The microfluidics of the eccrine sweat gland, including biomarker partitioning, transport, and biosensing implications

Salivary cortisol is frequently used as a biomarker of psychological stress. However, psychobiological mechanisms, which trigger the hypothalamus-pituitary-adrenal axis (HPAA) can only indirectly be assessed by salivary cortisol measures. The different instances that control HPAA reactivity (hippocampus, hypothalamus, pituitary, adrenals) and their respective modulators, receptors, or binding proteins, may all affect salivary cortisol measures. Thus, a linear relationship with measures of plasma ACTH and cortisol in blood or urine does not necessarily exist. This is particularly true under response conditions. The present paper addresses several psychological and biological variables, which may account for such dissociations, and aims to help researchers to rate the validity and psychobiological significance of salivary cortisol as an HPAA biomarker of stress in their experiments.

The present work describes the first example of real-time noninvasive lactate sensing in human perspiration during exercise events using a flexible printed temporary-transfer tattoo electrochemical biosensor that conforms to the wearer's skin. The new skin-worn enzymatic biosensor exhibits chemical selectivity toward lactate with linearity up to 20 mM and demonstrates resiliency against continuous mechanical deformation expected from epidermal wear. The device was applied successfully to human subjects for real-time continuous monitoring of sweat lactate dynamics during prolonged cycling exercise. The resulting temporal lactate profiles reflect changes in the production of sweat lactate upon varying the exercise intensity. Such skin-worn metabolite biosensors could lead to useful insights into physical performance and overall physiological status, hence offering considerable promise for diverse sport, military, and biomedical applications.