Microneedle Sensor Research Articles

Biocompatible Core-Shell Microneedle Sensor Filled with Zwitterionic Polymer Hydrogel for Rapid Continuous Transdermal Monitoring.

Microneedle (MN)-based electrochemical biosensors hold promising potential for noninvasive continuous monitoring of interstitial fluid biomarkers. However, challenges, such as instability and biofouling, exist. This study proposes a design employing hollow MN to encapsulate a zwitterionic polymer hydrogel sensing layer with excellent biocompatibility and antifouling properties to address these issues. MN shell isolates the internal microporous sensing layer from subcutaneous friction, and the hydrogel filling leverages the MNs' three-dimensional structures, enabling high-dense loading of biorecognition elements. The hollow MNs are successfully fabricated from high-molecular-weight polylactic acid via drawing lithography, exhibiting sufficient strength for effective epidermis penetration. Additionally, a high-performance gold nanoconductive layer is successfully deposited inside the MN hollow channel, establishing a stable electrical connection between the polymer MN and the hydrogel sensing layer. To support the design, numerical simulations of position-based diffusive analyte solutes reveal fast-responsive electrochemical signals attributed to the high diffusion coefficient of the hydrogel and the concentrated structure of the hollow channel encapsulation. Experimental results and numerical simulations underscore the advantages of this design, showcasing rapid response, high sensitivity, long-term stability, and excellent antifouling properties. Fabricated MN sensors exhibited biosafety, feasibility, and effectiveness, with accurate and rapid in vivo glucose monitoring ability. This study emphasizes the significance of rational design, structural utilization, and micro-nanofabrication to unlock the untapped potential of MN biosensors.

Unveiling Potassium and Sodium Ion Dynamics in Living Plants with an In-Planta Potentiometric Microneedle Sensor.

Potassium and sodium ions (K+ and Na+) play crucial roles in influencing plant growth and health status. Unfortunately, current strategies to determine the concentrations of such ions are destructive for the plants because it is necessary to collect/extract the sap for further analysis and produce either scattered or delayed results. Here, we introduce a new potentiometric dual microneedle sensor for nondestructive, real-time, and continuous monitoring of K+ and Na+ concentrations in living plants. The developed sensors show a response time <5 s, close-to-Nernstian slope (∼55 mV dec-1), resiliency to five insertions on the stem, good repeatability (max. %RSD = 0.3%) and reversibility (max. %RSD = 3%), appropriate continuous operation for 24 h, and linear range of responses that cover expected plant physiological levels (5-50 mM for Na+ and 50-120 mM for K+). Moreover, the accuracy was successfully investigated by comparing the results provided by the microneedle sensors to those obtained by a standard reference method (e.g., ion chromatography). Finally, we demonstrate that the developed analytical device is capable of tracking K+ and Na+ transportation from the hydroponic solution to the stem within 5-10 min. This research will contribute to establishing a new generation of analytical platforms for smart agriculture offering real-time information.

A high-performance wearable microneedle sensor based on a carboxylated carbon nanotube-carbon nanotube composite electrode for the simultaneous detection of uric acid and dopamine

Uric acid (UA) and dopamine (DA) are crucial biomarkers in the human body. However, there is limited research on detecting UA or DA in subcutaneous interstitial fluid (SISF), and no studies on their simultaneous detection in SISF. Here, we developed an electrochemical microneedle sensor based on the carboxylated carbon nanotubes/multiwalled carbon nanotubes (CCNT/CNT) composite electrode for the simultaneous detection of UA and DA. Highly carboxylated CCNT was prepared and mixed with CNT and organosilicon-modified acrylic resin to form the CCNT/CNT composite. The carboxyl functional groups of CCNT enhance the electrode’s adsorption of UA and DA, while CNT improves electron conduction. The sensor achieves a wide linear detection range for UA (5–600 μM) with high sensitivity (7.13 μA μM−1 cm−2) and linearity (R2 = 0.994), as well as a wide linear range for DA (2–200 μM) with high sensitivity (13.31 μA μM−1 cm−2) and linearity (R2 = 0.994). The sensor achieved simultaneous detection of DA and UA in artificial interstitial fluid (ISF), with sensitivity and linearity remaining nearly unchanged compared to that in PBS, and is not affected by high levels of ascorbic acid (AA). Finally, the sensor successfully detected changes in UA levels in ISF of subjects before and after alcohol consumption in vivo, demonstrating its practical application capabilities. This study presents a practical strategy for detecting human biomarkers characterized by low cost, easy fabrication and excellent performance of electrode materials, and a well-designed microneedle sensor preparation scheme, making it highly promising for further research extension.

Fabrication of graphene field effect transistors on complex non-planar surfaces

Graphene field effect transistors (GFETs) are promising devices for biochemical sensing. Integrating GFETs onto complex non-planar surfaces could uncap their potential in emerging areas of wearable electronics, such as smart contact lenses and microneedle sensing. However, the fabrication of GFETs on non-planar surfaces is challenging using conventional lithography approaches. Here, we develop a combined spray-coating and photolithography setup for the scalable fabrication of GFETs on non-planar surfaces and demonstrate their application as integrated GFETs on microneedles. We optimize the setup to pattern ∼ 67 μm long GFET channels across the microneedle tips. Graphene is deposited between photo-patterned electrodes by spray-coating a liquid-phase exfoliated graphene ink while monitoring the channel resistance to achieve the required conductivity. The formation of the GFET channels is confirmed by SEM and EDX mapping, and the GFETs are shown to modulate in solution. This demonstrates an approach for manufacturing graphene electronic devices on complex non-planar surfaces like microneedles and opens possibilities for wearable GFET microneedle sensors for real-time monitoring of biomarkers.

Fully integrated microneedle biosensor array for wearable multiplexed fitness biomarkers monitoring

Fitness monitoring has become increasingly important in modern lifestyles; the current fitness monitoring always relies on physical sensors, making it challenging to detect pertinent issues at a deeper level when exercising. Here, we report a fully integrated wearable microneedle sensor that simultaneously measures fitness related biomarkers (e.g., glucose, lactate, and alcohol) during physical exercise. Such a sensor integrates a biocompatible 3D-printed microneedle array that can comfortably access skin interstitial fluid and a small circuit for signal processing and calibration, and wireless communication. The microneedle array features good biocompatibility and highly sensitive biochemical sensors that can detect even the slightest variations within the biomarkers of this fluid. On-body experimental results indicate that such a sensor can monitor fitness-related biomarkers across multiple subjects and support multi-day monitoring, with results showing a good correlation with commercial devices. The data was transmitted to a smartphone via Bluetooth and uploaded to cloud platforms for further health assessment. This study has the potential to boost intelligent wearable devices in sports health.

A high-performance wearable microneedle sensor based on a prussian blue-carbon nanotube composite electrode for the detection of hydrogen peroxide and glucose

Hydrogen peroxide (H2O2) plays an important role in biology, and biomarker detection in skin interstitial fluid (ISF) has recently shown great promise. However, limited sensors were designed for H2O2 detection in ISF. Therefore, a high-performance wearable H2O2 microneedle sensor with a Prussian blue (PB)/carbon nanotube (CNT) composite electrode and a glucose microneedle sensor based on this electrode were developed. Two types of H2O2 microneedle sensors with different CNT and PB ratios were prepared with ultra-wide detection ranges (1 μΜ–2800 mM/100 nΜ–2200 mM) in accordance with the Hill equation. Ultra-high sensitivity (954.1 μA mM−1 cm−2 and 451 μA mM−1 cm−2) and excellent linearity (R2=0.999) were exhibited in linear ranges of 1 μM to 10 mM and 100 nM to 10 mM, respectively·H2O2 sensors exhibit good selectivity and essentially unattenuated detection signals in artificial ISF. The glucose enzyme-catalyzed microneedle sensor based on the CNT/PB composite electrode also exhibited a great detection range (0.5 mM–180 mM) in accordance with the Hill equation and ultra-wide linear range (0.5 mM–40 mM) with high sensitivity (16.56 μA mM−1 cm−2) and linearity (R2=0.995) in artificial ISF. To verify the practical applicability of the sensors, H2O2 levels in normal and tumor cells were successfully detected by the H2O2 sensor. The glucose levels in ISF before and after eating were successfully detected by the glucose sensor in vivo. The present work developed a low-cost, easy-to-prepare H2O2 sensor and a related enzyme-catalyzed sensor with excellent performance, which hold promising prospects for application across diverse fields.

All-Fiber Flexible Electrochemical Sensor for Wearable Glucose Monitoring.

Wearable sensors, specifically microneedle sensors based on electrochemical methods, have expanded extensively with recent technological advances. Today's wearable electrochemical sensors present specific challenges: they show significant modulus disparities with skin tissue, implying possible discomfort in vivo, especially over extended wear periods or on sensitive skin areas. The sensors, primarily based on polyethylene terephthalate (PET) or polyimide (PI) substrates, might also cause pressure or unease during insertion due to the skin's irregular deformation. To address these constraints, we developed an innovative, wearable, all-fiber-structured electrochemical sensor. Our composite sensor incorporates polyurethane (PU) fibers prepared via electrospinning as electrode substrates to achieve excellent adaptability. Electrospun PU nanofiber films with gold layers shaped via thermal evaporation are used as base electrodes with exemplary conductivity and electrochemical catalytic attributes. To achieve glucose monitoring, gold nanofibers functionalized by gold nanoflakes (AuNFs) and glucose oxidase (GOx) serve as the working electrode, while Pt nanofibers and Ag/AgCl nanofibers serve as the counter and reference electrode. The acrylamide-sodium alginate double-network hydrogel synthesized on electrospun PU fibers serves as the adhesive and substance-transferring layer between the electrodes. The all-fiber electrochemical sensor is assembled layer-by-layer to form a robust structure. Given the stretchability of PU nanofibers coupled with a high specific surface area, the manufactured porous microneedle glucose sensor exhibits enhanced stretchability, superior sensitivity at 31.94 μA (lg(mM))-1 cm-2, a broad detection range (1-30 mM), and a significantly low detection limit (1 mM, S/N = 3), as well as satisfactory biocompatibility. Therefore, the novel electrochemical microneedle design is well-suited for wearable or even implantable continuous monitoring applications, thereby showing promising significant potential within the global arena of wearable medical technology.

In situ measurement of the three-dimensional distribution of labile copper in sediment pore water using a microneedle sensor with nanoporous structure

Conventional ex situ analytical methods for sediment pore water are susceptible to disruptions in the speciation equilibrium of metals due to changes in external conditions. This study introduced an innovative in situ method for detecting the three-dimensional distribution of labile copper (CuLabile) in sediment pore water with high spatial resolution using a highly stable microneedle electrochemical sensor. The sensor featured a nanoporous tip structure and embedded gold nanomaterials with excellent electrocatalytic performance. The nanoporous structure could prevent the nanomaterials from falling off because of friction during the in situ detection process in sediments. The sensor exhibited good detection performance under different salinity conditions with a detection limit of 0.2 nM. Vertical and three-dimensional distributions of CuLabile in sediment pore water were successfully obtained using the in situ microneedle sensor. The results showed that the concentration of CuLabile was in the range of 5.2–43.5 nM, with a maximum value at a depth of approximately 4 cm, while there was almost no difference in the horizontal direction of a specific sediment sample column. Furthermore, this functional sensor could be extended to the in situ detection of other labile metals in sediment pore water.

A wearable microneedle patch incorporating reversible FRET-based hydrogel sensors for continuous glucose monitoring

Continuous glucose monitors are crucial for diabetes management, but invasive sampling, signal drift and frequent calibrations restrict their widespread usage. Microneedle sensors are emerging as a minimally-invasive platform for real-time monitoring of clinical parameters in interstitial fluid. Herein, a painless and flexible microneedle sensing patch is constructed by a mechanically-strong microneedle base and a thin layer of fluorescent hydrogel sensor for on-site, accurate, and continuous glucose monitoring. The Förster resonance energy transfer (FRET)-based hydrogel sensors are fabricated by facile photopolymerizations of acryloylated FRET pairs and glucose-specific phenylboronic acid. The optimized hydrogel sensor enables quantification of glucose with reversibility, high selectivity, and signal stability against photobleaching. Poly (ethylene glycol diacrylate)-co-polyacrylamide hydrogel is utilized as the microneedle base, facilitating effective skin piercing and biofluid extraction. The integrated microneedle sensor patch displays a sensitivity of 0.029 mM−1 in the (patho)physiological range, a low detection limit of 0.193 mM, and a response time of 7.7 min in human serum. Hypoglycemia, euglycemia and hyperglycemia are continuously monitored over 6 h simulated meal and rest activities in a porcine skin model. This microneedle sensor with high transdermal analytical performance offers a powerful tool for continuous diabetes monitoring at point-of-care settings.

Corn-inspired high-density plasmonic metal-organic frameworks microneedles for enhanced SERS detection of acetaminophen

Effective monitoring of acetaminophen (APAP) dosage is crucial for preventing antipyretic abuse, ensuring therapeutic efficacy, and minimizing toxic effects. However, existing self-monitoring methods are limited. In this study, we designed a plasmonic microneedle (MN) sensor for real-time nondestructive monitoring of acetaminophen levels in dermal interstitial fluid (ISF) by employing a handheld Raman spectrometer. The fabricated MN sensor incorporated a high-density plasmonic MOFs known as HDPM, which unique structure of large specific surface area, specific pore structure as well as high density gold nanospheres packing enabled the excellent performance of selective ISF drug enrichment and surface-enhanced Raman scattering (SERS). The maximum electric field enhancement factor of the HDPM nanostructure could be calculated as 5.73 × 107. The developed HDPM@MNs was characterized with a core-shell type “soft on the outside and rigid on the inside” structure, which exhibited sufficient hardness and flexibility to penetrate the dermal tissue with little damage, and robust SERS enhancement effect in APAP detection without any interfering peaks. Through a hydrogel drug simulation experiment, the sensor demonstrated robust capabilities for acetaminophen enrichment and monitoring, exhibiting excellent stability and repeatability. The quantitative detection window spanned from 1 to 100 μM, with a low detection limit reaching 0.45 μM. Furthermore, by monitoring the concentration of acetaminophen in the interstitial fluid of rat skin at different doses and for different administration times, the HDPM@MNs can be used to determine the pharmacokinetics of acetaminophen in rats and the physiological characteristics associated with various dosage regimens. This work not only holds promise for drug monitoring but also provides a novel approach for nondestructive monitoring of other crucial low-abundance physiological markers.

Wearable microneedle sensor with replaceable sensor base for multi-purpose detecting in skin interstitial fluid

The microneedle array provides a painless method for penetrating the skin to reach interstitial fluid (ISF), making it suitable for drug delivery and monitoring of key biomarkers. While current microneedle sensors integrate sensitive components for detecting specific biomarkers, challenges remain in their development, particularly in diversifying detection capabilities and ensuring durability. To address these challenges, we present a detachable microneedle platform capable of monitoring various biomolecules through colorimetric and electrochemical sensing. This platform is also compatible with the Urit uric acid tester, which, when coupled with the microneedles (MNs), demonstrates satisfactory stability results. The design features a mortise-tenon structure that significantly enhances the durability of the biosensor, while the provision of insertable cavities increases its versatility. By pairing with a variety of test strips, the platform can be customized to meet specific sensing requirements for a range of metabolites, aiding in disease management and regulation.

Non-Invasive Multi-Gas Detection Enabled by Cu-CuO/PEDOT Microneedle Sensor.

Metal-oxide-based gas sensors are extensively utilized across various domains due to their cost-effectiveness, facile fabrication, and compatibility with microelectronic technologies. The copper (Cu)-based multifunctional polymer-enhanced sensor (CuMPES) represents a notably tailored design for non-invasive environmental monitoring, particularly for detecting diverse gases with a low concentration. In this investigation, the Cu-CuO/PEDOT nanocomposite was synthesized via a straightforward chemical oxidation and vapor-phase polymerization. Comprehensive characterizations employing X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-ray diffraction (XRD), and micro Raman elucidated the composition, morphology, and crystal structure of this nanocomposite. Gas-sensing assessments of this CuMPES based on Cu-CuO/PEDOT revealed that the response current of the microneedle-type CuMPES surpassed that of the pure Cu microsensor by nearly threefold. The electrical conductivity and surface reactivity are enhanced by poly (3,4-ethylenedioxythiophene) (PEDOT) polymerized on the CuO-coated surface, resulting in an enhanced sensor performance with an ultra-fast response/recovery of 0.3/0.5 s.

Recent Advancements in Flexible Biosensors for Continuous Glucose Monitoring

Continuous glucose monitoring (CGM) systems have significantly improved diabetes management by continuously monitoring glucose levels in real-time. However, the existing CGM devices, which use inflexible and invasive sensors, pose difficulties in user comfort and can lead to skin irritation. This review explores recent advancements in flexible CGM technologies, highlighting their potential to overcome these limitations. Flexible CGM sensors, utilizing biofluids like sweat, tears, and interstitial fluid, offer a minimally invasive and more comfortable alternative. Recent developments and innovations in materials and fabrication techniques have brought these sensors closer to commercialization, showing noteworthy progress in their design and functionality. We examine flexible CGM prototypes, including sweat-based epidermal sensors, tear-based smart contact lenses, and interstitial fluid-based microneedle sensors. The review concludes by discussing prospects, emphasizing the need for continued innovation and improved manufacturing processes to achieve successful commercialization.

Intradermal Lactate Monitoring Based on a Microneedle Sensor Patch for Enhanced In Vivo Accuracy.

Lactate is an important diagnostic and prognostic biomarker of several human pathological conditions, such as sepsis, malaria, and dengue fever. Unfortunately, due to the lack of reliable analytical decentralized platforms, the determination of lactate yet relies on discrete blood-based assays, which are invasive and inefficient and may cause tension and pain in the patient. Herein, we demonstrate the potential of a fully integrated microneedle (MN) sensing system for the minimally invasive transdermal detection of lactate in an interstitial fluid (ISF). The originality of this analytical technology relies on: (i) a strategy to provide a uniform coating of a doped polymer-based membrane as a diffusion-limiting layer on the MN structure, optimized to perform full-range lactate detection in the ISF (linear range of response: 0.25-35 mM, 30 s assay time, 8 h operation), (ii) double validation of ex vivo and in vivo results based on ISF and blood measurements in rats, (iii) monitoring of lactate level fluctuations under the administration of anesthesia to mimic bedside clinical scenarios, and (iv) in-house design and fabrication of a fully integrated and portable sensing device in the form of a wearable patch including a custom application and user-friendly interface in a smartphone for the rapid, routine, continuous, and real-time lactate monitoring. The main analytical merits of the lactate MN sensor include appropriate selectivity, reversibility, stability, and durability by using a two-electrode amperometric readout. The ex-vivo testing of the MN patch of preconditioned rat skin pieces and euthanized rats successfully demonstrated the accuracy in measuring lactate levels. The in vivo measurements suggested the existence of a positive correlation between ISF and blood lactate when a lag time of 10 min is considered (Pearson's coefficient = 0.85, mean difference = 0.08 mM). The developed MN-based platform offers distinct advantages over noncontinuous blood sampling in a wide range of contexts, especially where access to laboratory services is limited or blood sampling is not suitable. Implementation of the wearable patch in healthcare could envision personalized medicine in a variety of clinical settings.

Transdermal Sensing of Enzyme Biomarker Enabled by Chemo-Responsive Probe-Modified Epidermal Microneedle Patch in Human Skin Tissue.

Wearable bioelectronics represents a significant breakthrough in healthcare settings, particularly in (bio)sensing which offers an alternative way to track individual health for diagnostics and therapy. However, there has been no notable improvement in the field of cancer, particularly for skin cancer. Here, a wearable bioelectronic patch is established for transdermal sensing of the melanoma biomarker, tyrosinase (Tyr), using a microneedle array integrated with a surface-bound chemo-responsive smart probe to enable target-specific electrochemical detection of Tyr directly from human skin tissue. The results presented herein demonstrate the feasibility of a transdermal microneedle sensor for direct quantification of enzyme biomarkers in an ex vivo skin model. Initial performance analysis of the transdermal microneedle sensor proves that the designed methodology can be an alternative for fast and reliable diagnosis of melanoma and the evaluation of skin moles. The innovative approach presented here may revolutionize the landscape of skin monitoring by offering a nondisruptive means for continuous surveillance and timely intervention of skin anomalies, such as inflammatory skin diseases or allergies and can be extended to the screening of multiple responses of complementary biomarkers with simple modification in device design.

An integrated wearable differential microneedle array for continuous glucose monitoring in interstitial fluids

Monitoring biomarkers in human interstitial fluids (ISF) using microneedle sensors has been extensively studied. However, most of the previous studies were limited to simple in vitro demonstrations and lacked system integration and analytical performance. Here we report a miniaturized, high-precision, fully integrated wearable electrochemical microneedle sensing device that works with a customized smartphone application to wirelessly and in real-time monitor glucose in human ISF. A microneedle array fabrication method is proposed which enables multiple individually addressable, regionally separated sensing electrodes on a single microneedle system. As a demonstration, a glucose sensor and a differential sensor are integrated in a single sensing patch. The differential sensing electrodes can eliminate common-mode interference signals, thus significantly improving the detection accuracy. The basic mechanism of microneedle penetration into the skin was analyzed using the finite element method (FEM). By optimizing the structure of the microneedle, the puncture efficiency was improved while the puncture force was reduced. The electrochemical properties, biocompatibility, and system stability of the microneedle sensing device were characterized before human application. The test results were closely correlated with the gold standard (blood). The platform can be used not only for glucose detection, but also for various ISF biomarkers, and it expands the potential of microneedle technology in wearable sensing.

A honeycomb-like micro-needle sensor with gold nanoparticles embedded for the determination of hexavalent chromium in seawater

As one of the most toxic heavy metals, hexavalent chromium (Cr(VI)) is a typical environmental pollutant that directly affects human health. In this work, a novel micro-needle sensor with honeycomb-like structure and embedded gold nanoparticles was proposed for the stable and sensitive determination of Cr(VI). The honeycomb-like structure provided larger specific surface area to support more gold nanoparticles which exhibited excellent catalytic activity towards the electrochemical reduction of Cr(VI) to Cr(Ⅲ). In addition, gold nanoparticles were embedded into the holes of the honeycomb-like structure, which can effectively prevent them from falling off from the electrode surface due to the protection of the holes. Combining the honeycomb-like structure of micro-needle electrode with the excellent catalytic activity of gold nanoparticles, the fabricated sensor exhibited outstanding stability and sensitivity for the cathodic stripping determination of Cr(VI). A linear range of Cr(VI) from 0.5 to 300 μmol/L was obtained, with the detection limit of 0.15 μmol/L. The recovery of the sensor for the determination of hexavalent chromium in seawater samples was also investigated. Moreover, the sensor might be further adapted for the determination of other heavy metals with different nanoparticles embedded.

Microneedle Sensors for Point-of-Care Diagnostics.

Point-of-care (POC) has the capacity to support low-cost, accurate and real-time actionable diagnostic data. Microneedle sensors have received considerable attention as an emerging technique to evolve blood-based diagnostics owing to their direct and painless access to a rich source of biomarkers from interstitial fluid. This review systematically summarizes the recent innovations in microneedle sensors with a particular focus on their utility in POC diagnostics and personalized medicine. The integration of various sensing techniques, mostly electrochemical and optical sensing, has been established in diverse architectures of "lab-on-a-microneedle" platforms. Microneedle sensors with tailored geometries, mechanical flexibility, and biocompatibility are constructed with a variety of materials and fabrication methods. Microneedles categorized into four types: metals, inorganics, polymers, and hydrogels, have been elaborated with state-of-the-art bioengineering strategies for minimally invasive, continuous, and multiplexed sensing. Microneedle sensors have been employed to detect a wide range of biomarkers from electrolytes, metabolites, polysaccharides, nucleic acids, proteins to drugs. Insightful perspectives are outlined from biofluid, microneedles, biosensors, POC devices, and theragnostic instruments, which depict a bright future of the upcoming personalized and intelligent health management.

Biopolymer-protected graphene-Fe3O4 nanocomposite based wearable microneedle sensor: toward real-time continuous monitoring of dopamine.

Neurological disorders can occur in the human body as a result of nano-level variations in the neurotransmitter levels. Patients affected by neuropsychiatric disorders, that are chronic require continuous monitoring of these neurotransmitter levels for effective disease management. The current work focus on developing a highly sensitive and personalized sensor for continuous monitoring of dopamine. Here we propose a wearable microneedle-based electrochemical sensor, to continuously monitor dopamine in interstitial fluid (ISF). A chitosan-protected hybrid nanomaterial Fe3O4-GO composite has been used as a chemical recognition element protected by Nafion antifouling coating layer. The morphological and physiochemical characterizations of the nanocomposite were carried out with XRD, XPS, FESEM, EDAX and FT-IR. The principle of the developed sensor relies on orthogonal detection of dopamine with square wave voltammetry and chronoamperometric techniques. The microneedle sensor array exhibited an attractive analytical performance toward detecting dopamine in phosphate buffer and artificial ISF. The limit of detection (LOD) of the developed sensor was observed to be low, 90 nM in square wave voltammetry and 0.6 μM in chronoamperometric analysis. The practical applicability of the microneedle sensor array has been demonstrated on a skin-mimicking phantom gel model. The microneedle sensor also exhibited good long-term storage stability, reproducibility, and sensitivity. All of these promising results suggest that the proposed microneedle sensor array could be reliable for the continuous monitoring of dopamine.

Colorimetric microneedle sensor using deep learning algorithm for meat freshness monitoring

Currently, the developed testing methods determining meat freshness are time-consuming, inconvenient, or have high specialty requirements. Herein, we proposed a colorimetric microneedle sensor (CMS) using a deep learning algorithm for visualized meat freshness monitoring. The CMS was obtained by molding edible hydrogels containing pH-responsive anthocyanins, which change colors because of the structure change of anthocyanins in response to pH. When attached to meat, the CMS was capable of penetrating the meat and extracting tissue fluids by capillary action. With meat spoilage, the pH of the tissue fluid gradually rose, leading to a change in CMS from pink to purple and finally to dark blue. Thus, according to variations of CMS colors, in situ and visualized detection of meat freshness was achieved. Further, a deep learning algorithm was applied to integrate with CMS to form a smartphone application (App), allowing for more convenient and accurate freshness detection. Images of CMS attached to the meat with different freshness were collected to form a training source as the input of the convolutional neural network (CNN). Through convolving CMS color features, the meat freshness classified as “fresh”, “less fresh”, and “spoiled” was finally outputted. With the incorporation of CNN, the App enabled users to identify the freshness of meat from stored photos or real-time images of CMS-labeled meats in a fast, accurate, portable, and universal way. This visualized detection strategy of CMS combined with an algorithm-integrated App has a promising potential for wide applications such as food safety, health monitoring, and environmental protection.