Physical modification methods focus on using techniques such as heating, mechanical stirring, and ultrasonic treatment to adjust the physical form and structure of biomass raw materials, thereby optimizing their performance in adhesive systems.

Mechanical stirring increases the contact area between biomass feedstock and solvents or other additives, enabling more uniform dispersion of the feedstock in the solvent and reducing its particle size^[16]. For biomass feedstocks such as cellulose, stirring can disrupt their fiber bundle structure, allowing them to dissolve or disperse more effectively within the system. Thermal treatment is also a method of physical modification, altering the material's physical state or internal structure through heating. For some crystalline biomass materials, partial melting or rearrangement of crystalline regions can enhance molecular chain mobility. Ultrasonic treatment, by generating localized high temperatures, high pressures, and shock waves, can effectively break up agglomerations of biomass feedstock, improving its dispersibility in adhesive systems. This allows for rapid and uniform dispersion within the system, thereby enhancing the stability and homogeneity of the adhesive.

Chemical modification approaches utilize chemical reactions such as esterification^[17],etherification^[18],and crosslinking^[19]to alter the chemical structure of biomass molecules by introducing functional groups or forming functional chemical bonds, thereby significantly enhancing key performance indicators of adhesives, such as bonding strength, water resistance, and stability.

Biomass-based raw materials (such as starch, cellulose, etc.) contain abundant hydroxyl groups, which can undergo esterification reactions with organic acids or acid anhydrides, introducing hydrophobic groups into the biomass macromolecular chains and thereby enhancing the water resistance of adhesives. Taking starch as an example, the hydroxyl groups in its molecules react with acetic anhydride to produce acetylated starch. Due to the reduced hydrophilicity of acetylated starch, the adhesives prepared from it exhibit improved stability in humid environments. Etherification is a chemical transformation process that incorporates ether linkages into the molecular chain structure of biomass; common etherifying agents include ethylene oxide and chloroethanol. For instance, the hydroxyl groups in cellulose molecules can undergo ring-opening addition reactions with ethylene oxide to form hydroxyethyl cellulose^[20]. In wood bonding applications, hydroxyethyl cellulose adhesives can better interact with the hydroxyl groups on cellulose and hemicellulose molecules at the wood surface, forming a strong bond. Crosslinking reactions enable the active groups in biomass-based adhesives to connect with each other, thus constructing a three-dimensional network structure. Formaldehyde, glutaraldehyde, and isocyanates are commonly used crosslinking agents in such systems. Glutaraldehyde, as a crosslinking agent, can condense with amino groups in soybean protein molecules to form a stable crosslinked network^[21], thereby creating a robust crosslinked structure. The formation of this crosslinked structure not only effectively enhances the cohesive strength of the adhesive and improves its bonding strength but also significantly boosts its water resistance and heat resistance, allowing it to demonstrate superior overall performance in practical applications.

The composite modification strategy leverages the advantageous properties of different biomass materials or combinations of biomass materials with synthetic polymers, employing multiple mechanisms such as physical blending and chemical bonding to develop adhesive products with superior performance and greater functional diversity, thereby meeting various complex industrial application requirements.

The rigidity and chemical stability inherent in lignin, combined with the flexibility exhibited by cellulose, result in an adhesive that not only demonstrates excellent bonding strength but also outstanding flexibility. In practical applications for wood bonding, this adhesive can maintain its integrity without fracturing even when subjected to deformation under a certain degree of external force, ensuring a stable and reliable bond between wood pieces. It is worth noting that biomass raw materials naturally possess renewable and biodegradable properties, which are fully retained in the composite adhesive, highlighting its environmentally friendly characteristics.

Biomass raw materials and synthetic polymers are combined through physical blending or chemical bonding. The latter approach effectively addresses the deficiency of insufficient bonding strength in biomass-based adhesives, significantly enhancing the overall performance of the adhesives and enabling their application in fields with stringent bonding requirements. After the introduction of synthetic polymers, the composite materials still partially retain the advantageous properties of biomass raw materials, such as biodegradability. Some composite adhesives can gradually degrade in the natural environment after disposal, thereby reducing environmental pollution. Meanwhile, the renewable nature of biomass raw materials also provides composite adhesives with certain sustainable advantages in terms of raw material supply.

Lignin, as one of the major components of plant biomass, is the most abundant source of aromatic compounds in nature and has long been produced as a byproduct in the pulping and papermaking process. Currently, significant progress has been made in converting natural lignin into aromatic monomers or polymeric materials through strategies such as depolymerization, fractionation, and modification. Among these, the preparation of bio-based adhesives from lignin has become a research hotspot. Song et al.^[22]first extracted a catechol monomer, 4-propylcatechol (C-lignin-derived monomer), from the seed coat of Sapium sebiferum, which was then copolymerized with bio-based butyl acrylate to prepare a class of catechol-functionalized copolymers. Through depolymerization, derivatization, and copolymerization, catechol monomers participate in adhesive polymerization. This adhesive can bond hydrophobic polytetrafluoroethylene sheets with a bonding strength exceeding 2.0 MPa, and even after immersion in seawater, the bonded steel achieves a tensile strength of 8.1 MPa. In addition to depolymerizing C-lignin into catechol monomers, another approach involves converting the methoxy groups of guaiacyl (G) and syringyl (S) units in conventional lignin into phenolic hydroxyl groups, thereby obtaining the catechol moiety found in 3,4-dihydroxyphenylalanine (DOPA). Using silver ammonia solution to oxidize lignin can synthesize catechol structures^[23]. The resulting catechol/quinone structures facilitate hydrogen bonding and hydroxylamine bonding, enabling durable and reversible adhesion^[24].

In addition to depolymerizing lignin into monomers for modification, lignin can also be directly modified. Hu et al.^[25]inspired by the structural similarities between plant lignin and mussels, successfully developed a novel lignin-based bioadhesive through demethylation and amino acid grafting modification techniques (Figure 1). The study showed that the demethylation process significantly converted guaiacol and ferulate structures in AL into catechol structures with enhanced adhesive properties. Compared to acidic amino acids, grafting basic amino acids resulted in a stronger synergistic adhesive effect with the catechol structure. After optimizing the synthesis route, the adhesion of AL-Lys-D to skin tissue was 5.31 times higher than that of AL. This enhanced effect is primarily attributed to the Schiff base reaction between the amino groups of amino acids and the carbonyl groups of skin keratin, which requires low energy and is more favorable under acidic and dry conditions.

With the continuous development of the adhesive industry, the bonding performance of adhesives in aquatic environments has become an important area of ongoing exploration and research for scientists. Zhao et al.^[26]proposed an innovative method for preparing underwater adhesives (Figure 2),which uses biomass pulping residues and lignin sulfonate (LS) as the main raw materials, synthesized with polyamide-epichlorohydrin (PAE-Cl). The resulting adhesive (LSPAE) not only possesses environmentally friendly characteristics but also exhibits outstanding underwater adhesion performance. Through electrostatic interactions and a hydrophilic stabilization mechanism, LSPAE adhesive forms a unique water-insoluble state, enabling it to instantly adhere to various substrate surfaces in aquatic environments and undergo a natural curing process, thereby significantly enhancing its wet bonding strength. Further studies have shown that within a relatively broad pH range (6 ~ 10), even under high-salt conditions (1.0 mol/L NaCl) or high-temperature conditions (100 ℃), after 30 days of continuous immersion, the wet tensile strength of the LSPAE adhesive remains stable without any significant performance degradation. The intrinsic mechanism underlying the excellent performance of the LSPAE adhesive is primarily attributed to PAE's unique self-curing properties, which play a crucial role in both the curing process and the final performance of the adhesive. Meanwhile, a robust and high-strength complex structure is formed between PAE and lignin sulfonate, with their strong interaction acting as a solid protective barrier, effectively enhancing the adhesive's overall water resistance and durability, ensuring outstanding performance in complex, variable, and demanding real-world applications.

Li et al.^[27]prepared a demethylated lignin-based epoxy resin adhesive that significantly improved bonding strength while maintaining waterproof properties. After modifying lignin using the LiBr/HBr system, the phenolic hydroxyl content of demethylated lignin (DL) increased to 6.19 mmol/g, and the tensile strength of the modified adhesive rose to 62.50 MPa, considerably higher than that of the adhesive modified with raw lignin (38.89 MPa). Due to its excellent cross-linking density, the DL-modified adhesive retained more than 85% of its tensile shear strength after being immersed in water for 48 hours or frozen at -25 ℃ for 48 hours. Shuai et al.^[28]designed a method to convert industrial lignin into an industrially applicable adhesive through hydrogenation-deoxygenation and acid-mediated hydroxyalkylation (Figure 3). This lignin-based formaldehyde-free adhesive exhibited superior lightness and strong bonding performance compared to traditional LPF resins. Mechanism studies revealed that chemical cross-linking of lignin via hydroxyalkylation, forming diarylmethane structures and physical interlocking interactions at the wood veneer interface, contributed to the strength of plywood. This two-step strategy provides a new pathway for preparing biomass-based adhesives from by-products of pulping or biomass refining industries. Meanwhile, Mojgan Nejad et al.^[37]also achieved groundbreaking progress, successfully developing a lignin-glyoxal composite biomass-based, formaldehyde-free wood adhesive. This study utilized unfunctionalized lignin and glyoxal to simultaneously replace petroleum-based phenol and formaldehyde, highlighting the potential of glyoxal in producing green adhesives with toxicity levels far lower than those emitted by indoor wood panels. Furthermore, corn stover lignin, as a renewable biomass-based polyphenol feedstock in the green adhesive formulation, demonstrated excellent performance, offering significant application prospects for the high-value utilization of lignin.

Polysaccharide-based adhesives use polysaccharides such as cellulose, starch, and chitosan—substances that are widely available and abundant in nature—as key raw materials. Polysaccharides are extremely widespread in nature, relatively inexpensive to obtain, and possess excellent biocompatibility and biodegradability. Their application in the adhesive field offers significant advantages and broad prospects, providing an ideal direction for developing green and sustainable adhesive products. Lu et al.^[29]proposed a method for preparing adhesives based on cellulose aqueous solutions. This adhesive exhibits wide substrate applicability, performing exceptionally well in bonding various substrates such as wood shavings, straw, and waste paper, with particularly great potential in wood bonding. Its superior adhesion and stability offer new insights for wood processing and related industries. Additionally, this cellulose adhesive demonstrates good temperature resistance, maintaining stable bonding performance within a certain temperature range, which greatly expands its application scenarios. At the same time, the adhesive boasts excellent biodegradability, gradually breaking down into harmless substances by microorganisms in natural environments, effectively reducing environmental burden and aligning with current sustainable development principles. Yang et al.^[30]developed a novel cellulose-based wood adhesive (Figure 4). They first oxidized natural cellulose with sodium periodate to form dialdehyde cellulose (DAC), selecting tris(2-aminoethyl)amine and diethylenetriamine as models for branched polyamines (3N) and linear diamines (2N), respectively. Subsequently, through Schiff base reactions between branched polyamines or linear diamines and DAC, imine-containing cellulose adhesives were prepared. By controlling the reaction ratio between DAC and amines, a series of different cellulose-based adhesive formulations could be obtained. The adhesive performance was evaluated via dry/wet lap-shear strength tests, achieving shear strengths of up to 2.7 MPa. This research not only provides a potential method for preparing eco-friendly, high-performance bioadhesives with branched cross-linking structures but also broadens the application of natural cellulose in wood products.

Starch-based wood adhesives are widely used in wood product manufacturing as an environmentally friendly and formaldehyde-free alternative. However, their poor water resistance, low bonding strength, and inability to inhibit bacterial/fungal growth limit their practicality. To address these issues, Chen et al^[31]prepared a physicochemical double-crosslinked network structure among citric acid, starch, and wood by physically entangling the wood structure and chemically crosslinking starch molecules (Figure 5). Citric acid inhibits the activity of various intracellular enzymes and denatured proteins in microbial cells, thereby suppressing bacterial and fungal growth. In water resistance cycling tests, the water resistance of the resulting esterified starch plywood was improved by 47.6% and 13.2% compared to starch plywood and commercially available phenolic resin plywood, respectively. Additionally, Wang et al^[32]used ethylene glycol diglycidyl ether as a crosslinking agent to promote the formation of a crosslinked network structure between starch and gelatin, with the incorporation of self-assembled modified reed fibers successfully creating a dense macromolecular network structure under the action of the crosslinking agent. This resulted in improvements in the adhesive's shear strength, water resistance, rheological properties, and thermal stability, with dry and wet strengths of 6.46 and 1.32 MPa, respectively. The dual physical and chemical crosslinking approach for starch adhesives provides a straightforward strategy for preparing inexpensive, waterproof, and antifungal/antibacterial starch adhesives, thus facilitating the wider adoption of starch-based adhesives.

Protein-based biomass feedstocks mainly come from soybean protein and collagen derived from animals and plants. These are natural macromolecular compounds rich in amino acids. In the field of adhesives, they offer excellent bonding properties while also being environmentally friendly and biodegradable.

Inspired by the phenomenon of underwater bioadhesion exhibited by proteins in nature, Tong et al.^[33]prepared a green underwater bioadhesive hydrogel (Figure 6) using gelatin and tannic acid (TA), two naturally occurring substances that are difficult to blend together, through an effective step-by-step immersion method without any polymerization reaction. After treatment with urea solution or heating, this hydrogel can achieve stable instant adhesion on various substrates and in complex solution environments, with an adhesive strength reaching 100–103 kPa. As the treatment temperature increased from 50 ℃ to 60 ℃, the adhesive strength significantly rose from 400 kPa to 1100 kPa. Further increasing the temperature to 70 ℃ and 80 ℃ resulted in adhesive strengths of 1250 kPa and 1500 kPa, respectively. In 10 repeated tests, the adhesive strength remained consistently between 1.2 and 1.5 MPa, demonstrating its excellent reproducibility and stable strong-adhesion properties.

Chu et al.^[34]successfully synthesized a novel adhesive whose main ingredients include soybean meal (SM), active sodium alginate (aSA), nano-hydroxyapatite (nHA), and polyamide-epichlorohydrin (PAE-Cl) resin. Among these, sodium alginate, as a reactive biomass-based crosslinking agent, can form covalent crosslinked structures with the SM matrix under room temperature conditions, endowing this adhesive with excellent applicability on various substrates. The wet shear strength for wood reaches as high as 0.76 MPa, significantly exceeding that of commercially available wood adhesives, demonstrating the adhesive's outstanding bonding performance. Due to the unique reactivity of PAE resin at temperatures above 60 ℃, this adhesive maintains efficient bonding capability across a broad temperature range from room temperature to 150 ℃, greatly expanding its application scenarios and possibilities in extreme environments. Furthermore, thanks to its dense structure and abundant nitrogen- and phosphorus-containing components, this adhesive exhibits excellent flame-retardant properties, with a limiting oxygen index reaching 31.2%, providing strong assurance for applications in numerous fields with stringent flame-retardant requirements.

Developing a soy flour adhesive that is formaldehyde-free, exhibits excellent coating properties, mold resistance, and flame retardancy, while maintaining high bonding strength, presents a significant challenge. Inspired by the powerful adhesive performance of gecko toe hairs with their brush-like structure, Gao et al.^[35]synthesized a brush-like polymer containing catechol groups, which improved the adhesion behavior of soy flour adhesives. Drawing on the high-strength characteristics of oyster biomineralization structures, calcium phosphate was precipitated onto soy protein and combined with the brush-like polymer to create a biomimetic mineralized structure. This approach enabled the adhesive to retain high bonding strength while also achieving mold resistance and flame retardancy. Compared to the unmodified adhesive, the dry and wet shear strengths of plywood bonded with this adhesive increased by 118.8% and 750%, respectively.

Fang et al.^[36]prepared multifunctional CA/TA/FC nanospheres (Figure 7) using cinnamaldehyde (CA), tannic acid (TA), and ferric chloride (FC). The soybean meal (SM) adhesive was synergistically modified through a multiple-network and nano-filling strategy, utilizing the nanospheres and mercapto-functionalized nanotubes (HNTs@SH). The crosslinked network of the resulting SM-CA/TA/FC-HNTs@SH adhesive was enhanced by Schiff base reaction, acetalization, disulfide bond (S—S) formation/exchange, electrostatic interactions, and hydrogen bond formation. Compared to the SM adhesive, the SM-CA/TA/FC-HNTs@SH adhesive exhibited increases in dry/wet shear strength by 47.0% and 160.7%, respectively, and its water resistance improved by 9%. Additionally, its cost was reduced by 26.7% compared to the commonly used SM-PTGE adhesive.