Chemical Approaches for Reliable Superhydrophobic Coatings: Synthesis and Applications

A doctoral dissertation conducted under a convention for the joint supervision of thesis at Aalto University (Finland) and Indian Institute of Technology Madras (India) for the degree of Doctor of Science (Technology) at Aalto University and the degree of Doctor of Philosophy at Indian Institute of Technology Madras.Biologically available surfaces having unique liquid repelling properties have become some of the most intensely pursued materials in the past few years. In this context, several living creatures have already been studied in great detail. Surface roughness (nano/microscale) and chemical functionalities being the underlying reason for such phenomena, artificially designed these surfaces have already proven their application potential in different fields of science and technology, directed to solving many global challenges. Although various methods have been proposed for the development of these bio-mimicked surfaces, most of the time, complex multi-step processes, affordability of initial materials, large-scale production, and durability restrict the use of these in day-to-day life. Again, use of organic solvents pose concerns related to safety, environmental pollution, operational cost, storage and transport. From an industrial point of view, these limit the commercialization of these materials and their bulk production. In view of the current scenario, our work mostly focused on the development of coating materials through green and industrially adaptable or feasible ways that provide durable liquid repelling films over diverse array of substrates. Water being an economic and environmentally accepted solvent system, fabrication of these materials was carried out solely in water at room temperature. With regard to primary ingredients, use of affordable materials like clay and cellulose enhanced the possibility of scale up. Along with the mechanical and chemical durability of the coated surfaces, application domains of these materials were identified and demonstrated. In the introduction (Chapter 1), evolution of the concept of "bio-mimicry" followed by its influence on the development of liquid repelling surfaces are discussed. Objectives and motivations of the work are presented in Chapter 2. Chapter 3 covers the details of ingredients, synthesis, experiments and fabrication of the working prototypes. Chapter 4 (publication I) & 5 (publication II) focus on the fabrication of waterborne superhydrophobic coating materials from cellulose nanofiber and clay. Fabrication of durable multi-functional waterproof paper was demonstrated for both cases, needed for various upcoming technologies including microfluidics and various paper-based technologies. In addition, publication II showed the under oil superhydrophobic property of the coated material. Chapter 6 (publication III) reports the fabrication of transparent solid slippery surface and its use in atmospheric water capture through a working prototype. Chapter 7 summarizes the above-mentioned results and gives a future perspective on this research area. We believe that the methodologies and materials presented here can be successfully implemented to various technologies or translated to useful products or devices that will be helpful for human civilization.

Molecular design of an ultra-strong tissue adhesive hydrogel with tunable multifunctionality

Designing adhesive hydrogels with optimal properties for the treatment of injured tissues is challenging due to the tradeoff between material stiffness and toughness while maintaining adherence to wet tissue surfaces. In most cases, bioadhesives with improved mechanical strength often lack an appropriate elastic compliance, hindering their application for sealing soft, elastic, and dynamic tissues. Here, we present a novel strategy for engineering tissue adhesives in which molecular building blocks are manipulated to allow for precise control and optimization of the various aforementioned properties without any tradeoffs. To introduce tunable mechanical properties and robust tissue adhesion, the hydrogel network presents different modes of covalent and noncovalent interactions using N-hydroxysuccinimide ester (NHS) conjugated alginate (Alg-NHS), poly (ethylene glycol) diacrylate (PEGDA), tannic acid (TA), and Fe3+ ions. Through combining and tuning different molecular interactions and a variety of crosslinking mechanisms, we were able to design an extremely elastic (924%) and tough (4697 kJ/m3) multifunctional hydrogel that could quickly adhere to wet tissue surfaces within 5 s of gentle pressing and deform to support physiological tissue function over time under wet conditions. While Alg-NHS provides covalent bonding with the tissue surfaces, the catechol moieties of TA molecules synergistically adopt a mussel-inspired adhesive mechanism to establish robust adherence to the wet tissue. The strong adhesion of the engineered bioadhesive patch is showcased by its application to rabbit conjunctiva and porcine cornea. Meanwhile, the engineered bioadhesive demonstrated painless detachable characteristics and in vitro biocompatibility. Additionally, due to the molecular interactions between TA and Fe3+, antioxidant and antibacterial properties required to support the wound healing pathways were also highlighted. Overall, by tuning various molecular interactions, we were able to develop a single-hydrogel platform with an “all-in-one” multifunctionality that can address current challenges of engineering hydrogel-based bioadhesives for tissue repair and sealing

Molecular design of an ultra-strong tissue adhesive hydrogel with tunable multifunctionality.

Designing adhesive hydrogels with optimal properties for the treatment of injured tissues is challenging due to the tradeoff between material stiffness and toughness while maintaining adherence to wet tissue surfaces. In most cases, bioadhesives with improved mechanical strength often lack an appropriate elastic compliance, hindering their application for sealing soft, elastic, and dynamic tissues. Here, we present a novel strategy for engineering tissue adhesives in which molecular building blocks are manipulated to allow for precise control and optimization of the various aforementioned properties without any tradeoffs. To introduce tunable mechanical properties and robust tissue adhesion, the hydrogel network presents different modes of covalent and noncovalent interactions using N-hydroxysuccinimide ester (NHS) conjugated alginate (Alg-NHS), poly (ethylene glycol) diacrylate (PEGDA), tannic acid (TA), and Fe3+ ions. Through combining and tuning different molecular interactions and a variety of crosslinking mechanisms, we were able to design an extremely elastic (924%) and tough (4697 kJ/m3) multifunctional hydrogel that could quickly adhere to wet tissue surfaces within 5 s of gentle pressing and deform to support physiological tissue function over time under wet conditions. While Alg-NHS provides covalent bonding with the tissue surfaces, the catechol moieties of TA molecules synergistically adopt a mussel-inspired adhesive mechanism to establish robust adherence to the wet tissue. The strong adhesion of the engineered bioadhesive patch is showcased by its application to rabbit conjunctiva and porcine cornea. Meanwhile, the engineered bioadhesive demonstrated painless detachable characteristics and in vitro biocompatibility. Additionally, due to the molecular interactions between TA and Fe3+, antioxidant and antibacterial properties required to support the wound healing pathways were also highlighted. Overall, by tuning various molecular interactions, we were able to develop a single-hydrogel platform with an all-in-one multifunctionality that can address current challenges of engineering hydrogel-based bioadhesives for tissue repair and sealing

Rational Design of Immunomodulatory Hydrogels for Chronic Wound Healing

With all the advances in tissue engineering for construction of fully functional skin tissue, complete regeneration of chronic wounds is still challenging. Since immune reaction to the tissue damage is critical in regulating both the quality and duration of chronic wound healing cascade, strategies to modulate the immune system are of importance. Generally, in response to an injury, macrophages switch from pro-inflammatory to an anti-inflammatory phenotype. Therefore, controlling macrophages' polarization has become an appealing approach in regenerative medicine. Recently, hydrogels-based constructs, incorporated with various cellular and molecular signals, have been developed and utilized to adjust immune cell functions in various stages of wound healing. Here, the current state of knowledge on immune cell functions during skin tissue regeneration is first discussed. Recent advanced technologies used to design immunomodulatory hydrogels for controlling macrophages' polarization are then summarized. Rational design of hydrogels for providing controlled immune stimulation via hydrogel chemistry and surface modification, as well as incorporation of cell and molecules, are also dicussed. In addition, the effects of hydrogels' properties on immunogenic features and the wound healing process are summarized. Finally, future directions and upcoming research strategies to control immune responses during chronic wound healing are highlighted

Engineering a highly elastic bioadhesive for sealing soft and dynamic tissues

Injured tissues often require immediate closure to restore the normal functionality of the organ. In most cases, injuries are associated with trauma or various physical surgeries where different adhesive hydrogel materials are applied to close the wounds. However, these materials are typically toxic, have low elasticity, and lack strong adhesion especially to the wet tissues. In this study, a stretchable composite hydrogel consisting of gelatin methacrylol catechol (GelMAC) with ferric ions, and poly(ethylene glycol) diacrylate (PEGDA) was developed. The engineered material could adhere to the wet tissue surfaces through the chemical conjugation of catechol and methacrylate groups to the gelatin backbone. Moreover, the incorporation of PEGDA enhanced the elasticity of the bioadhesives. Our results showed that the physical properties and adhesion of the hydrogels could be tuned by changing the ratio of GelMAC/PEGDA. In addition, the in vitro toxicity tests confirmed the biocompatibility of the engineered bioadhesives. Finally, using an ex vivo lung incision model, we showed the potential application of the developed bioadhesives for sealing elastic tissues

Engineering Tough, Injectable, Naturally Derived, Bioadhesive Composite Hydrogels

Engineering mechanically robust bioadhesive hydrogels that can withstand large strains may open new opportunities for the sutureless sealing of highly stretchable tissues. While typical chemical modifications of hydrogels, such as increasing the functional group density of crosslinkable moieties and blending them with other polymers or nanomaterials have resulted in improved mechanical stiffness, the modified hydrogels have often exhibited increased brittleness resulting in deteriorated sealing capabilities under large strains. Furthermore, highly elastic hydrogels, such as tropoelastin derivatives are highly expensive. Here, gelatin methacryloyl (GelMA) is hybridized with methacrylate-modified alginate (AlgMA) to enable ion-induced reversible crosslinking that can dissipate energy under strain. The hybrid hydrogels provide a photocrosslinkable, injectable, and bioadhesive platform with an excellent toughness that can be tailored using divalent cations, such as calcium. This class of hybrid biopolymers with more than 600% improved toughness compared to GelMA may set the stage for durable, mechanically resilient, and cost-effective tissue sealants. This strategy to increase the toughness of hydrogels may be extended to other crosslinkable polymers with similarly reactive moieties

Singlet Oxygen Generation from Polymer Nanoparticles–Photosensitizer Conjugates Using FRET Cascade

Herein, we demonstrate π-conjugated polymer nanoparticles–photosensitizer conjugates for singlet oxygen generation via FRET cascade, which would be useful for photodynamic therapy. Rose Bengal (RB) molecules are attached on the surface of coumarin 153 (C153)-dye-doped poly­[<i>N</i>-vinyl carbazole] (PVK) polymer nanoparticles, where polymer nanoparticles act as efficient light-absorbing antenna materials. The energy funneling from C153 to RB at the excitation of the PVK host (340 nm) is confirmed by shortening of decay time of C153 and disappearing of its rise time. Again, it is evident that the efficient multistep energy transfer occurs from host PVK to RB dye molecules through C153 dye molecules to generate singlet oxygen (¹O₂) in solution. In addition, photo-oxidation of 2-chlorophenol provides quantitative evidence of singlet oxygen generation in different systems indirectly. The estimated singlet oxygen quantum yield for RB-attached C153-dye-doped PVK polymer nanoparticles is 21%. The present investigations should pave the way for future development of different photodynamic and theranostic devices

Waterborne Fluorine-Free Superhydrophobic Surfaces Exhibiting Simultaneous CO2 and Humidity Sorption

| openaire: EC/H2020/725513/EU//SuperRepelRecent progress in the field of superhydrophobic materials has proven their potential to solve many problems of the contemporary society. However, the use of such materials to capture moisture and CO2 from air, to help reduce the impact of global climate change is not explored. In addition, most of the time, fabrication of these materials needs organic solvents and fluorinated molecules involving multiple steps that hinder the use of nonwettable materials in everyday life. Herein, a waterborne, fluorine-free, robust superhydrophobic material synthesized at room temperature through a one-step chemical-modification process is reported, which exhibits moisture and CO2 capturing capability. While covalently grafted low surface energy hydrocarbon molecules control the bulk superhydrophobicity, the incorporated amine functionalities facilitate moisture and CO2 adsorption as these molecules (H2O and CO2) can easily diffuse through hydrocarbon assemblies. Being polar, H2O molecules are observed to readily interact with amine groups and favor the adsorption process. Synthesized material shows an approximate CO2 adsorption of 480 ppm (10.90 mmol L−1) in ambient conditions having 75% humidity. Multifunctionality along with durability of this material will help expand the applications of superhydrophobic materials.Peer reviewe

Organic Solvent-Free Fabrication of Durable and Multifunctional Superhydrophobic Paper from Waterborne Fluorinated Cellulose Nanofiber Building Blocks

In view of a great demand for paper-based technologies, nonwettable fibrous substrates with excellent durability have drawn much attention in recent years. In this context, the use of cellulose nanofibers (CNFs), the smallest unit of cellulosic substrates (5-20 nm wide and 500 nm to several microns in length), to design waterproof paper can be an economical and smart approach. In this study, an eco-friendly and facile methodology to develop a multifunctional waterproof paper via the fabrication of fluoroalkyl functionalized CNFs in the aqueous medium is presented. This strategy avoids the need for organic solvents, thereby minimizing cost as well as reducing safety and environmental concerns. Besides, it widens the applicability of such materials as nanocellulose-based aqueous coatings on hard and soft substrates including paper, in large areas. Water droplets showed a contact angle of 160° (±2°) over these surfaces and rolled off easily. While native CNFs are extremely hydrophilic and can be dispersed in water easily, these waterborne fluorinated CNFs allow the fabrication of a superhydrophobic film that does not redisperse upon submersion in water. Incorporated chemical functionalities provide excellent durability toward mechanochemical damages of relevance to daily use such as knife scratch, sand abrasion, spillage of organic solvents, etc. Mechanical flexibility of the chemically modified CNF composed paper remains intact despite its enhanced mechanical strength, without additives. Superhydrophobicity induced excellent microbial resistance of the waterproof paper which expands its utility in various paper-based technologies. This includes waterproof electronics, currency, books, etc., where the integrity of the fibers, as demonstrated here, is a much-needed criterion.Peer reviewe

Ferrofluid Microdroplet Splitting for Population-Based Microfluidics and Interfacial Tensiometry

Ferrofluids exhibit a unique combination of liquid properties and strong magnetic response, which leads to a rich variety of interesting functional properties. Here, the magnetic-field-induced splitting of ferrofluid droplets immersed in an immiscible liquid is presented, and related fascinating dynamics and applications are discussed. A magnetic field created by a permanent magnet induces instability on a mother droplet, which divides into two daughter droplets in less than 0.1 s. During the splitting process, the droplet undergoes a Plateau–Rayleigh-like instability, which is investigated using high-speed imaging. The dynamics of the resulting satellite droplet formation is shown to depend on the roughness of the supporting surface. Further increasing the field results in additional splitting events and self-assembly of microdroplet populations, which can be magnetically actuated. The effects of magnetization and interfacial tension are systematically investigated by varying magnetic nanoparticles and surfactant concentrations, and a variety of outcomes from labyrinthine patterns to discrete droplets are observed. As the splitting process depends on interfacial tension, the droplet splitting can be used as a measure for interfacial tension as low as 0.1 mN m−1. Finally, a population-based digital microfluidics concept based on the self-assembled microdroplets is presented.Peer reviewe