Inactivation of microbes using compressed carbon dioxide—An environmentally sound disinfection process for medical fabrics

Abstract

High-pressure (HP) CO₂ treatment was applied to disinfect a fabric which is frequently used in hospitals. The physical properties of the treated textile were evaluated and found not to be negatively influenced. Subsequently, the significance of the main parameters influencing the disinfection process were determined using multi-factor analysis of variance. Efficacy of the developed technique was demonstrated and optimised for Gram-negative Escherichia coli and Gram-positive Micrococcus luteus. Water addition was found to be crucial for the reduction of both bacteria species. Complete inactivation was achieved at temperatures as low as 20 °C for E. coli and 65 °C for M. luteus, respectively. The effective pressure required for the disinfection was only 50 bar. Finally, based on the experimentally revealed results, an empirical non-linear model was developed describing the inactivation of E. coli and M. luteus in the low-temperature process using highly compressed liquid, gaseous or supercritical CO₂.

Introduction

In daily life, microorganisms are ubiquitous. However, only some of them are pathogens, causing infections. Fungi, moulds, vegetative bacteria and their persistent forms – spores – along with viruses, represent a wide range of possible threats for the human body. Infection paths can be direct contact, air and jointly used vehicles such as furniture, crockery and textiles [1]. This work focuses on the ‘oblique contact’ infection path stressing indirect transfer of microbes via contaminated objects namely medical textiles.

Nosocomial infections are those related to a stay in hospital [2] and Sheng et al. [3] regard this kind of infection to be very important. Therefore, in hospitals, many textiles have to meet high standards of disinfection [4]. Depending on the intended location or application for the textiles (e.g. bed sheets, clothes for staff in operating theatre and wound covers) and the material properties (natural or artificial fibres, e.g. polyester and manner of production), they often have to be treated intensively.

Conventional disinfection processes have major drawbacks. This becomes particularly clear in textile disinfection, where heat-sensitive material has to be treated frequently. Most physical treatment methods such as autoclaving damage the structure and reduce the product's lifetime, resulting in extra costs. Textural properties of textiles with their numerous and non-homogenous cavities also make it difficult to disinfect a fabric thoroughly by UV radiation or microwaves. The problem concerning disinfection by chemical means is three-fold. First, diffusion of the applied chemical regularly is very slow and increases treatment time. Second, residual chemicals can be harmful to the patients’ health and can cause skin irritation; as a result, wound covers and most implants cannot be treated chemically. Finally, most chemical disinfectants are flammable and toxic and constitute a significant danger to workers’ health and the environment [5].

The demand for an alternative low-temperature method is obvious. Preservation of physical properties and inertness of the treated material are basic requirements, which cannot be met by either ethylene oxide or gamma sterilisation [6], [7]. Furthermore, penetration of three-dimensional textile material – containing small pores and possibly constrictions at points of intersection of threads – must be assured. Evidently, conventional textiles and numerous (bio-)materials have similar characteristics concerning texture and porosity and could therefore be treated similarly. Moreover, as the field of tissue engineering expands, the bio-materials used will require mild inactivation methods for disinfection.

High-pressure (HP) CO₂ treatment (50–500 bar) is a promising technique for disinfection that is already in use for various liquid food products such as orange juice and egg yolk. It allows the combination of mild treatment parameters [8], thus protecting fabric texture. Highly compressed gaseous, liquid or supercritical CO₂ (T > 31 °C, p > 74 bar) is generally suitable for this process [9] but has not been investigated in detail for textile disinfection yet [10], [11], [12]. Alternative solvents – in particular highly compressed carbon dioxide – are of major importance for a wide range of applications. Due to its very advantageous physical properties (relatively high solvent power at elevated pressures, but low solvent power at atmospheric pressure), it is easy to design an integrated downstream process. Additionally, carbon dioxide is non-flammable, non-polar, inert, readily available and in the supercritical state it has favourable transport and pore-diffusion properties due to its low viscosity [13]. It can therefore accelerate processes which are limited by mass-transfer velocity or which have to overcome the obstacle of surface tension.

Industrial-scale plants using highly compressed CO₂ are well-established for the treatment of coffee and tea, extraction of aromatic compounds, for nutritional purposes and dry cleaning [9]. High-pressure dry cleaning using CO₂ has become well-known in recent years and several industrial-scale plants have started to work in US [14], [15], [16] and Europe [17]. Most importantly, the deficiencies concerning safety and environmental health issues of conventional dry cleaning can be minimised by using CO₂.

In this paper, we present the efficacy of the HP CO₂ disinfection on two species of bacteria attached to textile material. Most former evaluation approaches focussed on microbes in solution [18] but this required evaluation of microbes on textiles, so a novel evaluation method was developed and established. The significance of the most important parameters influencing the inactivation success was quantified through the application of statistical tools and the disinfection was described using an empirical model. Even though damage to natural or synthetic fibres by HP CO₂ was not expected [19], the effect of the HP treatment on textile performance was assessed against standard requirements as demanded by textile manufacturers.