Global trade in animal leather accounted for only 0.091% of the total international market in 2021, reaching a substantial value of $242.85 billion in 2022. ³⁴ Notable exporters in this trade were Italy ($3.55 billion), the United States ($1.88 billion), Brazil ($1.45 billion), China ($1.07 billion), and Germany ($734 million) in terms of exports. In comparison, China ($3.42 billion) and Italy ($2.3 billion) were significant importers of animal leather. ³⁵ Despite being economically significant for many nations, the tanning industry poses significant environmental challenges owing to solid waste and liquid and gaseous effluents, causing detrimental impacts on air, water, and soil quality.

Solid tannery waste can be divided into removed hair, untreated skin residues, waste from tanned skin, leather trimming, and processed sludge. These residues are abundant and rich in fats ³⁶ and proteins. ³⁷ Depending on their chemical composition, they can be recycled for diverse purposes, provided they undergo proper treatment and characterization.

Moreover, regarding water usage in the tanning process, an incredible 50,000 kg of water is required to process just one kilogram of cowhide. ³⁸ Also, tannery effluents are hazardous to decontaminating because of their chromium, sulfide, heavy metal, and organic matter content. ^{38 , 39} These effluents also exhibit elevated Chemical Oxygen Demand (COD) and Biochemical Oxygen Demand (BOD), ⁴⁰ and even after undergoing advanced chemical and physical treatments, they show low degradability indices. ⁴¹ In addition, wastewater can permeate through underground layers, as confirmed by research conducted in India, Iran, and Bangladesh, where groundwater samples near tanneries showed high concentrations of Cu, Cr, Pb, Zn, Ni, Al, and As, with Cr registering the highest concentration, ranging from 0.01 to 2.07 mg/L. ^{42 – 50} Undoubtedly, this risks ecosystem stability. ⁵¹

The environmental impact of tannery waste has been under scrutiny for decades, with substantial evidence highlighting the high toxicity of such residues to plants ⁵² and animals ^{53 – 55} and bioaccumulation exacerbating this concern. The hazard posed by these residues relies on the presence of contaminating substances, such as sulfides, ⁵⁶ chromium (III), chromium (VI), ^{57 , 58} lead, and other heavy metals. The immediate and severe effects of tannery waste on the environment and human health are undeniable. ⁵⁹ In recent years, substantial efforts have been made to remediate, recycle, and reuse various tannery wastes. In this context, repurposing solid tannery waste for glue production is a viable strategy for reducing waste and generating environmentally friendly products within a circular economic framework.

Adhesives derived from natural sources have had the oldest known historical use in all civilizations for at least 200000 years. ⁶⁰ Evidence of glue residues dates to 1350 BCE, as observed in wood decorations found in King Tutankhamun’s tombs. Additionally, indications of glue usage have been found in ancient civilizations such as Greece and Rome. ⁶¹ Furthermore, historical records highlight its presence in Edo period paintings in Japan and artifacts from the Joseon dynasty in Korea. ⁶² Today, natural glues have specific services, such as artistic applications, historical conservation, cardboard, packaging, ⁶³ and the creation of new adhesives.

Historically, glue was primarily produced from animal collagen derived from hides, bones, and connective tissues. Industrially, animal glues come from slaughterhouses that provide animal hides, blood, ^{64 , 65} and other sources of proteins ⁶⁶ that can be extracted by hydrolysis. ⁶⁷ Traditionally, to recover animal glue, animal parts, primarily bones from horses, cattle, other livestock, and fishes, are boiled for extended periods in water to obtain collagen, which solidifies into glue upon cooling. ⁶⁸

Other glue sources include water-resistant rennet casein and acidic casein. Rennet casein is produced by coagulating rennets with skim milk at 30°C and acidic casein. ⁶⁹ In contrast, lactic acid casein is derived from inoculating milk with certain bacteria such as Streptococcus lactis, Streptococcus cremoris, and Lactococcus lactis subspecies cremoris. ^{69 , 70} Chitosan is another natural glue obtained from ground crab and shrimp shell waste, processed through acid or alkali treatment. ^{71 , 72} Marine organisms such as mussels, barnacles, and tubeworms secrete protein adhesives that effectively adhere to hydrated underwater surfaces owing to the high proportion of amino acids with phenolic hydroxyl chemical groups. ⁷³ These secretions open ample avenues for developing water-resistant adhesives for various purposes.

In Asian cultures, the isinglass is the purest form of fish glue derived from the swim bladder membranes of sturgeons. ⁷⁴ Bone, fish, and hide adhesives have low moisture resistance, which affects the properties of the bond and notably decreases its elasticity and tensile strength. ⁷⁵

Adhesives have been derived from plant resins, saps, natural rubber, ⁷⁶ starches, ⁷⁷ natural gums, latex, soy, lignin, algae, and cellulose. ⁷⁸ Types of glue from starches such as wheat and rice are commonly used in paper and woodworking applications. The mixture of corn starch with hydrolyzed acrylic emulsion and uzkhitan glue warp threads ⁷⁹ and some starches serve as cohesive elements to create conductive glue for electrode materials. ⁸⁰ The functionality of these glues from starch can be improved with additives; for example, the water resistance increases with polymerized lignosulfonates. ⁸¹

The adhesive industry has become a specialized field of science, developing numerous innovative adhesive products. To create customized formulas for specific applications, it is crucial to understand the various existing adhesive types. This differentiation forms the basis for the evolution of collagen-modified adhesives. While this review does not delve extensively into the categorization of adhesives, Figure 2 provides a concise overview of the different segments within the adhesive industry, serving as a foundational reference for understanding the different adhesive types.

With the evolution of industrial processes, there is a need for more efficient and cost-effective adhesives. ⁸² Tanneries, which process animal hides to produce leather, generate significant amounts of waste rich in collagen, particularly trimmings, and shavings, whether tanned or not. ⁸³ Rather than discarding these by-products, innovators have realized the potential to utilize this waste for glue production.

Collagen is a complex protein with approximately 28 types. ^{84 , 85} This protein is characterized by a unique structure consisting of three parallel polypeptide strands with a left-handed, polyproline II-type (PPII) helical conformation that coils together to form a right-handed triple helix. This structure necessitates that every third residue be glycine, leading to a consistent XaaYaaGly sequence throughout all collagen types. Within this sequence, the amino acids at the Xaa and Yaa positions are (2S)-proline (28%) and (2S,4R)-4-hydroxyproline (38%), respectively, making ProHypGly the predominant triplet, occurring at 10.5% in collagen. ⁸⁶ This formation provides strength and flexibility owing to the high proline and hydroxyproline contents, which prevent the protein from assuming a globular shape. Numerous polar groups in collagen enhance the chain interactions. ^{87 , 88}

The source and preparation method of collagen largely determines its physical, chemical, and mechanical properties. When its chains are shortened, differences originating from various sources diminish, leading to coiled proteins with reduced molecular weight. Native collagen, with a molecular weight of 285–300 KDa, undergoes significant structural changes upon hydrolysis. After denaturation, its triple-helix structure transforms into a random coil form owing to the dissociation of the hydrogen bonds. As a result of this process, hydrolyzed collagen consists of numerous peptides with much lower molecular weights (3–6 KDa). ⁸⁹ The best glues contain collagen Type I because they retain high adherence, ease of gel formation, and an excellent structure to form bonds with other substances compared to simple peptides. The use of collagen is supported by its widespread use in industrial adhesives due to its abundance, strong fibrillar structure, and effective binding properties. ⁹⁰ Proteomic analyses have shown that most animal glues rely mainly on Type I and III collagen extracted from common domestic species, confirming their superior performance over simpler protein fragments. ⁹¹ Collagen variability is influenced by its origin—whether it comes from skin, connective tissue, cartilage, or bones—as well as by the age and species of the animal, in Figure 3, a schematic representation of five types of collagen is presented. Type I human collagen is predominantly found in the skin, bone, teeth, tendons, ligaments, vascular ligatures, and various organs ( Figure 3a). ⁹² Type II collagen is primarily located within cartilage ( Figure 3b), ⁹³ while Type III collagen is commonly sourced from the skin, muscles, and blood vessels ( Figure 3c). ⁹⁴ Type IV collagen is present in the basement membrane’s epithelial-secreted layer and the basal lamina ( Figure 3d). ⁹⁵ Additionally, Bos Taurus Type IV collagen is depicted in the schematic ( Figure 3e). ⁹⁶

The adhesion of the protein glue to wood depends on polar and nonpolar group interactions. Amino acids such as glutamic acid, tyrosine, and proline form hydrogen bonds. However these groups often remain inaccessible owing to internal bonds caused by forces such as van der Waals, hydrogen bonds, and hydrophobic interactions. Consequently, basic proteins have limited adhesion and require chemical changes to expose polar protein molecules. ^{97 , 98} Furthermore, intramolecular cross-linking is achieved through the oxidative removal of amine groups from specific amino acids within proteins to develop high-strength collagen adhesives. This process leads to the formation of aldehydes, a phenomenon known as the Schiff base protein cross-linking. ⁹⁹

Residues suitable for collagen extraction include untanned skin trimmings and tanned leather shavings. ¹⁰⁰ Tanneries sell untanned residues to gelatin factories, undergoing a relatively straightforward transformation.

In contrast, tanned residues, such as wet blue and leather trimmings, require more intricate extraction processes ¹⁰¹ because tanned wastes are intertwined collagen strands with agents like tannins, chromium, and alum. ¹⁰² The extraction processes commonly applied to these residues are acid or base hydrolysis at near boiling temperatures or complex enzymatic hydrolysis. ¹⁰³ However, recent investigations have employed multiple combined techniques to enhance collagen yield recovery while mitigating energy consumption. ¹⁰⁴ Collagen extraction typically involves the pre-treatment, hydrolysis, and purification processes.

Pre-treatment tannery waste

The main goal of pre-treatment is to disrupt covalent cross-links between collagen molecules because they do not break down even in boiling water. ¹⁰⁵ Trimmings and untanned skin must be liberated from chemicals and dirt. These materials are then processed to remove all traces of hair, fat, and flesh, ensuring they can absorb more components for further acid or alkaline treatments. ¹⁰⁶

Acid Pre-treatment: This method immerses washed and chopped skin pieces in dilute acid. The acid causes the skin to swell and hydrolyze the cross-links. Acid pre-treatment suits fragile skin with less fiber intertwinement, such as porcine and fish skin. ¹⁰⁷

Alkaline Pre-treatment: Dilute alkalis such as sodium hydroxide, calcium hydroxide, and hydrogen peroxide ¹⁰⁸ are used. Alkalis is effective for extracting collagen from thick and hard materials. Despite being lengthy, sodium hydroxide treatment is preferred because it swells the skin, aiding alkali diffusion into the tissue matrix. Alkalis also hydrolyses unwanted components. Lower hydroxide concentrations at suitable temperatures retain the acid-soluble collagen and its native structure. ¹⁰⁹

Collagen Extraction processes from tannery wastes

Prolonged boiling for collagen recovery is energy-intensive and inefficient, making it unsuitable for contemporary fabric treatment. Consequently, there is a pressing need to refine the process of extracting collagen from tannery wastes. The most prevalent method for such extraction involves hydrolysis using different agents, as shown in Table 1. There is also a growing interest in advancing novel techniques to enhance extraction yields. Moreover, establishing mathematical-physical models is crucial for improving the efficiency of shaving hydrolysis to reduce the need for extensive experimental trials, as emphasized by Vaskova and Vasek ¹¹⁰ whose model introduced a parameter simulation aimed at deriving hydrolyzed collagen from tannery shavings using a flow reactor.

Post-extraction purification and characterization

The purification of chromium-extracted collagen is crucial to ensure its suitability for subsequent applications. ¹²² Chromium (III) is the most used as a tanning agent. The chrome tanning reaction predominantly targets the carboxyl groups of collagen, which are assumed to be located at the aspartic and glutamic side chains. Kinetics showed quicker Cr (III) reactions with aspartic acid, whereas thermodynamics revealed a stronger tendency of glutamic acid for stable Cr (III) complexes involving Cr-O-Cr bridges. These bridges contribute to bridging the gaps between the collagen chains in the structure of the hide, thereby enabling tanning. ¹²³ Hence, the remarkable stability of leather underscores the necessity of devising methods that can disrupt the Cr-O-Cr complex while preserving collagen integrity.

The purification of collagen can be achieved by removing chromium through chemical methods ^{124 , 125} such as alkali hydrolysis using 6% magnesium oxide and 1.0% sodium carbonate at 70°C for 48 h, followed by 1% bate enzyme hydrolysis to eliminate almost 80% of chromium from wet blue samples. ¹²⁶ New technologies include ultrasonic dechroming at a maximum of 200 MHz to reduce Cr by 70.2%. ^{127 , 128} However, better results were achieved by applying sonication to the waste and adding EDTA at a ratio of 1:3 at 80°C for 30 min to achieve a chromium removal efficiency of 98%. ¹²⁹

These results show that purification of the extracted collagen is possible and practical. The efficiency and yield of dechroming depends on the nature of the waste and the process applied. This field of investigation offers abundant research opportunities and has considerable potential for prolific research.

There are two primary methods for utilizing natural adhesives. One involves their direct use for adhesive purposes, though this approach remains limited in applications. The other combines collagen with additional materials to produce copolymers with enhanced properties. For example, Negash et al. fabricated glue directly from hide-trimming waste ¹³⁰ through a sequential treatment process of lime soaking, washing, and acid neutralization. Extraction between 60 and 70 °C for 2.5–3.5 hours yielded an optimal formulation at 60 °C for three hours. This glue exhibited a viscosity of 90 centipoises, moisture content of 14.6%, ash content of 2.23%, density of 1259 kg/m ³, yield of 32%, pH of 5.98, and shear strength of 260 MN—values superior to the reference glue and suitable for restoration of artworks and historical artifacts. ¹³¹ Moreover, the precise composition of these glues can be determined, which is essential for accurately recreating original adhesives and ensuring careful restoration without compromising valuable pieces. ^{132 – 134}

Formaldehyde-based adhesives have been widely employed due to their strong bonding performance; however, their use is increasingly restricted because of formaldehyde emissions and stricter regulations such as the European Emission Standards E1 and E0. ^{135 , 136} A promising alternative incorporates enzymatically extracted collagen from chromed tannery waste into formaldehyde resin formulations. Introducing hydrolysate at a 5% mass fraction nearly doubled the content of methylene bridges compared to methylene oxide bridges (–CH ₂–O–CH ₂–), as demonstrated by thermogravimetric analysis. ^{135 , 137} This change potentially reduces methylene oxide formation—a key precursor of formaldehyde emissions—while enhancing mechanical strength due to increased molecular weight. Cross-link stability was also maintained under neutral conditions and in the presence of phthalic acid as a curing agent. ^{138 , 139}

Matyašovský et al. ¹⁴⁰ further demonstrated that collagen hydrolysates modified with urea can substantially reduce formaldehyde emissions when used as additives in urea-formaldehyde (UF) adhesives. By incorporating these collagen components with dialdehyde-modified hardeners, the formaldehyde content in bonded plywood decreased by up to 50% in laboratory tests and approximately 30% in industrial production without significant loss of mechanical strength. These findings reinforce that carefully engineered collagen hydrolysates offer environmental advantages and improve compatibility with UF resins, supporting their use as partial replacements for synthetic adhesives in wood-based applications subject to stringent emission standards.

Building on these insights, Sedlicik et al. ¹⁴¹ evaluated UF adhesives modified with collagen hydrolysate from chrome-tanned leather. Adhesives prepared by adding 5% modified hydrolysates underwent condensation at 100 °C for up to 45 minutes. Shear strength testing on beech plywood per EN 314-1 confirmed that all samples met standard requirements, achieving a maximum of 2.07 MPa at 19% humidity, despite a modest reduction compared to unmodified UF resin. Notably, adding 3–8% collagen hydrolysate effectively lowered formaldehyde emissions into the E1 classification, with FT-IR spectroscopy supporting chemical interactions between UF resin and collagen.

To avoid formaldehyde altogether, Islam et al. ¹⁴² developed sustainable adhesives for particleboard by comparing native collagen (Type A), acid-extracted collagen (Type B), and PVA-crosslinked collagen (Type C). Type C adhesive achieved a gel time of 4.2 minutes and a high shear strength of 5.31 MPa. In contrast, Type B without additives reached 3.98 MPa, demonstrating the benefits of PVA incorporation. However, even this optimized formulation remained below the 9.5 MPa strength typical of commercial urea-formaldehyde resins, suggesting further improvements are needed to match industrial benchmarks.

Additional studies focused on modifying gelatin-based glues with functional polymers, and for example, incorporating epoxy-terminated hyperbranched polymers (EHPAE) and sodium dodecyl sulfate substantially improved shear strength from 0.92 MPa in unmodified gelatin to 2.285 MPa in EHPAE-III adhesives. ¹⁴³ Although performance fell short of the 2.469 MPa achieved by commercial adhesives, this approach fulfilled EN standards for footwear, highlighting the potential of epoxy crosslinking to reinforce protein-based adhesives.

Yang et al. ¹⁴⁴ produced a wood adhesive by grafting waterborne polyurethane onto gelatin derived from tannery waste. The resulting WPUG copolymer combined high dry bonding strength (4.21 MPa), a contact angle of 111.5°, tensile strength of 32.91 MPa, and excellent thermal stability exceeding 350 °C.

Acrylic–collagen latex adhesives were also developed via emulsion copolymerization with acrylic acid and butyl acrylate, yielding flexible films with tunable properties. Neutralized formulations like A-C 25 N became sticky and highly extensible, demonstrating promising tack adhesion despite relatively low stress–strain performance (0.0115 MPa). ¹⁴⁵

Liu et al. ¹⁴⁶ developed a collagen adhesive (CPP-G) through an anhydrous condensation process combining tricyanogen chloride with collagen-degrading polypeptides for corrugated cardboard applications. The optimal product showed a high percentage of solubility rate of 97% and conformed to Chinese standard GB/T6544-2008 S-1.1 grade. Its viscosity (0.256 Pa·s), thermal stability (220–260 °C), and initial adhesion (90%) surpassed many commercial references, while 48-hour water resistance remained comparable.

Despite these advances, many collagen-based adhesives are still vulnerable to water and redissolve upon heating, limiting use in humid conditions. To address this, Zhou et al. ¹⁴⁷ created collagen hydrolysate–silane coupling agent hybrids (CSH). By combining hydrolysate extracted by alkali hydrolysis with silane crosslinkers (GPDMS, GPTMS, GPTES), they achieved adhesives with dry strength up to 1.57 MPa and wet strength up to 0.95 MPa—exceeding the Chinese Class II plywood standard. These results highlight the importance of crosslinker selection and dosage in improving water resistance.

Biodegradable adhesives incorporating collagen with polyvinyl alcohol and glycerol have also shown promise. Under optimal conditions identified by neural network analysis (65 °C, 3.2% PVA, 4.2% glycerol), peel strength reached 12.5 N/mm, with biodegradability and adhesion comparable to chemical adhesives. ¹⁴⁸

Finally, Wang et al. ¹⁴⁹ demonstrated that combining waterborne polyurethane with click-chemistry functional groups significantly improved gelatin adhesives. While plain gelatin had low strength (<0.1 MPa) and poor thermal stability (~310 °C), adding polyurethane increased strength to ~0.17 MPa, and the MWPU-FGE/GE formulation further improved shear strength (~0.68 MPa), peel strength (~1 N/mm), thermal resistance, and wet adhesion. This bio-inspired adhesive outperformed commercial water-based products after extended curing, suggesting a promising sustainable alternative.

Table 2 provides a comprehensive summary of quantitative performance data for a broad range of collagen-based adhesives evaluated in recent years, including their measured bonding strength, viscosity, thermal stability, and water resistance compared to conventional commercial adhesives. The data illustrates that several advanced formulations, particularly those incorporating crosslinkers or chemical modifications, achieve bonding strengths comparable to or exceeding synthetic resin benchmarks in dry conditions. Notably, adhesives based on crosslinked collagen hydrolysates and hybrid systems consistently demonstrate higher mechanical performance than unmodified gelatin or simple protein dispersions.

However, a recurring limitation evident across nearly all compositions is reduced resistance to water exposure. Even adhesives with excellent initial shear or peel strength often show significant loss of cohesion or increased solubility after prolonged immersion or humidity cycling. This variability underscores the importance of further optimizing formulation strategies to enhance water resistance, such as blending hydrophobic resins, introducing moisture-stable crosslinking networks, or applying surface treatments to improve dimensional stability.

These findings are relevant to diverse application areas: certain adhesives are tailored for wood panel bonding, others for corrugated packaging, and some for bio-based composites. Overall, the evidence consolidated here demonstrates that collagen adhesives have significant potential as sustainable alternatives to petrochemical resins. Nevertheless, achieving long-term moisture durability remains the main technical challenge and is critical for future development.

The opportunity to manufacture collagen-based adhesives arises at an opportune time, with promising data indicating significant market growth in the hydrolyzed collagen sector, projected to nearly double in value from 2023 to 2032, reaching an estimated $2.34 billion by 2032. ^{150 , 151} This is particularly pronounced in North America and Europe, where diverse industries, including food and cosmetics, are heavily invested. Furthermore, the adhesives market ranked as the 281st most traded global commodity, with a trade value of $14.2 billion in 2021. Additionally, the adhesive sector demonstrated an impressive growth rate of 21.3% within a single year, underscoring its dynamic and expanding nature. ^{152 , 153} Given the significant growth in the collagen sector, there is potential to establish a sustainable and profitable niche within the global adhesives industry by utilizing tannery waste as a collagen source. This approach could cater to non-edible or medical sectors, although the recovery process costs must be mitigated. The integration could revolutionize the supply chain by aligning waste utilization with market expansion opportunities.

It is feasible and profitable to produce gelatin from chrome shavings in a pilot plant operating with specific equipment, as demonstrated by Cabeza et al. ¹⁵⁴ In 24 hours, processing 9072 kg of chrome shavings can yield over 900 kg of gelatin daily at approximately $0.52 to $0.57 per kg. Meanwhile, the same year, commercially available low-quality gelatins were $3.20 per kg. ¹⁵⁵ These studies also illustrated that recovering collagen from tannery wastes can generate additional revenue from the reclaimed chrome and savings related to landfill disposal. In 2023, animal glue prices range between $1.5 to $4 per kilogram, ¹⁵⁶ making it cost-effective to recycle collagen at an industrial scale even today, especially considering the abundance of low-cost leather solid waste, the maturity of hydrolysis-based extraction technologies, and the added value from chromium recovery and landfill cost savings. ¹⁵⁷ The competitive price of collagen extracted from tannery wastes makes the industrial production of adhesives and glues possible, especially given the new technology that today is apt to improve collagen yield recovery from tannery wastes, ¹⁵⁸ as seen in Table 1.