Sweat monitoring beneath garments using passive, wireless resonant sensors interfaced with laser-ablated microfluidics

Adam R. Carr,Hector F. Angus,Nigel F. Reuel,Nathaniel E. Kallmyer,Yash H. Patel,Charles R. Neff,Sadaf Charkhabi

doi:10.1038/s41746-020-0270-2

Abstract

Sweat loss can help determine hydration status of individuals working in harsh conditions, which is especially relevant to those who wear thick personal protective equipment (PPE) such as firefighters. A wireless, passive, conformable sweat sensor sticker is described here that can be worn under and interrogated through thick clothing to simultaneously measure sweat loss volume and conductivity. The sticker consists of a laser-ablated, microfluidic channel and a resonant sensor transducer. The resonant sensor is wirelessly read with a handheld vector network analyzer coupled to two, co-planar, interrogation antennas that measure the transmission loss. A sweat proxy is used to fill the channels and it is determined that the sensor can orthogonally determine the sweat conductivity and volume filled in the channel via peak transmission loss magnitude and frequency respectively. A four-person study is then used to determine level of sensor variance caused by local tissue dielectric heterogeneity and sensor-reader orientation.

Highlights

The development of sweat analytical devices has been an active area of research in recent years[1,2]
This study reports the fabrication of a resonant sticker that can analyze sweat rate and sweat conductivity, which correlates to overall ion concentration
This study presents an initial prototype of a wireless resonant sweat sensor for monitoring undercoat/protective equipment (PPE) perspiration

Summary

INTRODUCTION

The development of sweat analytical devices has been an active area of research in recent years[1,2]. Even with the recent advances in sweat sensor development one application that proves difficult to implement is undercoat perspiration This is due to either the transduction method requiring a direct line of sight (e.g., optical), or bulky sensor implementation with multiple powered parts and integrated circuit boards. These become especially problematic for applications where the user is already encumbered with bulky gear and thick PPE, such as firefighters. One sensor type that could decrease the size of wireless sweat rate sensors for applications through thick PPE is resonant sensors[23] As their name denotes, these are simple circuits that resonate at a specific frequency dependent on their effective inductance and capacitance. The model showed that the laser ablation technique is adequate for channels with an aspect

RESULTS

DISCUSSION

METHODS

CODE AVAILABILITY