Textile waste-based biosensors for medical monitoring

Comparison of mechanical and chemical properties of silk, cotton and chitosan

Textile waste-derived biosensors, notably those utilizing silk, cellulose, and chitosan, demonstrate potential in disease-specific applications, particularly in glucose monitoring and other analyte detections. Silk is beneficial for creating biosensors used in glucose monitoring in locations with limited resources. This allows for the visual detection of glucose levels without the need for external instruments. Cellulose and chitosan can be combined to create nanocomposites that have a wide range of uses, especially in the field of biosensors for addressing health-related issues. Moreover, the utilization of silk yarn in textile weaving processes for biosensor production offers the development of compact electrochemical sensors with biocompatible characteristics and optimized reagent consumption.

These characteristics enable them to be used in a variety of applications across different fields, such as textiles and biomedical devices. ³ Table 1 compares the mechanical and chemical properties of silk, cotton, and chitosan for biosensor applications.

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Table 1.

Comparative properties of silk, cotton and chitosan.

Properties	Silk	Cellulose	Chitosan
Composition	Silk fibroin (protein)	Cellulose (polysaccharide)	Chitin (polysaccharide)
Tensile Strength	High	Moderate to High	Moderate
Flexibility	Excellent	Good	Moderate
Biocompatibility	Excellent	Good	Good
Antimicrobial	Limited antimicrobial properties	Limited antimicrobial properties	Strong antimicrobial properties
Degradability	Slow	Slow	Fast
Applications	Surgical sutures, tissue engineering	Textiles, drug delivery, tissue engineering	Wound healing, drug delivery, tissue engineering

Silk-based sensors

Silk is a natural fiber produced by spiders and silkworms using non-covalent interactions to build fibrous proteins. ¹¹ The chemical structure of the silk biopolymer is shown in Figure 4. Because of its remarkable mechanical properties, such as toughness, flexibility, and robustness, silk is an ideal material for producing smart and biomedical devices.^{12 –14}

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Figure 4.

Chemical structure of silk biopolymer. ¹⁵

Chemical modifications have been used to create intelligent electrical devices from silk-based hydrogels, fibers, nanowires, and films. The material is preferred for its biodegradability, biocompatibility, low-cost, and excellent mechanical characteristics.^16,17 In recent years, silk has been used primarily as a matrix for electronics, with SF (silk fibroin) becoming increasingly famous as a useful substance for the creation of flexible sensors. These sensors include humidity sensors, temperature sensors, pressure sensors, and electrochemical sensors.^{18 –20} A dual E-sensor for pressure and temperature sensor is shown in Figure 5.

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Figure 5.

E- Sensor for pressure and temperature monitoring. ²¹

Kadumudi et al. have combined silk fibroin with reduced GO (graphene oxide) to create a composite material called “Care Gum.” This material uses phenolic glue to allow hierarchical H-bonding and sacrificial bonding. When compared to its newer equivalent, the hierarchical bonding technique has superior mechanical toughness, an elongation capability of approximately 25,000%, excellent conformability, and three-dimensional printability. Further, electrical conductivity increases tenfold, and Young’s modulus increases fourfold. These unique features have been used to create a robust and self-healing E-glove for hand motion detection. ²²

He et al. ²³ created a hydrogel that is not only flexible but also conductive. They achieved this by mixing SF, PA (polyacrylamide), GO and Poly (3,4-ethylene dioxythiophene)-poly (styrene sulfonate) in the right proportions. This PSGP hydrogel is highly elastic and compressible, making it ideal for creating dual-strain and pressure sensors with wide applications and constant stability. The sensor can detect a variety of physiological data from the body, including joint movement, facial expressions, heart rate, and respiration. This type of sensor is suitable for the epidermis and does not irritate, making it safe for long-term use.

Liu et al. ²⁴ have invented a highly conformal, flexible, adjustable, and durable adhesive made of micro-structured SF protein-based adhesive (MSFA). The MSFA can maintain a consistent and stable binding force on the skin, even in humid or moist environments, and can be easily removed without causing any significant pain. MSFA’s conformal and customizable adhesion to the skin can improve the sensitivity and recyclability of epidermal strain sensors. Considering the growing trend of personalized health care, the MSFA has the potential to be a beneficial adhesive for multiple skin-sensing equipment. A composite film made of SF and graphene by casting has been developed. This film has excellent mechanical and electrical properties, making it ideal for wearable and implantable biomedical applications. The SF/graphene composite film serves as an internal sensor that can monitor various vital signs in real time. Its high sensitivity and stability make it an excellent tool for medical diagnostics and research.

Silk-based sensors play a critical role in monitoring body temperature to ensure it stays within the normal range of 36°C–37°C, as abnormal fluctuations may signal issues with thermoregulation. Disruptions in body temperature control can result in diseases or even death due to disrupted chemical procedures. Carbon nanotubes (CNTs) have exceptional thermal and electrical conductivity, chemical stability, and distinctive structural properties, making them a great material for wearable temperature sensors. Researchers have created various sensor types using CNTs and silk fibers, including the XSBR/SSCNT sensor, CNT/SNF ultrathin electronic tattoo, and fibrous sensors filled with CNTs and ionic liquid. These sensors have shown improved mechanical properties, electrical conductance, and precise temperature sensing capabilities. Moreover, incorporating dielectric elastomer, glycerol, and MXene into the SF/CNT system has enhanced flexibility and temperature monitoring. Additionally, materials such as electrospun silk nanofibers, reduced graphene oxide (rGO), and cellulose nanocrystals (CNC) have been combined with silk to develop sensors with high electrical conductivity, mechanical stability, and thermochromic properties for intuitive temperature display. These advancements in silk-based sensors hold great promise for wearable sensor applications. ²¹

Cellulose-based sensors

Cellulose possesses diverse characteristics such as biocompatibility, biodegradability, hydrophilicity and chemical modifiability.^25,26 Its chemical structure is illustrated in Figure 6. It is used to create paper and plastic and can yield a variety of cellulose derivatives. Superior mechanical qualities, strong thermal stability, and simplicity of modification make cellulose an environmentally beneficial material. It can be used in various forms to create cellulose-functionalized materials for different applications. Combining cellulose with different materials like metal and metal oxide nanoparticles, carbon nanomaterials, and conducting polymers results in distinct composite properties that enable the construction of intelligent sensors and electronic devices.^{27 –29} Due to its unique structures and qualities, cellulose is gaining popularity for its potential use in detecting electronic equipment.^{30 –32}

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Figure 6.

Chemical structure of cellulose biopolymer. ⁴⁴

Nanocellulose (NC) is a promising nanomaterial derived from cellulose used in sensing and biomedical applications. Neubauerova et al. ³³ developed a Nano cellulose-based biosensor that can detect glucose in urine samples from diabetic patients. The team incorporated glucose oxidase (GO_x) into the cellulose/NC-based substrate to enhance color development in the presence of H₂O₂. This is the first time that NC has been paired with an enzyme biosensor to monitor glucose levels in people with diabetes. Wang et al. ³⁴ used polymerizable deep eutectic solvents and natural bacterial cellulose to construct a reinforced cellulose nanofiber ionic conductor. The composite showed remarkable mechanical stability and was used to create a sensor for detecting movements such as coughing and drinking. These materials have various sensitivities to external stimuli, making them ideal for use as hypersensors to detect human physical activity.

Chen et al. ³⁵ developed CECAs (compressible and elastic carbon aerogels) for use in skin-implantable electronics and wearable technology (Figure 7). They created a lightweight CECA with superior sensing abilities by joining MXene (Ti₃C₂) nanosheets using BC as a nano binder. It can capture output data throughout a wide range of pressures, making the CECA suitable for flexible wearable sensors to monitor biosignals.

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Figure 7.

Cellulose-based sensor for detecting the jugular venous pulse of humans. ³⁵

Cellulose nanofibre (CNF) aerogels is being increasingly utilized in biomedical fields, particularly in tissue repair. Researchers have developed nanobiotechnology aerogels using materials for example, chitosan nanoparticles, CNFs, alginate and silver. CNFs have been implemented in biomedical applications like scaffold fabrication, antimicrobial wound dressing, biosensing, medical implants, drug delivery, and vascular grafts.^36,37 CNFs form aerogels during medication absorption. The potential applications of CNF aerogels in numerous biological and medical research have been positively impacted by their attributes, which include biocompatibility, biodegradability, low levels of toxicity, and sustainability. CNF aerogel has been tested for biosensing, tissue engineering, and drug delivery.^38,39

Nanotechnology has enabled the use of 3D bio aerogels to address the instability and sensitivity issues associated with one-D and two-D materials. For wound dressing, cotton-based nanocellulose aerogel has been employed as a biosensor surface. ⁴⁰ Scientists have produced a biosensor for heavy metals using genetically modified bacteria and CNF films. The CPC-CNF films release fluorescence that is sensitive to heavy metals and can identify free copper ions in human blood serum. ⁴¹

Chitosan- based sensors

Chitosan (CS) is a naturally renewable polymer made from chitin, the world’s second most abundant biopolymer (Figure 8). Chitosan-based materials are low-cost and have excellent biocompatibility, biodegradability, regulated mechanical characteristics, and various biological characteristics.^{42 –44} Wang et al. ⁴⁵ created flexible e-skins made of a bionic CS/MX hybrid film for detecting pressure and monitoring humidity. The hybrid film-based e-skins showed low toxicity and can differentiate between various physiological activities.

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Figure 8.

Chemical structure of chitosan biopolymer. ¹⁵

The researchers used self-standing glycine-chitosan piezoelectric sheets to make a flexible, biodegradable pressure sensor. Self-assembled glycine biomolecules in water-based CS form piezoelectric sheets. They are composed of a stable glycine spherulite framework enclosed in an amorphous CS. For biomedical uses, this biodegradable film might be used as a biodegradable sensor. ⁴⁶ Hroncekova et al. ⁴⁷ found sarcosine as a potential glandular prostate cancer biomarker utilizing a Ti₃C₂TX biosensor. Jang et al. ⁴⁸ produced a tunable color filter that uses a metal-insulator-metal (MIM) multilayer employing chitosan hydrogel as the insulator. This color filter when combined with a photovoltaic (PV) cell, can act as a humidity sensor. The CH film is sandwiched within two extremely thin layers of silver (Ag) on a glass substrate.

Hydrogel which is similar to human skin, can be used for health monitoring as a smart ionic skin. It enables the detection of external stimuli and signal transmission in biology. ⁴⁹ Shi and Wu ⁵⁰ developed a smart hydrogel skin patch (Figure 9) with excellent conductivity and biosafety. Bioelectronic pressure and temperature sensing capabilities are provided by this patch which is capable of distinguishing multiple physiological signals simultaneously.

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Figure 9.

Hydrogel-based smart skin patch. ⁵¹

Fabrication process of silk-based biosensors

Silk-based biosensors are cutting-edge devices that utilize the distinctive characteristics of silk for the identification of biological and chemical substances. The fabrication process is based on piezoresistive sensors for wearable. ⁵²

Materials Used- Silk Fibroin, derived from silkworm cocoons, is used to provide a biocompatible and stable matrix for the biosensor. Hollow Carbon Nanospheres (HCNs) act as conductive elements and enhance the sensor’s performance. Fresh rose petals are incorporated as a mold for creating microstructural polydimethylsiloxane (PDMS) films. PDMS, a flexible and transparent polymer commonly used in microfabrication, plays an essential role in the construction of the biosensor. Additionally, chemicals such as CaCl₂ and formic acid are utilized during the manufacturing process.

Preparation of HCNs- Silica spheres are ammoniated and coated with glucose. The coated silica spheres are then treated hydrothermally. The silica core is etched out using hydrofluoric acid (HF) to obtain hollow carbon nanospheres.

Molding Process- Fresh rose petals are used as a natural mold. Microstructural PDMS films are created using the rose petals as a template. These PDMS films serve as the substrate for the subsequent steps.

Sensor Assembly- Two layers of HCNs/silk fibroin composite films are assembled face-to-face, and silver paste electrodes are integrated into the films. The entire assembly is then packaged with medical-grade polyurethane (PU) tape.

Fabrication process of cellulose-based biosensors

Cellulose-based biosensors are receiving significant attention due to their biocompatibility, biodegradability, and versatility. The fabrication process and applications of these sensors:

Cellulose, the most abundant natural biopolymer, serves as an excellent substrate for strain and pressure sensors. It is accessible in a variety of forms, including fibers, nanocrystals and nanofibers. Researchers use a variety of ways to turn cellulose into hydrogels, aerogels, films, paper, and textiles that can potentially be used as sensors.

Extraction of cellulose from cotton waste- Cotton waste can be converted into valuable nanomaterials, which is significant for environmental protection. Cotton is abundant in cellulose, making it ideal for cellulose nanocrystals (CNC) production. Various extraction strategies have been investigated due to the unique features of CNC. The crystallinity of CNC recovered from cotton waste is determined by the pretreatment procedures used and the processing conditions. While CNC is an excellent biodegradable reinforcement material, processing mechanisms of cotton waste into functional materials are rarely reported. The idea of cotton-derived CNC has been examined as a reinforcement material to improve the functional qualities of composite films for packaging applications. Cotton waste can be used to extract nanocrystals through chemical, mechanical, or enzymatic processes. CNCs have excellent physical and mechanical properties, and their efficiency for reinforcement applications depends on their crystallinity and aspect ratio. CNCs have been successfully extracted from cotton linters, denim wastes, and viscose yarn through various methods resulting in higher crystallinity. The extraction of CNCs from cotton waste is low-cost and produces value-added products. ⁵³

Several studies have examined the method of extraction of CNCs from cotton waste. Some investigations obtained high degrees of crystallization and thermally stable operation of CNCs, while others have reported lower values due to the nature of the waste material and the hydrolysis process. The CNCs have the potential for use in hydrophilic nanocomposites and have been found to have high water-holding capacity. CNCs’ high aspect ratio contributes to better mechanical and barrier properties. ⁵⁴

Chemical pretreatment is a frequent way to extract CNC from cotton waste. CNCs’ high aspect ratio contributes to better mechanical and barrier properties. The process involves washing, pulping, bleaching, and hydrolysis. Sulfuric acid hydrolysis produces high crystallinity CNC, which is highly efficient when combined with bleaching. Ultrasonic-assisted hydrolysis improves the distribution of CNC in the polymer matrix. Enzymatic hydrolysis, sonication, and high-pressure mechanical homogenization can also produce high-quality CNC. ⁵⁵

Fabrication Methods- The production of cellulose membranes involves several processes. Vacuum filtration is used to prepare cellulose membranes by filtering a cellulose suspension under vacuum to form a membrane. Another method, solution regeneration, involves dissolving cellulose in a solvent and then regenerating the solution to form a membrane. Electrospinning is also employed, where cellulose nanofibers are electrospun into membranes. Additionally, cellulose solution casting entails casting a cellulose solution onto a substrate and drying it to form a membrane. Lastly, phase inversion is a process in which cellulose is precipitated from a solution by changing the solvent composition.

Functionalization and Applications- Researchers modify the surface of cellulose by functionalizing it with receptors, labeling units (e.g. fluorophores, redox molecules), and other functional groups. Cellulose-based sensors find applications in wearable health monitoring, soft robotics, human-machine interactions, and more. The choice of substrate and functional materials significantly influences sensor characteristics, such as sensitivity, response time, strain range, stability, and hysteresis.

Fabrication process of chitosan-based biosensors

Chitosan is a significant and extensively accessible biomaterial naturally created for the restoration of its resources. Chitosan is derived from chitin, which is recovered from marine waste materials. It is produced by the process of deacetylation. Chitosan is very effective at adsorbing harmful ions in water due to its many hydroxyl and amine groups. ⁵⁶ The fabrication of chitosan-based biosensors for medical monitoring ⁵⁷ :

Obstetrics and gynecology

Biopolymers have become increasingly popular in various applications, primarily medical ones, during the last few years. The fields of obstetrics and gynecology require the use of materials that are safe and non-toxic for sensitive parts of a woman’s body and her newborn baby, as well as those that do not cause infections or gene alterations. Healthcare professionals who utilize screening, examination, and pre- and post-operation materials must be more knowledgeable about the characteristics, health effects, and environmental impact of each type. Numerous advantages and benefits of adopting biopolymer-based materials for various obstetric and gynecologic applications can be found by understanding the fundamental concepts of such materials. ² An ideal biopolymer for medical applications should possess various characteristics, such as non-toxicity, non-inflammatory, and non-immunological responses, easy sterilization, acceptable shelf life, and ease of processing to its final form. Many biopolymers are non-toxic, biocompatible, and capable of boosting cellular survival and proliferation.

Ramphul et al. ⁵⁸ claim that biopolymers like cellulose have a high degree of hydrophilicity due to the presence of many OH groups, which promotes cellular connections. The cytotoxicity and biocompatibility of biopolymer-based tissue scaffolds were investigated. The scaffold had no detrimental effects on human or mouse cell lines. ⁵⁹

Biopolymeric-based materials have gained popularity in recent years in a variety of forms, including hydrogels, aerogels, films, sutures, surgical implants, examination materials, scaffolds for tissue engineering, and medication delivery. This is because, in contrast to traditional materials based on petroleum, metal, or ceramic, many biopolymers exhibit unusual and exceptional adaptability. ⁶⁰ Researchers are exploring the potential benefits of using biopolymers in obstetrics and gynecology, including vaginal drug delivery, cosmetic and personal care items, examination tools and equipment, bioplastics, smart gynecological sutures, and wound healing products.^{61 –65}

Pregnancy-related renal colic is the most common non-obstetric reason for hospitalization. It is a sickness that affects both the mother and the growing fetus. To address this issue during pregnancy, a biopolymeric stent was developed. Can et al. ⁶⁶ found the immediacy, dependability, and success of urgent stent placement during pregnancy. Additionally, the biopolymeric stent was found to have a low rate of complications and to be highly successful in treating pregnant women’s persistent discomfort in their flanks. ⁶⁷ Unlike conventional stents, biopolymeric stents have been demonstrated to break down into simple substances that naturally occur in the human body and can be surgically removed, which have several main difficulties, including microbial biofilm blockage and stent migration. ⁶⁸

Ethylcellulose was used as a precursor material by Cazorla Luna et al. to construct vaginal mucoadhesive bilayer films that regulate the release of antiviral agents. The researchers disclosed that biopolymers present a viable choice for women to safeguard themselves against multiple sexually transmitted infections, including HIV. ⁶⁹ Biopolymer-based materials have been used to address the restrictions of ordinary synthetic polymeric goods, like limited retention duration, pain, and a lack of optimal prevention and treatment strategies, which leads to a high risk of recurrence for vaginal sickness. ⁷⁰

Chronic wounds

Chronic wounds are distinguished by abnormal extracellular matrix reorganization, a lack of re-epithelialization at the wound boundary, and ongoing inflammation. They often become “stalled” in the inflammatory phase, leading to unusually high proteolytic activity, extracellular matrix tissue and degradation of growth factors, which are crucial for cell proliferation and wound closure. Globally, about 40 million people are affected by chronic wounds, demonstrating a substantial medicinal challenge.

Defective extracellular matrix reorganization is an indicator of chronic wounds and prolonged inflammation that results in high proteolytic activity. ⁷¹ Several studies have been conducted to develop dressings that can remove harmful levels of proteases from chronic wounds.^{72 –75} A cotton-based biosensor for chronic wounds is illustrated in Figure 11. Edwards et al. ⁷⁶ researched peptide-immobilized cellulose analogs to detect proteases and create interfaces with protease-sequestrant materials for the development of dressings that could sequester and detect proteases.

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Figure 11.

Cotton-based biosensor for chronic wounds. ⁷⁸

Cellulosic and nano-cellulosic materials were evaluated as biosensors to detect and sequester proteases in chronic wounds. The transducers have high porosity and absorbent properties necessary for wound exudate uptake. The NC biosensors can detect HNE at concentrations found in chronic wounds, but the sensitivity of pNA is limited. Its sensitivity concentration needs to be expanded to target all chronic wound types. ⁷⁷

Smart bandages with biosensors monitor wound variables such as temperature and pH, sending data wirelessly for remote tracking. They offer electrical stimulation and regulated drug release, which reduces the need for frequent dressing changes and problems while allowing for personalized therapy to improve patient results. Recent developments in wearable technology include colorimetric and electrochemical sensors that may detect changes in biomarkers electrically or visually, allowing for real-time wound state monitoring. They are integrated with cell phones and alert both patients and physicians. Smart sensor selection parameters include cost, reusability, clothing compatibility, sensitivity, detection limits, accuracy, and precision. ⁷⁹

Sweat fluids

Sensor materials can be used in garments to detect biochemical changes in physiological fluids like sweat, tears, saliva, and urine.^{80 –82} According to LeGrys, the primary component of sweat is water, with trace amounts of other biological components like urea, lactate, pyruvate, amino acids, and potassium and sodium ions. ⁸³ Sweat samples can be analyzed to monitor pathological diseases, dehydration, and hyponatremia, which can aid in rehydration and remineralization.^84,85 Sweat fluid may also contain drug opiates, amphetamines, cocaine, buprenorphine, and cannabinoids, which can be utilized as a diagnostic tool to evaluate a person’s health. ⁸⁶

Real-time monitoring of sweat variations on-site can give a preliminary physiologic assessment. The Colorimetric sensors are easy to fabricate and operate and can be identified by the naked eye, which makes them especially attractive. Colorimetric probes are recognized as one of the most effective methods to identify major hazards or variations in the surrounding environmental pollutants. They are low-cost and simple to use.^87,88 Using colorimetric methods to examine sweat is a helpful diagnostic tool, as it is non-invasive, easily accessible, and can provide real-time detection results without requiring complicated instruments or trained personnel.^{89 –91}

Real-time biochemical variation efficiency monitoring of human body sweat fluid is possible because of a new, user-friendly, long-lasting, and flexible halochromic cotton-based sensor. According to Saravanan et al., ⁹² the cotton sensors matrix has a high porosity, comfortable, flexible, and has a potentially high treatment surface area. Researchers created a colorimetric swab in calcium alginate by encapsulating a molecular switching hydrazone probe immobilized on cotton. The color of the microencapsulated cotton probe altered from bright yellow to purple. The color-fastness and air permeability were shown by the morphological analysis of the colorimetric sensor on a pad of dry cotton fiber. Therefore, this sensor can be used to assess drug tests in real-time and to ascertain the fluid content of perspiration. ⁹³

In summary, incorporating sensing materials into clothing and using colorimetric sensors to examine sweat can provide a non-invasive and easy-to-use diagnostic tool to detect pathological disorders, dehydration, hyponatremia, as well as drug abuse. Furthermore, the development of a novel, simple, and flexible halochromic cotton-based sensor can provide real-time monitoring of sweat changes on-site, which is highly beneficial for a preliminary physiological evaluation.

Pressure sensors

Flexible pressure sensors are devices that transform external force changes into electrical signals. They are becoming increasingly popular due to their potential use in wearable electronics, soft robotics, health monitoring, and human-machine interfacing.^{94 –96} Such sensors are usually based on piezoresistive, piezoelectric, capacitive, and triboelectric sensing mechanisms. ⁹⁷

Researchers have developed a graphene-based fiber with electrical conductivity of 0.136 S/m. However, there are challenges in using graphene-based fibers, such as their lower conductivity, poorer stability and durability of conductive polymers, and the risk of pulmonary toxicity of carbon nanotubes. ⁹⁸ Previous studies have introduced various types of pressure sensors. For instance, Pan et al. ⁹⁹ have developed a highly sensitive resistive pressure sensor based on a hollow sphere micro-structured conducting polymer thin film. Bae et al. ¹⁰⁰ have reported on a resistive pressure sensor that uses a hierarchical graphene array as an electrode material and PDMS as a flexible substrate .

Song et al. ⁹⁷ used a dip-coating approach to create a flexible sensor that was conductive, resilient, highly sensitive, and stable. This pressure sensor may be modified by adjusting the amount of polyurethane and silver nanowires. It is suitable for real-time assessments since it has a low detection limit, high sensitivity, and working stability. These sensors can be integrated into wearable devices such as fitness trackers, smart clothing, and health monitoring patches. They can provide real-time information on pressure changes, making them useful for monitoring vital signs, posture, and physical activity.

Urea recognition

Chronic kidney disease (CKD) is defined as the steady decline of the kidneys over time. The body may accumulate waste and fluid when the kidneys aren’t functioning properly. Kidney failure can eventually result from CKD, which can also cause heart disease and excessive blood pressure.^{101 –103} The recognition of urea via a sensitive surface is an intriguing technology. It is possible to improve the sensitivity of the instrument by increasing the sensing material’s surface area to volume ratio. Cotton gauze is ideal for making solid-state sensing devices due to its high surface area-to-volume ratio and fibrous porous structure. These devices can achieve both fast detection as well as excellent sensitivity, even on an industrial scale. The gauze’s lightweight, translucent fabric with an open weave allows for rapid adsorption and diffusion of the analyte onto the active detection site. ¹⁰⁴ To analyze various metabolites, enzyme-based processes are employed to transform a specific analyte into a fluorescent and/or colorimetrically recognizable structure. To ensure the proper functioning of the sensing device, the various elements must be closely immobilized within the host detection system. ¹⁰⁵

Researchers developed a novel sensor system using microcapsules containing calcium alginate, urease enzyme, and tricyanofuran hydrazone. This system detects urea by monitoring the enzymatic reaction catalyzed by urease. As urea breaks down into alkaline ammonia, the pH changes, causing the tricyanofuran hydrazone to shift from light yellow to purple. The sensor can determine urea concentration in biological fluids like blood and urine. It exhibits a detection range from 0.1 to 250 ppm under atmospheric conditions. Additionally, cotton gauze strips coated with this sensor could lead to the creation of implantable chromogenic smart bandages. ¹⁰⁶