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MetaBioLiq

Baicheng Chen, John Nolan, Xinyu Zhang

University of California San Diego

MobiCom’24 | November 18-22, 2024

A Wearable Passive Metasurface Aided mmWave Sensing Platform for BioFluids

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Motivation

Electrolyte Crystals

Wireless in Sweat Sensing

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Commercial product

[1] The effect of electrical stimulation on skin vulnerability to irritants

[2] https://nixbiosensors.com/

[3] https://hdroptech.com/

45 minute warm up time

Electrode irritation

Patch replacement cost

Power supply

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Wireless liquid sensing

  1. Requires impractical volume of liquid
  2. Not tested on sweat concentration of liquid

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Sweat sensing

  1. Sweat content:
    1. 99% water, 1% salt/fat, and other micro-substances (i.e., hormone).
  2. Sweat Rate
    • Up to a quarter of sweat evaporates a day, when exercising, 0.8-1.4 L per hour.
    • Micro-liters per inch square

RF Signal

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MetaBioLiq System Overview

  1. Micro-liters of sweat sensing
  2. Sweat rate and sweat metabolite sensing
  3. 3D printed Resonant Sensor
  4. mmWave radar

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Challenges

  1. Low RCS
    1. Micro-liters of fluid, sub-millimeter-thick layer
    2. Large body reflection
  2. Low SNR
    • Water attenuates signal
    • Sweat microscopic substances

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Wireless RF Resonance

  1. Tune the structure of our resonance sensor
  2. Empirically find resonance frequency

Saline Levels

Resonant Sensor

Glucose Levels

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Optimization Challenge

Transfer Matrix Model Optimization

  1. Input: layer design, liquid placement, environment interference
  2. Output: each layer’s geometric parameter

4-Layers -> Optimal

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Optimization result

Maximize signal from sweat. Minimize signal from skin.

Focusing on Sweat

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Evaluation Setup

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MetaBioLiq Sensor

24 Artificial Sweat Samples

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Deciding the form factor

Compact

Robust

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Per Person Variation

On Arm

On Table

Resonant Frequency Drift Correction

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Evaluation Results

Accurate

Robust

24 Sweat Mixtures

Continuous Sensing

Predicted Liquid

Ground Truth Label

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Conclusion

  1. MetaBioLiq
    1. mmWave Microliter-level fluid sensing capability
    2. Detailed sweat content/rate analysis
    3. Fully passive 3D printable dielectric material

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  1. More details in our paper ^v^