Ecoresorbable Radio-Frequency Platform for Humidity and Temperature Sensing ^†

Abstract

Aiming to reduce electronic waste in sensing applications, we report on an eco-friendly printed microstrip line to sense relative humidity (RH) and temperature. The device is made of zinc resonators on paper used as transducers, which are coated with beeswax as an encapsulant, and uses konjac as a humidity sensitive coating. The multi-resonating structure, operating in the S-band, shows reproducible temperature and humidity sensing from 15 °C to 35 °C and 30% to 70% RH with sensitivities up to 1.9 MHz/°C and 2.0 MHz/%RH, respectively. This combination of transducing and sensing materials is promising for the realization of disposable environmental sensors.

1. Introduction

The monitoring of environmental parameters is essential for the safety of perishable goods’ and to prevent waste. With the increase in connected devices for IoT applications such as tracking the logistics of products, the need for a more eco-friendly approach to reduce electronic waste is of importance. Compared to the state of the art [1], we report on a study conducted to measure temperature and relative humidity using a chipless approach implemented with entirely biodegradable materials.

2. Materials and Methods

A sensing platform composed of a multi-resonator, operating at a radio frequency from 1 GHz to 4 GHz, was developed using eco-friendly and degradable materials. A zinc microstrip line made of three resonators was screen-printed using 500 nm zinc nanoparticle ink on an 800 μm thick paper substrate and sintered using a hybrid sintering process [2] to reach high conductivity. The transducers were then encapsulated using beeswax melted at 80 °C to prevent interference from moisture. One of the resonators was used for temperature sensing, another was coated with the mucilage of a super absorbent plant-based polymer (Konjac Glucomannan) at 10% concentration in deionized water for humidity sensing, and the last one was used as reference. The most appropriate resonant frequency of the microstrip resonator for sensing temperature was determined and the position of the coating giving the highest humidity sensitivity was identified. The temperature and RH sensing data were acquired using a Vector Network Analyzer (VNA) with the device under test having a copper-tape ground plane and the device was placed in a climatic chamber plane with a commercial humidity and temperature sensor used as reference. The temperature sensitivity of the eco-friendly sensing platform was assessed from 15 °C to 35 °C and the humidity characterization was conducted from 30% up to 70% RH.

3. Results and Discussion

Each resonator acted as a stopband filter resulting in specific resonance frequencies along the recorded frequency spectrum. The S12 signal across the fabricated sensing microstrip line was recorded under varying temperature and humidity levels. Temperature detection was achieved by exploiting how the electrical conductivity of the microstrip line changed with temperature, with a temperature coefficient of resistance (TCR) of ~0.0032 ppm/°C for the zinc layer [2], leading to a shift in the resonance peak. The achieved sensitivities to temperature were 0.7 MHz/°C, 0.8 MHz/°C and 1.9 MHz/°C for the resonator at 1.2 GHz, 2.0 GHz and 3.3 GHz, respectively, as seen in Figure 1a.

Similarly for humidity, konjac coated on higher-frequency resonators led to higher average sensitivity: 0.85 MHz/%RH, 2.0 MHZ/%RH and 2.2 MHz/%RH for 1.2 GHz, 2.0 GHz and 3.3 GHz resonators, respectively. Since the temperature sensitivity at 3.3 GHz was twice that at 2.0 GHz while only a small increase was observed for humidity, the 2 GHz resonator was chosen to be coated with 30 μm thick konjac. As humidity increased, water molecules were adsorbed into the coating, locally changing the dielectric properties and thus the dielectric permittivity near the resonator. This change in permittivity resulted in a change in the resonant frequency proportional to the capacitance of the structure. We found a sensitivity ranging from 0.5 up to 6.8 MHz/%RH from 30% to 70% RH for the coated resonator, while the beeswax encapsulation prevented the two other resonators’ resonant frequency from shifting, as seen in Figure 1b.