The Role of Surface Modification on Bacterial Adhesion of Bio-implant Materials

Machining, Characterization, and Applications

Santhosh Kumar S, Somashekhar S. Hiremath

• Publisher: CRC Press
• Year: 2020
• eBook ISBN:
  9781000098112
• Topic: Physical Sciences
• Subtopic: Clinical Medicine

The Role of Surface Modification on Bacterial Adhesion of Bio-implant Materials: Machining, Characterization, and Applications, explores the relationship between the surface roughness of artificial implants used for hard tissue replacement and their bacterial adhesion. It summarizes the reason for the failure of implants, the mechanisms of bacterial formation on implant surfaces, and the fundamental and established methods of implant surface modification techniques. It provides readers with an organized and rational representation about implant manufacturing and mechanical surface modification. It also explores the use of developed unidirectional abrasive flow finishing processes to finish biomaterials at the nano-level. It is an invaluable guide for academics, graduate students, biomaterial scientists, and manufacturing engineers researching implants, related infections, and implant manufacturing.

Key Features:

• Explores implant related infections
• Discusses surface modification techniques
• Contains information on the mechanical finishing processes and complete guide on developed cutting edge unidirectional abrasive flow finishing technology

Chapter 1

Introduction to Bio-Implants

1.1 Introduction

Bio-implants are artificial medical devices used to replace the missing biological system and the damaged biological system, and also to support or enhance the existing biological structure. These implants can be of many types, such as load-bearing implants used in orthopaedic applications, dental implants used in restoring the functionality and appearance of natural teeth, implants used in cardiovascular system, and many more. Metallic biomaterials are widely used among other materials, especially in load-bearing implants and internal fixation devices, due to their superior characteristics such as high tensile strength, high yield strength, resistance to fatigue loading, resistance to creep, high corrosion resistance, and biocompatibility. Most popular metallic biomaterials in use today are stainless steel, cobalt-chromium alloys, titanium, and titanium alloys. These materials are used in cardiovascular, orthopaedic, dentistry, craniofacial, and otorhinology implants. The implants are usually manufactured using milling, casting, forging, compression moulding, powder metallurgy, and rapid prototype techniques; further features are generated using drilling, electrical discharge machining (EDM), electron beam machining (EBM), laser beam machining (LBM), and ultrasonic machining (USM).

According to statistics in the United States, around 1.32 million implant-related infections are registered every year and amount to devastating consequences that expose patients to high morbidity and mortality. The implant infections are mainly associated with bacterial adhesion and biofilm formation. Bacterial adhesion and biofilm formation, in turn, are influenced by several micro-environmental factors. Some of such micro-environmental factors listed in the literature are mass transport, surface charge, surface conditioning, hydrophobicity, surface topography, and surface roughness. This chapter emphasises on the different types of implants, materials classifications, and bacteria-related infections under various sections.

1.2 Classifications of Bio-Implants

Bio-implants are prostheses devices used to regularize physiological functions. They are made up of biosynthetic materials like collagen, and tissue-engineered products like artificial skin or tissues. Most bio-engineered products like cardiac pacemakers and orthopaedic artificial implants are also covered under bio-implants because they are implanted entirely in the patient’s body. Bio-implants are mostly classified at a broader level such as orthopaedic, cardiovascular, dental, ophthalmic, and neurostimulation implants; these are listed in Table 1.1.

Table 1.1 Broad classifications of bio-implants (Global Trends & Forecasts till 2017)

Implant system	Different types under each implants system
Cardiovascular Implants	Pacing devices Implantable cardiac pacemakers Stents and related implants Coronary stents Peripheral stents Stent-related implants Structural cardiac implants Heart valves Tissue heart valves Mechanical heart valves Implantable heart monitors
Spinal Implants	Thoracolumbar implants Intervertebral spacers Machined allograft spacers Cervical implants Implantable spinal stimulators
Orthopaedics and Trauma	Reconstructive joint replacements Knee replacement implants Hip replacement implants Extremities Shoulder implants Elbow replacements Ankle implants Other joint replacements fusion products Orthobiologics Trauma implants Sports medicine
Dental Implants	Dental screw Abutment Crown and bone graft
Ophthalmic Implants	Intraocular lens Glaucoma and other lenses
Neurostimulators Implants	Cortical stimulators Deep brain stimulators Sacral nerve stimulators Spinal cord stimulators Vagus nerve stimulators

1.3 Materials Used in Implants

The materials used in the implants should be a biocompatible, corrosion resistance, and wear resistance; they should have excellent mechanical properties and better Osseo-integration; and should not create any effect on biological system/tissue (Mahajan and Sindhu, 2018). All the available materials on earth cannot be used as biomaterials. Researchers have developed plenty of materials that can be used as biomaterials and are continually working towards developing new biocompatible materials. Some of the factors affecting implant biomaterial are chemical factors: these include three basic types of corrosion: general, pitting, and crevice; surface specific factors: the events at the bone-implant interface can be divided into the behaviour of the implant material, the host response; electrical factors: physiochemical methods, morphologic methods, and biochemical methods; mechanical factor: modulus of elasticity, tensile or compressive forces, and elongation and metallurgical aspects. The biomaterials are classified under metals, ceramics, and polymers. Some of the major materials used in the implants are listed as follows:

Metals: Titanium and its alloys are the metals of choice for dental and orthopaedic implants. Some of the commonly used metals are stainless steel, titanium and titanium alloys, gold, cobalt-chromium alloys, zirconium, niobium, tantalum, and so on.

Ceramics: Ceramics are non-organic, non-metallic, and non-polymeric materials manufactured by compacting and sintering at elevated temperatures. The entire implant may be made of ceramic, or this may be applied as a coat to a metallic core. Some of the ceramics biomaterials are aluminium oxide, zirconium oxide, hydroxyapatite, tricalcium phosphate, tetracalcium phosphate, calcium pyrophosphate, fluorapatite, brushite, carbon glass, pyrocarbon, and bioglass.

Polymers: Polymers are most commonly used for liners and small implants. These materials can be used alone or mixed with some other ceramics. Commonly used polymers are polyethylene, polyamide, ultra-high molecular weight polyethylene (UHMW-PE), polymethyl methacrylate, polypropylene (PP), polytetrafluoro-ethylene, silicone rubber (SR), and polyurethane (Sykaras et al., 2000).

1.4 Implant Failure – Infections

Orthopaedic joint implants such as hip, knee, shoulder, ankle, and elbow prosthesis are used for hard tissue replacement. These are load-bearing joints subjected to a high level of mechanical stress, fatigue, and wear in the normal daily activity of humans. The artificial implants used for these applications should possess structural integrity as well as surface compatibility with the surrounding biological environment for prolonged survivability without complications and damage (Choudhury et al., 2017; Lysaght and O’Loughlin, 2000; Navarro et al., 2008; Ribeiro et al., 2012). Most of the dental implants also fail due to poor primary stability, bacterial infection, manufacturing defect, and improper selection of surgical protocol. Despite the adoption of advanced technology in manufacturing these new implants and in surgical and medical management procedures, the number of revisions of total knee and hip arthroplasties keeps on increasing over the period (Moriarty et al., 2016).

A significant cause for the implant failure is a fracture, prosthetic dislocation, loosening, excessive wear rate at mating surfaces and its associated debris, and pre-surgical contamination/infection (i.e., bacterial adhesion). American Joint Replacement Registry annual report (2018) collected between 2012and2017 showed that 8.2% of 47,378 hip arthroplasties and 7.9% of 40,488 knee arthroplasties are due to the infection and inflammatory reactions. The National Joint Registry for England, Wales, Northern Ireland, and the Isle of Man surgical data of 2018 report shows that the overall 0.72% of hip replacement, 0.93% of knee replacement, and 6% of shoulder replacement results in implants-related infections. Similarly, it was reported in the Australian Orthopaedic Association National Joint Replacement Registry (AOA NJRR) annual report (2018) that, overall, less than 1% of revision of knee and hip arthroplasties accounts due to infection.

1.5 Bacterial Adhesion and Biofilm Formation

Bacterial infections on implants are usually initiated through the adhesion of bacteria to the implant surface by means of physiochemical interaction between the implant surface and bacteria, that is van der Waals forces, electrostatic forces, and gravitational forces (Arciola et al., 2015; Koseki et al., 2014). The reversible bacterial adhesion is followed by colonization and formation of biofilm that forms a layer of bacteria binding irreversibly to the implant surface that is later difficult to remove from the implants (Chan et al., 2017; Koseki et al., 2014). These adhered bacteria further forma colony of the bacteria that later on forms a thick layer of the dense mass of the bacteria called biofilms.

The bacterial adhesion on the implant surface depends on the

1. Properties of the implant material such as surface topography, surface roughness, surface chemistry, and surface energy;
2. Properties of the bacteria such as surface charge, surface hydrophobicity (contact angle, θ> 90°)/hydrophilicity (contact angle, θ< 90°); and appendages; and
3. Properties of the surrounding environment, such as the type of antibiotics and its associated flow condition, temperature, pH value, a period of exposure, chemical treatment, and bacterial concentration. Figure 1.1 shows the schematic diagram of bacterial adhesion and the effects of implant material properties.

Figure 1.1 Schematic diagram of bacterial adhesion and the effect of implant material properties (Song et al., 2015).

1.6 Effect of Surface Roughness on Implants Infection and Wettability

In particular, surface roughness and topography are the most influencing parameters because studies show that surface roughness increases the surface area of the material, in turn, increases bacterial adhesion. Here, the focus has been made on the relation between the surface roughness and the bacterial adhesion, which is critically important from a clinical perspective when the work surfaces are finished with the finishing process. The depression, grooves, pits, scratches, and crevices in the rough surfaces influences the bacterial adhesion and acts as favourable sites for colonization and biofilm formation (An and Friedman, 2000; Barbour et al., 2007; Cox et al., 2017; Hocevar et al., 2014; Ribeiro et al., 2012; Wassmann et al., 2017; Yoda et al., 2014). These topographical features depend on the type of machining, finishing, coating, and surface treatment procedure followed. The near-net-shape of the bio-implants with the required surface finish is achieved by various finishing steps after the primary machining process. After the pre-fabrication, implants are subjected to the grinding process at the end. Final finishing of these surfaces is performed using either polishing integrated with multi-axis CNC machines or vibratory abrasive polishing, free abrasive polishing, fixed abrasive polishing, belt polishing, and so on. These processes have all been comprehensively used in the past decades (Bohinc et al., 2016; Cox et al., 2017; Kang and Fang, 2018; Turger et al., 2013) to remove the finer irregularities. It was estimated that polishing processes typically accounted for 10% to 15% of the total manufacturing cost and detailed review on different manufacturing and finishing processes used in implant manufacturing are detailed in a study conducted (Petare and Jain, 2018).

It is understood that, despite the adoption of advanced technology in manufacturing the implants and in surgical and medical management procedures, there are still a large number of implants that are subject to failure due to the surface finish achieve...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Structure of the Book
• Abbreviations
• Notations
• Chapter 1. Introduction to Bio-Implants
• Chapter 2. Surface Modification Techniques
• Chapter 3. Abrasive Flow Finishing: Introduction and Literature Survey
• Chapter 4. A Novel Approach for Finishing Various Implants: UAFF Process
• Chapter 5. Effect of UAFF Process Parameters on Wettability and Bacterial Adhesion
• Chapter 6. Summary and Conclusions
• References
• Index