The "Aminometer" - a finger-sized prototype for total L-amino acid measurements from human serum.

The total pool of blood's serum circulating L-amino acids (ΣAA) is an underused biomarker that integrates muscle protein turnover, nutritional status and metabolic disorders. Despite increasing clinical relevance, current methods for quantifying ΣAA, including colorimetry/fluorimetry and chromatography, are laboratory-bound, slow, and unsuitable for decentralized or extreme environments such as space missions. We report a self-contained, portable, calibration-free prototype device - Aminometer - for rapid quantitative analysis of ΣAA concentration in 7.2 μL of blood serum within 33 min. The prototype integrates a paper-based sample volume fixation zone, a dried enzymatic catalyst (t1/2 = 84 d), a prefilled reaction chamber, a Prussian-blue modified carbon electrode and a reusable wireless transmitter with an integrated battery-powered potentiostat. The total system weighs 13 g and measures 48 × 16 × 8 mm. The prototype was tested on serum samples and showed a good agreement with reference assay (Pearson's r = 0.85, n = 23) demonstrating the first portable ΣAA monitoring system for a variety of applications including point-of-use assessment of muscle and nutritional status, point-of-care testing in resource-limited settings, and monitoring astronauts' nitrogen balance during long-duration space missions.