Adhesives

Phenol–resorcinol–formaldehyde (PRF) is a cold-setting wood adhesive whose cured form is intrinsically brittle and whose bonding effectiveness deteriorates with storage age due to the loss of reactive methylol groups. This study investigated the potential of nanocellulose (NC) reinforcement to enhance toughness and offset aging-related losses. Before the hardener was added, three NC types—cellulose nanocrystals (CNC) and bleached and unbleached cellulose nanofibers (BNF, UBNF)—were homogenized into fresh (NP) and 4-year-aged (OP) PRF resin at one and three weight percent. The resin was then cast and gradually post-cured. Differential scanning calorimetry and rheometry were used to track curing activity, and flexural characteristics were assessed on cured specimens. The inclusion of NC increased the temperature at which gelation began, prolonging the working time of the resin. In comparison to neat PRF, CNF (BNF/UBNF) integration enhanced flexural strength by up to 39%, while CNC addition increased flexural modulus by up to 57%. Fibril aggregation was responsible for the 12–20% decrease in BNF/UBNF strength at three weight percent loading. Aged PRF was regularly outperformed by fresh PRF composites. These findings support the use of NC reinforcement in wood–PRF composites for extrusion-based additive manufacturing by demonstrating that it can toughen PRF and restore characteristics lost due to resin aging.

Background: This study evaluated the bonding performance of four universal adhesive systems applied to sound dentin (SD) and artificially caries-affected dentin subjected to biomimetic remineralization with casein phosphopeptide–amorphous calcium phosphate (CPP–ACP), before and after artificial aging. Methods: One hundred and four human molars were allocated according to dentin substrate (SD or biomimetically remineralized [BRD]), adhesive system (FuturaBond^® M+, Scotchbond™ Universal, Prime&Bond Active^®, and One Coat7 Universal^®), and aging (24 h or 10,000 thermocycles). Artificially caries-affected dentin was produced by pH cycling and subsequently remineralized with CPP–ACP. Adhesive interface morphology was assessed by FEG-ESEM, while shear bond strength (SBS) and failure modes were also evaluated. Statistical significance was set at p ≤ 0.05. Results: Adhesive layer thickness was significantly affected by the adhesive system, dentin substrate, and aging (p < 0.001), with a significant adhesive × substrate interaction (p < 0.001). Resin tag length was affected by the adhesive system (p = 0.03), substrate (p < 0.001), and aging (p < 0.001), with significant adhesive × substrate (p = 0.03) and adhesive × substrate × aging interactions (p = 0.001). SBS was significantly higher on SD than on BRD (p < 0.001) and decreased after thermocycling (p < 0.001). Conclusions: Universal adhesives exhibited substrate- and aging-dependent behaviour. BRD showed lower SBS than SD, while morphological differences did not consistently correspond to higher bond strength. Further studies including untreated artificially caries-affected dentin are needed to determine the specific contribution of CPP–ACP remineralization.

Biological adhesion systems represent some of the most structurally and functionally versatile interfacial solutions in nature, enabling organisms to attach, locomote, and manipulate surfaces across environments ranging from dry rock faces to turbulent marine substrates. This review surveys the structural and mechanistic diversity of biological adhesion, organizing known systems into two principal categories: dry and wet adhesion. Dry adhesion, exemplified by geckos, spiders, and beetles, relies primarily on physical contact forces, most notably van der Waals interactions, arising from hierarchical micro- and nanoscale surface architectures that maximize real contact area without the use of adhesive secretions. Wet adhesion, as demonstrated by marine mussels, octopuses, and tree frogs, operates through biochemical crosslinking, negative-pressure suction, and fluid-displacement mechanisms that act specifically at aqueous or fluid-saturated interfaces. The engineering relevance of these systems is substantial: bioinspired adhesives derived from gecko-like hierarchical fibrillar surfaces have found application in climbing robots, transfer printing, and reversible electronic attachment; mussel-inspired catechol chemistry has enabled a generation of underwater sealants and surgical tissue adhesives; and octopus- and remora-inspired suction technologies inform the design of soft robotic grippers and marine sensor platforms. Despite significant advances, challenges remain in scalable fabrication, contamination resistance, and translating organism-level performance to engineered systems at commercially viable scales. This manuscript reviews each biological model with respect to its structural form, functional mechanism, adhesion classification, documented engineering applications, and inherent limitations, concluding with a synthesis of future research directions critical to advancing the field of bioinspired adhesive materials and systems.