# Power-Free Sweat Sample Concentration Using a Silica-Gel-Packed PDMS Microchannel

## TL;DR

A new passive method using silica gel in a microchannel can concentrate sweat samples without power, aiding sensitive diagnostics.

## Contribution

A power-free, room-temperature sample concentration method using silica-gel-packed PDMS microchannels is introduced.

## Key findings

- Silica gel in PDMS microchannels efficiently removes solvent vapor for sample concentration.
- A 1.4-fold concentration was achieved in DPBS solution and 1.2-fold in artificial sweat.
- The method requires no external power and integrates easily with microfluidic devices.

## Abstract

In recent years, diagnostic technologies that utilize noninvasively collected sweat have garnered significant interest. However, the concentration of components in sweat is lower than that in blood, making the introduction of a concentration step as a sample pretreatment crucial for achieving highly sensitive detection. In this study, we developed a PDMS-based microchannel filled with silica gel, a desiccant particle, to concentrate liquid samples at room temperature without requiring an external power source or heating. The evaluation of the basic characteristics of the fabricated microchannel confirmed that filling it with silica gel efficiently removed the solvent vapor from the liquid samples. In concentration tests using the fluorescent dye uranine as a model for sweat sugar, a maximum 1.4-fold concentration was achieved in DPBS solution and a 1.2-fold concentration in artificial sweat at room temperature. In contrast, no similar concentration effect was observed in microchannels without silica gel packing. The proposed silica-gel-packed PDMS microchannel features a simple structure and requires no external equipment, making it easily integrable with existing microfluidic devices as a sample pretreatment module. This method is considered useful as a passive and simple sample concentration technique for the analysis of low-molecular-weight components in sweat.