Polydimethylsiloxane based dry adhesives produced using a replica molding technique.

Dry adhesives have gained considerable interest due to their applications in a wide variety of areas. This study used a replica molding technique to produce micron-sized pillars on the surface of polydimethylsiloxane (PDMS) and investigated their dry adhesion behaviour. Shear adhesion for the produced samples is measured using a tensile testing machine. For this purpose, the sample was initially brought into contact with a glass slide. Following this, the shear adhesion was determined by measuring the shear stress required to slide the sample along the glass slide. Peel adhesion of the samples was measured using an in-house designed and built peel fixture. The force required to peel the sample from the surface of the fixture was measured to determine the peel strength. The shear adhesion and peel tests were also conducted on neat PDMS to determine the effect of surface micropillars on the adhesion performance of the samples. The results show that the shear adhesion strength was 0.12 N cm-2, while the shear adhesion strength of neat PDMS was determined to be 0.02 N cm-2. Similarly, the peel strength of the samples was recorded to be 0.15 N cm-2 compared to 0.05 N cm-2 recorded for neat PDMS.

Recent Progress on the Use of Stimulus-Responsive Materials for Dry Adhesive Applications.

Stimulus-responsive dry adhesives, inspired by the adhesive mechanisms displayed by the fibrillar structures present on the feet of geckos, have emerged as a promising area of research for applications such as robotic grippers and climbing robots. These stimulus-responsive dry adhesives exhibit some unique capabilities, as their ability to adhere to and detach from surfaces can be controlled with the help of an external stimulus. For example, studies have developed magnetic field-responsive dry adhesives and show that the adhesion of these materials can be turned on and off by controlling the applied magnetic field. Light-responsive adhesives have also been developed and shown to reverse their adhesion using infrared light as the stimulus. Such materials show tremendous promise in pick-and-place systems for handling delicate objects and microelectronic products. The focus of this article is to review the stimulus-responsive materials that have been used to develop dry adhesives. The mechanisms adopted by these stimulus-responsive materials to switch their adhesion are discussed. Applications of stimulus-responsive dry adhesives are presented, and last, the future perspective of these materials is discussed.

Evaluation of Physicochemical Properties of a Hydroxyapatite Polymer Nanocomposite for Use in Fused Filament Fabrication.

Over the last decade, there has been an increasing interest in the use of bioceramics for biomedical purposes. Bioceramics, specifically those made of calcium phosphate, are commonly used in dental and orthopaedic applications. In this context, hydroxyapatite (HA) is considered a viable option for hard tissue engineering applications given its compositional similarity to bioapatite. However, owing to their poor mechanobiology and biodegradability, traditional HA-based composites have limited utilisation possibilities in bone, cartilage and dental applications. Therefore, the efficiency of nano HA (nHA) has been explored to address these limitations. nHA has shown excellent remineralising effects on initial enamel lesions and is widely used as an additive for improving existing dental materials. Furthermore, three-dimensional printing (3DP) or fused deposition modelling that can be used for creating dental and hard tissue scaffolds tailored to each patient's specific anatomy has attracted considerable interest. However, the materials used for producing hard tissue with 3DP are still limited. Therefore, the current study aimed to develop a hybrid polymer nanocomposite composed of nHA, nanoclay (NC) and polylactic acid (PLA) that was suitable for 3DP. The nHA polymer nanocomposites were extruded into filaments and their physiochemical properties were evaluated. The results showed that the addition of nHA and NC to the PLA matrix significantly increased the water absorption and contact angle. In addition, the hardness increased from 1.04 to 1.25 times with the incorporation of nHA. In sum, the nHA-NC-reinforced PLA could be used as 3DP filaments to generate bone and dental scaffolds, and further studies are needed on the biocompatibility of this material.

Rose Petal Mimetic Surfaces with Antibacterial Properties Produced Using Nanoimprint Lithography.

In this study, we produced bioinspired micro/nanotopography on the surface of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) films and demonstrated that these films display antibacterial properties. In the first step, structures that are found on the surface of a rose petal were copied on the surface of PVDF-HFP films. Following this, a hydrothermal method was used to grow ZnO nanostructures on top of this rose petal mimetic surface. The antibacterial behavior of the fabricated sample was demonstrated against Gram-positive Streptococcus agalactiae (S. agalactiae) and Gram-negative Escherichia coli (E. coli) as model bacteria. For comparison purposes, the antibacterial behavior of a neat PVDF-HFP film was also investigated against both bacterial species. The results show that the presence of rose petal mimetic structures on PVDF-HFP helped the material to display improved antibacterial performance against both S. agalactiae and E. coli compared to the antibacterial performance of neat PVDF-HFP. The antibacterial performance was further enhanced for samples that had both rose petal mimetic topography and ZnO nanostructures on the surface.

Biomimetic Rose Petal Structures Obtained Using UV-Nanoimprint Lithography.

This study aims to produce a hydrophobic polymer film by mimicking the hierarchical micro/nanostructures found on the surface of rose petals. A simple and two-step UV-based nanoimprint lithography was used to copy rose petal structures on the surface of a polyurethane acrylate (PUA) film. In the first step, the rose petal was used as a template, and its negative replica was fabricated on a commercial UV-curable polymer film. Following this, the negative replica was used as a stamp to produce rose petal mimetic structures on UV curable PUA film. The presence of these structures on PUA influenced the wettability behavior of PUA. Introducing the rose petal mimetic structures led the inherently hydrophilic material to display highly hydrophobic behavior. The neat PUA film showed a contact angle of 65°, while the PUA film with rose petal mimetic structures showed a contact angle of 138°. Similar to natural materials, PUA with rose petal mimetic structures also displayed the water pinning effect. The water droplet was shown to have adhered to the surface of PUA even when the surface was turned upside down.

Modeling Impact Mechanics of 3D Helicoidally Architected Polymer Composites Enabled by Additive Manufacturing for Lightweight Silicon Photovoltaics Technology.

When silicon solar cells are used in the novel lightweight photovoltaic (PV) modules using a sandwich design with polycarbonate sheets on both the front and back sides of the cells, they are much more prone to impact loading, which may be prevalent in four-season countries during wintertime. Yet, the lightweight PV modules have recently become an increasingly important development, especially for certain segments of the renewable energy markets all over the world-such as exhibition halls, factories, supermarkets, farms, etc.-including in countries with harsh hailstorms during winter. Even in the standard PV module design using glass as the front sheet, the silicon cells inside remain fragile and may be prone to impact loading. This impact loading has been widely known to lead to cracks in the silicon solar cells that over an extended period of time may significantly degrade performance (output power). In our group's previous work, a 3D helicoidally architected fiber-based polymer composite (enabled by an electrospinning-based additive manufacturing methodology) was found to exhibit excellent impact resistance-absorbing much of the energy from the impact load-such that the silicon solar cells encapsulated on both sides by this material breaks only at significantly higher impact load/energy, compared to when a standard, commercial PV encapsulant material was used. In the present study, we aim to use numerical simulation and modeling to enhance our understanding of the stress distribution and evolution during impact loading on such helicoidally arranged fiber-based composite materials, and thus the damage evolution and mechanisms. This could further aid the implementation of the lightweight PV technology for the unique market needs, especially in countries with extreme winter seasons.

Characterising Penetrometer Tip Contact during Concrete Condition Assessment.

Concrete condition-assessing penetrometers need to be able to distinguish between making contact with a hard (concrete) surface as opposed to a semi-solid (corroded concrete) surface. We investigated whether different shaped tips of a cylindrical penetrometer were better than others at maintaining contact with concrete and not slipping. We designed a range of simple symmetric tip shapes, controlled by a single superellipse parameter. We performed a finite element analysis of these parametric models in SolidWorks before machining in stainless steel. We tested our penetrometer tips on a concrete paver cut to four angles at 20∘ increments. The results indicate that the squircle-shaped tip had the least slippage when used for concrete condition assessment.

Impact-Resistant and Tough 3D Helicoidally Architected Polymer Composites Enabling Next-Generation Lightweight Silicon Photovoltaics Module Design and Technology.

Lightweight photovoltaics (PV) modules are important for certain segments of the renewable energy markets-such as exhibition halls, factories, supermarkets, farms, etc. However, lightweight silicon-based PV modules have their own set of technical challenges or concerns. One of them, which is the subject of this paper, is the lack of impact resistance, especially against hailstorms in deep winter in countries with four seasons. Even if the front sheet can be made sufficiently strong and impact-resistant, the silicon cells inside remain fragile and very prone to impact loading. This leads to cracks that significantly degrade performance (output power) over time. A 3D helicoidally architected fiber-based polymer composite has recently been found to exhibit excellent impact resistance, inspired by the multi-hierarchical internal structures of the mantis shrimp's dactyl clubs. In previous work, our group demonstrated that via electrospinning-based additive manufacturing methodologies, weak polymer material constituents could be made to exhibit significantly improved toughness and impact properties. In this study, we demonstrate the use of 3D architected fiber-based polymer composites to protect the silicon solar cells by absorbing impact energy. The absorbed energy is equivalent to the energy that would impact the solar cells during hailstorms. We have shown that silicon cells placed under such 3D architected polymer layers break at substantially higher impact load/energy (compared to those placed under standard PV encapsulation polymer material). This could lead to the development of novel PV encapsulant materials for the next generation of lightweight PV modules and technology with excellent impact resistance.

Helicoidally Arranged Polyacrylonitrile Fiber-Reinforced Strong and Impact-Resistant Thin Polyvinyl Alcohol Film Enabled by Electrospinning-Based Additive Manufacturing.

In this study, we demonstrate the use of parallel plate far field electrospinning (pp-FFES) based manufacturing system for the fabrication of polyacrylonitrile (PAN) fiber reinforced polyvinyl alcohol (PVA) strong polymer thin films (PVA SPTF). Parallel plate far field electrospinning (also known as the gap electrospinning) is generally used to produce uniaxially aligned fibers between the two parallel collector plates. In the first step, a disc containing PVA/H2O solution/bath (matrix material) was placed in between the two parallel plate collectors. Next, a layer of uniaxially aligned sub-micron PAN fibers (filler material) produced by pp-FFES was directly collected/embedded in the PVA/H2O solution by bringing the fibers in contact with the matrix. Next, the disc containing the matrix solution was rotated at 45∘ angular offset and then the next layer of the uniaxial fibers was collected/stacked on top of the previous layer with now 45∘ rotation between the two layers. This process was continued progressively by stacking the layers of uniaxially aligned arrays of fibers at 45∘ angular offsets, until a periodic pattern was achieved. In total, 13 such layers were laid within the matrix solution to make a helicoidal geometry with three pitches. The results demonstrate that embedding the helicoidal PAN fibers within the PVA enables efficient load transfer during high rate loading such as impact. The fabricated PVA strong polymer thin films with helicoidally arranged PAN fiber reinforcement (PVA SPTF-HA) show specific tensile strength 5 MPa · cm3· g-1 and can sustain specific impact energy (8 ± 0.9) mJ · cm3· g-1, which is superior to that of the pure PVA thin film (PVA TF) and PVA SPTF with randomly oriented PAN fiber reinforcement (PVA SPTF-RO). The novel fabrication methodology enables the further capability to produce even further smaller fibers (sub-micron down to even nanometer scales) and by the virtue of its layer-by-layer processing (in the manner of an additive manufacturing methodology) allowing further modulation of interfacial and inter-fiber adherence with the matrix materials. These parameters allow greater control and tunability of impact performances of the synthetic materials for various applications from army combat wear to sports and biomedical/wearable applications.

Tensile Properties of Composite Reinforced with Three-Dimensional Printed Fibers.

This study used melt-electrospinning writing to fabricate three-dimensional fiber constructs by embedding them in a polyvinyl alcohol (PVA) matrix to obtain thin composite films. Fourier transform infrared spectroscopy (FTIR) and dynamic scanning calorimetry (DSC) were used to demonstrate an interaction between the polycaprolactone (PCL) fibrous phase and the PVA matrix phase. Following this, the mechanical deformation behavior of the composite was investigated, and the effect of reinforcement with three-dimensional fibrous constructs was illustrated. The specific strength of the composite was found to be five times higher than the specific strength of the neat PVA matrix. Additionally, the specific toughness of the composite was determined to be roughly four times higher than the specific toughness determined for the neat PVA matrix. These results demonstrate the potential of using melt-electrospinning writing for producing three-dimensional fibrous constructs for composite reinforcement purposes.

Emerging Developments in the Use of Electrospun Fibers and Membranes for Protective Clothing Applications.

There has been increased interest to develop protective fabrics and clothing for protecting the wearer from hazards such as chemical, biological, heat, UV, pollutants etc. Protective fabrics have been conventionally developed using a wide variety of techniques. However, these conventional protective fabrics lack breathability. For example, conventional protective fabrics offer good protection against water but have limited ability in removing the water vapor and moisture. Fibers and membranes fabricated using electrospinning have demonstrated tremendous potential to develop protective fabrics and clothing. These fabrics based on electrospun fibers and membranes have the potential to provide thermal comfort to the wearer and protect the wearer from wide variety of environmental hazards. This review highlights the emerging applications of electrospinning for developing such breathable and protective fabrics.

Inner Profile Measurement for Pipes Using Penetration Testing.

Penetration testing has been used to measure material properties for over fifty years. Currently, it is under-utilised as a contemporary scientific and engineering tool for investigating the condition of pipes whose inner surface has been exposed to chemical attack. We describe the design, development and calibration of a portable probe which uses a penetrative strain gauge load cell to measure where the semi-solid surface starts and stops within a pipe. We also describe the results of field tests of the probe in concrete sewers, affected by internal corrosion, where the probe proved to be a fast and reliable method for collecting pipe profile information. The results indicate significant benefit in the use of penetrometers to perform concrete sewer condition assessment.

Electrospun Silver Coated Polyacrylonitrile Membranes for Water Filtration Applications.

The scarcity of drinking water and the contamination of water sources in underdeveloped countries are serious problems that require immediate low-tech and low-cost solutions. In this study, we fabricated polyacrylonitrile (PAN) porous membranes coated with silver nanoparticles (AgNP) and demonstrated their use for water filtration and water treatment applications. The membranes were prepared by electrospinning a PAN solution and treating in a hydroxylamine (NH2OH) aqueous solution to form ⁻C(NH2)N⁻OH groups that were used for functionalization (Ag⁺ ions) of the membrane. The coordinated silver ions were then converted to silver nanoparticles. The microstructure of the membrane, water permeability, antimicrobial effect (using Escherichia coli), and particulate filtration capabilities were studied. This study verified that the membrane demonstrated a 100% reduction for Gram-negative bacteria with an effective filtration rate of 8.0 mL/cm² min. Furthermore, the membrane was able to eliminate 60% of latex beads as small as 50 nm and over 80% of the 2 µm beads via gravity filtration. This study demonstrated that PAN⁻AgNP membranes can be employed as antimicrobial membranes for the filtration of water in underdeveloped countries.

Gecko-Inspired Dry Adhesive Based on Micro-Nanoscale Hierarchical Arrays for Application in Climbing Devices.

The unusual ability of geckos to climb vertical walls underlies a unique combination of a hierarchical structural design and a stiffer material composition. While a dense array of microscopic hierarchical structures enables the gecko toe pads to adhere to various surfaces, a stiffer material (ß-keratin) composition enables them to maintain reliable adhesion over innumerable cycles. This unique strategy has been seldom implemented in engineered dry adhesives because fabrication of high-aspect-ratio hierarchical structures using a stiffer polymer is challenging. Herein, we report the fabrication of high-aspect-ratio hierarchical arrays on flexible polycarbonate sheets (stiffness comparable to that of ß-keratin) by a sacrificial-layer-mediated nanoimprinting technique. Dry-adhesive films comprising the hierarchical arrays showed a formidable shear adhesion of 11.91 ± 0.43 N/cm2. Cyclic adhesion tests also showed that the shear adhesion of the adhesive films reduced by only about 20% after 50 cycles and remained nearly constant until about 200 cycles. Most importantly, the high-aspect-ratio hierarchical arrays were integrated onto the feet of a miniature robot and the locomotion on a 30° inclined surface was demonstrated.

Measuring the Pull-Off Force of an Individual Fiber Using a Novel Picoindenter/Scanning Electron Microscope Technique.

We employed a novel picoindenter (PI)/scanning electron microscopy (SEM) technique to measure the pull-off force of an individual electrospun poly(vinylidene fluoride) (PVDF) fibers. Individual fibers were deposited over a channel in a custom-designed silicon substrate, which was then attached to a picoindenter. The picoindenter was then positioned firmly on the sample stage of the SEM. The picoindenter tip laterally pushed individual fibers to measure the force required to detach it from the surface of substrate. SEM was used to visualize and document the process. The measured pull-off force ranged between 5.8 ± 0.2 µN to ~17.8 ± 0.2 µN for individual fibers with average diameter ranging from 0.8 to 2.3 µm. Thus, this study, a first of its kind, demonstrates the use of a picoindenter to measure the pull-off force of a single micro/nanofiber.

Thermoresponsive Cellulose Acetate-Poly(N-isopropylacrylamide) Core-Shell Fibers for Controlled Capture and Release of Moisture.

In this study, we used core-shell electrospinning to fabricate cellulose acetate-poly(N-isopropylacrylamide) (CA-PNIPAM) fibrous membranes and demonstrated the ability of these fibers to capture water from a high humid atmosphere and release it when thermally stimulated. The wettability of the fibers was controlled by using thermoresponsive PNIPAM as the shell layer. Scanning electron and fluorescence microscopes are used to investigate the microstructure of the fibers and confirm the presence of the core and shell phases within the fibers. The moisture capturing and releasing ability of these core-shell CA-PNIPAM fibers was compared with those of the neat CA and neat PNIPAM fibers at room temperature as well as at an elevated temperature. At room temperature, the CA-PNIPAM core-shell fibers are shown to have the maximum moisture uptake capacity among the three samples. The external temperature variations which trigger the moisture response behavior of these CA-PNIPAM fibers fall within the range of typical day and night cycles of deserts, demonstrating the potential use of these fibers for water harvesting applications.

Cellulose Acetate-Poly(N-isopropylacrylamide)-Based Functional Surfaces with Temperature-Triggered Switchable Wettability.

Temperature-triggered switchable nanofibrous membranes are successfully fabricated from a mixture of cellulose acetate (CA) and poly(N-isopropylacrylamide) (PNIPAM) by employing a single-step direct electrospinning process. These hybrid CA-PNIPAM membranes demonstrate the ability to switch between two wetting states viz. superhydrophilic to highly hydrophobic states upon increasing the temperature. At room temperature (23 °C) CA-PNIPAM nanofibrous membranes exhibit superhydrophilicity, while at elevated temperature (40 °C) the membranes demonstrate hydrophobicity with a static water contact angle greater than 130°. Furthermore, the results here demonstrate that the degree of hydrophobicity of the membranes can be controlled by adjusting the ratio of PNIPAM in the CA-PNIPAM mixture.

Bio-inspired electrospun micro/nanofibers with special wettability.

Inspired by the extreme wetting states displayed by the natural materials, various techniques have been widely investigated to fabricate superhydrophobic and superhydrophilic surfaces. Electrospinning has gained huge amount of interest as fibers with suitable combination of surface chemistry and surface roughness can be easily obtained. This study provides a comprehensive overview of the progress that has been made on electrospun fibers that display superhydrophobicity, superhydrophilicity or a combination of both. The article discusses various modification techniques that can be implemented to obtain fibers with surface heterogeneity for improving its hydrophobicity or hydrophilicity. Both nanometer size of the fibers and secondary nanoscale structures ensure that the fibers have suitable surface topography to exhibit extreme wetting states. Additionally, for the first time, we critically review and identify the role of intrinsic structures such as crystallinity and chain orientation on the wettability of the fibers. We highlight some new emerging application areas that are being explored using superhydrophobic and superhydrophilic fibers. Further, methods for fabricating smart materials with special wettability are also discussed. Such fibers with special wettability show tremendous promise for water harvesting, unidirectional water collection and oil-water filtration applications.

Use of facile mechanochemical method to functionalize carbon nanofibers with nanostructured polyaniline and their electrochemical capacitance.

A facile approach to functionalize carbon nanofibers [CNFs] with nanostructured polyaniline was developed via in situ mechanochemical polymerization of polyaniline in the presence of chemically treated CNFs. The nanostructured polyaniline grafting on the CNF was mainly in a form of branched nanofibers as well as rough nanolayers. The good dispersibility and processability of the hybrid nanocomposite could be attributed to its overall nanostructure which enhanced its accessibility to the electrolyte. The mechanochemical oxidation polymerization was believed to be related to the strong Lewis acid characteristic of FeCl3 and the Lewis base characteristic of aniline. The growth mechanism of the hierarchical structured nanofibers was also discussed. After functionalization with the nanostructured polyaniline, the hybrid polyaniline/CNF composite showed an enhanced specific capacitance, which might be related to its hierarchical nanostructure and the interaction between the aromatic polyaniline molecules and the CNFs.

Self-assembly of graphene onto electrospun polyamide 66 nanofibers as transparent conductive thin films.

A simple method was developed to assemble graphite oxide (GO) densely onto electrospun (ES) polyamide 66 (PA66) nanofibrous membranes, used as a guide for the deposition of graphene nanosheet (GNS) conductive networks for preparing transparent conductive thin film (TCF). The main advantage of this technique by comparison with previous methods is that graphene does not form a uniform coating, but a percolated conductive network, when guided by PA66 nanofiber templates. A low surface coverage of the transparent substrate by GNS resulted in high transmittance. Polyvinylpyrrolidone-stabilized GO (PVP-GO) was prepared as a modifier for improving the adsorption to the nanofibers. The resulting PVP-GO material could adsorb well on PA66 nanofibers due to stronger hydrogen bonds. Hence, a lower sufficient concentration of PVP-GO (0.050 wt%) solution was required than that for GO solution (0.100 wt%) to fabricate a complete conductive path through a possible enriched adsorption process. For TCF applications, a reduction step is essential because as-deposited GO is non-conductive. In this work, we reduced GO to GNS by a combination of chemical reduction and thermal annealing. The TCF optical transmittance also could be improved after thermal annealing at 350 °C above the PA66 melting point. Light scattering by PA66 nanofibers was found as the main cause of reduced transmittance. A fused film, obtained after electrospinning PA66 solution for 120 s, and immersing in 0.050 wt% PVP-GO solution, exhibits a surface resistance of 8.6 × 10³ Ω/square, while maintaining 88% light transmittance.