A colored powder dye in the paper allows visualization of the liquid front while it progresses toward the far end of the paper geometry. first time. Keywords:COVID19, health monitoring medical devices, interstitial fluids, microneedles, painless microsampling Modern analytical technologies enable biomarker monitoring in microsamples, and thus exploration of less invasive matrices than blood. In this article, a microneedlebased, costeffective, painless device is proposed to collect interstitial fluid from the skin. This solution challenges the use of invasive techniques for applications where microsampling could reveal the pursued information, while minimizing patients discomfort and logistic requirements. == 1. Introduction == Sampling of biological fluids is essential for patient monitoring, diagnostics, and screening. Today, venous blood sampling followed by plasma analysis is the gold standard for these applications. However, such procedure entails the use of invasive and painful methods, such as venipuncture, which needs to be performed by professional phlebotomists typically at a healthcare facility. In addition to the inconvenience for patients and the cost related to personnel and infrastructures, liquid blood needs to be kept refrigerated and handled as a biohazard, adding up logistic requirements. The COVID19 pandemic has promoted and accelerated the demand for more convenient and patientcentric approaches. For example, GK921 sampling procedures had to be moved closer to the pointofcare (PoC), such as homes GK921 or decentralized centers, where patients could selfperform a procedure to collect the desired specimen.[1]Among different sampling techniques, finger pricking can be selfperformed to obtain smaller amounts of capillary blood at the PoC. However, samples must be stored in a format that is stable over time, and that can GLUR3 readily be analyzed using standard equipment thereafter. Currently, the storage of capillary blood in analyticalgrade paper in the form of dried blood spots (DBS) is gaining interest for use in analytical practice in different fields.[2,3,4,5]The sample, once dried in paper, is stable, not considered biohazardous, and can be shipped at room temperature without special handling.[6] However, also capillary blood is still associated with pain and risk of infection. Instead, dermal interstitial fluid (ISF) offers a more accessible specimen as it can be sampled from the skin tissue in a minimally invasive manner.[7,8,9,10,11,12]ISF is an emerging specimen for monitoring biomarkers and biomolecules also due to its physiological relevance. Studies have shown that ISF provides the possibility to detect a large variety of analytes commonly found in blood plasma.[9,13]Dermal ISF is of particular relevance because of its proximity to blood and the altogether strong correlation with blood plasma dynamics, especially for low molecular weight species such as metabolites, drugs, and unbound hormones, which can rapidly diffuse through the capillary walls into the interstitial space.[14]Moreover, due to the sensitivity of modern analytical tools, microsamples of 1 1 L can reliably be analyzed. This enables the practical use of new alternative matrices such as ISF which, while it cannot be sampled in large volumes from patients, can provide other advantages for the patients, in terms of comfort and reduced risks.[15]Today, ISF is widely used for continuous glucose monitoring in diabetic patients, where sensors are transdermally inserted and worn over time.[16,17]Even though the use of ISF as a measurement matrix has gained substantial interest in the research and medical communities, there is currently still a lack of commercial solutions and of userfriendly ISF sampling techniques available in general. This hinders a wider translational adoption of ISF as an analytical specimen in research and healthcare. To access dermal ISF, microneedles offer a minimally invasive solution to penetrate the epidermis and provide access to the dermis.[10]Due to their size, microneedles are painless both during insertion and after removal, potentially improving patient acceptance and compliance. Microneedles also reduce the risks of infections by drawing minute amounts of fluid and producing only microscopic wounds that do not cause long term damage to GK921 the skin.[18,19]Therefore, microneedles provide an attractive solution for repeated sampling for chronic patients if compared to collecting venous blood or performing finger pricks.[20]Stateoftheart methods for ISF sampling consist in prototypestage devices based on vacuumgenerated suction,[10,21,22]mechanically applied overpressure,[23,24]or absorbing porous microneedles.[25,26,27,28,29]In fact, unlike blood, sampling ISF requires an external pressure gradient to be applied to extract it from the skin tissue into an external collecting device. Major issues with previously proposed devices are for example their bulkiness,[21,30]userdependence,[31]and lack of scalable manufacturability and analytical compatibility,[26,28,29,32]which today makes them hardly implementable as costeffective and disposable PoC devices in translational applications. Ultimately, the realization.
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