Ubiquitous Distribution of Salts and Proteins in Spider Glue Enhances Spider Silk Adhesion

Modern orb-weaving spiders use micron-sized glue droplets on their viscid silk to retain prey in webs. A combination of low molecular weight salts and proteins makes the glue viscoelastic and humidity responsive in a way not easily achieved by synthetic adhesives. Optically, the glue droplet shows a heterogeneous structure, but the spatial arrangement of its chemical components is poorly understood. Here, we use optical and confocal Raman microscopy to show that salts and proteins are present ubiquitously throughout the droplet. The distribution of adhesive proteins in the peripheral region explains the superior prey capture performance of orb webs as it enables the entire surface area of the glue droplet to act as a site for prey capture. The presence of salts throughout the droplet explains the recent Solid-State NMR results that show salts directly facilitate protein mobility. Understanding the function of individual glue components and the role of the droplet\u27s macro-structure can help in designing better synthetic adhesives for humid environments

Role of Hygroscopic Low Molecular Mass Compounds in Humidity Responsive Adhesion of Spider’s Capture Silk

The aggregate glue in spider webs is composed of hygroscopic low molecular mass compounds (LMMCs), glycoproteins and water. The LMMCs absorb atmospheric water and solvate the glycoproteins to spread and adhere to flying insects upon contact. The glue viscosity varies with humidity and there is an optimum range of viscosity where the adhesion is maximum. LMMCs composition and the humidity at which glue viscosity is optimized vary greatly among spider species. These findings suggest that spiders adapt to forage in diverse habitats by “tuning” LMMCs composition or how LMMCs interact with glycoproteins to control water uptake and adhesion. To test these hypotheses, we analyzed the LMMCs for spiders from diverse habitats and performed water uptake studies on intact glue droplets, isolated glue constituents, and synthetic LMMCs. Even though glue droplets showed differences in water uptake among spider species, we found no differences among species in hygroscopicity of natural or synthetic LMMCs mixtures. This demonstrates that LMMCs composition alone is insufficient to explain interspecific differences in water uptake of spider glues and instead support the hypothesis that an interaction between LMMCs and glycoproteins mediate differences in water uptake and adhesion

Spiders Tune Glue Viscosity to Maximize Adhesion

Adhesion in humid conditions is a fundamental challenge to both natural and synthetic adhesives. Yet, glue from most spider species becomes stickier as humidity increases. We find the adhesion of spider glue, from five diverse spider species, maximizes at very different humidities that matches their foraging habitats. By using high-speed imaging and spreading power law, we find that the glue viscosity varies over 5 orders of magnitude with humidity for each species, yet the viscosity at maximal adhesion for each species is nearly identical, 10⁵–10⁶ cP. Many natural systems take advantage of viscosity to improve functional response, but spider glue’s humidity responsiveness is a novel adaptation that makes the glue stickiest in each species’ preferred habitat. This tuning is achieved by a combination of proteins and hygroscopic organic salts that determines water uptake in the glue. We therefore anticipate that manipulation of polymer–salts interaction to control viscosity can provide a simple mechanism to design humidity responsive smart adhesives

Quantitative assessment of visual microscopy as a tool for microplastic research: Recommendations for improving methods and reporting.

Microscopy is often the first step in microplastic analysis and is generally followed by spectroscopy to confirm material type. The value of microscopy lies in its ability to provide count, size, color, and morphological information to inform toxicity and source apportionment. To assess the accuracy and precision of microscopy, we conducted a method evaluation study. Twenty-two laboratories from six countries were provided three blind spiked clean water samples and asked to follow a standard operating procedure. The samples contained a known number of microplastics with different morphologies (fiber, fragment, sphere), colors (clear, white, green, blue, red, and orange), polymer types (PE, PS, PVC, and PET), and sizes (ranging from roughly 3-2000 μm), and natural materials (natural hair, fibers, and shells; 100-7000 μm) that could be mistaken for microplastics (i.e., false positives). Particle recovery was poor for the smallest size fraction (3-20 μm). Average recovery (±StDev) for all reported particles >50 μm was 94.5 ± 56.3%. After quality checks, recovery for >50 μm spiked particles was 51.3 ± 21.7%. Recovery varied based on morphology and color, with poorest recovery for fibers and the largest deviations for clear and white particles. Experience mattered; less experienced laboratories tended to report higher concentration and had a higher variance among replicates. Participants identified opportunity for increased accuracy and precision through training, improved color and morphology keys, and method alterations relevant to size fractionation. The resulting data informs future work, constraining and highlighting the value of microscopy for microplastics