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Cost-Effective Capacitance Sensor Platform for Wearable Applications

Henry Czupryna and Daniel van der Weide, Department of Electrical and Computer Engineering

Motivation

As the Internet of Things (IoT) expands into wearable health and environmental monitoring, a core challenge remains: creating sensors that are low-cost, zero-maintenance/batteryless, while still being widely deployable. Most solutions depend on power sources that limit their usability and scalability. Our project explores an alternative leveraging Near-Field Communication (NFC) technology as both a power and data interface for passive sensing in everyday materials.

System Overview

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Mobile Application:

Developed a cross-platform (iOS & Android) smartphone application.

• Initiate NFC communication
• Retrieve and process capacitance data
• Interpret sensor results and display to the user
• Log and store historical readings

Example: Smart Diaper

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Results

Our prototype demonstrated a clear correlation between diaper saturation and capacitance change.

• Capacitance increased proportionally with moisture levels
• Reference measured via HP 4285A Precision LCR Meter

Wearable Substrate:

We explored multiple substrate options:

• Paper: Ultra-low cost, ideal for disposables
• PCB: Durable and easily mass-produced
• Fabrics: Can also offer direct integration into garments

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NFC Chip/Antenna - NTAG 223 DNA StatusDetect

Manufactured by NXP Semiconductors, originally designed for package tamper detection, we creatively repurposed this chip to interface with a custom capacitive sensor.

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Capacitance-Sensing Element:

The core element that interacts with the environment to produce a measurable capacitance change. Experimented with various designs and materials:

• Simple copper wires (most accurate, simple)
• Conductive inks (low-cost, high-volume)
• Etched onto copper-clad PCB (most configurable)

Disposable diapers today rely on hydrochromic paint that visually changes color when wet, a method that is binary, imprecise, and not accessible remotely.

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Our prototype smart diaper enables continuous, quantifiable, and contact-free monitoring of moisture content using our NFC-capacitance sensor platform. This has practical benefits for hospital care and nursing homes where nurses can scan the diaper using a smartphone to determine changing needs, without disturbing the patient.

Objective

Demonstrate the technical feasibility of interfacing an NFC chip with a custom capacitance sensor on a flexible substrate integrated with a mobile application and showcase its relevance through a real-world medical use case.

• Cost-effective and suitable for mass-production
• Harvests power directly from scanning smartphone

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Skin

Substrate

Sensing Element

Chip

Antenna

Conclusion & Future Work

Our research demonstrates the successful repurposing of NFC technology into cost-effective, battery-free capacitive sensors. The smart diaper application validates both the technical feasibility and practical utility of this approach.

Future work will explore:

• Optimization of sensor design for enhanced sensitivity
• Additional healthcare and consumer applications

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Cost Analysis

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• Total estimated component cost: $0.40 - $0.65 per unit at scale
• Comparison with capacitance sensor with bluetooth interface: 75% – 90% cost reduction

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Mobile Application

NFC Field

Chip

Antenna

DEMO

Parallel Copper Wires

sealed with Tape

Diaper with Conductive Ink Traces

PCB with Copper Traces

Block Diagram of Chip

Prototype used for Testing

Mobile App UI

Schematic of the working principle of textile based tactile sensor; finger as dielectric (a) and finger as conductive (b) [1]

[1] A. S. R. Reddy, S. S. Kumar, and S. R. S. Prabaharan, "Fabrication of interdigitated capacitor on fabric as tactile sensor," Mater. Today: Proc., vol. 44, pp. 1–5, 2021, doi: 10.1016/j.matpr.2021.01.007