It is well-known that the sulfonation degree (DS) of aromatic syntan is an important factor affecting its retanning performances. But the quantitative relation between DS and syntan property and the influencing mechanism of DS on syntan property are not clarified. In this work, five phenolic formaldehyde syntans (PFSs) with the same polymerization degree but varying DS were prepared to investigate the effect of DS on the properties of syntan and crust leather. It was found that the absolute value of zeta potential and the particle size of PFS decreased with increasing DS in aqueous solution. Molecular dynamic simulation results proved that the DS of PFS was a major contributor to electrostatic interaction and hydrogen bonding in the PFS–water system and greatly affected the aggregation and dispersion of PFS in aqueous solution. The PFS with a low DS was prone to aggregate to large particles in aqueous solution because of low intermolecular electrostatic repulsion and less hydrogen bonds and therefore can be used to increase the thickness and tightness of leather. The PFS with a high DS presented a small particle size with more anionic groups in aqueous solution, thereby sharply decreasing the positive charge of leather surface and facilitating the penetration of the post-tanning agents into the leather. These results might be scientifically valid for rational molecular design of syntans and more productive use of syntans in leather making.

Polyurethanes have been widely used in many fields due to their remarkable features such as excellent mechanical strength, good abrasion resistance, toughness, low temperature flexibility, etc. In recent years, room-temperature self-healing polyurethanes have been attracting broad and growing interest because under mild conditions, room-temperature self-healing polyurethanes can repair damages, thereby extending their lifetimes and reducing maintenance costs. In this paper, the recent advances of room-temperature self-healing polyurethanes based on dynamic covalent bonds, noncovalent bonds and combined dual or triple dynamic bonds are reviewed, focusing on their synthesis methods and self-healing mechanisms, and their mechanical properties, healing efficiency and healing time are also described in detial. In addition, the latest applications of room-temperature self-healing polyurethanes in the fields of leather coatings, photoluminescence materials, flexible electronics and biomaterials are summarized. Finally, the current challenges and future development directions of the room-temprature self-healing polyurethanes are highlighted. Overall, this review is expected to provide a valuable reference for the prosperous development of room-temperature self-healing polyurethanes.

Acellular dermal matrix (ADM) is derived from natural skin by removing the entire epidermis and the cell components of dermis, but retaining the collagen components of dermis. It can be used as a therapeutic alternative to “gold standard” tissue grafts and has been widely used in many surgical fields, since it possesses affluent predominant physicochemical and biological characteristics that have attracted the attention of researchers. Herein, the basic science of biologics with a focus on ADMs is comprehensively described, the modification principles and technologies of ADM are discussed, and the characteristics of ADMs and the evidence behind their use for a variety of reconstructive and prosthetic purposes are reviewed. In addition, the advances in biomedical applications of ADMs and the common indications for use in reconstructing and repairing wounds, maintaining homeostasis in the filling of a tissue defect, guiding tissue regeneration, and delivering cells via grafts in surgical applications are thoroughly analyzed. This review expectedly promotes and inspires the emergence of natural raw collagen-based materials as an advanced substitute biomaterial to autologous tissue transplantation.

Effect of retanning on the thermal stability of leather is eliciting increasing attention. However, the relationship between the hydrophilicity of retanning agents and the heat resistance of leather and the corresponding mechanism remain unclear. Herein, phenolic formaldehyde syntans (PFSs) were selected as models to explore the effect of the hydrophilicity of retanning agents on the thermal stability of retanned leather. The thermal stability of leather was closely correlated to the hydrophilic group content (sulfonation degree) of PFSs. As the sulfonation degree increased, the water absorption rate of PFSs and their retanned leathers decreased, whereas the thermal stability of leather increased. Molecular dynamics simulation results proved that the introduction of PFSs could reduce the binding ability of collagen molecules with water and thus decreased the water molecules around the PFS-treated collagen. These results may provide guidance for the tanners to select retanning agents reasonably to improve the thermal stability of leather.

The use of natural polysaccharide crosslinkers for decellularized matrices is an effective approach to prepare cardiovascular substitute materials. In this research, NaIO4 was applied to oxidize konjac glucomannan to prepare the polysaccharide crosslinker oxidized konjac glucomannan (OKGM). The as-prepared crosslinker was then used to stabilize collagen-rich decellularized porcine peritoneum (DPP) to construct a cardiovascular substitute material (OKGM-fixed DPP). The results demonstrated that compared with GA-fixed DPP and GNP-fixed DPP, 3.75% OKGM [1:1.5 (KGM: NaIO4)]-fixed DPP demonstrated suitable mechanical properties, as well as good hemocompatibility, excellent anti-calcification capability, and anti-enzymolysis in vitro. Furthermore, 3.75% OKGM [1:1.5 (KGM: NaIO4)]-fixed DPP was suitable for vascular endothelial cell adhesion and rapid proliferation, and a single layer of endothelial cells was formed on the fifth day of culture. The in vivo experimental results also showed excellent histocompatibility. The current results demonstrted that OKGM was a novel polysaccharide cross-linking reagent for crosslinking natural tissues featured with rich collagen content, and 3.75% OKGM [1:1.5 (KGM: NaIO4)]-fixed DPP was a potential cardiovascular substitute material.

Nowadays, diverse leather usage conditions and increasing demands from consumers challenge the leather industry. Traditional leather manufacturing is facing long-term challenges, including low-value threshold, confined application fields, and environmental issues. Leather inherits all the biomimetic properties of natural skin such as flexibility, sanitation, cold resistance, biocompatibility, biodegradability, and other cross-domain functions, achieving unremitting attention in multi-functional bio-based materials. Series of researches have been devoted to creating and developing leather-based flexible multi-functional bio-materials, including antibacterial leather, conductive leather, flame-retardant leather, self-cleaning leather, aromatic leather, and electromagnetic shielding leather. In this review, we provide a comprehensive overview of the commonly used leather-based functional materials. Furthermore, the possible challenges for the development of functional leathers are proposed, and expected development directions of leather-based functional materials are discussed. This review may promote and inspire the emerging preparation and applications of leather for flexible functional bio-based materials.

Based on the demand of carbon peak and carbon emission reduction strategy, divinyl-terminated polydimethylsiloxane (ViPDMSVi), poly(methylhydrosiloxane) (PMHS), divinyl-terminated polymethylvinylsiloxane (ViPMVSVi), and fumed silica were used as primary raw materials, polydimethylsiloxane (PDMS) synthetic leather coating was in situ constructed by thermally induced hydrosilylation polymerization on the synthetic leather substrate. The effect of the viscosity of ViPDMSVi, the active hydrogen content of PMHS, the molar ratio of vinyl groups to active hydrogen, the dosage of ViPMVSVi and fumed silica on the performance of PDMS polymer coating, including mechanical properties, cold resistance, flexural resistance, abrasion resistance, hydrophobic and anti-fouling properties were investigated. The results show that ViPDMSVi with high vinyl content and PMHS with low active hydrogen content is more conducive to obtaining organosilicon coating with better mechanical properties, the optimized dosage of ViPMVSVi and fumed silica was 7 wt% and 40 wt%, respectively. In this case, the tensile strength and the broken elongation of the PDMS polymer coating reached 5.96 MPa and 481%, showing reasonable mechanical properties for leather coating. Compared with polyurethane based or polyvinyl chloride based synthetic leather, the silicon based synthetic leather prepared by this method exhibits excellent cold resistance, abrasion resistance, super hydrophobicity, and anti-fouling characteristics.

Supramolecular peptides exhibit obvious similarities with collagen fibers in terms of self-assembly characteristics, nanofibrous structure, and responsiveness to external stimuli. Here, a series of supramolecular peptides were developed by altering the amino acid sequence, enabling the self-assembly of three types of 4-biphenylacetic acid (BPAA)-tripeptides into fibrous hydrogel through hydrogen bonding and π–π stacking under the influence of ion induction. Transmission electron and scanning electron microscopies revealed that the diameter of the fiber within nanofibrous hydrogels was ~ 10 and ~ 40 nm, respectively, which was similar with the self-assembled collagen fibers. For this reason, these hydrogels could be considered as a biomimetic extracellular substitute. Meanwhile, the gelation concentration induced by ions was even lower than 0.66 wt%, with an elastic modulus of ~ 0.27 kPa, corresponding to a water content of 99.34 wt%. In addition, the three supramolecular hydrogels were found to be good substrates for L929 cell adhesion and MC-3T3 cell proliferation. The overall results implied that BPAA-based hydrogels have a lucrative application potential as cell carriers.

Restrictions on heavy metals, especially chromium, have encouraged alternative tanning systems that can reduce environmental and human health risks from conventional chrome-based tanning. In this work, metal-free combination tanning was developed by using vegetable tannins and a triazine-based syntan containing active chlorine groups (SACC). Specifically, the relationship between leather performance (e.g., hydrothermal stability and organoleptic properties) and technical protocols (e.g., types and dose of tannins) was systematically established. The optimized protocol involving a unique procedure (i.e., 10% SACC pre-tanning, shaving, and 25% wattle tanning) endowed the leather with high shrinkage temperature (~ 92 °C) and met the Chinese standards for shoe upper leather (QB/T 1873-2010). Our method not only produces zero chrome-containing solid wastes, but also uses ~ 75% less tannin for leather manufacture. The excellent leather performance was ascribed to the synergistic effects, where SACC and wattle diffused into collagen fibrils and may bind to collagen via covalent, hydrogen and ionic bonding, locking the hierarchical structure of collagen from microfibrils to fiber bundles. Moreover, we summarized these findings and proposed a diffusion-binding-locking mechanism, providing new insights for current tanning theory. Together with the biodegradable spent tanning liquor, this approach will underpin the development of sustainable leather manufacture.

This study aimed to identify and classify the type of plants used for tanning historical leathers using cost-effective Fourier transform infrared (FTIR) spectroscopy. The investigation was carried out on five plants (oak bark, sumac, valonia, tara, and gallnut) and four historical leather samples from book bindings dating back to the Qajar period. Tannin extraction from both plants and leathers was performed using acetone–water solvent, and the samples were then subjected to FTIR spectroscopy. Pre-processing of the spectra included baseline correction, smoothing, and normalization. Principal component analysis (PCA) was used to identify the source of tannins based on FTIR results. FTIR was found to provide a good separation of condensed tannins from hydrolysable ones. However, PCA analysis allowed for the separation and identification of the type of plant used for tannin extraction. The examination of historical leather samples revealed that the primary classification based on the type of tannin is possible, but accurate identification faces challenges due to structural changes and degradation over time.

Manufacture of eco-friendly chrome-free leather is of great significance for realizing sustainable development of leather industry. Conventional tanning theory believes that it is impossible to convert raw hide to leather without the utilization of cross-linking agent (e.g., chrome salts) among collagen fibers in raw hide. Here, we developed a brand-new leather manufacture strategy that relied on the composite dehydration media enabled self-driven directional dehydration mechanism to accomplish chrome-free leather manufacture for the first time, rather than followed the classic cross-linking mechanism that has been obeyed for more than one century in leather industry. We demonstrated that the essence of leather making is to regulate the water content in raw hide rather than to form cross-linkage among collagen fibers. The composite dehydration media comprised of anhydrous ethanol and molecular sieves (3A activated zeolite powder) successfully guaranteed continuous self-driven directional dehydration of raw hide by establishing stable water concentration gradient between raw hide and ethanol, which significantly increased the dispersity of collagen fibers in raw hide (with the water content reduced from 56.07% to 5.20%), thus obtaining chrome-free leather that is more ecological than chrome-tanned leather due to the elimination of any tanning agent. The as-prepared chrome-free leather exhibited outstanding tear force (174.86 N), tensile strength (24.56 N mm−2), elongation at break (53.28%) and dry-thermal stability, superior to chrome-tanned leather. Notably, the used composite dehydration media was recyclable for chrome-free leather manufacture, therefore facilitating an environmentally benign leather manufacture process. Our investigations are expected to open up a new conceptual leather making strategy that is applicable for realizing substantial manufacture of eco-friendly leather.

Wet-salted skin, as a special artificial high-salt environment, is rich in protein, fat, collagen and other nutrient substrates, and is a rich resource of halotolerant and halophilic microorganisms. However, knowledge gaps regarding the microbial community structure and inter taxa associations of wet-salted skin are large. In this study, the spatiotemporal dynamics and community structure of microorganisms present on wet-salted goatskins were investigated using 16S rRNA gene amplicon sequencing and culturable technique. Alpha diversity analysis based on Sobs, Chao, Ace and Shannon indices revealed that microbial diversity on the wet-salted goatskins exhibited a trend of ‘down → up → down → flat’ with time. During preservation, genera belonging to the bacteria domain such as Acinetobacter, Weissella and Streptococcus were slowly dying out, whereas those belonging to halophilic archaea such as Natrialba and Haloterrigena were gradually flourishing. Moreover, to resist high-salt stress, microorganisms on the wet-salted goatskin gradually migrated from the outside to the inside, eventually leading to the microbial diversity inside the skin being the same as or even higher than that on the skin surface. Venn diagram analysis revealed that the strains of some genera, including Psychrobacter, Salimicrobium, Salinicola, Ornithinibacillus, Halomonas, Bacillus and Chromohalobacter, were distributed throughout the interior and exterior of the wet-salted goatskin and existed during various periods. Accordingly, 45 protease-producing halophilic or halotolerant microorganisms were isolated and screened from the wet-salted goatskin using the gradient dilution plate method. Importantly, 16S rRNA genes of some bacteria exhibited less than 99.5% similarity to valid published species, indicating that they likely are novel species and have a good potential for application.

The production of high-valued organonitrogen chemicals, especially N-heterocycles, requires artificial N2 fixation accompanied by the consumption of fossil resources. To avoid the use of these energy- and resource-intensive processes, we develop a sustainable strategy to convert nitrogen-rich animal biomass into N-heterocycles through a thermochemical conversion process (TCP) under atmospheric pressure. A high percentage of N-heterocycles (87.51%) were obtained after the TCP of bovine skin due to the abundance of nitrogen-containing amino acids (e.g., glycine, proline, and l-hydroxyproline). Animal biomass with more diverse amino acid composition (e.g., muscles) yielded higher concentrations of amines/amides and nitriles after TCP. In addition, by introducing catalysts (KOH for pyrrole and Al2O3 for cyclo-Gly–Pro) to TCP, the production quantities of pyrrole and cyclo-Gly–Pro increased to 30.79 mg g−1 and 38.88 mg g−1, respectively. This approach can be used to convert the significant animal biomass waste generated annually from animal culls into valued organonitrogen chemicals while circumventing NH3-dependent and petrochemical-dependent synthesis routes.

Collagen made a tremendous impact in the field of regenerative medicine as a bioactive material. For decades, collagen has been used not only as a scaffolding material but also as an active component in regulating cells' biological behavior and phenotype. However, animal-derived collagen as a major source suffered from problems of immunogenicity, risk of viral infection, and the unclear relationship between bioactive sequence and function. Recombinant humanized collagen (rhCol) provided alternatives for regenerative medicine with more controllable risks. However, the characterization of rhCol and the interaction between rhCol and cells still need further investigation, including cell behavior and phenotype. The current study preliminarily demonstrated that recombinant humanized collagen type III (rhCol III) conformed to the theoretical amino acid sequence and had an advanced structure resembling bovine collagen. Furthermore, rhCol III could facilitate basal biological behaviors of human skin fibroblasts, such as adhesion, proliferation and migration. rhCol III was beneficial for some extracellular matrix-expressing cell phenotypes. The study would shed light on the mechanism research of rhCol and cell interactions and further understanding of effectiveness in tissue regeneration.

Hide and skin are complex tissue where the most abundant component is collagen. Matrix metalloproteinases and bacterial collagenases are two kinds of collagenases that can cleave the triple-helical domain of native fibrillar collagens. In this paper, the family members and domain composition of matrix metalloproteinases and bacterial collagenases are summarized. The catalytic mechanism of collagen hydrolysis by collagenases is described, and the methods adopted to date for investigating and regulating collagenases and their inhibitors are reviewed. Furthermore, the applications of collagenases and their inhibitors in biomedicine, food processing and the enzymatic unhairing process in the leather-making industry are presented.

Modern leather industries are focused on producing high quality leather products for sustaining the market competitiveness. However, various leather defects are introduced during various stages of manufacturing process such as material handling, tanning and dyeing. Manual inspection of leather surfaces is subjective and inconsistent in nature; hence machine vision systems have been widely adopted for the automated inspection of leather defects. It is necessary develop suitable image processing algorithms for localize leather defects such as folding marks, growth marks, grain off, loose grain, and pinhole due to the ambiguous texture pattern and tiny nature in the localized regions of the leather. This paper presents deep learning neural network-based approach for automatic localization and classification of leather defects using a machine vision system. In this work, popular convolutional neural networks are trained using leather images of different leather defects and a class activation mapping technique is followed to locate the region of interest for the class of leather defect. Convolution neural networks such as Google net, Squeeze-net, RestNet are found to provide better accuracy of classification as compared with the state-of-the-art neural network architectures and the results are presented.

Leather is a collagen-based biomass prepared from raw skins or hides by a series of unit operations, in which the unhairing and fiber opening are extremely important operations. However, the conventional Na2S/Ca(OH)2 system used in unhairing and fiber opening has given rise to the pollution to the environment. It is necessary to develop substitute technology for the Na2S/Ca(OH)2. In the present study, 1-allyl-3-methylimidazolium chloride ([AMIm]Cl) was used to cooperate with dispase for cycle unhairing and one-pot beamhouse to recycle waste bovine hides and compared with conventional processing. During those processes, the mechanism of [AMIm]Cl-dispase synergistic unhairing and collagen fibers opening were studied. Besides, plant hazard, organic matter and [AMIm]Cl of wastewater from [AMIm]Cl-dispase process were respectively investigated and separated to evaluate the environmental and economic benefits of the [AMIm]Cl-dispase process. As a result, enzyme activity after unhairing by [AMIm]Cl-diapase system for using 5 times is higher than that by KCl-dispase system, and needs lower unhairing time, which is because of rapid penetration of [AMIm]Cl-dispase solution in bovine hides. For this reason, the tensile strength and elastic modulus of tanned leather from [AMIm]Cl-dispase process are higher than those from the KCl-diapase and conventional processes, and its hydrothermal shrinkage temperature is comparable to that of the conventional one. Because of the 58.13% lower wastewater discharge (WD), 66.60% lower total solids (TS), 97.23% lower ammonia nitrogen (NH3-N), non-toxic wastewater and organic matter recovery in wastewater are reached from [AMIm]Cl-dispase process, which is expected to be an alternative to the conventional process to reduce environmental pollution and realize the sustainable development of technology for leather manufacturing.

Recently, metal–organic frameworks (MOFs) have received considerable attention as highly efficient adsorbents for dye wastewater remediation. However, the immobilization of MOFs on the substrate surfaces to fabricate easy recyclable adsorbents via a facile route is still a challenge. In this work, ZIF-67/cotton fibers as adsorbents for dye removal were prepared in a large-scale using a simple coordination replication method. The successful fabrication of the ZIF-67/cotton fibers was confirmed by FTIR, XRD, XPS, SEM and BET analysis, respectively. As expected, the as-prepared ZIF-67/cotton fibers exhibited high adsorption capacity of 3787 mg/g towards malachite green (MG). Meanwhile, the adsorption kinetics and isotherm obeyed the pseudo-second-order kinetics and Langmuir model, respectively. Moreover, its removal efficiency towards MG was not significantly influenced by the pH and ionic strength of aqueous solution. Most importantly, the ZIF-67/cotton fibers can remove MG from synthetic effluents, and it can be easily regenerated without filtration or centrifugation processes, with the regeneration efficiency remaining over 90% even after 10 cycles. Additionally, the ZIF-67/cotton fibers presented excellent antimicrobial performance against E. coli and S. aureus. Hence, the distinctive features of the as-prepared ZIF-67/cotton fibers make it promisingly applicable for the colored wastewater treatment.

Amphoteric polymer can be used as retanning agent in leather manufacture. It is particularly useful in chrome-free tanning systems since it can regulate the charge properties of chrome-free leather and enhance the fixation of anionic post-tanning chemicals in leather. However, the aggregation and precipitation of amphoteric polymer retanning agents around the isoelectric point (pI) hinder their wide application. Herein, we synthesized five amphoteric acrylic polymers (AAPs) by free radical copolymerization with acrylic acid and five different cationic acrylic monomers. The effect of cationic monomer structure on the aggregation behavior of AAPs was investigated. The aggregation of AAPs in aqueous solution showed pH and concentration dependence. Light scattering analysis showed that Poly (AA-co-MAPTAC) and Poly (AA-co-DMAPMA) were in the shape of coiled linear flexible chains with small particle size (Rg 7.6 nm and 14.8 nm, respectively) near the pI. However, Poly (AA-co-DAC), Poly (AA-co-DMC) and Poly (AA-co-DMAEMA) were in the shape of hollow spheres and exhibited serious aggregation. Quantum chemical calculations suggested that the amide groups in the cationic monomers MAPTAC and DMAPMA enhanced the nucleophilicity of AAPs. Thus the corresponding AAPs could carry a large number of cationic charges to slow their aggregation when the pH just climbed over the pI. The results are expected to provide theoretical reference for the synthesis and widespread application of AAPs.