Advanced collagen nanofibers-based functional bio-composites for high-value utilization of leather: A review

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This review describes how UFL will expand the scope of applications and significantly improve the ontological value of natural leather. Significantly, UFL is expected to boost the sustainable development of conventional leather industry and the next-generation of advanced functional bio-based materials.

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Nội dung Text: Advanced collagen nanofibers-based functional bio-composites for high-value utilization of leather: A review

Journal of Science: Advanced Materials and Devices 6 (2021) 153e166 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Review Article Advanced collagen nanoﬁbers-based functional bio-composites for high-value utilization of leather: A review Youyou Wang a, b, 1, Manhui Zheng a, b, 1, Xinhua Liu a, b, c, *, Ouyang Yue a, b, Xuechuan Wang c, **, Huie Jiang a, b a College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science & Technology, Wei Yang District, Xi'an 710021, Shaanxi, China b National Demonstration Center for Experimental Light Chemistry Engineering Education, Shaanxi University of Science & Technology, Weiyang District, Xi'an 710021, Shaanxi, China c Institute of Biomass & Functional Materials, Shaanxi University of Science & Technology, Weiyang District, Xi'an 710021, Shaanxi, China a r t i c l e i n f o a b s t r a c t Article history: Natural leather with outstanding biomimetic properties is the main application of animal skin. However, Received 10 October 2020 the traditional tannery industry is facing serious issues of sustainability, such as low value addition and Received in revised form high environmental pollution. Upscaled functional leather (UFL) inherits all the natural properties of 21 January 2021 leather and combines it with other cross-domain functions, due to which it has gained considerable Accepted 3 February 2021 Available online 13 February 2021 attention as advanced functional bio-based materials. This review brieﬂy introduces the multi- hierarchical structure and preponderant properties of leather as a natural biomass. Further emphasis is on UFL, which includes electromagnetic (EM) wave absorbing leather, infrared absorbing leather, X-ray Keywords: Functional leather shielding leather, ﬂexible conductive leather, ﬂame-retardant leather, antibacterial leather, self-cleaning Biomass leather as well as water-, oil-, and soil-repellent leather. Furthermore, the fabrication methods of these Bio-based materials UFLs, their merits and their respective applications are comprehensively discussed. Moreover, possible Sustainability challenges and outlooks for the development of UFL are proposed. This review describes how UFL will expand the scope of applications and signiﬁcantly improve the ontological value of natural leather. Signiﬁcantly, UFL is expected to boost the sustainable development of conventional leather industry and the next-generation of advanced functional bio-based materials. © 2021 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/). 1. Introduction [2]. Leather products have always played an important role in the lives of humans. According to reports, leather production would Animal skin is a byproduct of meat industry, which is further reach 19 billion square feet in 2020 globally and, hence, leather processed into leather by employing a series of physical, chemical, industry holds a lot of promise. Leather is a unique material pos- and mechanical methods [1]. Manufacturing of leather has been sessing many properties [3], like softness, air and water vapor one of the earliest activities of mankind. By the end of the Neolithic permeability, good hygroscopicity and wearability, ﬂexibility, period, people in the eastern Mediterranean region started using genuine and unique beauty, etc. Moreover, it possesses the ad- leaves, fruits, rhizomes, and other plant materials to process animal vantages of biomass materials, such as biodegradability and skins and transformed them into more durable and useful materials excellent biocompatibility. This is in contrast to polymer substrates with inferior mechanical properties and wearability, which can potentially cause damage to the environment owing to their poor * Corresponding author. College of Bioresources Chemical and Materials Engi- biodegradability. Leather is used not only for manufacturing neering, Shaanxi University of Science & Technology, Wei Yang District, Xi'an products of daily use, like clothes, shoes, bags, decorative articles, 710021, Shaanxi, China. etc. [4], but it is also shifting its focus towards functional materials. ** Corresponding author. However, with increased impetus on environmental protection E-mail addresses: liuxinhua@sust.edu.cn (X. Liu), wangxc@sust.edu.cn (X. Wang). world-wide, wastewater, sludge, and solid waste from tannery have Peer review under responsibility of Vietnam National University, Hanoi. gradually become the three technological bottlenecks for sustain- 1 Youyou Wang and Manhui Zheng contributed equally to this work. able development of leather industry. The cost of leather https://doi.org/10.1016/j.jsamd.2021.02.001 2468-2179/© 2021 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Y. Wang, M. Zheng, X. Liu et al. Journal of Science: Advanced Materials and Devices 6 (2021) 153e166 production increases greatly while tackling these eco- In addition, special functional materials can be compounded along environmental issues, making the traditional leather industry with tanning agents, retanning agents, or fatliquoring agents dur- challenging. It is crucial for the leather industry to signiﬁcantly ing leather processing, to prepare corresponding functional leather. enhance the value of the resultant leather products along with Herein, we brieﬂy introduce the multi-hierarchical structure ensuring environmental safety. and properties of leather, taking into account the histological and With developments in science and technology and improve- structural characteristics of natural collagen and animal skin. The ment in the standard of living, leather processing has become review mainly focuses on the existing functional and natural cleaner and more eco-friendly [5,6]. Basic properties of leather leathers, prepared from animal skins as raw bio-based materials. products, such as fullness, air permeability, cold resistance, ﬂexural Novel functional leathers, such as electromagnetic (EM) wave strength, etc., have been improved. Owing to a large variety of shielding leather [12], thermal camouﬂage leather [13], X-ray species, different outputs and functionality, there are various types shielding leather [14] and conductive leather for smart devices [3], of functional leathers. Recently, materials with advanced functions ﬂame retardant leather, antibacterial leather, self-cleaning leather, have become more popular, due to their conductivity, ﬂame have been explored. Moreover, conventional functional leathers, retardancy, self-cleaning property, antibacterial property, electro- such as waterproof, anti-fouling, and oil-proof leather have also magnetic shielding effectiveness, wave absorptivity, and X-ray made a lot of progress. With developments in UFL, the scope of shielding ability. Functional leathers such as conductive leather, applications of leather will further expand and there will be a ﬂame retardant leather, self-cleaning leather, antibacterial leather, dramatic increase in the ontological value of natural leather waterproof leather, anti-fouling leather, oil-proof leather, electro- biomass. Signiﬁcantly, functional leather will boost the sustainable magnetic shielding leather, thermal camouﬂage leather, and X-ray development of conventional leather industry and also of next- shielding leather have been successfully developed by combining generation advanced functional bio-based materials. with other functional materials. Functional leather is prepared using conventional processing techniques, wherein functional 2. Multi-hierarchical structure and properties of skin chemicals with speciﬁc properties are added either during pro- cessing or ﬁnishing of leather. The functional materials are either Leather, a natural bio-product with multi-layered hierarchical linked to the active groups of collagen ﬁbers in leather or directly structure, has been in focus since a long time. Indeed, leather is mixed with materials such as ﬁlm-forming agents for leather ﬁn- derived from natural collagen, which is an elementary building ishing. They remain on the leather by forming chemical linkages or block of collagen-rich tissues. The conformation of natural collagen by physical adhesion and impart speciﬁc functions to the resultant that accounts for more than 50% of the skin, includes ﬁve hierar- leather with. For example, ﬂame retardancy is imparted during chical structures: primary structure (amino acid triplet), secondary leather processing of ﬂame retardant leather [7e9]. Moreover, structure (a-helix), tertiary structure (triple helix), quaternary conductive material is added to produce conductive leather [10,11]. structure (ﬁber), and supramolecular aggregation, based on the Fig. 1. Schematic diagram showing the histological structure of animal skin (1). Reproduced with permission [16]. Copyright © 1969, Springer Nature. Illustration of the multi- hierarchical structure of nature collagen (2), Reproduced with permission [17]. Copyright © 2019, Elsevier Science B.V. 154
Y. Wang, M. Zheng, X. Liu et al. Journal of Science: Advanced Materials and Devices 6 (2021) 153e166 biosynthesis and in vivo assembly processes of collagen into its makes it diversely functional. In addition to the basic functions of hierarchical ﬁbrillar architectures [15], which are shown in Fig. 1. typical biomass materials, such as excellent biodegradability and Type I collagen, which is mainly present in animal skin, pos- biocompatibility, it also has good mechanical properties, thermal sesses three a chains. Each chain consists of more than 1000 amino stability, is hygienic, and shows wear resistance. The multi- acid residues, with repetition of permutations along the entire hierarchical structure makes it possible to impart it with multiple chain and having proline and hydroxyproline in a higher propor- functions by regulating its structure. tion. Proline is conformationally metastable, due to which the abundant number of Gly-Pro-Hyp (Gly ¼ glycine, Pro ¼ proline, 3. Functional leather Hyp ¼ hydroxyproline) triplets in collagen triple helices, confer it with well-deﬁned ﬂexibility [17]. Therefore, the alignment of Gly- 3.1. EM wave absorption leather (EMI shielding materials) Pro-Hyp triplets within the ﬁbrillar structure provides ﬂexibility to the ﬁbrils [18]. Meanwhile, the secondary bonds formed between As a result of high increase in emissions from antenna and peptide chains, as well as the intramolecular and intermolecular electronics equipment, EM radiations have become a matter of covalent bonds formed by aldol condensation, aldolamine concern. EMI shielding materials are essential for solving the condensation, and aldol histidine crosslinking reactions between problem of EM pollution [23,24]. This has led to intensive research the collagen molecules, stabilize the natural triple-stranded in materials related to EMI shielding, such as carbon-based polymer conformation. Hence, natural collagen has a very high tensile nanocomposites [25], novel metallic materials [26], MXene-based strength and structural stability [19,20]. It should be noted that the materials [27], etc. The main mechanisms of shielding of EM basic tissue structure of natural collagen is collagen bundles [21]. waves are reﬂection, absorption, and multiple reﬂections [28]. These collagen bundles interpenetrate and intertwine into one Conductive metals such as silver, gold, and copper have the ability another to form a three-dimensional network, imparting the to reﬂect EM waves, whereas ZnO, SiO2, and TiO2 with high leather with outstanding mechanical properties [17]. Furthermore, dielectric constants, have the ability to absorb EM rays. Moreover, this dense felt work of collagen at a macro level endows the porous foams and functional polymer nanocomposites with large rawhide and its products (leather) with special mechanical prop- interfaces have the ability to shield EMI through multiple re- erties [22], good air permeability, ﬂexibility and water vapor ﬂections [29]. Lightweight, foldable, and conductive composite permeability. In fact, owing to the multi-hierarchical structure of paper for EMI shielding, has been researched [30]. Leather, which is collagen, the hierarchical organization at any structural level of usually worn and decorated, can also be used to prepare EMI collagen material allows for the intervention and interplay of a shielding materials. A comparison of the preparation methods and series of design features, which is very signiﬁcant for speciﬁc ap- performances of different EMI shielding materials is summarized in plications. All in all, the multi-layer structure of natural leather Table 1. Table 1 Typical examples of EMI based on different materials. EMI shielding Key components Preparation method Shielding Features Disadvantages Ref. materials effectiveness (dB) MXene aerogel/ Nature wood-derived Freeze-dried and ~71.3 dB Ultra-light, green and Poor wearability [31] WPC composites porous carbon (WPC) annealed efﬁcient skeleton 3D MXene aerogel Hybrid aerogel ﬁlm Strong aramid nanoﬁbers Facial blade coating and ~54.4 dB Hydrophobicity Poor wearability [32] of FC-ANF/CNT (ANF) freeze-drying Self-cleaning Poor mechanical ﬁlms Conductive carbon Exceptional Joule heating properties nanotubes (CNT) performance Poor shielding effect Hydrophobic ﬂuorocarbon (FC) resin EMA/K-CB Ketjen carbon black (K-CB) Dropwise K-CB solution ~33.9 dB Low percolation threshold Poor mechanical [33] composites Ethylene methyl acrylate to the EMA solution properties copolymer (EMA) Metallic nanomesh Single-layer metallic Ultraviolet lithography >40 dB High transmittance (85% at Not light enough and [34] nanomesh (Cu, Cr) and the ion beam 550 nm) inﬂexible A transparent dielectric etching technique Wide broadband range (0.5 substrate (glass) e40 GHz) PANI-based Polyethylene terephthalate In-situ polymerization ~23.95 dB Bending durability (10,000 Poor shielding effect [30] composite paper (PET) paper polyaniline deformation cycles) (PANI) MXene-decorated MXene nanosheets Spray-drying ~36 dB Joule heating, strain sensing Poor shielding effect [35] fabrics Plain weave cotton fabrics performance Flexibility and breathability EMI materials Natural collagen ﬁbers Thermal treatment 36e57 dB Lightweight Poor mechanical [36] based on Ferromagnetic Low-cost properties. collagen ﬁbers nanoparticles (NPs) Flexible Skin collagen ﬁber (CF) Electroplated ~31.0 dB [37] NieFeeP coating EM waves LM Coating MNPs onto LM 4.58 wt % of coated Lightweight May slightly affect the [12] absorption Metal NPs (MNPs) MNPs Excellent processability texture of leather leather 76.0 dB Flexibility and breathability LM Spaying Cu@Ag 5.17 v% of Cu@Ag High-performance [38] Cu@Ag nanoﬂakes nanoﬂakes on LM nanoﬂakes Wearable 100 dB 155
Y. Wang, M. Zheng, X. Liu et al. Journal of Science: Advanced Materials and Devices 6 (2021) 153e166 Table 1 shows that compared to other EMI shielding materials, systems or human bodies to comply with special technical re- leather-based materials possess good EMI shielding properties. quirements of stealth. With continuous progress in infrared sur- Additionally, they also inherit the preponderant properties of nat- veillance technologies [42,43], thermal cloaking and camouﬂage ural leather, such as lightweight, excellent processability, ﬂexibility, have gained much attention [44e47]. The preparation methods and breathability, etc. On one hand, due to their high electrical con- performances of different thermal camouﬂage materials are ductivity, metal matrix materials can effectively reﬂect EM rays. On compared and summarized in Table 2. the other hand, leather matrix (LM) is a natural material with Although leather is one of the oldest clothing materials, it is still dielectric properties. It has a hierarchical supraﬁbrillar structure popular, due to its good ﬂexibility, comfortable wearability, high and abundant number of dipoles, which improve the energy water-vapor permeability, and high mechanical strength. Signiﬁ- dissipation of EM rays [12]. The microstructure containing bundles cantly, the intricacy at each level (nano to micro and macro) in the of 3D collagen nanoﬁbers can signiﬁcantly induce multiple diffused hierarchy of leather structure has an amazingly strong multiple reﬂections of EM rays, which dramatically increase the trans- diffuse reﬂection ability and an efﬁcient insulation property [13]. mission routes of microwaves inside the LM. This promoted the Wang et al. [13] demonstrated the in situ growth of SiO2 NPs on the dipole relaxation-induced dielectric loss to EM microwaves 3D hierarchical leather scaffolds (Fig. 3-(1)(2)), which not only [39e41]. Therefore, metal nanoparticles (MNPs) can be coated on increased the porosity of leather, but could also trap stagnant air as the LM to obtain EMI shielding composite materials that possess a heat insulation material more effectively. In addition, it could also both EM rays absorptivity and reﬂectivity [12]. This makes it su- prevent the infrared absorbing groups (-NH2) of the leather from perior to conventional polymer matrices that lack dielectric loss absorbing infrared radiation. This improved the infrared reﬂection ability. For instance, Cu@Ag nano-coating was applied on the sur- ability of leather from hot objects. The prepared multifunctional face of leather to obtain a lightweight, high-performance, and thermal camouﬂage leather had the advantages of high ﬂexibility wearable EMI shielding leather (Fig. 2) [38]. This coating had good (Fig. 3-(3) a), good hydrophobicity (Fig. 3-(3) h), ﬂame retardancy stability with no obvious cracking after bending 25,000 times. (Fig. 3-(3) i), high porosity, low thermal diffusivity, and thermal conductivity. It can be potentially used in wet environmental con- 3.2. Infrared absorption leather (Thermal camouﬂage materials) ditions (Fig. 3-(3) (e-g)) and shows an outstanding long-term thermal camouﬂage performance for human body (Fig. 3-(3) j). Thermal camouﬂage materials, also known as high-temperature Moreover, it can be tailored into diverse complex patterns with infrared camouﬂage materials or infrared stealth materials can be intact infrared stealth ability (Fig. 3-(3) (b-d)), endowing leather used to weaken the characteristic signals of infrared weapon with new functions. Fig. 2. (1) Schematic representation showing the lightweight and high-performance electromagnetic radiation shielding composites based on a surface coating of Cu@Ag nano- ﬂakes on a LM. (2) Photos of ﬂexible LM-Cu@Ag samples (10 cm 10 cm) (a), cross-section microstructure of LM-Cu@Ag (b), top view of SEM image of LM-Cu@Ag (c), conductivity of LM-Cu@Ag with different volumetric fractions (0e5.17 v%) of Cu@Ag nanoﬂakes (d), and the corresponding log value of the conductivity with the threshold of Cu@Ag nanoﬂakes, fc ¼ 0.41 v% (Inset Fig. 4 (2) d), photos of meter-scaled LM-Cu@Ag (e), the wearable clothing tailored from meter-scaled LM-Cu@Ag (f, g). (3) Hierarchical ﬁbrous structure of the LM (a), SEM images of different levels of microstructure morphology of the LM (bee), propose shielding mechanism of LM-Cu@Ag to microwaves (f). Reproduced with permission [38]. Copyright © 2016, Royal Society of Chemistry. 156
Y. Wang, M. Zheng, X. Liu et al. Journal of Science: Advanced Materials and Devices 6 (2021) 153e166 Table 2 Typical examples of thermal camouﬂage based on different materials. Thermal Key components Preparation methods Camouﬂage performance Features Disadvantages Ref. camouﬂage materials Wearable Flexible thermoelectric Integrating a layer The metabolic heat from human Adaptive within a wide Complex construction [44] thermal device (TED) [48] skin cannot be seen by infrared range of background Poor wearing comfort camouﬂage Cloth (fabric) camera in 16e38 C temperature (7 Ce15 C) device Phase change material Thermal conductivity Anti-IR reﬂection Property (PCM) layer (30 W m2 K1) Wearable High-emissivity elastomer Infrared Parafﬁn Insitu polymerization Emissivity is 0.575 Better ﬂame retardancy Potential formaldehyde [49] camouﬂage Urea-formaldehyde Reduce temperatures by 5 Washable hazard fabric resin e10 C Flexibility and bending Fabric resistance Leather SiO2 NPs In situ grown on the (Kirchhoff law) ε ¼ 0.63 Hydrophobicity Slightly affect the [13] thermal Leather ﬁbrous scaffold of Low thermal diffusivity Flame-resistance softness of the resultant camouﬂage leather (1.24 107 m2 s1) Arbitrary tailorability leather armor Low thermal conductivity Wearable (0.04 W m1 K1) Flexibility Smaller surface temperature difference (DT) Fig. 3. (1) The schematic illustration showing the preparation from (a) leather to (b) leather/SiO2 and (c) SiO2-leather/SiO2, (d) FESEM image showing the cross-section of leather/ SiO2, FESEM images of leather/SiO2 showing the (e) microﬁber bundle, (f) microﬁber and (g) nanoﬁber, (h) TEM and (i) HAADF-STEM images of primary nanoﬁber contained in leather/SiO2, (j) FTIR spectra of cowhide, leather and leather/SiO2.(2) The graphical abstract of the multifunctional thermal camouﬂage armor by direct editing the natural insulation structure of leather. (3) (a) Photograph of SiO2-leather/SiO2, thermal images of SiO2-leather/SiO2 with different patterns covered on the surface of plastic tanker ﬁlled by water with the temperatures of (b) 38 C, (c) 50 C and (d) 72 C, thermal images of hydrophobic SiO2-leather/SiO2 (e) before, (f) ongoing and (g) after water pouring on its surface, (h) the surface wetting behaviors of hydrophobic SiO2-leather/SiO2, (i) photograph of hydrophobic SiO2-leather/SiO2 burning by the ﬂame of alcohol lamp, (j) surface temperature and thermal images of thermal camouﬂage armor of SiO2-leather/SiO2 dressed by people. Reproduced with permission [13]. Copyright © 2019, Elsevier Ltd. 3.3. X-ray shielding leather (X-ray protection materials) the K or L absorption edges of an element play a major role in the attenuation of X-ray photon energy and have signiﬁcant attenua- X-rays have been ubiquitously utilized in everyday life and in- tion effect on X-ray photons of similar energy. Therefore, materials dustrial processes. However, long-term exposure to X-rays can with high mass densities (r) and high atomic numbers (Z) are cause biological effects, can directly or indirectly affect human considered to be effective X-ray shielding materials [52]. tissues, and can also have harmful effects on human health [50,51]. Commonly used shielding materials include lead plate, cement, Thus, effective radiation shielding materials protect people from alloy, and lead rubber. However, poor ﬂexibility, high toxicity and excessive exposure to X-rays in an X-ray environment. Generally, high speciﬁc gravity result in poor wear-ability. Meanwhile, 157
Y. Wang, M. Zheng, X. Liu et al. Journal of Science: Advanced Materials and Devices 6 (2021) 153e166 Fig. 4. (1) Graphical abstract of X-ray shielding by Biz@BixLayNL;(2) Photos of foldable and ﬂexible Bi28.2@Bi3.48La3.48NL samples (10 cm 10 cm). Reproduced with permission [55]. Copyright © 2020, American Chemical Society. (3) Graphic illustration of the facile two-step “impregnationdesolvation” strategy for the preparation of the BiINP- LM sample (A). SEM images of LM (B), BiINP-LM (C) and elemental mappings of Bi and I (D). Digital photographs of the as-prepared BiINP-LM (E). Reproduced with permission [54]. Copyright © 2020, American Chemical Society. polymer-based composite materials have low air permeability and address the serious challenges of protection against X-rays softness, which makes it difﬁcult for them to meet the re- (Fig. 4). NL, owing to its multi-layer woven structure and quirements of wearability. porous nature, can achieve multiple attenuation of photon en- Signiﬁcantly, natural leather (NL) can be considered as a ergy. Various functional groups such as amino and carboxyl prospective material, having many application potentials, to groups can be chemically modiﬁed to stabilize elements with Fig. 5. (1) SEM images showing the surface of (a) untreated and treated leathers (b) through single (aniline 0.30 M, APS 0.40 M, HCl 0.20 M, polymerization time 4 h and 5 ± 2 C), (c) double (aniline 0.15 M, APS 0.20 M, HCl 0.10 M, 4 h and 5 ± 2 C) in-situ polymerization of aniline and the corresponding FTIR spectrum (d). (2) Effect of concentration of (a) aniline, (b) oxidant and (c) hydrochloric acid on conductivity and color of treated leathers. (3) Percentage reﬂectance values against visible wavelength of (a) control and experimental leathers treated with single and double in-situ polymerization of aniline; (b) magniﬁed plot for the experimental leathers treated with single and double in-situ polymerization of aniline. (4) Photos of treated and untreated Sheep Nappa leather. Reproduced with permission [58]. Copyright © 2015, John Wiley and Sons. 158
Y. Wang, M. Zheng, X. Liu et al. Journal of Science: Advanced Materials and Devices 6 (2021) 153e166 high atomic number. In addition, LM contains some amount of ethylenedioxythiophene), and carbon nanotubes to the surface of moisture, which helps to absorb the heat generated by photons, leather to obtain a capacitive touch screen panel, which displayed thus greatly reducing the aging problem caused by photon ra- good conductivity and working performance. Polypyrrole possesses diations. Furthermore, compared with other X-ray protection high conductivity, is easy to synthesize, has good environmental materials, it has superior ﬂexibility, tailorability, and good stability, and less toxicity and hence can be used to prepare wearability. Thus, Bi/Ce NPs or sodium tungstate can be incor- conductive leather [60,61]. Wegene et al. [3] reported that a coating porated into NL using retanning method to fabricate a novel X- of polypyrrole on leather, prepared by in situ polymerization of ray protection material based on leather [14,53]. Xuepin Liao pyrrole, can simultaneously impart conductivity and color to et al. [54] engineered an advanced leather-based bismuth io- leather. dide nanoparticle-loaded X-ray shielding material (BiINP-LM) employing an “ImpregnationDesolvation” strategy. In short, X- 3.4.2. Flexible sensors, electronic skin (e-skin) ray shielding leather opens up new avenues for wearable X-ray Flexible wearable sensors and electronic skin have a broad range protection materials. of application prospects in personalized health monitoring systems, humanecomputer interactions, and soft robotics [62e68]. 3.4. Conductive leather Currently, a variety of materials have been developed to construct ﬂexible sensors. The preparation methods and performances of 3.4.1. Touch screen gloves different ﬂexible sensors based on various materials are summa- Conductive leather was created for applications in smart or rized in Table 3. From the table (Table 3), it is evident that the as- advanced products. Gloves made of conductive leather can be used prepared ﬂexible leather-based strain sensor shows good perfor- to operate touch-screen devices such as smart phones, iPod, tablet mance in terms of linearity, hysteresis effect, response time, sta- computers, etc. [3,56,57], making their use more convenient. W. bility, and durability. Notably, the homogeneously layered structure Jima Demisie et al. [58] reported the synthesis of self-colored of leather improves the linearity between the electrical signal and (bluish green) and conducting leather by varying the concentra- the strain applied on the leather-based sensor. Therefore, a leather- tions of aniline, hydrochloric acid, and APS (Fig. 5). Hong [59] based sensor having ﬁbrous microstructure, not only possesses applied electrically conductive materials, polyaniline, poly-(3, 4- inherent air permeability, mechanical property, biocompatibility, Table 3 Typical examples of ﬂexible sensor based on different materials. Type Key components Preparation Sensing performance Features Disadvantages Ref. method Response time Calibration curve and recovery time Bio-inspired multi- Collagen aggregates Two-step assembly 110 ms Two linear regions Wearing, Poor mechanical [71] functional sensor Polyaniline and accordingly Degradability, properties Acidiﬁed multi-walled gauge factor (GF) carbon nanotube (H- are 10,5.2 MWNTs) CB/leather strain sensor Conductive nanomaterials Filtration Compressive Three linear regions Flexible and stretchable Only suitable [70] carbon black (CB) bending and accordingly GF wearability application for Leather 80 ms are 10.57, 21.49, compressive and Tensile bending 7.29 tensile stress 100 ms Conductive MXene Ethylene glycol (EG) Immersing Not mentioned Two linear regions Self-healing capability Poor mechanical [72] nanocomposite MXene nanocomposite and accordingly GF properties organo-hydrogel hydrogel (MNH) are 5.02,44.85 (MNOH) sensor Ag/PDMS Small silver particles and Physically grinding, 125 ms Three linear regions Multidimensional Detection range is [73] PDMS mixing and accordingly GF sensing relatively narrow are 10,71,550 Weak vibration detecting Paper-based (PB) strain Printing paper Dip-coating Not mentioned Compression and Renewability, Only suitable for [74] sensor CB/CNT/methylcellulose tension strain Biodegradability and low tension or suspension (±0.7%) and low cost compression strain Hydrophobic fumed silica accordingly GF are detection. (Hf-SiO2) suspension 7.5,2.6 Wearable breathable Carbonized metal-organic Dip-coating Response 22 ms Three linear regions Wearable, Complex structure [75] pressure sensor C- framework (C-MOF) Screen printing Recovery 20 ms and corresponding Highly sensitive MOF/PANIF@PU Polyaniline nanoﬁber sensitivities are Broad-range and (PANIF) 15.81 kPa 1, breathable Polyurethane (PU) sponge 158.26 kPa 1, and 1 Interdigitated electrode- 81.59 kPa coated fabric Multifunctional Leather Filtration 40 ms Two linear regions Flexible Poor washing [10] pressure sensor (a- Nanomaterials acidiﬁed and accordingly Wearability performance CNT s/leather) carbon nanotubes (a-CNTs), sensitivities are silver nanowires (Ag- 32.42 kPa1, NWs)) 8.03 kPa1 Interdigitated electrode arrays 159
Y. Wang, M. Zheng, X. Liu et al. Journal of Science: Advanced Materials and Devices 6 (2021) 153e166 Fig. 6. (1) Schematic of the design principle of leather based e-skin. (2) Leather based display and user-interactive e-skin provide an instantaneous visual response. (a) Structure and (b, c) photographs of leather-based display. (def) Photographs showed that by tuning the applied force from light touch (e) to deep press (f) the brightness of display increased simultaneously. (g) Structure of leather-based user-interactive e-skin. (3) Flexible and wearable pressure sensor based on conductive leather. (a) Schematic illustration of the fabrication procedure of ﬂexible pressure sensor. (b) Photograph of the watchband shaped e-skin directly above the artery of the wrist. (c) SEM image of the cross-section morphology of a-CNTs/leather. (d) A schematic diagram of a-CNT s/leather under different pressures. (e) Current responses to various pressures. SEM images of the surface (f, g) and cross-section (h, i) morphology of a-CNTs/leather at different magniﬁcations. (j) Measurement of the physical force of wrist pulses under normal condition (z72 beats min1). (k) Sensing mechanism: current changes in response to loading and unloading pressure. Scale bars in (c), (g) 500 nm; (f) 2 mm; (h) 200 mm; (i) 1 mm. Reproduced with permission [10]. Copyright © 2018. The Authors published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Fig. 7. (1) Leather morphology by SEM. (A) and (B) grain surface micrographs at 100 and 25,000 magniﬁcation, respectively. (C) and (D) cross section micrographs at 2500 and 10,000 magniﬁcation, respectively. (2) Elemental analysis of sample: Leather-NaþMMT_6%. (3) SEM magniﬁcations of leather (A, B), leather with 1(C, D), 3 (E, F) and 6 mass% NaþMMT (G, H).(4) Vertical test evaluation of ﬂammability of leather samples with different levels of ﬂame retardants (A) Leather, (B) Leather- NaþMMT _1%, (C) Leather- NaþMMT _3% and (D) Leather- NaþMMT _6%. Reproduced with permission [81]. Copyright © 2014, Elsevier B.V. 160
Y. Wang, M. Zheng, X. Liu et al. Journal of Science: Advanced Materials and Devices 6 (2021) 153e166 and good wearability of leather, but also displays good 3.5. Flame retardant leather performance. The construction of a leather-based ﬂexible sensor mainly in- As a natural polymeric material, leather itself has certain ﬂam- volves the selection of functional materials, that is, active sub- mability. However, applications of leather for furniture, interior stances. By combining different functional materials with leather, it decoration, aircraft, automotive, etc. require it to have certain is possible to make leather “active” that can repurpose or even ﬂame-retardancy, in order to reduce the possibility of ﬁre. There- outperform the properties of real skin. Carbon materials such as fore, making leather ﬂame retardant is highly important. Flame carbon black, carbon nanotubes, and graphene possess good elec- retardant can be added during leather retanning, ﬁnishing, and trical conductivity, high chemical and thermal stability, as well as other processing steps. It can be incorporated into the leather ﬁbers low toxicity. Hence, they can be used as functional materials, which by either physical adsorption or by grafting onto leather by show great application potential in wearable electronics devices chemical bonding to achieve ﬂame-retardancy of the resultant [69]. Huang et al. [70] used leather as a substrate to fabricate a leather. It increased the limiting oxygen index (LOI) and time to strain sensor by ﬁltration of an aqueous dispersion of conductive ignition (TTI), and shortened the ﬂame combustion time [76]. nanomaterials. The surfaces of collagen ﬁber clusters in the Melamine ﬂame retardant is a nitrogen-based compound, with low multilayer structure of the leather matrix easily adsorb conductive toxicity, low corrosivity, has low smoke generation and high ther- materials and endow it with electrical conductivity. Zou et al. [10] mal stability, and, hence, is an environment-friendly ﬂame retar- reported simple and designable leather based e-skin (Fig. 6), dant material [77]. Yang et al. [78] found that ﬂame retardant ﬁbers wherein leather served as a unique platform for loading acid made by treatment of goat-skin with melamine-based ﬂame treated carbon nanotubes (a-CNTs). Then, the prepared conductive retardant could increase the activation energy and LOI and signif- leather was stitched together with leather-patterned interdigitated icantly improve the thermal stability and ﬂame retardancy of electrode arrays using a sewing machine to fabricate a ﬂexible and leather. Besides, montmorillonite (MMT) also has ﬂame retardancy wearable pressure sensor. [79]. Its mechanism of ﬂame retardancy is mainly manifested in the Fig. 8. (1) Visual comparison of untreated (a) and silver treated natural leather (b). (2) Agar diffusion tests on Escherichia coli and Staphylococcus aureus. (3) SEM analysis of the cross section of silver treated sample at 80 (a) and 1000 (b, c) showing the presence of silver particles under the outer layer. Reproduced with permission [83]. Copyright © 2012, Springer Nature. 161
Y. Wang, M. Zheng, X. Liu et al. Journal of Science: Advanced Materials and Devices 6 (2021) 153e166 fact that MMT can promote carbon formation and act as a barrier antibacterial leather. Currently, most of the preparation during combustion [80]. G. Sanchez-Olivares et al. [81] added so- methods of antibacterial leather involve the blending of the dium montmorillonite (NaþMMT) during the retanning process of antibacterial agent with ﬁnishing agent to make a composite wet blue leather. Owing to the pores and large number of inter- ﬁlm with antibacterial property. Bao et al. [82] studied the ﬁlm- stitial voids between ﬁbers (Fig. 7), the NaþMMT particles were forming properties of ZnO/polyacrylate composite emulsions, homogeneously distributed in the leather. Therefore, the ﬂame prepared by physical blending of polyacrylate and zinc oxide retardancy and mechanical strength of leather were simulta- having different morphologies and found that they had excel- neously improved. lent hygienic and anti-bacterial performance, but poor frictional resistance. Pollini et al. [83] could, through an in situ photore- 3.6. Antibacterial leather duction of silver solution, evenly distribute silver clusters, which could well-cover the surface of natural leather and Antibacterial leather is a new type of functional leather improve its abrasion resistance (Fig. 8). Signiﬁcantly, this inno- having bacteriostatic and bactericidal properties. Antimicrobial vative silver deposition technology on LM is a promising agents are added during the processing of leather or to the method to contain contamination in public transport vehicles. resultant leather. The leather products show bacteriostasis, In addition to the above-mentioned examples, a composite wherein they kill or inhibit the reproduction of bacteria glued coating of positively charged chitosan (CS) and negatively to leather over a certain period of time. Different antibacterial charged gallic acid modiﬁed silver NPs (GA@AgNPs) was fabri- agents such as MNPs, photocatalytic antibacterial agents, cated on a leather surface by layer-by-layer assembly with good organic antibacterial agents, natural antibacterial agents, and reusability, which was ideal for antibacterial ﬁnishing of dia- composite antibacterial agents can be used for the fabrication of betic leather shoes [84]. Fig. 9. (1) AgeTiO2 NPs and AgeNeTiO2 NPs powders. (2) Self-cleaning effect of AgeNeTiO2 NPs, AgeTiO2 NPs and TiO2 NPs on leather surface exposed to UV and visible light (Orange II (OII), Methylene blue (MB) dyes). (3) The sensitivity test of leather surfaces against Escherichia coli ATCC 25922 (a, b, c) and Staphylococcus aureus ATCC 25923 (d, e, f). B areas are leather surfaces (a, b, d and e samples are treated and c and f are untreated samples) and A areas are medium outside leather surface. Reproduced with permission [89]. Copyright © 2016, De Gruyter. 162
Y. Wang, M. Zheng, X. Liu et al. Journal of Science: Advanced Materials and Devices 6 (2021) 153e166 Fig. 10. (1) States of a water droplet on the leather surface: (a) untreated, (b) VTES sprayed sample, (c) treated under 50 W for 300s. (2) SPM images of the surfaces of (a) untreated, (b) VTES sprayed and (c) plasma treated leather samples. Reproduced with permission [93]. Copyright © 2014, American Leather Chemists Association. (3) Photograph of water- repellent leather. 3.7. Self-cleaning leather modern leather products, especially for military shoes and uphol- stery crust leather. Leather products inevitably get stained during daily wear and Due to the low surface tension of organosilicon compounds, use. It is inconvenient to wash them, and so self-cleaning per- they are commonly used as waterprooﬁng agents, smoothing formance of leather has become an important factor that needs agents, and softening agents for leather. Presently, traditional to be considered. Intelligent bionic surfaces have been tried in leather manufacturing generally uses organosilicon compounds as the leather industry, which involve imparting a high roughness leather ﬁnishing agents, fatliquors, and retanning agents to factor to leather by compounding leather with nano-solid par- improve the hydrophobicity of leather. Feng et al. [93] reported the ticles. This further improves the hydrophobic and oleophobic polymerization and deposition of vinyltriethoxysilane on the sur- properties, due to which leather does not get wet and remains face of upholstery crust leather by using low-pressure cold plasma non-oily with self-cleaning property. Nano-TiO2 particles provide technology, which improved the hydrophobicity of the leather photo-induced hydrophilicity and photocatalytic properties [85], surface (Fig. 10). It did not affect the appearance and sanitary per- photodegrade organic contaminants, and can be used to prepare formance of leather, which was difﬁcult to obtain. In order to obtain self-cleaning ﬁlms or coatings [86e88]. C. Gaidau, A et al. [89] leather coating with high hydrophobicity and good stability, Huang reported the synthesis of AgeNeTiO2 co-doped nanomaterials by et al. [94] combined the three-dimensional rough structure of the electrodeposition of Ag on NeTiO2 NPs, which improved the skin itself, ﬁrstly through in-situ hydrolysis of butyl titanate to photocatalysis, hydrophilicity, and antibacterial property of the generate TiO2 to enhance the roughness of the leather, and then use leather surface. Importantly, the leather surfaces treated with polydimethylsiloxane with low surface energy, high strength and AgeNeTiO2 NPs showed advanced self-cleaning properties, good softness to deposit nano-TiO2 leather was wrapped and when exposed to ultraviolet/visible light (Fig. 9), through the modiﬁed to prepare leather with a static contact angle of 164.6. hydrophilic mechanism to decompose organic soil. The modiﬁ- cation of TiO2 with dopant ions renders it sensitive to visible 3.8.2. Water-, oil-, and soil-repellent leather light and the highly active free radicals generated on the surface Besides leather with outstanding water-repellency, there also under the inﬂuence of ultraviolet/visible light irradiation exists multi-functional leather with combined water-, oil-, and soil- participate in the oxidation reaction. This promotes the repellency, which is termed “tri-repellent” leather. The common destruction of organic pollutants and deactivates microbes [90], way to obtain “tri-repellent” leather is to add ﬁnishing agents, thus providing better antimicrobial resistance. Moreover, Fe and which also have the property of “tri-substrate-repellency”, to N co-doped TiO2, used in the preparation of FeeNeTiO2 nano- leather [95]. materials, not only endow leather with self-cleaning properties, but also enhance the hydrophobicity of the leather surface [91]. 4. Conclusion and outlook Furthermore, silica-doped TiO2 NPs also have obvious advantages of thermal resistance and catalytic performance in visible light Leather, the primary product of rawhide, has for centuries been for ﬁnished leather with self-clearing properties [92]. used for several purposes. This review brieﬂy introduces the multi- hierarchical structure and properties of leather and summarizes 3.8. Other functional leathers and analyzes the existing functional leathers. The focus was on EW- absorbing leather, infrared-absorbing leather, X-ray shielding 3.8.1. Waterproof leather leather, conductive leather, ﬂame-retardant leather, antibacterial Waterproof leather refers to the leather surface and ﬁbers that leather, self-cleaning leather, and conventional functional leathers, are not wetted by water and whose water contact angle is greater such as waterproof leather and water-, oil-, and soil-repellent than 90 . Waterprooﬁng is one of the essential requirements of leather. 163
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