Bacterial Cellulose: Natural Biomaterial for Medical and Environmental Applications

ABSTRACT Extensive research has been conducted during 20th century to discover renewable natural polymers that are sustainable. Cellulose represents one such biomaterial which is abundant, renewable, and biodegradable. Both plant and microbial biomass can be processed to make cellulose. Bacterial cellulose (BC) is a prospective natural polymer produced by certain bacteria during their growth phase. BC is hydrophilic biopolymer, fibrous (20–100 nm diameter), and biocompatible. In contrast to plant cellulose, BC has the benefit of being in form, which is both, highly crystalline and extremely pure. This review provides a crisp summary of the synthesis and functionalization of BC, and its applications in the fields of biomedicine and the environment.


Introduction
Cellulose has been epitomized as one of the ample renewable biopolymers found globally with physical and mechanical properties that permits applications in an array of biomedical and environmental platforms.The structural chemistry of cellulose was reported for the first time in 1837 by French agrochemist Anselme Payen.Cellulose is a polysaccharide composed of glucose units linked to each other by β-1,4 glycosidic bonds.Acid hydrolysis breaks cellulose into individual glucose units by breaking β-1,4 glycosidic bonds, which is not possible in normal physiological environment Wang et al., (2019).Individual glucose units present in the cellulose consists of three hydroxyl groups with degree of polymerization, which further depends on origin and treatment given to the cellulose.Weight-average degree of polymerization (DPw) for plant cellulose like cotton, corn, wheat straw, jute and wood pulp is 15,000, 1700, 2600, 1900 and 4000 DPw, respectively while the degree of polymerization for different bacterial species is 16,000 (Hallac & Ragauskas, 2011).In one of the recent studies, the importance of degree of polymerization on the mechanical strength of cellulose was reported, wherein the properties of cellulose type Iα was compared with other biopolymers like chitosan.Further, it was also reported that polymerization degree of cellulose was up to 8000 due to which cellulose fibrillar structure has the enhanced mechanical strength (Dahman, 2009).Effect of different growth conditions such as method of cultivation, fermentation time and different growth media were also reported to influence average degree of polymerization of BC with viscosity measurement technique (Shi et al., 2013).
Cellulose is found in crystalline form as Cellulose I, II, III and IV.Cellulose I is naturally derived from plant, bacterial and algal sources characterized with parallel chains stacked together with hydrogen and Vander Waal bonds (Seddiqi et al., 2021) Cellulose II and III are derived from chemical modification of Cellulose I, further Cellulose IV is produced by thermal treatment.
Bacterial cellulose (BC) has a chemical structure like vegetal cellulose, but BC does not contain plant cell wall components like hemicelluloses, pectin and lignin and is obtained in a pure form; further it has high crystallinity and degree of polymerization (Ul-Islam et al., 2019;Sharma et al., 2020).Acetobacter xylinum (A. xylinum) is the first reported species to synthesize BC extracellularly (Brown, 1886).BC is also reported to be synthesized by different microbial species like Rhizobium, Glucoacetobacter, Sarcina, and Agrobacterium (Römling & Galperin, 2015;Ullah et al., 2016).Gluconobacter xylinus is used extensively for BC production due to its high production yield.Amongst the algae, Siphonocladales and Cladophorales are reported to synthesize cellulose (Mihranyan, 2011).Production of BC is controlled by oxygen supply and carbon source concentration (van Zyl & Coburn, 2019).The biochemistry of cellulose synthesis varies across the species.Cellulose was reported to be produced in between outer membrane and cytoplasmic membrane in Acetobacter xylinum and Komagataeibacter sp with enzymes glucokinase, phosphoglucomutase, and UDP-glucose pyrophosphorylase cellulose synthase (Jonas and Farah, 1998;van Zyl & Coburn, 2019).
BC has shown tremendous potential for applications in various fields such as food packaging, environmental remediation, energy production, drug delivery and biomedical applications (Choi et al., 2022;Gregory et al., 2021;Liu & Mo, 2020).BC is extracellular and intrinsically pure (Mohite & Patil, 2014).Further production of BC is not impacted by climatic variations and can be easily purified.Recently extensive research is in progress for upscaling the production of BC using different fermentation strategies.BC fibers are 100 times thinner than plant cellulose and exist in a highly porous 3-D network that is highly biocompatible (Basu et al., 2018;Cazón & Vázquez, 2021).Thus, it has garnered special interest of the scientific community (El-Gendi et al., 2022;Gregory et al., 2021).Moreover, BC crystallinity is better than plant cellulose.BC structure is composed of nanofibers (2-4 nm) staked together, which imparts the properties of elasticity, water holding capacity and moldability.Surface functionalization of BC is also possible due to the vacant hydroxyl groups on it surface (Popa et al., 2022).Upon treatment with sodium hydroxide, BC undergoes mercerization wherein a unique structure of anti-parallel packing (Cellulose II) of fibers is observed with increased Young's modulus with 118 GPa for single BC filament which is comparable to steel (Popa et al., 2022).This leads to reduction in diameter of BC filaments and exhibits high surface area in comparison to plant cellulose.Further this leads to increase in mechanical properties and stress and strain behavior making it suitable for biomedical applications (Lai et al., 2021).
BC is not only eco-friendly but also possess excellent mechanical properties and can be used to create antimicrobial packaging as plastic alternate (Cazón & Vázquez, 2021).BC has stable biofilm formation and supporting properties that can enhance the performance of microbial treatment systems (Choi et al., 2022;Mohite & Patil, 2014) and has been extensively reported for biomedical applications like wound healing graft (Picheth et al., 2017), electronic platforms (Qin et al., 2021), food packaging and artificial skin grafts (Azeredo et al., 2019).In addition, BC has also been extensively studied for its potential applications in the environmental field, such as dye degradation, heavy metal detection and remediation, pathogen detection, and biosensing (Choi et al., 2022;Gregory et al., 2021).
The major emphasis of this review is to provide a concise summary of the production methods for BC and the biomedical and environmental applications thereof for further research.The multidimensional nature of BC provides a wide scope for applications in a variety of fields.to achieve this, we searched various global scientific databases such as Google Scholar (https://scholar.google.com/),Scopus (https://scholar.google.com/),Web of Science (https://www.webofscience.com/),ScienceDirect (https://www.sciencedirect.com/)and PubMed (https://pubmed.ncbi.nlm.nih.gov/), which are efficient, and reliable tools to search, collect, and collate scientific data.A preliminary search on all these databases using the key word "BC" collectively yielded 20,60,000 documents including research and review articles, book chapters and patents.We adopted a Funnel approach to narrow down the research areas by employing specific keywords that address the scope of the review.This further reduced the number of relevant documents to approximately 38,000.We further reduced the document search by searching the documents in the past 5 years (2018-2023) which yielded 21,400 documents.This indicates that considerable research is being carried out on BC in the past five years.We selected relevant full text articles to structure the current review to explore and consolidate the recent research pertaining to BC synthesis methods, and its biomedical and environmental applications.Figure 1 illustrates a schematic abstract of the review article highlighting the focus on synthesis methods, specific biomedical and environmental applications of BC.

Bacterial cellulose synthesis methods
BC synthesis needs optimization for its economical production and commercial application.Numerous reports highlighted the research in variations of culture techniques, media and conditions wherein the Hestrin Schramm (HS) medium was inoculated with Gluconacetobacter persimmonis and incubated for 14 days, in static culture to observe the pellicle formation at air-liquid interface (Hegde et al., 2013;Keshk, 2014Ruka et al., 2014).Further the effect of pH, speed of agitation, nutrient availability & viscosity and oxygen supply on BC production was studied (Mohammadkazemi et al., 2015).The effect of growth conditions on morphology and properties of BC has also been reported (Ruka et al., 2014).
BC production is currently performed using two methods namely-static and agitated method (Chawla et al., 2009).Depending on morphologies and properties of bacterial culture; production varies (Pang et al., 2020).When grown in static conditions, BC takes the form of a gelatinous pellicle; however, in agitated conditions, it forms into lumpy masses.There are pros and cons of both types of culture methods, for example in static culture higher genetic stability is observed while in agitated culture mutants can be formed (Ross et al., 1991;Sani & Dahman, 2010).Based on the Young's modulus measurements, BC produced in static fermentation has superior mechanical properties than of agitated fermentation (Krystynowicz et al., 2002).Production of BC depends majorly on the carbon source in culture media like sucrose, fructose, molasses, arabitol, and mannitol, nitrogen sources such as peptone, yeast extract, and corn steep liquor.Moreover, different types of agricultural and wastewater residues are also been reported for BC production (Buldum et al., 2018;Velásquez-Riaño and Bojacá, 2017).Figure 2 depicts the two main fermentation methods for BC synthesis.

Static fermentation
BC is usually produced by a static fermentation method at the liquid-gas interface wherein carbon dioxide generated in bacterial metabolism gets entrapped in the cellulose membrane (Sharma et al.,  2021).Briefly the production vessels are filled with sterile nutrient medium, further the appropriate amount of inoculum is added in this sterile media.Optimum fermentation conditions are maintained at 28°C temperature and pH between 4-7 for 2 weeks incubation to achieve desired thickness of the BC pellicle.BC coloration immediately after harvest is yellow, and turns pale white after repeated washing with 2% sodium hydroxide in warm water (Sharma et al., 2021).The pellicle is further washed with distilled water repeatedly till the pH lowers to neutral.BC productivity in static culture is greatly dependent on the surface area of growth vessel and not by the volume and depth of media (Hsieh et al., 2016).To overcome this limitation, intermittent feeding method was introduced, wherein regular addition of growth media was fed to culture flask (Blanco Parte et al., 2020).Once BC of particular thickness is attained the cell growth stops due to depletion of oxygen and nutrients in the medium (Hsieh et al., 2016).Static production of BC using glycerol as a sole substrate and its kinetic studies was reported using Gluconobacter xylinus (KCCM 41,431) (Dikshit & Kim, 2020).The standard HS medium used for BC production is not a cost-effective option for scale-up production, thus many researchers have tried using various cheaper alternatives (Zheng et al., 2020).In one such report BC production from fresh oil palm frond juice by Acetobacter xylinum 0416 has increased the yield 6× than the standard HS medium (Azmi et al., 2021).Additionally, various agricultural and industrial waste residues like pineapple peels, alcohol waste, rotten fruit culture, beet molasses etc have been used for economic BC production (Kongruang, 2008Velásquez-Riaño & Bojacá, 2017) (Stumpf et al., 2018;Wu and Liu, 2013;Singh et al., 2017).Many studies have highlighted the application of wastewaters for BC production, in one of such studies, wastewater from lipid fermentation (yeast biomass) was utilized from Gluconacetobacter xylinus (Huang et al., 2014).This wastewater had high COD value and the study was focused on evaluating the chances of biological conversion of low value carbohydrate polymer to high value carbohydrate polymer (Huang et al., 2014).In another study use of crude distillery effluent having high COD value (87,433 mg/l) was utilized for synthesizing BC by a new isolated bacterium Komagataeibacter saccharivorans.Alternative method of production of BC from tobacco waste as carbon source by Acetobacter xylinum ATCC 23,767 was reported (Ye et al., 2019).
Industrial production of BC has been reported with Gluconobacter xylinus-wherein glucose, fructose, starch, alcohol, D-arabitol and organic acids were used.The BC yield from glucose, fructose and glycerol was similar, while cellulose yield for D-arbitol was six times more as compared to rest of the carbon sources.The applications of the BC synthesized in this method was reported in skin therapy, artificial blood vessels and tablet modification (Keshk, 2014;Jing Wang et al., 2019).Production of BC with cost effective and eco-friendly bioprocess approach reported as static intermittent fed-batch (SIFB) (El-Gendi et al., 2022).In one of the recent studies; loaf sponge (Lufa aegyptiaca) was used for immobilizing Gluconacetobacter kombuchae and production of BC with these immobilized bacteria which is a novel and sustainable approach of production (Rahman et al., 2021).

Agitated fermentation
Production of BC in static culture has two main limitations namely high production cost and low yield, thus, agitated culture method is the best alternative for scaling up the yield (Czaja et al., 2004).Production of BC in static culture lowers as the cellulose pellicle on the surface becomes thick in size, which limits the passage of sufficient oxygen to the culture bacteria, agitated culture makes a better alternative for this limitation (Wang et al., 2019).The main advantage of agitated fermentation is uniform oxygen and nutrient supply to all the culture organisms.Nonetheless, oxygen alone is not a single governing factor for high production of BC, but genetic stability and fewer mutations are other factors that affect BC yield (Zywicka et al., 2015).In agitated fermentation, shaking speed and composition of the culture are responsible for the variations in shape, size and yield of BC (Gu & Catchmark, 2012).Gluconacetobacter xylinus CGMCC 2955, has been reported to produce BC spheres with diameters between 3 and 5 mm when produced by fermentation up to 72 h, with 160 rpm and temperature of 30 °C (Zhu et al., 2011).In yet another study, different strains of Gluconacetobacter xylinus (DSM 46,602,DSM 5602 and DSM 466,040) were evaluated for effect of different shaking parameters wherein; uniform BC was obtained with orbital shaker (100 rpm) while at 150-250 rpm BC was formed with circular shape and lesser diameter (Brandes et al., 2017;Zywicka et al., 2015).Komagataeibacter xylinus (KX, TISTR 086, 428, 975, and 1011) was reported to produce BC pellicles as membranous sheets in static culture while special or asterisk-like shape in agitated culture.Further, strain K975 has more BC production ability than strain KX in agitated culture; moreover, K428 and K086 has low ability to produce BC but BC produce here had a fascinating smaller diameter with larger surface area (Singhsa et al., 2018).In a similar study nine different strains that belonged to the Gluconacetobacter genus, isolated from waste and fresh vinegar were evaluated for BC production at static and agitated conditions, and were found to have capability of commercial production (Aydin & Aksoy, 2014).Synthesis of BC under shaking condition also produce a disordered structure of cellulose crystallites (Czaja et al., 2004).

Industrial scale fermentation
Industrial scale production of BC has been reported as an alternative static and agitated fermentation methods due to the limitations in using them (Blanco Parte et al., 2020).Recent reports have highlighted use of stirred tank, airlift bioreactor and rotating disk bioreactors (Blanco Parte et al., 2020), wherein stirred tank was reported to show high volumetric mass transfer coefficient, moreover increase in stirring rate results in higher cell densities which further results in higher BC production (Serafica et al., 2002).Airlift fermenter has added advantage over stirred tank fermenter as it produces less shear stress (Gorgieva, & Trček, 2019).Further rotating disk bioreactor addresses the limitation from stirred tank and airlift fermenter.In rotating disk bioreactor there is a central shaft attached to a circular plate for growth of BC pellicle which leads to more uniform growth than that of stirred tank and airlift bioreactors (Gorgieva, & Trček, 2019).Enzymatic saccharification is yet another efficient method adopted to scale-up the yield of BC on industrial scale.Skiba et al., (2020) reported a pilot scale-up study with a reported BC yield of 80.5 tons (98%-wet hydrogel) per 100 tons of oat hulls chemically pre-treated followed by enzymatic saccharification using commercially available enzymes (Skiba et al., 2020).

Bacterial cellulose applications
BC has attracted the attention in research and development due to its unique properties and diverse applications.Applications of BC are broadly categorized as biomedical and environmental, apart from these it is also been used in food industry, ethanol production and battery separation (Cacicedo et al., 2016;Wang et al., 2019).Functionalization and modification of BC has been reported for in situ and ex situ applications.The BC can either be modified while its formation (in situ) or after the harvest (ex situ) (Stumpf et al., 2018).The following sections provide a detailed discussion on applications of BC.

Biomedical applications
The ease of production of BC, hydrophilicity, purity, transparency, degree of crystallinity and the ability to tune its physiochemical properties make it very appealing.It is not only biocompatible and biodegradable but also nontoxic, allows gas permeation, has high liquid holding capacity, high nanoporosity and prevents microbial infection (Yoshino et al., 2013;Zheng et al., 2020).Such qualities make BC superior to its counterparts in biomedical applications.The potential biomedical applications of BC are depicted in Figure 3 and discussed in the subsequent sections.

Wound healing
Any good wound dressing material must allow gas permeability, prevent microbial infection, maintain optimal moisture level in the wound, adsorb wound exudates and increase the process of reepithelialization at the target site (Portela et al., 2019).Since BC per se is not bacteriostatic or bactericidal, it is frequently used in the form of composites (Picheth et al., 2017).The study by Maneerung et al., (2008), showcased that silver-nanoparticles impregnated BC showed 99.7% and 99.9% reduction in viable E. coli and S. aureus, respectively, which are generally found in contaminated wounds (Maneerung et al., 2008).Similarly, BC with enhanced antibacterial function can be produced with the introduction of antibiotics, inorganic antimicrobials or organic antimicrobials as it has a high surface area with excellent porosity (Zheng et al., 2020).BC/antibiotic composites (with antibiotics such as amikacin and ceftriaxone) displayed larger zones of inhibition by penetrating bacterial cell membranes and effectively suppressing the bacterial growth (Volova et al., 2018).
Another reason for the focus on BC is its intrinsic mechanical or physical properties.Biocompatibility was observed in BC as it not only displays zero cytotoxicity to living tissue but also prevented immune reactions against it when demonstrated using various human fibroblast cell lines (Mohamad et al., 2017).Moreover, due to its high water holding capacity and slow drug release rate, high tensile strength and excellent flexibility, it behaves as a perfect vehicle for loading liquid drugs or any bioactive compounds along with maintaining proper moisture balance (Sulaeva et al., 2015;Ul-Islam et al., 2015).This helps to decrease pain, inflammation, and scarring.
Commercialization of BC wound dressings depend on their ultimate application, be it in the form of a moisture membrane or a dry film.These films vary in their thickness, exudate absorption capacity and the types of wounds they target: ulcers, abrasions, lesions, lacerations etc.For instance, the Suprasorb X® wound dressing is a wet pellicle utilized in balancing moisture in chronic wound lesions with low to moderate levels of exudates while the Nanoderm® wound dressing is a dry film used protecting skin-lost lesions (Zhong, 2020).
Further, BC from Acetobacter xylinum was valuable to treat slight second degree and deep dermal second degree burns with large area.Due to BC membranes, a moist condition for tissue regeneration was maintained, safe drug delivery occurred and the wound was positively isolated from the environment.The ultrafine network in the 3D structure determined the unique property of BC to absorb large volumes of liquid while maintaining mechanical strength.Clinical trials on 34 patients were done after accomplishing positive in vivo results on rat models as healing process was accelerated and fluid loss from the burns was minimized.The BC membrane absorbed the wound exudates properly and showed no hypersensitive reactions when tested on patients.BC dressing accelerated epidermal growth in minor wounds and reduced the time needed for scab demarcation in deep wounds (Czaja et al., 2007).

Cancer treatment
Cancer comes second in the list of the leading causes of mortality worldwide.It can originate due to successive mutation in the genes, exposure to carcinogens or lifestyle choices (Hassanpour and Dehghani, 2017).Conventional cancer therapies include surgery, chemotherapy, and radiotherapy, used either individually or in combination with each other.Recent advances have led to exploration in targeted drug therapy, stem cell therapy, hormone therapy, immunotherapy, sonodynamic therapy, ablation therapy etc (Debela et al., 2021).Although drug delivery techniques have been utilized frequently, obstacles such as multiple drug resistance, drug-related cytotoxicity, and insufficient drug concentration and penetration at target site arise.
Nanostructured lipid carriers (NLCs) loaded with cationic Dox (NLCs-H) or neutral Dox (NLCs-N) drug -BC hydrogel hybrid material was evaluated for localized cancer therapy (Cacicedo et al., 2018).NLC-H showed low encapsulation efficiency of 48% with a fast drug release while NLC-N showed higher encapsulation at 97% with a sustained drug release.When this hybrid system was examined on MDA-MB-231 breast cancer cell line, the IC 50 of Dox determined by MTT assays displayed that in its free form, the value was higher as compared to Dox internalized in the nanostructures.Since the release kinetics and encapsulation efficiencies differed for neutral and cationic Dox, a specific formulation of these loaded into BC membranes (BC-NLC-NH) was checked on an in vivo model without any side effects (Cacicedo, Castro et al., 2016;Cacicedo, Islan et al., 2018).
Sometimes, the success of surgical resection, chemotherapy, and radiation therapy are all threatened by the fact that metastatic lesions are frequently numerous and resistance to such traditional therapies is encountered.Photodynamic therapy is suggested as an alternative, which involves the uptake of a photosensitzer molecule which can be excited by a particular wavelength and subsequent production of reactive oxidant species (ROS).ROS in turn can destroy cell membranes, proteins and DNA, crucial for cytotoxicity efficiency (Santos et al., 2019).C 60 , a fullerene derivative and an attractive photosensitizer for photodynamic therapy (PDT), when combined with BC to form BCC 60 composite can be applied for cancer therapy.Properties of BC such as optical characteristics, swelling capacity, good biocompatibility etc. make it a suitable carrier of the fullerene derivative for PDT.The dehydration-rehydration method used to fabricate the composite solved the issue of poor solubility of C 60 in polar solvents.To add onto, antibacterial assessment of the composite was done against S. aureus and E. coli in which increasing the C 60 content under light radiation yielded high antibacterial activity of 95% coupled with high cell death toward human derived epidermoid carcinoma cells (Chu et al., 2018).
BC composites co-precipitated with magnetic nanoparticles also play an important role in drug delivery systems (Chaabane et al., 2020).The production and application of a novel magnetic material [Fe (DABC-EDABzl) Cl 2 ] through a co-precipitation method in basic conditions for cancer treatment has been described.In the process, DABC was chemically altered by ethylenediamine (EDA), benzyl (Bzl) and ferrous ions (Fe), followed by manufacture of the magnetic nanoparticles.Antimicrobial activity of the nanoparticles was evaluated by measuring zones of inhibition against bacterial and fungal strains, and an in vitro cytotoxicity assay (MTT assay) was performed on colon cancer cells.In vivo anti-tumor activity was observed in female BALB/c mice with subcutaneous tumors, which provided further evidence in support of these results.This novel nanomaterial could be further used for cancer treatment (Chaabane et al., 2020).

Drug delivery
BC's incomparable retention capacity conjointly with its nanofibrillar structure make it an indispensable carrier for a sustained release of antibiotics, analgesics, anti-inflammatories, hormones and anticancer agents (R Rebelo et al., 2018;Trovatti et al., 2012).Moreover, it allows for transparency, light transmission and gas permeability which are essential attributes for medical applications (Lai et al., 2021).(3-Glycidoxypropyl) trimethoxysilane [BC-GPTS (H)] and BC-chitosan nanocomposites integrated with cyclodextrin were loaded with either diclofenac sodium or ciprofloxacin in ophthalmological lenses.To assess cytotoxicity and genotoxicity, Chinese hamster ovary (CHO-KI) cells were used.The study demonstrated that therapeutic contact lenses with full transparency were made from organic-inorganic hybrid compound of BC without any mutagenic effects on the cells, where diclofenac sodium was necessary to eliminate drug toxicity (Coelho et al., 2019).
Furthermore, Luo et al. invented a novel BC-graphene oxide (BC/GO) nanocomposite where ibuprofen (IBU) was successfully loaded and released in a non-Fickian diffusion manner during in vitro studies (Luo et al., 2017).Although GO can be easily functionalized, loaded with drugs and has a large surface area owning to its nanosheet, the barrier that it faces when used on its own is that it tends to agglomerate due to nonspecific binding to proteins.The IBU loaded-nanocomposite (IBU-BC /GO) was manufactured via the in situ process and proved to be superior to IBU-BC as GO played an important role in the controlled release of a promising novel drug delivery system (Luo et al., 2017).
Another instance outlines a comparable experiment where methylene blue (MB) was used as a drug model and loaded into a BC-gelatin hydrogel composite which possessed properties such as chemical resistance, mechanical tenability, and thermal stability (Yin et al., 2015).The hydrogel is perfect for drug delivery application due to the estimated swelling ratio of 400-600% and a semi-interpenetrating polymer network.As compared to the neat gelatin hydrogel and BC hydrogels, BC filled the gelatin hydrogel forms denser composite matrix, it led to a burst drug release during the initial stage of the study, followed by a slower, gradual diffusion required for a sustainable drug release over a long time period of more than 40 h (Treesuppharat et al., 2017).
A controlled drug release system was created using a pH-electroactive hydrogel of BC and polyaniline (PANI) (BC/PANI) which depicted a Korsemyer-Peppas kinetic model with free diffusion.As the drug berbine hydrochloride (BH) shows ineffective absorption via oral and intravenous route of drug administration, it was impregnated onto the BC/PANI composite (which had a sandwiched morphology) and checked for its release under different pH (2.2-11) and voltage (0-0.5 V).The study concluded that the drug release was slower in acidic conditions which could be applied to drug release at a target site such as the small intestine where the pH is 6.8 and faster when a small voltage was given.Moreover, the multilayer sandwich structure had different porosities at each layer which led to tunable release of BH (Li et al., 2018).

Other biomedical applications
Alternative applications of BC find extensive use in ophthalmology, diagnostic and wearable sensors.For instance, in ophthalmology, BC (and/or its composites) was adopted in terms of substrate for retinal pigment epithelium (Goncalves et al., 2015), novel tissue-engineered corneal transplantation (Sepúlveda et al., 2016) and tissue substitute in rabbit's cornea (Han et al., 2020).Additionally, BC nanocomposites are also used for cartilage and bone regeneration (Kumbhar et al. 2017).To add onto, the umbrella of diagnostic sensors includes BC-based biosensors combined with metal nanoparticles, metal oxides, enzymes etc (Torres et al., 2020),.piezoelectric immune-chip coated with BC nanocrystals for dengue detection (Pirich et al., 2017), bienzymatic glucose biosensor from gold nanoparticles-BC nanofibre nanocomposite (Wang et al., 2010).Wearable sensors are currently receiving a lot of attention.Some examples of these sensors include a dual-mode BC based sensor for monitoring human motion (Huang et al., 2020); compressible and elastic carbon aerogels (CECAs, using BC as a nano-binder) as piezoresistive sensors used to monitor a range of biological signals (Chen et al., 2019); and an electrochemical biosensor as a noninvasive point-of-care testing device (Gomes et al., 2020).

Environmental applications
The extensive and versatile utilization of BC in environmental applications is a function of its many attributes such as high porosity, high mechanical strength, water holding capacity and high adsorbing ability (Bethke et al., 2018;Mohite and Patil, 2014).It also provides a highly branched robust framework for functionalization with different other organic chemicals and nanomaterials and used for water and wastewater remediation (Herbert and Nikita Tawanda, 2022).Surface modification in BC further enhances its charge properties, making it suitable for utilization in the field of energy harvesting and clean water production (Liu & Mo, 2020;Zhang & Chi, 2022).Figure 4 depicts the environmental applications of BC and BC composites.In this section, we will focus on the environmental applications of BC.

Heavy metal detection and remediation
Heavy metal contamination and exposure is of great concern for the environment and human health, especially in developing countries that have limited resources and statutory policies for the mitigation of pollution.BC nanofibers (BCNFs) have been used as biosensors for the detection of heavy metals such as lead, cadmium, chromium, mercury, arsenic, selenium, and copper in water (Hosseini & Mousavi, 2021;Huang et al., 2018;Liu et al., 2021;Song et al., 2020;Zhang et al., 2021).Surface modification of BC with different advanced functional nanomaterials enables for the adsorption and detection of heavy metal ions and change in the optical or electrical properties of the BC nanocomposites (Wang et al., 2016;Yang et al., 2020;Zhang et al., 2021).BC-based nanocomposites also exhibit promising adsorption properties for heavy metal ions.Introduction of nanomaterials not only enhance the specific surface area of the polymer structures, but also improves the thermal stability and conductivity.Song et.al, describe the synthesis and characterization of a novel composite material made from BC, graphene oxide, and attapulgite clay (BC/PVA/GO/APT), which exhibits excellent adsorption capacity for copper and lead ions in aqueous solutions (Song et al., 2020).The resulting composite material exhibited high adsorption capacities for both copper and lead ions in aqueous solutions, with a maximum adsorption capacity of 150.79 mg/g for copper and 217.8 mg/g for lead.The material's adsorption kinetics and isotherms were also analyzed to understand the adsorption behavior, revealing that the adsorption process was rapid and followed a Langmuir isotherm model.Furthermore, the reusability and stability of the hydrogel indicate promising and sustainable use for the BC/PVA/GO/APT polymer hydrogel.BC can also be used to create membranes for simultaneous removal of multiple heavy metals (Hu et al., 2019;Huang et al., 2018;Qinghua et al., 2017).Qinghua, et al., (2017) describe the fabrication of a paper-like multifunctional purifier membrane possessing 3-D BC substrate grafted polyethyleneamine derived quaternary ammonium compound exhibiting high adsorption capacities toward various heavy metal ions such as lead, copper and hexavalent chromium in addition to anionic dyes such as congo red, methyl red and methyl orange (Y.Hu et al., 2019).BC can be blended with other materials, such as MOFs, chitosan, PVA, to create a composite membrane with high adsorption capacity for heavy metal ions (Li et al., 2020;Ma et al., 2019;Wu et al., 2023).The BC component provides a porous and stable structure, while the other components such as MOFs and chitosan add anionic and cationic functionality, making the composite membrane suitable for the adsorption of heavy metals.The use of BC for heavy metal detection and removal shows promise as a sustainable and effective method for remediating heavy metal-contaminated water.

Biosensors
The development of ultrasensitive, highly selective biosensors with high catalytic activity, low limit of detection and rapid response toward analyte are an urgent need considering the current environmental scenario.Biomarker-based as well as label free assays for rapid and on-site detection of contaminants is the emerging need to provide timely mitigation measures against increasing pollution load (Robson et al., 2020).BC nanofibers (BCNFs) have been used as biosensors for the detection of heavy metals such as lead, cadmium, and copper in water.In this method, the BCNFs are functionalized with thiol groups that bind to heavy metal ions, causing a change in the electrical properties of the material (Salehi et al., 2021).BC and nanocellulose have been widely used as biosensors for the detection of environmental pollutants due to their unique properties such as high surface area, porosity, and biocompatibility.BC nanofibers (BCNFs) have been used as a biosensor for the detection of heavy metals such as lead, cadmium, and copper in water.In this method, the BCNFs are functionalized with thiol groups that bind to heavy metal ions, causing a change in the electrical properties of the material that can be measured (Basu et al., 2018;Hassan et al., 2018).Nanocellulose films have been used as biosensors for the detection of VOCs such as ethanol, methanol, and acetone.In this method, the nanocellulose films are functionalized with enzymes that react with the VOCs, causing a change in the optical properties of the material that can be measured (Blanco et al., 2018).Nanocellulose films derived from BC have been used as biosensors for the detection of VOCs such as ethanol, methanol, and acetone.Nanocellulose films are functionalized with enzymes react with VOCs, causing a change in the optical properties of the material that can be measured (Narwade et al., 2022).In a study by Tang et al., (2022), hierarchically synthesized highly porous BC aerogels functionalized with enzymes that react with the pesticides are used to report the change in the electrical properties of the material (Tang et al., 2022).Additionally, pH responsive indicators and natural pigments can be conjugated with BC to detect changes in pH levels that can be estimated through colorimetric assays (Maruthupandy et al., 2021;Mehran et al., 2019).BC-based nanocomposites can also be used to create biosensors for the detection of heavy metals as well as pathogens in the environment (Muhamad et al., 2020;Xu et al., 2021).The surface of the BC can be modified with specific chemical groups that change in response to changes in gas concentrations.This change in the chemical groups can result in a change in the optical or electrical properties of the BC, which can be detected and used to monitor changes in gas concentrations.

Microplastics remediation
Microplastic pollution is a major environmental concern, and various methods are being explored to mitigate its impact.BC has been recently investigated for its potential in microplastic adsorption due to its high surface area, biocompatibility, and sustainability (Faria et al., 2022).BC aerogels were prepared and tested for their ability to adsorb microplastics from water (Zhuang et al., 2023).The results showed that the BC aerogels had a high adsorption capacity for microplastics, with a maximum adsorption capacity of 160.1 mg/g (Wang et al., 2021).report the modification of BC with magnetite nanoparticles for enhanced microplastic adsorption.The results showed that the modified BC had a higher adsorption capacity for microplastics compared to unmodified BC.In this study, BC membranes were prepared and tested for their ability to remove microplastics from water.The results showed that the BC membranes had a high removal efficiency for microplastics, with a maximum removal efficiency of 96.6%.Another study reported the preparation of BC membranes with a 96.6% removal efficiency for microplastics (Mendonca et al., 2023).

Other environmental applications
Bacterial cellulose also finds alternate applications in a variety of environmental applications such as pathogen detection (Patrícia et al., 2018;A. Rahman et al., 2022;Samaneh et al., 2020), sustainable and antimicrobial food packaging and food wrapper (Albuquerque et al., 2020;Aydin & Aksoy, 2014;Bandyopadhyay et al., 2019), biodegradable fabrics and accessories (da Silva & Gouveia, 2015;Provin et al., 2021), semiconductors, flexible & conducting polymers that can be used for device and battery fabrication.Additionally, major effort is taking place in the field of energy harvesting by using triboelectric nanogenerators that utilize the flexibility, ease of surface charge modification and especially, the piezoelectric properties of BC (Liu & Fu, 2021;Li & Jie, 2019).

Further scope
BC has proved as an excellent and sustainable alternative to synthetic polymers owing to its high purity and crystallinity and superior mechanical tunability.However, production of BC at commercial level has many limitations, like culture maintenance, high media cost and effect of environmental factors on size of the pellicle formed.To overcome these issues, future research can be focused on production of BC at lower cost with environmental waste products such as fruit and vegetable waste as substrate which has high fructose content and by implementing better optimized fermentation techniques.Additionally, the ease of fermentation process may prove to be advantageous for up-scaling the process in commercial settings.Another challenging aspect for large scale production of BC is maintaining the genetic structure and retaining the inherent properties of the culture bacteria.This can be achieved by performing biochemical and genetic investigations at specific stages in the fermentation process to ensure integrity.Numerous nanomaterials like carbon dots, metal and metal oxide nanoparticles can be entrapped in BC matrix and can be utilized in increasing the electrochemical properties for sensing making it more applicable in various biomedical and environmental applications, which are not yet reproduced at a larger scale.Moreover, BC can be complexed with other biopolymers and frameworks (MOF, COF, ZIF, etc.) to achieve enhanced surface area and adsorptivity.Researchers can add in efforts for utilization of these composites at industrial and commercial level to make them more robust and reliable for use.Biocompatibility, mechanical attributes and liquid holding capacity of the BC make it an ideal material for utilization in biomedical applications and wearable sensors.Furthermore, its customized composites can be functionalized with drugs and other medicinal compounds for targeted drug delivery, cancer therapy and tissue engineering.Detection of environmental pollutants with the BC matrix as a platform can be focused, as most of the current methods reported are toxic, expensive, complex, and nonbiodegradable.Detection of a specific pollutant in vast matrices like water, has challenges like selectivity and sensitivity of the detection system, which can be taken into consideration for success in research work.To summarize, low-cost commercial production of BC in the future may deliver a sustainable biomaterial in abundance, with the potential for application in a variety of biomedical and environmental fields.

Highlights
• Energy efficient and Scale-up synthesis methods described for production of Bacterial Cellulose • Utilization of fruit waste & agro-waste as a sustainable substrate for BC synthesis • Application of bacterial cellulose as biocompatible and tensile matrix in biomedical protocols • Use of bacterial cellulose as a biodegradable bioadsorbent for environmental sensing and remediation

Figure 1 .
Figure 1.Schematic of the review article.

Table 1 .
Table 1 summarizes a few examples of fruit wastes used as carbon source.Different types of fruit wastes used as carbon source for bacterial cellulose production.