Mechanisms of sampling interstitial fluid from skin using a microneedle patch

Abstract

Although interstitial fluid (ISF) contains biomarkers of physiological significance and medical interest, sampling of ISF for clinical applications has made limited impact due to a lack of simple, clinically useful techniques that collect more than nanoliter volumes of ISF. This study describes experimental and theoretical analysis of ISF transport from skin using microneedle (MN) patches and demonstrates collection of >1 µL of ISF within 20 min in pig cadaver skin and living human subjects using an optimized system. MN patches containing arrays of submillimeter solid, porous, or hollow needles were used to penetrate superficial skin layers and access ISF through micropores (µpores) formed upon insertion. Experimental studies in pig skin found that ISF collection depended on transport mechanism according to the rank order diffusion < capillary action < osmosis < pressure-driven convection, under the conditions studied. These findings were in agreement with independent theoretical modeling that considered transport within skin, across the interface between skin and µpores, and within µpores to the skin surface. This analysis indicated that the rate-limiting step for ISF sampling is transport through the dermis. Based on these studies and other considerations like safety and convenience for future clinical use, we designed an MN patch prototype to sample ISF using suction as the driving force. Using this approach, we collected ISF from human volunteers and identified the presence of biomarkers in the collected ISF. In this way, sampling ISF from skin using an MN patch could enable collection of ISF for use in research and medicine.

Keywords: biomarker; dermal interstitial fluid sampling; medical diagnostics; microneedle patch; skin.

Fig. 1.

ISF collection by diffusion into hydrogel MNs and skin µpores. (A) Representative patch containing 10 × 10 array of dried hydrogel MNs made of PVA. Magnified views of representative (B) dried hydrogel MN (520 µm tall) before insertion into skin, (C) hydrated hydrogel MN after swelling in pig skin for 12 h ex vivo, and (D) two stainless steel MNs (750 µm tall).

Fig. 2.

ISF collection by capillary action through porous and single-capillary MNs. (A) Representative porous paper MN sandwiched between two stainless steel MNs for mechanical support (750 µm tall). (B) Representative porous paper MN that has absorbed ISF from pig skin soaked in fluorescein to facilitate ISF imaging. (C) Representative hollow stainless steel MN (750 µm tall) with a single hollow capillary to draw fluid out of skin. (D) ISF volume sampled from pig skin after 20 min: porous paper backing with no MNs (negative control), porous paper MNs, and hollow, single-capillary MNs of 33 gauge (108-µm i.d.), 30 gauge (159-µm i.d.), and 25 gauge (260-µm i.d.). Data show mean ± SD (n = 4); *P < 0.05.

Fig. 3.

ISF collection by osmosis through µpores. (A) Representative MN coated with maltose (bar shows coated region). (B) ISF volume collected from pig skin ex vivo punctured with coated and uncoated MNs followed by application of aqueous solutions of different osmotic strength for 20 min. Data show mean ± SD (n ≥ 4); *P < 0.05, **P < 0.01.

Fig. 4.

ISF collection by pressure-driven convective flow through µpores. ISF collected from MN-punctured pig skin ex vivo after 20 min of suction (−85 kPa) (A) for skin under different degrees of tension and (B) for skin covered by different absorbent papers. (C) ISF collected from pig skin ex vivo after different durations and levels of suction and positive pressure applied to skin. Data show mean ± SD (n ≥ 3); *P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 5.

ISF collection from human volunteers by suction through µpores. Representative images of skin (A) immediately after suction show faint erythema and no evidence of edema or bleeding (dashed white circles identify sites of ISF collection through 50 µpores) and (B) 24 h after ISF collection show resolution of erythema. (C) Representative image showing ISF droplets on skin surface after collection by suction through µpores.