**2. Antimicrobial and anticorrosive titanium dioxide coating on stainless steel to reduce hospital‐acquired infection**

#### **2.1. Background**

The increasing incidence and host risk of device‐related infections that result in morbidity and even mortality have been noted for some time, particularly regarding the spread of antibiotic‐ resistant bacteria, such as methicillin‐resistant *Staphylococcus aureus* and bursting *Clostridium difficile.* These hospital‐acquired infections are a worldwide problem [1]. The outbreaks of SARS and avian influenza have also drawn attention to novel preventative measures, includ‐ ing the development and application of antimicrobial materials, to enhance the conventional disinfection concept. This movement compelled us to develop an antimicrobial technique for medical implements in clinical use.

Antimicrobial or antibacterial refers to the inhibition of bacterial growth and reproduction [2]. Antimicrobial functions can be performed by essential materials themselves or through the use of coating materials. One example of an essential antimicrobial alloy material is stainless steel that has been doped with copper. This material forms when ε‐copper precipitates in a steel matrix; specifically, copper ions can be dissolved into a surface‐passivated chromium oxide film. Such creates an antimicrobial effect on the stainless steel surface, resulting in the inhibition of bacterial growth [3]. The similar antimicrobial metal alloys, such as copper‐con‐ taining ferritic stainless steel [4], martensitic stainless steel [5], and austenitic stainless steel [6], were also developed. On the other hand, for the antimicrobial purpose on coating materi‐ als, the idea of coatings containing with copper, silver, zinc, and other antimicrobial active metals was considered [7]. Unfortunately, such substance may induce the corrosion reaction because of the undesired Galvanic effect between two metals, which may be unsustainable during service. In this regard, TiO2 with anatase (A‐TiO2 ) phase may be the promising candi‐ dates for antimicrobial purposes.

The antimicrobial effects of TiO<sup>2</sup> are attributed to its photocatalytic characteristics, as discov‐ ered by Fujishima and Honda [8]. The photocatalytic process of TiO2 involves the generation of electron‐hole pairs when the material is exposed to light that emits energy exceeding the band gap energy of TiO2 . The aggressive superoxide ions (O2−) are generated by the elec‐ tron attack, and the holes accelerate hydroxyl radical (•OH) formation on the material surface [9, 10]. These active radicals subsequently inhibit the growth of germs and bacteria that are known to be antimicrobially active through the direct oxidation of intracellular coenzyme, reducing the respiratory activity and thereby causing cell death [11].

In the present study, arc ion plating (AIP) was used to deposit a TiO2 coating on common med‐ ical‐grade AISI 304 stainless steel. The antimicrobial efficacy of the TiO<sup>2</sup> ‐coated stainless steel specimens was then evaluated according to the JIS standard. The corrosion resistance of the TiO2 coating was also examined to determine whether such films can be stable in a physiologi‐ cal environment. The results suggest that this modification may be effective as an antimicro‐ bial surface coating for medical implements to reduce the risk of hospital‐acquired infections.

#### **2.2. Preparation of antimicrobial and anticorrosive TiO2 films**

steel, titanium, and their alloys are considered especially promising materials for surgical instruments and implants of many types and sizes. Polymeric materials have also garnered considerable interest in research and development as soft‐ and hard‐tissue replacements, on the basis of the ease of manufacturing and modifying such materials, and their appropriate

When biomedical materials come in contact with physiological tissue and body fluids, vari‐ ous interactions, such as corrosive reaction, inflammation, and host response, are triggered. For this reason, knowing and understanding the surface properties of biomedical materials are crucial. Unfortunately, metallic materials are easily influenced by corrosion damage due to electrochemical reactions; additionally, the bioinertness and hydrophobic surface prop‐ erties render polymeric materials unfavorable for cell adhesion. Long‐term clinical experi‐ ments have also indicated that the primary causes of implant failure include not only unstable

To overcome the aforementioned problems, a surface modification technique that uses a mul‐

bial, and bioactive properties for the underlying biomaterial. These versatile natural features of

medical material is provided. The two main topics discussed in the next section are as follows: • Antimicrobial and anticorrosive titanium dioxide coating on stainless steel to reduce hos‐

• Bioactive titanium dioxide coating on polyetheretherketone for spinal implant application.

**2. Antimicrobial and anticorrosive titanium dioxide coating on stainless** 

The increasing incidence and host risk of device‐related infections that result in morbidity and even mortality have been noted for some time, particularly regarding the spread of antibiotic‐ resistant bacteria, such as methicillin‐resistant *Staphylococcus aureus* and bursting *Clostridium difficile.* These hospital‐acquired infections are a worldwide problem [1]. The outbreaks of SARS and avian influenza have also drawn attention to novel preventative measures, includ‐ ing the development and application of antimicrobial materials, to enhance the conventional disinfection concept. This movement compelled us to develop an antimicrobial technique for

Antimicrobial or antibacterial refers to the inhibition of bacterial growth and reproduction [2]. Antimicrobial functions can be performed by essential materials themselves or through the use of coating materials. One example of an essential antimicrobial alloy material is stainless steel that has been doped with copper. This material forms when ε‐copper precipitates in a

are attributed to its stable bonding structure, photocatalytic characteristics, and negatively

) coating is introduced to provide anticorrosive, antimicro‐

coating modification in the field of bio‐

physical, chemical, and mechanical properties.

104 Application of Titanium Dioxide

tifunctional titanium dioxide (TiO2

pital‐acquired infection.

medical implements in clinical use.

**2.1. Background**

TiO2

implant fixation to bone tissue, but also bacterial infection.

charged surfaces. In this paper, a brief overview of TiO2

**steel to reduce hospital‐acquired infection**

TiO2 deposition was conducted using a typical AIP technique and involved three steps: argon ion bombardment, bottom titanium layer deposition, and TiO<sup>2</sup> coating deposition. The ion bom‐ bardment was performed to clean and mildly preheat the substrate, followed by the bottom tita‐ nium layer deposition, which enhanced the adhesion between the substrate and TiO2 coating.

The wide acceptance indicates that an A‐TiO2 phase structure is the key factor for maximizing the antimicrobial efficiency of TiO<sup>2</sup> . This corresponds to a specific condition with 100% oxy‐ gen pressure at 0.5 Pa by using the AIP technique with a cathode target voltage of 20 V and a cathode target current of 90 A. Under this optimized deposition condition, the proportion of A‐TiO2 in the TiO2 coating has been reported to be 76.8% [12–14].

#### **2.3. Antimicrobial characteristics of TiO2 ‐coated stainless steel**

The JIS Z2801:2000 [15] was employed as a standard to test the antimicrobial efficacy of TiO2 ‐coated stainless steel specimens. The bacterial strains used in this test were Gram‐posi‐ tive *Staphylococcus aureus* (*S. aureus*, ATCC 6538P) and Gram‐positive *Escherichia coli* (*E. coli*, ATCC 8739) with an initial concentration of 4.0 × 105 bacteria/mL. In the antimicrobial test, the specimens were divided into three groups: group A and group B consisted of uncoated stainless steel specimens, and group C consisted of TiO2 ‐coated stainless steel specimens. The specimens in group A immediately underwent serial dilution and plate culture after inoculation, while the specimens of groups B and C were incubated with exposure to fluo‐ rescent lighting for 24 h. The fluorescent lamp used was a regular daily‐living light source that emitted mainly visible light and had a weak emission of 365 nm. Antimicrobial activity (*R*) of the specimens in all three groups was then calculated.

As revealed in **Figure 1** [13], the petri dishes corresponding to groups A and B (the uncoated stainless steel specimens) presented significant numbers of *S. aureus* and *E. coli* bacterial colo‐ nies, respectively; by contrast, the TiO2 ‐coated stainless steel specimens in group C did not show a significant amount of bacterial colonies. This qualitatively describes the antimicrobial ability of the TiO2 coating. Although only one out of the three petri dishes corresponding to each group is pictured in **Figure 1**, those not shown revealed a similar situation; this confirms the statistical accuracy of the antimicrobial test.

For both *S. aureus* and *E. coli*, the numbers of viable bacteria for groups A, B, and C are com‐ pared in **Figure 2** [13]. The group A specimens showed 2.85 × 10<sup>5</sup> and 1.06 × 10<sup>5</sup> viable bacteria cells, respectively, for *S. aureus* and *E. coli*, whereas the group B specimens showed 1.04 × 10<sup>4</sup> and 1.36 × 10<sup>4</sup> viable bacteria cells, respectively, for *S. aureus* and *E. coli*. By contrast, the group C specimens showed no bacterial colonies (10 bacteria cells) for *S. aureus* and 4.30 × 10<sup>1</sup> viable bacteria cells for *E. coli*. Based on these results, the TiO2 ‐coated stainless steel specimens pre‐ sented *R* values of 3.0 and 2.5, respectively, for *S. aureus* and *E. coli*. Such values are far beyond the index of 2 stipulated for the JIS test standard.

**Figure 1.** *S. aureus* and *E. coli* colonies formed on petri dishes after 24 h on the (a) group A stainless steel specimens, (b) group B stainless steel specimens, and (c) TiO2 ‐coated stainless steel specimens [13].

Anticorrosive, Antimicrobial, and Bioactive Titanium Dioxide Coating for Surface-modified Purpose... http://dx.doi.org/10.5772/intechopen.68854 107

the specimens were divided into three groups: group A and group B consisted of uncoated

The specimens in group A immediately underwent serial dilution and plate culture after inoculation, while the specimens of groups B and C were incubated with exposure to fluo‐ rescent lighting for 24 h. The fluorescent lamp used was a regular daily‐living light source that emitted mainly visible light and had a weak emission of 365 nm. Antimicrobial activity

As revealed in **Figure 1** [13], the petri dishes corresponding to groups A and B (the uncoated stainless steel specimens) presented significant numbers of *S. aureus* and *E. coli* bacterial colo‐

show a significant amount of bacterial colonies. This qualitatively describes the antimicrobial

each group is pictured in **Figure 1**, those not shown revealed a similar situation; this confirms

For both *S. aureus* and *E. coli*, the numbers of viable bacteria for groups A, B, and C are com‐

cells, respectively, for *S. aureus* and *E. coli*, whereas the group B specimens showed 1.04 × 10<sup>4</sup>

sented *R* values of 3.0 and 2.5, respectively, for *S. aureus* and *E. coli*. Such values are far beyond

**Figure 1.** *S. aureus* and *E. coli* colonies formed on petri dishes after 24 h on the (a) group A stainless steel specimens, (b)

‐coated stainless steel specimens [13].

C specimens showed no bacterial colonies (10 bacteria cells) for *S. aureus* and 4.30 × 10<sup>1</sup>

coating. Although only one out of the three petri dishes corresponding to

viable bacteria cells, respectively, for *S. aureus* and *E. coli*. By contrast, the group

‐coated stainless steel specimens.

‐coated stainless steel specimens in group C did not

and 1.06 × 10<sup>5</sup>

‐coated stainless steel specimens pre‐

viable bacteria

viable

stainless steel specimens, and group C consisted of TiO2

(*R*) of the specimens in all three groups was then calculated.

pared in **Figure 2** [13]. The group A specimens showed 2.85 × 10<sup>5</sup>

bacteria cells for *E. coli*. Based on these results, the TiO2

the index of 2 stipulated for the JIS test standard.

group B stainless steel specimens, and (c) TiO2

nies, respectively; by contrast, the TiO2

the statistical accuracy of the antimicrobial test.

ability of the TiO2

106 Application of Titanium Dioxide

and 1.36 × 10<sup>4</sup>

**Figure 2.** Viable bacteria numbers of *S. aureus* and *E. coli* for (a) group A stainless steel specimens, (b) group B stainless steel specimens, and (c) TiO2 ‐coated stainless steel specimens [13].

To further investigate the antimicrobial mechanism of a TiO2 coating, the bacterial micro‐ structure was observed using transmission electron microscopy (TEM; JEOL JEM‐1230). This closer examination revealed that most of the *S. aureus* cells were retained their integrity as the cells were inoculated on bare stainless steel with the exposure to fluorescent light for 24 h; moreover, the complete cell structure, including the cell wall, cytoplasmic membrane, cytoplasma, and nucleoid, was observed. The cells were undergoing mitosis, as presented in **Figure 3(a)** [14], was also found. These results indicate that the inoculated *S. aureus* cells on bare stainless steel were not deactivated by the fluorescent light. However, for the *S. aureus* cells on the TiO2 ‐coated stainless steel specimens, detachment of the cell wall from the cell membrane was frequently observed in the microscopic field (**Figure 3(b)** [14]). As has been noted elsewhere [16–18], the cell walls in these specimens are attacked by superoxide ions and

**Figure 3.** Cell structures of *S. aureus* inoculated on (a) bare stainless steel and (b) TiO2 ‐coated stainless steel specimens, following continuous exposure to a fluorescent lamp for 24 h. (The arrow indicates detachment of cell wall from the cell membrane.) [14].

hydroxyl radicals, and lipid peroxidation caused polyunsaturated phospholipids in the cell membrane to be destroyed; similarly, the degeneration of the membranes in the present study caused the detachment of the cell walls from the cell membranes.

A high percentage of the *E. coli* cells inoculated on bare stainless steel and exposed to fluo‐ rescent light for 24 h also retained their integrity, as depicted in **Figure 4(a)** [14]. By contrast, a large amount of *E. coli* cell fragments were observed following inoculation on TiO2 ‐coated stainless steel specimens and exposure to fluorescent light for 24 h, as presented in **Figure 4(b)** [14]. This occurred because *E. coli* cell walls are too thin to protect against attack by superox‐ ide ions and hydroxyl radicals, resulting in massive death. A closer examination of the *E. coli* cells reveals that the nucleoid structures in the cytoplasma tend to give way to features of condensation, as indicated by the arrow in **Figure 4(b)** The degeneration of *E. coli* in response to photocatalysis found in the present study is similar to the degeneration that was observed in response to the antimicrobial effects of silver ions [16].

#### **2.4. Anticorrosive characteristics of TiO2 ‐coated stainless steel**

A potentiodynamic polarization test was carried out in a potentiostat (EG&G 263 A) accord‐ ing to the ASTM G44–99 standard [19] to evaluate the corrosion resistance of a TiO2 coating in a 3.5 wt.% sodium chloride electrolyte. A saturated silver/silver chloride electrode was used as the reference, with a platinum counter electrode; a TiO2 ‐coated stainless steel specimen was inserted as the working electrode.

**Figure 5** illustrates the potentiodynamic polarization curves of bare stainless steel and TiO2 ‐coated stainless steel specimens [20]. The corrosive potential (*E*corr) and corro‐ sive current (*I*corr) were −0.54 V and 6.0 × 10−8 A/cm2 , respectively, for the bare stainless steel

Anticorrosive, Antimicrobial, and Bioactive Titanium Dioxide Coating for Surface-modified Purpose... http://dx.doi.org/10.5772/intechopen.68854 109

**Figure 4.** Cell structures of *E. coli* inoculated on (a) bare stainless steel and (b) TiO2 ‐coated stainless steel specimens, following continuous exposure to a fluorescent lamp for 24 h. (The arrows indicate the condensation features of the nucleoid) [14].

hydroxyl radicals, and lipid peroxidation caused polyunsaturated phospholipids in the cell membrane to be destroyed; similarly, the degeneration of the membranes in the present study

following continuous exposure to a fluorescent lamp for 24 h. (The arrow indicates detachment of cell wall from the cell

A high percentage of the *E. coli* cells inoculated on bare stainless steel and exposed to fluo‐ rescent light for 24 h also retained their integrity, as depicted in **Figure 4(a)** [14]. By contrast,

stainless steel specimens and exposure to fluorescent light for 24 h, as presented in **Figure 4(b)** [14]. This occurred because *E. coli* cell walls are too thin to protect against attack by superox‐ ide ions and hydroxyl radicals, resulting in massive death. A closer examination of the *E. coli* cells reveals that the nucleoid structures in the cytoplasma tend to give way to features of condensation, as indicated by the arrow in **Figure 4(b)** The degeneration of *E. coli* in response to photocatalysis found in the present study is similar to the degeneration that was observed

A potentiodynamic polarization test was carried out in a potentiostat (EG&G 263 A) accord‐

a 3.5 wt.% sodium chloride electrolyte. A saturated silver/silver chloride electrode was used

**Figure 5** illustrates the potentiodynamic polarization curves of bare stainless steel

‐coated stainless steel specimens [20]. The corrosive potential (*E*corr) and corro‐

ing to the ASTM G44–99 standard [19] to evaluate the corrosion resistance of a TiO2

**‐coated stainless steel**

‐coated

‐coated stainless steel specimens,

coating in

‐coated stainless steel specimen was

, respectively, for the bare stainless steel

a large amount of *E. coli* cell fragments were observed following inoculation on TiO2

caused the detachment of the cell walls from the cell membranes.

**Figure 3.** Cell structures of *S. aureus* inoculated on (a) bare stainless steel and (b) TiO2

in response to the antimicrobial effects of silver ions [16].

as the reference, with a platinum counter electrode; a TiO2

sive current (*I*corr) were −0.54 V and 6.0 × 10−8 A/cm2

**2.4. Anticorrosive characteristics of TiO2**

inserted as the working electrode.

and TiO2

membrane.) [14].

108 Application of Titanium Dioxide

**Figure 5.** Polarization curves of bare stainless steel and TiO2 ‐coated stainless steel specimens in a 3.5 wt.% sodium chloride solution [20].

specimens. Once the specimens had been coated with TiO2 , the *E*corr and *I*corr of the specimens were −0.42 V and 1.0 × 10−8 A/cm2 , respectively. Notably, TiO2 is an inorganic compound, and its inertness in corrosive environments (e.g., a sodium chloride solution) helps reduce the tendency and rate of substrate dissolution and species coating in an electrolyte. This increases the corrosive potential and decreases the corrosive current, as noted herein.

In summary, the research results show that A‐TiO2 adds effective antimicrobial characteristics to stainless steel. The key to providing efficient antimicrobial efficacy lies in the photocatalytic performance of the coating, which originates from the anatase phase. Furthermore, based on the TEM observation results, the antimicrobial mechanisms that inhibit *S. aureus* and *E. coli* bacteria under the photocatalytic action of A‐TiO2 are different; specifically, the antimicrobial efficacy of A‐TiO<sup>2</sup> against *E. coli* is more thorough. The A‐TiO2 coating also reduces the over‐ all rate of corrosion and increases the corrosion barrier, compared with the features of bare stainless steel.
