A Systematic Review on Potential Bio Leather Substitute for Natural Leather

Bio-based leather substitutes are an emerging class of ethically and environmentally responsible natural fabrics that are increasingly exceeding consumer aesthetic and functional expectations as an alternative to bovine and synthetic leathers. Tis literature review creates a clear and elaborative overview of conventional leather processing along with innovative potential bio-leather substitutes. Plant-driven, fungal-origin, bacterial-driven, and animal-origin bio-leathers are the current innovative research advances addressed in this literature. While traditional leather and its alternatives are sourced from animals and synthetic polymers, these renewable and sustainable leather substitutes are gained from bacterial cellulose, mycelium, plant cellulose, and animal cells using tissue engineering and other eco-friendly techniques. In conclusion, bio-based leather alternatives are eco-friendly, non-toxic, and sustainable and ultimately can substitute natural leather made by conventional processing.


Introduction
When animal skins and hides are physically and chemically treated, their protein structures are changed to create a strong and fexible natural product called leather [1].Due to its popularity in clothes, shoes, upholstery, and accessories and its longevity along with its natural tactile and visual qualities, including color, softness, and warmth [1,2], it is a common commodity with a market share predicted to reach about US$360 billion by 2025.Te usage of leather, which is traditionally considered as a byproduct of meat industry [3], has, however, come under fre from some reasons like being socially irresponsible and ecologically unsustainable [4,5], given the changing societal norms and growing emphasis on ecological responsibility.After all, raising cattle results in signifcant emissions of greenhouse gases deforestation by grazing, and environmental devastation from animal waste are among the challenges that the leather industry must tackle [6].
Due to toxic chemical involvement and large amounts of sludge waste discharge, while treating raw hides and skins, leather production is not ecologically sound.Tose problems have led to the creation of leather-like materials that are not made from animal products.On the other hand, faux leather alternatives made of polyvinyl chloride (PVC) and polyurethane (PU) have found a substantial market and signifcantly lessen the social and environmental issues traditionally connected to leather manufacturing [7].However, these synthetic leather substitutes often entail the use of risky ingredients in their manufacturing [8], as they are made from fossil fuels, making them nonbiodegradable, and experience the same constrained end-of-life possibilities as most plastics [9].
In the past few decades, research eforts have been made to substitute natural leather with bio-leather alternatives.Hide Biotech in London creates conventional leather substitutes using isolated collagen proteins.Bio-leather, which is derived from mycelial growth, is another newly emerging material that holds promise as a fnancially advantageous, socially conscious, and ecologically sustainable potential substitute to both natural and artifcial leather alternatives for use in furniture, garments, and footwear [9].
An eco-friendly cofee-dyed bacterial cellulose bio-leather [10] has also been developed as a sole substitute for conventional animal-driven leather.
Although several academic and industrial studies on alternatives to natural leather have been carried out, there is a gap on the systematic compilation of research developments on potential bio-based leather alternatives for natural leather.Terefore, the sole aim of this review is to assess and compile research advances in bio leather as an alternative to natural leather.

Overview of Conventional Leather Processing
2.1.Beamhouse Operations.Being the initial stage of leather processing, soaking aims to wash away curing salt from the salted skin, clear of surface dirt, and remove blood stains.It also rehydrates the skin structure which leads to the opening up of the fbers for further chemical processing.Glycosaminoglycans, nonstructural proteins, and lipids are removed from the skin during this process, and the fbril bundles in the fber structure are separated to split the fbers [11].Te foat (200-300%), temperature (30 °C), pH (8.5-9.5/10), duration (24-48 hours), and mechanical action (3-4 rpm) should all be met for the soaking process [11,12].Te rehydrated hide/skin is then soaked with sulphides and an alkaline solution to get rid of hairs and other keratinous material in hair burning unhairing.Te pelt swells as a result of the alkaline solution, which eventually allows the fber structure to open up at a pH of 12-12.5 [9].Tis enables more efcient chemical reactions and improved chemical penetration in the next stage of leather making.Te lime chemicals present in the lime liquor react with the hair root or base of the hair shaft and cause the hair to become looser.Tis thinning of the hair is caused by the amino acid cysteine's disulfde bond breakage, a process known as hair burning [11].To detach adherent fesh that was left after faying, the feshy side of the pelt undergoes a mechanical procedure known as feshing after the liming process [13].
Deliming lowers the pH of liquor from 12-12.5 to 8-9 by removing lime as well as other alkalis from the limed pelt, either through continuous water cleansing or chemical processing.While the lime found in the pelt cross-section can be eliminated using chemicals like ammonium chloride and ammonium sulphate, surface lime may be removed by repeated water washing [11].On the other hand, the bating procedure contributes to the manufacture of supple, stretchable, pliable, and fat fnished leather.Te fbrous structure of the pelt is opened by proteolytic enzymes as they are added as part of the process.Besides, the leftover lime in the pelt is also removed during batting.Te procedure is carried out between 35 and 40 °C, which is the ideal range for the enzymes [11].
Te basic goal of the pickling process is to modify the collagen to the specifcations needed by the tanning chemicals during the process of tanning.Te pH of pickle liquor is adjusted through the addition of an acid and salt [14].Common salt (5-10%) and sulfuric acid (0.6-1.5%) are the two pickling agents that are most often employed.While the purpose of the salt used is to inhibit acid swelling, the collagen matrix is protonated with the application of acids since the chrome tanning process exclusively uses ionized carboxyl groups [15].

Tanning Operation.
Te collagen protein of the pelt which is stabilized by this process does not putrefy and is thermal resistant [16].During this procedure, extra crosslinks are added to the collagen including bonding the protein's functional group with the active groups of the tanning chemicals.Te stability of the collagen matrix is evaluated by measuring the shrinkage temperature of the tanned leather.Adequate uptake and even dispersion of the tanning ingredients are characteristics of a desirable tanning outcome [17].Tere are various tanning materials and techniques, and the decision is mostly infuenced by the characteristics of the fnal leather, the price of the tanning material, and the abundance of plants used as tanning material.Among numerous tanning techniques employed to convert putrescible pelt to a material-resistant putrefaction vegetable tanning and nonmineral tanning techniques like chrome tanning methods are widely used.Following tanning, basifcation is carried out with sodium bicarbonate to raise the pH to support subsequent procedures including retanning, dying, and fat liquoring [18].Te basifcation procedure bonds the tanning substance to the leather, and the more tanning substance that is bound, the better the leather's thermal.

Post-tanning Operation.
After basifcation chrometanned and semichrome leathers are piled up for the night.Te pH then lowers, indicating the release of acid.Te ionization of neutral carboxyls, the ionization of positively charged amino (−NH 3+ ) groups, or even the hydrolysis of chrome itself may be the sources of this acid [19].Neutralization, a crucial step in the production of leather, eliminates the leather's acidity and helps dyes penetrate the leather matrix.Sodium carbonate and sodium bicarbonate are among the chemicals used to assist the neutralization process and the pH of the foat is adjusted between 5.4 and 6.5 [19,20].
Retanning chemicals are used to prevent the possibility of loose grain and to make the fnished leather level out, full, and elastic [20].Te sole purpose of this process is to fll any loose areas of the tanned leather, reduce shrinkage while drying, increase the penetration of anionic type fat liquors, dyestufs, and fnish adhesion, enhance some specialized attributes like sweat resistance, and fame retardancy.Syntans, inorganic mineral compounds like chromium, resins, and vegetable extracts are among the chemicals widely utilized for retanning.While vegetable retanning raises the thickness of the leather and enhances its useable surface area, acrylic resins are frequently employed as they ofer an excellent selective flling property to increase the leather's cutting value [21].
Fat liquoring process of leather making prevents fbers from adhering during drying resulted from the interaction between the fatty substance and the collagen's fbrous structure.Te water that is trapped between the smaller fbrils and the bundles of fbers thoroughly lubricates the leather.Te main aim of fat liquoring is to regulate the leather's level of softness and suppleness and ultimately to improve its strength attributes [8].Additionally, this aids in giving the leather's physical qualities including tensile strength, extensibility, wetting capabilities, waterproofness, and permeability to water vapor and air.On the other hand, leather dyeing introduces coloring agents to the leather matrix.To impart a deep, opaque color, dyes may be applied to the surface of the leather in the form of a solution or pigment, which may then be sprayed or distributed by hand.Scientists have discovered a wide variety of natural dyes as a result of growing environmental concerns, which led to a decrease in the usage of azo dyes causing potential risks to human beings through prolonged exposure to their potential colon carcinogens [22].

Finishing Operation.
To reduce the water content of wetprocessed leather and make it ready for fnal fnishing, leathers are often dried after prior fnishing.Te process of drying is regarded as one of the most crucial mechanical steps in the production of leather since the fnal texture and elasticity of the leather are obtained by drying.When preparing leather, many drying techniques including overhead dryers, vacuum dryers, and toggle dryers are employed [23].Due to the moderate drying temperature during toggle drying, the area yield is increased as well as the mechanical properties.Surface fnishes are applied after mechanical treatment in the fnishing process.Tis serves to both emphasize the inherent features of leather and conceal any surface faws that may have existed in the leather.Finishing may be accomplished using a variety of ingredients, including casein, nitrocellulose, polyurethane, acrylic, and other resin and polymer components combined with organic elements including oils, waxes, albumins, and cellulose esters.Shade coating, roller coating, and spraying are a few of the several fnishing techniques [21].
Pretanning and tanning procedures are in charge of raising the efuent's biological oxygen demand (BOD), chemical oxygen demand (COD), total dissolved solids (TDS), and other parameters.As a result, they are ultimately responsible for 80% to 90% of the pollution [23] generated by the leather industry.Both hazardous wastes like lime and chrome sludge as well as hazardous gases like hydrogen sulfde are produced during these operations.Furthermore, according to [24], posttanning operation of leather making is accountable for (6)(7)(8)(9)(10)(11), (24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40), and (8-15) kg/weight of rawhide suspended solids, COD, and BOD, respectively, making it the other pain in leather production that the tanners have to deal with.Industrial experts and academic researchers have created alternative bio-leather substitutes to address these and other environmental and human health issues associated with natural leather and synthetic leather made of polyvinyl chloride and polyurethane.Tese substitutes can ofer insights for environmentally friendly materials and aspects for sustainability.

Ecological and Health Impacts of Chrome Tanning
Tanneries are among the polluting industries mainly causing chromium pollution, and its efect is highly pronounced on regions where high concentrations of tanneries are located.For instance, it has been reported that more than 2500 tanneries were operating in India by 2009, of which 80% of them were engaged in the chrome tanning process [25].Hexavalent chromium (Cr 6+ ) and other toxic compounds are discharged via industrial wastewater to the environment.Tis hexavalent chrome is soluble, toxic, mutagenic, tetragenic, and known to have a number of negative efects on human health as a result of its high oxidizing potential.Te toxicity occurs in humans because of environmental pollution through soil and water contamination or due to occupational and nonoccupational exposure to heavy metals [26].Additionally, while Cr 3+ is more chemically stable and relatively inert, Cr 6+ is highly mobile and soluble in water [27], making it 1000 times more mutagenic and nearly 100 times more toxic than Cr 3+ .Metal toxicity results in serious morbidity and mortality.Because of its high toxicity, mutagenicity, and carcinogenicity, Cr 6+ can have negative efects on the environment and human health at even very low levels [28].Soluble Cr 6+ poses a signifcant carcinogenic risk if ingested.Tis is attributed to the low pH of the stomach as particulate chromate dissolves at low pH [29].
Chromium is toxic and mutagenic to microorganisms at concentrations between 10 and 12 mg•l −1 , which are inhibitory to the majority of soil bacteria in liquid media.

Bio Leather Alternatives for Natural Leather
4.1.Fungal Origin Bio-Leathers.Fungi are aerobic creatures that display carbon-neutral growth [30].Scholars from several felds are looking for more viable ways to mitigate environmental stress as a result of the increased demand for sustainable alternatives that are biodegradable and made from renewable sources [31].A network of tiny white flaments known as mycelium makes up the vegetative portion of a fungus.Te diameter of these hyphae ranges from 1 to 30 m, and they can branch and merge to form networks that cover kilometers [32,33] making them the biggest creatures on Earth.Te majority of agricultural waste, including sawdust and pistachio shells, is suitable for growing mushrooms.Te fruiting bodies of Fomitella species and Phellinus ellipsoideus were used to create mycelium-based leather.Fungi-based leather substitutes are ethically and environmentally responsible alternatives to natural leather [34].
As mycelia-based leather can be produced utilizing agrowaste substrates and lignocellulosic materials, it is more ecofriendly and sustainable than conventionally processed bovine, polyurethane, and PVC-based leather materials.Cattle require more than two years before they reach the optimum size to extract the skin while mushrooms grow exponentially and take weeks to entirely eat their substrate [34].Te current growth and advances in fungal-driven bioleather substitutes are directly linked to biotechnological Journal of Engineering companies such as MycoTech, Reishi ™ , MycoWorks, Bolt Treads Company, and so on that have developed sandals, handbags, shoes, watch bands, and wallets (Figure 1(a)).Industrially produced alternatives for leather made from fungus exhibit a comparable resistance to fading or running as does leather made from animals.Additionally, leather made from fungi has revealed heat degradation at 250 °C [34].Mushroom's mycelium binds substrate materials together as it develops, providing opportunities for composite growth.Several edible mushroom species and other natural materials can potentially create mycelium composites.According to [36], four diferent mushroom species Reishi (Ganoderma lucidum), Oyster (Pleurotus ostreatus), King oyster (P.eryngii), and Yellow oyster (P.citrinopileatus) have been applied on two fabric levels.
Te relation between density and compressive strength was presented to be linearly signifcant as increased density resulted in higher mechanical properties [36].Te growth of mycelium depends on environmental elements including temperature and humidity, which accelerates when these circumstances are maintained.As far as the production of mycelium is concerned, frst mycelia spores and the nutrient-rich sawdust combination are spread out across a sizable mat, where they develop into a thick, foamy mass.Te leftover residues are composted once the mycelium has been collected.Te resultant mycelium sheet is subsequently treated and colored to create Mylo ™ material, a bio-leather substitute used in the leather industry [36].To get the desired result, diferent physical and chemical treatments are employed during the production process.To enhance the amount of sections and water in the mushroom mycelium, a moisturizing/hydrating chemical (glycerol or sorbitol) can be used during the pretreatment stage.Ten, between fve seconds and six months, the tissue is immersed in alcohol, sodium hydroxide, and acid by vacuum infusion or injection.Ten, through deacetylating the chitin, crosslinking sites can be produced as this chemical process helps remove proteins from the body.Te third phase is physical treatment involving manual, hydraulic, hot, or cold pressing utilizing rollers to minimize the thickness of the fungal biomass.Te fnished product is then dried in a convection oven.After all, a plasticizer such as glycerol is added to the product to improve its fexibility and stretchability.Te required shade or patterns can be applied during the posttreatment phase before the fnal drying [34].

Bacterial Origin Bio-Leathers. Bacterial cellulose (BC) is
a substance naturally present in some bacteria [37].It could provide an alternative to plant cellulose if produced in large amounts via biotechnology.Microorganisms such as Acetobacter xylinus and Komagataeibacter are potential sources of BC, a pure cellulose biomaterial [38].It has the appearance of a gelatinous pellicle and exhibits good resilience.It is made up of three-dimensional interwoven nanofbers [39], which result in high porosity, strong water-holding capacity, and high crystallinity [40].Te perception of BC as a potential leather alternative is based on the industrial synthesis of cellulose fbers from bacteria belonging to the genus Komagataeibacter, which are consumed as part of the fermentation process for kombucha tea and other products [41].Tese microbes create cellulose pellicles that build up in the extracellular media by respiring aerobically, either by themselves or in cooperation with other bacteria and yeasts.
Tese can potentially be produced at the necessary thicknesses and, when dried, provide a fexible leatherlikematerial with comparable characteristics to those of animal leathers.Trough the industrial-scale fermentation of basic nutrients, large amounts of these pellicles might be produced.Te tactile characteristics of BC are comparable to those of high-end, stretchy, fne-fnished leather.A study conducted by [42] reported a bio-leather alternative from kombucha-derived bacterial cellulose formulated with sour whey, apple juice, and brewer's spent grain composites.Te resultant bio-based composites showed good shape stability and considerable fexibility with an average tensile strength and elastic modulus of 1.69 MPa and 100 MPa, respectively.

Plant-Driven Bio-Leathers.
Cellulose is an essential component of the cell walls of green plants which is a linear polysaccharide.Te strength and stifness of the plant is mainly due to microfbrils of cellulose arranged in cell walls [43].According to [44], cellulose is the most common organic polymer on earth and has a regenerative nature.In order to provide the fnished product stifness, plant-driven bio-leathers rely on the cellulose found in plant biomass.
Te production techniques for developing these kinds of leathers vary depending on the source of cellulose utilized due to the confdentiality surrounding the businesses.
Piatex ® , one of the most popular plant-based bio-leathers, is created from the fbers of pineapple leaves, a leftover from the pineapple industry.To generate a nonwoven mesh, the fbers from the leaves are removed, cleaned, dried, and combined with polylactic acid (PLA) from maize [45].Tis mesh is then coated to create the fnished product called Piatex ® .Desserto ® is the other kind of plant-based bio- leather made of cactus leaves.A compact layer, a foam layer, and a woven base consisting of either polyester, cotton, or a mix of the two make up Desserto ® [46, 47].Tis type of bio-leather is sustainable since it is manufactured from cactus that is grown and collected twice annually.After being washed and powdered, the cactus leaves are sun-dried for three days.After that, fbers and proteins are extracted from leaves to make a combination that is later transformed into Desserto ® leather [48].
As it is said to be PVC-free and contains about 28% bio polyurethane, this cactus-based leather has a lower environmental efect than conventional and synthetic leather [46,49].Te photosynthesis carried out by the cactus plantings reduces the overall emissions from this material, further enhancing its sustainability.Being a plant-based bioleather substitute, mango leather is made by mechanically separating the fesh from the seed and pulping the fesh afterward.Binding agents are then included, and the mixture is cooked in an oven.A leather fnisher takes sheets of material out of the oven and coats them [50].Te mango 4 Journal of Engineering leather is positioned on top of an organic cotton foundation and has satisfactory mechanical properties [51].

Animal Origin Bio-Leathers.
Te collagenous matrix found in animal skins can be customized to serve a variety of purposes as it is incredibly durable.Nowadays, there has been a great deal of attempts to create collagen matrix without using animal products.A discovery of laboratorycultured bio-leather announced by Modern Meadow is made from cow cells that are removed without adversely injuring the animal.In order to encourage collagen creation, these cells are multiplied in a lab and placed on the proper medium.Although the collagen structure obtained is extremely diferent from that of skin [35], fndings revealed that a bio-leather may still be created using this tissue-engineering technique (Figure 1(b)).On the other hand, hide Biotech in London makes leather alternatives out of isolated collagen proteins obtained from scales and other fsh debris [52].Te company's exclusive enzymatic and chemical procedure aids in the development of a transparent material by strengthening and constructing a protein network.Tere is no need for a separate dyeing or tanning process because dyes and fat liquors may be applied to the material as it is being created.

Global Production, Fermentation Media/ Biomass, and Market Size of Bio-Based Leathers
Te global leather industry is expected to grow signifcantly in the coming years.In light of this, the leather industry's 6 Journal of Engineering market size is anticipated to reach USD 708.7 billion to USD 738.61 billion by 2030, displaying a CAGR of 5.76% to 6.7% over the predicted timeframe.Te development can be attributed to the rising demand for luxury items, shifting fashion tastes, and expanding domestic and international tourism [53].On the other hand, as described in Figure 2, the global bio-based leather market was valued at USD 647.21 million in 2021 taking its share from the leather industry and is expected to grow at an alarming rate with a CAGR of 6.2% during the forecast period of 2030.Tis progress can be directly attributed to the signifcant growth in the global production of bio-based leathers signifying their pronounced scalability in the coming years though specifc details regarding the current production of bio-leathers have not been quantifed yet in the current literature.Te most crucial nutrients for the fermentation of bio-leathers, on the other hand, have been identifed by various scholars as being carbon sources, nitrogen sources, water, salts, and micronutrients [54].
It is important to note that the specifc composition of fermentation media for bio-leather production can vary depending on the desired properties of the fnal product and the microorganisms used.In a study aimed to develop ecofriendly bacterial cellulose bio-leather with improved durability using plant-based proteins, soy protein isolate, glucose, and yeast extract were used by researchers as the fermentation medium [55].However, the amount of biomass or fermentation media required for bio-leathers has not been quantifed yet based on the currently available literature.Te amount will generally depend on the specifc process and the type of material used for bio-leather manufacturing.

Biodegradation Studies and Life Cycle Assessment for Different Leather Categories
Te biodegradation of microbial cellulose and lecithintanned fabricated microbial bio-textiles has been studied in natural terrestrial environments for the amount of mass lost after 60 days in soil.Te results revealed that microbial cellulose and lecithin-tanned bio-leathers possessed an average mass loss of 74.45 ± 2.94 and 76.3 ± 4.3 percent, respectively, after 60 days [54].Another study has been conducted to develop an eco-friendly BC bio-leather with improved durability using plant-based proteins.Te researchers found that BC could undergo rapid and ecofriendly biodegradation without leaching toxic compounds into groundwater [55].Besides, a bio-leather produced by extracting cellulose fbers from tomato plant waste has been developed by an Indian alt-leather producer.Tis material is biodegradable and ofers a sustainable alternative to traditional leather.Strategies to improve the mechanical and moisture stability of bio-leather typically involve heavy metals and/or synthetic plasticizers and coatings, which can compromise biodegradability and introduce human and ecological toxicity [54].Synthetic polymers used in bioleathers such as polyurethane, poly (butylene adipate-co-terephthalate) may contain chemicals that can be harmful to humans and the environment [56].Tese chemicals can leach out of the material and have negative efects on living organisms.Moreover, these polymers are not easily broken down by natural processes, leading to their accumulation in the environment.Tis can contribute to pollution and long-term environmental damage.To address these issues, researchers  (a-d) Comparison with respect to environmental impact categories (e) carbon footprint of Reish ™ [57].are exploring alternative materials and production methods for bio-leathers.One approach is the use of microbial nanocellulose biotextiles, which ofer renewability, low toxicity, and functional properties.Tese biotextiles are made from natural materials and have the potential to be more biodegradable compared to synthetic polymers [54].By exploring these alternative materials and production methods, it is possible to reduce toxicity and improve the biodegradability of bio-leathers, making them more environmentally friendly.

Journal of Engineering
Life cycle assessment and key performance indicators have been employed as a standard and a valid evaluation metric to evaluate the environmental impact of bio-leathers in some scholarly published research [57].Life cycle assessment is a practical tool used to evaluate the life cycle environmental impacts of a product.Te global warming contributors in the life cycle of diferent leather and leather alternatives are quantifed and expressed as a carbon footprint.As stated in Figures 3(a)-3(d), the environmental impact of bio-leather production compared to traditional leather is generally considered to be more sustainable and less harmful since reduced use of harsh chemicals is involved which results in a reduced carbon footprint.In a study conducted to analyze the carbon footprint of fnished bovine leather in the cradle-to-gate scenario, leather thickness has been found to have a major impact on the carbon footprint of the fnished leather.
Accordingly, fnished bovine leather has been found to have 64.8, 74.5, and 79.6 kg of CO 2 equivalents per square meter carbon footprint for 1.5 ± 0.1 mm, 1.7 ± 0.1 mm, and 1.9 ± 0.1 mm leather thicknesses, respectively [58].On the other hand, the carbon footprint of bio-leather (Figure 3(e)) is generally much lower than conventional leather production.Te carbon footprint of bio-leather is 100 times lower than leather modeled as a coproduct.When compared to synthetic leather made from polyurethane-coated polyester and cotton, the carbon footprint of bio-leather is reduced by 96.8% and 94.3%, respectively.In addition, lifecycle assessment studies showed that bio-leather reduces greenhouse gas emissions by nearly 80% compared to conventional leather [59].

Mechanical Properties of Leather and Different Leather Alternatives
In a study, conducted by [42], diferent compositions of leather-like materials were fabricated using waste maple leaves and apple fruit pulp, mixed with additives such as kombucha biomass cellulose biodegradable polyesters and plasticizers.Scanning electron analysis of the result showed that the bio-fabricated materials were porous and breathable.However, the water absorptivity evaluation indicated that the values obtained were slightly higher compared to the required value (max.25%) for upper shoe soles by BASF.In another study, the durability of bacterial cellulose bio-leather was improved using soy protein isolate and mushroom plant-based proteins via a physical entrapment.Accordingly, the tensile strength of the resultant bio-leather was approximately 13 N/mm 2 which was better than that of cowhide leather.Te fexibility and crease recovery of BC bio-leathers were also improved after the entrapment of plant-based proteins [55].In an attempt to alleviate food waste and fashion pollution environmental issues, a textile material with leather-like properties from fungal biomass cultivated on bread waste was investigated.Te resulting leather-like material with only glycerol posttreatment showed a tensile strength of 7.7 MPa and an elongation at a break of 5%.Te physical properties of bovine and various leather alternatives have been stated in Table 1.

Concluding Remarks and Gaps for Future Work
Tis review attempts to highlight recent advances in biobased leather research that could eventually replace natural leather.Te goal of these bio-leather alternatives is to lessen the negative efects that the manufacture of natural and artifcial leathers has on human health and the environment.Since many chemicals used in the processing of natural leather have a negative infuence on human health and the environment, researchers and inventors have been more interested in the sustainability of the leather processing industry.According to the literature, bio-based leather substitutes can be derived from bacteria, fungi, plants, animals, and so on which can ofer a viable answer for problems related to the ecological and commercial sustainability of the industry.Because they do not need the use of hazardous chemicals in their synthesis, bio-leather alternatives are produced in a manner that is ecologically sound, biodegradable, and economical for producers.
To sum up, these new, greener, and bio-based natural leather alternatives will play a substantial role in the future development of ecologically responsible and sustainable leather manufacturing.Despite their priceless ecological and sustainability importance, most bio-driven leathers have low mechanical properties when we compare them with the properties of conventionally obtained natural leather.So, to fll the gap, researchers in this area should put their eforts into improving the mechanical properties of innovative biobased leathers.

Figure 1 :Figure 2 :Figure 3 :
Figure 1: Process and product overview of bio-based leathers.(a) Products made of leather like mushroom bio-material.Photo credit: (MycoTech (Bandung, Indonesia) and Bolt Treads Inc. (Emeryville, United States)) (b) production process of leather-like bio-material by tissue engineering [35].

Table 1 :
Comparison on mechanical properties of leather and various leather alternatives.