Ultrashort Pulsed Laser Treatment is Effective at Sterilizing Metal Surfaces for Planetary Protection

Authors

Kaleb McQuillan, University of Nebraska-LincolnFollow

First Advisor

Craig Zuhlke

Committee Members

Craig Zuhlke, Jessica A. Lee, Natale Ianno

Date of this Version

8-2024

Document Type

Thesis

Citation

A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science

Major: Electrical Engineering

Under the supervision of Professor Craig Zuhlke

Lincoln, Nebraska, August 2024

Comments

Copyright 2024, Kaleb McQuillan. Used by permission

Abstract

To prevent forward contamination from microbes aboard spacecraft intended for exploration of solar system bodies there is a need for effective sterilization methods. However, current techniques are both time-consuming and expensive. For example, dry heat sterilization requires removal from the assembly site and several days of treatment. Furthermore, some components such as optics and electronics are not compatible with current sterilization techniques. In this thesis, a novel femtosecond laser surface processing technique for the rapid sterilization of spacecraft hardware is reported. Femtosecond lasers produce extremely high photon fluxes (10²⁹ photons/s*cm², ~0.03 J/cm²) in extremely short pulses, which can inactivate even stress-tolerant microbial spores with minimal damage to the spacecraft surface. For the work reported in this thesis, aluminum coupons were inoculated with specific densities of Bacillus subtilis bacterial endospores. These coupons were treated with various laser illumination parameters. Afterward, metal coupon samples were assayed for viable spores using a polyvinyl alcohol (PVA) peel, serial dilution, and plating for colony-forming units (CFU). Results indicate that with high enough energy density and pulse counts, most bacterial spores are inactivated with minimal damage to the metal. The sterilization is dependent on both the fluence and pulse count. In addition, femtosecond pulses are more effective than longer pulse lengths for inactivation. These experiments have consistently achieved 4-log reduction in viable spores. Sterilization has been achieved on both flat metal coupons and non-flat surfaces with microchannels, with a slight reduction in sterilization efficiency on the non-flat surfaces. The application of air flow during laser processing was also investigated as a way to remove spores that are dislodged from the surface by the laser illumination, which would contribute to the reduction of spacecraft bioburden. With laser processing technology rapidly evolving, these results support the possibility of an extremely rapid, in-situ surface sterilization method for use in spacecraft assembly clean rooms.

Advisor: Craig Zuhlke