Designing Materials for Medical Devices

The following sections are included:

• Foreword
• Preface
• About the Editor
• Acknowledgments
• Contents

The success of a material to be used as a biomaterial in medical devices, is related to biocompatibility, the ease of fabrication to form into complicated shapes and the sterilizability of the material used. This chapter provides an overview of biomaterials engineering, paying particular attention on the effect of processing methods on the mechanical properties of biomaterials. The effects of grain refinement in metals and ceramics, molding conditions on polymeric wear, and composite lamination are discussed. This is because the ability of a biomaterial to be formed into shape easily often determines its success as a medical device in the long run. When it comes to the manufacturability of a biomaterial, processing techniques will affect the final property of the biomaterial, which means affecting the durability of the device. On this note, engineers need to examine the various processing effects that stem from grain refinement of steel, surface modification to molding conditions and adoption of new manufacturing methods such as 3D printing technology. Future direction seems to lead us to nanolaminate composites, which give better properties such as fracture toughness and wear enhancement. The era of tissue engineering also paves the way for new biomaterial processes to be developed and invented. The integration of different modalities from cells, biomaterials to medical imaging has opened up new challenges in the healthcare industry.

Metallic implant materials such as stainless steel, titanium, and cobaltbased alloys have found many applications as medical devices. This is due to their excellent mechanical properties, such as fatigue strength and fracture toughness. Their durability however is dependent on their corrosion and wear resistance. The heat treatment and manufacturing method also affect these properties. The issues of adverse cellular response to wear debris from fretting fatigue and contact motion in artificial joints continue to present many challenges to the design of medical implants. The leaching of metallic ions such as nickel during the corrosion process has caused considerable concerns. This has paved way to the development of new nickel-free alloys and amorphous metals that are more biocompatible.

The practice of using metals and alloys to repair or replace human body parts is now well established. Two of the most important parameters in determining the suitability of a material for biomedical applications are its biocompatibility and corrosion resistance. This chapter gives a basic introduction to the thermodynamic and electrochemical aspects behind corrosion, concentrating on the various forms of localized corrosion that are responsible for most in-vivo failures. This will be followed by a brief review of the successes and, in reality, the remarkably few failures of the traditional materials: mainly titanium alloys, cobalt–chrome alloys, amalgams and stainless steels. The desire to make use of a number of advanced materials, such as memory-shaped alloys, porous materials and rare earth magnets will then be discussed. Unfortunately, nearly all of these materials have inadequate corrosion resistance to be used directly in vivo without some form of protection. The chapter ends with some case histories of surgical implant failures.

When a metallic material is implanted into a human body, an immediate reaction occurs between its surface and the living tissues. In other words, the immediate reaction at this initial stage straightaway determines and defines a metallic material’s tissue compatibility. Since conventional metallic biomaterials are usually covered with metal oxides, surface oxide films on metallic materials play an important role not only against corrosion but also in tissue compatibility. The surface properties of a metallic material may be controlled using surface modification techniques. To develop and apply the appropriate surface modification technique, knowledge of the material’s surface composition is absolutely necessary. This is because surface modification is a process that improves surface property by changing the composition and structure while leaving the mechanical properties of the material intact. Since surface properties are critically important in biomaterials, issues closely related to this aspect of metallic biomaterials are discussed here: surface compositions of metallic biomaterials; how surface compositions change due to interaction with human tissues; how to control surface compositions and morphologies using surface modification techniques.

Dentistry is a science and art concerned with the prevention, diagnosis, and treatment of diseases of the teeth and adjacent tissues, and the restoration of missing dental and maxillo-facial structures. Every dental restorative procedure requires the use of materials. This chapter introduces the restorative materials used in clinical dentistry. Dental biomaterials can be broadly classified into ceramics, polymers, and metals. Many restorative materials are fixed permanently into the patient’s mouth or are removed only occasionally for cleaning. The materials have to with-stand the effects of a hostile environment. Temperature variations, wide variations in acidity or alkalinity, and high stresses all affect the durability of restorative materials. Most restorative materials are managed entirely by clinicians and their assistants. Some are, however, associated with the work of dental technologists or the use of computer-aided design and manufacturing (CAD/CAM) at dental laboratory to produce customized dental restoration to reconstruct patient-specific tooth anatomy. Successful restorative dentistry is dependent on the correct selection of material for a given application and the ability to carry out customized manipulative procedures to arrive at the optimum properties of the material.

An improved understanding of currently used bioceramics in human implants and bone replacement materials could contribute significantly to the design of new-generation prostheses and post-operative patient management strategies. Overall, the benefits of advanced ceramic materials in biomedical applications have been universally appreciated — specifically in terms of their strength, biocompatibility, and wear resistance. However, the supporting data is not sparse. Against this background, continuous development of new methods is pertinent — if not imperative — for a better understanding of the microstructure–property relationship as well as obtaining new directives to further improve ceramics as biomaterials. This chapter gives an overview of and reexamines key issues that concern both the processing and applications of ceramics as biomaterials.

Polymeric hydrogels are of special importance to polymeric biomaterials because of their favorable biocompatibility. Hydrogels are crosslinked macromolecular networks formed by hydrophilic polymers swollen in water or biological fluids. The crosslinks can be formed by either covalent bonds or physical cohesion forces that exist between the polymer segments. Polymeric hydrogels are primarily classified into chemical and physical hydrogels (based on the bonding type of the crosslinks), although they can also be classified in many other ways. Chemical hydrogels can be prepared by copolymerization of a monomer with a crosslinker or by crosslinking water-soluble polymers. Physical hydrogels can be made of natural biopolymers, thermosensitive synthetic polymers, amphiphilic triblock copolymers, or many other copolymers. Furthermore, polyelectrolyte complexes and polymer-cyclodextrin inclusion complexes can likewise form hydrogels. All chemical and physical hydrogels described in this chapter are of biomedical significance. The biomedical applications of these materials in drug delivery and tissue engineering are hence discussed.

The performance of a biomedical composite is affected by many factors, including those in composite design and composite manufacture. Understanding the roles of major influencing factors is very important for successfully developing a biomedical composite from design to clinical applications. This chapter provides concise coverage of biomedical composites, from the rationale, fundamentals, templates from nature, design, and manufacture to their clinical applications. New technologies, such as 3D printing, have been increasingly used in making high-performance and customized biomedical composite products. Clinical applications of biomedical composites have also been expanding, from using non-porous non-biodegradable or biodegradable composites for human tissue replacement to employing porous biodegradable composites for human tissue regeneration. Beyond human body tissue repair, biomedical composites can be found in areas such as oncology where composite nanoparticles, the so-called theranostics, are very useful for early detection and effective treatment of cancer. Biomedical composites are fascinating biomaterials and more and more biomedical products will be made out of these materials.

Composites are materials that contain two or more distinct constituent phases on a scale larger than that of atomic. Compared with traditional homogeneous materials such as metals, ceramics, and polymers, the main advantage of composites is that their mechanical, biological, and other physical properties can be tailored to the requirements of specific applications. This chapter focuses on composites that are suitable for biomedical applications. Various application practices documented in the literature have been summarized in this chapter. Some of the commonly used methods for composite fabrications are introduced. Attention has been given to the mechanics of composites: a study which aims at estimating the mechanical properties of different composites using only the material parameters and geometric information of their constituents. Some future trends are also given at the end of this chapter.

Restoration of gait in people with lower limb amputations requires the aid of an artificial limb often referred to as a prosthesis. This chapter discusses the methods and materials related to lower limb prosthetics. Attention is paid particularly to the interface between the residual limb and the prosthesis, i.e., the prosthetic socket, where man interfaces with machine. The basic considerations, background, and constraints of various state-of-the-art techniques applied to prosthetics are discussed. These include the function and safety of prostheses, computer-aided design and fabrication methods, and prosthetic sockets. The field of prosthetics relies heavily on the innovative use of existing materials often found in other industries. The choice of materials is primarily derived from novel methods used in prostheses fabrication or component design, in order to achieve both function and safety for the person with an amputation. In the current edition of this book chapter, we have added a small section on bone-anchored percutaneous implants, an alternative method to sockets for attaching a prosthesis to a residual limb.

Chitin is a unique biopolymer based on the N—acetyl—glucosamine monomer. Over the years, there has been much interest in exploiting this biopolymer and its variants for a myriad of biomedical applications. Chitin and its derivatives have been shown to be useful as wound dressing materials, drug delivery vehicles, and increasingly as candidates for tissue engineering. The potential for this biomaterial is immense and will continue to increase. Studies are underway to explore its utilization in new biomedical applications and investigate the chemistry of extending its capability.

The aim of regulation for medical devices is to ensure that the device is safe and efficacious for human beings. The regulations also improve access of countries to good-quality and safe medical devices by offering step-by-step guidance on strengthening their regulatory controls. All countries seeking to develop their regulatory capacity can apply these steps, protect the consumer, and provide a means for international trade. Most developed regions like the United States, China, and countries in the European Union have strict regulatory standards to protect the wellbeing of their citizen. The introduction to regulatory requirements for medical devices will give the reader and overview with case studies to understand how to apply regulatory knowledge.

Laser usage in medicine opens up a new field for life sciences and the clinical diagnoses and treatment of diseases. This chapter discusses what lasers bring to biomedicine from the foundational principles to clinical applications. Different types of lasers and their interactions with various materials are introduced. This chapter is an enlightening voyage in which lasers meet biomedicine.

The following section is included:

• Index