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Chemistry, Analytical
Hydrogel-based wearable electrochemical biosensors (HWEBs) are emerging biomedical devices that have recently received immense interest. The exceptional properties of HWEBs include excellent biocompatibility with hydrophilic nature, high porosity, tailorable permeability, the capability of reliable and accurate detection of disease biomarkers, suitable device–human interface, facile adjustability, and stimuli responsive to the nanofiller materials. HWEBs are gaining popularity due to their numerous potential applications in various fields, including medical, environmental, and industrial applications. They have the potential to revolutionize the way biological parameters are monitored.
- biocompatible polymer
- electrochemistry
- Hydrogel-based wearable biosensors
- Mechanical properties
- Conductivity
1. Introduction
The growing popularity of wearable biosensors in healthcare management stems from their capacity to continuously and instantly gather physiological data by means of noninvasive analysis of biochemical markers present in biofluids, such as sweat, tears, saliva, and interstitial fluid [1]. Flexible and stretchable biosensors are also gaining attention due to their enhanced signal validity, patient comfort, and excellent mechanical properties, which allow for effective skin–device interface coupling and skin monitoring [2].
Wearable biosensors made of various materials are being developed for non-invasive, wireless, and consistent human health monitoring, which can help diagnose diseases in their preliminary stage, potentially reducing the economic burden caused by chronic and acute diseases on humans [3,4]. Some of these applications include cardiovascular disease monitoring [5], biological signals monitoring, such as glucose [6], lactate [7], pH [8], and body electrolytes [9], as well as recording various physiological parameters, including heart rate [10], electrocardiogram signals [11], body temperature [12], and blood oxygen levels [13] in real-time.
Biosensors consist of different components and sensing mechanisms that define biointerfaces. There are various types of wearable biosensors available, including chemical and physical biosensors, which are based on their sensing platforms. Chemical biosensors use chemical reactions to detect and quantify analytes in biological samples, while physical biosensors employ mechanical and optical properties for the same purpose. Some of the most common sensing mechanisms used in wearable biosensors are electrochemical, mechanical, and optical biosensing [14]. Wearable electrochemical biosensors (WEBs), in particular, have demonstrated promising results in clinical applications, particularly for continuous monitoring of biological signals [15]. These biosensors have been designed to have a fast response time and a high sensitivity, making them ideal for detecting low levels of analytes in a sample. Additionally, they are portable and can be used in a variety of settings, from laboratories to remote locations.
Recently, hydrogel-based wearable electrochemical biosensors (HWEBs) as an innovative technology are becoming increasingly popular since they take advantage of hydrogels and WEB devices [16]. HWEBs are advanced sensing devices using hydrogel materials as the platforms for immobilizing biorecognition elements [17,18]. The high selectivity and sensitivity of HWEBs make them a promising alternative to traditional analytical methods [19].
Hydrogels as soft, biocompatible, biodegradable and usually hydrophilic materials with a weak mechanical strength but acceptable elasticity resembling human tissues can be simply integrated into wearable devices to offer a non-invasive and flexible platform for continuous monitoring [20]. Hydrogels with a unique structure, including a three-dimensional (3D) network of crosslinked polymers, can absorb and retain large amounts of water in their interstitial spaces whilst maintaining their structural integrity in the swollen state [21]. The hydrogel surface can also be functionalized by various functional groups to enhance their specificity toward the target analyte [22]. In addition, hydrogels can be functionalized with various biorecognition elements, including enzymes, antibodies, and nucleic acids, to specifically detect the analyte of interest [23]. Most hydrogels do not demonstrate high electrical conductivity by their inherent nature. However, their conductivity can be improved through certain methods, such as hybridization with conductive materials and functionalization with redox and biomolecule species.
HWEBs offer a unique combination of mechanical and chemical stability, biocompatibility, and high swelling capacity, which is essential for detecting biological analytes in complex environments [22,24]. The hydrogel surface can also be functionalized by various functional groups to enhance their specificity toward the target analyte [22]. The immobilized biomolecules in HWEBs can catalyze a corresponding redox reaction, leading to a change in the current, potential, or impedance at the electrode surface [22,25]. The softness of HWEBs can release mechanical stress on the biological elements, leading to more stable and reliable biosensors [26]. However, the development and implementation of these biosensors are limited by the availability of suitable platforms that can provide the necessary functionality and performance.
2. Applications of HWEBs
HWEBs are gaining popularity due to their numerous potential applications in various fields, including medical, environmental, and industrial applications. They have the potential to revolutionize the way biological parameters are monitored.
2.1. HWEB Platforms
There are a variety of platforms that have been developed for HWEBs, such as wearable patches [71], epidermal tattoos [273], microfluidic-based platforms [274], microneedle–based platforms [275], soft contact lens [276], and paper-based HWEBs [277]. Each of these platforms has unique advantages and disadvantages, and the choice of platform depends on the specific application and requirements of the device.
2.1.1. Wearable Patches
Wearable patches are flexible and conformable devices that can provide a stable and non-invasive interface for monitoring various biological signals, which can be applied directly to the skin [71,278]. Hydrogel-based wearable patches have been widely studied for their potential use in the electrochemical biosensors [239]. These devices are ideal for the continuous monitoring of biological parameters [71].
Transparent microfiber and nanofiber hydrogel patches are discussed by Jin et al. [71] for use in monitoring interstitial glucose levels continuously. The team created a new hydrogel using poly(vinyl alcohol)/β-cyclodextrin polymer that is transparent and flexible, making it well-suited for biosensor applications. These hydrogels exhibit a range of desirable properties, including excellent absorptivity, good mechanical properties, and high enzyme activity. Hydrogels are a suitable choice for biosensors because they possess excellent properties, such as high permeability and rapid electron transfer. This biosensor was found to have excellent sensing performance, with a wide linear range, high sensitivity (47.2 μA mM−1), low sensing limit (10 μM), and rapid response time (<15 s). In future clinical applications, the biosensor could be used to accurately measure glucose concentrations in human serum.
2.1.2. Epidermal Tattoos
Epidermal tattoos are being developed to meet the growing demand for non-invasive, continuous monitoring of biological parameters in real time. They are thin, flexible, and biocompatible devices designed to be worn directly on the skin like a temporary tattoo, which provides a stable and robust platform that is easy to use for the detection of biological signals which causes no pain or discomfort [273,279]. One of the main advantages of the tattoo platform is its non-invasive design. The tattoo platform can be worn without causing any discomfort or pain. This makes it ideal for use in long-term monitoring applications, where the user needs to wear the device for extended periods of time [280]. Another key benefit of using hydrogel-based tattoos as a platform for wearable biosensors is their electrochemical nature, which allows for the direct measurement of target analytes [19]. The tattoo platform is connected to a wearable device that contains the electronic components required to process the data from the biological materials [273,281].
Bandokar et al. [273] reported the development of a hydrogel-based tattoo glucose sensor for non-invasive electrochemical monitoring of glucose levels. The sensor is designed to be worn as a temporary tattoo and is the first of its kind to combine reverse iontophoretic extraction of interstitial glucose with an enzyme-based amperometric biosensor. The device can be easily worn on the skin and has the potential to provide continuous glucose monitoring without the need for finger-stick blood tests with a sensitivity of 23 nA μM−1 and a low limit of detection (3 μM). In vitro, studies were conducted to evaluate the sensor’s performance, and the results showed that the tattoo sensor has a linear response to physiologically relevant glucose levels with minimal interference from other electroactive species commonly found in interstitial fluid.
2.1.3. Microfluidic-Based Platforms
Microfluidic-based platforms combine the properties of hydrogels and microfluidics to create non-invasive wearable devices that can monitor various biological signals in real-time with high precision [282]. The use of hydrogel material in these biosensors also offers the potential for improved comfort and skin adherence [274]. These features make microfluidic platforms ideal for the development of wearable biosensors that can be used to monitor a variety of biological signals [274,283].
Bolat and colleagues [274] have developed a hydrogel-based wearable epidermal microfluidic device that facilitates the stimulation, collection, and analysis of sweat through a transdermal pilocarpine delivery system. This device is specifically designed for soft skin-mounted applications and offers non-invasive interfaces for on-demand sweat sampling and analysis. The microfluidic channel within the device enables real-time electrochemical monitoring of sweat glucose, and its layout was optimized using fluid dynamics. To enhance usability and functionality, the device was integrated with a wireless data transmission system. This integration enables the device to perform in real-time and allows for the monitoring of biomarkers in stimulated sweat samples, making it suitable for a range of healthcare and wellness applications.
2.1.4. Microneedle-Based Platforms
Microneedle-based platforms are a promising technology for the development of HWEBs, as they are stable, biocompatible, small, and minimally invasive. Furthermore, they can penetrate through the skin and monitor biological analytes painlessly and continuously [284,285]. The integration of microneedles with HWEBs provides a solution for continuous and non-invasive monitoring of a range of analytes, including glucose [284], lactate [286], pH [287], and body electrolytes [288]. They also offer a range of benefits, such as improving skin permeability [289], increasing wear comfort [290], and reducing the risk of irritation and discomfort for the patient [291].
The microneedles serve as a point of contact with the interstitial fluid, which can be used as a sample source for electrochemical analysis [292]. Such microstructures are small enough to be painless yet large enough to allow for efficient fluid exchange, making them ideal for long-term monitoring [293].
Zheng and colleagues [284] developed an innovative microneedle-based device that enables the monitoring of biomarkers, such as glucose and alcohol, in the interstitial fluid of the skin. This device consists of expandable microneedles and electrochemical test strips, which facilitate the accurate detection of glucose using an agarose hydrogel. The microneedles are connected to the test strips through a chitosan layer and are designed to penetrate the skin, allowing the extraction of ISF that subsequently flows to the test strip for analysis. Through in vitro experiments, the device demonstrated precise detection of glucose concentrations ranging from 0 to 12 mM and alcohol concentrations ranging from 0 to 20 mM. In addition, in vivo testing revealed the device’s capability for minimally invasive sampling of ISF and analysis of glucose levels in mice. The authors propose that this microneedle-based device offers a cost-effective and convenient approach for researchers to extract skin ISF for biomarker analysis.
2.1.5. Soft Contact Lens
Electrochemical Soft contact lenses are a new type of wearable biosensors that utilize tears as a biological fluid for detecting various analytes [294]. These biosensors are based on the principle of electrochemistry and can measure the electrical signals generated by the interactions between the biomolecule and bioreceptor [294,295].
Soft contact lens biosensors have several advantages over traditional wearable biosensors. First, they are non-invasive and painless, making them suitable for monitoring of biological parameters [1,276]. Second, they are biocompatible, which means they do not cause any adverse reactions when in contact with biological fluids [296]. Third, they have a high sensitivity and specificity, which allows for accurate and reliable measurement of the target analyte [1]. Despite their advantages, there are also some challenges associated with soft contact lens biosensors. One of the major challenges is the limited lifetime of the biosensor, which is a result of the gradual degradation of the hydrogel matrix and the sensing layer over time [296].
A smart contact lens can act as an effective and convenient interface between the human body and an electronic device for wearable healthcare applications. Keum et al. [276] developed a smart contact lens that can be used for continuous glucose monitoring and treating diabetic retinopathy. The device is made of a biocompatible polymer and contains thin, flexible electrical circuits, a microcontroller chip, and wireless communication for real-time electrochemical biosensing, on-demand controlled drug delivery, wireless power management, and data transmission. Using diabetic rabbit models, the researchers demonstrated that tear glucose levels measured by the smart contact lens were consistent with those obtained by conventional invasive blood glucose tests. They also showed that drugs could be released from reservoirs in response to electrochemical signals to treat diabetic retinopathy. Tear glucose in the range of 0 to 49.9 mg dL−1 can be measured accurately by employing this soft contact lens. This study successfully demonstrated the potential of smart contact lenses for noninvasive and continuous diabetic diagnosis and treatment.
2.1.6. Paper-Based and Textile-Based Platforms
Paper-based platforms and conductive papers have been identified as attractive options for wearable sensing applications due to their low cost, scalability, ease of disposal, and capillary transport capabilities. However, the paper microfluidic channel on these devices must be replaced frequently since they do not allow for long-term biomarker measurements [297,298]. Textiles are another promising platform for HWEBs, offering flexibility, versatility, breathability, stability, non-invasiveness, and comfort during wear [282,299,300]. A paper-based hydrogel electrochemical wearable biosensor is made by designing and fabricating the biosensor onto a paper-based substrate, preparing a hydrogel, and immobilizing a biomolecule onto it to detect the target analyte. Textile-based biosensors utilize hydrogel, which is soft, pliable, and biocompatible. This hydrogel can be functionalized with specific enzymes or other biomolecules to selectively detect specific analytes of interest [301].
Li et al. [277] developed an integrated and flexible hydrogel-based electrochemical paper patch that simultaneously detects electrophysiology and biochemical changes in sweat during exercise. The paper patch was self-assembled by a porous PEDOT:PSS hydrogel on a paper fiber, enabling it to serve as an electrocardiogram electrode with low impedance and a glucose sensor with ultra-high sensitivity (1018.2 μA mM−1 cm−2) and low LOD of 10.3 μM. Additionally, it provides excellent conductivity and hydrophilic properties, which are responsible for electron transmission and substance diffusion, respectively.
2.2. Biosensing Applications
IUPAC defines a biosensor as a device that detects chemical compounds, usually with electrical signals resulting from specific biochemical reactions mediated by enzymes, immune systems, tissues, organelles, or whole cell [302]. Biological components consist of enzymes, antibodies, nucleic acids (DNA/RNA), whole cells, etc., while physical components include transducers converting biological signals into electrical signals to be detected by electrical devices [303]. The biological recognition element of EBs specifically binds to the analyte of interest, and the transducer (e.g., electrodes) transfers the generated electrical signal to the electrical reader device, determining the concentration of the analyte [304]. These wearable devices incorporate electrodes made from conductive materials that are integrated into the hydrogel matrix. The electrodes are used to detect the concentration of various biochemicals, including glucose, lactate, and urea, among others, using electrochemical methods [257,305]. The integration of the electrodes into the hydrogel matrix allows for stable and long-term monitoring of biochemicals, as the hydrogel provides a protective layer that helps maintain the stability of the electrodes and prevents their degradation over time [239].
2.2.1. Catalytic HWEBs
Catalytic HWEBs consist of a hydrogel matrix and a biological enzyme, which acts as a catalyst to convert the analyte into an electrical signal [306]. They detect analytes by reducing or oxidizing them in the presence of enzyme catalysts. By electrochemical techniques, such as cyclic voltammetry or amperometry, the electrode surface redox potential can be measured [307]. Enzymatic HWEBs is the enzyme-based biosensor using enzymes as biological components due to their specificity and high reactivity. When the target molecule is present in the sample, it binds to the enzyme, leading to a conformational change that can be detected by a change in the electrical signal [308]. In recent years, electrochemical enzymatic biosensors have found applications in various fields, including clinical diagnosis, food safety, environmental monitoring, and pharmaceuticals [309].
Glucose
HWEBs for monitoring glucose are designed to be integrated into wearable devices, such as patches or flexible electrodes [253]. These biosensors usually use GOx as an enzyme that oxidizes glucose to gluconic acid and H2O2. The H2O2 produced can be detected electrochemically, allowing for the quantification of glucose levels in biological samples [252]. One of the approaches to designing and fabricating electrochemical enzymatic biosensors is electrode modification using conductive polymers or nanoparticles. Conductive polymers, such as PEDOT:PSS, can be used to enhance the sensitivity of biosensors. Nanoparticles like PB are electrocatalysts for Glucose conversion. The modified biosensor can improve biocompatibility, stability, and sensitivity, leading to improved glucose detection accuracy [51].
Lin et al. [253] developed a non-invasive sweat glucose sensor that utilizes hydrogel patches to rapidly collect natural perspiration without external stimulation. The hydrogel patch absorbs sweat from the hand and generates H2O2 proportional to the glucose concentration in the patch. This H2O2 is quickly reduced through a PB layer at a low overpotential while the sweat glucose sensor tracks the response signal hourly for long-term glucose monitoring. The observed signal showed a linear range of 6.25 μM to 0.8 mM and a LOD of 4 μM toward glucose concentrations with high specificity.
Lactate
Lactate is a common metabolic intermediate that is widely used as a marker of cellular metabolism. The most used enzyme in lactate biosensors is LOx, which converts lactate to pyruvate and H2O2. The H2O2 produced can then be oxidized at the electrode surface and change the electrical signals [310]. Enzymatic HWEBs for lactate detection are usually made by incorporating LOx into an electrochemical hydrogel, which serves as the matrix for enzyme immobilization and provides a conducive environment for the electron transfer [311]. An enzymatic biosensor electrode pad fitted to eyeglasses has been designed to measure lactate in human sweat during exercise. The biosensor displayed linearity for lactate concentrations up to 25 mM in phosphate-buffered pH 7.0 solutions. The amperometric profiles reflected changes in sweat lactate concentrations with the intensity of physical exercise when the biosensors were applied to an analysis of sweat lactate dynamics during cycling exercise [7].
Cholesterol
Enzymatic HWEBs for cholesterol detection can help individuals manage their health proactively and reduce the risk of cardiovascular disease By providing real-time and non-invasive monitoring of cholesterol levels [312]. The enzymes involved in this process are typically cholesterol oxidases, which catalyze the oxidation of cholesterol to cholest-4-en-3-one, generating hydrogen peroxide in the process. The hydrogen peroxide is then detected by an electrochemical transducer [313].
In a recent study, researchers reported the development of a wireless and soft smart contact lens that is capable of recording cholesterol levels in tear fluids in real time for monitoring patients with hyperlipidemia using a smartphone. In addition to an NFC chip, the sensor was also integrated with a stretchable antenna on the contact lens, allowing it to operate wirelessly and without batteries. In the soft contact lens, the silicone elastomer is used, which is biocompatible and suitable for medical applications. The protective film is a black-painted circular PI film and PDMS thin film. On a polyimide film substrate modified with cholesterol oxidase enzyme, a Cr/Au layer was deposited by thermal evaporation as a working electrode for the biosensor. Finally, The LOD of 9.91 μM was achieved for cholesterol detection [314].
Hydrogen peroxide (H2O2)
Hydrogen peroxide (H2O2) is a reactive oxygen species that plays an important role in various physiological and pathological processes, making it an important target for biosensor development [315]. Enzymatic electrochemical biosensors for H2O2 detection typically incorporate enzymes, such as horseradish peroxidase (HRP) or catalase, into the hydrogel matrix. These enzymes catalyze the reaction between H2O2 and a suitable electron acceptor, such as phenol, to produce water and oxygen, generating a change in the electrochemical signal [316].
A recent development Involves the creation of a self-standing electrochemical sensor that utilizes flexible hydrogels for the detection of H2O2 in liquid environments. The sensor design aimed to optimize the composition of PEDOT:PSS, hydrophilic polyurethane (HPU) hydrogel, and HRP to achieve mechanically stable sensors with desirable sensitivity and selectivity for hydrogen peroxide detection. In this design, PEDOT:PSS acts as the transducer, HPU serves as the hydrogel matrix, and HRP functions as the specific redox enzyme for H2O2. These sensors demonstrate remarkable stability, with response times of less than 6 s, and offer a detection range spanning from 100 µM to 101.6 mM [317]. The flexibility and mechanical stability of this hydrogel-based electrochemical sensor make it well-suited for wearable applications.
Alcohol
Alcohol dehydrogenase (ADH) and Aldehyde dehydrogenase (ALDH) are commonly used alcohol-oxidase (aOx) enzymes for alcohol detection as they catalyze the oxidation of alcohols to aldehydes and subsequently to carboxylic acids. Alcohol in the sample diffused to an electrode surface. Then is converted to acetaldehyde by ADH. In this process, acetaldehyde is further oxidized to acetic acid by ALDH, resulting in the production of electrons and protons. As electrons and protons are transferred to the electrode surface, an electrical signal proportional to the amount of alcohol in the sample is generated [318].
Researchers have developed a wearable tattoo-based alcohol biosensing system that can be used to detect alcohol levels in induced sweat without invasive measurement. Using an aOx enzyme and a printed PB electrode transducer, ethanol could be amperometrically detected in the sweat generated by the wearable prototype through transdermal delivery of the pilocarpine drug. A skin-compliant designed biosensor is highly selective and sensitive (0.362 µA mM−1) to ethanol [319].
2.2.2. Bioaffinity HWEBs
Bioaffinity HWEBs are promising devices for non-invasive continuous monitoring of biomolecules. In such devices, bioaffinity molecules, such as antibodies or aptamers, act as bioreceptors and are coated on top of hydrogel to selectively bind to the target biomolecules [320,321,322].
Immunosensor HWEBs
The immunosensors, also known as antibody-based sensors, rely on the specific interaction between an antibody and an antigen to produce a change in signal transduction. Immunosensors are extremely sensitive and can detect minor concentrations of biomolecules that allow for rapid and precise analysis. When the target analyte binds to the antibody, it triggers a conformational change in the antibody that can be detected by changes in the electrochemical properties of the sensor [18].
In order to detect cytokine levels in human serum for the identification of COVID-19 as a “symptom diagnostic biomarker” and to obtain real-time information about the individual’s health status, Shi and colleagues [320] have developed a cost-effective immunosensor based on a microfluidic paper-based system. This immunosensor serves to predict the health status of COVID-19. The foldable paper-based assay employs a magnetic immunoassay and streptavidin-horseradish peroxidase combined with tetramethyl benzidine/hydrogen peroxide (TMB/H2O2) to amplify the signal for electrochemical readout. The researchers enhanced the sensitivity of cytokine detection by modifying the working electrode with a hybrid of gold nanoparticles and polypyrrole hydrogel, which increased conductivity and improved the electron transfer rate. Operating in differential pulse voltammetry mode, this paper-based immunosensor exhibited excellent performance, with a dynamic range spanning from 5 to 1000 pg mL−1 and a lower detection limit of 0.654 pg mL−1. To evaluate its clinical application, the researchers tested the proposed immunosensor using human serum samples obtained from a hospital, and the results indicated its significant potential for early diagnosis of high-risk COVID-19 patients.
Nucleic Acid-based HWEBs
Nucleic acids, specifically DNA and RNA, play a crucial role in maintaining the genetic information of cells and viruses [323]. Nucleic acid-based Electrochemical biosensors have the advantage of being quick to respond and can be produced inexpensively due to their simple instrumentation. In these biosensors, DNA is attached to an electrode, and the resulting hybridization reaction causes a change in electrical properties, such as current, potential, impedance, or conductance, which can be measured in the biosensors [324]. Nucleic acid-based HWEBs make use of unique properties of nucleic acids, such as specific recognition and hybridization, to detect target biomolecules [325].
Yang et al. [326] introduced a wearable epidermal system that combines reverse iontophoresis and microneedles (MNs) to improve the sensitivity and capture efficiency of cell-free DNA from ISF. The system specifically targets Epstein–Barr virus cell-free DNA, which is a significant biomarker for nasopharyngeal carcinoma diagnosis. The wearable system can extract and sense the target DNA within 10 min, with a maximum capture efficiency of 95.4% and a detection limit of 1.1 copies per µL using a recombinase polymerase amplification electrochemical microfluidic biosensor. The study also validates the feasibility of the wearable system using immunodeficient mouse models. This new approach to minimally invasive ISF sampling provides a promising pathway for cancer screening and prognosis through the detection of cell-free DNA.
Aptamer-based HWEBs
An aptasensor is a type of biosensor that utilizes aptamers as the recognition element [327]. Aptamers are short synthetic single-stranded oligonucleotides (e.g., DNA or RNA molecules) that are designed to bind specifically to a target molecule with a high-affinity [328]. The aptamers in electrochemical aptasensors are typically attached to a conductive material, such as gold, which allows the interaction between the aptamer and target to be monitored electrically [329]. Aptamers are capable of binding to their target molecules with a higher affinity than antibodies because they can form more stable and specific interactions with their targets. This is due to their ability to adopt various 3D structures, which enhances their ability to bind to their targets with a high degree of specificity and affinity [330].
Karuppaiah and colleagues [321] have developed a novel method for detecting low concentrations of cortisol in human saliva samples. This method utilizes an aptamer-based electrochemical sensing platform with a hybrid hydrogel network. Detecting low cortisol concentrations in saliva, which is essential for assessing physiological stress, is challenging due to the interference from salivary proteins and mucin. In this approach, the aptamer is connected to a redox probe to generate a signal, while the hydrogel network incorporates gold nanocubes to enhance electrical conductivity. This hybrid hydrogel network effectively reduces the matrix effect, enabling the detection of physiologically relevant cortisol concentrations (0.1–50 ng mL−1) in human saliva samples.
This entry is adapted from the peer-reviewed paper 10.3390/bios13080823
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