Biosensors are being developed to provide rapid, quantitative, diagnostic information to clinicians in order to help guide patient treatment, without the need for centralised laboratory assays. The success of glucose monitoring is a key example of where technology innovation has met a clinical need at multiple levelsfrom the pathology laboratory all the way to the patient's home. However, few other biosensor devices are currently in routine use. Here we review the challenges and opportunities regarding the integration of biosensor techniques into body fluid sampling approaches, with emphasis on the point-of-care setting.

Vaccines delivered to the skin by microneedles – with and without adjuvants – have increased immunogenicity with lower doses than standard vaccine delivery techniques such as intramuscular (i.m.) or intradermal (i.d.) injection. However, the mechanisms behind this skin-mediated ‘adjuvant’ effect are not clear. Here, we show that the dynamic application of a microprojection array (the Nanopatch) to skin generates localized transient stresses invoking cell death around each projection. Nanopatch application caused significantly higher levels (~65-fold) of cell death in murine ear skin than i.d. injection using a hypodermic needle. Measured skin cell death is associated with modeled stresses ~1–10 MPa. Nanopatch-immunized groups also yielded consistently higher anti-IgG endpoint titers (up to 50-fold higher) than i.d. groups after delivery of a split virion influenza vaccine. Importantly, co-localization of cell death with nearby live skin cells and delivered antigen was necessary for immunogenicity enhancement. These results suggest a correlation between cell death caused by the Nanopatch with increased immunogenicity. We propose that the localized cell death serves as a ‘physical immune enhancer’ for the adjacent viable skin cells, which also receive antigen from the projections. This natural immune enhancer effect has the potential to mitigate or replace chemical-based adjuvants in vaccines.

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While advances in assay chemistry and detection continue to improve molecular diagnostics technology, blood samples are still collected using the 150-year-old needle/syringe method. Surface modified microprojection arrays have been developed as a novel platform for in vivo, needle-free biomarker capture. These devices are gold coated silicon arrays with >20,000 projections per cm 2 , which can be applied to the skin for tunable penetration into the epidermis or dermis. The microprojection array conceptually offers several advantages over the current methods including: minimally invasive sample collection, no need for sample processing and concentration of specific markers at the device surface for sensitive detection. In this study, Microprojection arrays were coated with antibodies to capture an early marker of dengue virus infection, NS1, from the skin of live mice. We also developed a complementary "total IgG" assay which could be used as a positive control for adequate penetration of the projections. Surface modifications designed for selective extraction were tested against standard microtiter plate ELISA. We also investigated the use of Protein G-mediated antibody immobilization in order to orient capture antibodies. While we found that capture efficiency could be improved, the direct EDC-based antibody immobilization resulted in a significantly higher surface density leading to a higher degree of NS1 capture. Using mice intravenously injected with recombinant dengue virus type 2 NS1 as a pseudomodel for dengue infection, NS1 was successfully extracted using microprojection arrays sampling from skin fluid, with a detection limit of 8 μg/mL.

Oral vaccination offers the promise of convenient, pain-free and self-administrable vaccine delivery. This is highly attractive in response to pandemic outbreaks where rapid mass vaccination is critical. Furthermore, oral vaccination produces mucosal, as well as systemic, immune responses, which protect against infection at mucosal surfaces. As the majority of pathogens enter the body through mucosal surfaces this may further enhance protection and minimize the spread of disease. The gastrointestinal (GI) tract presents a number of prospective mucosal inductive sites for targeting orally delivered vaccines, including the oral cavity, stomach and small intestine. Despite this, currently available oral vaccines are effectively limited to either live attenuated and inactivated vaccines against enteric diseases. The GI tract poses a number of challenges to the delivery of subunit and nucleic acid vaccines, including degradative processes that digest biologics and mucosal barriers that limit their absorption. This review summarizes the approaches currently under development and future opportunities for oral vaccine delivery to established (intestinal) and relatively new (oral cavity, stomach) mucosal targets. Special consideration is given to recent significant advances in oral biologic delivery that offer promise as future platforms for administration of oral vaccines. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 61 is January 8, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Surface modified microprojection arrays are a needle-free alternative to capture circulating biomarkers from the skin in vivo for diagnosis. The concentration and turnover of biomarkers in the interstitial fluid, however, may limit the amount of biomarker that can be accessed by microprojection arrays and ultimately their capture efficiency. Here we report that microprojection array insertion induces protein extravasation from blood vessels and increases the concentration of biomarkers in skin, which can synergistically improve biomarker capture. Regions of blood vessels in skin were identified in the upper dermis and subcutaneous tissue by multi-photon microscopy. Insertion of microprojection array designs with varying projection length (40-190 μm), density (5000-20,408 proj.cm(-2)) and array size (4-36 mm(2)) did not affect the degree of extravasation. Furthermore, the location of extravasated protein did not correlate with projection penetration to these highly vascularised regions, suggesting extravasation was not caused by direct puncture of blood vessels. Biomarker extravasation was also induced by dynamic application of flat control surfaces, and varied with the impact velocity, further supporting this conclusion. The extravasated protein distribution correlated well with regions of high mechanical stress generated during insertion, quantified by finite element models. Using this approach to induce extravasation prior to microprojection array-based biomarker capture, anti-influenza IgG was captured within a 2 min application time, demonstrating that extravasation can lead to rapid biomarker sampling and significantly improved microprojection array capture efficiency. These results have broad implications for the development of transdermal devices that deliver to and sample from the skin.

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