Wax Blends as Tunable Encapsulants for Soil-Degradable Electronics

Abstract

Printed biodegradable electronics potentially enable the monitoring of various soil parameters at a high spatial density while minimizing cost and waste. A tunable degradable encapsulant is a critical component in a soil-degradable electronic device, as it acts to delay the ingress of water, microbes, and other agents responsible for degradation of underlying functional materials. Here, blends of beeswax and commercial soy wax are presented as tunable biodegradable encapsulant materials for transient soil sensors. Using differential scanning calorimetry, we first show that the blends of the two waxes have limited miscibility, which enables programming of degradation times. Laboratory degradation tests in soil revealed that the longevity of encapsulated devices can be controlled by the ratio of the component soy and beeswax, with up to 100 days with 100% beeswax and less than 10 days with the addition of 25% soy wax by mass. Thicker coatings of 1.6 mm of 10% soy wax in beeswax blends are shown to protect devices for 12 weeks. Additionally, melt-processed beeswax encapsulants are used as a simple method to delay the degradation of otherwise rapidly biodegradable materials, such as wooden stakes, that could be used to house soil-degradable electronic devices.