Tunable Adhesion Research Articles

Switchable Wettability and Adhesion of Micro/Nanostructured Elastomer Surface via Electric Field for Dynamic Liquid Droplet Manipulation.

Dynamic control of liquid wetting behavior on smart surfaces has attracted considerable concern owing to their important applications in directional motion, confined wetting and selective separation. Despite much progress in this regard, there still remains challenges in dynamic liquid droplet manipulation with fast response, no loss and anti‐contamination. Herein, a strategy to achieve dynamic droplet manipulation and transportation on the electric field adaptive superhydrophobic elastomer surface is demonstrated. The superhydrophobic elastomer surface is fabricated by combining the micro/nanostructured clusters of hydrophobic TiO2 nanoparticles with the elastomer film, on which the micro/nanostructure can be dynamically and reversibly tuned by electric field due to the electric field adaptive deformation of elastomer film. Accordingly, fast and reversible transition of wetting state between Cassie state and Wenzel state and tunable adhesion on the surface via electric field induced morphology transformation can be obtained. Moreover, the motion states of the surface droplets can be controlled dynamically and precisely, such as jumping and pinning, catching and releasing, and controllable liquid transfer without loss and contamination. Thus this work would open the avenue for dynamic liquid manipulation and transportation, and gear up the broad application prospects in liquid transfer, selective separation, anti‐fog, anti‐ice, microfluidics devices, etc.

Controllable superhydrophobic surfaces with tunable adhesion on Mg alloys by a simple etching method and its corrosion inhibition performance

Through a simple chemical etching and surface modification, superhydrophobic surfaces with tunable water adhesion were prepared on rolled Mg-3Al-1Zn (AZ31) and Mg-9Al-1Zn (AZ91) alloy sheets. The different water adhesion is described by three different wetting model, i.e. Wenzel, Cassie impregnating, and Cassie model. An in-depth investigation on anti-corrosion performances of low adhesive superhydrophobic AZ31 and AZ91 surfaces reveals that dense microstructure can effectively improve the stability of Cassie state, thus facilitating durable anti-corrosion ability. Simultaneously, the superhydrophobic surfaces with tunable water adhesion are demonstrated to be feasible in the application of low-loss transport of microdroplet. In addition, the as-fabricated superhydrophobic surfaces show high-temperature stability and chemical stability in a large range of temperature and pH, respectively. It is expected that this research can shed light on designing new anti-corrosion and tunable adhesive superhydrophobic surfaces, and as-fabricated superhydrophobic Mg alloys are promising in medical instruments, transport systems, micro-fluidic devices, and so on.

Reversible Structure Engineering of Bioinspired Anisotropic Surface for Droplet Recognition and Transportation.

Surfaces with tunable liquid adhesion have aroused great attention in past years. However, it remains challenging to endow a surface with the capability of droplet recognition and transportation. Here, a bioinspired surface, termed as TMAS, is presented that is inspired by isotropic lotus leaves and anisotropic butterfly wings. The surface is prepared by simply growing a triangular micropillar array on the pre‐stretched thin poly(dimethylsiloxane) (PDMS) film. The regulation of mechanical stress in the PDMS film allows the fine tuning of structural parameters of the micropillar array reversibly, which results in the instantaneous, in situ switching between isotropic and various degrees of anisotropic droplet adhesions, and between strong adhesion and directional sliding of water droplets. TMAS can thus be used for robust droplet transportation and recognition of acids, bases, and their pH strengths. The results here could inspire the design of robust sensor techniques.

Design of tunable gelatin-dopamine based bioadhesives

Bioadhesives have a potential to modulate the wound closure process with significant biological outcomes. However, none of the currently commercialized adhesives are satisfactory in their performance. It is a challenging task to develop an adhesive system that can work on wet surface and enhances tissue repair and closure. In this study, we have fabricated a series of gelatin-dopamine (Gel-dop) conjugates and studied their adhesive properties after being chemically crosslinked using sodium periodate. The designed material was assessed for its adhesive properties including tensile, lap shear and peeling study by varying the degree of dopamine substitution. It was observed that the adhesive property has a direct correlation with increase in dopamine content until reaching a maximum and then a subsequent decrease. We tested the adhesive strength of the different formulations by varying the degree of substitution and compared against fibrin glue, which is considered as the gold standard of adhesives. The formulation with a moderate substitution degree demonstrated the optimal adhesive property than those formulations with lower and larger substitution degree. Further, the in vitro cytotoxicity study showed that this tunable Gel-dop adhesives are to non-cytotoxic, indicating a potential use in clinic applications. This study illustrates that adhesiveness can be regulated by changing the degree of dopamine substitution.

Sustainable Triblock Copolymers as Tunable and Degradable Pressure Sensitive Adhesives

Pressure sensitive adhesives (PSAs) are widely used in a variety of applications (e.g., sticky note and packing tape). As for most commercial polymer products, current PSA materials are primarily d...

Preparation of superhydrophobic surface with tunable adhesion on glass substrate

Superhydrophobic coatings help to reduce the surface contamination of glass and improve its performance. A SiO2 nanoparticles superhydrophobic surface with tunable adhesion is obtained on a glass substrate after modifying with PDMS. With the water contact angle of 155.1 ± 3°, the superhydrophobic surface also shows excellent self-cleaning property, good durability and thermal stability. This method is facile, cost-effective and easy to operate and will expand the potential applications of glass materials in different fields.

Manipulating small water droplets on a photo-switchable mat of fluorinated linear liquid crystal polymers

Manipulating small water droplets on an intelligent surface with tunable superhydrophobic adhesion has attracted considerable interests because of potential applications ranging from no-loss transfer to surface chemistry but remains challenging. Herein, we present a strategy to manipulate the small water droplet on a photo-switchable mat, which is fabricated by a fluorinated linear liquid crystal polymer via electrospinning. The electrospun mat with hierarchical microstructures exhibits fast and reversible superhydrophobic adhesion change, and still remains the superhydrophobicity after pinning and removing of the water droplets for several times. Diverse erasable and rewritable arrays on the vertically placed mat are formed by pinning the small water droplets to any high adhesive position after localized light irradiation. Moreover, after evaporation of the water droplets containing aggregation-induced emission materials, the mat exhibits bright fluorescence upon faint UV irradiation to indicate the trace with the same pattern as the original droplets. Such photo-switchable superhydrophobic mats would provide an open tunable surface for microdroplets manipulation.

Switchable Adhesion via Subsurface Pressure Modulation

Materials and devices with tunable dry adhesion have many applications, including transfer printing, climbing robots, and gripping in pick-and-place processes. In this paper, a novel soft device to achieve dynamically tunable dry adhesion via modulation of subsurface pneumatic pressure is introduced. Specifically, a cylindrical elastomer pillar with a mushroom-shaped cap and annular chamber that can be pressurized to tune the adhesion is investigated. Finite element-based mechanics models and experiments are used to design, understand, and demonstrate the adhesion of the device. Specifically, the device is designed using mechanics modeling such that the pressure applied inside the annular chamber significantly alters the stress distribution at the adhered interface and thus changes the effective adhesion strength. Devices made of polydimethylsiloxane (PDMS) with different elastic moduli were tested against glass, silicon, and aluminum substrates. Adhesion strengths (σ0) ranging from ∼37 kPa (between PDMS and glass) to ∼67 kPa (between PDMS and polished aluminum) are achieved for the nonpressurized state. For all cases, regardless of the material and roughness of the substrates, the adhesion strength dropped to 40% of the strength of the nonpressurized state (equivalent to a 2.5× adhesion switching ratio) by increasing the chamber pressure from 0.3σ0 to 0.6σ0. Furthermore, the strength drops to 20% of the unpressurized strength (equivalent to a 5× adhesion switching ratio) when the chamber pressure is increased to σ0.

Extracellular matrix mechanics regulate transfection and SOX9-directed differentiation of mesenchymal stem cells

Gene delivery within hydrogel matrices can potentially direct mesenchymal stem cells (MSCs) towards a chondrogenic fate to promote regeneration of cartilage. Here, we investigated whether the mechanical properties of the hydrogel containing the gene delivery systems could enhance transfection and chondrogenic programming of primary human bone marrow-derived MSCs. We developed collagen-I-alginate interpenetrating polymer network hydrogels with tunable stiffness and adhesion properties. The hydrogels were activated with nanocomplexed SOX9 polynucleotides to direct chondrogenic differentiation of MSCs. MSCs transfected within the hydrogels showed higher expression of chondrogenic markers compared to MSCs transfected in 2D prior to encapsulation. The nanocomplex uptake and resulting expression of transfected SOX9 were jointly enhanced by increased stiffness and cell-adhesion ligand density in the hydrogels. Further, transfection of SOX9 effectively induced MSCs chondrogenesis and reduced markers of hypertrophy compared to control matrices. These findings highlight the importance of matrix stiffness and adhesion as design parameters in gene-activated matrices for regenerative medicine. Statement of SignificanceGene-activated matrices (GAMs) are biodegradable polymer networks integrating gene therapies, and they are promising technologies for supporting tissue regeneration. Despite this interest, there is still limited information on how to rationally design these systems. Here, we provide a systematic study of the effect of matrix stiffness and cell adhesion ligands on gene transfer efficiency. We show that high stiffness and the presence of cell-binding sites promote transfection efficiency and that this result is related to more efficient internalization and trafficking of the gene therapies. GAMs with optimized mechanical properties can induce cartilage formation and result in tissues with better characteristics for articular cartilage tissue engineering as compared to previously described standard methods.

Abstract 112: Galvanotactic Smart Bandage For Wireless Wound Healing

Background: Approximately thirty million people in the United States suffer from diabetes. The prevalence of diabetic foot ulcers (DFUs) in this population is 13%. There is a pressing need to develop effective therapies to treat chronic wounds robustly. Current standard of care wound dressings are passive and cannot actively respond to variations in the wound environment. Smart bandages are well positioned to address these challenges with their ability to integrate (bio)sensors for real-time monitoring and active wound care treatment. Current smart bandage technologies have demonstrated significant promise in their ability to sense physiological conditions. This includes detecting pH of the wound, temperature, oxygen, moisture, mechanical and electrical changes. To our knowledge there have not been significant advancements in incorporating sensing technologies to deliver active wound care. We believe this can be achieved using a multidisciplinary approach combining electrical and chemical engineering with the fundamentals of cellular and biomolecular processes in wound healing directed towards high resolution, in situ tissue regeneration. Methods: A flexible printed wireless stimulator was designed and fabricated to deliver directional energy across a wound gradient. Subsequently a low impedance PEDOT:PSS electrode was designed to optimize the skin and stimulator interface, producing a robust gel with tunable adhesion properties. The smart bandage was evaluated in an excisional diabetic and C57BL6/J murine wound healing model. A parabiosis model was used to evaluate circulating cell migration into the wound bed. Single cell analyses were performed to evaluate changes in cell populations as a direct result of induced electrical stimulation. In vitro validation was performed to elucidate in vivo results. Results: Wireless electrical stimulation resulted in significantly accelerated wound closure, when compared to controls, in both a diabetic and C57BL6/J murine excisional wound healing model. Complete epidermal recovery was observed, with a thicker collagen network and increased dermal thickness. Greater neovascularization and appendage formation were observed in the treatment groups. Single cell analyses revealed higher proliferation and remodeling regulatory markers expressed across treated groups. In vitro co-culture validation experiments demonstrated accelerated proliferation, mitotic rate and tube formation when compared to controls. Conclusion: Our data demonstrates the functionality of a robust wireless interface for wound healing. This novel treatment modality will integrate AI processing components for the development of a closed-loop functional stimulator. By combining the domain expertise of nanofabrication, mechanotransduction, fibrosis and molecular/cellular analyses, we are developing a novel chronically stable and robust smart bandage that will pave the way for the next generation of palliative wound care.

Magnetically tunable adhesion of composite pads with magnetorheological polymer gel cores

Tunable and reversible dry adhesion has important applications in transfer printing, precision manufacturing, and intelligent robotics. In this paper, a novel composite pad is fabricated whose adhesion can be adjusted rapidly and reversibly by the magnetic field. The composite pad consists of a cylindrical polydimethylsiloxane (PDMS) shell and a magnetorheological polymer gel (MRPG) core. The mechanical properties of the MRPG core in the pads are sensitive to the magnetic field. So the adhesion of the composite pad can be changed due to the interface stress modulation by the magnetic field. The interface stress is calculated by using the finite element method to study the mechanism of magnetically controllable adhesion. The results show the smaller the edge stress, the larger the adhesion. So composite pads with four different core shapes are prepared to optimize the structure design. Pad-bump has the largest magnetically tunable adhesion interval. The adhesion of composite pads on uneven surfaces is also tested and the results show the adhesion of composite pads with soft cores is improved compared to the solid core pad. Finally, a magnetic field controlled pick-and-place demonstration experiment shows that the composite pad array has great application potential in magnetically controlled grabbing and release.

Tunable Adhesion on Superhydrophobic Copper Surfaces Using a Facile Controllable Dewetting Approach

AbstractSuperhydrophobic (SHPB) surfaces with tunable adhesion have attracted substantial research interest due to their broad applications. Surface adhesion is a critical parameter that is used to determine the dynamics of water droplets, including liquid pining and releasing. In this study, a facile dewetting strategy is developed to finely tune adhesion on SHPB copper surfaces in a controllable manner. Using this approach, various liquid manipulation behaviors are demonstrated. Copper nanofiber textures (CNFTs) with different surface morphologies, including nanofiber array, bundled nanofibers, and hierarchical micro‐/nanocomposite structures facilitate different dewetting processes of the silicone oil solution via different capillary pathways, which lead to different localization of the silicone oil. Consequently, the CNFTs exhibit tunable adhesion ranging from 8.5 to 104.7 µN, while exhibiting superhydrophobicity. It is proposed that the continuity of the three‐phase contact line plays a crucial role in determining the surface adhesion, which varies from a continuous state on nanofiber array to a discontinuous state on micro‐/nanocomposite textures. Taking advantage, various droplet manipulation behaviors, such as the loss‐less transfer, stretchable deformation, and the directional droplet rolling, are demonstrated using CNFTs with different adhesion. It is envisioned that the proposed strategy will elucidate the fabrication of functional SHPB surfaces.

Single-step plasma-induced hierarchical structures for tunable water adhesion

Smart surfaces in nature have been extensively studied to identify their hierarchical structures in micro-/nanoscale to elucidate their superhydrophobicity with varying water adhesion. However, mimicking hybrid features in multiscale requires complex, multi-step processes. Here, we proposed a one-step process for the fabrication of hierarchical structures composed in micro-/nanoscales for superhydrophobic surfaces with tunable water adhesion. Hierarchical patterns were fabricated using a plasma-based selective etching process assisted by a dual scale etching mask. As the metallic mesh is placed above the substrate, it serves the role of dual scale etching masks on the substrate: microscale masks to form the micro-wall network and nanoscale masks to form high-aspect-ratio nanostructures. The micro-walls and nanostructures can be selectively hybridized by adjusting the gap distance between the mesh and the target surface: single nanostructures on a large area for a larger gap distance and hybrid/hierarchical structures with nanostructures nested on micro-walls for a shorter gap distance. The hierarchically nanostructured surface shows superhydrophobicity with low water adhesion, while the hybrid structured surface becomes become superhydrophobic with high adhesion. These water adhesion tunable surfaces were explored for water transport and evaporation. Additionally, we demonstrated a robust superhydrophobic surface with anti-reflectance over a large area.

Tuning the Mechanical and Adhesion Properties of Carbon Nanotubes Using Aligned Cellulose Wrap (Cellulose Nanotube): A Molecular Dynamics Study.

Improving the adhesion properties of carbon nanotubes (CNTs) at the molecular scale can significantly enhance dispersion of CNT fibers in polymer matrix and unleash the dormant extraordinary mechanical properties of CNTs in CNT-polymer nanocomposites. Inspired by the outstanding adhesion, dispersion, mechanical, and surface functionalization properties of crystalline nanocellulose (CNC), this paper studies the mechanical and adhesion properties of CNT wrapped by aligned cellulose chains around CNT using molecular dynamic simulations. The strength, elastic modulus, and toughness of CNT-cellulose fiber for different cellulose contents are obtained from tensile and compression tests. Additionally, the effect of adding cellulose on the surface energy, interfacial shear modulus, and strength is evaluated. The result shows that even adding a single layer cellulose wrap (≈55% content) significantly decreases the mechanical properties, however, it also dramatically enhances the adhesion energy, interfacial shear strength, and modulus. Adding more cellulose layers, subsequently, deceases and increases mechanical properties and adhesion properties, respectively. In addition, analysis of nanopapers of pristine CNT, pristine CNC, and CNT-wrapped cellulose reveals that CNT-wrapped cellulose nanopapers are strong, stiff, and tough, while for CNT and CNC either strength or toughness is compromised. This research shows that cellulose wraps provide CNT fibers with tunable mechanical properties and adhesion energy that could yield strong and tough materials due to the excellent mechanical properties of CNT and active surface and hydrogen bonding of cellulose.

Tunable Adhesion from Stoichiometry-Controlled and Sequence-Defined Supramolecular Polymers Emerges Hierarchically from Cyanostar-Stabilized Anion-Anion Linkages.

Sequence-controlled supramolecular polymers offer new design paradigms for generating stimuli-responsive macromolecules with enhanced functionalities. The dynamic character of supramolecular links present challenges to sequence definition in extended supramolecular macromolecules, and design principles remain nascent. Here, we demonstrate the first example of using stoichiometry-control to specify the monomer sequence in a linear supramolecular polymer by synthesizing both a homopolymer and an alternating copolymer from the same glycol-substituted cyanostar macrocycle and phenylene-linked diphosphate monomers. A 2:1 stoichiometry between macrocycle and diphosphate produces a supramolecular homopolymer of general formula (A)n comprised of repeating units of cyanostar-stabilized phosphate-phosphate dimers. Using a 1:1 stoichiometry, an alternating (AB)n structure is produced with half the phosphate dimers now stabilized by the additional counter cations that emerge hierarchically after forming the stronger cyanostar-stabilized phosphate dimers. These new polymer materials and binding motifs are sufficient to bear normal and shear stress to promote significant and tunable adhesive properties. The homopolymer (A)n, consisting of cyanostar-stabilized anti-electrostatic linkages, shows adhesion strength comparable to commercial superglue formulations based on polycyanoacrylate but is thermally reversible. Unexpectedly, and despite including traditional ionic linkages, the alternating copolymer (AB)n shows weaker adhesion strength more similar to commercial white glue based on poly(vinyl acetate). Thus, the adhesion properties can be tuned over a wide range by simply controlling the stoichiometric ratio of monomers. This study offers new insight into supramolecular polymers composed of custom-designed anion and receptor monomers and demonstrates the utility of emerging functional materials based on anion-anion linkages.

Bioinspired, Microstructured Silk Fibroin Adhesives for Flexible Skin Sensors.

Wearable epidermal sensors are of great importance to the next generation of personalized healthcare. The adhesion between the flexible sensor and skin surface is critical for obtaining accurate, reliable, and stable signals. Herein we present a facile approach to fabricate a microstructured, natural silk fibroin protein-based adhesive for achieving highly conformal, comfortable, adjustable, and biocompatible adhesion on the skin surface. The microstructured fibroin adhesive (MSFA) exhibits reliable and stable bonding force on skin surfaces, even under humid or wet conditions, and can be easily peeled off from the skin without causing significant pain. Such an MSFA can greatly improve the sensitivity and reusability of epidermal strain sensors because of its conformal and tunable adhesion on skin surfaces. The MFSA has a great potential to be applied as a functional adhesive for various epidermal electronic sensors in the era of personalized healthcare.

Pillararene-based supramolecular membranes with the rose-petal effect and nanostructure-modulated tunable water adhesion

Pillararene-based supramolecular membranes with the rose-petal effect and nanostructure-modulated tunable water adhesion was fabricated via integrating electrospraying with the breath figure approach.

A reversible underwater glue based on photo- and thermo-responsive dynamic covalent bonds

A reversible underwater glue exhibits strong and widely tunable adhesion in response to orthogonal photo- and thermal stimuli.

From lotus effect to petal effect: Tuning the water adhesion of non-wetting rare earth oxide coatings

A facile post-deposition plasma treatment was developed to tune the wetting behavior of Yb2O3 coatings deposited via the solution precursor plasma spray process. The as-deposited coatings exhibited the lotus effect after vacuum treatment, with a water contact angle of ∼162° and a roll-off angle of ∼5°, due to the hierarchical columnar microstructure. When the plasma torch was scanned over the coatings as a post-deposition treatment, the rough top surfaces of the columns present in the as-deposited coatings became smooth, and the coatings then exhibited the petal effect, with a water contact angle of ∼150° to ∼130° and high, tunable adhesion to water. Analysis of the surface compositions showed that after vacuum treatment hydrocarbon adsorption on the as-deposited coatings and plasma-treated coatings was very similar. A mathematical model correlating surface structures with wetting behaviors was developed to elucidate the transition from the lotus effect to the petal effect.

Tunable Adhesion of Different Cell Types Modulated by Thermoresponsive Polymer Brush Thickness.

Tunable adhesion of different cell types on well-defined surfaces has attracted common interests in the field of biomaterial science and surface engineering. Herein, we demonstrate a new strategy for the regulation of cell adhesion by simply controlling the thickness of thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) brushes via surface-initiated atom transfer radical polymerization (ATRP). The adhesion of different cell types (4T1, HEK293, H9C2, HUVEC, and L929) can be easily modulated by varying the thickness of PNIPAAm brushes from 5.9 ± 1.0 nm (PN1) to 69.0 ± 5.0 nm (PN6). The fluorescent staining of different cell types on a variety of surfaces reveals that the thickness of PNIPAAm brushes would regulate the assembly of F-actin and the expression of vinculin and fibronectin, which are essential in regulating the adherent status of cells. Moreover, the cellular morphologies revealed that the adherent cells are well-spread, and multiple pseudopod extensions and protrusions can be observed at the margin of cells. This work provides a facile strategy for regulating tunable adhesion of different cell types, which may find applications in tissue engineering and regenerative medicine.