Adhesion of polymers

Adhesion advances: from nanomaterials to biomimetic adhesion and applications

The field of adhesion has revealed a significant impact on numerous applications such as wound healing, drug delivery, electrically conductive adhesive, dental adhesive, and wood industry. Nanotechnology has continued to be the primary means to achieve adhesion. Among them, biological systems based on the unique structure of the nano-levels have developed excellent adhesion capabilities after billions of years of evolution and natural selection. Therefore, the research on bionic adhesion inspired by biological systems has gradually emerged. This review firstly focuses on the mechanism of adhesion, and secondly reports the effects of different nanomaterials on adhesion properties. Then based on the structure of mussels, geckos, tree frogs, octopuses, and other organisms, the research progress of biomimetic nanotechnology to achieve adhesion is summarized. Finally, the applications, challenges, and future directions of nanotechnology in new adhesive materials are provided.

New Chemically Resistant Coating Systems with Progressive Incorporation of Hazardous Waste in Polyurethane and Epoxy Matrices

New types of highly chemically resistant coating systems, primarily intended for concrete and metal substrates, were designed and experimentally verified in the paper. Secondary raw materials in optimal amounts, including solidified hazardous waste (e.g., end product and cement bypass dust), were used as microfillers. The polymer coating systems, containing pre-treated hazardous waste (HW), showed high abrasion resistance and excellent adhesion to metal and concrete surfaces. Based on polyurethane and epoxy resins, the coatings can be used in environments where aggressive chemical media act, such as sewers and the chemical industry. The developed polymeric coating systems showed even better properties than the compared reference coating systems. The chemical resistance of the three-layer coating systems was evaluated both visually and based on changes in mechanical properties, such as hardness and adhesion. The microstructure of the coating systems was also monitored using a digital optical microscope and a scanning electron microscope with energy dispersive X-ray analysis (SEM-EDX) after chemical stress. It was observed that the particles of HW were fully incorporated into the polymer matrix of the coating systems.

Controlling unequal surface energy results caused by test liquids: the case of UV/O3 Treated PET

Ultraviolet/ozone (UV/O₃) treatment has been reported to be an effective method to modify properties such as wettability, adhesion or adsorption of plastic surfaces. The change in the surface is measured by contact angle analysis, which employs liquids and their surface tensions (ST) to estimate the surface energy (SE). We found two different practices in the scientific community: (1) the majority of researchers adopted the ST value of liquids from the literature, while (2) other researchers conducted real-time measurements in the lab under ambient conditions prior to SE estimation. To the best of our knowledge, there is no study that compares the difference between the two practices. One study was found to show different SE methods generating unequal SE values for the same substrate. However, there was no definitive conclusion backed by general thermodynamics rules. In this study, we presented (1) a statistical significance test that showed the literature and experimental ST values are significantly different, and studied (2) the effect of different liquid pairs on the SE estimation for UV/O₃ treated poly(ethylene terephthalate) (PET) substrate. Modification techniques such as atmospheric pressure plasma or chemical modification were studied previously to examine PET’s wettability and the SE. The UV/O₃ treatment was studied to improve adhesion and to modify its chemical properties for adsorption. In contrast, we studied (3) the effect of UV/O₃ on wettability at different timeframes and addressed (4) how to control unequal SE based on a method that was refined on a rigorous thermodynamic three-phase system. It must be noted that this method can be generalized to other types of solid surfaces to estimate thermodynamically self-consistent SE values. This work also provides (5) a web-based calculator that complements computational findings available to the readership in the data availability section.

Biopackaging Potential Alternatives: Bioplastic Composites of Polyhydroxyalkanoates and Vegetal Fibers

Initiatives to reduce plastic waste are currently under development worldwide. As a part of it, the European Union and private and public organizations in several countries are designing and implementing regulations for single-use plastics. For example, by 2030, plastic packaging and food containers must be reusable or recyclable. In another approach, researchers are developing biopolymers using biodegradable thermoplastics, such as polyhydroxyalkanoates (PHAs), to replace fossil derivatives. However, their production capacity, high production costs, and poor mechanical properties hinder the usability of these biopolymers. To overcome these limitations, biomaterials reinforced with natural fibers are acquiring more relevance as the world of bioplastics production is increasing. This review presents an overview of PHA–vegetal fiber composites, the effects of the fiber type, and the production method’s impact on the mechanical, thermal, barrier properties, and biodegradability, all relevant for biopackaging. To acknowledge the behaviors and trends of the biomaterials reinforcement field, we searched for granted patents focusing on bio-packaging applications and gained insight into current industry developments and contributions.

Influence of Simple and Double-Weave Structures on the Adhesive Properties of 3D Printed Fabrics

The double-weave structure of a fabric allows for the use of different materials and weave structures for the upper and lower layer, which can be advantageous in the functionalization of 3D printed textiles. Therefore, the aim of this research was to investigate the influence of simple and double-weave structures on the adhesion of 3D printed fabrics. From this perspective, we investigated the influence of different twill derivates and weft densities on the adhesion force. We produced fabrics specifically for this study and printed them with a polylactic acid filament using Fused Deposition Modeling technology. The T-peel test was performed to measure the adhesion, and the results were statistically analyzed. A morphological study of the surfaces and cross-sections of the 3D printed fabrics helped us interpret the results. We found that adhesion was higher for double fabrics when printed with a smaller z-distance, where the molten polymer reached the lower layer of the fabric and adhered to it. The opposite was confirmed when printing with a larger z-distance, where adhesion was higher for simple fabrics. Both weave and density had a significant effect on adhesion in all cases. Surprisingly, different twill derivatives generally had a greater influence on adhesion than density.

Improving Layer Adhesion of Co-Extruded Polymer Sheets by Inducing Interfacial Flow Instabilities

Co-extrusion is commonly used to produce polymer multilayer products with different materials tailoring the property profiles. Adhesion between the individual layers is crucial to the overall performance of the final structure. Layer adhesion is determined by the compatibility of the polymers at the interface and their interaction forces, causing for example the formation of adhesive or chemical bonds or an interdiffusion layer. Additionally, the processing conditions, such as temperature, residence time, cooling rate, and interfacial shear stress, have a major influence on the interactions and hence resulting layer adhesion. Influences of temperature and residence time are already quite well studied, but influence of shear load on the formation of an adhesion layer is less explored and controversially discussed in existing literature. In this work, we investigated the influence of different processing conditions causing various shear loads on layer adhesion for a two-layer co-extruded polymer sheet using a polypropylene and polypropylene talc compound system. Therefore, we varied the flow rates and the flow geometry of the die. Under specific conditions interfacial flow instabilities are triggered that form micro layers in the transition regime between the two layers causing a major increase in layer adhesion. This structure was analyzed using confocal Raman microscopy. Making use of these interfacial flow instabilities in a controlled way enables completely new opportunities and potentials for multi-layer products.

From macromonomers to bottlebrush copolymers with sequence control: synthesis, properties, and applications

Bottlebrush polymers (BBPs) are a type of comb-like macromolecules with densely grafted polymeric sidechains attached to the polymer backbones, and many intriguing properties and applications have been demonstrated due to...

Adhesion Optimization between Incompatible Polymers through Interfacial Engineering

Additive manufacturing technologies such as fused filament fabrication (FFF) open many possibilities in terms of product functionality, including the possibility to integrate a sensor in FFF parts to perform structural health monitoring. In this context, embedding fiber Bragg grating (FBG) sensors into 3D-printed polymeric structures for strain or temperature measurements has attracted increasing attention in recent years. Indeed, offering structural health monitoring functionality can optimize the maintenance cost and increase security compared with conventional materials. However, the transmission of strain and temperature between the polymeric matrix and the FBG polymer jacket requires optimal bonding between them. In this work, the two polymers of interest are polyimide (PI) and poly(lactic acid) (PLA) for the FBG jacket and printed polymer, respectively. The current study investigates the influence of different surface treatment methods on the adhesion between a PI film and a plate of PLA, with PLA and PI being incompatible polymers. The adhesion promotion applied to the PI surface relies on cleaning, plasma activation, roughness modification, or the use of adhesive nanocoating. Bilayer samples of PI-PLA are processed by welding PLA against the treated PI by heating, whereas the adhesion between PI and PLA is measured by peel testing. It is observed that the highest adhesion between PI and PLA is achieved by a combination of mechanical abrasion increasing roughness and the use of polydopamine as an adhesive. This finding is discussed based on a synergetic effect between mechanical interlocking and chemical interaction between the two counterfaces.

Me-Doped Ti-Me Intermetallic Thin Films Used for Dry Biopotential Electrodes: A Comparative Case Study

In a new era for digital health, dry electrodes for biopotential measurement enable the monitoring of essential vital functions outside of specialized healthcare centers. In this paper, a new type of nanostructured titanium-based thin film is proposed, revealing improved biopotential sensing performance and overcoming several of the limitations of conventional gel-based electrodes such as reusability, durability, biocompatibility, and comfort. The thin films were deposited on stainless steel (SS) discs and polyurethane (PU) substrates to be used as dry electrodes, for non-invasive monitoring of body surface biopotentials. Four different Ti–Me (Me = Al, Cu, Ag, or Au) metallic binary systems were prepared by magnetron sputtering. The morphology of the resulting Ti–Me systems was found to be dependent on the chemical composition of the films, specifically on the type and amount of Me. The existence of crystalline intermetallic phases or glassy amorphous structures also revealed a strong influence on the morphological features developed by the different systems. The electrodes were tested in an in-vivo study on 20 volunteers during sports activity, allowing study of the application-specific characteristics of the dry electrodes, based on Ti–Me intermetallic thin films, and evaluation of the impact of the electrode–skin impedance on biopotential sensing. The electrode–skin impedance results support the reusability and the high degree of reliability of the Ti–Me dry electrodes. The Ti–Al films revealed the least performance as biopotential electrodes, while the Ti–Au system provided excellent results very close to the Ag/AgCl reference electrodes.

Development of a Simple Spheroid Production Method Using Fluoropolymers with Reduced Chemical and Physical Damage

Establishing an in vitro–based cell culture system that can realistically simulate in vivo cell dynamics is desirable. It is thus necessary to develop a method for producing a large amount of cell aggregates (i.e., spheroids) that are uniform in size and quality. Various methods have been proposed for the preparation of spheroids; however, none of them satisfy all requirements, such as cost, size uniformity, and throughput. Herein, we successfully developed a new cell culture method by combining fluoropolymers and dot patterned extracellular matrix substrates to achieve size-controlled spheroids. First, the spheroids were spontaneously formed by culturing them two-dimensionally, after which the cells were detached with a weak liquid flow and cultured in suspension without enzyme treatment. Stable quality spheroids were easily produced, and it is expected that the introduction and running costs of the technique will be low; therefore, this method shows potential for application in the field of regenerative medicine.

2D Materials for Skin-Mountable Electronic Devices

Skin-mountable devices that can directly measure various biosignals and external stimuli and communicate the information to the users have been actively studied owing to increasing demand for wearable electronics and newer healthcare systems. Research on skin-mountable devices is mainly focused on those materials and mechanical design aspects that satisfy the device fabrication requirements on unusual substrates like skin and also for achieving good sensing capabilities and stable device operation in high-strain conditions. 2D materials that are atomically thin and possess unique electrical and optical properties offer several important features that can address the challenging needs in wearable, skin-mountable electronic devices. Herein, recent research progress on skin-mountable devices based on 2D materials that exhibit a variety of device functions including information input and output and in vitro and in vivo healthcare and diagnosis is reviewed. The challenges, potential solutions, and perspectives on trends for future work are also discussed.

Simulation Study of the Effects of Interfacial Bonds on Adhesion and Fracture Behavior of Epoxy Resin Layers

The adhesion and fracture behavior of tetraglycidyl-4,4'-diaminodiphenylmethane (TGDDM)/4,4'-diaminodiphenyl sulfone (44DDS)-bisphenol A diglycidyl ether (DGEBA)/44DDS layer interfaces were investigated by molecular dynamics (MD) simulation, mainly focusing on the role of covalent and noncovalent interactions. To accurately investigate the bond dissociation processes, the force field parameters of several bond potentials of the epoxy resin polymers were optimized by density functional theory calculations. In the MD simulations under a tensile load, small voids gradually developed without covalent bond dissociation in the plateau region. In the final large strain region, the stress rapidly increased with bond breaking, leading to failure. When the chemical bonds across the interface between the two layers were removed, the stress-strain curve in the initial elastic region was almost the same as that with interfacial bonds. This showed that the nonbonded interactions governed adhesion strength in the initial elastic region. In contrast, the bonded interactions at interfaces played important roles in the hardening regions because the bonded interactions made the major contribution to the fracture energies. We also investigated the effect of the etherification reaction in cross-linking. It was found that the etherification reaction mainly contributed to the behavior in the late region with large strain. These simulation results revealed that the nonbonded interactions, especially, van der Waals interactions, are important factors for adhesion of the different polymer layers in the small strain region up to the yield point.

Aqueous dispersion and tuning surface charges of polytetrafluoroethylene particles by bioinspired polydopamine-polyethyleneimine coating via one-step method

We propose a surface modification of poorly dispersive polytetrafluoroethylene (PTFE) particles via bioinspired polydopamine-polyethyleneimine (PDA-PEI) which conferred PTFE particles a uniform dispersion in aqueous medium. With increasing dopamine concentration in the reaction solution, dispersity of PTFE particles improved and the surface charges of particles changed from negative to positive due to an increase of surface coverage of PDA-PEI layers. Simplicity of the method here outlines an attractive route for surface modification of inert surfaces useful for large-scale applications.

Review on Interfacial Bonding Mechanism of Functional Polymer Coating on Glass in Atomistic Modeling Perspective

Atomistic modeling methods are successfully applied to understand interfacial interaction in nanoscale size and analyze adhesion mechanism in the organic-inorganic interface. In this paper, we review recent representative atomistic simulation works, focusing on the interfacial bonding, adhesion strength, and failure behavior between polymer film and silicate glass. The simulation works are described under two categories, namely non-bonded and bonded interaction. In the works for non-bonded interaction, three main interactions, namely van der Waals interaction, polar interaction, and hydrogen bonds, are investigated, and the contributions to interfacial adhesion energy are analyzed. It is revealed that the most dominant interaction for adhesion is hydrogen bonding, but flexibility of the polymer film and modes of adhesion measurement test do affect adhesion and failure behavior. In the case of bonded interactions, the mechanism of covalent silane bond formation through condensation and hydrolysis process is reviewed, and surface reactivity, molecular density, and adhesion properties are calculated with an example of silane functionalized polymer. Besides interfacial interactions, effects of external conditions, such as surface morphology of the glass substrate and relative humidity on the adhesion and failure behavior, are presented, and modeling techniques developed for building interfacial system and calculating adhesion strengths are briefly introduced.

Poly(ionic liquid)s Containing Alkoxy Chains and Bis(trifluoromethanesulfonyl)imide Anions as Highly Adhesive Materials

Adhesive materials have wide applications in diverse fields, but the development of a novel and multipurpose adhesive is a great challenge. This study demonstrates that conventional poly(ionic liquid)s (PILs) can be designed as highly efficient adhesives by simply introducing alkoxy moieties into the cationic backbone of PILs containing bis(trifluoromethanesulfonimide) (TFSI^- ) anions. The incorporated flexible alkoxy chain not only reduces the glass transition temperature of PILs but also endows these materials with strong hydrogen bonding interactions, which, together with the unique electrostatic interaction of the PILs, simultaneously contributes to a high cohesive energy and interfacial adhesive energy. Consequently, these alkoxy PILs are highly adhesive on various substrates such as glass, ceramic, stainless steel, aluminum, and polymers, in contrast to the nonadhesive behavior of conventional PILs. Photosensitive or electronically conductive composite adhesives are fabricated by virtue of the compatibility between ionic liquids and carbon nanotubes or silver nanofibers. Interestingly, the PIL-2-TFSI adhesive possesses a unique and reversible response to electric fields and achieves up to 35% improvement in adhesive strength.

Application of Floating TiO2 Photocatalyst for Methylene Blue Decomposition and Salmonella typhimurium Inactivation

The growing level of wastewater as well as pollution of freshwater by various bacteria are essential worldwide issues which have to be solved. In this contribution, nanocrystalline anatase TiO2 films deposited by magnetron sputtering on high-density polystyrene (HDPE) beads were applied as floating photocatalysts for Salmonella typhimurium bacterial inactivation in water for the first time. Additionally, the photocatalytic degradation of methylene blue dye in the presence of HDPE beads with TiO2 film under UV-B irradiation was investigated. The suitability to adopt such floating photocatalyst structures for practical applications was tested in cycling experiments. The detailed surface morphology, crystal structure, elemental mapping, surface chemical composition and bond analysis of deposited TiO2 films were investigated by scanning electron microscope, X-ray diffractometer and X-ray photoelectron spectroscope techniques. The bacterial viability as well as MB decomposition experiments showed promising results by demonstrating that 6% of bacterial colonies were formed after the first run and only about 1% after the next four runs, which is an appropriate outcome for practical applications. NPN uptake results showed that the permeability of the outer membrane was significantly increased as well.

A Strong Dual-Component Bioadhesive Based on Solventless Thiol-isocyanate Click Chemistry

Isocyanate is an efficient tissue anchor for engineering of strong bioadhesives. However, isocyanate-containing adhesives were seldom manufactured due to their requirement of water-free administration and time-consuming moisture-induced solidification. To address this issue, here, a solventless dual-component bioadhesive based on thiol-isocyanate cross-linking chemistry is reported. This dual-component bioadhesive consists of a hyperbranched polymer with thiol groups (HBPTE) and an isocyanate-modified polyethylene glycol (PEGNCO). HBPTE and PEGNCO are low-viscosity fluids at room temperature and hence could be used directly as adhesive components, in the absence of a catalyst and a solvent. The thiol-isocyanate click chemistry of components provides the HBPTE-PEGNCO mixture with a gelation time of 1.8-3 min, which makes it acceptable for practical applications. The abundance of isocyanate groups in the adhesive molecule provides strong bonding strength through formation of chemical linkages with reactive groups on the tissue. Moreover, in vitro and in vivo evaluations showed excellent biocompatibility of the HBPTE-PEGNCO adhesive. This dual-component bioadhesive based on solventless thiol-isocyanate click chemistry displayed a fast gelation time and excellent bonding performance, providing a pioneering idea for engineering isocyanate-containing bioadhesives.

Effect of Friction Stir Welding Techniques and Parameters on Polymers Joint Efficiency-A Critical Review

The objective of current work is to analyse the influence of different welding techniques and welding parameters on the morphology and mechanical strength of friction stir welds (FSW) in polymers, based on data collected in the literature. In the current work, only articles that provide data on the joint efficiency, or sufficient information to estimate it are considered. The process using conventional tool is presented and compared with new procedures developed for FSW of polymers, such as those using tools with heated stationary shoulder, preheating of the polymer or double-side passage of the tool. The influence of tool rotational speed (w), welding speed (v), tilt angle and geometry of the pin are discussed. This work focuses on the polymers most studied in the literature, polyethylene (PE) and polypropylene (PP). The use of external heating and tools with stationary shoulder proved to be of great importance in improving the surface finish, reducing defects, and increasing the mechanical strength of the welds. The increase in the w/v ratio increased the joint efficiency, especially when using conventional tools on PE. A trend was obtained for conventional FSW, but it was difficult to establish mathematical relationships, because of the variability of welding conditions.

Numerical Investigation of Deposition Characteristics of PLA on an ABS Plate Using a Material Extrusion Process

Three-dimensional prototypes and final products are commonly fabricated using the material extrusion (ME) process in additive manufacturing applications. However, these prototypes and products are limited to a single material using the ME process due to technical challenges. Deposition of plastic on another dissimilar plastic substrate requires proper control of printing temperature during an ME process due to differences in melting temperatures of dissimilar plastics. In this paper, deposition of PLA filament on an ABS substrate during an ME process is investigated using finite element analysis. A heat transfer finite element (FE) model for the extrusion process is proposed to estimate the parameters of the ME machine for the formulation of a heat flux model. The effects of printing temperature and the stand-off distance on temperature distributions are investigated using the proposed FE model for the extrusion process. The heat flux model is implemented in a proposed heat transfer FE model of single bead deposition of PLA on an ABS plate. From this FE model of deposition, temperature histories during the ME deposition process are estimated. The results of temperature histories are compared with experiments. Using the calibrated FE model, a proper heating temperature of ABS for deposition of PLA is evaluated.

Shear Bond Strength and Optical Properties of Resin Composite - Layered Provisional Poly(methyl methacrylate) (PMMA)

Aims:

To compare the shear bond strength between resin composite and Poly(methyl methacrylate) (PMMA) with various bonding protocols and to evaluate the optical properties of resin composite - layered provisional Poly(methyl methacrylate) (PMMA).

Materials and Methods:

Eighty cylindrical shape specimens were fabricated from self-polymerized provisional Poly(methyl methacrylate) (PMMA) and they were randomly divided into eight groups. Poly(methyl methacrylate) (PMMA) was mixed and bonded onto the specimens as a positive control group. Resin composite was bonded to MMA-wetted surface without bonding agent as a negative control group. All remaining groups were bonded to resin composite using different bonding agents (Scothbond Universal, Luxatemp glaze&bond, and HC Primer) with and without MMA wetting. Shear bond strength testing was performed using a universal testing machine. Various shades of 0.5 mm-thick resin composites were layered onto 1.5 mm-thick PMMA both light and dark shade, with the most effective bonding protocol. Color differences between resin composite and – layered provisional Poly(methyl methacrylate) (PMMA) were measured using Spectrophotometer.

Results:

Bonding resin composite onto Poly(methyl methacrylate) (PMMA) using luxatemp glaze, bond and HC Primer without methyl methacrylate wetting provided statistically significantly lower bond strength than those of the MMA-wetted Poly(methyl methacrylate) (PMMA) surface. The highest shear bond strength was achieved with the application of Scothbond Universal Adhesive regardless of MMA wetting. The colors of resin composite - layered provisional Poly(methyl methacrylate) (PMMA) were different from the original resin composite color with ΔE results greater than the acceptable threshold (>3.7).

Conclusion:

Resin composites were able to effectively bond to the MMA-wetted Poly(methyl methacrylate) (PMMA) surface with the application of a tested bonding agent. Layering Poly(methyl methacrylate) (PMMA) with 0.5 mm-thick resin composite could not modify the Poly(methyl methacrylate) (PMMA) shade to the original resin composite color.

Role of Chemical Functionality in the Adhesion of Aluminum and Isotactic Polypropylene

In the direct melt bonding of isotactic polypropylene (iPP) to aluminum (Al), the blending of a small amount of maleic anhydride-grafted PP (PPgMA) with iPP was found to induce a dramatic improvement of the strength of adhesion. The effect of blending PPgMA was, however, limited, maximizing at ∼20 wt % PPgMA. Incorporation of larger amounts of PPgMA reduced the strength of adhesion. We studied the mechanism of adhesion between Al and iPP by incorporating chemical functionality to the polymer side. The fracture surfaces produced by peeling off the interfaces were investigated by replicating the surface topographic features on a platinum thin film and analyzing them by scanning transmission electron microscopy (STEM) as well as by reconstructing three-dimensional (3D) surface structures with STEM tomography. The replica-STEM technique enabled us to visualize PP surface crystalline lamellar structures and their deformation upon the failure in 3D. We found that polymer/metal interfaces produced surface features in the failure that were similar to those associated with failure of entanglement-based polymer/polymer adhesion via chain pullout. A fractography study by replica-STEM suggested that the formation of a low-molecular-weight layer with low crystallinity at the interfacial region was responsible for the improvement of adhesion. The adhesion strength depended on the toughness of the "soft layer" and did not depend on the chemical bonding between PPgMA and Al. The interfacial chemical reaction between MA and the Al surface yielded PP with a grafted carboxylic acid (-COOH) group, which may have been excluded from the PP crystalline lamellae. We concluded that chemical bonding was not the primary reason for the improvement of adhesion, but it was necessary to induce the segregation of PPgMA in the interfacial region and the formation of the soft layer.

Design, Manufacture, and Performance Testing of Extrusion–Pultrusion Machine for Fiber-Reinforced Thermoplastic Pellet Production

This study aimed to develop an extrusion and pultrusion system for producing carbon fiber-filled thermoplastic pellets. The extruder delivers a plastic melt to an impregnation die in sufficient volume and is pulled out along with the fibers. The fibers pass in a sideways stretched condition through spreader pins attached in the melt pool, which can then be wetted optimally. The wetting effect was also improved by immersing fiber in a coupling agent solution at an elevated temperature before feeding to the extruder die. For machine performance testing, polypropylene was used as a matrix resin with the following parameters: a screw speed of 5 rpm, a die temperature of 210 °C, and a pulling speed of 56 mm/s. The pull-out test was conducted to assess the interfacial shear strength (IFSS) between fibers and matrix. Scanning electron microscopy (SEM) was applied to characterize the quality of fiber impregnation. SEM characterized a good bonding performance between carbon fiber and the matrix. The average IFSS of the results indicated a good resistance of fiber–matrix bonding against a pulling force. It proved that the combination of the extrusion–pultrusion system can produce high-quality filaments as a raw material of composite pellets.

Tissue Adhesives: From Research to Clinical Translation

Sutures, staples, clips and skin closure strips are used as the gold standard to close wounds after an injury. In spite of being the present standard of care, the utilization of these conventional methods is precarious amid complicated and sensitive surgeries such as vascular anastomosis, ocular surgeries, nerve repair, or due to the high-risk components included. Tissue adhesives function as an interface to connect the surfaces of wound edges and prevent them from separation. They are fluid or semi-fluid mixtures that can be easily used to seal any wound of any morphology - uniform or irregular. As such, they provide alternatives to new and novel platforms for wound closure methods. In this review, we offer a background on the improvement of distinctive tissue adhesives focusing on the chemistry of some of these products that have been a commercial success from the clinical application perspective. This review is aimed to provide a guide toward innovation of tissue bioadhesive materials and their associated biomedical applications.

Modeling and Performance Assessment of a Millimeter-Wave CVD Film-Graphene Based Antenna

The transfer of graphene onto dielectric substrates to form an antenna needs to be performed with care, as tears and cracks may appear on the samples transferred improperly. This paper studies a detailed method in estimating the performance of a millimeter-wave (MMW) antenna built using commercially-available graphene layers. In contrast to the widely studied monolayer graphene, the proposed antenna is produced based on the chemical vapor deposition (CVD) technique, which is to be secured onto the antenna substrate via adhesive material without removal of the transition metal substrate from the graphene layer sample. The antenna performance modeled using this technique indicated quite similar results to a model using ideal monolayer graphene in terms of gain, radiated power and radiation efficiency. Its performance benchmarked against a copper antenna operating at the same MMW frequency also indicated similar performance improvements as with another antenna designed using monolayer graphene.

UV-Cured Biodegradable Methacrylated Starch-Based Coatings

Promising UV-curable starch-based coatings were fabricated by utilizing methacrylated starch. The aqueous methacrylated starch solution was cast on a glass substrate, and UV-cured after drying. The efficiency of UV-curing process was monitored with gel percentage measurements. The thermal and mechanical properties of the fabricated UV-cured coatings were investigated through differential scanning calorimetry and tensile test and compared with the starch-based uncured casted coatings. A complete characterization of the surface properties was performed by means of pencil hardness, adhesion, solvent resistance, and surface tension measurements. The cross-linking by UV-curing significantly enhanced the mechanical and surface properties of the coating. The effect of UV-curing on the biodegradability of the coating was evaluated by following the enzymatic degradation by α-amylase by determining the amount of glucose and maltose released from the coatings. UV-cured methacrylated starch based coating with promising material and surface properties and retained biodegradation potential was demonstrated.

Changing compliance of poly(vinyl alcohol) tubular scaffold for vascular graft applications through modifying interlayer adhesion and crosslinking density

Poly(vinyl alcohol) (PVA) is a water-soluble polymer and forms a hydrogel that has been studied as a potential small-diameter (<6 mm) vascular graft implant. The PVA hydrogel crosslinked using sodium trimetaphosphate (STMP) has been shown to have many beneficial properties such as bioinert, low-thrombogenicity, and easy surface modification. Compared to conventional synthetic vascular graft materials, PVA has also shown to possess better mechanical properties; however, the compliance and other mechanical properties of PVA grafts are yet to be optimized compared to the native blood vessels. Mechanical compliance has been an important parameter to be studied for small-diameter vascular grafts, as compliance has been proposed to play an important role in intimal hyperplasia formation. PVA grafts are made using dip-casting a cylindrical mold into crosslinking solution. The number of dipping can be used to control the wall thickness of the resulting PVA grafts. In this study, we hypothesized that the number of dip layer and wall thickness, the chemical crosslinking, and interlayer adhesive strength could be important parameters in the fabrication process that would affect compliance. This work provides the relationship between the wall thickness, burst pressure, and compliance of PVA. Furthermore, our data showed that interlayer adhesion as well as chemical and physical crosslinking density can increase the compliance of PVA grafts.

Hyperbranched polymer with dynamic thiol-aldehyde crosslinking and its application as a self-healable bioadhesive

Bioadhesives crosslinked with dynamic bonds exhibit shear-thinning, self-healing, and on-demand detachment properties, but generally show a weak bonding performance due to their poor bulk strength. Obtaining a strong bioadhesive with reversible crosslinking remains a challenge. To address this issue, herein we engineered a dynamic thiol-aldehyde crosslinked solvent-free adhesive based on hyperbranched polymer. The adhesive was obtained by directly mixing a liquid hyperbranched polymer with thiol end groups (HBPTE) and benzaldehyde-terminated polyethylene glycol (PEGCHO) without any additional catalyst or solvent. The solvent-free strategy yielded a dense crosslinking structure with many aldehyde groups, so this HBPTE-PEGCHO adhesive can strongly bond to tissue and various non-biological substrates. In addition, the HBPTE-PEGCHO adhesive has self-healing and thermo-reversible bonding properties due to the dynamic thiol-aldehyde crosslinking matrix. In vivo wound healing experiments show that this HBPTE-PEGCHO adhesive is tissue-benign, suggesting it can be applied in clinical practice. Combining the hyperbranched polymer-based solvent-free strategy and dynamic thiol-aldehyde crosslinking chemistry provides a simple but effective way to engineer a multifunctional bioadhesive with the desired bonding performance.

Effects of Processing Variables of Extrusion-Pultrusion Method on the Impregnation Quality of Thermoplastic Composite Filaments

Carbon fibre-reinforced polypropylene composite filaments were fabricated via the extrusion–pultrusion method. One of the important factors influencing composites’ filament processability and structural properties is the impregnation quality, which can be represented by interfacial adhesion between the matrix and fibre. To improve the interfacial shear strength (IFSS) of the filament, four processing variables—melt temperature, pulling speed, number of pins in the impregnation die and fibre treatment—have been optimised using the Box–Behnken response surface methodology (RSM). Analysis of variance (ANOVA) was conducted to evaluate the linearity of the response surface models. Three levels were set for each independent variable. The melt temperature was varied at levels 190, 210 and 230 °C, while the pulling speed was set at three levels, namely, 40, 47 and 50 cm/min. The number of spreader pins was varied at 1, 2 and 3 pins, and there were three variations of the fibre treatment, namely, vinyltrimethoxysilane (VTMS), γ-aminopropyltriethoxy silane (APTS) and liquid nitrogen. Twenty-seven experimental runs were conducted, and a significant regression for the coefficient between the variables was obtained. The filament IFSS was measured by a customised pull-out test, and its surface morphology was characterised using a scanning electron microscope. ANOVA showed that fibre treatment significantly affected the IFSS due to their surface roughness, followed by pulling speed and melt temperature in quadratic order. Liquid nitrogen is recommended for carbon fibre treatment because of the high surface roughness, thereby providing a better matrix–fibre bonding effect. The results demonstrated that a melt temperature of 190 °C, pulling speed of 40 cm/min, three spreader pins and treatment of the fibre with liquid nitrogen afforded the optimum impregnation quality. It is important to keep a reasonable low processing temperature to obtain the geometrical stability of the product.

Polymer Infused Porous Surfaces for Robust, Thermally Conductive, Self-Healing Coatings for Dropwise Condensation

Hydrophobic coatings with low thermal resistance promise a significant enhancement in condensation heat transfer performance by promoting dropwise condensation in applications including power generation, water treatment, and thermal management of high-performance electronics. However, after nearly a century of research, coatings with adequate robustness remain elusive due to the extreme environments within many condensers and strict design requirements needed to achieve enhancement. In this work, we enable long-lasting condensation heat transfer enhancement via dropwise condensation by infusing a hydrophobic polymer, Teflon AF, into a porous nanostructured surface. This polymer infused porous surface (PIPS) uses the large surface area of the nanostructures to enhance polymer adhesion, while the nanostructures form a percolated network of high thermal conductivity material throughout the polymer and drastically reduce the thermal resistance of the composite. We demonstrate over 700% enhancement in the condensation of steam compared to an uncoated surface. This performance enhancement was sustained for more than 200 days without significant degradation. Furthermore, we show that the surfaces are self-repairing upon raising the temperature past the melting point of the polymer, allowing recovery of hydrophobicity and offering a level of durability more appropriate for industrial applications.

A Review on Anodizing of Aerospace Aluminum Alloys for Corrosion Protection

Aluminum alloys used for aerospace applications provide good strength to weight ratio at a reasonable cost but exhibit only limited corrosion resistance. Therefore, a durable and effective corrosion protection system is required to fulfil structural integrity. Typically, an aerospace corrosion protection system consists of a multi-layered scheme employing an anodic oxide with good barrier properties and a porous surface, a corrosion inhibited organic primer, and an organic topcoat. The present review covers published research on the anodic oxide protection layer principles and requirements for aerospace application, the effect of the anodizing process parameters, as well as the importance of process steps taking place before and after anodizing. Moreover, the challenges of chromic acid anodizing (CAA) substitution are discussed and tartaric-sulfuric acid anodizing (TSA) is especially highlighted among the environmentally friendly alternatives.

Evaluation of Durability of Transparent Graphene Electrodes Fabricated on Different Flexible Substrates for Chronic In Vivo Experiments

OBJECTIVE

To investigate chronic durability of transparent graphene electrodes fabricated on polyethylene terephthalate (PET) and SU-8 substrates for chronic in vivo studies.

METHODS

We perform systematic accelerated aging tests to understand the chronic reliability and failure modes of transparent graphene microelectrode arrays built on PET and SU-8 substrates. We employ graphene microelectrodes fabricated on PET substrate in chronic in vivo experiments with transgenic mice.

RESULTS

Our results show that graphene microelectrodes fabricated on PET substrate work reliably after 30 days accelerated aging test performed at 87 °C, equivalent to 960 days in vivo lifetime. We demonstrate stable chronic recordings of cortical potentials in multimodal imaging/recording experiments using transparent graphene microelectrodes fabricated on PET substrate. On the other hand, graphene microelectrode arrays built on SU-8 substrate exhibit extensive crack formation across microelectrode sites and wires after one to two weeks, resulting in total failure of recording capability for chronic studies.

CONCLUSION

PET shows superior reliability as a substrate for graphene microelectrode arrays for chronic in vivo experiments.

SIGNIFICANCE

Graphene is a unique neural interface material enabling cross-talk free integration of electrical and optical recording and stimulation techniques in the same experiment. To date, graphene-based microelectrode arrays have been demonstrated in various multi-modal acute experiments involving electrophysiological sensing or stimulation, optical imaging and optogenetics stimulation. Understanding chronic reliability of graphene-based transparent interfaces is very important to expand the use of this technology for long-term behavioral studies with animal models.