Graduate Thesis Or Dissertation


Design of Additively Fabricated Biodegradable Sensors for Soil Monitoring Public Deposited

Downloadable Content

Download PDF
  • Printed biodegradable electronics has the potential to enable the monitoring of various soil parameters at a high spatial density while minimizing cost and waste. The goal of the studies presented in this dissertation is to aid in closing the gap between the prior work in biodegradable electronics for biomedical applications and the need for distributed sensing for soil applications. More specifically, this dissertation will focus on the development of conductive inks and encapsulants with soil-sensing in mind.

    The development of controllably biodegradable, high-conductivity materials suitable for additive manufacturing under ambient conditions remains a challenge. In the first study, printable conductive pastes that employ poly(lactic acid) (PLA) as a binder and tungsten as a conductor are demonstrated. These composite conductors can provide enhanced stability in applications where moisture may be present, such as environmental monitoring or agriculture. Post-processing the printed traces using a solvent-aging technique increases their conductivity by up to two orders of magnitude, with final conductivities approaching 5000 S/m. Such techniques could prove useful when thermal processes including heating or laser sintering are limited by the temperature constraints of typical biodegradable substrates. Both accelerated oxidative and hydrolytic degradation of the printed composite conductors are examined, and a fully biodegradable capacitive soil moisture sensor is fabricated and tested.

    The second study briefly expands upon the previous one and investigates the more hydrophobic polymer, poly(ϵ-caprolactone), as a binder in the place of PLA. Similar soaking techniques yield an increase in conductivity, with conductivities approaching 7190 S/m. As ambient and higher temperatures are well above PCL’s glass transition temperature (Tg) of -60 °C, annealing or degradation of amorphous areas are hypothesized to be the cause of increased conductivity, rather than physical aging. PCL-W traces showed greater stability in water and hydrogen peroxide than PLA due to its hydrophobicity. Traces printed on fully biodegradable substrates, showed stability of conductivity in incubated soil for 20 days, although PCL showed evidence of biodegrading through this time.

    In the third study, blends of beeswax and commercial soy wax are presented as tunable biodegradable encapsulant materials for transient soil sensors. Using differential scanning calorimetry (DSC), blends of the two waxes are shown to 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 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, which could be used to house soil-degradable electronic devices. Finally, a prototype of a homemade soil decomposition sensor is presented, along with preliminary data in sand, soil, and amended soil.

    In the final study, lessons learned from the previous studies are considered and a novel, elegantly simple, soil decomposition sensor that relies on the biodegradation of a printed conductive trace comprising a poly(3-hydroxybutyrate- co-3-hydroxyvalerate) (PHBV) binder with a carbon flake conductor is presented. This sensor is instrumented with a low-cost Arduino-based data acquisition device that is able to send readings wirelessly. By varying the temperature of soils, and confirming microbial activity via CO2 efflux, a correlation is drawn between the rate of increase of resistance of these printed conductive traces and the microbial activity of the medium under test in less than 14 days. The sensors also show evidence of detecting a changepoint in microbial activity along with effectiveness in microbially active liquid media.

    In the concluding chapter, concepts from these four studies are synthesized into a roadmap for creating a set of design guidelines and heuristics for printed biodegradable microbial or enzymatic activity sensors. A preliminary mapping of enzymes of interest to candidate materials and a menu of possible sensor architectures are presented along with example heuristics to aid future design of such sensors.

Date Issued
  • 2022-07-25
Academic Affiliation
Committee Member
Degree Grantor
Commencement Year
Last Modified
  • 2022-09-16
Resource Type
Rights Statement