Many soft biomimetic synthetic adhesives, optimized to support macroscopic masses (∼kg), have been inspired by geckos, insects and other animals. Far less work has investigated bioinspired adhesion that is tuned to micro- and nano-scale sizes and forces. However, such adhesive forces are extremely important in the adhesion of micro- and nanoparticles to surfaces, relevant to a wide range of industrial and biological systems. Pollens, whose adhesion is critical to plant reproduction, are an evolutionary-optimized system for biomimicry to engineer tunable adhesion between particles and micro-patterned soft matter surfaces. In addition, the adhesion of pollen particles is relevant to topics as varied as pollinator ecology, transport of allergens, and atmospheric phenomena. We report the first observation of structurally-derived pressure-sensitive adhesion of a microparticle by using the sunflower pollen and stigma surfaces as a model. This strong, pressure-sensitive adhesion results from interlocking between the pollen's conical spines and the stigma's receptive papillae. Inspired by this behavior, we fabricated synthetic polymeric patterned surfaces that mimic the stigma surface's receptivity to pollen. These soft mimics allow the magnitude of the pressure-sensitive response to be tuned by adjusting the size and spacing of surface features. These results provide an important new insight for soft material adhesion based on bio-inspired principles, namely that ornamented microparticles and micro-patterned surfaces can be designed with complementarity that enable a tunable, pressure-sensitive adhesion on the microparticle size and length scale.

Sporopollenin, the polymer comprising the exine (outer solid shell) of pollen, is recognized as one of the most chemically and mechanically stable naturally occurring organic substances. The elastic modulus of sporopollenin is of great importance to understanding the adhesion, transport and protective functions of pollen grains. In addition, this fundamental mechanical property is of significant interest in using pollen exine as a material for drug delivery, reinforcing fillers, sensors and adhesives. Yet, the literature reports of the elastic modulus of sporopollenin are very limited. We provide the first report of the elastic modulus of sporopollenin from direct indentation of pollen particles of three plant species: ragweed (), pecan () and Kentucky bluegrass (). The modulus was determined with atomic force microscopy by using direct nanomechanical mapping of the pollen shell surface. The moduli were atypically high for non-crystalline organic biomaterials, with average values of 16 ± 2.5 GPa (ragweed), 9.5 ± 2.3 GPa (pecan) and 16 ± 4.0 GPa (Kentucky bluegrass). The amorphous pollen exine has a modulus exceeding known non-crystalline biomaterials, such as lignin (6.7 GPa) and actin (1.8 GPa). In addition to native pollen, we have investigated the effects of exposure to a common preparative base-acid chemical treatment and elevated humidity on the modulus. Base-acid treatment reduced the ragweed modulus by up to 58% and water vapour exposure at 90% relative humidity reduced the modulus by 54% (pecan) and 72% (Kentucky bluegrass). These results are in agreement with recently published estimates of the modulus of base-acid-treated ragweed pollen of 8 GPa from fitting to mechanical properties of ragweed pollen-epoxy composites.

We report a two-phase adhesive fluid recovered from pollen, which displays remarkable rate tunability and humidity stabilization at microscopic and macroscopic scales. These natural materials provide a previously-unknown model for bioinspired humidity-stable and dynamically-tunable adhesive materials. In particular, two immiscible liquid phases are identified in bioadhesive fluid extracted from dandelion pollen taken from honey bees: a sugary adhesive aqueous phase similar to bee nectar and an oily phase consistent with plant pollenkitt. Here we show that the aqueous phase exhibits a rate-dependent capillary adhesion attributed to hydrodynamic forces above a critical separation rate. However, the performance of this adhesive phase alone is very sensitive to humidity due to water loss or uptake. Interestingly, the oily phase contributes scarcely to the wet adhesion. Rather, it spreads over the aqueous phase and functions as a barrier to water vapor that tempers the effects of humidity changes and stabilizes the capillary adhesion.

Cellulose nanocrystals (CNCs) derived from renewable plant-based materials exhibit strong potential for improving properties of polymers by their dispersal in the polymer matrix as a composite phase. However, the hydrophilicity and low thermal stability of CNCs lead to compromised particle dispersibility in common polymers and limit the processing conditions of polymer–CNC composites, respectively. One route that has been explored is the modification of CNCs to alter surface chemistry. Acrylic materials are used in a broad class of polymers and copolymers with wide commercial applications. Yet, the available methods for adding groups that react with acrylics to enhance dispersion are quite limited. In this work, a versatile chemical modification route is described that introduces acryloyl functional groups on CNCs that can in turn be polymerized in subsequent steps to create acrylic–CNC composites. The hydroxyl group on CNC surfaces was reacted with the isocyanate moiety on 2-isocyanatoethyl methacrylate (IEM), a bifunctional molecule possessing both the isocyanate group and acryloyl group. The resulting modified CNCs (mCNCs) showed enhanced hydrophobicity and dispersibility in organic solvent relative to unmodified CNCs. Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy, solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, and elemental analysis verified the surface modification and allowed an estimation of the degree of modification as high as 0.4 (26.7% surface hydroxyl substitution CNC). The modified CNCs were copolymerized with methyl methacrylate, and the composites had improved dispersion relative to composites with unmodified CNCs and enhanced (104%) tensile strength at 2 wt % CNC when compared to the neat poly­(methyl methacrylate) (PMMA), indicating a benefit of the reactive acryloyl groups added to the CNC surface. Overall, the modification strategy was successful in functionalizing CNCs, opening possibilities for their use in organic media and matrices.