As the biggest organ in vertebrates, skin functions as the external barrier for the protection of inner organs, regulation of the body temperature etc. (Morgado et al., 2015). Skin tissue suffers from severe damages when high heat, pressure, genetic disorder and other related diseases occur, and might fail in self-regeneration unfortunately (Divyashri et al., 2022). The injured wounds are classified based on the cause, depth, complexity and time of healing (Iacob et al., 2020). Among which, the chronic wound (in comparison with acute wound) usually fails to heal after the treatment for more than 4 weeks, which poses great threat to human health (Sweeney et al., 2012). In consideration of this, proper wound dressings are highly demanded to protect the wound region and accelerate the healing process.

The first wound bandage can be traced back to 4000 years ago, which was made by honey or resin (Farahani and Shafiee, 2021). After thousands of years evolution, a variety of materials have been applied in wound dressing, such as hydrocolloids, films, foam and hydrogel. Ideal wound dressings should meet the requirements of good biocompatibility, moisture retention, appropriate mechanical property, non-adherent and proper exudate management (Liang et al., 2021). It should be noted that the feasible management of biofluid exerts a crucial role in wound care among all characteristics. Regarding to four phases of wound healing that include hemostasis, inflammation, proliferation, and remodeling, the control of bleeding in hemostasis stage is the primary task, followed by the management of exudates in the inflammatory and proliferative phases (Bernardes et al., 2021; Maleki et al., 2021). Although large efforts have been devoted, there still exist disturbing dilemmas for traditional dressings. For example, materials capable of favorable absorption property usually lead to the ingrowth of clotting and new granulation tissues, which might cause secondary injuries for the routine removal.

The development of superwettable materials opens new avenues for wound dressings. Derived from the natural superwettable phenomena, that include the typical examples of self-cleaning lotus leaf and slippery nepenthes, diverse types of superwetability have been explored: superhyrophilic, amphiphobic and Janus wettability (Ding et al., 2012; Zhu et al., 2017; Sun et al., 2021). Depending on this, the exquisite design of bio-interface can address the problems of existing dilemmas in wound dressing. However, the advancements in this area have rarely been combed. In light of this, the aim of this minireview is to summarize the recent strategies of superwettable bio-interfaces for wound care. Concentrating on the management of biofluids, we divided the content into the control of blood (hemostasis) and exudates. The advantages and disadvantages of each design were interpreted, such as superhydrophilic, superhydrophobic, slippery, inner hydrophilic-outer hydrophobic, inner hydrophobic-outer hydrophilic and multilayer materials (Figure 1). Finally, the perspective of this area was given. We have to state that common hydrogels with the intrinsic hydrophilic characteristic lack the feasible tailoring of wettability, and benefit the wound healing from other viewpoints (injectable, self-healing and smart response). This topic is beyond the scope of this review and will not be discussed here. The readers please refer to previous excellent reviews (Dong and Guo, 2021; Liang et al., 2021; Maleki et al., 2021). We believe this minireview will inspire the design of superwettable interfaces, and expand the application of these materials in other related areas.

The uncontrollable bleeding caused by severe trauma threatens the life of people, haemorrhage control is thus the principal step for wound management (Liu et al., 2022). In an attempt to reduce the high mortality of massive hemorrhage, the development of quick hemostatic materials is in high demand. Normally, the bleeding stops by the formation of blood clot with the main composition of platelets and fibrin (Versteeg et al., 2013). Hydrophilic materials, cotton gauze, as an example, possessing the advantage of high water affinity can absorb water in the blood and accelerate clotting (Zhu et al., 2018). By virtue of this, active hemostatic agents including proteins, polysaccharides and silicon-based materials were widely integrated in hydrophilic substrates for synergistic hemostasis by activating the coagulation cascade, which can be fabricated in the form of membranes, sponges, hydrogels and particles. This topic has been well-summarized by Prof. Guo and will not be discussed here (Liang et al., 2021).

Although large advancements have been achieved, normal hemostatic interfaces constructed by hydrophilic materials generally suffer from the following limitations: 1) massive blood loss due to the rapid capillary drainage of hydrophilic materials; 2) secondary injuries caused by the removal of wound dressing, considering the clotting has formed between bleeding tissue and materials. The delicate design of superwettable bio-interfaces provides an intriguing solution to tackle these problems.

Regarding to the massive blood loss, asymmetric or Janus dressings with superhydrophilic and superhydrophobic properties on two sides were adopted. For example, cotton fabric was endowed with Janus performance by spraying one side with hydrophobic SiO₂ nano-particles and ethyl-α-cyanoacrylate superglue, meanwhile the other side of fabric remained superhydrophilic (Sasaki et al., 2016). The superhydrophilic surface absorbed blood and expedited clotting while the superhydrophobic side prevented the permeation of water and blood. Zhu et al. reported that the Janus fabrics could reduce blood loss more than 50% in comparison with the common superhydrophilic gauzes and prolong the survival time in rat model (Zhu et al., 2018). The similar results were duplicated on Janus cellulose sponges (Cheng et al., 2020). It was explained that the hydrophobic layer prevented the blood penetration and posed proper pressure on the wound, which accounted for efficient blood clotting performance (Figure 2A). This strategy was further verified in self-assembled dipeptide aerogels (Li et al., 2020) and carboxymethyl chitosan/paraffin modified cellulose films (Wang et al., 2020). Moreover, the superhydrophobic external layer, acts as an armor and protects the inner wound from bacterial infection.

To avoid the secondary injuries, the affinity between bleeding tissue and materials should be weakened, and hydrophobic blood-repelling material seems an optimal choice (Liu et al., 2020). However, it should be noted that mere blood repellence fails in the control of massive bleeding. In this case, the active hemostatic agents were simultaneously incorporated. Li and co-workers creatively proposed an approach to address the dilemma of rapid blood coagulation and facial wound-dressing removal (Li Z. et al., 2019). Specifically, a super-hydrophobic surface with immobilized carbon nanofibers (CNFs) was developed, among which CNFs promoted quick fibrin growth and rapid clotting, while the air pocked generated minimal contact area and easy detachment of mature clots (Figure 2B). The peeling tension was 1-2 folds lower than the commercial products. Besides, the commercial zeolite gauze modified with a paraffin coating was rendered with blood repellent property and procoagulant performance as well, which was derived from the retained cation exchange capacity of zeolites (Zhang W. et al., 2021). Very recently, He et al. tailored the hydrophilicity-hydrophobicity balance of cotton gauze by modifying it with catechol compound with flexible long hydrophobic alkyl chain (He H. et al., 2022). The adhesion/blocking effect of catechol groups, blood wicking of cotton and hydrophobic effect of alkyl chains conjointly contributed to the outstanding hemostatic efficiency (rapid hemostatic, low blood loss and no secondary injuries). Following the similar idea, superhydrophilic/superhydrophobic alternate pattern was designed to realize the balance between hemostatic and reduced adhesion (Long et al., 2021).

Apart from the haemorrhage control in the first step of wound care, the management of exudate is also crucial to wound healing (Sweeney et al., 2012). Wound exudate is defined as the fluid produced around wound in the inflammatory and proliferative phases when haemostasis has been completed (Bernardes et al., 2021). For the acute wounds, the exudate is mainly composed of water electrolytes, protein-digesting enzymes, macrophages, inflammatory mediators and neutrophils, which allows the autolysis of damaged tissue, and provides nutrient for the metabolism of cells (Spear, 2012). The exudate is regarded positive in acute wounds and its production declines with the passage of time. In this case, the dressings should perform the protection and moist control roles (Priya et al., 2016; Shi et al., 2020). By contrast, the composition of chronic wound exudate shows distinct differences: it contains higher concentrations of pro-inflammatory cytokines, matrix metalloproteinases (MMP), and decreased levels of growth factors (Brett, 2006). In addition, the exudate damages peri-wound skin and its production in chronic wounds might be continuous and excessive due to inflammation (Cutting, 2003). Moreover, high-volume exudate could raise the risk of bacterial invasion because the saturated dressing functions as a portal. In this sense, it is of great significance to remove excessive exudate while maintain a moist environment for wound healing.

Water absorbent dressings made by hydrophilic materials supply abundant channels for exudate drainage and are great candidates for exudate management (Varaprasad et al., 2020). In general, the hydrophilic dressings were endowed with other functions including antibacterial and electro-activity (Mayandi et al., 2020; Li et al., 2021; Wang et al., 2022). For instance, superhydrophilic electrospun gelatin nanofibers were loaded with ε-polylysine (a broad-spectrum antibacterial agent) and crosslinked by polydopamine (PDA) for the treatment of second-degree burns (Mayandi et al., 2020). The gelatin mats absorbed the exudates and propelled the migration of early bacterial colonizers into the antibacterial trappers. Li et al. reported nanozyme composite cryogels with the capability of exudate absorption and acid triggered sterilization (Li et al., 2021). The macroporous structure and excellent hydrophilicity of cyogel generated rapid liquid absorption property. Furthermore, taking advantage of the pH-responsive amine groups and switchable Schiff base reaction, on the one hand, the polymer scaffold and Fe-MIL-88NH₂ nanozyme were positively charged to capture microbes through electrostatic interaction in acidic condition. On the other hand, the nanozyme conducted reversible release and rebinding behavior for dynamic killing of microbes. Wang and coworkers opened a new path for the utilization of exudate by transmitting endogenous bioelectricity and motivating the cascade release of growth factors (Wang et al., 2022). To achieve this goal, short nanofibrous sponges were modified by graphene oxide, which was then reduced to conductive graphene under the effect of PDA, and VEGF-carrying liposomes were then loaded. The above works represent the characteristic examples of hydrophilic bio-interfaces, which regulated the biofluid by simple absorption. However, the hygroscopic dressing might overhydrate the wounds and complicate the healing (Schuren et al., 2005). Besides, new granulation tissues grown in the hydrophilic mats further hinder the routine change of dressing. Therefore, other strategies based on superwettbility interfaces have been proposed.

In summary, the development of superwettable bio-interfaces motivates the application of these designs in wound care, which exhibit attractive advantages of biofluid management, drug delivery, easy to change etc. In view of the two primary biofluids during healing stages (blood and exudates), superwettable hemostatic bio-interfaces were first described, the management of exudates was then summarized. The blood management requires the quick hemostatic, no blood loss and secondary injuries, and the integration of superhydrophobic bio-interface satisfies the needs. Diverse strategies including hydrophobic/slippery, inner hydrophilic-outer hydrophobic, inner hydrophobic-outer hydrophilic and multilayer surfaces were applied for exudate management. These bio-interfaces all have distinctive merits and were reasonably verified in the specific wound model. Based on the shallow understanding of authors, the perspectives were proposed as follows: 1) The delicate control of exudates (the backflow and purification) needs in-deep investigation, especially the mechanism and accuracy. 2) Almost all chronic animal models in the reported papers differ from that in clinic (non-healing for more than 4 weeks), and the animal model resembling that of real case would be more meaningful. 3) The regeneration of skin appendices and scarless wound healing are still challenging. In a word, tailoring superwettability bio-interfaces provides inspiring solutions to tackle the existing problems in wound management. We hope that this minireview can give a comprehensive outline for readers and provide references for the design of superwettable materials in biomedical area.