, wettability has been widely accepted as the critical parameter for evaluating the stability and type of Pickering emulsions. In general, starch granules/particles with WCAs of 15°–90° and 90°–165° are inclined to form single-particle-layer-stabilized oil-in-water (O/W) and water-in-oil (W/O) Pickering emulsions, respectively (Fig. 4(a)). The intrinsic multihydroxy structure of native starch granules results in a high water wettability and contributes to the formation of O/W Pickering emulsions. Further chemical modification or complexation with hydrophobic substances (e.g., octenyl succinic anhydride, short-chain fatty acid, fatty acid chloride, and protein) can produce amphiphilic stabilizers that exhibit greater affinity for the oil phase than native starch does, which results in increased stability and even the conversion to the W/O type [83] (Fig. 4(b)).
In addition to hydrophilicity, the microstructures of native starch granules, such as their large particle size, isotropic near-spherical shape, and rigid structure, are responsible for their unstable emulsions. The relationships between the dimensions of the near-spherical starch particles and their detachment energy (ΔEs) and mass (m) at the interface can be described by Eqs. (8), (9), respectively [83]:
where ds and R are the mean surface diameter and radii of the spherical starch particles, resectively; γow is the oil/water interfacial tension; θw refers to the CA of the particles adsorbed at the oil/water interface; Cso is the starch/oil ratio in mg/mL; φ is the packing density at the interface; ρ is the starch density; and do is the mean surface diameter of the oil droplets. These equations indicate that large starch particles possess a high detachment energy but require a low starch mass to cover an equal interface area. Consequently, large starch granules induce incomplete packing, which contributes to interfacial destabilization, whereas small particles provide increased interfacial coverage and form denser layers (high mass) at the interface, which results in better interface performance (Fig. 4(b)) [83]. The superior emulsification capacity of small starch particles compared with that of large starch granules has also been repeatedly confirmed in previous research [[83], [84], [85]]. To increase the suitability of modified starch granules as Pickering stabilizers, a variety of methods, such as milling, nanoprecipitation, and acid hydrolysis, are applied to reduce the particle size to the nanoscale [86]. Several native starch granules with near-nanometer-scale sizes (e.g., rice, taro, and quinoa) are also good candidates [[87], [88], [89]]. Moreover, the detachment energy for rod-like (ΔEr) and disc-shaped (ΔEd) particles can be calculated via Eqs. (10), (11), (12), (13) [90]:
where a and b are the dimensions of the long and short semi-axes of nonspherical particles, respectively. ΔEr/d,w and ΔEr/d,o represent the detachment energy for particles with θw ≤ 90° and θw ≥ 90°, respectively. According to these equations, the inequality ΔEs < ΔEr < ΔEd holds irrespective of the wettability, which indicates the relatively stronger anchorage of anisotropic particles at the interface than that of isotropic spherical particles [90]. On the basis of these findings, we can infer that several irregular starch nanoparticles/crystals exhibit relatively better emulsifying performance [91,92]. However, this is seldom considered in the design of starch-based emulsions because of the low morphology controllability of starch nanoparticles/crystals. In addition, the positive contribution of deformability to the stability of starch Pickering emulsions is important. In deformable starch granules/particles, leached amylopectin and amylose (i.e., flexible polymers) form an entangled network at the interface, which provides sufficient tension and electrostatic/steric repulsion for emulsion stabilization (Fig. 4(b)) [82]. The deformable structure generally depends on the crystallinity and gelatinization degree of starch. Starch particles produced by nanoprecipitation may be more deformable than starch nanocrystals due to the high proportion of the amorphous region, and starch treated with high pressure, milling, or heating is gelatinized to different extents, which produces swollen granules/particles with flexible polymers [86]. Overall, the design of Pickering emulsions based on 0D starch materials focuses on the regulation of the adsorption capacity or detachment energy of granules/particles at the interface, which can be achieved through the modification of the chemical composition and microstructures. Specific methods and applications have been extensively reported in numerous studies and are therefore not covered in this review. Table 2 lists several representative highly cited studies for reference [[42], [43], [44], [45], [46], [47]].
Amphiphilic micelles containing superhydrophilic groups constitute another special starch-based superwettable colloidal system. Starch is one of the most promising materials for the fabrication of drug delivery platforms (e.g., micelles and vesicles) because of its excellent biocompatibility [93]. To impart amphiphilicity to starch, one must introduce hydrophobic and hydrophilic groups onto the starch chain to modulate its surface activity, which propels the formation of the interfacial structure. However, the surface of conventional amphiphilic micelles possesses a weak hydration layer formed by hydrogen bonding, which is prone to adherence by proteins and is labeled as a foreign body, resulting in rapid clearance before it reaches its target site. In contrast, starch micelles modified with a superhydrophilic zwitterionic sulfobetaine group acquire a strong hydration layer via electrostatic interactions, which prevents their contact with proteins and results in long-circulation vectors in vivo (Fig. 4(c)) [48]. In such systems, the micro/nanostructure and surface energy are no longer critical factors, and attention needs to be paid to the design of a molecular structure using superhydrophilic zwitterionic groups and the balance of the hydrophilic/oleophilic nature.
4.1.2. 2D coating from 0D granules/particles
Starch granules/particles are increasingly accepted as alternatives to conventional inorganic micro/nanoparticles in superwettable coatings, where they are responsible for providing surface roughness (Fig. 4(d) and Table 2). Specifically, granules/particles are first chemically modified to modulate their surface energy and then suspended in volatile solvents for the coating process. Upon solvent evaporation, the starch granules/particles randomly aggregate and accumulate to form hierarchical micro/nanostructures. However, the resulting surface roughness is fragile and vulnerable to damage, making this method more suitable for decorating the surfaces of porous materials, whose intrinsic superficial pore structures offer physical support for particle attachment. In some cases, chemical bonding is introduced to reinforce the attachment [94]. For example, Fe3O4 nanoparticles can be partially replaced with porous starch granules, and their composite particles can be prepared through oscillation and centrifugation. Further modification with hexadecyltrimethoxysilane (HDTMS) combined with spraying results in a magnetic superhydrophobic coating (WCA = 152.2°, SA < 9.0°). This coating results in five levels of micro/nanostructures on the surface of starch cryogels, including micropore walls, microstarch granules, microgranule aggregates, nanogaps, and nanomagnetic particles. HDTMS contributes to reducing surface energy and strengthening the adhesion of hybrid particles on the gel surface through Si–O–C covalent bonds [55]. Zhang et al. [51] used pH-responsive succinic anhydride and hydrophobic acetic anhydride to modify starch particles prepared by nanoprecipitation. The chemically modified nanoparticles were anchored to the surface of the filter paper via spray coating and glutaraldehyde crosslinking. The resulting paper exhibited well-defined micro/nanostructures combining microscale porous structures of fibers with nanoscale starch particle structures and showed switchable superhydrophilicity and underwater superoleophobicity (WCA = 0°, OCA < 150° at pH = 3–5; OCA > 150° at pH = 1 and 7–10) [51]. To simplify and improve manipulation, a strategy involving a “hydrophobic reagent with high adhesiveness plus starch granules/particles” has been proposed. The reagent serves both as a binder to link starch granules/particles to the substrate and as a surface energy modifier. This strategy is adaptable to a wide range of substrates, such as steel, glass, and paper. Currently, polydimethylsiloxane [49,53,54], polydopamine [50], and stearic acid [56] have been combined with starch granules/particles to create coatings. The roughness of these coatings attained the superwettability standard with the participation of starch granules/particles and could be well retained after mechanical treatment.
Particular attention must be paid to the effects of the mean particle size (Mz) and bulk density (BD) on wettability [52]. Large starch granules (Mz > 25 μm, BD > 0.6 g∙mL−1) scatter to form nonhierarchical microstructures, which cannot achieve superwettability, as seen with potato and wheat starches. Medium-sized (Mz = 10–25 μm, BD = 0.4–0.6 g∙mL−1) and small-sized (Mz = 5–10 μm, BD = 0.3–0.4 g∙mL−1) granules, such as those from corn and rice starches, are more suitable because of their dense distribution and hierarchical microstructures. Nano-starch (Mz < 1 μm, BD < 0.3 g∙mL−1) is the best choice for superwettable systems because it can be densely distributed on surfaces and randomly stacked as hierarchical micro/nanostructures. However, nano-starch with V-type crystallization may not be ideal because of its strong water affinity. Current studies have developed starch micro/nanoparticles of various shapes, sizes, and crystallinities, all of which are potential candidates for superwettable coatings.
4.1.3. 3D fillers from 0D particles-stabilized Pickering emulsions
Pickering emulsion polymerization is a common method for preparing microspheres. Starch nanoparticles are effective stabilizers for Pickering emulsions (Section 4.1.1), and the combined use of these technologies results in microspheres with unique structures. The surface of the filler created by stacking these spheres features hierarchical micro/nanostructures, including nanoparticles/pores, microspheres, and their aggregates. The interior of the filler contains interparticle and intraparticle pores that provide channels for fluid infiltration. These structural features align with the requirements for superwettability (Fig. 4(e) and Table 2). For example, acidified corn starch modified with stearoyl chloride and acetic anhydride was used to prepare starch nanoparticles via nanoprecipitation, which stabilized W/O emulsions containing acrylic acid and acrylamide. The addition of N,N′-methylenebisacrylamide crosslinked this system into microspheres (i.e., W/O Pickering emulsion polymerization reaction). The resulting spheres, with micron-sized particles and numerous nanobulges on their surfaces (derived from starch nanoparticles), exhibit low surface energy and hierarchical micro/nanostructures, contributing to superoleophilicity and under-oil superhydrophobicity (OCA ≈ 0°, WCA = 153°) in the final filler [57]. Thermo-responsive 2-hydroxy-3-butoxypropyl starches (HBPSs), obtained by modifying starch with butyl glycidyl ether, were used to stabilize O/W Pickering emulsions. These emulsions served as reaction vessels for ultraviolet crosslinking of the monomer trimethylolpropane triacrylate. The resulting microspheres were composed of numerous nanoparticles, and the filler was stacked, revealing a 3D dendritic network structure. The surface of the filler features micro-spheres and nano-particles/pores hierarchical structures, exhibiting superlipophilicity and under-oil superhydrophobicity. The thermo-responsiveness of HBPS allows for controlled variation in wettability with temperature, enables a switch between emulsification and demulsification, and provides an innovative strategy for producing microspheres via recyclable emulsion polymerization [58]. In contrast to colloidal systems and coatings, starch granules/particles are not the dominant component of 3D fillers, but they play an essential role in creating the special structure and surface energy needed for superwettability.
4.2. 1D starch fiber and its 2D fibrous membrane
Fibers inherently possess hierarchical micro/nanostructures (micro/nanoscale diameters and re-entrant curvature), which interweave to form membranes with high surface roughness. Additionally, interconnected pores exist between the interwoven fibers, providing tortuous paths for liquids to penetrate. These features enable the implementation of superwettable properties on fibrous membranes [95]. As natural organic fibers, starch micro/nanofibers and fibrous membranes have attracted attention because of their low cost, high flexibility, renewability, and eco-friendliness. Various spinning techniques, including conventional methods (e.g., melt spinning, wet spinning, and dry spinning) and modern methods (e.g., electrospinning, centrifugal spinning, and solution blowing), have been employed to fabricate starch fibers [96].
Currently, starch fibers prepared by electrospinning and centrifugal spinning have been successfully utilized as substrates for superwettable fibrous membranes (Table 2). Unlike conventional pressure-driven spinning, electrospinning depends on electrostatic forces to produce ultrafine micro/nanofibers with specialized multiscale outer and inner structures (e.g., cylindrical, bead-on-string, porous, and core–shell shapes), enabling the creation of various hierarchical micro/nanostructures [97]. Cai et al. [59] employed a temperature-assisted electrospinning technique to convert native corn starch (dissolved in dimethyl sulfoxide) into membranes consisting of randomly oriented nanofibers without any nonstarch components (Fig. 5(a)). The external structure of the resulting starch fibers varied with starch concentrations ranging from 15% to 30%, including bead shapes, bead-on-string shapes, smooth cylinders (mean diameter of (365 ± 97) nm), and uneven cylinders (mean diameter of (635 ± 198) nm). To convert the hydrophilicity to superhydrophobicity, stearic acid (dissolved in ethanol) was chosen to modify the surface of the starch nanofibrous membrane via a one-step immersion strategy. In this process, the carboxyl acid group of stearic acid docked with the hydroxyl group of starch through hydrogen bonding, whereas the alkyl chains facilitated primarily hydrophobic interactions and van der Waals forces to organize the stearic acid molecules. These noncovalent interactions synergistically led to the controlled self-assembly of stearic acid on the starch fibers, contributing to the low surface energy and the formation of hierarchical flower-like micro/nanostructures (i.e., the increase in surface roughness). By adjusting the concentration of stearic acid and treatment time, the surface roughness and hydrophobicity of the fiber surface could be regulated. The resulting stearic acid-modified starch fibrous membrane exhibited a high WCA of ∼134.7° (near superhydrophobicity). Water droplets can remain spherical and roll freely on this membrane.
In addition to electrospinning, centrifugal spinning is also an effective technique for producing micro/nanofibers. Centrifugal force is used to inject starch and form membranes with porous and well-aligned micro/nanostructures. Starches with high amylose contents are typically more suitable for the production of high-quality fibers via many spinning techniques, but amylopectin-rich (> 70%) native starches can achieve excellent fibrosis through centrifugal spinning [96]. For example, potato starch (amylopectin content: ∼80%) mixed with polyvinyl alcohol (1:1 w/w) can be spun into ultrafine cylindrical fibers with a mean diameter of ∼800 nm, forming a fluffy porous membrane via centrifugal spinning (Fig. 5(b)) [60]. This membrane was coated with an ultrathin 2D Voronoi-like bacterial cellulose nanonet through spray assembly to improve surface roughness and hydration capability. The stability of the starch matrix and the adhesion between layers were further enhanced by steam–etherification–crosslinking with glutaraldehyde. The combination of these chemical and physical treatments provided the necessary structural (biomimetic cobweb-like nanostructure) and chemical properties for superhydrophilicity (WCA = 0°). The underwater OCA and oil SA of the composite membrane also increased to ∼152° and decreased to ∼4°, respectively, indicating excellent selective superwettability.
While these findings demonstrate promising results, several technical challenges exist in the use of 1D starch fibers for superwettable systems. The integrity and formability of the fibers are closely linked to the solubility of starch in the spinning solvents. Thus, hydrophilization or hydrophobization is typically performed on preprepared fibrous membranes rather than raw starch. Furthermore, the surface micro/nanostructures formed from the interweaving of starch fibers remain limited, and these structures are held together by weak noncovalent intermolecular interactions. Hence, chemical modification is necessary to improve structural stability, hydrophilicity/hydrophobicity, and surface roughness, which typically relies on the self-assembly of external molecules (a mild strategy) on starch fibrous membrane surfaces. Despite these challenges, 1D starch fibers remain viable candidates for future applications in superwettable systems.
4.3. 2D thermoplastic starch films
To mitigate the environmental impact of plastics, biodegradable thermoplastic starch (TPS) films are considered an ideal alternative for short-lifetime applications, such as disposable organic electronics and packaging materials [98]. However, the hydrophilic nature of TPS films makes them susceptible to water-induced wetting, which can lead to irreversible mechanical failure in high-humidity environments. This issue can be explained through molecular self-association theories and Lewis site interactions (i.e., Lewis acid–base interactions and primarily hydrogen bonding) [99]. The 3D hydrogen-bonded network promotes the self-association of water molecules, allowing droplets to maintain a stable shape on hydrophobic surfaces. However, this network collapses when droplets approach surfaces rich in Lewis sites, where water molecule self-association is competed by surface interactions. The numerous polar hydroxyl (–OH) groups from starch provide dense Lewis sites on the surface of the TPS films, resulting in high water wettability (Fig. 6(a)) [52]. According to surface thermodynamics [100], the solid–liquid interfacial energy (σsl) can be described by a function of surface energy of solid (σs) and liquid (σl):
which is transformed into another form (Eq. (15)) on the basis of Young’s equation (Eq. (1)):
where θ is the Young CA, and θ and σs exhibit an inverse relationship. Moreover, the surface energy (σ) contains dispersion and polar components that are described by the following equations:
where σd and σp denote the surface energy of dispersion and polar components, respectively. These equations can be further complicated on the basis of the Lifshitz–van der Waals and Lewis acid–base interaction theories:
where σLW and σAB represent the Lifshitz–van der Waals and Lewis acid–base components of surface energy, respectively; σ+ and σ− refer to the electron-acceptor and electron-donor parameters, respectively. The high content of electron-acceptor hydroxyl groups implies high , , and σs values, which lead to the low θ values (i.e., high water wettability). Despite previous attempts to mask hydroxyl groups (e.g., acylation and plasma treatments with hydrophobic compounds), these methods have not achieved satisfactory water repellency. Water droplets can still adhere to the surface (WCA < 100°) and penetrate starch films [101]. Additionally, overmodification of hydroxyl groups should be avoided as these groups are critical for maintaining the mechanical properties and biodegradability of starch films. Therefore, superwettable starch films are typically superhydrophobic to mitigate their high water sensitivity, and a top-down approach is usually employed to minimize variations in film properties.
Unmodified TPS films have extremely smooth surfaces, in contrast with the rough superhydrophobic surfaces (Fig. 6(a)). Currently, surface coating with wax or eco-friendly hydrophobic micro/nanocomponents is a straightforward top-down strategy that can simultaneously provide low surface energy and hierarchical micro/nanostructures, which are essential for achieving superhydrophobicity (Table 2). For example, beeswax dissolved in a mixed solvent of acetic acid, hexane, and ethanol was sprayed onto the surface of a starch–cellulose composite film (WCA = 78°). After solvent evaporation, beeswax alone could impart a flower-like surface structure and low surface energy, allowing the composite film to meet the superhydrophobicity standard (WCA = 152°, SA = 6°) (Fig. 6(b)) [61]. Blending TPS films with low-density polyethylene (LDPE) can reduce the content of Lewis sites, increasing WCA from ∼40° to ∼75°. However, this modification still results in smooth surfaces that are easily wetted by water (WCA < 90°) [62]. The surface roughness and hydrophobicity were further enhanced by spray-depositing polystyrene 2-aminoethyl microspheres (MPSs) with a mean diameter of 5 µm. The refined texture derived from individual microspheres and the coarse texture formed by the alternation of microsphere aggregate-rich and aggregate-poor regions jointly generated well-pronounced hierarchical microstructures (Fig. 6(c)) [62]. The WCA increased to ∼152°, and the corresponding WCA hysteresis decreased to ∼2°. As an alternative to synthesized microspheres, hydrophobically modified starch granules/particles can also be used to decorate superhydrophobic TPS films (Section 4.1.2) [52]. Additionally, various choices exist in the morphology of hydrophobic micro/nanocomponents. Polycaprolactone (PCL) electrospun nanofibers, for example, enhance surface roughness by creating fibrous textures, transitioning the surface wettability of the TPS film from the Wenzel state to the Cassie–Baxter state [101]. The mean diameter of the fibers and coverage density (i.e., the distance between the fibers) are closely linked to the surface wettability, and these properties can be adjusted by varying the deposition time. Superhydrophobicity was achieved on a TPS film covered with PCL fibers (∼370 nm) after 2 min of deposition (Fig. 6(d)) [62]. Flaky ball-milled montmorillonite (MMT) combined with poly(dimethylsiloxane) (PDMS) has also been confirmed as a potential highly hydrophobic coating for TPS films [63]. The wettability and roughness of this coating depend on the content and particle size of the MMT. A coating with 30% MMT created a sea-island structure on the film surface, resulting in limited roughness (surface average roughness (Ra) = 7.34 µm, surface area (Sa) = 1.0524 × 10−6 m2) and hydrophobicity (WCA = 122.8°, SA is none). In contrast, a rougher surface (Ra = 13.46 µm, Sa = 1.3388 × 10−6 m2) approaching superhydrophobicity (WCA = 153.6°, SA > 10.0°) was achieved with 70% MMT content. Then, ball milling reduced the particle size of the MMT from 14.40 to 8.45 µm, further increasing the surface roughness (Ra = 16.70 µm) and specific surface area (Sa = 1.5954 × 10−6 m2), resulting in superhydrophobicity (WCA = 159.1°, SA < 4.0°) (Fig. 6(e)) [63]. Various aqueous liquids maintain spherical droplets on the above all TPS films, which easily bounce off the surface.
The mechanical durability of superhydrophobic TPS films is a key consideration, as surface micro/nanostructures and weak bonding between coatings and the film are prone to damage. These studies all highlight the coating attachment modes on the film surface: ① Polyethyleneimine (PEI) was used as an adhesive to tightly connect the beeswax coating and starch film via hydrogen bonding (Fig. 6(b)); ② heating the coating solution to 90 °C softens the substrate, allowing MPS microspheres to adhere securely to the TPS film surface (Fig. 6(c)); ③ residual solvents during electrospinning cause TPS films to swell, promoting tighter attachment of PCL fibers to the surface (Fig. 6(d)); and ④ PDMS, with its high stickiness, forms Si–O–C bonds with starch, providing dual binding forces between the particles (MMT and starch) and the TPS film (Fig. 6(e)). Most of these attachment modes cleverly utilize the physicochemical properties of starch. Notably, one study utilized the thermoplastic properties of starch to construct surface patterns that improve both hydrophobicity and mechanical durability. The biomimetic combination route of soft-imprint lithography (bottom-up) and spin coating (top-down) techniques was adopted to realize the lotus effect on TPS films (Fig. 6(f)) [64]. Positive microscale papillate hills were first imprinted from a negative PDMS mold that replicated the lotus leaf microstructure. PDMS and vinyl-modified silica nanoparticles were then sequentially spin-coated onto the film surface to mimic the nanostructures of wax crystals. After curing at 80 °C for 5 h, water droplets were unable to wet the surface, maintaining a spherical shape (WCA ≈ 151°, SA < 5°). The surface features and water repellency remained intact after various durability tests, such as water-impacting, knife-scratching, finger-pressing, and sandpaper-abrasion. The combination of lotus-like microstructures and the bonding strength from PDMS contributed to the high mechanical durability of the film. The development of these 2D superhydrophobic TPS films has introduced innovative solutions to address the high water sensitivity of TPS films. However, current research is limited, and further strategies are required for more advanced applications.
4.4. 3D starch porous systems
4.4.1. Starch-based porous materials
Compared with 2D membranes or films, 3D materials with macroporous, mesoporous, or microporous structures offer lower density, higher porosity, longer penetration channels, and larger specific surface areas. These properties contribute to enhanced adsorptive performance, making them ideal for integration with superwettable systems to achieve controllable liquid permeation and separation [102]. To reduce the reliance on nonbiodegradable synthetic polymers and fossil-derived materials, researchers have turned to starch as an alternative in the production of 3D porous materials [103]. The abundant hydroxyl groups in starch also enable functionalization for specific applications. Currently, 3D starch porous materials can be broadly classified into fillers (Section 4.1.3), foams, sponges, and dried hydrogels (e.g., supercritical CO2-dried aerogels, freeze-dried cryogels, and air-dried xerogels) [[104], [105], [106]]. Additionally, starch, as a rigid chain, can be integrated into flexible chain network structures to form high-performance dual-network 3D porous materials [107]. The original surface of these materials possesses a microscale structure derived from pore walls and a high surface energy generated by hydroxyl groups, both of which are essential for superwettability. However, further chemical and physical modifications are necessary to adjust the micro/nanostructures (either internal or surface) and surface energy to meet specific goals (Table 2). A superwettable aerogel can be produced by gelatinizing, retrograding, freeze-thawing, lyophilizing, and carbonizing (270 °C) a mixture of starch and carbon dots (CDs) [65]. Oxygenated groups on the CDs form hydrogen bonds with starch hydroxyl groups, promoting polymerization and crosslinking of the starch chains. Carbonization further induces the loss of hydrogen and oxygen, creating more pores and a thinner pore wall. These processes refine the porous structure, increasing the porosity (92.4%) and pore size, which accelerates water diffusion and transport. The introduction of CDs also modulates the chemical composition of the starch aerogel, increasing the surface energy with more oxygenated groups. The resulting carbonized CD-modified starch aerogel absorbs water droplets completely within 0.6 s (WCA = 0°) and becomes fully wetted by water within 20 s, demonstrating excellent superhydrophilicity (Fig. 7(a)) [65]. Wang et al. [66] developed a series of superhydrophobic/superoleophilic starch cryogels by incorporating organosilanes (e.g., methyltrichlorosilane and HDTMS). Methyltrichlorosilane (dissolved in toluene) was applied to the starch cryogel through a simple immersion strategy and hydrolysis–condensation reaction. It is grafted onto the C-2 position of starch molecules via a Si–O–C bond, providing nonpolar alkyl groups that form a dense low-surface-energy coating. Moreover, a honeycomb coral-like structure with micro-bowl and nano-bowl-wall structures formed on the cryogel surface by the self-assembly of methyltrichlorosilane through Si–O–Si bonds. These microstructures cooperated with the microholes and pore walls of the starch cryogel to create hierarchical micro/nanostructures. This combination of low surface energy and high surface roughness enabled the cryogel to achieve near-superhydrophobicity (WCA = 151.8°, SA = 14.6°) (Fig. 7(b)) [66,67]. HDTMS also hydrophobically modified the starch cryogel (WCA ≈ 131.0°) through a similar mechanism (e.g., chemical composition, reaction mechanism, and grafting site), but it was unable to construct the surface roughness needed for superhydrophobicity. Therefore, micro/nanoparticles are typically used in combination with HDTMS to provide extra micro/nanostructures for starch cryogels. A sol–gel process involving tetraethyl orthosilicate and HDTMS is commonly used for superhydrophobic modification and can be applied to starch cryogels via spray-coating, dip-coating, or one-step immersion methods [67,68]. In an alkaline medium, tetraethyl orthosilicate hydrolyzes and condenses into silica nanoparticles, increasing the surface roughness. HDTMS hydrophobizes both silica nanoparticles and starch cryogels via Si–O–C and Si–O–Si bonds, but also serves as a “bridge” to connect silica nanoparticles to form aggregates that attach to the surface of the starch substrate via O–Si–O bonds. The combination of nanoscale silica particles, microscale aggregates, inherent pore structure, and hydrophobic modification plays a synergistic role in achieving superhydrophobicity (WCA > 153.0°, SA < 8.0°) (Fig. 7(b)). Additionally, biodegradable or functional particles can replace some or all the silica to enhance sustainability or introduce intelligence into the system. A magnetic and superhydrophobic starch cryogel (WCA = 151.6°, SA = 4.3°, saturation magnetization = 2.15 emu∙g−1 (1 emu = 10−3 A·m2)) was fabricated by incorporating Fe3O4 nanoparticles into the above sol–gel process (Fig. 7(c)) [69,106]. This cryogel could float on the water surface when driven by a magnet. Furthermore, tetraethyl orthosilicate can be fully replaced by starch nanoparticles to achieve superhydrophobicity (WCA = 151.5°, SA = 8.0°) without the use of nonbiodegradable inorganic particles (Fig. 7(c)) [70]. The water droplets retained a nearly spherical shape on the surface of all the superhydrophobic starch cryogels, while the oil could be fully absorbed, demonstrating their selective wettability toward oil and water.
4.4.2. Starch food texture
3D porous structures are also found in starchy foods prepared through processes such as microwave heating, screw extrusion, freeze drying, and frying. These structures can be described as special food textures with high wettability and adsorbability. The initial step in the digestion of starchy food is wetting by saliva. The development of superhydrophilicity may increase digestibility and reduce chewing time, providing a better taste experience for consumers. Jiang et al. [71] proposed that the delightful taste of fresh popcorn is attributed to its superhydrophilicity and explored the mechanism behind its ultrafast wetting behavior, relating it to microstructure and chemical composition (Table 2). During the popping process, the leakage of water vapor pressure pushes out numerous polygonal (hexagonal) micropores that form three interpenetrating layers of porous networks. This 3D interpenetrating microporous structure facilitates capillary flow in both the horizontal and vertical directions at multiple scales. Additionally, the transformation from closed stacked starch structures (A-type crystallization) to the puffed structures (B-type crystallization) during the wetting process promotes water absorption and diffusion (i.e., a water-soluble crystalline form). These factors combined enable the popcorn to be fully wetted within 40 ms (Fig. 7(d)). Upon exposure to air, the semi-crystalline starch gradually transforms into an amorphous state, reducing its wettability and solubility. This research area is in its early stages, and further studies are needed to explore the relationship between 3D texture and wettability in starchy foods.
5. Design from the biotransformation of starch
As discussed in Section 4.2, 1D fibers are frequently used in superwettable systems. In the current study, bacterial cellulose fibers (BCFs), which are produced through a static cultivation method, are considered promising green 1D materials. Starch is a widely accepted carbon source in the metabolic pathway for BCF biosynthesis. The entire process, starting with starch and culminating in the creation of a superwettable system, involves eight key steps: starch recovery, starch gelatinization, enzymatic saccharification of starch, synthesis of uridine diphosphate glucose (UDP-glucose), cellulose molecular chain synthesis, cellulose crystallization and polymerization, chemical modification, and lyophilization (Fig. 8). Commercial starches, such as those derived from rice, potato, maize, wheat, and cassava, can serve as the starting material for BCF biosynthesis. However, these are not the preferred choices because of their ability to compete as staple food sources for humans. Therefore, starches recovered from food waste have emerged as viable candidates [108]. Gelatinization is an essential step for dispersing starch molecular chains into a medium, enabling high-degree biotransformation. Endo- and exo-amylases then work together to cleave the α-1,4- and β-1,6-glycosidic bonds of the starch chain [109,110]. The resulting glucose enters bacteria through membrane transporters, where it is catalyzed by glucokinase, phosphoglucomutase, and UDP-glucose pyrophosphorylase to form UDP-glucose, which is the precursor for cellulose synthesis [111]. A complex enzyme system consisting of BcsA, BcsB, BcsC, and BcsD subunits utilizes UDP-glucose to synthesize subfibrils, which are subsequently discharged from the cell. The BCF forms through the self-aggregation of these subfibrils [112]. Finally, physical treatment, chemical modification, and freeze-drying are successively applied to the BCF to achieve superwettability. Wang et al. [72] isolated a starch-rich solid from kitchen waste using hot-water extraction. After sterilization and gelatinization at high temperatures, the starch-rich fraction was thoroughly blended with α-amylase and amyloglucosidase for enzymatic saccharification. The resulting enzymatic hydrolysate, containing a high concentration of glucose, was subjected to static fermentation with Acetobacter xylinum to yield BCF. For physical treatment and chemical modification, the BCF hydrogels were cut into small pieces and immersed in a solution of stearic acid (dissolved in tertiary butanol) for the solvent exchange process. After 48 h of lyophilization, a superwettable BCF aerogel was obtained. The web-like microstructure of BCF, transformed from starch, combined with hydrophobic nanoscale symmetrical humps derived from the stearic acid crystals, resulted in superhydrophobicity–oleophilicity (WCA = 156.8°) (Table 2) [72].
6. Emerging applications of starch-involved superwettable systems
6.1. Functional biomaterials
The potential of starch in biomedical materials, including drug delivery platforms [113], wound dressings [114], and tissue scaffolds [115], has been widely recognized because of its superior biocompatibility. With the advancement of biomedical engineering, additional requirements have emerged for these materials. Surface functionalization, which facilitates interactions between biological entities (e.g., proteins, cells, and drugs) and biomaterials, has become a key strategy to meet these demands. Moreover, the complex wetting environment of the human body can impede the functionality of biomaterials and increase the likelihood of complications [3]. Thus, the design of biomaterials with wettability has been proposed to better adapt to biological wetting conditions and regulate related cellular behaviors. Superhydrophilic biomaterials, in particular, have been shown to promote tissue recovery and healing, enhance protein adsorption, improve cell adhesion and proliferation, and increase the biocompatibility of implants. Several specialized functions of starch-based superwettable biomaterials have been reported. Superhydrophilic starch micelles containing zwitterionic groups formed a strong hydration layer that prevented protein adhesion (i.e., “stealth” properties) and self-aggregation. This micelle-based delivery platform exhibited excellent hemocompatibility and extended circulation time, resulting in hydrophobic drugs (e.g., doxorubicin) achieving a high plasma area under the concentration curve and prolonged elimination half-life in vivo [48]. The asymmetric wettable starch/chitosan composite sponge features a superhydrophobic top surface that repels water, blood, and bacteria, whereas its superhydrophilic bottom surface exhibits excellent water/blood absorption and clotting capabilities. These properties make it a promising candidate for wound dressings [40]. Starch-based biomaterials designed using the superwettable approach are expected to play a significant role in the future of biomedical engineering.
6.2. Oral delivery system
Owing to their potential health benefits, numerous bioactive molecules, such as their antioxidant, hypoglycemic, and anti-inflammatory effects, are used as drugs or nutrients. However, many of these molecules have low aqueous solubility and poor process stability, leading to reduced efficiency and bioavailability [116,117]. Some of them also cause side effects, such as esophageal ulcers and gastrointestinal irritation, after long-term use [118]. Encapsulation and delivery systems offer an effective solution to these issues [119]. Oral delivery remains the most attractive route for drug/nutrient administration, providing a painless method of supply as a medicament or as a food formulation for long-term use. Compared with conventional emulsions stabilized by surfactants, the use of edible superwetting agents to construct delivery systems with superfunctional interfaces or ultralow interfacial tension represents a forward-looking approach. Among these systems, Pickering emulsions incorporating starch granules/particles are ideal for oral delivery because of their biocompatibility, nontoxicity, and nonirritating properties. Previous studies have demonstrated the remarkable suitability of starch-based Pickering emulsions for encapsulating and delivering bioactive molecules. For example, puerarin was loaded in O/W Pickering emulsions stabilized by octenyl succinic anhydride-modified Pueraria montana quinoa starch, resulting in high encapsulation efficiency and improved bioaccessibility [120]. Compared with native starch emulsions, Pickering emulsions stabilized by starch nanoparticles (corn, potato, and sago starch) presented greater loading efficiency and static stability of curcumin. Among these emulsions, the sago starch nanoparticle-based Pickering emulsion displayed the highest bioavailability [121]. Intelligent delivery systems with targeted or controlled release capabilities have also been developed. For example, β-carotene was encapsulated in high-internal-phase Pickering emulsions stabilized by thermo-responsive HBPS particles. The release behavior of this emulsion can be controlled by adjusting the temperature, with a high release rate below 30 °C and nearly zero release at higher temperatures (37.5 °C) [122]. Similarly, Pickering emulsions stabilized by citric acid-crosslinked corn starch nanoparticles exhibited pH-responsive behavior, with curcumin release increasing with pH, making it ideal for targeted delivery to the intestine during oral administration [123].
6.3. Emulsion polymerization
Starch granules/particles can act as green Pickering emulsifiers, replacing hazardous particles to provide microreaction vessels for one-pot polymer synthesis [124,125]. The resulting microspheres can be used as 0D materials to construct 2D or 3D superwettable systems. For example, stearoyl chloride/acetic anhydride-modified starch nanoparticles serve as vessels for polymerizing acrylic acid and acrylamide [57]. Thermo-responsive HBPS nanoparticles enable the construction and demolition of polymerization vessels through temperature control, offering an intelligent platform for polymerization [58]. The hydrophobic microspheres synthesized in these vessels can be applied as superhydrophobic 3D fillers for oil–water separation. Generally, starch-based Pickering emulsion polymerization is used to support the synthesis of materials for various practical applications. The related theory has been comprehensively covered in previous literature, so a detailed discussion is not included here.
6.4. Water treatment
Starch-involved superwettable systems have notable applications in water treatment, particularly in liquid–liquid separation and freshwater production [126]. Oil–water separation is a crucial technique for addressing industrial wastewater and oil spills. The development of superwettable porous materials for such purposes has attracted significant interest from researchers [1] (Table 2). Materials exhibiting superhydrophobicity/superoleophilicity can repel water from oil–water mixtures while allowing the filtration or absorption of oil. This phenomenon is referred to as water-blocking/oil-removing capacity. Starch-based coatings, fillers, and cryogels have been developed as such materials through surface chemical and structural modifications. The thermal decomposition and biotransformation of starch have also been used to prepare superhydrophobic/superoleophilic fibrous membranes and aerogels, respectively. However, superoleophilicity can lead to oil contamination after use, limiting durability and recyclability. In contrast, superhydrophilic/underwater superoleophobic materials, which feature high surface energy, prevent oil penetration while allowing water to pass through, thus exhibiting oil-blocking/water-removing capabilities. Several starch-based microsphere coatings and fibrous membranes fall into this category, with starch acting as an auxiliary to provide high surface energy owing to its intrinsic hydrophilicity. These superoleophobic materials avoid oil contamination, offering better durability and recyclability. Moreover, the use of starch-involved superwettable porous materials provides a more eco-friendly alternative for large-scale oil–water separation than conventional materials (e.g., ceramic membranes [127], graphene–polyurethane sponges [128], and TiO2 nanowire membranes [129]).
Among the various technologies used for freshwater production (e.g., reverse osmosis [130], water evaporation [131], and electrodialysis [132]), solar-powered water evaporation is a low-cost, eco-friendly, and efficient method that uses solar energy for water distillation/desalination. A recently developed carbonized CD-modified starch aerogel, with its inherent porous framework and superhydrophilicity, demonstrates superior water transportation, reduced water evaporation enthalpy, light harvesting, and thermal insulation [65]. These unique advantages make it an ideal candidate for solar water evaporators, showing the potential of superwettable starch porous materials in freshwater production.
6.5. Packaging technology
Conventional packaging technology has largely met the needs for product containment and protection; however, it faces several technological challenges [73]. The attachment of fluid items to packaging materials can result in up to 25% waste, leading to product loss, hygiene issues, and increased cleaning costs. Additionally, fogging on packaging surfaces not only reduces transparency but also creates favorable conditions for bacterial growth and reproduction. The incorporation of superwettability into packaging technology offers a viable strategy to prevent fluid and moisture adhesion, thereby mitigating these adverse effects. As a green, low-cost, and abundant natural polymer, starch has been developed into sustainable, biodegradable packaging materials [133], playing a key role in superwettable packaging (Table 2). TPS films and starch fibrous membranes can serve as biodegradable substitutes for plastic packaging, although their high hydrophilicity due to their abundant hydroxyl groups results in significant water affinity. To address this, hydrophobic micro/nanoparticles and fibers have been employed to decorate starch film surfaces. The air pockets trapped in surface micro/nanostructures reduce the SA, contributing to the increased repellency of aqueous fluids and facilitating a self-cleaning effect. This approach introduces eco-friendly superhydrophobic packaging to the market. Specific chemically modified starch granules/particles can replace conventional hydrophobic micro/nanoparticles to impart superhydrophobicity to various packaging materials. Notably, starch granules can act as carriers for active ingredients, enabling the development of intelligent packaging. For example, anthocyanin-loaded starch nanoparticles can be incorporated into superhydrophobic coatings, resulting in waterproof packaging that responds to changes in pH, thus serving as a freshness indicator for food [56]. Furthermore, the high adhesiveness of starch paste ensures strong bonding between micro/nanoparticles and substrates, enhancing the durability of superwettable packaging. While the biodegradability of starch-based superhydrophobic packaging has rarely been assessed, Niu et al. [61] reported that a starch–cellulose composite film coated with beeswax and PEI degraded by approximately 30% when buried in soil for 30 days, suggesting good biodegradability in natural environments. However, current studies have focused primarily on starch-involved superhydrophobic packaging. Given the inherent hydrophilicity of starch, it could be a promising material for designing superoleophobic packaging to reduce oil residue, but this area remains unexplored. Additionally, micro/nanocomponents are known to enhance the barrier properties of conventional packaging substrates [134], making investigating how micro-/nano-starch components in superwettable coatings improve these properties worthwhile.
6.6. Food taste modulation
With the increasing understanding of food oral processing and advancements in food technology, traditional food texture and rheological analyses are insufficient to fully capture changes in food taste within the wet oral environment [135,136]. Specifically, solid starchy foods with 3D porous structures tend to dissolve rapidly when exposed to saliva, which impacts their taste (e.g., chewiness, crunchiness, and smoothness). The interfacial wetting behavior of these foods is a significant factor influencing their taste perception [137]. Jiang et al. [71] investigated the mechanism behind the pleasant taste of popcorn during chewing, focusing on interfacial wettability. The 3D interpenetrating microporous structure of puffed starch combined with the water-soluble starch crystalline structure synergistically imparted superhydrophilicity (ultrafast wetting) to the popcorn. This rapid wetting, caused by saliva penetration, led to a swift collapse of the popcorn’s texture, which contributed to the unique taste experienced during chewing. This finding opens the door for the innovative microfabrication of starchy foods aimed at taste modulation based on the principle of wettability. Concurrently, achieving superwettability is deemed feasible through the optimization of the texture and components of foods for a superlative taste.
7. Conclusions and future perspectives
This review has detailed the recent developments in starch-involved superwettable systems, covering topics ranging from fundamental theories to design principles, from liquid–liquid colloidal systems to solid–liquid systems, and from applications where starch serves as the primary component to those where it acts as an auxiliary material. Emerging applications in the food texture and nonfood industries have also been discussed. Specifically, this review first provides an overview of the fundamental theories and design principles of superwettable systems as a foundation for starch-based designs. The unique contributions of starch to superwettable systems are then presented, including multiple aspects, such as dimensionality (e.g., colloidal systems, 0D granules/particles, 1D fibers, 2D films/fibrous membranes/coatings, and 3D fillers/porous materials/food texture), intrinsic properties (e.g., hydrophilicity, film-forming ability, adhesiveness, and thermal decomposition), and biotransformation. Given its superior biocompatibility and sustainability, starch has been incorporated into various emerging applications, such as functional biomaterials, food taste modulation, oral delivery systems, water treatment, emulsion polymerization, and packaging technology.
In response to environmental and human health concerns, starch offers a sustainable alternative to reduce the use of nonbiodegradable and bioincompatible polymers. Research on starch-involved superwettable systems has grown rapidly in recent years, but many of its attributes remain underexploited, limiting its application and diverse design in nonfood sectors. Compared with other natural polymers, starch has several unique intrinsic physicochemical properties. Exploiting these further could provide novel avenues for the development of superwettable systems. Some of these properties include:
(1) The left-handed helices in starch possess an inner cavity that can be linearly arranged by polyiodides, forming iodine–amylose complexes, which could introduce photothermal effects into superwettable systems.
(2) Debranched starch tends to aggregate on surfaces containing multiple hydroxyl groups through hydrogen bonding, offering the potential to increase the surface roughness required for superwettability.
(3) The solubility of starch in hot water implies that it can be removed from composites by boiling water, leaving behind pores that provide the necessary micro/nanostructures for superwettable systems.
(4) Superhydrophobic or zwitterionic superhydrophilic starch-based biomaterials may bypass amylase in human blood and intestines, prolonging their in vivo retention.
(5) Hydrophilic small molecules, such as dyes and phytochemicals, can be incorporated into superwettable systems through the physical loading of starch particles to achieve specific functions and appearances.
(6) The granular, crystalline, and molecular structures of starch are closely linked to its properties, which can influence the surface energy and roughness of the resulting materials. Thus, a comprehensive analysis of the relationship between the macro/microstructure of starch and surface wettability is essential for developing targeted design strategies for starch-based superwettable systems.
As the most widely consumed edible carbohydrate, starch has promising potential for the development of superwettable foods. The wettability of starchy foods governs their infiltration and hydrolysis by saliva. During oral processing, the initial mouthfeel, chewing time, texture changes, and swallowing speed are related to wettability, directly affecting the sensory quality of starchy foods. Furthermore, starch digestion begins in the oral cavity, and the interaction of salivary amylase with the food matrix influences starch digestion rates, indirectly reflecting nutritional features such as blood sugar levels and gastric emptying rates. Inspired by popcorn, the use of modern puffing and drying techniques to achieve an ultrafast wettable starch matrix with a 3D porous structure could lead to the creation of novel foods with enhanced taste. This could provide alternative food options for people with special needs, such as those with dysphagia, dysmasesia, or hypoglycemia. Additionally, the effects of superwettability on oral digestion and starch digestion kinetics deserve further exploration.
Despite these advancements, several technical challenges remain, hindering the design of starch-involved superwettable systems.
(1) Many 0D starch granules/particles used in superwettable systems have irregular morphologies, which can compromise the reproducibility and consistency of systems. More controlled spheroidization techniques, such as microfluidics, could address this issue.
(2) The surface roughness and chemical/mechanical durability of the 1D starch fibers and fibrous membranes reported thus far remain inadequate. New spinning technologies, such as focused rotary jet spinning, may be needed for these systems.
(3) The smooth surfaces of 2D TPS films could benefit from soft printing techniques to generate more biomimetic structures, enabling diverse superwettable properties.
(4) 3D-printed superwettable systems are a growing area of interest. Starch, as a printable ink for hot-melt extrusion in 3D printing, could represent a breakthrough technology in this field.
(5) In biotransformation, starch influences bacterial motility, cellulose synthesis, and polymerization. However, current studies focus on its impact on microstructures and mechanical properties, with limited exploration of its effects on wettability.
(6) Although approximately 64 superwettability states have been proposed, only approximately 10 states have been explored in starch-involved systems. Further development of additional states is needed.
(7) Additional functionalities, such as droplet manipulation, external stimulus responses, self-adaptation, and self-healing, are emerging directions but present significant challenges for starch-involved systems.
(8) The biodegradability of starch-based superwettable materials is crucial for industrial applications; however, it has rarely been evaluated. A systematic analysis of how surface wettability affects the biodegradability of starch-based materials is necessary.
The theoretical studies, design principles, and specific applications of starch-involved superwettable systems are inherently multidisciplinary. As such, collaboration across diverse fields, such as biology, physics, polymer chemistry, and food science, will be essential to fully explore the unique contributions of starch to superwettability.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was financially supported by the National Key Research and Development Program of China (2023YFD1600600 and 2021YFD1600105-03), the Postdoctoral Fellowship Program of China Postdoctoral Science Foundation (GZB20240665), the Special Funding for Postdoctoral Research Projects in Zhejiang (ZJ2024044), and the China Postdoctoral Science Foundation (2024M762848).
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