Self-Cleaning Properties of TiN/CrN Nanoscale Multi-layer Deposited on Surgical 420C Stainless Steel

Document Type : Research Paper

Authors

1 Department of Mechanical Engineering, Babol Noshirvani University of Technology, Babol, Iran

2 Department of Materials Engineering, Babol Noshirvani University of Technology, Babol, Iran

3 Department of Materials Engineering, Bu-Ali Sina University, Hamedan, Iran

Abstract

The present paper focuses on the investigation of self-cleaning properties based on studding of water repellency and blood repellency for TiN- and CrN single-layer and TiN/CrN nanoscale multi-layer coatings deposited via Cathodic Arc Evaporation (CAE) method on medical grade 420C stainless steel substrate. X-ray diffraction (XRD) method and Field Emission Scanning Electron Microscope (FESEM) was used to characterize microstructure. Surface roughness parameters were measured by using Taylor-Habsson method. Blood contact angle and water contact angle measurements test were applied to characterize the self-cleaning properties of the specimens. The analysis of sample data shows that coated specimens have more water contact angle values in comparison to bare steel. Among the coated samples, CrN single-layer has the highest water contact angle (80°) due to its lower surface roughness (Ra=0.189µm) among the other samples. Moreover, the findings of the paper prove that samples had inverse behavior against blood and water in the contact angle measurement test. Bare steel has higher blood contact angle (76.1°) and more blood repellency than the coated specimens. It seems that the different behavior of samples against water and blood in contact angle tests is due to the nature of two fluids. TiN/CrN multi-layer coating results had minor differences in water and blood contact angle tests (67.3° and 62.2° respectively).

Keywords


INTRODUCTION

Nosocomial infections (NIs) are important in terms of their potential for high mortality, morbidity and elevated hospital costs. The rate of nosocomial infections (NIs) reported in USA is 2.8% and in European countries is 2-5%. NIs caused 5,000 deaths per year in England and cause 17500-70000 annual deaths in the USA. These infections cost the UK’s National Health Service (NHS) one billion pounds each year which was the equivalent of 1% of the total national hospital budget and between 17 and 25 billion dollars added to health costs every year in the USA [1-2]. Various studies done in Iran have shown the rate of NIs varying from 8% to 10%. There are over 100,000 hospital beds in 830 hospitals in Iran that approximately 6 million patients admitted annually [3]

Based on the results released by the National Nosocomial Infections Surveillance (NNIS) on 100 hospitals with more than 200 beds each and 6,616,520 studied patients between 2007 and 2010, surgical site infections (26.8%) was one of the main cause of NIs in Iran [4].

Infections related to such biomedical instruments and devices are responsible for at least 1.5–7.2% post-surgery complications depending on the type of surgical procedure [5].

Surgical instruments tend to be contaminated to a certain level during surgeries by micro-organisms that inhabit the skin and organs. Surgical instruments could act as fomites for the pathogens of surgical site infection even if the surgical field is not apparently contaminated [6-8].

Proper reprocessing of reusable surgical instruments and medical devices is a critical infection prevention strategy. Cleaning is a key factor to overcoming these infections. An instrument that has not been properly cleaned cannot be effectively disinfected or sterilized and consequently using of the contaminated surgical instruments can result in further severe infection. The level of bacterial contamination is a vital parameter that will determine the time period of the sterilization or the disinfection and efficacy of these processes. The Occupational Safety and Health Administration (OSHA) defines decontamination as, “The use of physical or chemical means to remove, inactivate, or destroy blood-borne pathogens on a surface or item to the point where they are no longer capable of transmitting infectious particles and the surface or item is rendered safe for handling, use, or disposal” [8-11]. In this study the OSHA definition will be used to both cleaning and decontamination. Besides, to keeping (to keep) instruments free of contamination during surgeries, cleaning of instruments should occur as soon as possible after they are used [10]. Results of some studies have been proven that the cleaning of stainless steel surgical instruments during the first 6h after the surgery is essential in order to ensure effective disinfection and sterilization [11,12].

The process that should be followed for sterilization of reusable medical instruments is shown in Fig. 1.

The self-cleaning characteristic is related to the surface contact angle. Self-cleaning coatings are broadly categorized into two groups: hydrophilic and hydrophobic. Both of the groups clean themselves by the action of water. In a hydrophilic coating, the water is made to spread over the surface, which carries away the dirt, blood and other contaminating particles, whereas in the hydrophobic technique a water droplet contact with such a surface can easily roll across the surface on which it rests, collecting dust or other impurities [13-16].

Contact angle value which is dependent on surface roughness can determine the surface free energy and also efficiency of the self-cleaning coatings [17-19].

Surface roughness is a 2D parameter of a material surface that affected the self-cleaning process. Irregularities of the material surfaces normally promote blood adhesion and biofilm accumulation. This is due to the increased surface area and depressions in the roughened surfaces [5,7]. There are many different roughness parameters. Ra (arithmetical mean of surface roughness of every measurement within the total distance ½ roughness average) is the most universally used roughness parameter since it is easy to define and to measure providing a good general description of height variations [20,21].

Metals have a high surface energy and are negatively charged and hydrophilic as shown by water contact angles. Near a hydrophobic surface the water is less structured in terms of intermolecular hydrogen bonding between the water molecules, while near a hydrophilic surface water is more structured [7,22,23].

420C stainless steel is one of the most important materials used in the manufacture of general surgical instrument due to superior mechanical strength, ductility, elasticity, corrosion resistance and low cost [20,22,23]. However self-cleaning properties of this alloy is relatively poor [21,23]. As noted, an important way to create a self-cleaning surface is wetting controllability. The ability to control the wetting properties of stainless steel is acutely useful because this alloy has many applications in scientific and industrial fields [17]. Surface modification with advanced materials is an appropriate method to reduce the risk of infections caused by microorganisms and altering the self-cleaning phenomena [24].

Coating is an important area of surface modification. Single-layered and multi-layered coatings deposited by physical methods are increasingly used in all branches of industry practically for surgical instruments [25]. TiN and CrN are two conventional coatings in medical applications that have high hardness, good wear and corrosion resistance. These coatings are physiologically inert, non-toxic, non-carcinogenic and usable in implantable devices approved by the Food and Drug Administration (FDA) of USA [25-28].

Heretofore many researches have been done on fabrication of TiN- and CrN based coatings for enhancement of mechanical, corrosion resistance and antibacterial properties of stainless steel. However few studies have been done on fabrication of nanoscale TiN- and CrN based coatings deposited on 420C stainless steel with studding both blood repellency and water repellency for determination of self-cleaning properties to prevent infection in surgical instruments. Consequently, the main goal of current study is the determination of the effect of the nanoscale TiN- and CrN based coatings deposition on water repellency and blood repellency characteristics of 420C stainless steel with the aim of easier cleaning and thus decreasing the risk of contamination and infection of general surgical instruments.

MATERIALS AND METHODS

Substrate preparation

In this study 420C stainless steel with 3mm of thickness was chosen as a base metal. Optical Emission Spectrometer (OES) confirmed the chemical composition of this alloy (Table 1).

At first surface hardening of stainless steel specimen took place in salt furnace at 1037°C and were quenched in warm oil. Then specimen was tempered at 150°C for 20min for the aim of removing residual stresses. Finally samples were cut to a dimension of 40×60mm2. Samples were subjected to surface sanding, mirror polishing and finally surface passivation with electro polishing method. Hardness of the substrate was measured 980HV.

Deposition process

The samples were ultrasonically cleaned in acetone and ethanol solutions for 30min. This process was repeated thrice for removing all of the probable contamination on the surfaces. Samples then were introduced in the PVD chamber and subjected to etching and preheating in Argon glow discharge plasma for 15min. to clean up more levels of contamination and to enhance the adhesion of the coating on the substrate.

A CAE PVD equipment model YN Saleh (Yarnican Saleh, Tehran, Iran) was used to deposit the coatings. The working pressure in deposition system was kept at 1×10-3torr. Ti and Cr high purity metallic targets (99/99%) were mounted on vertically opposed cathodes. The Nitrogen gas was fed into the chamber while an AC power supply connected to substrate holders was used as bias output fixed at -100V during deposition process. Temperature of the substrates was 200°C and duty cycle of process was 50%. Substrates in the distance of 150mm were rotating in front of the metallic targets. Through hold activating one of the metallic targets and allowing Nitrogen gas to enter into the chamber, single-layered TiN and -CrN coatings were formed. For deposition of multi-layered TiN/CrN, the Ti and Cr targets were covered intermittently with a steel bulkhead. The total deposition time for all specimens was 90min. and the overall thickness also was about 1.5µm. To avoid rapid cooling,