ISOCTA
Institute for Scientific Operations, Cryogenics & Technical Applications

Plasma Sterilisation of Surgical Instruments

Figure 1: Atmospheric-pressure plasma jet in operation, treating a stainless-steel surgical scalpel blade inoculated with a calibrated suspension of Staphylococcus aureus. The He/O\u2082 plasma plume (visible as a faint violet glow) contacts the blade surface inside a Class II biosafety cabinet in Building 4.

Overview

This collaborative programme between the Biotechnology Division and the Plasma & Thermal Sciences Division evaluates atmospheric-pressure non-thermal plasma jets as a low-temperature method for sterilising heat-sensitive surgical instruments and biomaterial surfaces. The work is motivated by the limitations of conventional steam autoclaving for temperature-sensitive devices and by concerns about residual toxicity from chemical sterilants (e.g., ethylene oxide).

Plasma Source

The sterilisation experiments use a dielectric-barrier discharge (DBD) plasma jet operating at atmospheric pressure with helium as the carrier gas and oxygen as the reactive admixture (He + 0.5–2% O2 by volume). The jet is driven by a 20 kHz sinusoidal HV supply (5–10 kVpp) applied to a coaxial electrode configuration. The plasma plume extends approximately 5–15 mm from the nozzle exit and operates at near-ambient temperature (<40°C at the treatment plane), making it compatible with heat-sensitive materials.

The reactive chemistry is dominated by atomic oxygen, ozone, and metastable O2(1Δg) — species known to be effective against microbial targets through oxidative damage to cell membranes and intracellular components. The plume is characterised by optical emission spectroscopy on a sister system in the Plasma Division laboratory, providing identification and relative density of reactive species. [Plasma Diagnostics programme]

Microbiological Methods

Test Organisms

Standard test organisms include:

  • Staphylococcus aureus (ATCC 25923) — Gram-positive, represents common surgical-site infection pathogen
  • Escherichia coli (ATCC 25922) — Gram-negative, represents broader bactericidal testing
  • Bacillus atrophaeus spores — used as a biological indicator for sterilisation efficacy (high resistance)

Protocol

Sterile test coupons (stainless steel 316L, titanium Ti-6Al-4V, and medical-grade PEEK polymer) are inoculated with a calibrated bacterial suspension (~106 CFU/coupon), air-dried, and exposed to the plasma plume for treatment times ranging from 10 to 300 seconds. Post-treatment, viable organisms are recovered by sonication in saline + 0.1% Tween-80, serially diluted, and plated on tryptic soy agar for colony counting after 24-hour incubation at 37°C.

Figure 2: Log-linear survival curves for S. aureus on stainless steel coupons as a function of plasma treatment time, for three O\u2082 admixture fractions. The 2% O\u2082 condition achieves a 6-log reduction in under 120 seconds. Error bars represent \u00b11 standard deviation from triplicate experiments.

Key Results

  • A 6-log reduction in S. aureus CFU is achieved in <120 s at 2% O2 admixture on stainless steel substrates.
  • Treatment time for equivalent kill on PEEK polymer is approximately 50% longer, attributed to reduced thermal conductivity affecting local reactive species transport.
  • B. atrophaeus spores require ~300 s for 6-log reduction, consistent with the known resistance of spore coats to oxidative attack.
  • SEM imaging reveals no detectable surface damage to stainless steel or titanium at treatment times up to 600 s (5× the sterilisation dose). Cytotoxicity testing (ISO 10993-5) of treated PEEK shows no extractable cytotoxicity.

Publications

  • Volkova I., Okonkwo M. — "Characterisation of an atmospheric-pressure plasma jet for biomedical surface treatment." J. Phys. D: Appl. Phys. 37, 421–428 (2004). [abstract]
  • Santos M., Volkova I. — "Plasma sterilisation of surgical stainless steel: efficacy and surface effects." (In preparation, 2004.)

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